Letter pubs.acs.org/JPCL
Materials Properties and Solvated Electron Dynamics of Isolated Nanoparticles and Nanodroplets Probed with Ultrafast Extreme Ultraviolet Beams Jennifer L. Ellis,*,† Daniel D. Hickstein,*,† Wei Xiong,†,‡ Franklin Dollar,†,○ Brett B. Palm,§ K. Ellen Keister,†,■ Kevin M. Dorney,† Chengyuan Ding,†,△ Tingting Fan,† Molly B. Wilker,∥,▼ Kyle J. Schnitzenbaumer,∥,⬡ Gordana Dukovic,∥ Jose L. Jimenez,§ Henry C. Kapteyn,† and Margaret M. Murnane† †
JILA−NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, United States Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, United States § CIRES and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ∥ Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ‡
S Supporting Information *
ABSTRACT: We present ultrafast photoemission measurements of isolated nanoparticles in vacuum using extreme ultraviolet (EUV) light produced through high harmonic generation. Surface-selective static EUV photoemission measurements were performed on nanoparticles with a wide array of compositions, ranging from ionic crystals to nanodroplets of organic material. We find that the total photoelectron yield varies greatly with nanoparticle composition and provides insight into material properties such as the electron mean free path and effective mass. Additionally, we conduct time-resolved photoelectron yield measurements of isolated oleylamine nanodroplets, observing that EUV photons can create solvated electrons in liquid nanodroplets. Using photoemission from a time-delayed 790 nm pulse, we observe that a solvated electron is produced in an excited state and subsequently relaxes to its ground state with a lifetime of 151 ± 31 fs. This work demonstrates that femotosecond EUV photoemission is a versatile surfacesensitive probe of the properties and ultrafast dynamics of isolated nanoparticles.
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that the nanoparticles are larger than a few MFPs and can be isolated in an ultrahigh vacuum chamber.17−20 Although timeresolved EUV photoemission studies have been conducted on large isolated helium clusters,21,22 the ultrafast surface dynamics of isolated nanoparticles have been largely unexplored to date. Photoemission studies of isolated nanoparticles face the challenges of generating high-flux tabletop sources of ultrashort EUV pulses and preparing a high-density beam of nanoparticles in vacuum. In this work, we overcome both of these challenges and present the first experimental study of the properties and dynamics of fully isolated nanoparticles of varying composition using ultrashort pulses of EUV light. This breakthrough is accomplished through the combination of high-harmonic generation (HHG) to produce ultrashort pulses of EUV light, a velocity-map-imaging (VMI) photoelectron spectrometer to provide high electron-collection efficiency, and an aerodynamic lens for introducing collimated beams of nanoparticles into vacuum. We present two demonstrations of how this technique can provide new insights into nanosystems. First, we
anoparticles exhibit a surface-area-to-volume ratio many orders of magnitude higher than bulk materials, allowing them to serve as powerful catalysts for chemical reactions, both in the laboratory1−3 and as atmospheric aerosols.4−6 Such surface-catalyzed chemical reactions often involve molecular motions that take place on femtosecond and picosecond time scales,7−10 with associated electronic dynamics that can occur on attosecond time scales. To capture and understand these dynamics, new experimental techniques are needed to probe the surfaces of nanoparticles on femtosecond-to-attosecond time scales. Furthermore, theoretical models of nanoparticle surface dynamics are best validated with measurements performed in the absence of solvents or surface-deposition effects, creating a demand for measurements of completely isolated (gas phase) nanoparticles on ultrafast time scales. Photoemission following extreme ultraviolet (EUV) illumination is a well established and robust technique for probing the electronic structure of atoms, molecules, and the surfaces of bulk materials.11−14 EUV photoemission is intrinsically surface sensitive because the excited electrons have mean free paths (MFPs) of only a few nanometers.15,16 Consequently, only electrons from the several outermost atomic layers can escape a material and reach the detector. Thus, EUV photoemission could serve as an ideal probe of nanoparticle surfaces, provided © XXXX American Chemical Society
Received: December 14, 2015 Accepted: January 25, 2016
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DOI: 10.1021/acs.jpclett.5b02772 J. Phys. Chem. Lett. 2016, 7, 609−615
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Figure 1. (a) Experimental apparatus for extreme ultraviolet (EUV) photoemission from isolated nanoparticles. A nanoparticle aerosol passes through an aerodynamic lens, which forms a collimated beam of nanoparticles while leaving the carrier gas divergent. The isolated nanoparticles then enter the differentially pumped photoemission chamber, where they are ionized by femtosecond EUV pulses produced through high-harmonic generation (HHG). Electrodes guide the photoelectrons toward a microchannel-plate−phosphor-screen detector, which is imaged with a CCD camera (not shown). (b) EUV light is absorbed throughout the nanoparticle, creating hot electrons (yellow). However, because of short electron mean free paths (MFPs), only electrons excited near the surface can escape the nanoparticle. In liquid nanodroplets, the excited electrons that remain inside the nanoparticle become highly excited solvated electrons.
