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Physical Review A

Vibronic coherence between the sequential transitions in two-photon processes induced by phase-incoherent light beams

Yoshihiro Nomura, Yuichi Fujimura

Published Apr 1, 1992• 1 citation
Publisher

Abstract

Mechanisms of two-photon transitions of a molecular ensemble induced by stationary-phase-incoherent light beams are theoretically studied. The time evolution of the vibronic coherence created by the phase-incoherent light beams is analyzed by using the Liouville-space Feynman diagram properly taking into account vibronic dephasings. It is shown that effects of coherence between sequential transitions make a dominant contribution to the time profile of the pump-probe two-photon transition induced by the phase-incoherent light beams. The time profiles are calculated within a simple model to demonstrate differences in the time evolution of the vibronic coherence between the phase-incoherent and pulsed-coherent light spectroscopy. A theoretical consideration is also given to nonresonant two-photon fluorescence of coumarin 440 in solution excited by incoherent light beams, which shows subfemtosecond oscillation with the incident photon-field frequencies.

DOI
10.1103/physreva.45.4855

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Mechanisms of two-photon transitions of a molecular ensemble induced by stationary-phase-incoherent light beams are theoretically studied. The time evolution of the vibronic coherence created by the phase-incoherent light beams is analyzed by using the Liouville-space Feynman diagram properly taking into account vibronic dephasings. It is shown that effects of coherence between sequential transitions make a dominant contribution to the time profile of the pump-probe two-photon transition induced by the phase-incoherent light beams. The time profiles are calculated within a simple model to demonstrate differences in the time evolution of the vibronic coherence between the phase-incoherent and pulsed-coherent light spectroscopy. A theoretical consideration is also given to nonresonant two-photon fluorescence of coumarin 440 in solution excited by incoherent light beams, which shows subfemtosecond oscillation with the incident photon-field frequencies.
An expression for the probability of time-resolved three-photon ionization via coherently excited resonant states of a molecular system is derived in the perturbative density matrix formalism with the aid of the Liouville space Feynman diagram. It is shown how the time evolution of the molecular coherence in the first resonant state can be detected as a function of the delay time between the pumping and probing lasers through the probing two-photon ionization process. The three-photon ionization process consists of the simultaneous and sequential processes which are classified by the Liouville space Feynman diagrams. The possibility of appearance of quantum beats in the pump–probe three-photon ionization is discussed by performing model calculations of the probability of ionization. The role of the direct and redistributed processes is discussed. The redistributed process is induced by intramolecular processes in the resonant state. The theoretical treatment is applied to the (1+2) three-photon ionization of pyrazine observed by Knee et al. It is demonstrated that the fast decay component (110 ps) of the pyrazine spectra originates from the intramolecular dephasing of the vibronic coherence created by the pumping pulse laser, and this component appears only when the redistributed process (the ionization process through triplet levels) is absent.
Photoinduced biological processes occur via one photon absorption in natural light, which is weak, CW and incoherent, but are often studied in the laboratory using pulsed coherent light. Here we compare the response of a molecule to these two very different sources within a quantized radiation field picture. The latter is shown to induce coherent time evolution in the molecule, whereas the former does not. As a result, the coherent time dependence observed in the laboratory experiments will not be relevant to the natural biological process. Emphasis is placed on resolving confusions regarding this issue that are shown to arise from aspects of quantum measurement and from a lack of appreciation of the proper description of the absorbed photon.
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Two-dimensional (2D) spectroscopy has been intensively used to study electronic and vibronic coherences in biological systems and semiconductors. This technique studies coherent as well as incoherent signals that arise from the nonlinear interaction of a sequence of laser pulses. In this paper we present a direct evaluation of the 2D signal based on elementary quantum kinetics in order to compare with the common approximate diagrammatic approaches. Here we consider incoherent action signals such as fluorescence or photocurrent as the observable, which is easily accessible in a measurement. These observables are calculated by solving the time evolution of the density matrix in the Lindblad form, which can take into account all possible decoherence processes. The phase modulation technique is used to separate the relevant nonlinear signals from the other possible interaction pathways. The approach can be used to calculate 2D spectra of any quantum system. For our model system we find a good agreement for the quantum beating between the coupled states.
A consistent quantum theory of a two-pulse pump-probe experiment in the femto-and picosecond spectroscopy of diatomic molecules is developed. The theory aims at the use of weak quantum femtosecond light pulses to excite molecular coherence. It is suggested that the experiment be realized using a double-frequency excitation scheme that is feasible for Na 2 molecules. On the basis of the theory developed, the luminescence signals are calculated at different temperatures and different optical densities of the medium for the time range from 100 fs to ~ 100 ps. The results obtained show the presence of a strong dependence of the time and frequency spectra of a luminescence signal on the optical density of the medium at temperatures from 50 to 300 K. The proposed approach makes it possible to use the quantum coherence properties of optically dense media for more detailed study of the vibronic dynamics of diatomic molecules, in particular, detection of weak optical transitions.
The effect of parametric coupling between pump and probe beams in time-resolved optical Kerr effect is theoretically discussed for various experimental configurations. It is shown that with a cicularly polarized pump beam, the polarization changes induced on a nearly collinear probe beam are due to this coherence effect only. In other configurations, this parametric coupling can appear as an additional contribution to the observed signal, especially when pump and probe are derived from the same laser beam. It is pointed out that this coupling can occur not only when the pump and probe beams are at the same wavelength, but also when the difference or the sum of their frequencies can excite a Raman or a two-photon resonance in the medium.