Vibronic Dynamics of Photodissociating ICN from Simulations of Ultrafast X‐Ray Absorption Spectroscopy

Abstract Ultrafast UV‐pump/soft‐X‐ray‐probe spectroscopy is a subject of great interest since it can provide detailed information about dynamical photochemical processes with ultrafast resolution and atomic specificity. Here, we focus on the photodissociation of ICN in the 1Π1 excited state, with emphasis on the transient response in the soft‐X‐ray spectral region as described by the ab initio spectral lineshape averaged over the nuclear wavepacket probability density. We find that the carbon K‐edge spectral region reveals a rich transient response that provides direct insights into the dynamics of frontier orbitals during the I−CN bond cleavage process. The simulated UV‐pump/soft‐X‐ray‐probe spectra exhibit detailed dynamical information, including a time‐domain signature for coherent vibration associated with the photogenerated CN fragment.


Introduction
In this study,w ee xplore the potential of ultrafast UVpump/soft-X-ray-probe spectroscopy to gain fundamental understanding of dynamical photochemical processes with atomic specificity and ultrafast resolution. Ultrafast laser technology has revolutionized the field of chemistry by enabling powerful techniques to monitor elementary steps of chemical reactions. [1] Pioneering UV/Vis electronic pumpprobe studies of small molecules in the gas-phase,i ncluding the influential work of Zewail and co-workers, [1c] have provided valuable insights into the interplay between vibrational modes and chemical bond rearrangements.P hotoinduced chemical reactions in solution have also been studied extensively, [2] where solute-solvent couplings and spectral broadening typically limited the amount of structural information that could be obtained from acursory examination of the spectroscopic signals.U ltrafast vibrational spectroscopy allows probing specific vibrational modes of chemical bonds in transient species,w hich provides dynamical information about structural rearrangements. [2a,d] Ford elocalized vibrational mode patterns,t ransient changes in the electronic structure can only be obtained in terms of quantum chemical calculations of the corresponding transient electronic states. [3] Recent developments in ultrafast X-ray spectroscopy open new opportunities to probe changes in transient electronic structure during the course of achemical reaction. Ultrafast X-ray spectroscopies, [4] including X-ray absorption spectroscopy (XAS), [3a, 5] X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS) can enable ad irect characterization of the valence electronic structure dynamics. [6,7] Recent results probing occupied (and unoccupied) orbitals of organic and organometallic systems have provided key insights into the electronic structural rearrangements of av ariety of molecular systems undergoing photoinduced bond-breaking reactions. [2d,e,8] In this study,w ee xplore the capabilities of ultrafast UV-pump/soft-X-ray-probe spectroscopy to monitor the underlying dynamics of frontier molecular orbitals during the photolysis reaction of ICN. [9] Photoexcitation of the ICN continuum band with an ultraviolet pump pulse induces ultrafast dissociation of ICN into Ia nd CN fragments (Figure 1). Previous studies have determined that mainly two electronic states are involved in the dissociation paths: [10] the 3 P 0+ state that forms the I*( 2 P 1/2 ) fragment, and the 1 P 1 state that correlates with the high energy range of the band and dissociates into the I( 2 P 3/2 ) fragment. [9a,e] Thet wo photofragmentation pathways branch out at aconical intersection with branching ratios that change as af unction of the photoexcitation wavelength, as in many other photochemical reactions. [11] In this study we focus on photoexcitation from the ground state to the 1 P 1 state with negligible population transfer to the 3 P 1 state.W es imulate the photoinduced transient dynamics in the excited state to investigate the description that could be obtained from UV-pump/soft-X-ray-probe spectroscopy using wavepacket propagation on accurate potential energy surfaces [9a,e] and an electronic response evaluated with as imple approach based on ac ombination of the maximum overlap method (MOM) [12] and configuration interaction singles (CIS) [13] (see the Supporting Information). While our approach could also be implemented with more accurate electronic structure schemes, [3h,i] we show the method pro-vides the fundamental aspects of ICN photodissociation dynamics that could be probed by UV-pump/soft-X-rayprobe spectroscopy.
Our findings indicate that the femtosecond time resolution and atomic specificity of soft-X-ray spectroscopy [5,21] enable ad etailed molecular movie of the ICN photofragmentation dynamics to be captured, including the production of vibrationally hot CN fragments along the IÀCdissociation path during the ultrafast relaxation dynamics on the 1 P 1 excited electronic state.F urthermore,w es how the spectral signature can provide an unequivocal interpretation of the changes in bond-order parameters induced by transient electronic structure rearrangements responsible for the photodissociation. Figure 2s hows the predicted capabilities of UV-pump/ soft-X-ray-probe spectroscopy for characterization of timedependent changes in the electronic structure of ICN triggered by photoexcitation to the 1 P 1 state.T he transient X-ray absorption spectrum (TRXAS) in the CK-edge region was obtained by averaging the spectral lineshape over the nuclear wavepacket probability density (see the Supporting Information). [3e, 14] Thec omputed transient spectrum ( Figure 2B)p rovides clear fingerprints of the nuclear and electronic dynamics in the 1 P 1 excited state.The lower energy region of the spectrum is characterized at early times by two broad peaks that branch out into three bands within about 30 fs.V ibronic oscillations are observed after about 20 fs in the central and higher frequency bands.T he oscillations correspond to the CÀN vibration, as shown by the agreement between the timedependent modulation of intensities in the TRXAS and the  time-dependent expectation value of the C-N distance hr CN i ( Figure 2C). Theo scillation period is about 16 fs (ca. 2085 cm À1 ), in close agreement with the vibrational frequency of the CN radical. [15] These coherences will be revealed in the experimental TRXAS spectrum so long as the durations of the pump and probe pulses do not exceed the CN stretching period.

