Helium induced fine structure in the electronic spectra of anthracene derivatives doped into superfluid helium nanodroplets.

Helium induced fine structure in the electronic spectra of anthracene derivatives doped into superfluid helium nanodroplets D. Pentlehner and A. Slenczka Citation: The Journal of Chemical Physics 142, 014311 (2015); doi: 10.1063/1.4904899 View online: http://dx.doi.org/10.1063/1.4904899 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microsolvation in superfluid helium droplets studied by the electronic spectra of six porphyrin derivatives and one chlorine compound J. Chem. Phys. 138, 244303 (2013); 10.1063/1.4811199 Line broadening in electronic spectra of anthracene derivatives inside superfluid helium nanodroplets J. Chem. Phys. 133, 114505 (2010); 10.1063/1.3479583 Electronic polarization spectroscopy of metal phthalocyanine chloride compounds in superfluid helium droplets J. Chem. Phys. 127, 174308 (2007); 10.1063/1.2803186 Fine structure of the (S 1 ←S 0 ) band origins of phthalocyanine molecules in helium droplets J. Chem. Phys. 121, 9396 (2004); 10.1063/1.1804945 Helium nanodroplet isolation spectroscopy of perylene and its complexes with oxygen J. Chem. Phys. 120, 6792 (2004); 10.1063/1.1667462

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THE JOURNAL OF CHEMICAL PHYSICS 142, 014311 (2015)

Helium induced fine structure in the electronic spectra of anthracene derivatives doped into superfluid helium nanodroplets D. Pentlehner and A. Slenczkaa) Institut für Physikalische und Theoretische Chemie, Universität Regensburg, 93040 Regensburg, Germany

(Received 20 October 2014; accepted 10 December 2014; published online 7 January 2015) Electronic spectra of organic molecules doped into superfluid helium nanodroplets show characteristic features induced by the helium environment. Besides a solvent induced shift of the electronic transition frequency, in many cases, a spectral fine structure can be resolved for electronic and vibronic transitions which goes beyond the expected feature of a zero phonon line accompanied by a phonon wing as known from matrix isolation spectroscopy. The spectral shape of the zero phonon line and the helium induced phonon wing depends strongly on the dopant species. Phonon wings, for example, are reported ranging from single or multiple sharp transitions to broad (∆ν > 100 cm−1) diffuse signals. Despite the large number of example spectra in the literature, a quantitative understanding of the helium induced fine structure of the zero phonon line and the phonon wing is missing. Our approach is a systematic investigation of related molecular compounds, which may help to shed light on this key feature of microsolvation in superfluid helium droplets. This paper is part of a comparative study of the helium induced fine structure observed in electronic spectra of anthracene derivatives with particular emphasis on a spectrally sharp multiplet splitting at the electronic origin. In addition to previously discussed species, 9-cyanoanthracene and 9-chloroanthracene will be presented in this study for the first time. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4904899]

I. INTRODUCTION

Electronic spectroscopy of molecules in superfluid helium droplets provides information on both the dopant molecular species and the host system. The latter aspect is of particular interest since superfluid helium is not accessible by standard spectroscopic means. Superfluid helium is a so called quantum fluid, which in the form of helium nanodroplets serves as a host system with unique cryogenic properties.1–9 Almost any kind of molecular species can be dissolved into helium droplets and is cooled down to a temperature of only 0.38 K within pico-seconds. In contrast to classical solvents, rotationally resolved spectra are obtained in helium droplets which is a strong indication for superfluidity. Environmentally induced effects such as solvent shifts or line broadening are in most cases observed to be small compared to classical solvents or solid matrices. While this is the case in infrared and microwave spectra, electronic spectra can reveal helium induced features such as phonon wings (PWs), multiplet splitting, or severe line broadening. In some cases, these effects can even dominate the spectra thereby turning the use of helium droplets as host into a disadvantage for a spectroscopic investigation of the dopant species.10–12 As discussed in Ref. 11, the helium environment appears to be particularly sensitive on the change of the electron density distribution of the dopant species. Numerous molecules are known where the change of the electron density distribution induced by electronic excitation causes a rearrangement of the intramolecular nuclear coordinates. Rearrangement includes modifications of bond lengths and ana)Author to whom correspondence should be addressed. Electronic mail:

[email protected] 0021-9606/2015/142(1)/014311/8/$30.00

gles, isomerization, proton transfer, charge transfer, and other unimolecular photochemical processes. Forces responsible for such intramolecular rearrangements certainly also perturb the helium environment surrounding the dopant species. Thus, the response of the helium environment on the change of the electron density distribution appears to be the reason for many of the helium induced spectral features reported for electronic spectra of molecules in helium droplets. In order to learn about the dopant species and/or about the solvent, it is necessary to disentangle purely molecular contributions from helium induced features in the electronic spectra. Consequently, a better and finally quantitative understanding of the helium induced spectral fine structure is needed. In contrast to a classical solvent, a rotational fine structure of the dopant molecule can be resolved in superfluid helium droplets, even though it shows modified moments of inertia as compared to the isolated molecule.14 The low energy gap of elementary excitations of superfluid helium creates a characteristic phonon gap which should separate the zero phonon line (ZPL) from the PW.13 In addition, the superfluid host system provides rather homogeneous conditions which contrasts to the severe inhomogeneous spectral broadening found in solid matrices. These features are all met in the electronic spectrum of Glyoxal.13,14 However, in this respect, the spectrum of Glyoxal is unique because for all other molecules investigated so far in helium droplets the fine structure of the ZPL differs from the expected rotational fine structure and the spectral shape of the PW differs from the spectrum of elementary excitations of superfluid helium. There is consensus on an explanation of such deviations by an empirical model of a non-superfluid helium solvation layer covering the dopant molecule. The most extensive investigation on this phenomenon has been pub-

