Molecular dynamics simulation study of N-methylacetamide in water.: II.: Two-dimensional infrared pump-probe spectra

A theoretical description of two-dimensional (2D) IR pump-probe spectroscopy of a three-level system is presented by taking into account the system-bath interaction. By using the correlation function of the fluctuating amide I mode frequency of N-methylacetamide in D2O, which was obtained by carrying out both ab initio calculations and MD simulations, the time-resolved 2D pump-probe spectra as a function of pump-probe pulse delay time are calculated and compared with experiment. We found that the vibrational dephasing becomes homogeneous on the 2 ps time scale, which is a bit faster than the experimental result. It is theoretically shown that the degree of slant of 2D contours is linearly proportional to the correlation function of the fluctuating amide I mode frequency. Consequently, it is suggested that the 2D IR pump-probe spectroscopy can provide a direct information on the vibrational frequency fluctuation dynamics and on the magnitude of static inhomogeneity. (C) 2003 American Institute of Physics.