Structural information from two-dimensional fifth-order Raman spectroscopy

Two-dimensional (2D) fifth-order Raman spectroscopy is a coherent spectroscopy that can be used as a structural tool, in a manner analogous to 2D nuclear magnetic resonance (NMR) but with much faster time scale. By including the effect of dipole-induced dipole interactions in the molecular polarizability, it is shown that 2D Raman experiments can be used to extract distances between coupled dipoles, and thus elucidate structural information on a molecular level. The amplitude of cross peaks in the 2D Raman spectrum arising from dipole-induced dipole interactions is related to the distance between the two dipoles (r) and the relative orientation of the dipoles. In an isotropic sample with randomly distributed dipole orientations, such as a liquid, the cross peak amplitude scales as r(-6). In an anisotropic sample such as a solid, where the orientational averaging effects do not nullify the leading order contribution, the amplitude scales as r(-3). These scaling relationships have analogy to the dipole coupling relationships that are observed in solid state and liquid 2D NMR measurements. (C) 1999 American Institute of Physics. [S0021-9606(99)02425-3].