ARTICLES PUBLISHED ONLINE: 20 JULY 2014 | DOI: 10.1038/NCHEM.2006

Control of ultrafast molecular photodissociation by laser-field-induced potentials M. E. Corrales1, J. González-Vázquez2, G. Balerdi1, I. R. Solá1 *, R. de Nalda3 and L. Bañares1 * Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an additional reagent is also well established. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place. Here we explore the transition from the weak- to the strong-field regimes of laser control for the dissociation of a polyatomic molecule, methyl iodide. The control over the yield of the photodissociation reaction proceeds through the creation of a light-induced conical intersection. The control of the velocity of the product fragments requires external fields with both high intensities and short durations. This is because the mechanism by which control is exerted involves modulating the potentials around the light-induced conical intersection, that is, creating light-induced potentials.

T

he quantum-control revolution represents a paradigm shift in ultrafast spectroscopy1,2. Ultrashort laser pulses tuned to molecular resonances in the visible or the ultraviolet region can project the initial state of the system onto accessible electronically excited states. The dynamics following this event often includes photodissociation, as the wave packet evolves following the gradient of the potential energy surface (PES). The dynamics of polyatomic molecules in excited states is usually driven by internal conversion at conical intersections and intersystem crossings3. These intramolecular processes accelerate the redistribution of energy among different electronic and vibrational degrees of freedom. Similarly to what intramolecular vibrational energy redistribution signifies to the control of the nuclear dynamics in the ground electronic state4, internal conversion and intersystem crossing accelerate the redistribution of energy among the different electronic and vibrational states and hamper the control of the dynamics in the excited states. However, from a spectroscopic point of view, these processes allow the exploration of otherwise inaccessible regions of the molecular Hamiltonian. An important challenge remains in using intramolecular couplings to take advantage of the predesigned dynamics. A first step towards this goal is the creation of lightinduced conical intersections (LICI)5–8 together with other strongfield effects that allow the reshaping of the PES9–13. The dynamically modified PES in the presence of the laser is usually called the molecular light-induced potential (LIP)14. In this work we show experimentally and demonstrate theoretically the preparation of a LICI between an electronically excited valence state and the ground state of a prototypical polyatomic molecule, methyl iodide (CH3I). The LICI, and the modulated LIP around it, is used to control the electronic state (reaction channel) and the kinetic-energy distribution (KED) of the product fragments, decisive milestones towards the full detailed control of the photodissociation reaction of complex systems. The case study is the ultrafast photodissociation dynamics of CH3I in the first absorption A band. The experiments use a pump pulse to induce a one-photon (ultraviolet) absorption to a valence state and a strong infrared laser pulse to control the outcome of the reaction. To probe the dynamics

we use a resonant multiphoton ionizing laser pulse, and detection is performed with the velocity-map-imaging technique, which provides detailed information on product fragment KEDs as the dissociation event takes place. To compose a coherent picture of the dynamics of the system we compare the experimental findings with numerical solutions of the time-dependent Schrödinger equation (TDSE), in which we apply reduced dimensionality approximations. Over recent decades, photolysis of CH3I in the A band has been considered as a prototypical photodissociation reaction15–24. Its relative simplicity makes it liable to be investigated both experimentally and theoretically. However, the presence of a conical intersection between the two main valence states and the influence of fragment vibration on the dynamics provide interesting non-adiabatic dynamics23. Indeed, it was the first system to be studied by ion imaging17 and has produced a rich variety of studies over the years15–24. This work demonstrates, for the first time, a strongfield control on the direct A-band-photodissociation reaction in this molecule. There have been several theoretical proposals and some experimental attempts to control photochemical reactions mediated by internal conversion and intersystem crossing. A straightforward approach is to bypass the conical intersection by steering the nuclear wave packet to other bright electronic states on certain sectors of the PES25. The experimental implication of such an approach is the need for pulse sequences tuned to the desired resonances and the shaping of the pump pulse to avoid internal conversion or intersystem crossing right after the Franck–Condon window. This method is, however, very challenging in polyatomic molecules, in which the wave-packet spread and the conical intersection seams can affect large regions of the PES. Recently, Sussman et al. used strong infrared fields to exploit the non-resonant dynamic Stark effect (NRDSE)26. In this case the nuclear wave function remains in the same electronic state, but the field distorts its structure. In the direct non-adiabatic photodissociation of IBr, the NRDSE was strong enough to greatly alter the branching of the reaction that takes place

