Dynamics Behind the Long-Lived Coherences of I2 in Solid Xe

The absorption spectrum of I2 in solid Xe shows resolved zero-phonon lines and phonon side bands near the origin of the B←X transition (550–625 nm). The long-lived |B⟩⟨X| coherence in this energy range (T 2 = 600 fs on average) emerges as vibrationally unrelaxed fluorescence in resonance Raman (RR) spectra. Upon excitation in the structureless continuum at 532 nm, the oscillatory RR progression exhibits electronic dephasing time of T 2 = 150 fs. Two RR progressions with markedly different vibrational coherence on the X-state are observed. The main progression of sharp overtones (T 2 > 21 ps) is assigned to molecules trapped in double-substitution sites. The minor progression, which shows dephasing times T 2 = 6–0.6 ps for v = 1–8, is assigned to molecules in triple-substitution sites. The line progressions allow a detailed characterization of the solvated B- and X-state potentials. Time-resolved coherent anti-Stokes Raman scattering is used to probe selected vibrational coherences on the X-state. Assignments are obtained through molecular dynamics simulations, which reproduce the relative dephasing rates between the two sites, clarify the role of rotation-translation dynamics, and enable quantum dynamics simulations of the spectra by the potentials of mean force that accurately describe the molecule–surrounding interactions.