Vibrational Wave Packet Motion Research Articles

Differential Shannon Entropies and Mutual Information in a Curve Crossing System: Adiabatic and Diabatic Decompositions.

The differential Shannon entropy provides a measure for the localization of a wave function. We regard the vibrational wave packet motion in a curve crossing system and calculate time-dependent entropies. Using a numerical example, we analyze how localization inside diabatic and adiabatic states can be accessed and discuss the differences between these two representations. In order to do so, we extend the usual entropy definition and introduce novel state-selective entropies. These quantities contain information on the form of the nuclear density components on the one hand and on the state population on the other, and it is shown how the contribution of the population can be removed. Having the state-selective entropies at hand, two additional functions derived from these, namely, the conditional entropy and the mutual information, are determined and compared. We find that these quantities relate closely to correlation effects rooted in different electronic properties of the system.

Ultrafast Vibrational Wave Packet Dynamics of the Aqueous Guanine Radical Anion Induced by Photodetachment.

Studying the ultrafast dynamics of ionized aqueous biomolecules is important for gaining an understanding of the interaction of ionizing radiation with biological matter. Guanine plays an essential role in biological systems as one of the four nucleobases that form the building blocks of deoxyribonucleic acid (DNA). Guanine radicals can induce oxidative damage to DNA, particularly due to the lower ionization potential of guanine compared to the other nucleobases, sugars, and phosphate groups that are constituents of DNA. This study utilizes femtosecond optical pump-probe spectroscopy to observe the ultrafast vibrational wave packet dynamics of the guanine radical anion launched by photodetachment of the aqueous guanine dianion. The vibrational wave packet motion is resolved into 11 vibrational modes along which structural reorganization occurs upon photodetachment. These vibrational modes are assigned with the aid of density functional theory (DFT) calculations. Our work sheds light on the ultrafast vibrational dynamics following the ionization of nucleobases in an aqueous medium.

Ultrafast Extreme Ultraviolet Photoelectron Spectroscopy of Nonadiabatic Photodissociation of CS2 from 1B2 (1Σu+) State: Product Formation via an Intermediate Electronic State.

We studied nonadiabatic dissociation of CS2 from the 1B2 (1Σu+) state using ultrafast extreme ultraviolet photoelectron spectroscopy. A deep UV (200 nm) laser using the filamentation four-wave mixing method and an extreme UV (21.7 eV) laser using the high-order harmonic generation method were employed to achieve the pump-probe laser cross-correlation time of 48 fs. Spectra measured with a high signal-to-noise ratio revealed clear dynamical features of vibrational wave packet motion in the 1B2 state; its electronic decay to lower electronic state(s) within 630 fs; and dissociation into S(1D2), S(3PJ), and CS fragments within 300 fs. The results suggest that both singlet and triplet dissociation occur via intermediate electronic state(s) produced by electronic relaxation from the 1B2 (1Σu+) state.

Ultrafast vibrational wave packet dynamics of the aqueous tyrosyl radical anion induced by photodetachment.

The ultrafast dynamics triggered by the photodetachment of the tyrosinate dianion in aqueous environment shed light on the elementary processes that accompany the interaction of ionizing radiation with biological matter. Photodetachment of the tryosinate dianion yields the tyrosyl radical anion, an important intermediate in biological redox reactions, although the study of its ultrafast dynamics is limited. Here, we utilize femtosecond optical pump-probe spectroscopy to investigate the ultrafast structural reorganization dynamics that follow the photodetachment of the tyrosinate dianion in aqueous solution. Photodetachment of the tyrosinate dianion leads to vibrational wave packet motion along seven vibrational modes that are coupled to the photodetachment process. The vibrational modes are assigned with the aid of density functional theory (DFT) calculations. Our results offer a glimpse of the elementary dynamics of ionized biomolecules and suggest the possibility of extending this approach to investigate the ionization-induced structural rearrangement of other aromatic amino acids and larger biomolecules.

