A model study of vibrational excitation of carbon dioxide molecule by slow electrons: the role of wave packet sliding from the ridge

The method of an accurate calculation of vibrational excitation cross sections of a two-mode molecule by slow electrons within the framework of the local theory (the ‘boomerang’ model) is applied for a model study of the excitation of the symmetrical stretching vibrational modes of carbon dioxide in the two-mode approximation (i.e., only the symmetrical stretch and bending modes are included in the consideration). It is shown that the 'boomerang’ oscillations in the cross section are strongly suppressed due to the decay of the one-dimensional ‘boomerang' state caused by the anion wave packet sliding from the linear configuration ridge. Consequently, the bending motion in CO₂ molecule should to be taken into account even if only the processes without the final bending excitation are considered. A simple quasi one-dimensional model describing the system sliding from the ridge is put forward which treats this phenomenon as a decay of the initial wave packet via the series of diabatic resonant states related to unstable trajectory.     

Optical control of excited-state vibrational coherences of a molecule in solution: The influence of the excitation pulse spectrum and phase in LD690

Spectral and phase shaping of femtosecond laser pulses is used to selectively excite vibrational wave packets on the ground (S0) and excited (S1) electronic states in the laser dye LD690. The transient absorption signals observed following excitation near the peak of the ground-state absorption spectrum are characterized by a dominant 586 cm(-1) vibrational mode. This vibration is assigned to a wave packet on the S0 potential energy surface. When the excitation pulse is tuned to the blue wing of the absorption spectrum, a lower frequency 568 cm(-1) vibration dominates the response. This lower frequency mode is assigned to a vibrational wave packet on the S1 electronic state. The spectrum and phase of the excitation pulse also influence both the dephasing of the vibrational wave packet and the amplitude profiles of the oscillations as a function of probe wavelength. Excitation by blue-tuned, positively chirped pulses slows the apparent dephasing of the vibrational coherences compared with a transform-limited pulse having the same spectrum. Blue-tuned negatively chirped excitation pulses suppress the observation of coherent oscillations in the ground state.     

Vibrational analysis of excited and ground electronic states of all-trans retinal protonated Schiff-bases

We report on vibrational coherence dynamics in excited and ground electronic states of all-trans retinal protonated Schiff-bases (RPSB), investigated by time-resolved Degenerate Four-Wave-Mixing (DFWM). The results show that wave packet dynamics in the excited state of RPSB consist of only low-frequency (<800 cm(-1)) modes. Such low-frequency wave packet motion is observed over a broad range of detection wavelengths ranging from excited state absorption (～500 nm) to stimulated emission (>600 nm). Our results indicate that low-frequency coherences in the excited state are not activated directly by laser excitation but rather by internal vibrational energy redistribution. This is supported by the observation that similar coherence dynamics are not observed in the electronic ground state. Challenging previous experimental results, we show that the formation of low-frequency coherence dynamics in RPSB does not require significant excess vibrational energy deposition in the excited state vibrational manifolds. Concerning ground state wave packet dynamics, we observe a set of high-frequency (>800 cm(-1)) modes, reflecting mainly single and double bond stretching motion in the retinal polyene-chain. Dephasing of these high-frequency coherences is mode-dependent and partially differs from analogous vibrational dephasing of the all-trans retinal chromophore in a protein environment (bacteriorhodopsin).     

Coherent low-frequency motions of hydrogen bonded acetic acid dimers in the liquid phase

Ultrafast vibrational dynamics of cyclic hydrogen bonded dimers and the underlying microscopic interactions are studied in temporally and spectrally resolved pump-probe experiments with 100 fs time resolution. Femtosecond excitation of the O-H and/or O-D stretching mode gives rise to pronounced changes of the O-H/O-D stretching absorption displaying both rate-like kinetic and oscillatory components. A lifetime of 200 fs is measured for the v=1 state of the O-H stretching oscillator. The strong oscillatory absorption changes are due to impulsively driven coherent wave packet motions along several low-frequency modes of the dimer between 50 and 170 cm(-1). Such wave packets generated via coherent excitation of the high-frequency O-H/O-D stretching oscillators represent a clear manifestation of the anharmonic coupling of low- and high-frequency modes. The underdamped low-frequency motions dephase on a time scale of 1-2 ps. Calculations of the vibrational potential energy surface based on density functional theory give the frequencies, anharmonic couplings, and microscopic elongations of the low-frequency modes, among them intermolecular hydrogen bond vibrations. Oscillations due to the excitonic coupling between the two O-H or O-D stretching oscillators are absent as is independently confirmed by experiments on mixed dimers with uncoupled O-H and O-D stretching oscillators.     