1 Departamento de Química Física (Unidad Asociada de I+D+i al CSIC), Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain, 2 Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, Spain, 3 Instituto de Química Física Rocasolano, CSIC, C/ Serrano 119, 28006 Madrid, Spain. * e-mail: [email protected]; [email protected]

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through an avoided crossing between the PESs involved. One advantage of this approach is its universality. There is no need to drive the photodissociation reaction through other excited states (that is, through other transition states) because of the mild frequency dependence of the process. By creating LICIs and modulating LIPs, the use of near-resonant or resonant strong infrared fields significantly changes the nature of the excited electronic state, which is now described as a mixture of different valence states5,27–30. In this work we show that dissociation can even proceed in the ground state. As internal conversion depends on the relative velocity of the fragments, the LICI can be used as a momentum filter on the velocity components of the dissociating wave packet on the different reaction channels31. However, additional control of the KED of the products is only possible by dynamic nonlinear effects induced by ultrashort infrared pulses, for which the duration must be of the order of the passage through the LICI. These effects can also be regarded as experimental signatures of the existence of LIPs.

Results and discussion The idea that underlies the creation and control of a LICI is sketched in Fig. 1. A 100 fs, 268 nm pump pulse, εp(t) (peak intensity below 1 TW cm−2), shines a molecular beam sample of CH3I in He. Under these conditions, the only bright state accessed is the 3Q0 electronic state15,20,21,23 (hereafter, Ve). The main photodissociation channel leads to ultrafast direct fragmentation into CH3 fragments (mostly in the ground vibrational state, ν = 0) and spin–orbit excited I*(2P1/2) atoms. The kinetic energy of the CH3(ν = 0) fragments in the centre of the mass frame of the system peaks at around 1.15 eV. A second reaction channel to CH3(ν = 0) fragments and ground-state I(2P3/2) atoms, characterized by a CH3 centre of mass kinetic energy 2.0 eV, can be accessed through a conical intersection between the 3Q0 and 1Q1 electronic states near the Franck–Condon window23. In the absence of any control field, velocity-map images of the CH3 fragments (acquired through a (2 + 1) 786

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Figure 1 | Potential energy curves of the relevant electronic states involved in the photodissociation of CH3I in the A band along the R C–I coordinate. The 3Q0 (red line) and 1Q1 (yellow line) potential energy curves are taken from Xie et al.22. The ground-state potential energy curve (green line) was calculated according to the ab initio method described in the text. A pump pulse at 268 nm was used to prepare a wave packet in the 3Q0 state. An intense ultrashort 804 nm control laser pulse was time delayed (τ) with respect to the pump pulse. This control pulse creates the LICI, here shown at the crossing of 3Q0 and the X˜ 1 A1 ground state, which has been shifted by the energy of the 804 nm photon (blue line). For sufficiently strong fields the control pulse also induces a strong modulation of the potentials around the LICI to create the corresponding LIPs (dashed red and blue lines).

