Overview of Resonance Raman Spectroscopy
Raman spectroscopy is a light scattering technique which probes the vibrational energy levels of molecules. Incident light from a laser is inelastically scattered by the sample, undergoing a shift to lower frequency when the sample makes a transition from a lower initial state i to a higher final vibrational state f. This is called Stokes scattering, while the less probable anti-Stokes scattering results from downward vibrational transitions and leads to scattered light which is higher in frequency than the incident light. The energy difference between the initial and final states of the molecule is related to the difference in the incident n0 and scattered ns light frequencies by n0 -ns = (Ef – Ei )/h, where h is Planck’s constant. Typically, the molecule starts out in the ground (v = 0) vibrational state within the ground electronic state, and makes a transition to the first (v = 1) excited vibrational state. This is called the fundamental transition, but hot bands, originating in excited vaibrational states, are also possible as are overtones (an excitation of two or more quanta of the same vibrational mode) and combinations (involving different normal modes). The difference between the incident and scattered light frequencies is the vibrational frequency. A typical Raman spectrum reveals peaks in scattered light intensity at frequency shifts Dn = n0 – ns corresponding to vibrational transitions of various normal modes of the sample. As a tool for determining molecular structure, Raman spectroscopy provides information which is complementary to that from infrared spectroscopy, revealing functional groups and molecular symmetry. All the information from an ordinary Raman spectrum pertains to the molecule in its ground electronic state. The intermediate state n is a virtual state of the moelcule, rather than a real excited electronic state. This virtual state exists as a result of the perturbation of the incident photon. Quantum mechanically it is a superposition of all the excited states of the molecule.
Things get more interesting when the exciting light is resonant with an electronic absorption band of the sample. Now the intermediate state is a real rather than virtual state. This is the realm of resonance Raman spectroscopy, in which the intensities of the Raman-active normal modes as a function of excitation wavelength provide insight into the fate of the molecule in its excited electronic state. Most vibrational modes which are strong in the resonance Raman spectrum derive their intensities from a geometry change of the molecule in the excited electronic state, illustrated by a shift in the position of the excited state potential energy curve relative to the ground state as shown in the figure. This displacement in the ground and excited state potential energies is also responsible for the contribution of certain vibrational modes (said to be “Franck-Condon active”) to the electronic absorption and emission spectra. The intensity of a vibrational transition in resonance Raman is larger for larger displacements. Thus resonance Raman can reveal the structure of the molecule in its excited electronic state.
Immediately after the upward transition that results from the incident photon, the wavepacket that represents the initial vibrational wavefunction begins to evolve on the upper potential surface. Dynamics that disturb this wavepacket on a very short (sub-picosecond) time scale control the intensity of the Raman peak. Thus resonance Raman spectroscopy can provide information about the dynamics of the excited electronic state.
As shown in the figure, resonance Raman spectroscopy depends on the observation of the Raman spectrum at various incident laser frequencies. The peak frequencies generally do not depend on incident frequency as they are properties of the ground electronic state, however, the relative intensities vary as the laser is tuned across the electronic absorption band. The Raman excitation profile is a graph of Raman intensity of a particular vibrational transition as a function of incident frequency.
“Resonance Raman Spectroscopy”, pp. 534-556, Chapter in the Handbook of Vibrational Spectroscopy, Vol. 1, J. M. Chalmers and P. R. Griffiths, Eds., John-Wiley and Sons, New York, 2002.