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lished for phthalocyanine.15–21,11 Instead of a dopant molecule dissolved into a superfluid helium droplet, one has to deal with a dopant-helium complex, henceforth addressed as solvation complex, dissolved into a superfluid helium droplet. Since the solvation layer consists of an arrangement of individual helium atoms, one has to take into account the possibility of more than only a single configuration, also addressed as different isomers, of the solvation complex which justifies the non-rotational fine structure of the ZPL. Such configurations correspond to what is called sites in matrix isolation spectroscopy. Moreover, the solvation layer as part of the host system exhibits van der Waals modes which belong to the PW and, thus, justify any spectral shape other than that of the spectrum of purely superfluid helium. Since the helium solvation layer is determined by the shape of the dopant species and, in particular, by its electron density distribution, the spectral shape of the ZPL and of the PW are dopant specific. Related molecular species are expected to show similarities in the helium induced features of ZPL and eventually also of the PW. Consequently, a systematic study of, for example, a series of derivatives of a spectroscopically well understood molecule may help to interpret the helium induced spectral features and, thereby, to learn more about microsolvation in superfluid helium droplets. Motivated by this idea, the present paper is part of an extended investigation of anthracene derivatives10,11,22,23 which was preceded by similar studies on pyrromethene dye molecules24 as well as porphyrin derivatives.25 The present paper reports on electronic spectroscopy of 9-cyanoanthracene (9-CNA) and 9-chloroanthracene (9-CA) doped into superfluid helium droplets and complements our investigation of a series of anthracene derivatives.10,11,22,23 The helium induced fine-structure of electronic and vibronic transitions are analyzed individually and in comparison with other anthracene derivatives in order to identify spectral features which are common to all or unique for individual anthracene derivatives. Such features may provide a guide line for testing theoretical models for microsolvation in superfluid helium droplets.

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temperatures chosen for 9-CA and 9-CAN were about 25 ◦C and 40 ◦C, respectively, and, thus, optimized for single molecule doping. About 8 cm behind the pick-up unit, the doped droplet beam was crossed perpendicularly by an excimer laser (Lambda Physics LEXTRA 100) (50 Hz) pumped pulsed dye laser (Lambda Physics LPD3000) beam. The band width of the laser was about 0.2 cm−1. The wavelength of the pulsed dye laser was not calibrated for absolute values. However, as found by comparison with a single mode ring dye laser equipped with a high resolution wavemeter, the pulsed dye laser frequency reading needed an offset of −5 cm−1 to match the same molecular resonances with an accuracy of ±1 cm−1. The molecular fluorescence was collected perpendicular to both the droplet beam axis and the laser beam axis and imaged onto the cathode of a photomultiplier tube (PMT) (Hamamatsu R94302). The PMT signal was amplified (Ortec VT120) and fed into a boxcar averager (Scientific Instruments SRS250). The averaged signal was digitized (Scientific Instruments SRS245) and recorded together with the corresponding laser wavelength. Simultaneously, the laser intensity was recorded in order to eliminate the intensity profile of the dye laser. Laser stray light collected by the imaging optics was minimized by placing an edge filter in front of the PMT. For dispersed emission spectra, the laser frequency was kept fix in resonance with a molecular transition, and the fluorescence was imaged onto the entrance slit of a spectrograph. The spectrograph consists of a grating spectrometer (LOT Oriel MS257) and a charge coupled device (CCD) camera (ANDOR iDUS). The CCD chip consists of 256 × 1024 pixels and was operated in full vertical binning mode providing spectra with 1024 data points. The spectral resolution can be estimated by the frequency section covering one camera pixel horizontally which was 4.0 cm−1 at the blue and 5.8 cm−1 at the red end of the spectrum of 9-CNA. The chemicals 9-CNA and 9-CA were purchased from Aldrich with a purity of at least 96% and were used without further purification. The data of additional anthracene derivatives reported in Refs. 10, 11, 22, and 23 which will be addressed in the discussion below have been recorded using the same experimental setup.

II. EXPERIMENT

The experimental setup consists of a two chamber vacuum machine equipped with a pulsed helium droplet source described in Ref. 26. Helium droplets were formed by expansion of pure helium (6.0 purity) under a stagnation pressure of 80 bars through an Even-Lavie valve modified for cryogenic operation with an orifice of 60 µm in diameter. The valve was attached to a closed cycle cryostat and cooled to a temperature of 21-22 K producing droplets with an average size of about 105 He atoms. The valve, which can be operated at a repetition rate of up to 1 kHz, was synchronized to the repetition rate of the laser system limited to 50 Hz. At a distance of 5 cm downstream, the droplet beam passed through a home built trombone shaped skimmer with an opening aperture of 6 mm in diameter and 13 cm further downstream through a pick-up cell. The pick-up cell was resistively heated for sublimating the solid sample. The temperature was adjusted for optimizing single molecule doping of the helium droplets. The

