Vibrational Pre-excitation Research Articles

Vibrationally resolved two-photon electronic spectra including vibrational pre-excitation: Theory and application to VIPER spectroscopy with two-photon excitation.

Following up on our previous work on vibrationally resolved electronic absorption spectra including the effect of vibrational pre-excitation [von Cosel et al., J. Chem. Phys. 147, 164116 (2017)], we present a combined theoretical and experimental study of two-photon-induced vibronic transitions in polyatomic molecules that are probed in the VIbrationally Promoted Electronic Resonance experiment using two-photon excitation (2P-VIPER). In order to compute vibronic spectra, we employ time-independent and time-dependent methods based on the evaluation of Franck-Condon overlap integrals and Fourier transformations of time-domain correlation functions, respectively. The time-independent approach uses a generalized version of the FCclasses method, while the time-dependent approach relies on the analytical evaluation of Gaussian moments within the harmonic approximation, including Duschinsky rotation effects. For the Coumarin 6 dye, two-dimensional 2P-VIPER experiments involving excitation to the lowest-lying singlet excited state (S1) are presented and compared with corresponding one-photon VIPER spectra. In both cases, coumarin ring modes and a CO stretch mode show VIPER activity, albeit with different relative intensities. Selective pre-excitation of these modes leads to a pronounced redshift of the low-frequency edge of the electronic absorption spectrum, which is a prerequisite for the VIPER experiment. Theoretical analysis underscores the role of interference between Franck-Condon and Herzberg-Teller effects in the two-photon experiment, which is at the root of the observed intensity distribution.

Simulation of the dynamics of vibrationally mediated photodissociation for deuterated pyrrole

The dynamics of photodissociation for vibrationally pre-excited deuterated pyrrole molecules is simulated using ab initio multiple cloning (AIMC) approach. Total kinetic energy release (TKER) spectra and dissociation times are calculated. The results for pyrrole and deuterated pyrrole molecules with and without vibrational pre-excitation are compared. Calculations show that, as expected, the kinetic energy of additional dissociation fragments is lower in deuterated pyrrole and mostly located in the upper-middle part of the TKER spectrum. However, despite lower energy of dissociative bond vibrations, pre-excitation of deuterated pyrrole leads to higher dissociation yield increase than in pyrrole and significantly shortens dissociation time.

High-Resolution Photoelectron Spectroscopy of Vibrationally Excited OH.

The effect of vibrational pre-excitation of anions on their photoelectron spectra is explored, combining slow photoelectron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) with tunable IR radiation to pre-excite the anions. This new IR cryo-SEVI method is applied to OH- as a test system, where the R(0) transition of the hydroxyl anion (3591.53 cm-1) is pumped. Vibrational excitation induces a 30% depletion in photodetachment signal from the v = 0, J = 0 ground state of the anion and the appearance of all five allowed, rotationally resolved photodetachment transitions from the OH- (v = 1, J = 1) level, each with peak widths between 1 and 2 cm-1. By scanning the IR laser, IR cryo-SEVI can also serve as a novel action technique to obtain the vibrational spectrum of OH-, giving an experimental value for the R(0) transition of 3591(1.2) cm-1.

Simulation of the effect of vibrational pre-excitation on the dynamics of pyrrole photo-dissociation.

Photo-dissociation dynamics is simulated for vibrationally pre-excited pyrrole molecules using an ab initio multiple cloning approach. Total kinetic energy release (TKER) spectra and dissociation times are calculated. It is found that pre-excitation of N-H bond vibrations facilitates fast direct dissociation, which results in a significant increase in the high-energy wing of TKER spectra. The results are in very good agreement with the recent vibrationally mediated photo-dissociation experiment, where the TKER spectrum was measured for pyrrole molecules excited by a combination of IR and UV laser pulses. Calculations for other vibrational modes show that this effect is specific for N-H bond vibrations: Pre-excitation of other modes does not result in any significant changes in TKER spectra.

Peptide Bond Ultraviolet Absorption Enables Vibrational Cold-Ion Spectroscopy of Nonaromatic Peptides.

Peptide-bond VUV absorption is inherent to all proteins and peptides. Although widely exploited in top-down proteomics for photodissociation, this absorption has never been spectroscopically characterized in the gas phase. We have measured VUV/UV photofragmentation spectrum of a single peptide bond in a cryogenically cold protonated dipeptide. Although the spectrum appears to be very broadband and structureless, vibrational pre-excitation of this and even larger cold peptides significantly increases the UV dissociation yield for some of their photofragments. We use this effect to extend the technique of IR-UV photofragmentation vibrational spectroscopy, developed for aromatic peptides, to nonaromatic ones and demonstrate measurements of conformation-specific and nonspecific IR spectra for di- to hexa-peptides.

