UV Resonance Raman Spectroscopy

Normal, or non-resonant Raman scattering utilizes excitation wavelengths far from any absorption bands. It is an inelastic scattering process, in which the incident photon gives/gains some energy to/from the target molecule. If the resulting photon frequency is lower then we call it a Stokes shifted photon. If the photon frequency is higher then we call it an anti-Stokes shifted photon. The energy gained/lost by the molecule, usually as vibrational excitation, is equal to that lost/gained by the photon. The absence of dissipative processes during the "instantaneous" scattering typically leads to sharp spectral features, even for complex molecules.

Raman scattering may be viewed as an excitation from the ground electronic state to a continuum virtual level followed by a return to a different level of the ground state as illustrated in Fig. 1a) below. This simple picture must be viewed with caution, as the scattering is a simultaneous, not sequential process. The traditional method for determining the scattering intensities to various vibrational levels uses the approach based on the Kramers-Heisenberg-Dirac (KHD) sum-over-states and a vibronic coupling picture, which is a time independent formulation.

When the laser frequency is tuned near or on an absorption band, resonance Raman scattering results (see Fig. 1b)). In this case, the scattering intensity increases, sometimes by orders of magnitude, and there is selective enhancement of some vibrational modes. Also, overtone progressions may appear (higher ground state vibrational modes). During the scattering process, the nuclei experience forces associated with the excited state potential energy surface (PES) and move from their equilibrium positions, giving rise to the observed resonant Raman scattering. Overtones are particularly prominent when the motion on the excited PES corresponds to a particular vibrational mode of the ground state. This includes the beginning of bond-breaking motion, which leads to photo-dissociation. The intensities of features associated with different vibrational modes provide information about the excited PES.

Overtones provide a clear signal of wave packet propagation on the excited PES. Consider that we start with an ensemble of identically, or nearly so, prepared states.

Observation of long Raman scattering overtone progressions in ozone and methyl iodide led to application of the wave packet propagation theory to Raman spectroscopy of molecules undergoing direct photodissociation. When methyl iodide is excited by 250 nm radiation, the molecule undergoes a transition from the bound ground state surface to an excited state surface, which is repulsive for the C-I bond. The C and I nuclei immediately begin to separate on the time scale of a few femtoseconds. As the bond stretches, there is a finite probability that the incident photon will become inelastically scattered and the molecule returned to the ground state. Because the bond is stretched, some of the nuclei are displaced relative to their ground state equilibrium positions, and the molecule is left in a vibrationally excited level. The energy of the resonance Raman scattered photon is equal to the energy of the incident photon minus the vibrational energy deposited in the molecule. The more the bond is stretched, the greater the frequency shift of the scattered photon. The resulting series of sharp Raman emission lines corresponding to increasing quanta of energy deposited in a particular vibrational mode is known as an overtone progression. The motion of the nuclei is not observed in real time, but rather as scattered photon energy loss. Analysis of the frequency-dependant data, however, provides the evolution of the breaking bond on a femtosecond time scale. Rapid motion on the excited PES leads to highly enhanced overtone progressions. A caveat is associated with this intuitive picture: Raman scattering is a coherent process, so all three steps, excitation, propagation, and emission occurs simultaneously. The experiment does not have the means to time resolve the emission; the total time integrated emission into each level is recorded. Although the time-dependant model presents a rather different physical picture than the time independent sum-over-states KHD theory, they have been shown to be equivalent.