III. EXPERIMENTAL RESULTS

The fluorescence excitation spectrum of 9-CNA recorded in helium droplets shown in panel (c) of Fig. 1 starts with an intense sharp peak which is followed for the next 1800 cm−1 by a series of much less intense but similarly sharp peaks (note the gap in the ordinate). This pattern reflects the corresponding gas phase spectrum27–30 where the leading and most intense peak was assigned as the electronic origin. The absolute value for the frequency position of the corresponding peak in panel (c) of Fig. 1 is 26 074 cm−1. It is shifted by 97 cm−1 to the red with respect to that from the gas phase.30 Table I lists frequency position and intensity of all transitions with respect to the electronic origin. These values correspond to vibrational frequencies and are complemented by corresponding gas phase data from two literature sources as indicated. Finally, the helium induced shift of vibrational frequencies with respect to the corresponding gas phase data was deter-


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J. Chem. Phys. 142, 014311 (2015) TABLE I. Relative wavenumbers ν − ν 0 (cm−1) in excitation and peak intensities of 9-CNA in helium droplets with ν 0 = 26 074 cm−1 and I0 = 1. Only the leading intense peak of the fine structure is listed (cf. text). For comparison, the most intense transitions of 9,10-CNA in the gas phase ν v i b (jet) are listed from Refs. 27 and 30. ν − ν 0 / cm−1 (droplet)

FIG. 1. Fluorescence excitation spectrum of anthracene (a), 9,10-DCA with additional spectral section recorded from a 9-CA sample (b), 9-CNA (c), and dispersed emission spectrum of 9-CNA (d) all in helium droplets. The value of ν 0 amounts to 27 622 cm−1 for anthracene, 25 889 cm−1 for 9,10-DCA, and 26 074 cm−1 for 9-CNA. The excitation spectrum was recorded with a laser intensity of about 10 µJ/pulse to avoid saturation and was normalized to the laser intensity. In the additional spectral section plotted in panel (b), only two signals can be assigned to 9-CA (cf. text).

mined to less than 3% which is the typical small value found for vibrational frequencies of molecules in helium droplets. By comparison with bare anthracene shown in panel (a) of Fig. 1, vibrational modes involving the cyano group can readily be identified. For example, the 216 cm−1 mode which is missing for bare anthracene can be assigned to a cyano bending mode. The intensity pattern of a fluorescence excitation spectrum normalized to the corresponding absorption spectrum reveals the fluorescence quantum yield of the state specific decay path of the molecular compound.28–30 The decay path of molecules in an excited state is known to be affected by the helium environment. In the case of 9,10-dichloroanthracene (9,10DCA) whose spectrum is shown in panel (b) together with a section recorded for a 9-CA sample, the helium droplet was found to promote the fluorescence quantum yield so that the intensity pattern in the excitation spectrum is closer to the absorption spectrum in the gas phase than to the fluorescence excitation spectrum in the gas phase.23 Also in the case of 9-CNA, the fluorescence excitation spectrum in helium droplets reflects a promotion of radiative decay paths and a suppression of non-radiative channels. The intensity pattern appears to be an intermediate between the corresponding absorption and fluorescence spectra in the gas phase.28–30 This issue will be addressed again at the end of the next paragraph.

0 ··· 216 378 432 448 594 627 661 672 755 810 824 843 888 970 1004 1020 1030 1039 1049 1071 1119 1129 1132 1162 1185 1238 1246 1313 1330 1344 1359 1361 1371 1378 1408 1465 1484 1502 1521 1539 1560 1579 1721 a Reference b Reference

I/I0 (droplet)

ν v i b a / cm−1 (jet)

ν v i b b / cm−1 (jet)

shift / cm−1 (droplet)

1.000 ··· 0.144 0.35 0.008 0.033 0.028 0.023 0.011 0.053 0.031 0.003 0.007 0.005 0.005 0.005 0.006 0.002 0.017 0.009 0.015 0.005 0.003 0.002 0.004 0.093 0.008 0.004 0.018 0.015 0.005 0.038 0.043 0.21 0.032 0.043 0.012 0.008 0.016 0.009 0.007 0.036 0.014 0.051 0.02

0 128 214 376 429 448 593 645 ··· 661 751 ··· ··· 868 (?) 882 966 ··· 1019 ··· ···

0 128 217 377 ··· ··· 588 ··· ··· ··· 753 ··· ···

0 ··· +2/−1 +2/+1 +3 0 +1/+5 +18 ··· +11 +4/+2 ··· ··· (−25) +6 +4 ··· +1 +3 ··· +4 +3 ··· ··· ··· +4/+2 +4 ··· +4/0 +3 ··· ··· +3 +6 ··· +3/−18 ··· +1 ··· ··· ··· +4 ··· +4/+2 −9/−17

1068 ··· ··· ··· 1158 1181 ··· 1242 1310 ··· ··· 1356 1367 ··· 1375 ··· ··· ··· ··· ··· 1535 ··· 1575 1730

··· ··· ··· ··· 1027 ··· 1045 ··· ··· ··· ··· 1160 ··· ··· 1246 ··· ··· ··· ··· ··· ··· 1396 ··· 1464 ··· ··· ··· ··· ··· 1577 1738

27. 30.