Effect of vibrational pre-excitation on sub-femtosecond structural evolution of water cation in 2A1 state

We study the effects of vibrational excitation of neutral water molecule on sub-femtosecond nuclear dynamics of the 2A1 electronic state of cationic water and its isotopomers that are induced by high-harmonic generation (HHG) process. Both the photoelectron spectra and the autocorrelation functions of electronically excited states of M2O+ (M = H, D, and T) that are produced by Franck-Condon ionization of vibrationally pre-excited M2O to its 2A1 electronic state are calculated. HHG signals are also calculated from the square of the absolute value of autocorrelation functions. In addition, the ratio of the HHG signal of D2O+ to that of H2O+ and that between T2O+ and H2O+ are calculated with respect to time, for varying initially prepared vibrational state of the corresponding neutral molecule. Vibrational dynamics are notably strong on the 2A1 state of the cationic species, as the initial vibrational quantum number increases. The HHG signal of a heavier isotopomer is larger than that of water when the initial vibrational state is on its ground state. The expectation values of the bond length and bond angle show quasiperiodic oscillations with respect to time, which are ascribed to the observed overall rise in the HHG signals. Exceptionally strong vibrational dynamics observed on the 2A1 state potential energy surface of the cationic water molecule can be attributed to the excitation of water bending mode upon Franck-Condon ionization of the corresponding neutral molecule. Here, the observed peaks in the ratios of the HHG signals are theoretically explained in terms of time-evolving molecular structures at two different turning points of the 2A1 surface. We anticipate that the present computational method of solving time-dependent Schrödinger equation would be of use to provide explanations on both ultrafast and chiroptical HHG spectroscopy of polyatomic molecules with certain handedness.

Vibrationally resolved electronic spectra including vibrational pre-excitation: Theory and application to VIPER spectroscopy.

Vibrationally resolved electronic absorption spectra including the effect of vibrational pre-excitation are computed in order to interpret and predict vibronic transitions that are probed in the Vibrationally Promoted Electronic Resonance (VIPER) experiment [L. J. G. W. van Wilderen et al., Angew. Chem., Int. Ed. 53, 2667 (2014)]. To this end, we employ time-independent and time-dependent methods based on the evaluation of Franck-Condon overlap integrals and Fourier transformation of time-domain wavepacket autocorrelation functions, respectively. The time-independent approach uses a generalized version of the FCclasses method [F. Santoro et al., J. Chem. Phys. 126, 084509 (2007)]. In the time-dependent approach, autocorrelation functions are obtained by wavepacket propagation and by the evaluation of analytic expressions, within the harmonic approximation including Duschinsky rotation effects. For several medium-sized polyatomic systems, it is shown that selective pre-excitation of particular vibrational modes leads to a redshift of the low-frequency edge of the electronic absorption spectrum, which is a prerequisite for the VIPER experiment. This effect is typically most pronounced upon excitation of modes that are significantly displaced during the electronic transition, such as ring distortion modes within an aromatic π-system. Theoretical predictions as to which modes show the strongest VIPER effect are found to be in excellent agreement with experiment.

Predistortion amplified in the excited state

Abstract If a molecule is distorted by steric factors or by its environment along a photochemically active coordinate, vertical excitation puts it to a down-slope in the upper state, so that it is accelerated into the same direction. If the photoreaction is barrierless or nearly so, this acceleration can be decisive for selecting one of several possible paths. This principle is used to explain the accordancy principle, for which examples in photochemical electrocyclic reactions are abundant. It also rationalizes the selection between several allowed pericyclic reactions and/or cis–trans isomerizations including the regioselectivity in rhodopsin. Because the Woodward–Hoffmann rules are activated only with delay, the origin of the stereospecificity is reconsidered. In a generalization of the model, the predistortion is generated in a preceding electronic state Sn or on transition from there to the lowest excited singlet state S1; another variant uses selective vibrational preexcitation.

Dynamical Resonances Accessible Only by Reagent Vibrational Excitation in the F + HD→HF + D Reaction

Experimental limitations in vibrational excitation efficiency have previously hindered investigation of how vibrational energy might mediate the role of dynamical resonances in bimolecular reactions. Here, we report on a high-resolution crossed-molecular-beam experiment on the vibrationally excited HD(v = 1) + F → HF + D reaction, in which two broad peaks for backward-scattered HF(v' = 2 and 3) products clearly emerge at collision energies of 0.21 kilocalories per mole (kcal/mol) and 0.62 kcal/mol from differential cross sections measured over a range of energies. We attribute these features to excited Feshbach resonances trapped in the peculiar HF(v' = 4)-D vibrationally adiabatic potential in the postbarrier region. Quantum dynamics calculations on a highly accurate potential energy surface show that these resonance states correlate to the HD(v' = 1) state in the entrance channel and therefore can only be accessed by the vibrationally excited HD reagent.

