Vibrationally mediated photodissociation of jet-cooled CH₃CF₂Cl: A probe of energy flow and bond breaking dynamics

A double resonance scheme was employed to photodissociate jet-cooled CH₃CF₂Cl in a time-of-flight mass spectrometer. First, the molecule was promoted to the second (3v_CH) or third (4v_CH) methyl overtone by direct IR excitation. Subsequently, a UV laser beam at ∼235 nm was used to both dissociate the molecule and tag the Cl ²P_3/2 [Cl] and Cl ²P_1/2 [Cl*] photofragments by (2 + 1) resonantly enhanced multiphoton ionization. The photofragment action spectra revealed multiple peak structures in both overtone regions, attributed to couplings of the C-H stretches and to methyl stretch-deformation Fermi resonances. The cooling of the samples afforded narrowing of the peaks, relative to room temperature photoacoustic spectra, due to the reduced rotational and vibrational congestion, and enabled observation of a new splitting in the high frequency peak of the second overtone. This splitting apparently resulted from a local resonance of the mixed stretch-deformation state with a close lying dark state. The time scales for vibrational energy redistribution in the local resonance and between the mixed stretch-deformations were evaluated. These time scales and the measured Cl*/Cl branching ratios were compared to those of other hydrohalocarbon compounds. The Cl*/Cl ratio was also compared to that of the nearly isoenergetic vibrationless ground state 193 nm photodissociation and found to be different, demonstrating the effect of vibrational pre-excitation on the photodissociation dynamics.