Absolute chlorine and hydrogen atom quantum yield measurements in the 193.3 nm photodissociation of CH3CFCl2 (HCFC-141b)

Abstract

Articles you may be interested inUltraviolet photochemistry of buta-1,3-and buta-1,2-dienes: Laser spectroscopic absolute hydrogen atom quantum yield and translational energy distribution measurementsThe dynamics of chlorine and hydrogen atom formation in the 193.3 nm gas-phase laser photolysis of room-temperature 1,1-dichloro-1-fluoroethane, CH 3 CFCl 2 ͑HCFC-141b͒, were studied by means of the pulsed-laser-photolysis and laser-induced fluorescence ͑LIF͒ ''pump-and-probe'' technique. Nascent ground-state Cl( 2 P 3/2 ) and spin-orbit excited Cl*( 2 P 1/2 ) as well as H( 2 S) atom photofragments were detected under collision-free conditions by pulsed Doppler-resolved laser-induced fluorescence measurements employing narrow-band vacuum ultraviolet probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in krypton. Using HCl photolysis as a reference source of well-defined Cl( 2 P 3/2 ), Cl*( 2 P 1/2 ), and H atom concentrations, values for the chlorine-atom spin-orbit branching ratio ͓Cl*͔/͓Cl͔ϭ0.36Ϯ0.08, the total chlorine atom quantum yield (⌽ ClϩCl * ϭ1.01Ϯ0.14), and the H atom quantum yield (⌽ H ϭ0.04Ϯ0.01) were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the mean relative translational energy of the chlorine fragments could be determined to be E T(Cl) ϭ157Ϯ12 kJ/mol and E T(Cl * ) ϭ165 Ϯ12 kJ/mol. The corresponding average values 0.56 and 0.62 of the fraction of total available energy channeled into CH 3 CFClϩCl/Cl* product translational energy were found to lie between the limiting values 0.36 and 0.85 predicted by a soft impulsive and a rigid rotor model of the CH 3 CFCl 2 →CH 3 CFClϩCl/Cl* dissociation processes, respectively. The measured total chlorine atom quantum yield along with the rather small H atom quantum yield as well as the observed energy disposal indicates that direct C-Cl bond cleavage is the most important primary fragmentation mechanism for CH 3 CFCl 2 after photoexcitation in the first absorption band.

Lyman-α photodissociation of CH3CFCl2 (HCFC-141b): Quantum yield and translational energy of hydrogen atoms

“…Thus, photodissociation at 193 nm preferentially leads to C-Cl bond breaking which is supported by the experimentally measured Cl atom quantum yield values which are near unity. 3 Unfortunately, the VUV spectrum of CH 3 CFCl 2 is not reported at 121⋅6 nm. However, for very similar molecule i.e.…”

“…At 193 nm, the main photodissociation channel is found to be formation of Cl atom with quantum yield of near unity and a minor channel leading to H atom formation (φ H = 0⋅05). 3,4 Rupture of C-Cl and C-H has been explained by mixed character for the σ* antibonding orbital. At the shorter wavelengths, the Rydberg excitations come into play and the spectra become richer.…”

4 Photodissociation in the gas phase

“…Although its production was stopped in 1996, the release into atmosphere still continues because of the long-term use of the previously manufactured products. La ¨uter et al 18 determined the absolute quantum yield of chlorine and hydrogen atoms at 193 nm. The total chlorine atom quantum yield (W Cl1Cl* ) was found to be 1.01, and the H atom quantum yield W H ~0.04.…”

“…In the Hartley band, ozone primarily dissociates through the spin conserved channels, Chakraborty and Bhattacharya 89 addressed the issue of enrichment of heavier isotopomer in stratospheric ozone. They studied 16 O/ 17 O/ 18 O mass-dependent photodissociation in the visible region (Chappuis band, probed at 520 and 630 nm) and in the UV (Hartley band, probed at 253.6 and 184.9 nm). In the former region, only (14.2) occurs.…”