Abstract
Resonance Raman spectra provide a valuable probe into molecular excited-state structures and properties. Moreover, resonance enhancement is of importance for the chemical contribution to surface-enhanced Raman scattering. In this work, we introduce a simplified sum-over-states scheme for computing Raman spectra and Raman excitation profiles. The proposed sum-over-states approach uses derivatives of electronic excitation energies and transition dipole moments, which can be efficiently computed from time-dependent density functional theory. We analyze and interpret the resonance Raman spectra and Raman excitation profiles of nucleic acid bases using the present approach. Contributions of individual excited states under strictly resonant and nonresonant conditions are investigated, and smooth interpolation between both limiting cases is obtained.
Highlights
Resonance enhancement of Raman scattering, which occurs whenever the frequency of the incident radiation approaches molecular excitation frequencies, was reported some 20 years after the initial experimental observation of the Raman effect.[1,2] The large degree of enhancement spanning several orders of magnitude is useful for detection of the inherently inefficient spontaneous Raman scattering
We reduce the summation over vibronic states of the KHD dispersion relation to a summation of electronic states, similar to the parametric method of Shorygin and co-workers, and apply the double harmonic approximation, which is commonly used in calculations of vibrational spectra
While truncation of the sum-over-states is a potential source of error not present in the finite-lifetime approach,[30,31] we find that convergence is sufficiently fast even in the non-resonant regime for nucleic acid bases considered here
Summary
Resonance enhancement of Raman scattering, which occurs whenever the frequency of the incident radiation approaches molecular excitation frequencies, was reported some 20 years after the initial experimental observation of the Raman effect.[1,2] The large degree of enhancement spanning several orders of magnitude is useful for detection of the inherently inefficient spontaneous Raman scattering. This approximation requires only excitation energies, transition dipole moments, and their respective geometric derivatives to be computed for the electronic excited states included in the sum-over-states expression. Since the shape of resonance Raman spectra is sensitive to the relative position of the frequency of the incident light in the electronic excitation spectrum, we correct for the systematic error in excitation energies with PBE0.
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