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Figure 2 | Appearance of a dissociation channel mediated by a LICI. a, The three panels show Abel-inverted CH3+ images obtained on 268 nm photodissociation of CH3I under the effect of a moderately intense (5 TW cm−2) and relatively long duration (3.9 ps FWHM) control pulse for three values of the relative delay. Ionization (probe) of CH3(ν = 0) is realized at a long pump–probe delay time of 30 ps with a third pulse at 333.5 nm by (2 + 1) REMPI. The molecular response in the absence of the control field is identical to that shown in the left image, in which the main ring corresponds to the CH3(ν = 0) + I*(2P1/2) channel for dissociation on the 3Q0 potential energy curve (Ve) and the weaker external ring to the CH3(ν = 0) + I(2P3/2) channel caused by the intrinsic conical intersection with the 1Q1 PES. The middle and right images show an additional channel at a lower speed caused by the control field. The double arrow shows the direction of the polarization of the three laser pulses. b,c, Comparison between the experimental (b) and theoretical (c) CH3 centre-of-mass KEDs as a function of the pump (268 nm)–control (804 nm) delay time. The most-intense contribution corresponds to the main part of the wave packet that evolves adiabatically along the 3Q0 surface correlating to CH3(ν = 0) + I*(2P1/2). To emphasize the region in which the control is exerted, the contribution that corresponds to the CH3(ν = 0) + I(2P3/2) and appears at higher kinetic energies is not shown. With an overlapping control pulse, part of the wave packet is dumped to the ground-state surface, which correlates to CH3(ν = 0) + I(2P3/2). We can distinguish these contributions in both maps at 1.15 and 0.62 eV, respectively. The experimental control pulse has a chirp of 42 cm−1 ps−1. The theoretical control pulse has a peak intensity of 3 TW cm−2, a duration of 3.9 ps and its instantaneous frequency is adjusted to match the experimental chirp.

resonance-enhanced multiphoton ionization (REMPI) process at 333.5 nm (see Methods))23 are identical to those shown in the left image of Fig. 2a. NATURE CHEMISTRY | VOL 6 | SEPTEMBER 2014 | www.nature.com/naturechemistry

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Control with LICIs. We first analyse the control conditions under moderately intense pulses. In the experiment, εc(t) is Gaussian with a pulse energy of 330 µJ and a full-width-half-maximum (FWHM) pulse duration of 3.9 ps (peak intensity ∼5 TW cm−2). It is therefore much longer in time than the pump pulse and it has, additionally, a negative chirp of ∼42 cm−1 ps−1. At a much later time (∼30 ps) a probe laser centred at 333.5 nm ionizes the CH3(ν = 0) fragments produced in the photodissociation event by (2 + 1) REMPI. Figure 2a shows CH3+ Abel-inverted images at three different delay times of the control pulse with respect to the pump. In the left image, the control pulse precedes the pump pulse by more than 4 ps and causes no modification of the image, which contains the two well-known major I*(2P1/2) and minor I(2P3/2) channels (inner and outer anisotropic rings)23. In the middle and right images, a new contribution appears with a lower kinetic energy (∼0.6 eV, inner anisotropic ring) caused by transit through the LICI in a manner equivalent to that described by Kim et al.7. Figure 2b shows the CH3(ν = 0) fragment KED as a function of the time delay between the pump and control pulses in the experiment for the new and main channels (for simplicity, the natural CH3(ν = 0) + I(2P3/2) channel is not shown in this plot), and the corresponding numerical simulation is depicted in Fig. 2c. The maps reveal two contributions that correlate with dissociation channels CH3(ν = 0) + I*(2P1/2) and CH3(ν = 0) + I(2P3/2), at 1.15 eV (KEDE) and 0.62 eV (KEDG), respectively. The signal from the slow fragments tilts linearly to higher kinetic energies at negative time delays. The interpretation of the results, backed up by the numerical simulations, is clear. The pump pulse creates a wave packet in the dissociative state, Ve. At some C–I internuclear distance, Rc , the control pulse frequency ωc(t) is in resonance with the ground state, Vg , which creates the LICI. Part of the wave packet is transferred to Vg , where it dissociates because the average kinetic  is enough to overcome the energy acquired evolving in Ve , T,  > Vg (R → ∞) − Vg (Rc ). As εc(t) is negaremaining bond energy, T tively chirped, when it precedes the pump pulse ωc(t) will be lower and the LICI will occur at larger bond distances. Thus both  and Vg(Rc) will be higher and the KEDG will be blue shifted. T The opposite occurs when εp(t) precedes εc(t). These results show that intensity, chirp and delay are the actors that control the outcome of the photodissociation reaction. The intensity of the pulse controls the ratio of the products; the carrier frequency (or the instantaneous frequency at the time the wave packet is around the LICI, when using chirped pulses) selects the kinetic energy of the products in Vg and controls the kinetic energy release of the dissociation that occurs in the ground state of the molecule (the KEDG).

