Vibrationally Mediated Photodissociation of Jet-Cooled CH₃CF₂Cl:  A Probe of Energy Flow and Bond Breaking Dynamics

Abstract

A double resonance scheme was employed to photodissociate jet-cooled CH 3 CF 2 Cl in a time-of-flight mass spectrometer. First, the molecule was promoted to the second (3ν CH ) or third (4ν 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 2 P 3/2 [Cl] and Cl 2 P 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. † Part of the special issue "Donald Setser Festschrift".

Photodissociation dynamics of vibrationally excited CH2Cl2 molecules

“…Nonetheless, the general appearance of the features and the relative intensities of Cl and Cl à in the different bands of the action spectra seem to be comparable. This agrees with the behavior encountered in the VMP of CHF 2 Cl and CHFCl 2 [23,24], but differs from that observed in VMP of CH 3 Cl [14], where the action spectra of 35 Cl à and 37 Cl à were different. Moreover, the band Fig.…”

“…Since the combined energy used in the VMP of CH 2 Cl 2 , $51 180 cm À1 , is only slightly less than that used in the 193 nm photodissociation, 51 813 cm À1 , it seems unreasonable that the difference in energy led to the variation in the branching ratio. It should be noted that an increase in the branching into Cl à was also observed in the VMP of other chlorine containing molecules, including CH 3 Cl, CHD 2 Cl, CH 3 CF 2 Cl, and CH 3 CFCl 2 [15,16,23,24], but not in that of CHF 2 Cl [26]. Therefore, the alteration of the branching ratio is probably due to the varied non-adiabatic dynamics in VMP, which determines the fragment spin-orbit splitting during the departure from the molecule.…”

“…In this manner, the redistribution time in CH 2 Cl 2 pre-excited with three C-H quanta is estimated to be in the 0.20-0.25 ps range. These times are in between those encountered in molecules with an isolated C-H chromophore, CHFCl 2 , CHF 2 Cl, CHCl 3 , and CHF 3 and in ethane derivatives, CH 3 CF 2 Cl and CH 3 CFCl 2 excited to the second and third overtones [23]. This is since in the former the energy redistribution between C-H stretches and bends mostly takes less than 0.1 ps and in the extreme case less than 0.2 ps, while in the latter it is in the range of 0.25-0.65 ps.…”

“…An increase in the branching ratio into Cl à was also observed by Lambert and Dagdigian [15,16] in vibrationally mediated photodissociation (VMP) [21,22] of CH 3 Cl and CHD 2 Cl pre-excited to the fourth overtone of the C-H stretch, with a larger increase in the former. Furthermore, we encountered alteration of the branching ratio in the VMP of CH 3 CF 2 Cl and CH 3 CFCl 2 , as compared to the almost isoenergetic vibrationless ground state photodissociation [23][24][25]. However, this is not always the case as revealed by VMP of CHF 2 Cl, which did not affect the Cl à /Cl branching ratio probably due to a smaller variation in the photolysis dynamics relative to that of vibrationless ground state molecules [26].…”

“…The experiments were performed with the previously described setup [22][23][24][25][26][27][28]. The photodissociation was carried out in a home-built timeof-flight mass spectrometer (TOFMS) providing sensitive and mass-selective detection.…”