The Quantum Theory of Transient Resonance Raman Scattering including Light Source and Observation Process

正美 楠

Yu. P. Malakyan

A theory is developed for transient hyper-Raman scattering and four-wave parametric processes in a gaseous medium subjected to an ultrashort pump pulse field. Investigations are made of the intensity and spectrum of the infrared and ultraviolet radiation generated in a medium as a result of these processes, bearing in mind the transient properties of the pump (phase modulation, pulse profile). It is shown that the initial nonlinearity of the pump field and the parametric processes result in new effects not observed in transient stimulated Raman scattering. For a large two-photon detuning, the hyper-Raman forward scattering is suppressed because of destructive interference with stimulated parametric generation of ultraviolet radiation. The gain of the infrared and ultraviolet radiation depends on the pump pulse profile. Under exact two-photon resonance conditions in the absence of any phase modulation of the pump, infrared and ultraviolet pulses are coherently split into various subpulses and a time-dependent Autler–Townes doublet is observed in their spectrum.

Milan Pospíchal, Jan Peřina

Toshiaki Hattori, Akira Terasaki, Takayoshi Kobayashi

A new method to measure picosecond or subpicosecond vibrational-dephasing dynamics with the use of incoherent or broadband light is presented theoretically and experimentally. It is based on a transient coherent Raman process with three beams. In our experimental scheme, two of them are incoherent light from a single source, and a delay time between the two is variable. A third beam has a higher frequency than the incoherent light by a vibrational energy in a molecule of interest, and is coherent in the delay-time range of the measurement. The delay-time dependence of coherent Stokes Raman scattering (CSRS) intensity offers the information about the coherence dynamics of the vibrational transition with a resolution time determined only by the correlation time of the incoherent light. For the theoretical calculation, a three-level model of the molecular system with homogeneous broadening is used, and delay-time dependences of the CSRS signal are calculated for off-resonance and resonance cases. Dephasing dynamics of the 2915-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode in dimethylsulfoxide was observed experimentally by the new method with the use of nanosecond laser pulses, and the result agrees well with that obtained with picosecond pulses.

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.

Milan Pospíchal