Exploring potential reactivity by optical polarization dependent coherent vibrational spectroscopy

Identifying reaction coordinates is a central challenge across all fields of chemistry. While the advancement of femtosecond spectroscopic techniques has enabled direct identification of reaction coordinates in ultrafast processes, new experimental or analytical approaches are required to access such information in photoinitiated processes of broader interest. In this study, we theoretically demonstrate a spectroscopic method capable of identifying vibrational modes with potential reactivities in non-reactive photochemical and photophysical systems. The method is based on femtosecond transient absorption spectroscopy and leverages the polarization dependence in wavepacket propagation arising from the position-dependent electronic character within the Born-Oppenheimer approximation. The vibrational wavepacket associated with the reactive mode exhibited clear dependencies on both polarization and detection frequency, whereas the non-reactive mode showed only weak polarization dependence. This contrast highlights a fundamental distinction between reactive and non-reactive modes in terms of their polarization-sensitive spectral responses. Leveraging this property, it may be possible to extract reaction coordinate information even for general photochemical and photophysical processes.