Probing dynamical symmetries by bicircular high-order harmonic spectroscopy beyond the Born-Oppenheimer approximation

We explore the possibility of bicircular high-order harmonic spectroscopy to probe the laser-induced dynamics of molecules in a non-Born-Oppenheimer treatment. The numerical solutions of the time-dependent Schr\odinger equation for aligned ${\mathrm{H}}_{2}$ and its isotopologs in $\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ bicircular fields show that the intensity ratio between ${\mathrm{D}}_{2}$ and ${\mathrm{H}}_{2}$ for harmonic orders $3q$ is lower than that for orders $3q\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{4pt}{0ex}}(q\ensuremath{\in}\mathbb{N})$. Based on the strong-field approximation, we demonstrate that the interplay of vibrational wave-packet motion and dynamical-symmetry breaking leads to the different ratio. In general, the vibrational motion causes the ratio between isotopologs to increase with $q$ for both harmonic orders $3q$ and $3q\ifmmode\pm\else\textpm\fi{}1$. On the other hand, the emission of orders $3q$ is possible only because of the alignment-induced breaking of the dynamical symmetry. The faster nuclear motion in ${\mathrm{H}}_{2}$ enhances the symmetry breaking, resulting in the lower ${\mathrm{D}}_{2}$/${\mathrm{H}}_{2}$ ratio for the orders $3q$. Therefore, the harmonic orders $3q$ give access to the attosecond probing of dynamical symmetries in molecules.

Long-Lived Nuclear Coherences inside Helium Nanodroplets.

Much of our knowledge about dynamics and functionality of molecular systems has been achieved with femtosecond time-resolved spectroscopy. Despite extensive technical developments over the past decades, some classes of systems have eluded dynamical studies so far. Here, we demonstrate that superfluid helium nanodroplets, acting as a thermal bath of 0.4K temperature to stabilize weakly bound or reactive systems, are well suited for time-resolved studies of single molecules solvated in the droplet interior. By observing vibrational wave packet motion of indium dimers (In_{2}) for tens of picoseconds, we demonstrate that the perturbation imposed by this quantum liquid can be lower by a factor of 10-100 compared to any other solvent, which uniquely allows us to study processes depending on long nuclear coherence in a dissipative environment. Furthermore, tailor-made microsolvation environments inside droplets will enable us to investigate the solvent influence on intramolecular dynamics in a wide tuning range from molecular isolation to strong molecule-solvent coupling.

Time-Resolved Photoelectron Imaging of Acetone with 9.3 eV Photoexcitation

Ultrafast electronic relaxation following 9.3 eV photoexcitation of gaseous acetone was investigated with time-resolved photoelectron imaging spectroscopy. An intense photoionization signal due to a transition from the 41A1(π,π*) state to the D1(π-1) cationic state diminishes within 50 fs, owing to vibrational wave packet motion leaving our observation energy window. Additional photoionization signals were assigned to transitions from Rydberg states with principal quantum numbers of 3-8 to the D0(n-1) cationic state, created by strong vibronic couplings with the bright 41A1(π,π*) state. The deactivation processes of the 41A1(π,π*) and Rydberg states are discussed based on their decay profiles obtained from a time-energy map of photoelectron kinetic energy distributions.

Theoretical studies of strong-field photoionization of CH3I

The CH3I molecule serves as an important model system in photochemistry. The photoionization usually includes the laser-molecule interaction and ionization process. Theoretical simulations of the laser-induced ionization process are, however, difficult due to multi-electrons involved. In this work, the ionization process of CH3I is studied based on the Frank-Condon principle. Three two-dimensional potential energy surfaces along the CI bond length and the HCI umbrella angle are constructed for the ground electronic state of CH3I and the ground and first excited states of CH3I+, with internally contracted multireference configuration interaction method and the aug-cc-pVTZ basis set used for the atoms C and H and the aug-cc-pvtz-pp basis set for the atom I. The vibrational frequencies of the CI stretching mode and the umbrella mode of CH3 are calculated for both CH3I and CH3I+, and are consistent with the experimental measurements. Furthermore, based on the Ammosov-Delone-Krainov ionization model, dynamics calculations are carried out to simulate ionization process induced by R-selective depletion mechanism and the vibrational wave packet motion on CH3I+ PESs.