Quasibound continuum states in SiF4 (D2A1) photoionization: photoelectron-vibrational coupling

The authors report a fully vibrationally resolved photoelectron spectroscopy investigation of a nonplanar molecule studied over a range of excitation energies. Experimental results for all four fundamental vibrational modes are presented. In each case significant non-Franck-Condon effects are seen. The vibrational branching ratio for the totally symmetric mode nu1+ is found to be strongly affected by resonant excitation in the SiF4+ (D2A1) photoionization channel. This is shown to be the result of two distinct shape resonances, which for the first time have been both confirmed by theoretical calculations. Vibrationally resolved Schwinger photoionization calculations are used to understand the vibronic coupling for the photoelectrons, both using ab initio and harmonic vibrational wave functions.     

Ultrafast dynamics of halogens in rare gas solids

We perform time resolved pump-probe spectroscopy on small halogen molecules ClF, Cl2, Br2, and I2 embedded in rare gas solids (RGS). We find that dissociation, angular depolarization, and the decoherence of the molecule is strongly influenced by the cage structure. The well ordered crystalline environment facilitates the modelling of the experimental angular distribution of the molecular axis after the collision with the rare gas cage. The observation of many subsequent vibrational wave packet oscillations allows the construction of anharmonic potentials and indicate a long vibrational coherence time. We control the vibrational wave packet revivals, thereby gaining information about the vibrational decoherence. The coherence times are remarkable larger when compared to the liquid or high pressure gas phase. This fact is attributed to the highly symmetric molecular environment of the RGS. The decoherence and energy relaxation data agree well with a perturbative model for moderate vibrational excitation and follow a classical model in the strong excitation limit. Furthermore, a wave packet interferometry scheme is applied to deduce electronic coherence times. The positions of those cage atoms, excited by the molecular electronic transitions are modulated by long living coherent phonons of the RGS, which we can probe via the molecular charge transfer states.     

Coherent ultrafast photoelectron spectroscopy in Br2: an alternative method for measuring Delta v not equal 0 transitions in Rydberg-to-Rydberg excitations

A two-state vibrational wave packet is prepared in a low-lying 4d[12](1 or 2) Rydberg state of jet cooled Br(2) (4d, v(')=3 and v(')=4) by two-photon excitation with 266.5 nm pulses from an ultrafast laser. The wave packet is detected by autoionization following excitation with time-delayed 800 nm pulses to the n=8 (v(+)=4) and n=9 (v(+)=3) Rydberg states in the (2)Pi(12) angular momentum core state. Autoionization of each state occurs to the (2)Pi(32) state of the ion through spin-orbit ionization. Photoelectron spectroscopy is used to differentiate between the n=8 and n=9 ejected photoelectrons. Detection of the wave packet recurrences via the n=8 and n=9 Rydberg states reveals a pi phase-shift difference of the recurrences between the two final states. In each case, Delta v not equal 0 transitions are observed since wave packet recurrences are detected. By fitting the observed phase change of the recurrences with a simple model for the overlap amplitudes and assumptions about the potentials, we estimate, within the context of the model, that approximately 0.6% of the transitions may be attributed to Delta v= +/- 1 transitions between the initial Rydberg superposition state and the final Rydberg detection state.     

Terahertz normal mode relaxation in pentaerythritol tetranitrate

Normal vibrational modes for a three-dimensional defect-free crystal of the high explosive pentaerythritol tetranitrate were obtained in the framework of classical mechanics using a previously published unreactive potential-energy surface [J. Phys. Chem. B 112, 734 (2008)]. Using these results the vibrational density of states was obtained for the entire vibrational frequency range. Relaxation of selectively excited terahertz-active modes was studied using isochoric-isoergic (NVE) molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Dependence of the relaxation time on the initial modal excitation was considered for five excitation energies between 10 and 500 kT and shown to be relatively weak. The terahertz absorption spectrum was constructed directly using linewidths obtained from the relaxation times of the excited modes for the case of 10 kT excitation. The spectrum shows reasonably good agreement with experimental results. Dynamics of redistribution of the excited mode energy among the other normal modes was also studied. The results indicate that, for the four terahertz-active initially excited modes considered, there is a small subset of zero wave vector (k = 0) modes that preferentially absorb the energy on a few-picosecond time scale. The majority of the excitation energy, however, is transferred nonspecifically to the bath modes of the system.     

Analysis of wave packet motion in frequency and time domain: oxazine 1

Wave packet motion in the laser dye oxazine 1 in methanol is investigated by spectrally resolved transient absorption spectroscopy. The spectral range of 600-690 nm was accessible by amplified broadband probe pulses covering the overlap region of ground-state bleach and stimulated emission signal. The influence of vibrational wave packets on the optical signal is analyzed in the frequency domain and the time domain. For the analysis in the frequency domain an algorithm is presented that accounts for interference effects of neighbored vibrational modes. By this method amplitude, phase and decay time of vibrational modes are retrieved as a function of probe wavelength and distortions due to neighbored modes are reduced. The analysis of the data in the time domain yields complementary information on the intensity, central wavelength, and spectral width of the optical bleach spectrum due to wave packet motion.     