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In addition, the reaction channel that yields ground-state I(2P3/2) atoms can be opened using an 804 nm control laser pulse, εc(t), by creating a LICI between Ve and the ground state (Vg). When crossing this LICI, part of the wave packet proceeds to dissociation in the ground state, Vg. Depending on the nature of the control pulse, it can change the molecular photochemistry by significantly modifying three features of the reaction: (1) the branching ratio of the reaction (that is, the electronic state of the iodine fragments); (2) the KED of the new (LICI-induced) opened channel, which takes place at the ground state (KEDG), and (3) the KED of the main (pump-induced) exit channel, which takes place on the excited state (KEDE). The first two elements of control can be achieved under moderately strong laser fields. As explained below, they basically rely on a pump–dump process in which both the intermediate and final states belong to the continuum. Full control over the fragment kinetic energies on both channels requires additional nonlinear interactions that can only be explained in simple terms in the frame of LIPs.

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Figure 3 | Control of the release of kinetic energy in the main dissociation channel. a,b, The plots depict the comparison between experimental (a) and simulated (b) 2D intensity maps that show the evolution of the KED of the CH3(ν = 0) fragment as a function of the pump (268 nm)–control (804 nm) delay time. Control pulses of high intensity (80 TW cm−2) and short duration (50 fs FWHM) were employed. The ionization (probe) of CH3 for detection is realized at a long pump–probe delay time of 30 ps with a third pulse at 333.5 nm by (2 + 1) REMPI. As in Fig. 2, the main channel corresponds to the part of the wave packet that evolves adiabatically along the 3Q0 (Ve) surface, which correlates with CH3(ν = 0) + I*(2P1/2). However, it can be seen that application of a short control pulse causes distortion of the KEDE to lower values (see text). The additional channel caused by dynamics on a laser-induced potential, correlating with CH3(ν = 0) + I(2P3/2), can be observed at lower kinetic energies as a broad contribution centred at 0.5 eV. Inset: Abel-inverted CH3+ image that corresponds to a pump-control delay time of around 0 fs.

Control with LIPs. Control of the kinetic energy (or velocity) of the products in the main channel (the KEDE) with a constant pump laser frequency is more demanding and requires stronger and shorter control fields. Figure 3a shows the experimental KED versus delay time in both channels using the same pump pulse at 268 nm (100 fs FWHM) and a short 50 fs (FWHM) Gaussian control pulse at 804 nm with a peak intensity of ∼80 TW cm−2. At these intensities the control pulse induces some degree of multiphoton dissociative ionization to yield some amount of CH3+ ions. Thus, it is necessary to subtract the contribution of these ions from that produced when the third (probe) laser pulse at 333.5 nm is added to detect the neutral CH3(ν = 0) fragments. The corresponding numerical simulation is depicted in Fig. 3b. We observe a remarkable broadening and a clear bending of the KEDE, particularly to lower kinetic energies, and the appearance of a much broader KEDG than that obtained with weaker and longer control fields. The numerical simulation takes into account the experimental resolution on kinetic energy. In addition to the main features, the simulation shows that the KEDG is slightly tilted with a positive slope, with a larger kinetic-energy at longer delay times. To the best of our knowledge, this is the first case that reports control over the kinetic-energy release of a direct photodissociation reaction.