The photoelectrons ejected from the nanoparticles are detected by a VMI photoelectron spectrometer, which consists of three parallel-plate electrodes in the Eppink−Parker geometry.30−32 We operated the VMI in spatial imaging mode,27,33,34 which allows us to measure the total photoelectron yield from isolated nanoparticles with increased signal-to-noise ratios. Further experimental details are described in the Supporting Information (SI). Because of the surface sensitivity of EUV photoemission,35−37 we avoided chemically synthesized nanoparticles, which must be passivated with surface ligands to remain suspended in solution.24 Instead, nanoparticles with bare surfaces can be produced through the aerosolization process itself, simply by using solutes that completely dissolve into a solvent without the use of ligands, for example, dissolving table salt (NaCl) into water.17,24,26 The atomizer produces an aerosol of droplets from a solution, which dry to produce nanoparticles of solute. The size of the nanoparticles produced therefore is determined by the concentration of the solute and the initial droplet size distribution, which is influenced primarily by the solvent. We produced nanoparticle aerosols from an array of different compounds, including organic compounds and ionic crystals, using three different solvents (hexane, water, and acetone). We measured the distribution of particle sizes in each case (see SI, section S2), finding the average diameter to be constant to within a factor of 1.4 for particles originating from the same solvent. Across all of the nanoparticle aerosols investigated, the average particle diameter ranged from 65 to 150 nm (Figure 2a−c). The static photoelectron yields upon EUV irradiation show dramatic material-dependent differences (Figure 2d), which cannot be accounted for by the variation in the nanoparticle size distributions (i.e., total available volume or total available surface area) (see SI, section S3). Even within a given solvent, where the nanoparticle diameter distributions are nearly identical, there is large variation in the photoelectron yield. To understand the origin of the observed variations in photoelectron yield, we can consider the specifics of the photoemission process. Photoemission can be described in terms of a three-step model, where each step contributes to the
demonstrate that EUV photoelectron yields can provide a surface-specific probe of nanoparticle properties. Specifically, we measure the static EUV photoelectron yield from ∼100 nm diameter nanoparticles with a wide variety of compositions ranging from organic materials to ionic crystalsand find that the photoelectron yield changes by more than an order of magnitude depending on the composition of the nanoparticles. We attribute this difference in photoelectron yield to varying electron MFPs and interfacial scattering (electron effective mass) in different nanoparticle systems. Second, we conduct time-resolved photoemission measurements of isolated nanoparticles using EUV light, finding that the absorption of EUV photons can create excited-state solvated electrons in oleylamine nanodroplets. This study was enabled through the combination of capabilities from atmospheric science (aerodynamic lens), physical chemistry (VMI photoelectron spectrometer), and extreme nonlinear optical science (HHG EUV source), as depicted in Figure 1. We utilize a compressed gas atomizer (TSI, model 3076) to produce an aerosol of nanoparticles, which is then introduced into the vacuum chamber via an aerodynamic lens23 (Aerodyne), resulting in a collimated beam of isolated nanoparticles streaming through vacuum. The atomizer is extremely versatile and can produce nanoparticles with a wide range of compositions, including ionic crystals, organic liquids, and clusters of chemically synthesized nanoparticles.17,24−27 A beam of EUV photons produced through HHG is gently focused onto the nanoparticle beam. We use the majority of the output of a Ti:sapphire regenerative amplifier (8 mJ, 790 nm) to produce the second harmonic (1.5 mJ, 395 nm) in a beta-barium borate crystal (β-BBO, 200 μm thickness), which is coupled into an argon-filled waveguide to drive HHG emission. The use of shorter-wavelength driving lasers for HHG achieves higher conversion efficiencies than traditional 790 nm driven HHG, producing more output EUV photons and concentrating that flux into a single narrowband harmonic (at 22 eV), instead of producing a spectral comb of many high harmonic orders.28,29 This single-bright harmonic makes 2ω driven HHG an ideal tool for photoemission, providing bright ultrafast-pulses of spectrally narrow EUV light on the tabletop. 610
DOI: 10.1021/acs.jpclett.5b02772 J. Phys. Chem. Lett. 2016, 7, 609−615
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Figure 2. Size distributions of isolated nanoparticles measured with a scanning mobility particle sizer and total photoelectron yield measured using 22 eV EUV high harmonics. Nanoparticles are produced through compressed gas atomization and originate from solutions of (a) hexane, (b) water, and (c) acetone. Although differences in the atomization process lead to variation in nanoparticle size distribution between the three solvents, within a given solvent, the distributions are quite similar. Therefore, the differences in photoelectron yield between various compounds (d) are a result of different material properties, which result in different electron MFPs and different probabilities for the electron to transmit through the surface. acac = acetylacetonate (C5H7O2).
final photoelectron yield.11,38,39 First, a photon is absorbed and a hot electron is created within the nanoparticle. In this experiment, the probability for creating a hot electron is proportional to the material absorption at 22 eV, since any photon that is absorbed must create a hot electron. Second, the hot electron travels through the material with a probability of scattering that is given by the electron MFP in that material. Third, the electron impinges upon the nanoparticle surface, where the electron can escape into the vacuum or reflect back into the nanoparticle. The probability to escape depends on the geometry of the nanoparticle and the electron effective mass. In the specific case that the nanoparticle size is much greater than the electron MFP, the escape probability is largely independent of geometry. Differences in the material absorption at 22 eV (see SI, Figure S3) do not account for the observed variations in photoelectron yield, demonstrating that considering only the first step in the three-step model of photoemission is not sufficient to explain the experimental observations. The remaining differences in photoelectron yield must therefore be related to electron transport and scattering at the interface, making this technique a sensitive probe of the associated materials properties, namely, the electron MFP and effective mass. Additionally, the surface sensitivity of EUV photoemission is illustrated by the large difference in photoelectron yield between KI nanoparticles produced from solvation in acetone versus water. Despite the similarity in the particle size distributions from the two solvents (Figure 2b,c), the photoelectron yield from KI nanoparticles produced in water is 20 times lower than from KI nanoparticles produced using acetone as the solvent. Because this difference in photoelectron yield cannot be explained by the variations in the nanoparticle size distributions, it must be due to a difference in the
nanoparticle surfaces. The vapor pressure of water is more than an order-of-magnitude lower than acetone,40 so it is quite likely that, although the acetone evaporates completely, several monolayers of water remain on the nanoparticle surface. This thickness of water is on the same order-of-magnitude the attenuation length of electrons in water41 and could account for the dramatic decrease in the photoelectron yield. Alternatively, the morphology or surface roughness of the KI nanoparticles produced through the atomization process could vary depending on the solvent of origin, changing the available surface area for photoemission. In either case, this demonstrates that EUV photoemission is an effective probe of the chemical environments and the surfaces of nanoparticles. Having investigated the surface sensitivity of EUV photoemission from nanoparticles, we further demonstrate that this technique can probe femtosecond dynamics in nanoparticles. We performed EUV-pump−near-IR-probe spectroscopy on oleylamine (C18H35NH2) nanodroplets, monitoring the total photoelectron yield as a function of the time delay between 790 nm and EUV pulses. We find a pronounced enhancement in the yield when the pulses are overlapped in time, a significant decay on the femtosecond time scale (151 ± 31 fs, Figure 3a), and then a long-lived enhancement of the yield when the EUV pulse precedes the 790 nm pulse (≫100 ps, Figure 3b). We observe no such dynamics or enhancement in photoelectron yield when instead the 790 nm pulse precedes the EUV pulse. These dynamics likely result from the formation of excited solvated electrons in the oleylamine nanodroplet and their subsequent relaxation to their ground state. Solvated electrons were first observed in ammonia in 190842 and have recently attracted much attention because of their high reactivity,43−46 for example, their role in dissociative electron-transfer reactions. Since their discovery in ammonia, they have been observed in 611