Electronic coherences and vibrational wave-packets in single molecules studied with femtosecond phase-controlled spectroscopy

Employing femtosecond pulse-shaping techniques we investigate ultrafast, coherent and incoherent dynamics in single molecules at room temperature. In first experiments single molecules are excited into their purely electronic 0-0 transition by phase-locked double-pulse sequences with pulse durations of 75 fs and 20 nm spectral band width. Their femtosecond kinetics can then be understood in terms of a 2-level system and modelled with the optical Bloch equations. We find that we observe the coherence decay in single molecules, and the purely electronic dephasing times can be retrieved directly in the time domain. In addition, the Rabi-frequencies and thus the transition dipole moments of single molecules are determined from these data. Upon excitation of single molecules into a vibrational level of the electronically excited state also incoherent intra-molecular vibrational relaxation is recorded. Increasing the spectral band width of the excitation pulses to up to 120 nm (resulting in a transform-limited pulse width of 15 fs) coherent superpositions of excited state vibrational modes, i.e. vibrational wave packets, are excited. The wave-packet oscillations in the excited state potential energy surface are followed in time by a phase-controlled pump-probe scheme, which permits to record wave packet interference, and to determine the energies of vibrational modes and their coupling strengths to the electronic transition.     

Initial excited-state structural dynamics of 2'-deoxyguanosine determined via UV resonance Raman spectroscopy

The resonance Raman spectra of 2'-deoxyguanosine, a DNA nucleoside, were measured in aqueous solution at wavelengths throughout its 260 nm absorption band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism with two electronic states yielded the initial excited-state structural dynamics in both states. The vibrational modes containing the N(7)═C(8) stretching and C(8)-H bending internal coordinates were found to exhibit significant initial structural dynamics upon photoexcitation to either state and are coincident with the photochemical reaction coordinate involving the formation of the 2'-deoxyguanosine cation radical.     

Following photoinduced dynamics in bacteriorhodopsin with 7-fs impulsive vibrational spectroscopy

Sub-10-fs laser pulses are used to impulsively photoexcite bacteriorhodopsin (BR) suspensions and probe the evolution of the resulting vibrational wave packets. Fourier analysis of the spectral modulations induced by transform-limited as well as linearly chirped excitation pulses allows the delineation of excited- and ground-state contributions to the data. On the basis of amplitude and phase variations of the modulations as a function of the dispersed probe wavelength, periodic modulations in absorption above 540 nm are assigned to ground-state vibrational coherences induced by resonance impulsive Raman spectral activity (RISRS). Probing at wavelengths below 540 nm-the red edge of the intense excited-state absorption band-uncovers new vibrational features which are accordingly assigned to wave packet motions along bound coordinates on the short-lived reactive electronic surface. They consist of high- and low-frequency shoulders adjacent to the strong C=C stretching and methyl rock modes, respectively, which have ground-state frequencies of 1008 and 1530 cm-1. Brief activity centered at approximately 900 cm-1, which is characteristic of ground-state HOOP modes, and strong modulations in the torsional frequency range appear as well. Possible assignments of the bands and their implication to photoinduced reaction dynamics in BR are discussed. Reasons for the absence of similar signatures in the pump-probe spectral modulations at longer probing wavelengths are considered as well.     

One-color two-photon mass-analyzed threshold ionization spectroscopy of ethyl bromide through a dissociative intermediate state

Mass-analyzed threshold ionization (MATI) spectra of ethyl bromide were obtained using one-color two-photon ionization through a dissociative intermediate state. Accurate values for the adiabatic ionization energy have been obtained, 83099+/-5 and 85454+/-5 cm(-1) for the X1 2E and X2 2E states of the ethyl bromide cation, respectively, giving a splitting of 2355+/-10 cm(-1). Compared with conventional photoelectron data, the two-photon MATI spectrum exhibited a more extensive vibrational structure with a higher resolution, mainly containing the modes involving the dissociation coordinate. The observed modes were analyzed and discussed in terms of wave packet evolving on the potential-energy surface of the dissociative state.     

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.     