Important information on the dopant to helium interaction can be deduced from the fine structure of each individual transition and in the present case most clearly from the intense electronic origin. Plotted to a magnified scale, this fine structure is shown in panel (a) of Fig. 2. The spectral section of only 9 cm−1 shows the intense sharp peak accompanied by


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FIG. 2. Excitation spectra of the electronic origin (a) (recorded for low (thick line) and high (thin line) laser intensity) and selected vibronic transitions (b)-(d) of 9-CNA in helium droplets on an expanded scale. ν 0 = 26 074 cm−1 . Line shapes were fitted with a Lorentzian (red lines) with line widths of 0.71, 0.35, and 0.45 cm−1 for the 216 (b), 378 (c), and 1162 cm−1 (d) mode, respectively.

a series of similarly sharp, however, much less intense peaks with most pronounced maxima shifted by 0.8, 1.7, 2.6, and 3.6 cm−1 and one at −0.5 cm−1 with respect to the most intense peak. This fine structure can neither be fitted by a rotational fine structure nor has any counterpart in the gas phase. For the electronic origin (panel (a)), the most intense peak is readily saturated at low laser intensity while the tiny signals show

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linear response to the laser intensity over additional two orders of magnitude of the laser intensity. The difference in the saturation behavior reveals oscillator strengths which are largest for the intense peak and significantly smaller for all the tiny signals. As shown for three examples in Fig. 2 (panels (b)–(d)), the helium induced fine structure repeats for all vibronic transitions of 9-CNA, however, with a mode specific broadening. As in the case of phthalocyanine,11 the intense peak of vibronic transitions can be fitted perfectly by a Lorentzian with a line width of 0.71, 0.35, and 0.45 cm−1 for the 216, 378, and 1162 cm−1 modes, respectively, while the line width at the electronic origin of about 0.2 cm−1 reflects the apparatus function. Compared to the higher energy modes, the cyano bending mode at 216 cm−1 is significantly broadened, and compared to the gas phase spectra, the peak intensity of the cyano bending mode is reduced by a factor of 2/3. However, the reduction in the peak intensity is compensated by the increase in the line width so that the intensity pattern throughout the entire spectrum as read from the peak area is similar to the intensity pattern reported for the corresponding gas phase spectrum. Under gas phase conditions, the fluorescence quantum yield of the 216 cm−1 mode is almost unity,28–30 and the fluorescence decay times are almost independent of the excitation energy up to at least 590 cm−1 excess energy.31,27,32 Under these conditions, the fluorescence excitation spectrum is identical to the absorption spectrum. The information obtained for the electronically excited state are complemented by data on the electronic ground state as deduced from dispersed emission spectra. In contrast to a gas phase experiment, dispersed emission spectra of (organic) molecules in helium droplets are usually independent of the resonance chosen for excitation.17,22 The decay path of dopant species excited in excess to the electronic origin starts in most cases with the dissipation of the excess energy into the helium droplet which is followed by the decay of the ground level of S1 whose radiation is recorded. The dispersed emission recorded for 9-CNA is shown in panel (d) of Fig. 1 with a frequency axis inverted at the electronic origin in order to compare the vibrational fine structure with that in the excitation spectrum. Within the experimental accuracy of absolute frequency measurement and spectral resolution, the electronic origin of dispersed emission and fluorescence excitation coincide. The vibrational fine structure is in agreement with that recorded in the gas phase upon excitation at the electronic origin.31,27,30 Furthermore, it is similar to the emission spectra of anthracene and 9,10DCA though the cyano group adds the totally symmetric low frequency mode around 220 cm−1 and weak C-N stretching modes at around 2200 cm−1 (not shown in Fig. 1; cf. Table II). Further, the reduction of symmetry from D2h for anthracene to C2v for 9-CNA lifts some degeneracies, among others of vibrations in the range of the C–C stretching and the C–H bending modes.33 In addition to 9-CNA, 9-CA has been investigated exhibiting the same C2v symmetry. The purity of the 9-CA sample which was labeled to 96% was confirmed by a nuclear magnetic resonance spectrum. Thereby, a contamination of the 9-CA sample with 9,10-DCA could be detected similar as reported in Ref. 34. 9,10-DCA is known to have a fluorescence quantum yield of almost 100% (Ref. 30) in the gas phase


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TABLE II. Relative wavenumbers ν 0 − ν (cm−1) in the emission spectrum of 9-CNA in helium droplets with ν 0 = 26 106 cm−1 and their relative intensities I/I0. Gas phase spectra are shown in Refs. 31, 27, and 29 though vibrational frequencies are not reported. ν 0 − ν / cm−1 (droplet) 0 228 385 457 615 682 776 1068 1172 1268 1344 1412 1491 1570 1641 1804 1871

I/I0 (droplet)

ν 0 − ν / cm−1 (droplet)

I/I0 (droplet)

1.00 0.20 0.35 0.06 0.08 0.11 0.06 0.04 0.15 0.29 0.10 0.51 0.09 0.37 0.20 0.22 0.07

1952 2098 2190 2251 2338 2435 2591 2685 2751 2835 2907 2983 3067 3140 3220 3362 3518

