Missing mode effect in resonance Raman excitation profile

Abstract

Vibrational structure in the resonance Raman excitation profile (REP) of adenosinetriphosphate (ATP) in aqueous solution has been observed for both the 1333 and 1482 cm−1 modes, even though the absorption spectrum is structureless. The ∼2100 cm−1 spacing of the vibrational structure in both cases is much larger than the normal mode frequencies of the molecule. It is possible for such a vibrational structure to develop in a quasidiatomic (single mode) case, based on the reflection principle of the product wave functions ‖〈Q‖1〉〈0‖Q〉‖2 appropriate for the fundamental REP, if the (dimensionless) displacement Δ between excited and ground state potentials is sufficiently large (Δ≳1) and the absorptive component of the REP dominates. However, this is not the case for ATP, where we show that the observed vibrational structure is due to an interplay of multiple modes. This ‘‘missing mode effect’’ (MIME) for REP, as in the case of luminescence spectra of large molecules, can be explained using the time-dependent correlation function picture of electronic transitions. The harmonic model is used and it is shown how the MIME frequency in the REP arises. The phenomenon is illustrated with ATP, where the harmonic parameters are derived from its resonance Raman spectrum at an excitation frequency of 260 nm, which are then used to calculate the absorption and REPs to compare with the experimental results.