Elucidating the origins of multimode vibrational coherences of polyatomic molecules induced by intense laser fields

Strong-field laser–molecule interaction forms much of the basis for initiating and probing ultrafast quantum dynamics. Previous studies aimed at elucidating the origins of vibrational coherences induced by intense laser fields have been confined to diatomic molecules. Furthermore, in all cases examined to date, vibrational wave packet motion is found to be induced by R-selective depletion; wave packet motion launched by bond softening, though theoretically predicted, remains hitherto unobserved. Here we employ the exquisite sensitivity of femtosecond extreme ultraviolet absorption spectroscopy to sub-picometer structural changes to observe both bond softening-induced vibrational wave packets, launched by the interaction of intense laser pulses with iodomethane, as well as multimode vibrational motion of the parent ion produced by strong-field ionization. In addition, we show that signatures of coherent vibrational motion in the time-dependent extreme ultraviolet absorption spectra directly furnish vibronic coupling strengths involving core-level transitions, from which geometrical parameters of transient core-excited states are extracted.

Sub-10-fs control of dissociation pathways in the hydrogen molecular ion with a few-pulse attosecond pulse train.

The control of the electronic states of a hydrogen molecular ion by photoexcitation is considerably difficult because it requires multiple sub-10 fs light pulses in the extreme ultraviolet (XUV) wavelength region with a sufficiently high intensity. Here, we demonstrate the control of the dissociation pathway originating from the 2pσu electronic state against that originating from the 2pπu electronic state in a hydrogen molecular ion by using a pair of attosecond pulse trains in the XUV wavelength region with a train-envelope duration of ∼4 fs. The switching time from the peak to the valley in the oscillation caused by the vibrational wavepacket motion in the 1sσg ground electronic state is only 8 fs. This result can be classified as the fastest control, to the best of our knowledge, of a molecular reaction in the simplest molecule on the basis of the XUV-pump and XUV-probe scheme.

ChemInform Abstract: Multidimensional Electronic Spectroscopy of Photochemical Reactions

AbstractReview: 242 refs.

Control of electronic populations in porphycene with chirped ultrashort pulses

The degree of optical excitation of porphycene in solution can be controlled by manipulating linear chirp of high intensity ultrashort laser pulses while keeping their energies and spectra constant. Transient absorption, which reflects molecular populations in the ground and the first singlet excited state, varies by over 100% between optimal positively and negatively chirped excitation pulses. The results are discussed invoking coherent vibrational wavepacket dynamics. Full Text: PDF References J. Waluk, Porphycenes: Spectroscopy, Photophysics, and Tautomerism. In CRC Handbook of Organic Photochemistry and Photobiology, M. Oelgemoeller, A. Griesbeck, F. Ghetti, Eds. (CRC Press: Boca Raton, FL, 2012), pp 809-829. CrossRef P. Fita, N. Urbańska, C. Radzewicz, J. Waluk, "Ground- and Excited-State Tautomerization Rates in Porphycenes", Chem. Eur. J. 15, 485 (2009). CrossRef P. Ciąćka, P. Fita, A. Listkowski, M. Kijak, S. Nonell, D. Kuzuhara, H. Yamada, C. Radzewicz, and J. Waluk, J. Phys. Chem. B 119, 2292 (2015). CrossRef P. Nuernberger, G. Vogt, T. Brixner and G. Gerber, "Femtosecond quantum control of molecular dynamics in the condensed phase", Phys. Chem. Chem. Phys. 9, 2470 (2007). CrossRef G. Cerullo, C.J. Bardeen, Q. Wang, C.V. Shank, "High-power femtosecond chirped pulse excitation of molecules in solution", Chem. Phys. Lett. 262 362 (1996). CrossRef C.J. Bardeen, Q. Wang, C.V. Shank, "Selective Excitation of Vibrational Wave Packet Motion Using Chirped Pulses", Phys. Rev. Lett. 75, 3410 (1995). CrossRef P. Tournois, "http://dx.doi.org/10.1016/S0030-4018(97)00153-3", Opt. Comm. 140, 245 (1997). CrossRef W. Demtroder, Laser spectroscopy: basic concepts and instrumentation, 3rd ed. (Springer Press, Berlin, Heidelberg, New York, 2003). CrossRef A. Starukhin, E. Vogel, J. Waluk, J. Phys. Chem. A 102, 9999 (1998). CrossRef P. Fita, C. Radzewicz, J. Waluk, J. Phys. Chem. A, 112, 10753 (2008). CrossRef