0.15 0.05 0.05 0.06 0.04 0.05 0.09 0.12 0.07 0.22 0.08 0.15 0.09 0.08 0.11 0.06 0.06

whereas the corresponding value for 9-CA measured at room temperature in solution is less than 10%.35 The fluorescence excitation spectrum of the 9-CA sample recorded in helium droplets was dominated by the signal of the minor impurity 9,10-DCA as a result of the much larger fluorescence quantum yield. An unequivocal identification of the very weak 9CA signal required discrimination of the intense 9,10-DCA signal which was accomplished by recording a helium droplet spectrum of a commercial 9,10-DCA sample (full spectrum in Fig. 2(b)) in comparison to that of a 9-CA sample (additional spectral section in Fig. 2(b)). Thus, the electronic origin at 26 726 cm−1 and an additional vibrational mode of 374 cm−1 of 9-CA marked by vertical arrows in panel (b) of Fig. 2 could be identified. Most likely, the latter corresponds to the most intense vibrational mode of anthracene at 385 cm−1.10,22 The electronic origin of 9-CA is shown in Fig. 3 with a frequency axis scaled to the most intense peak at 26 726 cm−1. Compared to the gas phase, it is shifted by 66 cm−1 to the red.36 Similar as for 9-CNA, the peaks assigned to the electronic origin and to a vibronic transition (not shown in Fig. 3) are accompanied by additional weak features shifted by 0.8, 2.1, and 3.6 cm−1 to the blue (cf. Fig. 3). Even a red shifted peak can be identified at the red tail of the intense peak.

IV. DISCUSSION

The excitation and emission spectra of 9-CNA recorded in helium droplets (cf. Fig. 1, panels (c) and (d)) reflect very much what was recorded under gas phase conditions. This concerns the vibrational frequencies as well as the similarity of these frequencies in excitation and emission. Both observations are the typical signature of a rigid molecule. In the low frequency section of the helium droplet spectrum, the 128 cm−1 mode reported for the gas phase is entirely missing. According

FIG. 3. Excitation spectra of the electronic origin of 9-CA in helium droplets. ν 0 = 26 726 cm−1.

to Ref. 32, this is an in-plane bending mode of the cyano group. The relative peak intensity reported for this mode in the gas phase26–28 should suffice to overcome the detection limit of the helium droplet experiment. As was frequently observed in helium droplets, transitions involving low energy and large amplitude vibrational modes suffer significant line broadening whereby the peak intensity is greatly reduced. This is empirically explained by helium induced damping of such modes as exemplified for butterfly modes of polycyclic aromatic hydrocarbons compounds such as tetracene,37 pentacene37 and perylene20 and, more recently, in an extensive study for torsional and bending modes of various substituted derivatives of pyrromethene dye molecules.11,24 This also accounts for the reduced peak intensity of the 216 cm−1 mode discussed above. In the ultimate case which we presume for the 128 cm−1 mode of 9-CNA, vibrational mode damping causes line broadening to such an extent, that the peak intensity is below the detection limit. In contrast, all the other vibronic transitions which correspond to vibronic transitions also recorded for bare anthracene appear as sharp and intense peaks which reflect a less effective damping of such in plane modes. It should be noted that similar observations have been reported for the corresponding modes of 9,10-DCA.23 For vibronic excitations, the helium mediated decay path starts with dissipation of vibrational excess excitation energy, and the fluorescence stems exclusively from the radiative decay of the ground level of S1. If only this helium enforced decay path is active, the intensities of all transitions in the fluorescence excitation spectrum are determined by the fluorescence quantum yield of the ground level of S1. Under these conditions, the intensity pattern within the spectrum is governed solely by the excitation probability and, therefore, is identical to the absorption spectrum. Fluorescence intensities of individual transitions in the helium droplet spectrum, which are reduced compared to the absorption spectrum reveal participation of non-radiative decay paths which compete with the initial step of relaxation to the ground level of S1.


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One of the key features providing insight into microsolvation in superfluid helium droplets is the helium induced fine structure which in many cases was assigned partly to the ZPL and partly to the PW. The fine structure in the ZPL is empirically explained by the presence of configurational variants of a solvation complex whose van der Waals modes contribute to the fine structure of the PW. Commonly accepted criteria for assigning ZPL and PW are first spectral shape, second spectral position, third saturation behavior (which is related to the oscillator strength), and fourth the frequency shift of emission with respect to the excitation. The observation of emission coincident with the excitation frequency is an unequivocal indication for a ZPL at the electronic origin. It is also evident that the PW carries excitation energy in excess to the purely molecular electronic origin. Therefore, a PW appears blue shifted to the ZPL and exhibits exclusively emission red shifted with respect to its excitation frequency. However, the observation of exclusively red shifted emission does not prove against the ZPL of an electronic origin. As will be shown in the following, neither of these four criteria nor all together suffice to unequivocally assign ZPL and PW within the helium induced fine structure. The resonances of van der Waals modes of the helium solvation complex are possibly as sharp as the preceding ZPL. Moreover, van der Waals modes cover the same frequency range as the variation of the solvent shift for configurational variants of the solvation complex. Thus, ZPL and PW of an ensemble of numerous solvation complexes may spectrally overlap. In addition, the ZPL may show exclusively red shifted emission in case the electronic origin leads into a highly metastable configuration of the solvation complex which undergoes relaxation prior to radiative decay. The same scenario may shift the transition probability from the ZPL to the PW similar as within a FranckCondon-progression of vibrational bands. In the ultimate case, the PW may exhibit a higher oscillator strength than the ZPL. Consequently, one has to be very careful in the assignment of ZPL and PW within a highly structured helium induced fine structure. With this in mind, the corresponding fine structure of 9-CNA and 9-CA will be analyzed and compared to other anthracene derivatives previously investigated in our laboratory with the same experimental setup. The basic system of the entire study is bare anthracene whose helium induced fine structure at the electronic origin of the fluorescence excitation spectrum starts with a pronounced doublet which is followed by a second less pronounced one (cf. Fig. 4(a)). Dissimilarities of the leading double peak feature to that reported for tetracene remain hidden when looking only at the excitation spectrum.38,22 In short, each of the two doublets exhibits its own emission spectrum and its own saturation behavior. This reveals a common emissive state and common oscillator strength within the first and the second doublet, respectively, whereby the oscillator strength is much larger for the first doublet.22 In Ref. 22, we tentatively assigned both doublets to ZPLs. According to the discussion in the previous paragraph, the experimental data do not exclude an assignment of each doublet to a ZPL followed by a PW. In addition to anthracene, Figs. 4(b)–4(f) show the corresponding fine structure of five additional anthracene derivatives. The anthracene derivatives are sorted according to the molecular

