Real-Time Observation of Fermi Resonances in the S State of Phenol.

Fermi resonances in the first electronically excited (S) state of phenol have been observed in real time. Quantum beats associated with coherent superposition of Fermi resonant eigenstates are manifested as temporal oscillations of the ionization cross sections of which the amplitudes are strongly dependent on the total ionization energy. This indicates that coherently excited eigenstates are effectively decomposed into their zeroth-order states, providing the unique opportunity for the investigation of nonstationary state dynamics in real-time. Energy gaps (Δν̃) of eigenstates within the laser coherence width have been most precisely determined up to date, giving Δν̃ ∼ 3.302 ± 0.001 or 1.655 ± 0.001 cm for the 1/410b or 12/8a Fermi doublets, respectively. Dephasing rate suddenly increases as the S internal energy becomes above ∼1500 cm, revealing the important role of energy randomization dynamics during the H atom tunneling process of phenol in S.