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Figure 4 | Numerical simulations of the dissociation reaction in four different scenarios with respect to the nature of the control field. a–d, Time-dependent fragment relative velocity distributions (left panels) obtained by integrating the TDSE using a 1D model within the Floquet representation for different control pulses with the pump pulse the same in all cases. The blue and red vertical lines indicate the final relative velocity of the fragments for the CH3(ν = 0) + I(2P3/2) and CH3(ν = 0) + I*(2P1/2) channels, respectively, expected by energy conservation (that is, in the absence of the control pulse during the dynamics) after excitation with the pump pulse. a, No control pulse. b, Long (3.9 ps FWHM) and moderately intense (5 TW cm−2) top-hat control pulse. c, Long (3.9 ps FWHM) and strong (85 TW cm−2) control pulse. d, Strong (85 TW cm−2) and short (50 fs FWHM) control pulse, time delayed by 60 fs with respect to the pump pulse. The right panels show the molecular potentials and LIPs that explain the populations and shapes of the asymptotic KEDs. Arrows sketch the main contributing processes along with the shifts in the velocities that lead to the observed broadening and shifting of the asymptotic distributions (see the text for the explanation); the thickness of the arrows represents the branching ratio between the different dissociation pathways.

Theoretical model of the control mechanism. The main features of the control over the kinetic energy of the fragments can be explained with the aid of a simplified one-dimensional (1D) model in the dressed-state picture using a Floquet representation. In Fig. 4 we show the time evolution of the photodissociating wave packets in momentum space for different pulse conditions after εp(t) is at maximum and until the wave packets reach the asymptotic states. This picture provides the transient KED of the fragments. Supplementary Videos 1a–d provide movies of the wave packets that evolve, both in coordinate and in momentum representations. 788

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Without the control pulse the dynamics occur on the unperturbed molecular potentials (see Fig. 4a). In the first tens of femtoseconds the wave packet rapidly gains a large momentum in Ve (not apparent in the figure). Immediately afterwards, the kinetic energy of the excited wave packet is reduced slightly because the potential has a small minimum, and then the molecule dissociates directly. With a relatively weak (5 TW cm−2) and long (3.9 ps) control pulse overlapping with the pump pulse and continuing until the wave packet reaches the asymptotic state (Fig. 4b), part of the wave packet crosses through the LICI and dissociates in the ground state. The asymptotic KEDG is similar to the KEDE, but shifted to the red in momentum space. This corresponds to the description of the results shown in Fig. 2, in which an additional dissociation channel appears, shifted to a lower momentum with respect to the main channel. Quantum control can be achieved on the relative yield of the photodissociation reaction, although only for very strong control fields can the ground-state channel dominate over photodissociation in the main (direct) channel. For very strong (85 TW cm−2) and relatively long (3.9 ps) fields, the dynamics basically occurs on the LIPs (Fig. 4c). The relation between LIPs and molecular potentials is depicted in Fig. 4 (and also in Fig. 1). The excited LIP, V ea, correlates to Ve before Rc , and to Vg after Rc. However, the ground LIP, V ag, correlates to Vg before Rc and to Ve after it. Both LIPs show a smooth curvature around the LICI. In this representation, the wave packet excited on Ve moves coherently on a single LIP, V ea. Therefore, the only component that survives asymptotically when the transition dipole moment decays is the ground state, Vg. It is remarkable that, in this case, a complete asymptotic wave-packet transfer to the ground state is achieved. However, the asymptotic KED, except for a small shift, remains fairly similar to that obtained with weaker fields. The dynamic effects of the LIP, momentum shifts or time delays with respect to the evolution in the molecular potentials are transient, and do not leave signatures on the asymptotic results. To observe dynamic effects and control the KEDE, the key ingredient is to freeze the transient effect at some particular time by suddenly switching from the LIP to the molecular potential (or vice versa). This can only be achieved with strong and ultrashort control pulses that act in the temporal vicinity of the crossing through the LICI. Experimentally, these conditions were tested for short (50 fs FWHM) control pulses with intensities of 80 TW cm−2, and the results are shown in Fig. 3a. Figure 4d shows the time evolution of the photodissociating wave packet for a selected time delay between the pump and control pulses of τ = 60 fs. The FWHM of the pump pulse εp(t) is τp = 50 fs and, depending on the momentum component, the wave packet takes from around 50 to 100 fs to reach the LICI (at Rc). In the simulation, the intensity of the control pulse is 85 TW cm−2 and its FWHM is 50 fs. The choice of a τ = 60 fs time delay is motivated by the need to have rapid changes in the intensity of the control pulse during the transit of the wave packet through the LICI. In particular, for a 60 fs time delay the fastest momentum components of the wave packet reach Rc before the control pulse εc(t) acts, to yield I*(2P1/2), whereas the slowest momentum components cross Rc when the control pulse is at a maximum, dissociating (following the LIP) in Vg and yielding I(2P3/2). Under these conditions, the pulse sequence acts as a momentum filter that discriminates the velocity components of the dissociating wave packet on the different reaction channels. In addition to the filtering (narrowing of the KED), there is a red shifting of the KEDE (see Fig. 4d). This is a typical signature of nonadiabatic effects. Right after the crossing, some intermediate momentum components increase as a result of the abrupt ramp up in εc(t) from a very small to a large intensity. This causes the wave packet to switch suddenly from the excited potential Ve to NATURE CHEMISTRY | VOL 6 | SEPTEMBER 2014 | www.nature.com/naturechemistry