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FIG. 4. Excitation spectra at the electronic origin of selected anthracene derivatives: (a) anthracene, (b) 9,10-DCA, (c) 9-CA, (d) 9-CNA, (e) 9-MA, and (f) 1-MA recorded with the same laser and detection scheme and at laser intensities low enough to avoid saturation effects. ν 0 as given in Refs. 22, 10, and 23. For 9-MA, the experimental spectrum (thick line) can be seen as combination of two identical contributions one of which is added as thin line which includes the first intense peak. (for more details see text).

symmetry. In panel (b), 9,10-DCA exhibits the D2h symmetry of bare anthracene while the symmetry of 9-CA (c) and 9-CNA (d) is reduced to C2v . The two methyl substituted compounds 9-methylanthracene (9-MA) (e) and 1-MA (f) are both of Cs symmetry. While all of these derivatives show a helium induced fine structure with sharp resonances, none of them reflect the spectral shape observed for anthracene. However, these derivatives reveal similarities among each other. Except for 9-MA, all substituted anthracene derivatives show only a single leading peak instead of a doublet. Furthermore, this peak is followed by a series of tiny nevertheless sharp peaks. The spectrum recorded for 9-MA can be interpreted as an overlap of two identical such spectra one starting with the first intense peak and a second with the second intense peak. The first such spectra is shown by the first intense peak continuing with the thin line in panel (e) of Fig. 4. This can empirically be explained by a tunneling doublet within the highly symmetric intramolecular torsional potential of the methyl group which creates two systems of opposite parity as explained in Ref. 10. This model can similarly be applied explaining the two doublets of bare anthracene assuming tunneling splitting for the


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solvation complex which is accomplished by the high symmetry in the electron density distribution of bare anthracene as seen by the helium solvation layer. The vanishing of the doublet splitting has also been observed for a cluster of anthracene with H2O22 similar as reported for tetracene.39,40 The spectrum of 9,10-DCA proves that the presence or absence of the characteristic doublet recorded for bare anthracene does not correlate with the symmetry of the corresponding dopant species. Unfortunately, the signal level and the spectral resolution of the spectrograph did not allow to obtain further information from the dispersed emission spectra of 9-MA. Most probably, an explanation of the helium induced fine structure lies in the individual electron density distribution of each dopant species which determines the dopant to helium interface and, thus, the possible configurations of a solvation complex as well as its dynamics upon electronic excitation. Finally, the weak nevertheless sharp spectral features accompanying the intense peak of the substituted anthracene derivatives need to be discussed. Despite of the variations in the details of the spectral intensity distribution, the similarity among the five anthracene derivatives is remarkable. For all of the anthracene derivatives, the saturation behavior revealed a reduced oscillator strength for the tiny signal to the blue similar as exemplified for 9-CNA in the top panel of Fig. 2. As outlined above, we have no spectroscopic criterion which allows for a definite distinction between two assignments, namely, configurational variants or van der Waals modes of a solvation complex. Thus, both explanations need to be taken into account. Important information is available from spectroscopic data of anthracene-helium clusters in the gas phase. The complex of anthracene with a single helium atom shows van der Waals modes at 8 cm−1 and 22 cm−1.41,42 For additional three helium atoms, three van der Waals modes are reported at 2.0, 3.6, and 7.0 cm−1.41,42 The same paper reports vanishing of spectrally sharp van der Waals modes for clusters with seven and more helium atoms. Since the density of states increases with the number of particles of a system, there is no reason to expect a reappearance of isolated van der Waals modes for a dopant-helium solvation complex embedded into a superfluid helium droplet. Thus, the extrapolation of such gas phase experiments to anthracene in helium droplets speaks, in general, against an assignment to van der Waal modes as part of the PW. On the other hand, experiments on hetero-clusters of anthracene derivatives have shown that the variation of the oscillator strength among clusters of different stoichiometries and even among configurational variants is safely below an order of magnitude.43 However, the embedding into a superfluid droplet accompanied by the low temperature condition (T = 0.37 K) is known to stabilize local minima configurations which remain hidden in seeded beam experiments or for clusters generated in a classical high temperature solvent. Tiny variations of the electron density distribution as induced by electronic excitation of the chromophore may change these local minima into a barrier. Consequently, the variation of the oscillator strength of configurational variants of solvation complexes or other hetero-clusters generated inside superfluid helium droplets may be much larger as compared for those reported in Ref. 43. Despite all the speculative scenarios, the present understanding of cluster formation inside superfluid

