VIBRATIONAL ECHO SPECTROSCOPY - SPECTRAL SELECTIVITY FROM VIBRATIONALCOHERENCE

Theory and experimental data are presented which illustrate a new meth od for performing two-dimensional vibrational spectroscopy using ultra fast pulsed infrared lasers, called vibrational echo spectroscopy (VES ). The VES technique can generate a vibrational spectrum with backgrou nd suppression using the nonlinear vibrational echo pulse sequence. Th e vibrational echo pulse sequence is used with the delay between the e xcitation pulses fixed while the excitation wavelength is varied. A de tailed theory of VES is presented which calculates the full third orde r nonlinear polarization including rephasing and nonrephasing diagrams . Finite width laser pulses are used and the calculations are performe d for a model spectrum with two or more peaks. Two mechanisms that can result in background and peak suppression are illustrated. The mechan isms are based on differences in homogeneous dephasing times (T-2) or transition dipole matrix element magnitudes. Although the VES line sha pe differs from the absorption line shape, it is possible to recover t he absorption line shape from the VES. The method is demonstrated expe rimentally on the vibrational mode of CO (center at 1945 cm(-1)) bound to the active site of the protein myoglobin (Mb-CO). The protein and solvent produce a large absorption background while the VES spectrum o f Mb-CO is background free. Calculations are able to reproduce the exp erimental Mb-CO VES line shape. (C) 1998 American Institute of Physics . [S0021-9606(98)01137-4].