Resonance CARS spectroscopy of the ground and low‐lying excited electronic states of nickel octaethylporphin solution

Nanosecond time domain transient resonant CARS (RCARS) techniques have been used to investigate photophysical transformations of Ni‐octaethylporphin (Ni‐OEP) in tetrahydrofuran (THF) solution. For the first time the RCARS spectra have been obtained from the short‐lived (280 ps) low‐lying excited electronic state 3 B 1 g , populated under the effect of intense resonant laser pulses. The amplitude‐polarization sensitive techniques of nondegenerate RCARS have been used to perform symmetry species assignments for the Raman‐active vibrational modes in the 1 A 1 g and 3 B 1 g electronic states. Only non‐totally symmetric in‐plane vibrations of the porphyrin skeleton with the b 1 g , b 2 g and a 2 g symmetry species in the D 4 h point symmetry group show up in the coherent scattering spectra under excitation within the visible Q absorption bands. The character of electronic resonance enhancement for the RCARS process has been studied when the excitation laser frequencies match one‐photon resonances with the frequencies of vibronic transitions between the 3 B 1 g and 3 Q ″ excited states, and excitation and probing nondegenerate RCARS profiles have been measured for the Raman modes at 1562 and 1627 cm −1 . It has been found that the Raman modes intensity in the RCARS spectra from the 3 B 1 g state is borrowed from the Herzberg–Teller scattering when resonances with the 3 B 1 g ↔ 3 Q ″ transition take place. The data of the saturation degenerate RCARS experiments combined with the numerical analysis of the stationary rate equations for populations of the low‐lying electronic states made it possible to reveal the influence of the resonant radiation intensity on the RCARS efficiency.