Ground-state Vibrational Coherence Research Articles

Does electronic coherence enhance anticorrelated pigment vibrations under realistic conditions?

The light-harvesting efficiency of a photoactive molecular complex is largely determined by the properties of its electronic quantum states. Those, in turn, are influenced by molecular vibrational states of the nuclear degrees of freedom. Here, we reexamine two recently formulated concepts that a coherent vibronic coupling between molecular states would either extend the electronic coherence lifetime or enhance the amplitude of the anticorrelated vibrational mode at longer times. For this, we study a vibronically coupled dimer and calculate the nonlinear two-dimensional (2D) electronic spectra that directly reveal electronic coherence. The time scale of electronic coherence is initially extracted by measuring the antidiagonal bandwidth of the central peak in the 2D spectrum at zero waiting time. Based on the residual analysis, we identify small-amplitude long-lived oscillations in the cross-peaks, which, however, are solely due to groundstate vibrational coherence, regardless of having resonant or off-resonant conditions. Our studies neither show an enhancement of the electronic quantum coherence nor an enhancement of the anticorrelated vibrational mode by the vibronic coupling under ambient conditions.

Observing ground state vibrational coherence and excited state relaxation dynamics of a cyanine dye in pure solvents.

Using a degenerate pump probe technique at 800 nm, Ground State Vibrational Coherence (GSVC) of a cyanine dye (IR780) is explored in various solvents. Five representative solvents were studied, of which GSVC is present in three: methanol (MeOH), ethanol (EtOH) and dichloromethane (DCM). The maximum number of damped oscillations was observed in DCM solvent and the oscillations persisted for 1.2 ps. However, GSVC is absent in two solvents: acetonitrile (ACN) and dimethyl sulfoxide (DMSO). For nondegenerate pump probe studies with a 400 nm pump and a supercontinuum probe, GSVC was absent for IR780 in all of the five solvents studied. Instead these studies showed excited state absorption corresponding to S1 → Sn absorption bands in the 450 to 650 nm range. Decay profiles for these correspond to the higher electronic states (Sn, n ≥ 2) in contrast to the degenerate case involving S1. Complete dynamics of IR780 with 30 fs pulses are reported by invoking three time-constants to fit the kinetic traces that can, in turn, be assigned as: vibrational cooling, intermediate state time, and ground state recovery (GSR). All these dynamics exhibit noticeable solvent dependence. A plateau region is also observed in the early part of the GSR dynamics, strongly suggesting the existence of an intermediate state between the ground (S0) and excited singlet (S1) states.

Origin of long-lived quantum coherence and excitation dynamics in pigment-protein complexes.

We explore the mechanism for the long-lived quantum coherence by considering the discrete phonon modes: these vibrational modes effectively weaken the exciton-environment interaction, due to the new composite (polaron) formed by excitons and vibrons. This subsequently demonstrates the role of vibrational coherence which greatly contributes to long-lived feature of the excitonic coherence that has been observed in femtosecond experiments. The estimation of the timescale of coherence elongated by vibrational modes is given in an analytical manner. To test the validity of our theory, we study the pigment-protein complex in detail by exploring the energy transfer and coherence dynamics. The ground-state vibrational coherence generated by incoherent radiations is shown to be long-survived and is demonstrated to be significant in promoting the excitation energy transfer. This is attributed to the nonequilibriumness of the system caused by the detailed-balance-breaking, which funnels the downhill migration of excitons.

Two-dimensional photon-echo spectroscopy at a conical intersection: A two-mode pyrazine model with dissipation

The multi-dimensional electronic spectroscopy of ultrafast nuclear dynamics at conical intersections (CI) is an emerging field of investigation, which profits also from the recent extension of the techniques to the UV domain. We present a detailed computational study of oscillatory signatures in two-dimensional (2D) photon-echo spectroscopy (also known as 2D electronic spectroscopy, 2DES) for the two-mode pyrazine model with dissipation. Conventional 2D signals as well as the resulting beating maps are considered. Although of a reduced character, the model captures quite well all the main signatures of the excited-state dynamics of the molecule. Due to the ultrafast relaxation via the CI and no excited-state absorption from the low-lying dark state, the oscillatory components of the signal are found to be predominantly determined by the ground state bleach contribution. They reflect, therefore, the ground-state vibrational coherence induced in the Raman active mode. Beating maps provide a way to experimentally differentiate between ground state bleach and stimulated emission oscillatory components. The ultrafast decay of the latter constitutes a clear indirect signature of the CI. In the considered model, because of the sign properties of the involved transition dipole moments, the dominance of the ground-state coherence leads to anti-correlated oscillations of cross peaks located at symmetric positions with respect to the main diagonal.

Isolated Ground-State Vibrational Coherence Measured by Fifth-Order Single-Shot Two-Dimensional Electronic Spectroscopy.

Vibrations play a critical role in many photochemical and photophysical processes in which excitations reside on the electronically excited state. However, difficulty in assigning signals from spectroscopic measurements uniquely to a specific electronic state, ground or otherwise, has exposed limitations to their physical interpretation. Here, we demonstrate the selective excitation of vibrational coherences on the ground electronic state through impulsive Raman scattering, whose weak fifth-order signal is resonantly enhanced by coupling to strong electronic transitions. The six-wave mixing signals measured using this technique are free of lower-order cascades and represent correlations between zero-quantum vibrational coherences in the ground state and single-quantum coherences between the ground and electronic states. We believe that this technique has the potential to shed much-needed insight onto some of the mysteries regarding the origin of long-lived coherences observed in photosynthetic and other coupled chromophore systems.