Ultrafast dual photoresponse of isolated biological chromophores: link to the photoinduced mode-specific non-adiabatic dynamics in proteins

The anionic wild-type Green Fluorescent Protein (GFP) chromophore defines an entire class of naturally occurring chromophores, which are based on the oxydized tyrosine side chain. The GFP chromophore exhibits an enriched photoinduced non-adiabatic dynamics in the multiple excited-state decay channels. Deactivation includes vibrational resonant photodetachment and internal conversion. Here, we provide detailed insight into the efficiency of different vibrational modes in promoting a selective photoresponse in the bare GFP chromophore anion. We introduce a general theoretical model that is capable of accounting for the alternative non-equivalent pathways in internal conversion, and we outline the factors, by which the photo-initiated response may be altered in this channel. The topography around the planar minimum in S1 and the two distinct types of the S1/S0 conical intersections obtained through high-level ab initio calculations provide direct support to the proposed model. There are mode-selective ways to control the photoresponse and to direct it towards a single excited-state decay channel. By tuning the excitation wavelength, the photoresponse may be directed towards the ultrafast non-statistical electron emission coupled with vibrational (de)coherence, whereas a vibrational pre-excitation in the ground state may lead to the ultrafast non-statistical internal conversion through a conical intersection. We also discuss the implication of our results to the photo-initiated non-adiabatic dynamics in the proteins.

Photoisomerization Control Mechanisms in Protonated Schiff Bases

We have performed ab initio excited-state molecular dynamics simulations of an isolated photo-excited protonated Schiff base (C1-N2=C3-C4=C5-C6) to search for mechanisms that control its photoisomerization outcome, such as the bond selectivity and (trans, cis) conformation. We observe that the photo-excited molecule twists around the N2C3 bond (∼80% cases of the thermal ensemble) or the C4C5 bond, and relaxes back to the ground electronic state with either a trans or cis outcome. First, we show that a significant initial distortion of several selected dihedral angles can preferentially guide the excited-state dynamics towards twisting of the C4C5 bond. Next, we examine if the bond selectivity can be controlled by the vibrational pre-excitation of the molecule along individual normal modes. We find that pre-excitation of only one of the modes, which contains a prominent propelling motion of the C4C5 bond with respect to the neighboring C3C4 single bond, leads to twisting of the C4C5 bond. Normal mode decomposition of the ground state thermal ensemble shows that in starting structures in which this same mode is pre-excited by 1-2 kBT thermal energy, the twisting of C4C5 occurs with a 30-50% probability. Finally, we find that the (trans, cis) outcome of the reaction can be controlled by selective pre-twisting of several dihedral angles, while keeping other degrees of freedom thermally excited. This choice was justified by the observed pre-twisting of retinal chromophore in rhodopsin, which exhibits 65% cis to trans transition. In the thermal ensemble with such pre-twisted dihedrals, we observe on the excited state potential energy surface synchronized twisting of CN2C3C and HN2C3H torsional angles surrounding the isomerizing N2C3 bond, which significantly increases the fraction of reactive (cis to trans) trajectories. These observations provide crucial understanding of natural photoisomerization mechanisms and their potential use in synthetic molecules.

Six-dimensional quasiclassical and quantum dynamics of H2 dissociation on the c(2 × 2)-Ti/Al(100) surface

Based on a slab model of H(2) dissociation on a c(2 × 2) structure with Ti atoms in the first and third layers of Al(100), a six-dimensional (6D) potential energy surface (PES) has been built. In this PES, a molecular adsorption well with a depth of 0.45 eV is present in front of a barrier of height 0.13 eV. Using this PES, H(2) dissociation probabilities are calculated by the classical trajectory (CT), the quasiclassical trajectory (QCT), and the time-dependent wave-packet (TDWP) method. The QCT study shows that trajectories can be trapped by the molecular adsorption well. Higher incident energy can lead to direct H(2) dissociation. Vibrational pre-excitation is the most efficient way to promote direct dissociation without trapping. We find that both rotational and vibrational excitation have efficacies close to 1.0 in the entire range of incident energies investigated, which supports the randomization in the initial conditions making the reaction rate solely dependent on the total (internal and translational) energy. The H(2) dissociation probabilities from quantum dynamics are in reasonable agreement with the QCT results in the energy range 50-200 meV, except for some fluctuations. However, the TDWP results considerably exceed the QCT results in the energy range 200-850 meV. The CT reaction probabilities are too low compared with the quantum dynamical results.

