Structural information from two-dimensional fifth-order Raman spectroscopy

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