Results and Discussion
Of note,the CN vibrations are also exhibited in the NKedge TRXAS (Supporting Information, Figure S1), although just weakly observed in the IK -edge,w hich reflects atomic specificity as an important advantage offered by ultrafast UVpump/soft-X-ray-probe spectroscopy.G iven the non-bonded nature of the 1s core electrons,t he K-edge absorption bands are typically separated by tens or hundreds of eV for different atoms.T he energy separation enables one to probe specific atoms,w ith pulses tuned at their corresponding frequencies, to monitor the local dynamics of individual fragments in the system. In this context, we find that UV/X-ray pump-probe spectroscopy can provide unique insights into the nuclear motion of specific vibrations,a sd emonstrated here for the ultrafast relaxation dynamics of ICN on the femtosecond time scale.
TheI À Cb ond dissociation is clearly observed in the TRXAS ( Figure 2B). During the first 30 fs after UV photoexcitation the three bands in the spectrum display asignificant frequency shift. In particular,t he lower energy signal branches into two bands,o ne of which shows av ery pronounced red-shift of about 9eV. These shifts are originated in the rotation of the CN fragment ( Figure 2D)and elongation of the I À Cb ond ( Figure 2E). Moreover,r ight before 30 fs, when the X-ray absorption bands complete their branching process and reach their asymptotic values,there is asignificant drop in the IÀCb ond order,c onsistent with the breaking of the I À Cb ond (Figure 2A). This demonstrates that the TRXAS provides direct information about the nature of chemical bonding and the vibronic behavior of molecular fragments photogenerated in the sub-100 femtosecond time scale.
It is worth mentioning that the energy resolution of the TRXAS,s hown in Figure 2, is only limited by the intrinsic core-hole lifetime,whereas in an experiment the signal would have to be convoluted with the instrumental resolution. To estimate the minimum spectral resolution needed for experimental observation of such features,w ec onvoluted the original spectrum with aGaussian pulse of varying full width at half maximum (FWHM) (see the Supporting Information). We observe that unequivocal signs of both the I À Cd issociation dynamics and the CN vibrational motions are retained by the broadened TRXAS with pulses of up to aF WHM of about 3eV( Supporting Information, Figure S2). Spectral broadening with FWHM > 4.5 Vm asks the dynamical information of the photodissociation process by merging all peaks into ab road band centered at 295 eV.S ince as pectral resolution of 1.2 eV at the carbon K-edge has already been reported for current laser-based table-top UV/X-ray pumpprobe setups (utilizing extreme high-order harmonic radiation and with clear options for further improvements) [16] and resolution at large scale facilities is about an order of magnitude better, [2b, 17] measuring the predicted oscillatory features should be clearly within experimental reach.
Then ear-K-edge condition enables interpretation of the TRXAS signals in terms of the ensuing dynamical evolution of frontier orbitals induced by UV photoexcitation. The natural transition orbitals (NTOs) of the carbon near-K-edge excitations of ICN in the 1 P 1 excited state are the predominant components of the state occupied by the 1s core excitation. Hence,the NTOsprovide avaluable description of the frontier orbitals occupied upon X-ray absorption.
To clearly visualize the rich information on the electronic structure dynamics that is encoded in the TRXAS,F igure 3 shows asimplified view of the spectral evolution, taking four representative configurations of ICN along the photodissociation pathway (which correspond to the expectation values of the nuclear coordinates obtained from the ICN time-dependent wavepacket). Each peak is depicted with its corresponding NTO to establish ac onnection between the electronic rearrangements and the spectral changes.
Thei nitial ICN configuration, corresponding to the ground state equilibrium geometry,o rapump-probe delay time of 0f s, reveals aX AS spectrum composed of p* antibonding frontier orbitals with X-ray absorption energies in the 295-300 eV range and a s*a ntibonding orbital at 298 eV (Figure 3, bottom). All of these orbitals are significantly affected by the photodissociation process,asevidenced by their energy shifts towards the asymptotic values at t > Figure 3. Carbon K-edge X-ray absorption spectra and natural transition orbitals (NTOs) associated with each group of transitions of ICN from the 1 P 1 excited state at representative configurations during the early-time photodissociation dynamics. Spectra at different times are shifted vertically for clarity.