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photon process, making it relatively insensitive to any prior excitation of the nanodroplet caused by the 790 nm pulse. In this work, we presented measurements of isolated nanoparticles using EUV light from a tabletop high-harmonicgeneration source. We found that the EUV light is a sensitive probe of the nanoparticle surface and that the total photoelectron yield from the nanoparticles is strongly dependent on the material properties, namely the electron MFP and effective mass. Although these total photoelectron-yield measurements cannot distinguish the contributions of the second and third steps in the three-step model of photoemission, measuring the change in photoelectron yield with nanoparticle size could be used to deconvolve them.57 Therefore, this technique provides an exciting new opportunity to directly measure material properties like electron MFPs and effective masses for a wide array of materials that cannot readily be studied with other techniques. The capability to measure these material properties for a wide array of unconventional materials can greatly enhance our understanding of the behavior of hot electrons in materials, including solvated electrons in nanodroplets.58−60 We also demonstrated a pump−probe measurement of the femtosecond electron dynamics that occur after a nanodroplet is irradiated by high-flux femtosecond pulses of EUV light, which create excited-state solvated electrons within the nanodroplet. These solvated electrons can be probed by subsequent ionization by a near-IR pulse, revealing that the solvated electrons are created in an excited state and then relax to their ground state within 151 ± 31 fs. They remain as ground-state solvated electrons for ≫100 ps, producing a longlived enhancement in the photoelectron yield. Here, the spatial imaging mode of the VMI spectrometer was used to obtain a higher signal-to-noise ratio; however, future studies (at higher laser repetition rates) may obtain fully energy and angularly resolved photoelectron distributions, providing further insight into nanoparticle surface dynamics. This technique can scale readily to smaller nanostructures, higher photon energies, and attosecond pulse durations, paving the way for a convenient tabletop probe of ultrafast dynamics of nanoparticles and their surfaces. Driving the HHG process with 790 nm or longer wavelengths produces higher energy photons and attosecond pulse trains or even isolated attosecond bursts.61,62 Higher-energy probe photons provide access to core levels of the atoms within nanoparticles, providing elemental and chemical specificity. Additionally, the energy dependence of electron MFPs can be exploited by tuning the EUV photon energy to tune the depth into the nanoparticle that photoemission probes. Furthermore, methods of generating bright circularly polarized EUV light through novel HHG techniques63−67 will allow the study of chiral and magnetic nanoparticles. Therefore, future improvements to the apparatus may provide a breakthrough method for understanding the femtosecond and attosecond dynamics of nanoparticle surfaces.
Figure 3. Photoelectron yield from oleylamine nanodroplets as a function of time delay between a 790 nm pulse and a 22 eV EUV pulse. Negative time delays have nonzero photoelectron yield because there are static signals from both the pump and the probe included here, photoelectrons originating from the probe alone are due to multiphoton ionization from the nanodroplets. (a) On femtosecond time scales, the electron yield is enhanced when the pulses are overlapped and drops exponentially to an equilibrium value with a time constant of 151 ± 31 fs. This decay corresponds to the time scale for excited-solvated electron states to relax to their ground state. See Supporting Information for fitting details. (b) When the EUV pulse precedes the 790 nm pulse, the photoelectron yield is enhanced (lifetime ≫ 100 ps), indicating that electrons in the nanodroplet have been excited into a long-lived solvated-electron ground state by the EUV irradiation, which makes subsequent ionization by the 790 nm pulse more probable.
many different liquids, including water, alcohols, and acetonitrile.47−50 Solvated electrons are typically created after electrons have been promoted into the conduction band of a liquid.47,51,52 When an EUV photon ionizes a molecule within a nanodroplet, a high-energy electron is produced, which typically cannot directly escape the droplet. Instead, it scatters within the droplet, relaxing through the conduction band until the electron becomes solvated by the surrounding oleylamine.53,54 Solvated electrons are relatively loosely bound, with typical ground-state binding energies ranging from about −3.3 eV (water)47,48 to −1.5 eV (ammonia)54 relative to the vacuum. Therefore, solvated electrons are much more easily ionized by the 790 nm (1.57 eV) pulse than electrons that are still in the ground state of oleylamine molecules, leading to an enhancement in the photoelectron yield when the 790 nm pulse arrives after an EUV pulse. The long persistence of this enhancement is in agreement with previously observed lifetimes of solvated electrons.48,55 Furthermore, the initial 151 ± 31 fs decay is consistent with previous measurements of the time scale for excited-solvated electrons to be created and decay to the ground 1s state in ammonia.54,56 The greater photoelectron yields observed when 790 nm pulses ionize excited states of the solvated electron can be understood through two separate mechanisms. One explanation is that the excited states are more delocalized than the ground state, leading to a larger photoionization cross section. Another possibility is that the binding energy of the groundstate solvated electron in oleylamine is greater than 1.57 eV. In that case, two 790 nm photons are required to ionize a groundstate solvated electron, while only one is necessary for excited states, resulting in a greater probability of photoionization from the excited state. We observe no measurable dynamics when the 790 nm pulse precedes the EUV pulse because 22 eV photons can easily ionize unexcited oleylamine in a single
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02772. Experimental details for the photoelectron yield measurements; details of the nanoparticle aerosol diameter distribution measurements; photoelectron yield data adjusted for material absorption at 22 eV, total available volume, and total available surface area; discussion of the 612
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possibility of hydrates; fitting procedure to extract the lifetime of the excited state solvated electrons. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Present Addresses ○
Department of Physics and Astronomy, University of California Irvine, Irvine, California 92697, United States. ■ Department of Physics and Astronomy, Earlham College, Richmond, Indiana 47374, United States. △ Cymer, Inc., San Diego, California 92127, United States. ▼ Department of Chemistry, Luther College, Decorah, Iowa 52101, United States. ⬡ Department of Chemistry, Transylvania University, Lexington, Kentucky 40508, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Eleanor Campbell for many substantive and helpful discussions. This work was completed at JILA. J.E., D.H., W.X., F.D., K.D., C.D., T.F., H.K., and M.M. acknowledge support from the DOE Office of Basic Energy Sciences (AMOS program, DE-FG02-99ER14982). J.E. acknowledges support from the National Science Foundation Graduate Research Fellowship (DGE-1144083). B.P. and J.J. were supported by DOE (BER/ASR) DE-SC0011105. B.P. acknowledges support from a U.S. EPA STAR Graduate Fellowship (FP-91761701-0). M.W., K.S., and G.D. acknowledge support from the Air Force Office of Scientific Research under AFOSR award No. FA9550-12-1-0137.
