Two-dimensional spectroscopy and harmonically coupled anharmonic oscillators

Experimentally it is established that the fourth-order anharmonicity plays significant roles in many molecules. In high resolution spectroscopy, it is known that a picture of local (anharmonic) modes with harmonic couplings b etween modes gives rise to a quartic Darling-Dennison coupling between norm al modes. It has been shown that this order of anharmonicity can be selecti vely studied via seventh-order off-resonant spectroscopy, which probes the three-time response function (J. Chem. Phys. 106 (1997) 1687). In this pape r, we further explore the effect of the fourth-order anharmonicity in the s eventh-order signal, or in the three-time response function (which can be c omplementary investigated via third-order infrared spectroscopy), when the anharmonicity causes mode mixing; we obtain a fairly compact analytical res ult and numerically present the signal from CH stretch vibrations in methyl ene chloride as two-dimensional (2D) contour maps. It is found that purely kinetic coupling between anharmonic local oscillators can give rise to cros s-peaks in the 2D spectra. (C) 2001 Elsevier Science B.V. All rights reserv ed.