Angewandte Chemie
Research Articles 20046 www.angewandte.org 30 fs.T he carbon 1s!s*t ransition is distinct from the other peaks in the TRXAS since the s*orbital has the symmetry of the dissociating I À Cbond, which explains its higher sensibility to the dissociation process.T he red-shift associated with this transition is aconsequence of the anti-bonding IÀCcharacter. In contrast, during the photodissociation process,t he CN rotation breaks the p*c haracter of the carbon 1s!p* transitions,w hich evolve to be mainly localized on the CN photofragment. As ac onsequence,t he carbon 1s!p*t ransitions experience as mall blue shift. These results show that the XAS spectral shifts can be employed as ad irect observable of the symmetry of frontier orbitals.
Thev ibronic oscillation associated with the CN photofragment is much more evident in the carbon 1s!p* transitions.T he vibrating CN triple bond has both the p symmetry of the C1s!p*b ands and the s symmetry of the C1s!s*t ransition and therefore exhibits significant oscillations that correlate with the CN vibration. As discussed above,t he NTOsa ssociated with the C1s!p*p eaks are mainly localized on the vibrating CN fragment, with av ery small delocalization on I, while for the C1s!s*t ransitions the orbitals are spread over the entire ICN molecule.T his is reflected in the higher sensitivity to the CN vibrations displayed by the C1s!p*b ands in comparison with the C1s!s*bands.Therefore,wefind that the TRXAS provides valuable information on both the vibronic dynamics and the localization of the orbitals involved in the corresponding Xray absorption transitions.

Conclusion
We have shown the capabilities of UV-pump/soft-X-rayprobe spectroscopy to monitor ultrafast nuclear dynamics and transient electronic structure rearrangements during the photodissociation of ICN in the 1 P 1 excited state.O ur findings suggest that currently available instrumental resolution with pulses with FWHM 1.2 eV of laser-based tabletop UV/X-ray pump-probe setups should be sufficient to resolve the predicted oscillatory features and capture the molecular movie of the ultrafast photodissociation process, since the excited state dynamics can be clearly identified for as pectral broadening below about 3eV. Thep resent work reveals the potential of UV/soft-X-Ray pump-probe spectroscopy to resolve the dynamics of the frontier orbitals responsible for chemical bonding and vibrational motions with atomic specificity.T hese unique capabilities should benefit the exploration of ap lethora of chemical transformations,including conformational changes associated with isomerization reactions,b ond dissociation, recombination and charge transfer processes.
Future work will be focused on the extension of the present computational scheme to analyze ultrafast dynamics in more complex systems.A lthough the use of exact wavepacket propagation is unfeasible for condensed phases, despite recent numerical advances, [18] the development of alternative dynamical representations,including semiclassical approximations [19] and/or hybrid QM/MM Born-Oppenheimer schemes, [20] will enable exploration of the use of UV-pump/ soft-X-ray-probe spectroscopy to elucidate chemical dynamics in condensed phases,s uch as biological systems or processes in solution. In particular, the development of proper modeling of rotational and vibrational cooling, geminate recombination, isomerization to iodoisocyanide (INC), hydrogen abstraction by the cyano radical fragment, and coherent control of ICN in solution [9f, g, 21] will be rich topics for further exploration.