화학공학소재연구정보센터
Journal of Chemical Physics, Vol.101, No.3, 1865-1877, 1994
Molecular-Spectroscopy with Light-Pulses of Arbitrary Pulse-Shape and Field-Strength - A Nonperturbative Approach
We present a novel approach to molecular spectroscopy with light pulses of arbitrary strength and duration. The key quantity is the frequency-resolved net energy transfer dE/d omega which reveals at which frequencies energy is transferred from the field to the molecule (absorption) or from the molecule to the field (stimulated emission). it is shown that dE/d omega can be expressed as the Fourier transform of the cross-correlation function of the molecular polarization P(t) and the time derivative of the applied field; dE/dt. In this sense, it is formally equivalent to the absorption cross section under weak-field conditions which, as commonly known, can be represented as the Fourier transform of the autocorrelation function S(t). The time-dependent polarization P(t) is determined by exact integration of the time-dependent Schrodinger equation including the light-matter interaction to all orders. It is shown that under weak-field conditions the expression for dE/d omega reduces to the well-known cross section formula in the time-dependent; picture of spectroscopy; multiplied by the spectral intensity of the light pulse. Therefore, we consider the expression for the frequency-resolved energy transfer, which is valid for arbitrary electric fields, as the natural extension of the absorption cross section in the weak-held limit. Furthermore, dE/d omega is shown to be formally equivalent to the change of the spectral intensity, Delta I/(omega), of an optical pulse after transmission through a sample; the latter being derived by solving Maxwell’s equations under well-known approximations. The theory is applied to a simple one-dimensional model with two electronic states and the frequency-resolved energy transfer is investigated as a function of the field strength. For sufficiently strong fields, dE/d omega exhibits transitions between essentially all vibrational levels in the ground and al! states in the excited electronic manifold, The new expression distinguishes between absorption and emission and that is clearly seen in the spectra.