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DOI: 10.1021/acs.jpclett.5b02772 J. Phys. Chem. Lett. 2016, 7, 609−615
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The Journal of Physical Chemistry Letters (27) Ellis, J. L.; Hickstein, D. D.; Schnitzenbaumer, K. J.; Wilker, M. B.; Palm, B. B.; Jimenez, J. L.; Dukovic, G.; Kapteyn, H. C.; Murnane, M. M.; Xiong, W. Solvents Effects on Charge Transfer from Quantum Dots. J. Am. Chem. Soc. 2015, 137, 3759−3762. (28) Wang, H.; Xu, Y.; Ulonska, S.; Robinson, J. S.; Ranitovic, P.; Kaindl, R. A. Bright High-Repetition-Rate Source of Narrowband Extreme-Ultraviolet Harmonics beyond 22 eV. Nat. Commun. 2015, 6, 7459. (29) Eich, S.; Stange, A.; Carr, A. V.; Urbancic, J.; Popmintchev, T.; Wiesenmayer, M.; Jansen, K.; Ruffing, A.; Jakobs, S.; Rohwer, T.; et al. Time- and Angle-Resolved Photoemission Spectroscopy with Optimized High-Harmonic Pulses Using Frequency-Doubled Ti:Sapphire Lasers. J. Electron Spectrosc. Relat. Phenom. 2014, 195, 231−236. (30) Eppink, A. T. J. B.; Parker, D. H. Velocity Map Imaging of Ions and Electrons Using Electrostatic Lenses: Application in Photoelectron and Photofragment Ion Imaging of Molecular Oxygen. Rev. Sci. Instrum. 1997, 68, 3477−3484. (31) Parker, D. H.; Eppink, A. T. J. B. Photoelectron and Photofragment Velocity Map Imaging of State-Selected Molecular Oxygen Dissociation/ionization Dynamics. J. Chem. Phys. 1997, 107, 2357−2362. (32) Hickstein, D. D.; Ranitovic, P.; Witte, S.; Tong, X.-M.; Huismans, Y.; Arpin, P.; Zhou, X.; Keister, K. E.; Hogle, C. W.; Zhang, B.; et al. Direct Visualization of Laser-Driven Electron Multiple Scattering and Tunneling Distance in Strong-Field Ionization. Phys. Rev. Lett. 2012, 109, 073004. (33) Johnsson, P.; Rouzée, A.; Siu, W.; Huismans, Y.; Lépine, F.; Marchenko, T.; Düsterer, S.; Tavella, F.; Stojanovic, N.; Redlin, H.; et al. Characterization of a Two-Color Pump-Probe Setup at FLASH Using a Velocity Map Imaging Spectrometer. Opt. Lett. 2010, 35, 4163−4165. (34) Stei, M.; von Vangerow, J.; Otto, R.; Kelkar, A. H.; Carrascosa, E.; Best, T.; Wester, R. High Resolution Spatial Map Imaging of a Gaseous Target. J. Chem. Phys. 2013, 138, 214201. (35) Winter, B.; Weber, R.; Hertel, I. V.; Faubel, M.; Jungwirth, P.; Brown, E. C.; Bradforth, S. E. Electron Binding Energies of Aqueous Alkali and Halide Ions: EUV Photoelectron Spectroscopy of Liquid Solutions and Combined Ab Initio and Molecular Dynamics Calculations. J. Am. Chem. Soc. 2005, 127, 7203−7214. (36) Bauer, M. Femtosecond Ultraviolet Photoelectron Spectroscopy of Ultra-Fast Surface Processes. J. Phys. D: Appl. Phys. 2005, 38, R253−R267. (37) Siffalovic, P.; Drescher, M.; Heinzmann, U. Femtosecond TimeResolved Core-Level Photoelectron Spectroscopy Tracking Surface Photovoltage Transients on P − GaAs. Europhys. Lett. 2002, 60, 924− 930. (38) Hüfner, S. Photoelectron Spectroscopy: Principles and Applications, 3rd ed.; Springer-Verlag: Berlin/Heidelberg/New York, 2003. (39) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. Photochemistry of Colloidal Semiconductors. 20. Surface Modification and Stability of Strong Luminescing CdS Particles. J. Am. Chem. Soc. 1987, 109, 5649− 5655. (40) William, M. Haynes. CRC Handbook of Chemistry and Physics, 96th ed.; CRC Press: Boca Raton, FL, 2015. (41) Suzuki, Y.-I.; Nishizawa, K.; Kurahashi, N.; Suzuki, T. Effective Attenuation Length of an Electron in Liquid Water between 10 and 600 eV. Phys. Rev. E. Stat. Nonlin. Soft Matter Phys. 2014, 90, 010302. (42) Kraus, C. A. Solutions of Metals in Non-Metallic Solvents; IV. 1 Material Effects Accompanying the Passage of an Electrical Current through Solutions of Metals in Liquid Ammonia. Migration Experiments. J. Am. Chem. Soc. 1908, 30, 1323−1344. (43) Simons, J. How Do Low-Energy (0.1−2 eV) Electrons Cause DNA-Strand Breaks? Acc. Chem. Res. 2006, 39, 772−779. (44) Wang, C.-R.; Nguyen, J.; Lu, Q.-B. Bond Breaks of Nucleotides by Dissociative Electron Transfer of Nonequilibrium Prehydrated Electrons: A New Molecular Mechanism for Reductive DNA Damage. J. Am. Chem. Soc. 2009, 131, 11320−11322. (45) Arnold, F. Solvated Electrons in the Upper Atmosphere. Nature 1981, 294, 732−733.