J. Chem. Phys. 142, 014311 (2015)

helium droplets does not exclude a large variation of the oscillator strength among the ZPLs of configurational variants. A continuation of experiments on molecules in helium droplets but also of supersonic jet experiments with the focus shifted to larger clusters as described in Refs. 41–43 and, in particular, on the development of van der Waal modes might help to solve this experimental puzzle. It should be mentioned that in addition to the six anthracene derivatives listed in Fig. 4, we have investigated 2-MA and 9-phenylanthracene (9-PA).22 These two compounds differ from the other six compounds in exhibiting severe line broadening throughout the entire electronic spectrum including the electronic origin. This observation can only be explained by the perturbation of the helium solvation layer induced by the change of the electron density distribution accompanying electronic excitation. Consequently, any possible helium induced fine structure remains hidden.

V. CONCLUSION

As part of a systematic study of microsolvation in superfluid helium droplets, previous investigations of anthracene and anthracene derivatives10,11,22,23 are amended by new data on 9-CNA and 9-CA. As for bare anthracene derivatives, the electronic excitation and emission spectra reveal the typical signature of a rigid molecule, and the helium induced fine structure at the electronic origin differs significantly from that recorded for anthracene. However, for all of the five derivatives shown in Figs. 4(b)–4(f), this fine structure looks surprisingly similar. It starts with an intense sharp peak which is followed by a series of much less intense and similarly sharp peaks. Interestingly, as for many other compounds, a signal representing the PW of the superfluid helium droplet body was not observed. The observed fine structure appears to be a characteristic feature of microsolvation of these anthracene derivatives in helium droplets. At present, an unequivocal assignment of the helium induced fine structure to either ZPLs of configurational variants of solvation complexes or their accompanying PWs is not possible. This aporia might be solved by detailed investigations of van der Waals modes of clusters of the various dopant species with helium atoms as created in a seeded beam expansion. Such investigations have been carried out among others by Even and Jortner41,42 and might experience a fruitful revival and continuation for deciphering microsolvation in superfluid helium droplets. As an alternative approach to studying anthracene derivatives, we have also studied the modification of the helium induced fine structure of clusters consisting of a single anthracene molecule and a well defined number of Ar atoms which will be the subject of a forthcoming paper. Finally, for theoretical modeling of the helium induced fine structure of these anthracene derivatives, one needs to keep in mind that the assignment of ZPL and PW is an open question. 1J.

P. Toennies and A. F. Vilesov, Annu. Rev. Phys. Chem. 49, 1 (1998). P. Toennies and A. F. Vilesov, Angew. Chem., Int. Ed. 43, 2622 (2004). 3M. Y. Choi, G. E. Douberly, T. M. Falconer, W. K. Lewis, C. M. Lindsay, J. M. Merritt, P. L. Stiles, and R. E. Miller, Int. Rev. Phys. Chem. 25, 15 (2006).

2J.


014311-8 4J.


Küpper and J. M. Merritt, Int. Rev. Phys. Chem. 26, 249 (2007). Stienkemeier and K. K. Lehmann, J. Phys. B: At., Mol. Opt. Phys. 39, R127 (2006). 6J. Tiggesbäumker and F. Stienkemeier, Phys. Chem. Chem. Phys. 9, 4748 (2007). 7A. Slenczka and J. P. Toennies, “Chemical dynamics inside superfluid helium nanodroplets at 0.37 K,” in Low Temperatures and Cold Molecules, edited by I. A. M. Smith (World Scientific, Singapore, 2008), p. 345. 8A. Hernando, M. Barranco, M. Pi, E. Loginov, M. Langlet, and M. Drabbels, Phys. Chem. Chem. Phys. 14, 3996 (2012). 9N. Pörtner, J. P. Toennies, A. F. Vilesov, and F. Stienkemeier, Mol. Phys. 110, 1767 (2012). 10D. Pentlehner, C. Greil, B. Dick, and A. Slenczka, J. Chem. Phys. 133, 114505 (2010). 11D. Pentlehner, R. Riechers, A. Vdovin, G. M. Pötzl, and A. Slenczka, J. Phys. Chem. A 115, 7034 (2011). 12E. Loginov, D. Rossi, and M. Drabbels, Phys. Rev. Lett. 95, 163401 (2005). 13M. Hartmann, F. Mielke, J. P. Toennies, and A. F. Vilesov, Phys. Rev. Lett. 76, 4560 (1996). 14N. Pörtner, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys. 117, 6054 (2002). 15A. Slenczka, B. Dick, M. Hartmann, and J. P. Toennies, J. Chem. Phys. 115, 10199 (2001). 16B. Dick and A. Slenczka, J. Chem. Phys. 115, 10206 (2001). 17R. Lehnig and A. Slenczka, J. Chem. Phys. 118, 8256 (2003). 18R. Lehnig and A. Slenczka, J. Chem. Phys. 120, 5064 (2004). 19R. Lehnig and A. Slenczka, ChemPhysChem 5, 1014 (2004). 20R. Lehnig and A. Slenczka, J. Chem. Phys. 122, 244317 (2005). 21R. Lehnig, J. A. Sebree, and A. Slenczka, J. Phys. Chem. A 111, 7576 (2007). 22D. Pentlehner and A. Slenczka, Mol. Phys. 110, 1933 (2012). 23D. Pentlehner and A. Slenczka, J. Chem. Phys. 138, 024313 (2013). 24A. Stromeck-Faderl, D. Pentlehner, U. Kensy, and B. Dick, ChemPhysChem 12, 1969 (2011). 5F.