Dissipative dynamics within the electronic friction approach: the femtosecond laser desorption of H2/D2 from Ru(0001)

An electronic friction approach based on Langevin dynamics is used to describe the multidimensional (six-dimensional) dynamics of femtosecond laser induced desorption of H(2) and D(2) from a H(D)-covered Ru(0001) surface. The paper extends previous reduced-dimensional models, using a similar approach. In the present treatment forces and frictional coefficients are calculated from periodic density functional theory (DFT) and essentially parameter-free, while the action of femtosecond laser pulses on the metal surface is treated by using the two-temperature model. Our calculations shed light on the performance and validity of various adiabatic, non-adiabatic, and Arrhenius/Kramers type kinetic models to describe hot-electron mediated photoreactions at metal surfaces. The multidimensional frictional dynamics are able to reproduce and explain known experimental facts, such as strong isotope effects, scaling of properties with laser fluence, and non-equipartitioning of vibrational, rotational, and translational energies of desorbing species. Further, detailed predictions regarding translations are made, and the question for the controllability of photoreactions at surfaces with the help of vibrational preexcitation is addressed.

Nonadiabatic Quantum Dynamics and Laser Control of Br2 in Solid Argon

A five-dimensional reaction surface-vibronic coupling model is introduced to describe the B- to C-state predissociation dynamics of Br(2) occupying a double substitutional lattice site in a face-centered cubic argon crystal at low temperatures. The quantum dynamics driven by a Franck-Condon vertical excitation is investigated, revealing the role of matrix cage compression for efficient nonadiabatic transitions. Vibrational preexcitation of the Br(2) bond in the electronic ground state can be used to access a different regime of predissociation which does not require substantial matrix compression because the Franck-Condon window shifts into the energetic range of the B-C level crossing. Using optimal control theory, it is shown how vibrational preexcitation can be achieved via a pump-dump-type mechanism involving the repulsive C state.

The Cl + H2 → HCl + H Reaction Induced by IR + UV Irradiation of Cl2 in Solid para-H2: Quantum Model Simulation

Recent experimental investigations by the group of D. T. Anderson (Kettwich, S. C.; Raston, P. L.; Anderson, D. T. J. Phys. Chem. A 2009, 113, DOI 10.1021/jp811206a) show that the reaction Cl + H(2) --> HCl + H in the para-H(2) crystal can be induced by infrared (IR) + ultraviolet (UV) coirradiations causing vibrational pre-excitation of the molecular reactant, H(2)(v=1), and generation of the atomic reactant, Cl((2)P(3/2)), by near-resonant photodissociation of a matrix-isolated Cl(2) molecule in the C (1)Pi(u) state, respectively. The corresponding reaction probability P(v=1) for the reactants Cl + H(2)(v=1) is approximately 0.15; this is approximately 25 times larger than P(v=0) for Cl + H(2)(v=0) (as initiated by pure UV irradiation). We present a simple three-step quantum model which accounts for some important parts of the experimental results and allows predictions for other scenarios, for example, UV photodissociation of the Cl(2) molecule by a laser pulse. The first step, vibrational pre-excitation of H(2), yields the molecular initial state which is described using the Einstein model of the para-H(2) crystal. The second step, photodissociation of Cl(2), generates the Cl((2)P(3/2)) atom approaching H(2)(v=1). In the third step, Cl reacts with H(2)(v=1) much more efficiently than with H(2)(v=0) close to threshold. The ultrashort time domains (approximately 100 fs) of steps 2 plus 3 support one- and then two-dimensional models of photodissociation of Cl(2) by short laser pulses and of the subsequent reaction of the system Cl-H-H embedded in frozen environments. The widths of the corresponding wave function describing the translational motion of the reactants is revealed as a significant parameter which is determined not only by the duration of the laser pulse but, even more importantly, by the width of the Gaussian-type distribution of the center of mass of the H(2) molecule in its Einstein cell. As a consequence, the resulting P(v) are quite robust versus variations of the UV pulse durations, allowing extrapolations to continuous wave irradiation. Quantum dynamics simulations of the reaction reveal that the experimental results are due to energetic and dynamical effects.

