Vibrationally excited states of CH3CFCl2: Intramolecular vibrational redistribution and photodissociation dynamics

Utilizing vibrationally mediated photodissociation of jet-cooled CH3CFCl2 e nabled monitoring of the second (3 nu(CH)) and third (4 nu(CH)) overtones o f the methyl in the ground electronic state. The excited molecules are phot odissociated by similar to 235 or 243.135 nm photons that further tag Cl(P- 2(3/2))[Cl] and Cl(P-2(1/2))[Cl*] isotopes or H photofragments via mass sel ective (2 + 1) resonantly enhanced multiphoton ionization. The vibrational spectra are characterized by a multiple peak structure related to C-H stret ches and to Fermi resonating levels involving the CH3 deformation. The cool ing in the expansion reduces the rotational and vibrational congestion and affords a determination of the splittings and the upper limits for homogene ous broadening of the transitions. The highest-frequency peak of 4 nu(CH) e xhibits an additional splitting, related to coupling of the mixed stretch-d eformation states with other modes of the molecule. The yield of all three photofragments increases as a result of preexcitation, demonstrating that t he energy is not preserved in the excited bond but rather flows to the C-Cl bond. The initial vibrational state preparation not only enhances C-Cl and C-H bond cleavage but also affects the Cl*/Cl branching ratio, as compared to the nearly isoenergetic one-photon 193 nm photolysis of vibrationless g round state CH3CFCl2, implying that it alters the photodissociation dynamic s. (C) 2000 American Institute of Physics. [S0021-9606(00)01424-0].