J. Chem. Phys. 142, 014311 (2015) 25R.

Riechers, D. Pentlehner, and A. Slenczka, J. Chem. Phys. 138, 244303 (2013). 26D. Pentlehner, R. Riechers, B. Dick, A. Slenczka, U. Even, N. Lavie, R. Brown, and K. Luria, Rev. Sci. Instrum. 80, 043302 (2009). 27S. Hirayama, J. Chem. Phys. 85, 6867 (1986). 28A. Amirav, J. Jortner, S. Okajima, and E. C. Lim, Chem. Phys. Lett. 126, 487 (1986). 29A. Amirav, Chem. Phys. 124, 163 (1988). 30A. Amirav, C. Horwitz, and J. Jortner, J. Chem. Phys. 88, 3092 (1988). 31S. Hirayama, K. Shobatake, and K. Tabayashi, Chem. Phys. Lett. 121, 228 (1985). 32M. Kono, Y. Kubo, S. Hirayama, and K. Shobatake, Chem. Phys. Lett. 198, 214 (1992). 33S. R. Langhoff, C. W. Bauschlicher, D. M. Hudgins, S. A. Sandford, and L. J. Allamandola, J. Phys. Chem. A 102, 1632 (1998). 34R. Zimmermann, C. Lermer, D. Lenoir, and U. Boesl, “Isomer selective ionization of chlorinated PAHs: Detection of impurities in technical 9monochloroanthracene,” AIP Conf.Proc. 329, 527–530 (1995). 35S. Hirayama, Y. Iuchi, F. Tanaka, and K. Shobatake, Chem. Phys. 144, 401 (1990). 36C.-H. Lin and T. Imasaka, Talanta 42, 1111 (1995). 37M. Hartmann, A. Lindinger, J. P. Toennies, and A. F. Vilesov, J. Phys. Chem. A 105, 6369 (2001). 38S. Krasnokutski, G. Rouillé, and F. Huisken, Chem. Phys. Lett. 406, 386 (2005). 39A. Lindinger, J. P. Toennies, and A. F. Vilesov, Chem. Phys. Lett. 429, 1 (2006). 40S. Kuma, H. Nakahara, M. Tsubouchi, A. Takahashi, M. Mustafa, G. Sim, T. Momose, and A. F. Vilesov, J. Phys. Chem. 115, 7392 (2011). 41U. Even, J. Jortner, D. Noy, N. Lavie, and C. Cossart-Magos, J. Chem. Phys. 112, 8068 (2000). 42U. Even, I. Al-Hroub, and J. Jortner, J. Chem. Phys. 115, 2069 (2001). 43A. Penner, A. Amirav, J. Jortner, and A. Nitzan, J. Chem. Phys. 93, 147 (1990).


Laser ionization and spectroscopy of Cu in superfluid helium nanodroplets.

Vortex-induced aggregation in superfluid helium droplets.

Quantum interferences in the photodissociation of Cl2(B) in superfluid helium nanodroplets ((4)He)N.

Formation of bimetallic clusters in superfluid helium nanodroplets analysed by atomic resolution electron tomography.

Formation of dianions in helium nanodroplets.

Photoinduced molecular dissociation and photoinduced recombination mediated by superfluid helium nanodroplets.

Electron Diffraction of Superfluid Helium Droplets.

Doubly charged CO2 clusters formed by ionization of doped helium nanodroplets.

One- and two-color resonant photoionization spectroscopy of chromium-doped helium nanodroplets.

Electron impact ionization and multiphoton ionization of doped superfluid helium droplets: A comparison.

Solvation of intrinsic positive charge in superfluid helium.

Complexation of allyl radicals and hydrochloric acid in helium nanodroplets.

IR spectroscopy of pyridine-water structures in helium nanodroplets.

Effect of kinetic energy on the doping efficiency of cesium cations into superfluid helium droplets.

Formation of Au and tetrapyridyl porphyrin complexes in superfluid helium.

Communication: Electron diffraction of ferrocene in superfluid helium droplets.

Preparation of ultrathin nanowires using superfluid helium droplets.

Helium mass flow through a solid-superfluid-solid junction.

Solvation of molecules in superfluid helium enhances the "interaction induced localization" effect.

Building Carbon Bridges on and between Fullerenes in Helium Nanodroplets.

Visualization of two-fluid flows of superfluid helium-4.

Dynamics of excited sodium atoms attached to helium nanodroplets.

Helium Droplets Doped with Sulfur and C60.

Detection of Negative Charge Carriers in Superfluid Helium Droplets: The Metastable Anions He- and He2-