Enhanced photodesorption by vibrational pre-excitation: Quantum model simulations for Cs/Cu(1 1 1)

We present quantum model simulations for the laser-induced desorption of Cs from Cu(1 1 1) induced by a charge transfer from the Cu surface state to an unoccupied resonance state of the Cs-adsorbate. Based on empirical two-state model potentials along the Cs–Cu distance quantum nuclear dynamics were performed including electronic relaxation to account for the ultrashort lifetime of the resonance state. In order to increase the negligible desorption yield obtained from direct UV excitation the system was vibrationally pre-excited by a THz-laser pulse. Using an “jumping” wavepacket incoherent averaging scheme we calculated the desorption probability with respect to the time evolution of a vibrational wavepacket on the electronic ground state. The desorption probability is significantly increased for short Cs–Cu distances, but it is almost negligible for the stretched Cs–Cu bond. Vibrational pre-excitation enhances the photodesorption of Cs from Cu(1 1 1), but only if the timing of the laser-induced charged transfer is chosen correctly.

Nonadiabatic quantum dynamics of Br 2 in solid Ar: A four-dimensional study of the B to C state predissociation

Quantum dynamics simulations are performed for a diatomics-in-molecules based model of Br 2 in solid Ar which incorporates four nuclear degrees of freedom and four electronic states. The nuclear motions comprise two large amplitude coordinates describing the Br 2 bond distance and an effective symmetry-preserving matrix mode. Two symmetry-lowering harmonic modes are added in the spirit of linear vibronic coupling theory. Initiating the dynamics on the B state by means of an ultrafast laser pulse, nonadiabatic transitions to the two degenerate C states are monitored and the effect of vibrational preexcitation in the electronic ground state is investigated.

Efficient, highly selective laser isotope separation of carbon-13

We recently demonstrated an original approach to highly selective laser isotope separation of carbon-13 that employs vibrational overtone pre-excitation of CF3 H together with infrared multiphoton dissociation [O.V. Boyarkin, M. Kowalczyk, T.R. Rizzo, J. Chem. Phys. 118, 93 (2003)]. The practical implementation of this approach was complicated by the long absorption path length needed for the overtone excitation laser beam. In the present work, we employ a low overtone level for the pre-excitation that shortens this pathway, facilitating engineering of the process. We propose an optimal configuration of the isotope separation scheme and consider a realistic example of a separation unit for isotopic enrichment of carbon-13 to 94–98%. The photon energy expenditure of 97 eV per separated atom is much lower than that of the current commercial laser technology, making this process economically feasible.

Coherent spin control of matrix isolated molecules by IR+UV laser pulses: quantum simulations for ClF in Ar.

Two coherent sequential IR+UV laser pulses may be used to generate two time-dependent nuclear wave functions in electronic excited triplet and singlet states via single (UV) and two photon (IR+UV) excitation pathways, exploiting spin-orbit coupling and vibrational pre-excitation, respectively. These wave functions evolve from different Franck-Condon domains until they overlap in a domain of bond stretching with efficient intersystem crossing. Here, the coherence of the laser pulses is turned into optimal interferences of the wave packets, yielding the total wave packet at the target place, time, and with dominant target spin. The time resolution of spin control is few femtoseconds. The mechanism is demonstrated by means of quantum model simulations for ClF in an Ar matrix.

Control of breaking strong versus weak bonds of BaFCH3 by femtosecond IR + VIS laser pulses: theory and experiment

Intense (≈80 GW cm−2) ultrashort (≈100 fs) infrared (IR) laser pulses may be employed for excitation of a high frequency (≈3500 cm−1) local mode vibration in a molecule. Subsequently, an intense (16–256 GW cm−2), ultrashort visible (VIS) laser pulse yields electronic excitation with near adiabatic transfer of the vibrational energy, which has been accumulated by the IR pulse. The net result of these sequential IR + VIS laser pulses may be the breaking of a strong molecular bond close to the pre-excited one. In contrast, exclusive excitation by just a visible laser pulse breaks a competing weak bond. The effects of IR + VIS laser pulse control may be considered as an extension of vibrationally mediated chemistry, from ns pulses or continuous wave (cw) excitations to sub-ps laser pulses, and from direct vibrational pre-excitation of the bond to be broken to a neighboring bond, thus exploiting intramolecular vibrational redistribution (IVR) from the pre-excited local mode to the bond to be broken in the electronic excited state. The mechanism is demonstrated by quantum simulations for the model system BaFCH3, where BaF-, FC- and CH3 play the roles of the weak and strong bonds to be broken, and the vibrationally pre-excited CH3 stretch. The theoretical predictions are confirmed experimentally. Various extensions of the control by IR + VIS laser pulses include the control of the branching ratio of weak versus strong bond breaking, as well as isotopomer selectivity depending on the vibrational pre-excitations.