Photodissociation Of Acetaldehyde Research Articles

Dynamics of ethylene photodissociation from rovibrational and translational energy distributions of H2 products

The dynamics of H2 elimination from ethylene was studied via a pump and probe technique utilizing an ultrahigh resolution vacuum ultraviolet laser system. H2 product internal and translational energy distributions were obtained for the photodissociation at 193 nm. The distribution of energy in H2 product from the dissociation of (1,1)-dideuteroethylene is also presented. Two separate H2 elimination channels are inferred: a 1,1 elimination producing the vinylidene radical and a 1,2 elimination producing the acetylene molecule. Differences between the vibrational, rotational, and translational energy distributions for these two channels are discussed and correlations between product internal and translational energy are presented. A comparison with ab initio calculations of the transition state configurations for these processes is made. We suggest that the H2 elimination process may be nonstatistical in nature. The D atom elimination from C2D4 was examined and kinetic energy distribution for this product measured.

Photodissociation of ethylene at 193 nm

The photodissociation of ethylene at 193 nm was studied by measuring the product translational energy distributions for the H+C2H3 and H2+C2H2 channels. In agreement with previous workers, it was determined that atomic and molecular elimination occur in relatively equal amounts. Using 1,1 D2CCH2 and 1,2 cis HDCCDH, it was shown that both acetylene and vinylidene are formed and that the acetylene/vinylidene ratio is approximately 2/3 in the molecular elimination. This H2 elimination channel has a translational energy distribution peaked at around 20 kcal/mol, indicating that it is a concerted process with a substantial exit barrier. It was found that the H atom elimination channel is best described as a simple bond rupture occurring after internal conversion of the electronically excited molecule to the vibrationally excited ground state ethylene. Some of the primary C2H3 product has sufficient internal energy to spontaneously decompose to H+HC≡CH. At higher laser intensity a large fraction of the C2H3, however, absorbs another photon and fragments to H+H2C=C: (1A1 and 3B2).

The dynamics of H 2 elimination from ethylene

The dynamics of H 2 elimination in the photodissociation of ethylene at 193 nm were investigated through measurements of the translational energy distribution and rovibrational state distribution of H 2 products. Using 1,1-D 2C=CH 2 and 1,2-HDC=CDH, it was shown that both four-centered and three-centered elimination of H 2 could take place with an acetylene to vinylidene formation ratio of approximately two-thirds. Limited 1,2 hydrogen atom migration occurred during the fragmentation, presumably through an ethylidene-type structure. The relatively high rotational excitation of the H 2 fragment suggests that the transition is not symmetric; as the two ethylene hydrogen atoms approach each other and reach the transition state, one CH bond should be significantly longer than the other. The vibrational energy distributions can be roughly characterized by a vibrational temperature of 4800 K and an average translational energy, which is dependent on the rovibrational state of H 2, exceeding 20 kcal mol −1.

The hydrogen atom channel in the photodissociation of ethylene

The photodissociation of ethylene at 193 nm is studied by laser induced fluorescence of the hydrogen atom product. The quantum yield for producing deuterium atoms from C2D4 is 0.19±0.04. Their average translational energy is 7.3±1.3 and 15.1±1.4 kcal/mol at 193 and 157 nm, respectively. Considerable evidence, both experimental and theoretical, shows that dissociation takes place from a hot internally equilibrated ground state molecule. A remarkable isotope effect is found. The ratio of H atoms to D atoms released from CH2CD2 is 2.2±0.2, whereas that from t-CHDCHD is 1.2±0.1. An RRKM model explains these ratios. For CH2CD2 the vibrational frequencies of the transition state for releasing H atoms are quite different from those for the release of D atoms. In t-CHDCHD, the transition state frequencies are nearly the same; there is a cancellation between differing zero point energies and moments of inertia.

Formation of vibrationally excited HCO in the photodissociation of acetaldehyde at 266 nm

The formation and decay kinetics of HCO(0, 0, 0) and vibrationally excited HCO were measured following the 266 nm photolysis of acetaldehyde (CH 3CHO) vapor. Time-resolved intracavity laser spectroscopy was used to detect the absorption of HCO with 1–2 μs time resolution. The kinetic properties of HCO were modeled using simulations derived from measured kinetic parameters and from best-fit comparisons. The two components observed in the formation of HCO(0, 0, 0) were identified with its production by direct photolysis and by intermolecular energy transfer from vibrationally excited HCO (second-order rate coefficient k = (1.1 ± 0.3) × 10 −11 molecule −1 cm 3 s −1 for acetaldehyde collisions). Contributions from rotationally excited HCO were not observed.

Laser-induced fluorescence emission from HCO produced by 308 nm excimer laser photodissociation of acetaldehyde

The LIF emission spectrum of the HCO Ā 2A″-X̄ 2A′ transition has been analyzed, and ν 1″ = 2432 ± 20 cm −1 was determined. The HCO(X̄) radicals generated by a photolysis pulse from the 5 Torr CH 3CHO sample disappear by second-order kinetics involving radical—radical reactions.

Wavelength dependence of HCO formation in the photolysis of acetaldehyde

The wavelength dependence of HCO(0,0,0) formation in the photodissociation of acetaldehyde was measured using narrow (0.1 nm) bandwidth laser excitation and time-resolved intracavity laser detection (TRILD). A sharp energetic onset at 320 ± 1 nm (89.3 ± 0.3 kcal) for HCO(0,0,0) formation was found. The maximum concentration of HCO(0,0,0) occurs between 100 and 250 μs after excitation depending on the wavelength of excitation