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the corresponding LIP, Vga, which correlates to Ve after Rc. As shown in the right panel of Fig. 4d, Vga has a strong attractive well that reduces the momentum of the wave packet as it evolves, instead of increasing it, as it would do on a purely repulsive potential. The effect is not as large as might be expected because of the presence of the small well in Ve , but it is enough to induce the red-shift bending of the KEDE that is observed in the experimental results (see Fig. 3a). However, as the adiabaticity is broken, there is a small probability that the wave packet appears on the other LIP, Vea, which correlates asymptotically with Vg. In contrast to strong and long control pulses for which the wave packet moves on Vea all the time (Fig. 4c), for strong and short control pulses the wave packet comes from Ve , where it gains more momentum than it would have in Vea. This additional momentum is then diabatically (that is, suddenly) transferred to Vea (Fig. 4d), which correlates asymptotically to Vg after Rc , leading to a blue shift of the KEDG. A timely breakdown of the adiabaticity is, therefore, an essential tool to manipulate both the ground- and the excited-state KEDs, one that can be accessed by controlling the time delay between the ultrafast pump and control pulses of the required time duration.

Two electronic states were included in the calculation, the so-called 3Q0 potential taken from the analytical model of Morokuma and co-workers19 with the corrections of Xie et al.22, and the adiabatic spin–orbit ground-state potential calculated by a consistent ab initio methodology, using state-average complete active space corrected by multireference configuration interaction. The active space includes six electrons and five orbitals. The basis set employed was the Dunning basis set aug-cc-pVTZ and the relativistic effects were taken into account by using the pseudopotential developed by Peterson et al.37. All the calculations were performed using the MOLPRO program38. The permanent dipole moments were considered to be zero (because the frequency of the laser fields is much larger than the vibrational quanta) and the transition dipole moment was approximated by an analytical function that reproduced the results of Alekseyev et al.39 and Shapiro40. The model is thus very similar to one previously developed23,36, without explicitly considering the bending mode ν6 , but recalculating the ground state of the molecule in a consistent way (not using harmonic approximations). The TDSE was solved by propagating the wave function with the Split-Operator method41–43 combined with the fast Fourier transform technique44 with a time step of 0.05 fs in a 2D grid of 512 points for the R coordinate (between 2 and 20 Bohr) and 64 points for the umbrella mode (r ranging between –1.5 and 1.5 Bohr). For interpretation purposes, a simple model without the umbrella mode was designed, and the TDSE was solved to obtain the results in Fig. 4 using the Floquet representation, which allows the calculation of the LIPs in the presence of strong fields.