Enhancement of Vibronic and Ground-State Vibrational Coherences in 2D Spectra of Photosynthetic Complexes

A vibronic-exciton model is applied to investigate the recently proposed mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom. We study a dimer system to elucidate the role of resonance coupling, site energies, vibrational frequency and energy disorder in the enhancement of vibronic-exciton and ground-state vibrational coherences, and to identify regimes where this enhancement is significant. For a heterodimer representing two coupled bachteriochloropylls of the FMO complex, long-lived vibronic coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic energy difference. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that, due to the dephasing of the former, both type of coherences have a similar magnitude at longer population time.

Influence of the Chemical Design on the Coherent Photoisomerization of Biomimetic Molecular Switches

Ultrafast transient absorption spectroscopy reveals the effect of chemical substitutions on the photoreaction kinetics of biomimetic photoswitches displaying coherent dynamics. Ground state vibrational coherences are no longer observed when the excited state lifetime exceeds 300fs.

Dissipative Laser-Driven Hydrogen-Bond Dynamics in Deuterated o-Phthalic Acid Monomethylester

The intramolecular nuclear wave packet dynamics of carboxy-deuterated o-phthalic acid monomethylester following ultrafast excitation of the OD stretching vibration in the hydrogen bond is investigated. On the basis of quantum chemical calculations of the normal modes as well as the anharmonicities, a three-dimensional model is proposed comprising the OD stretching and bending as well as a low-frequency mode which modulates the hydrogen bond geometry. The interaction with the remaining intramolecular and solvent degrees of freedom is incorporated within a quantum master equation formulation. The linear absorption spectrum is shown to be dominated by a Fermi resonance, with the actual line widths depending on pure dephasing as well as on a fourth-order relaxation process. The latter involves two intramolecular vibrational quanta plus a solvent phonon. It proves to be vital for the explanation of the very fast relaxation of the fundamental OD stretching excitation. Further, it is shown that in the OD ground state vibrational coherences with respect to the low-frequency mode are completely dephased only after about 2 ps. These results are discussed in the context of recent infrared pump−probe and four-wave-mixing experiments.

Theory of dynamic absorption spectroscopy of nonstationary states. 4. Application to 12-fs resonant impulsive Raman spectroscopy of bacteriorhodopsin

A time-dependent theory for femtosecond dynamic absorption spectroscopy is used to describe the creation and observation of molecular ground-state vibrational coherence through the resonance impulsive stimulated Raman mechanism. Model calculations show that the oscillatory absorption signal that arises from this ground-state coherence is maximized for a limited range of pulse lengths and that there is a complex relationship between the probe wavelength and the strength of the spectral oscillations. The generalized time-dependent linear susceptibility of the nonstationary system created by the impulsive pump pulse is defined and used to discuss the strong dependence of the measured signals on the properties of the probe pulse. Finally, calculations are presented to analyze the high-frequency oscillations ({approximately}20-fs period) recently observed in the transient absorption spectra of light-adapted bacteriorhodopsin (BR{sub 568}) following excitation with a 12-fs optical pulse. At the probe wavelengths used in this experiment, the contribution of stimulated emission is negligible at long times because of the extremely rapid excited-state isomerization; as a result, the spectral oscillations observed after this time are due to the impulsive excitation of coherent vibrations in the ground state. The transient response observed for BR{sub 568} is calculated using a 29-mode harmonic potential surface derived from amore » prior resonance Raman intensity analysis. Both the oscillatory signals and their dependence on the probe wavelength are satisfactorily reproduced. 68 refs., 11 figs.« less

Application of chirped ultrashort pulses for generating large-amplitude ground-state vibrational coherence: a computer simulation

Computer simulations are presented to show that frequency-chirped ultrashort pulses, when properly employed, may be more effective than their compressed analogs for generating large-amplitude vibrational coherence in molecular systems. A one-dimensional model, which was used previously to simulate impulsive optical excitation of CsI, is used [ Chem. Phys. Lett.158, 238 ( 1988)]. A negatively chirped pulse (for which the light is red-shifted with time) is devised to excite the population initially onto the excited surface. As this population propagates in the repulsive upper state, the later red-shifted portions of the pulse follow the change in the resonance conditions and effectively dump the population back down into excited vibrational states of the ground surface. These results are compared with the dynamics simulated with the compressed and positively chirped analogs, and the experimental realization of this scheme is discussed.

Large amplitude ground state vibrational coherence induced by impulsive absorption in CsI. A computer simulation

Computer simulations are presented to show that impulsive optical excitation of CsI, in various vibrational levels, can give rise to large-amplitude vibrational coherence in the ground electronic state. It is further demonstrated that using transient absorption spectroscopy, such motion may be directly detected, and that this scheme should be experimentally realizable using currently available laser sources. The quantum mechanical propagation method used in this study, proves to be well suited for the simulation of optical excitation of small molecules, under conditions where the assumptions of time-dependent perturbation theory are violated.