Multidimensional Electronic Spectroscopy of Photochemical Reactions.

Coherent multidimensional electronic spectroscopy can be employed to unravel various channels in molecular chemical reactions. This approach is thus not limited to analysis of energy transfer or charge transfer (i.e. processes from photophysics), but can also be employed in situations where the investigated system undergoes permanent structural changes (i.e. in photochemistry). Photochemical model reactions are discussed by using the example of merocyanine/spiropyran-based molecular switches, which show a rich variety of reaction channels, in particular ring opening and ring closing, cis-trans isomerization, coherent vibrational wave-packet motion, radical ion formation, and population relaxation. Using pump-probe, pump-repump-probe, coherent two-dimensional and three-dimensional, triggered-exchange 2D, and quantum-control spectroscopy, we gain intuitive pictures on which product emerges from which reactant and which reactive molecular modes are associated.

Evidence for the purely electronic character of primary electron transfer in purple bacteria Rh. Sphaeroides

A quantum-chemical calculation of the excited electronic states of a Rh. Sphaeroides reaction centre was performed. We discovered a new excited electronic state which can participate in electron transfer (ET). The energy gradient calculations showed that photoexcitation activates only high-frequency vibrational modes. This contradicts the widely accepted picture of ET resulting from vibrational wave packet motion. An alternative model is suggested where ET has a purely dissipative character and occurs only due to pigment--protein interaction. With this model, we demonstrate that oscillations in the femtosecond spectra can be caused by the new electronic state and non-Markovian character of dissipative dynamics.

Direct observation of coherent oscillations in solution due to microheterogeneous environment.

We report, for the first time, direct observation of coherent oscillations in the ground-state of IR775 dye due to microheterogeneous environment. Using ultrafast near-infrared degenerate pump-probe technique centered at 800 nm, we present the dynamics of IR775 in a binary mixture of methanol and chloroform at ultra-short time resolution of 30 fs. The dynamics of the dye in binary mixtures, in a time-scale of a few fs to ~740 ps, strongly varies as a function of solvent composition (volume fraction). Multi-oscillation behavior of the coherent vibration was observed, which increased with decreasing percentage of methanol in the dye mixture. Maximum number of damped oscillations were observed in 20% methanol. The observed vibrational wavepacket motion in the ground-state is periodic in nature. We needed two cosine functions to fit the coherent oscillation data as two different solvents were used. Dynamics of the dye molecule in binary mixtures can be explained by wavepacket motion in the ground potential energy surface. More is the confinement of the dye molecule in binary mixtures, more is the number of damped oscillations. The vibrational cooling time, τ2, increases with increase in the confinement of the system. The observed wavepacket oscillations in ground-state dynamics continued until 1.6 ps.