Received 20 March 2014; accepted 16 June 2014; published online 20 July 2014

Conclusions The preparation of a LICI and the control of the photodissociation dynamics of a polyatomic system by modulating the LIPs around it are demonstrated experimentally and theoretically. We show that the application of tailored control infrared pulses of enough intensity allows significant control of the product branching ratios and fragment kinetic energies. Two key aspects of the control exerted on the system are identified first in the creation of the LICI and second in the collapse of the adiabaticity through the application of rapidly varying field amplitudes. The work presented here is a demonstration that laser pulses intense enough to dress the PESs constitute a powerful means of steering molecular dynamics to the desired targets with capabilities beyond weak-field approaches based on wave-packet-dynamics control through resonant transitions.

Methods Experimental. A detailed description of the experimental set-up used in these experiments can be found elsewhere23,32 and only a brief summary is given here. Photodissociation of CH3I was initiated by a short (∼100 fs FWHM) ultraviolet laser pulse at 268 nm, obtained by frequency tripling part of the output of an amplified Ti: sapphire system that produced 804 nm, 50 fs, 1 kHz pulses. The control pulse was an arm of the laser output at 804 nm; experiments were conducted in two distinct time/intensity regimes. The conditions of high intensity (∼80 TW cm−2) and short duration were obtained directly from the laser output. Control laser pulses with moderate intensity (∼5 TW cm−2) and relatively long duration (3.9 ps FWHM) were obtained by stretching the pulses in a diffraction grating pair. In both cases, a third pulse was added 30 ps later. This pulse, synthesized in an optical parametric amplifier, was centred at 333.5 nm, and produced efficient (2 + 1) REMPI of the vibrational ground state (ν = 0) of the CH3 product of the reaction. All the laser beams with parallel polarizations, controllable energies, focusing geometries and delays were propagated collinearly and focused with a 25 cm focal-length lens into the interaction region of the vacuum chamber where they intersected a supersonic molecular beam of CH3I seeded in He produced by a 1 kHz piezoelectric valve. The ions generated in the interaction region were extracted perpendicularly and accelerated by an electrostatic lens system working in a velocitymap-imaging configuration33,34. At the end of a 50 cm time-of-flight tube they were projected onto a microchannel plate detector coupled to a phosphor screen. Finally, a Peltier-cooled 12-bit charge-coupled device camera recorded the phosphorescence emitted by the phosphor screen as raw images that were later inverted using the Abel transform pBasex methodology35 to extract the desired information (translational energy and angular distribution). Theory. The photodissociation dynamics of CH3I in the presence of the pump and control pulses was simulated by solving the TDSE in a 2D model, which included the dissociation coordinate RC–I (distance between the I atom and the centre of mass of the CH3 fragment) and the umbrella mode of CH3 , r (approximated as a stretching mode between the C atom and the H3 centre of mass). These modes account for the most-important features of the dynamics in the A band23,36. The remaining coordinates were frozen at the equilibrium distance.

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Acknowledgements This work was financed by the Spanish Ministry of Economy and Competitiveness (MINECO) through grants CTQ2008-02578, CTQ2012-37404-C02-01 and CTQ201236184, Consolider program SAUUL CSD2007-00013 and the European Union Initial Training Networks ‘Ultrafast control of quantum systems by strong laser fields’ (FASTQUAST, PITN-GA-2008-214962). This research was performed within the Unidad Asociada ‘Química Física Molecular’ between the Departamento de Química Física of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Científicas (CSIC). J.G-V. thanks the Spanish MINECO for a Juan de la Cierva grant and the PIM2010ECC-00751 project for financial support. The facilities provided by the Centro de Asistencia a la Investigación de Láseres Ultrarrápidos at UCM and the computational resources of the ‘Trueno’ cluster at CSIC are acknowledged.

Author contributions M.E.C. and J.G-V. contributed equally to the paper. M.E.C. and G.B. performed the experiments and analysed the experimental data. J.G-V. designed and performed the calculations. I.R.S. conceived and designed the calculations and is responsible for the theoretical interpretation of the results. R.N. and L.B. conceived and designed the experiments. All authors contributed to writing sections of the paper.

Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to I.R.S. and L.B.

Competing financial interests

The authors declare no competing financial interests.

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Control of ultrafast molecular photodissociation by laser-field-induced potentials.

Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an addition...
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