H-bond spectroscopy as the level crossing problem: Evolution of nonstationary probe spectra

The previously developed level crossing model has been employed to study the relaxation of transmission spectra of hydrogen-bonded species. We investigated the spectral transformation after the saturation of intramolecular vibrational transition by a strong light pulse as well as the signal accumulation in the course of light action. From the shape of the transmission spectrum, composed from the negative ("superabsorption") and positive ("bleaching") components, the first moment of the latter was calculated whose shift in time provides the information about the relaxation of the H-bond to the equilibrium length. The limiting cases of long and short exciting pulse as well as fast and slow vibrational relaxation were investigated. When the latter is slow the kinetics of the first moment relaxation is similar to that obtained experimentally, especially after short pumping. On the contrary, the kinetics of signal accumulation and dissipation is better to study at long pumping when it is closer to stationary response, at any time. From this response, the signal dependence on light power can be obtained, which is the main characteristic of the saturation phenomenon. (C) 2000 American Institute of Physics. [S0021-9606(00)50110-X].