Aviva Melchior

C–Cl and C–H bond cleavage in 193 nm photodissociation of CH3CF2Cl and CH3CFCl2

The relative amounts of Cl(2P3/2) [Cl], Cl(2P1/2) [Cl*], H, and HCl produced by 193 nm photodissociation of CH3CF2Cl and CH3CFCl2 were studied by a laser pump-and-probe technique. An excimer laser photodissociated the parent molecules and a frequency doubled tunable dye laser probed the photofragments via (2+1) resonantly enhanced multiphoton ionization coupled with a time-of-flight mass spectrometer. Only atomic products were observed; no HCl was found. The profiles were well fitted to Gaussians. The Cl*/Cl branching ratios were almost identical, 0.18±0.04 and 0.16±0.04, while the H/(Cl+Cl*) ratios were 0.67±0.21 and 0.18±0.07, for CH3CF2Cl and CH3CFCl2, respectively. The mechanism for the production of the atomic species is discussed.

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

Utilizing vibrationally mediated photodissociation of jet-cooled CH3CFCl2 enabled monitoring of the second (3νCH) and third (4νCH) overtones of the methyl in the ground electronic state. The excited molecules are photodissociated by ∼235 or 243.135 nm photons that further tag Cl(2P3/2)[Cl] and Cl(2P1/2)[Cl*] isotopes or H photofragments via mass selective (2+1) resonantly enhanced multiphoton ionization. The vibrational spectra are characterized by a multiple peak structure related to C–H stretches and to Fermi resonating levels involving the CH3 deformation. The cooling in the expansion reduces the rotational and vibrational congestion and affords a determination of the splittings and the upper limits for homogeneous broadening of the transitions. The highest-frequency peak of 4νCH exhibits an additional splitting, related to coupling of the mixed stretch–deformation states with other modes of the molecule. The yield of all three photofragments increases as a result of preexcitation, demonstrating that t...

Alteration of Cl spin–orbit branching ratios via photodissociation of pre-excited fundamental CH3 stretch of CH3CFCl2

Abstract The fundamental symmetric CH 3 stretch ( ν 2 ) in the ground electronic state of CH 3 CFCl 2 is efficiently prepared by stimulated Raman excitation. The excited molecules are photodissociated by ∼235 nm photons that further tag Cl( 2 P 3/2 ) and Cl( 2 P 1/2 ) [Cl * ] photofragments via (2+1) resonantly enhanced multiphoton ionization. The yield of both photofragments increases as a result of pre-excitation, indicative of energy flow from the excited C–H to the C–Cl bond and hence a better Franck–Condon overlap between the components of the wavefunction along the C–Cl coordinate. The Cl * /Cl branching ratio is 0.60±0.09 in ∼235 nm photolysis of vibrationally excited molecules and 0.40±0.07 in that of vibrationless ground state CH 3 CFCl 2 .

Overtone spectroscopy of methyl C–H stretch vibration in CH3CF2Cl and CH3CFCl2

Photoacoustic spectra of the second (3νCH), third (4νCH), and fourth (5νCH) overtones of the methyl C–H stretches in CH3CF2Cl and CH3CFCl2 were measured. The spectra are characterized by a multiple peak structure of partially resolved triplets and quartets with an anomalous linewidth decrease in the 4νCH region. The results are interpreted in terms of a simplified local mode model for C–H stretching vibrations, including also the stretch-deformation Fermi resonances. The model accounts for most spectral features and allows determination of the time scale for vibrational redistribution.

Photodissociation and intramolecular dynamics of vibrationally excited CHF2Cl

Photodissociation of jet-cooled, vibrationally excited CHF2Cl molecules was studied in a time-of-flight mass spectrometer to elucidate bond rupture and intramolecular dynamics. The molecules were first excited with infrared photons to the N=3, N=7/2, and N=4 C–H stretch-bend polyad components, representing stretch-bend mixed states. They were then dissociated via promotion to excited electronic states by ∼235 or 243.135 nm photons, which also tagged 35Cl(2Pj) and 37Cl(2Pj) or H photofragments, respectively, by (2+1) resonantly enhanced multiphoton ionization. Comparison of the photofragment yield spectra to the simultaneously measured room-temperature infrared absorption spectra revealed significant narrowing of the former due to the reduction of rotational inhomogeneous structure. These spectra, and particularly the band contraction, afforded observation of resonance splitting in the vicinity of the 31, 34, 41, and 44 components, reflecting redistribution times in the range of 1–18 ps. These times manife...

STATE-RESOLVED DYNAMICS OF THE O(1D) + CHF2CL OH + CF2CL REACTION

Abstract The dynamics of the O ( 1 D ) + CHF 2 Cl → OH + CF 2 Cl reaction were investigated at a center-of-mass (c.m.) collision energy of 0.64 eV using O( 1 D) atoms generated by laser photolysis of N 2 O at 193 nm. OH products were detected under single collision conditions by laser-induced fluorescence and nascent OH( v = 0, 1) rotational distributions were derived. The OH( v = 0)/OH( v = 1) branching ratio wwas determined to be 1.19. The OH spin-orbit states as well as the λ-double states were found to be equally populated in the reaction. The absolute reaction cross section σ R (E c.m. = 0.64 eV ) = 0.21 ± 0.05 A 2 , was determined and a possible reaction mechanism is discussed.