(46) Jonah, C. D.; Bartels, D. M.; Chernovitz, A. C. Primary Processes in the Radiation Chemistry of Water. Int. J. Radiat. Appl. Instrumentation. Part C. Radiat. Phys. Chem. 1989, 34, 145−156. (47) Siefermann, K. R.; Liu, Y.; Lugovoy, E.; Link, O.; Faubel, M.; Buck, U.; Winter, B.; Abel, B. Binding Energies, Lifetimes and Implications of Bulk and Interface Solvated Electrons in Water. Nat. Chem. 2010, 2, 274−279. (48) Yamamoto, Y.; Suzuki, Y.-I.; Tomasello, G.; Horio, T.; Karashima, S.; Mitríc, R.; Suzuki, T. Time- and Angle-Resolved Photoemission Spectroscopy of Hydrated Electrons Near a Liquid Water Surface. Phys. Rev. Lett. 2014, 112, 187603. (49) Shreve, A. T.; Elkins, M. H.; Neumark, D. M. Photoelectron Spectroscopy of Solvated Electrons in Alcohol and Acetonitrile Microjets. Chem. Sci. 2013, 4, 1633−1639. (50) Ehrler, O. T.; Neumark, D. M. Dynamics of Electron Solvation in Molecular Clusters. Acc. Chem. Res. 2009, 42, 769−777. (51) Elles, C. G.; Jailaubekov, A. E.; Crowell, R. A.; Bradforth, S. E. Excitation-Energy Dependence of the Mechanism for Two-Photon Ionization of Liquid H(2)O and D(2)O from 8.3 to 12.4 eV. J. Chem. Phys. 2006, 125, 044515. (52) Laenen, R.; Roth, T.; Laubereau, A. Novel Precursors of Solvated Electrons in Water: Evidence for a Charge Transfer Process. Phys. Rev. Lett. 2000, 85, 50−53. (53) Savolainen, J.; Uhlig, F.; Ahmed, S.; Hamm, P.; Jungwirth, P. Direct Observation of the Collapse of the Delocalized Excess Electron in Water. Nat. Chem. 2014, 6, 697−701. (54) Vöhringer, P. Ultrafast Dynamics of Electrons in Ammonia. Annu. Rev. Phys. Chem. 2015, 66, 97−118. (55) Buchner, F.; Lübcke, A.; Heine, N.; Schultz, T. Time-Resolved Photoelectron Spectroscopy of Liquids. Rev. Sci. Instrum. 2010, 81, 113107. (56) Lindner, J.; Unterreiner, A.-N.; Vöhringer, P. Femtosecond Relaxation Dynamics of Solvated Electrons in Liquid Ammonia. ChemPhysChem 2006, 7, 363−369. (57) Goldmann, M.; Miguel-Sánchez, J.; West, A. H. C.; Yoder, B. L.; Signorell, R. Electron Mean Free Path from Angle-Dependent Photoelectron Spectroscopy of Aerosol Particles. 2015, arXiv:1502.05699. arXiv.org e-Print archive. http://arxiv.org/abs/ 1502.05699 (accessed December 2015). (58) Lee, G. H.; Arnold, S. T.; Eaton, J. G.; Sarkas, H. W.; Bowen, K. H.; Ludewigt, C.; Haberland, H. Negative Ion Photoelectron Spectroscopy of Solvated Electron Cluster Anions, (H2O) N- and (NH3) N-. Z. Phys. D: At., Mol. Clusters 1991, 20, 9−12. (59) Alfano, J. C.; Walhout, P. K.; Kimura, Y.; Barbara, P. F. Ultrafast Transient-Absorption Spectroscopy of the Aqueous Solvated Electron. J. Chem. Phys. 1993, 98, 5996. (60) Elkins, M. H.; Williams, H. L.; Shreve, A. T.; Neumark, D. M. Relaxation Mechanism of the Hydrated Electron. Science 2013, 342, 1496−1499. (61) Popmintchev, T.; Chen, M.-C.; Arpin, P.; Murnane, M. M.; Kapteyn, H. C. The Attosecond Nonlinear Optics of Bright Coherent X-Ray Generation. Nat. Photonics 2010, 4, 822−832. (62) Chen, M.-C.; Mancuso, C.; Hernández-García, C.; Dollar, F.; Galloway, B.; Popmintchev, D.; Huang, P.-C.; Walker, B.; Plaja, L.; Jaroń-Becker, A. A.; et al. Generation of Bright Isolated Attosecond Soft X-Ray Pulses Driven by Multicycle Midinfrared Lasers. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, E2361−E2367. (63) Fleischer, A.; Kfir, O.; Diskin, T.; Sidorenko, P.; Cohen, O. Spin Angular Momentum and Tunable Polarization in High-Harmonic Generation. Nat. Photonics 2014, 8, 543−549. (64) Kfir, O.; Grychtol, P.; Turgut, E.; Knut, R.; Zusin, D.; Popmintchev, D.; Popmintchev, T.; Nembach, H.; Shaw, J. M.; Fleischer, A.; et al. Generation of Bright Phase-Matched CircularlyPolarized Extreme Ultraviolet High Harmonics. Nat. Photonics 2014, 9, 99−105. (65) Lambert, G.; Vodungbo, B.; Gautier, J.; Mahieu, B.; Malka, V.; Sebban, S.; Zeitoun, P.; Luning, J.; Perron, J.; Andreev, A.; et al. Towards Enabling Femtosecond Helicity-Dependent Spectroscopy with High-Harmonic Sources. Nat. Commun. 2015, 6, 6167. 614
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The Journal of Physical Chemistry Letters (66) Hickstein, D. D.; Dollar, F. J.; Grychtol, P.; Ellis, J. L.; Knut, R.; Hernández-García, C.; Zusin, D.; Gentry, C.; Shaw, J. M.; Fan, T.; et al. Non-Collinear Generation of Angularly Isolated Circularly Polarized High Harmonics. Nat. Photonics 2015, 9, 743−750. (67) Mancuso, C. A.; Hickstein, D. D.; Grychtol, P.; Knut, R.; Kfir, O.; Tong, X.-M.; Dollar, F.; Zusin, D.; Gopalakrishnan, M.; Gentry, C.; et al. Strong-Field Ionization with Two-Color Circularly Polarized Laser Fields. Phys. Rev. A: At., Mol., Opt. Phys. 2015, 91, 031402.
