Semi-classical Theory of Transient Resonance Raman Scattering--Excitation,Relaxation and Observation

正美 楠

M. G. Zakaraya, Gia G. Maisuradze

T. Takagahara

Resonance Raman scattering and luminescence under an intense excitation field are studied for both stationary and transient excitation regimes. By means of the Green function theory of resonance Raman scattering and luminescence we explain the characteristic features of the recent experiments on two-photon resonance Raman scattering via excitonic molecule in CuCl where the resonance dip and the asymmetry of the Raman yield around the resonance were observed. We ascribe the origin of the resonance dip of the Raman yield to the resonance enhancement of collisional damping of excitonic molecule and exciton states. The asymmetry of the Raman yield can be naturally understood in terms of the frequency and wave-vector dependent relaxation constants. Furthermore an approach is proposed which incorporates the coherence and correlation among excitonic molecules and excitons in their non-equilibrium state and clarifies their effects on the optical spectra.

Masami Kusunoki

The quantum mechanical description of the transient resonance Raman scattering including both the excitation and observation processes is presented. To obtain the general formula for the actually observable transient emission spectrum, the following way of observation is proposed; the use of a spectroscope for wave transformation and a photon-counter behind it for particle transformation. On its basis, the transient emission spectrum and its average over the duration time of photon-counting are explicitly defined especially for systems small compared with the spacial extent of an incident pulse. It will be shown that the transient emission spectrum conforms to the minimal case of the Heisenberg uncertainty relation through the restriction for the emission time coming from the Fourier transform of the transfer function of the spectroscope. The theory is applied to a strongly coupled localized electron-phonon system. It will be shown that what aspects of the correlation between absorption and emission are disclosed depends strongly on both the ways of excitation and observation.

T. Takagahara

The effects of the finite correlation time of the external modulation on the secondary-emission spectra are clarified emphasizing the deviation from the motional narrowing limit or the impact approximation in the case of large off-resonance excitation. The dependences of the excitation spectrum of the secondary emission on the modulation parameters and on the inhomogeneous broadening are studied systematically. It is found that the excitation spectrum of the luminescence shows a faster decrease than that of the Raman scattering as the off-resonance frequency increases. The present theory is relevant for explaining the recent experiment by Watanabe, Kinoshita, and Kushida [Chem. Phys. Lett. 126, 197 (1986)] on the \ensuremath{\beta}-carotene in solutions. A general theory of the time-resolved spectra of the secondary emission is formulated to incorporate the effect of the finite correlation time. The luminescence component shows a slow temporal decay with the longitudinal relaxation rate. It is found for the first time that the difference is unexpectedly large between the transient behavior of the luminescence intensity and that of the intermediate-state population for the slow-modulation regime and that the difference indicates the effect of finite correlation time of the frequency modulation. On the other hand, the transient behavior of the Raman scattering contains a fast component that follows the envelope of the excitation-pulse intensity and the same slow component as the luminescence. The ratio of the slow component to the total intensity depends sensitively on the modulation parameters and the off-resonance frequency. The modulation parameters of the system in question can be estimated from these dependences in combination with the spectral characteristics of the emission under stationary excitation.

P. P. Shorygin