Intramolecular dynamics in the photofragmentation of initially vibrationally excited CH ₂Cl ₂

The intramolecular dynamics of dichloromethane was studied by applying vibrationally mediated photodissociation and photoacoustic spectroscopy. The parent species were initially excited to the second, third, and fourth C-H stretch overtone regions and subsequently photodissociated by ∼235 nm photons that also tagged the ground, Cl( ²P _3/2) [Cl], and spin-orbit-excited, Cl( ²P _1/2) [Cl*], state photofragments via resonantly enhanced multiphoton ionization. The Cl and Cl* action spectra revealed enhancement of their yield as a result of rovibrational excitation with a Cl*/Cl ratio of about 1/2. The action and photoacoustic spectra manifested a multiple-peak structure, which in terms of normal- and local-mode models is shown to be related to overtones of C-H stretches or combination bands of C-H stretches and bends. The measured time-of-flight profiles together with the determined Cl*/Cl branching ratios suggest fast dissociation and involvement of upper states of A′ and A″ symmetry, mixing via curve crossing and releasing both Cl and Cl*. The determined Cl*/Cl ratios in photodissociation of vibrationally excited CH ₂Cl ₂ are higher than those obtained previously in 193 nm photodissociation of the vibrationless ground state, implying higher nonadiabaticity for the former.