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Materials Properties and Solvated Electron Dynamics of Isolated Nanoparticles and Nanodroplets Probed with Ultrafast Extreme Ultraviolet Beams Jennifer L. Ellis,*,† Daniel D. Hickstein,*,† Wei Xiong,†,‡ Franklin Dollar,†,○ Brett B. Palm,§ K. Ellen Keister,†,■ Kevin M. Dorney,† Chengyuan Ding,†,△ Tingting Fan,† Molly B. Wilker,∥,▼ Kyle J. Schnitzenbaumer,∥,⬡ Gordana Dukovic,∥ Jose L. Jimenez,§ Henry C. Kapteyn,† and Margaret M. Murnane† †
JILA−NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, United States Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093, United States § CIRES and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ∥ Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ‡
S Supporting Information *
ABSTRACT: We present ultrafast photoemission measurements of isolated nanoparticles in vacuum using extreme ultraviolet (EUV) light produced through high harmonic generation. Surface-selective static EUV photoemission measurements were performed on nanoparticles with a wide array of compositions, ranging from ionic crystals to nanodroplets of organic material. We find that the total photoelectron yield varies greatly with nanoparticle composition and provides insight into material properties such as the electron mean free path and effective mass. Additionally, we conduct time-resolved photoelectron yield measurements of isolated oleylamine nanodroplets, observing that EUV photons can create solvated electrons in liquid nanodroplets. Using photoemission from a time-delayed 790 nm pulse, we observe that a solvated electron is produced in an excited state and subsequently relaxes to its ground state with a lifetime of 151 ± 31 fs. This work demonstrates that femotosecond EUV photoemission is a versatile surfacesensitive probe of the properties and ultrafast dynamics of isolated nanoparticles.
N
that the nanoparticles are larger than a few MFPs and can be isolated in an ultrahigh vacuum chamber.17−20 Although timeresolved EUV photoemission studies have been conducted on large isolated helium clusters,21,22 the ultrafast surface dynamics of isolated nanoparticles have been largely unexplored to date. Photoemission studies of isolated nanoparticles face the challenges of generating high-flux tabletop sources of ultrashort EUV pulses and preparing a high-density beam of nanoparticles in vacuum. In this work, we overcome both of these challenges and present the first experimental study of the properties and dynamics of fully isolated nanoparticles of varying composition using ultrashort pulses of EUV light. This breakthrough is accomplished through the combination of high-harmonic generation (HHG) to produce ultrashort pulses of EUV light, a velocity-map-imaging (VMI) photoelectron spectrometer to provide high electron-collection efficiency, and an aerodynamic lens for introducing collimated beams of nanoparticles into vacuum. We present two demonstrations of how this technique can provide new insights into nanosystems. First, we
anoparticles exhibit a surface-area-to-volume ratio many orders of magnitude higher than bulk materials, allowing them to serve as powerful catalysts for chemical reactions, both in the laboratory1−3 and as atmospheric aerosols.4−6 Such surface-catalyzed chemical reactions often involve molecular motions that take place on femtosecond and picosecond time scales,7−10 with associated electronic dynamics that can occur on attosecond time scales. To capture and understand these dynamics, new experimental techniques are needed to probe the surfaces of nanoparticles on femtosecond-to-attosecond time scales. Furthermore, theoretical models of nanoparticle surface dynamics are best validated with measurements performed in the absence of solvents or surface-deposition effects, creating a demand for measurements of completely isolated (gas phase) nanoparticles on ultrafast time scales. Photoemission following extreme ultraviolet (EUV) illumination is a well established and robust technique for probing the electronic structure of atoms, molecules, and the surfaces of bulk materials.11−14 EUV photoemission is intrinsically surface sensitive because the excited electrons have mean free paths (MFPs) of only a few nanometers.15,16 Consequently, only electrons from the several outermost atomic layers can escape a material and reach the detector. Thus, EUV photoemission could serve as an ideal probe of nanoparticle surfaces, provided © XXXX American Chemical Society
Received: December 14, 2015 Accepted: January 25, 2016
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Figure 1. (a) Experimental apparatus for extreme ultraviolet (EUV) photoemission from isolated nanoparticles. A nanoparticle aerosol passes through an aerodynamic lens, which forms a collimated beam of nanoparticles while leaving the carrier gas divergent. The isolated nanoparticles then enter the differentially pumped photoemission chamber, where they are ionized by femtosecond EUV pulses produced through high-harmonic generation (HHG). Electrodes guide the photoelectrons toward a microchannel-plate−phosphor-screen detector, which is imaged with a CCD camera (not shown). (b) EUV light is absorbed throughout the nanoparticle, creating hot electrons (yellow). However, because of short electron mean free paths (MFPs), only electrons excited near the surface can escape the nanoparticle. In liquid nanodroplets, the excited electrons that remain inside the nanoparticle become highly excited solvated electrons.