Effective chromophore potential, dissipative trajectories, and vibrational energy relaxation: Br2 in Ar matrix

The intramolecular wave packet dynamics on the electronic B (3pi0) potential of Br2 in solid argon is induced and interrogated by femtosecond pump-probe spectroscopy. An effective potential of the chromophore in the solid is derived from the wave packet period for different excitation photon energies. Deep in the potential well, it is consistent with vibrational energies from wavelength-resolved spectra. It extends to higher energies, where the vibrational bands merge to a continuum, and even beyond the dissociation limit, thus quantifying the cage effect of the argon matrix. This advantage of pump-probe spectroscopy is related to a reduced contribution of homogeneous and inhomogeneous line broadenings. The vibrational energy relaxation rates are determined by a variation of the probe window spatial position via the probe quantum energy. A very large energy loss in the first excursion of the wave packet is observed near the dissociation limit. This strong interaction with the argon matrix is directly displayed in an experimental trajectory.     

A six-dimensional wave packet study of the vibrational overtone induced decomposition of hydrogen peroxide

A converged quantum wave packet study is presented for the unimolecular reaction HOOH → OH+OH induced by the fifth OH-overtone excitation employing an ab initio potential energy surface. All six internal vibrational degrees of freedom are explicitly represented in this simulation for total angular momentum zero. It is found that the decay of the survival probability and of the autocorrelation function is non-exponential and that the long time dynamics is likely due to the superposition of a number of resonance states. The simulated overtone spectrum and rotational product distribution is in good agreement with experimental measurements. It is concluded that: (1) the reaction dynamics is non-statistical on a 30 ps timescale, (2) the observed line width is roughly a factor of 1.7 larger than implied by the reactive lifetimes suggesting that a significant portion of the linewidth is due to intramolecular vibrational energy relaxation, and (3) the quantum reaction rate is suppressed by about a factor of two relative to its classical counterpart.     

Proton relay reaction in green fluorescent protein (GFP): Polarization-resolved ultrafast vibrational spectroscopy of isotopically edited GFP

The complex transient vibrational spectra of wild type (wt) GFP have been assigned through polarization anisotropy measurements on isotopically edited proteins. Protein chromophore interactions modify considerably the vibrational structure, compared to the model chromophore in solution. An excited-state vibrational mode yields information on excited-state electronic structure. The proton relay pathway is characterized in more detail, and the protonation of the remote E222 residue is shown to occur in a concerted step. Modifications to protein vibrational modes are shown to occur following electronic excitation, and the potential for these to act as a trigger to the proton relay reaction is discussed.     

Experimental investigation of the Jahn-Teller effect in the ground and excited electronic states of the tropyl radical. Part II. Vibrational analysis of the A 2E"3-X 2E"2 electronic transition

Laser-induced fluorescence (LIF) and laser-excited dispersed fluorescence (LEDF) spectra of the cycloheptatrienyl (tropyl) radical C7H7 have been observed under supersonic jet-cooling conditions. Assignment of the LIF excitation spectrum yields detailed information about the A-state vibronic structure. The LEDF emission was collected by pumping different vibronic bands of the A 2E"3<--X 2E"2 electronic spectrum. Analysis of the LEDF spectra yields valuable information about the vibronic levels of the X 2E"2 state. The X- and A-state vibronic structures characterize the Jahn-Teller distortion of the respective potential energy surfaces. A thorough analysis reveals observable Jahn-Teller activity in three of the four e'3 modes for the X 2E"2 state and two of the three e'1 modes for the A 2E"3 state and provides values for their deperturbed vibrational frequencies as well as linear Jahn-Teller coupling constants. The molecular parameters characterizing the Jahn-Teller interaction in the X and A states of C7H7 are compared to theoretical results and to those previously obtained for C5H5 and C6H6+.     

The ultraviolet spectrum of OCS from first principles: Electronic transitions, vibrational structure and temperature dependence

Global three dimensional potential energy surfaces and transition dipole moment functions are calculated for the lowest singlet and triplet states of carbonyl sulfide at the multireference configuration interaction level of theory. The first ultraviolet absorption band is then studied by means of quantum mechanical wave packet propagation. Excitation of the repulsive 2?(1)A(') state gives the main contribution to the cross section. Excitation of the repulsive 1?(1)A(") state is about a factor of 20 weaker at the absorption peak (E(ph) ≈ 45?000?cm(-1)) but becomes comparable to the 2?(1)A(') state absorption with decreasing energy (35?000?cm(-1)) and eventually exceeds it. Direct excitation of the repulsive triplet states is negligible except at photon energies E(ph) < 38?000?cm(-1). The main structure observed in the cross section is caused by excitation of the bound 2?(3)A(") state, which is nearly degenerate with the 2?(1)A(') state in the Franck-Condon region. The structure observed in the low energy tail of the spectrum is caused by excitation of quasi-bound bending vibrational states of the 2?(1)A(') and 1?(1)A(") electronic states. The absorption cross sections agree well with experimental data and the temperature dependence of the cross section is well reproduced.