Raman dispersion spectroscopy (RADIS): 1—Phenomenology

RADIS is a novel spectroscopic technique for the investigation of vibronic states. It differs from the common alternative absorption spectroscopy in two significant ways: (a) there is a distinct RADIS spectrum for every band in the Raman spectrum versus just one absorption spectrum; (b) the observables in RADIS are determined by a tensor quantity, and so polarization measurements are useful and significant also for isotropic systems like molecules in solution. In this paper the phenomenology of RADIS is discussed, with special emphasis on molecules in solution. There are three different RADIS spectra for isotropic systems, namely (i) an intensity spectrum, (ii) a linear polarization spectrum and (iii) a circular polarization spectrum. From these one may obtain three different intensity spectra, one for each tensor invariant. A parametrization of these spectra is given. It is shown that the spectra have the form of a simple superposition of Lorentzians, with resonance frequencies determined by the energies of the adiabatic molecular states. Inhomogeneous broadening is treated by means of simple spectral convolution. It is shown that when the inhomogeneous lineshape is Lorentzian, the RADIS intensity retains its Lorentzian form, while a Gaussian lineshape leads to RADIS spectra described by the complex error function.