The photoelectrons ejected from the nanoparticles are detected by a VMI photoelectron spectrometer, which consists of three parallel-plate electrodes in the Eppink−Parker geometry.30−32 We operated the VMI in spatial imaging mode,27,33,34 which allows us to measure the total photoelectron yield from isolated nanoparticles with increased signal-to-noise ratios. Further experimental details are described in the Supporting Information (SI). Because of the surface sensitivity of EUV photoemission,35−37 we avoided chemically synthesized nanoparticles, which must be passivated with surface ligands to remain suspended in solution.24 Instead, nanoparticles with bare surfaces can be produced through the aerosolization process itself, simply by using solutes that completely dissolve into a solvent without the use of ligands, for example, dissolving table salt (NaCl) into water.17,24,26 The atomizer produces an aerosol of droplets from a solution, which dry to produce nanoparticles of solute. The size of the nanoparticles produced therefore is determined by the concentration of the solute and the initial droplet size distribution, which is influenced primarily by the solvent. We produced nanoparticle aerosols from an array of different compounds, including organic compounds and ionic crystals, using three different solvents (hexane, water, and acetone). We measured the distribution of particle sizes in each case (see SI, section S2), finding the average diameter to be constant to within a factor of 1.4 for particles originating from the same solvent. Across all of the nanoparticle aerosols investigated, the average particle diameter ranged from 65 to 150 nm (Figure 2a−c). The static photoelectron yields upon EUV irradiation show dramatic material-dependent differences (Figure 2d), which cannot be accounted for by the variation in the nanoparticle size distributions (i.e., total available volume or total available surface area) (see SI, section S3). Even within a given solvent, where the nanoparticle diameter distributions are nearly identical, there is large variation in the photoelectron yield. To understand the origin of the observed variations in photoelectron yield, we can consider the specifics of the photoemission process. Photoemission can be described in terms of a three-step model, where each step contributes to the
demonstrate that EUV photoelectron yields can provide a surface-specific probe of nanoparticle properties. Specifically, we measure the static EUV photoelectron yield from ∼100 nm diameter nanoparticles with a wide variety of compositions ranging from organic materials to ionic crystalsand find that the photoelectron yield changes by more than an order of magnitude depending on the composition of the nanoparticles. We attribute this difference in photoelectron yield to varying electron MFPs and interfacial scattering (electron effective mass) in different nanoparticle systems. Second, we conduct time-resolved photoemission measurements of isolated nanoparticles using EUV light, finding that the absorption of EUV photons can create excited-state solvated electrons in oleylamine nanodroplets. This study was enabled through the combination of capabilities from atmospheric science (aerodynamic lens), physical chemistry (VMI photoelectron spectrometer), and extreme nonlinear optical science (HHG EUV source), as depicted in Figure 1. We utilize a compressed gas atomizer (TSI, model 3076) to produce an aerosol of nanoparticles, which is then introduced into the vacuum chamber via an aerodynamic lens23 (Aerodyne), resulting in a collimated beam of isolated nanoparticles streaming through vacuum. The atomizer is extremely versatile and can produce nanoparticles with a wide range of compositions, including ionic crystals, organic liquids, and clusters of chemically synthesized nanoparticles.17,24−27 A beam of EUV photons produced through HHG is gently focused onto the nanoparticle beam. We use the majority of the output of a Ti:sapphire regenerative amplifier (8 mJ, 790 nm) to produce the second harmonic (1.5 mJ, 395 nm) in a beta-barium borate crystal (β-BBO, 200 μm thickness), which is coupled into an argon-filled waveguide to drive HHG emission. The use of shorter-wavelength driving lasers for HHG achieves higher conversion efficiencies than traditional 790 nm driven HHG, producing more output EUV photons and concentrating that flux into a single narrowband harmonic (at 22 eV), instead of producing a spectral comb of many high harmonic orders.28,29 This single-bright harmonic makes 2ω driven HHG an ideal tool for photoemission, providing bright ultrafast-pulses of spectrally narrow EUV light on the tabletop. 610
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Figure 2. Size distributions of isolated nanoparticles measured with a scanning mobility particle sizer and total photoelectron yield measured using 22 eV EUV high harmonics. Nanoparticles are produced through compressed gas atomization and originate from solutions of (a) hexane, (b) water, and (c) acetone. Although differences in the atomization process lead to variation in nanoparticle size distribution between the three solvents, within a given solvent, the distributions are quite similar. Therefore, the differences in photoelectron yield between various compounds (d) are a result of different material properties, which result in different electron MFPs and different probabilities for the electron to transmit through the surface. acac = acetylacetonate (C5H7O2).
final photoelectron yield.11,38,39 First, a photon is absorbed and a hot electron is created within the nanoparticle. In this experiment, the probability for creating a hot electron is proportional to the material absorption at 22 eV, since any photon that is absorbed must create a hot electron. Second, the hot electron travels through the material with a probability of scattering that is given by the electron MFP in that material. Third, the electron impinges upon the nanoparticle surface, where the electron can escape into the vacuum or reflect back into the nanoparticle. The probability to escape depends on the geometry of the nanoparticle and the electron effective mass. In the specific case that the nanoparticle size is much greater than the electron MFP, the escape probability is largely independent of geometry. Differences in the material absorption at 22 eV (see SI, Figure S3) do not account for the observed variations in photoelectron yield, demonstrating that considering only the first step in the three-step model of photoemission is not sufficient to explain the experimental observations. The remaining differences in photoelectron yield must therefore be related to electron transport and scattering at the interface, making this technique a sensitive probe of the associated materials properties, namely, the electron MFP and effective mass. Additionally, the surface sensitivity of EUV photoemission is illustrated by the large difference in photoelectron yield between KI nanoparticles produced from solvation in acetone versus water. Despite the similarity in the particle size distributions from the two solvents (Figure 2b,c), the photoelectron yield from KI nanoparticles produced in water is 20 times lower than from KI nanoparticles produced using acetone as the solvent. Because this difference in photoelectron yield cannot be explained by the variations in the nanoparticle size distributions, it must be due to a difference in the
nanoparticle surfaces. The vapor pressure of water is more than an order-of-magnitude lower than acetone,40 so it is quite likely that, although the acetone evaporates completely, several monolayers of water remain on the nanoparticle surface. This thickness of water is on the same order-of-magnitude the attenuation length of electrons in water41 and could account for the dramatic decrease in the photoelectron yield. Alternatively, the morphology or surface roughness of the KI nanoparticles produced through the atomization process could vary depending on the solvent of origin, changing the available surface area for photoemission. In either case, this demonstrates that EUV photoemission is an effective probe of the chemical environments and the surfaces of nanoparticles. Having investigated the surface sensitivity of EUV photoemission from nanoparticles, we further demonstrate that this technique can probe femtosecond dynamics in nanoparticles. We performed EUV-pump−near-IR-probe spectroscopy on oleylamine (C18H35NH2) nanodroplets, monitoring the total photoelectron yield as a function of the time delay between 790 nm and EUV pulses. We find a pronounced enhancement in the yield when the pulses are overlapped in time, a significant decay on the femtosecond time scale (151 ± 31 fs, Figure 3a), and then a long-lived enhancement of the yield when the EUV pulse precedes the 790 nm pulse (≫100 ps, Figure 3b). We observe no such dynamics or enhancement in photoelectron yield when instead the 790 nm pulse precedes the EUV pulse. These dynamics likely result from the formation of excited solvated electrons in the oleylamine nanodroplet and their subsequent relaxation to their ground state. Solvated electrons were first observed in ammonia in 190842 and have recently attracted much attention because of their high reactivity,43−46 for example, their role in dissociative electron-transfer reactions. Since their discovery in ammonia, they have been observed in 611