The photochemical ring opening reaction of chromene as seen by transient absorption and fluorescence spectroscopy

In this paper we investigate the photochromic ring-opening reaction of 2,2-diphenyl-5,6-benzo(2H)chromene. In particular, we study the uncertainties and contradictions in various published reaction models using a combination of transient absorption and fluorescence spectroscopy with femtosecond time resolution. We propose a simplified reaction scheme which is in good agreement with theoretical studies. Here, photoexcitation populates a Franck-Condon state, whose fast vibrational wave packet motion, vibrational relaxation, bond-alternation and/or solvent rearrangement processes occur on the sub-picosecond timescale. Our data suggest that the resulting excited state minimum with picosecond lifetime still features structural characteristics of the closed form. Subsequently, the ring-opened photoproducts are formed in a concerted step from the excited state. The velocity of the photoreaction hence only depends on the time that the molecule needs to reach the transition region between the ground and excited states where the crucial bond breakage occurs.

Ultrafast Excited State Intramolecular Proton Transfer Dynamics of 1-Hydroxyanthraquinone in Solution

Proton transfer reaction is one of the most fundamental processes in chemistry and life science. Excited state intramolecular proton transfer (ESIPT) has been studied as a model system of the proton transfer, since it can be conveniently initiated by light. We report ESIPT reaction dynamic of 1-hydroxy-anthraquione (1-HAQ) in solution by highly time-resolved fluorescence. ESIPT time of 1-HAQ is determined to be <TEX>$45{\pm}10$</TEX> fs directly from decay of the reactant fluorescence and rise of the product fluorescence. High time resolution allows observation of the coherent vibrational wave packet motion in the excited state of the reaction product tautomer. The coherently excited vibrational mode involves large displacement of the atoms, which shortens the distance between the proton donor and the acceptor. With the theoretical analysis, we propose that the ESIPT of 1-HAQ proceeds barrierlessly with assistance of the skeletal vibration, which in turn becomes excited coherently by the ESIPT reaction.

Vibrational vs. electronic coherences in 2D spectrum of molecular systems

Two-dimensional spectroscopy has recently revealed the oscillatory behavior of the excitation dynamics of molecular systems. However, in the majority of cases there is considerable debate over what is actually being observed: excitonic or vibrational wavepacket motion or evidence of quantum transport. In this letter we present a method for distinguishing between vibrational and excitonic wavepacket motion, based on the phase and amplitude relationships of oscillations of distinct peaks as revealed through a fundamental analysis of the two-dimensional spectra of two representative systems.

Noisy Optical Pulses Enhance the Temporal Resolution of Pump-Probe Spectroscopy

Time-resolved measurements of quantum dynamics are based on the availability of controlled events that are shorter than the typical evolution time scale of the processes to be observed. Here we introduce the concept of noise-enhanced pump-probe spectroscopy, allowing the measurement of dynamics significantly shorter than the average pulse duration by exploiting randomly varying, partially coherent light fields consisting of bunched colored noise. These fields are shown to be superior by more than a factor of 10 to frequency-stabilized fields, with important implications for time-resolved experiments at x-ray free-electron lasers and, in general, for measurements at the frontiers of temporal resolution (e.g., attosecond spectroscopy). As an example application, the concept is used to explain the recent experimental observation of vibrational wave-packet motion in D(2)(+) on time scales shorter than the average pulse duration.

Excited‐State Dynamics of CS2 Studied by Photoelectron Imaging with a Time Resolution of 22 fs

The ultrafast dynamics of CS2 in the 1B2(1Σu+) state was studied by photoelectron imaging with a time resolution of 22 fs. The photoelectron signal intensity exhibited clear vibrational quantum beats due to wave packet motion. The signal intensity decayed with a lifetime of about 400 fs. This decay was preceded by a lag of around 30 fs, which was considered to correspond to the time for a vibrational wave packet to propagate from the Franck–Condon region to the region where predissociation occurred. The photoelectron angular distribution remained constant when the pump–probe delay time was varied. Consequently, variation of the electronic character caused by the vibrational wave packet motion was not identified within the accuracy of our measurements.