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

The previously developed level crossing model has been employed to study th e relaxation of transmission spectra of hydrogen-bonded species. We investi gated the spectral transformation after the saturation of intramolecular vi brational transition by a strong light pulse as well as the signal accumula tion in the course of light action. From the shape of the transmission spec trum, composed from the negative ("superabsorption") and positive ("bleachi ng") 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 la tter 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 l ong pumping when it is closer to stationary response, at any time. From thi s response, the signal dependence on light power can be obtained, which is the main characteristic of the saturation phenomenon. (C) 2000 American Ins titute of Physics. [S0021-9606(00)50110-X].