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photon process, making it relatively insensitive to any prior excitation of the nanodroplet caused by the 790 nm pulse. In this work, we presented measurements of isolated nanoparticles using EUV light from a tabletop high-harmonicgeneration source. We found that the EUV light is a sensitive probe of the nanoparticle surface and that the total photoelectron yield from the nanoparticles is strongly dependent on the material properties, namely the electron MFP and effective mass. Although these total photoelectron-yield measurements cannot distinguish the contributions of the second and third steps in the three-step model of photoemission, measuring the change in photoelectron yield with nanoparticle size could be used to deconvolve them.57 Therefore, this technique provides an exciting new opportunity to directly measure material properties like electron MFPs and effective masses for a wide array of materials that cannot readily be studied with other techniques. The capability to measure these material properties for a wide array of unconventional materials can greatly enhance our understanding of the behavior of hot electrons in materials, including solvated electrons in nanodroplets.58−60 We also demonstrated a pump−probe measurement of the femtosecond electron dynamics that occur after a nanodroplet is irradiated by high-flux femtosecond pulses of EUV light, which create excited-state solvated electrons within the nanodroplet. These solvated electrons can be probed by subsequent ionization by a near-IR pulse, revealing that the solvated electrons are created in an excited state and then relax to their ground state within 151 ± 31 fs. They remain as ground-state solvated electrons for ≫100 ps, producing a longlived enhancement in the photoelectron yield. Here, the spatial imaging mode of the VMI spectrometer was used to obtain a higher signal-to-noise ratio; however, future studies (at higher laser repetition rates) may obtain fully energy and angularly resolved photoelectron distributions, providing further insight into nanoparticle surface dynamics. This technique can scale readily to smaller nanostructures, higher photon energies, and attosecond pulse durations, paving the way for a convenient tabletop probe of ultrafast dynamics of nanoparticles and their surfaces. Driving the HHG process with 790 nm or longer wavelengths produces higher energy photons and attosecond pulse trains or even isolated attosecond bursts.61,62 Higher-energy probe photons provide access to core levels of the atoms within nanoparticles, providing elemental and chemical specificity. Additionally, the energy dependence of electron MFPs can be exploited by tuning the EUV photon energy to tune the depth into the nanoparticle that photoemission probes. Furthermore, methods of generating bright circularly polarized EUV light through novel HHG techniques63−67 will allow the study of chiral and magnetic nanoparticles. Therefore, future improvements to the apparatus may provide a breakthrough method for understanding the femtosecond and attosecond dynamics of nanoparticle surfaces.
Figure 3. Photoelectron yield from oleylamine nanodroplets as a function of time delay between a 790 nm pulse and a 22 eV EUV pulse. Negative time delays have nonzero photoelectron yield because there are static signals from both the pump and the probe included here, photoelectrons originating from the probe alone are due to multiphoton ionization from the nanodroplets. (a) On femtosecond time scales, the electron yield is enhanced when the pulses are overlapped and drops exponentially to an equilibrium value with a time constant of 151 ± 31 fs. This decay corresponds to the time scale for excited-solvated electron states to relax to their ground state. See Supporting Information for fitting details. (b) When the EUV pulse precedes the 790 nm pulse, the photoelectron yield is enhanced (lifetime ≫ 100 ps), indicating that electrons in the nanodroplet have been excited into a long-lived solvated-electron ground state by the EUV irradiation, which makes subsequent ionization by the 790 nm pulse more probable.
many different liquids, including water, alcohols, and acetonitrile.47−50 Solvated electrons are typically created after electrons have been promoted into the conduction band of a liquid.47,51,52 When an EUV photon ionizes a molecule within a nanodroplet, a high-energy electron is produced, which typically cannot directly escape the droplet. Instead, it scatters within the droplet, relaxing through the conduction band until the electron becomes solvated by the surrounding oleylamine.53,54 Solvated electrons are relatively loosely bound, with typical ground-state binding energies ranging from about −3.3 eV (water)47,48 to −1.5 eV (ammonia)54 relative to the vacuum. Therefore, solvated electrons are much more easily ionized by the 790 nm (1.57 eV) pulse than electrons that are still in the ground state of oleylamine molecules, leading to an enhancement in the photoelectron yield when the 790 nm pulse arrives after an EUV pulse. The long persistence of this enhancement is in agreement with previously observed lifetimes of solvated electrons.48,55 Furthermore, the initial 151 ± 31 fs decay is consistent with previous measurements of the time scale for excited-solvated electrons to be created and decay to the ground 1s state in ammonia.54,56 The greater photoelectron yields observed when 790 nm pulses ionize excited states of the solvated electron can be understood through two separate mechanisms. One explanation is that the excited states are more delocalized than the ground state, leading to a larger photoionization cross section. Another possibility is that the binding energy of the groundstate solvated electron in oleylamine is greater than 1.57 eV. In that case, two 790 nm photons are required to ionize a groundstate solvated electron, while only one is necessary for excited states, resulting in a greater probability of photoionization from the excited state. We observe no measurable dynamics when the 790 nm pulse precedes the EUV pulse because 22 eV photons can easily ionize unexcited oleylamine in a single
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02772. Experimental details for the photoelectron yield measurements; details of the nanoparticle aerosol diameter distribution measurements; photoelectron yield data adjusted for material absorption at 22 eV, total available volume, and total available surface area; discussion of the 612
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possibility of hydrates; fitting procedure to extract the lifetime of the excited state solvated electrons. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Present Addresses ○
Department of Physics and Astronomy, University of California Irvine, Irvine, California 92697, United States. ■ Department of Physics and Astronomy, Earlham College, Richmond, Indiana 47374, United States. △ Cymer, Inc., San Diego, California 92127, United States. ▼ Department of Chemistry, Luther College, Decorah, Iowa 52101, United States. ⬡ Department of Chemistry, Transylvania University, Lexington, Kentucky 40508, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Eleanor Campbell for many substantive and helpful discussions. This work was completed at JILA. J.E., D.H., W.X., F.D., K.D., C.D., T.F., H.K., and M.M. acknowledge support from the DOE Office of Basic Energy Sciences (AMOS program, DE-FG02-99ER14982). J.E. acknowledges support from the National Science Foundation Graduate Research Fellowship (DGE-1144083). B.P. and J.J. were supported by DOE (BER/ASR) DE-SC0011105. B.P. acknowledges support from a U.S. EPA STAR Graduate Fellowship (FP-91761701-0). M.W., K.S., and G.D. acknowledge support from the Air Force Office of Scientific Research under AFOSR award No. FA9550-12-1-0137.
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