Search

The electric field of a light wave accumulates a π phase shift as it passes through a focus. We show here how this effect, known as the Gouy phase, may be used to control the branching ratio of a unimolecular reaction when the products are formed with different numbers of photons. We demonstrate this control method for the ionization and dissociation of vinyl chloride, using absorption of 177 and 532 nm photons to induce a pair of interfering paths. Excellent agreement between the observed and calculated phase shift as a function of the axial coordinate of the laser focus indicates that fragmentation occurs via a ladder switching mechanism. The axial dependence of the modulation depth is evidence of loss of coherence at higher internal temperatures of the molecule. [ABSTRACT FROM AUTHOR]

Velocity-aligned Doppler spectroscopy was used to measure the speed distribution functions of HCl(v″=0–2,J″) produced in the 193 nm photodissociation of vinyl chloride. A mechanism which is consistent with our present data as well with our earlier work is three center elimination of HCl followed by concerted but nonsynchronous isomerization of the vinylidene fragment, with some of the isomerization energy converted to kinetic energy of the recoiling fragments. In order to explain the vibrational dependence of the translational and rotational energy distributions, the mechanism is assumed to be vibrationally adiabatic. In addition, magic angle Doppler spectroscopy was used to measure the speed distribution functions of Cl atoms produced in the photodissociation of vinyl chloride and the three dichloroethylene isomers. Bimodal energy distributions were observed for both spin–orbit states, with the Cl(2P1/2) fragment having more kinetic energy than Cl(2P3/2). The data are consistent with competitive reaction on two potential energy surfaces. Adiabatic correlation, with some scrambling in the asymptotic region, provide a qualitative explanation of the spin–orbit branching ratios. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The elimination of H2 in the photodissociation of mono- and di-chloroethylenes was studied with a pump-and-probe technique. A 193 nm excimer laser was used to photodissociate the parent molecules, and a tunable dye laser was used to probe the H2 fragment by 2+1 resonance-enhanced multiphoton ionization (REMPI). The nascent rotational state distributions of H2(X 1Σ+g,v″=0–4) were extracted from the REMPI spectra, and were found to have Boltzmann-type distributions. The maximum and average translational energies for some of the rovibrational levels of H2 were measured using magic angle Doppler spectroscopy. The translational energy of the fragments plus the internal energy of H2 was found to exceed the available energy for a three-center elimination mechanism. It is concluded that a migration mechanism plays a significant role in H2 elimination. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have used partially deuterated vinyl chloride to elucidate the photodissociation mechanism of this molecule. We have found that 75% of the HCl fragments are produced by three-center α,α elimination. Surprisingly, the rotational energy distributions of HCl are the same for both elimination pathways. One possible explanation is that H atom migration in the parent molecule results in similar transition states for the two pathways. Alternatively, dissociation and fragment isomerization may be strongly coupled so that the product angular momentum distribution is determined late in the reaction coordinate. Very different rotational state distributions were observed for v‘=0 and v‘>=0. We speculate that this dichotomy is caused by formation of a hydrogen-bonded π complex between HCl(v‘>=0) and acetylene. We have also determined that HCl+ ions are produced primarily by an α,β mechanism. This mechanism consists of electronic excitation of vinyl chloride, followed by photoelimination and photoionization of electronically excited HCl*. Finally, we determined that H atoms are preferentially produced by detachment from the β carbon, as predicted from the relative stabilities of the α- and β-chlorovinyl radicals. [ABSTRACT FROM AUTHOR]

Doppler profiles of chlorine and hydrogen atomic fragments produced in the photodissociation of mono- and dichloroethylenes at 193 nm have been measured in a pump-and-probe experiment using 2+1 resonance-enhanced multiphoton ionization. In a second experiment, the angular distributions of the Cl fragments produced from chloroethylenes at 235 and 238 nm were measured using a perfect-focusing mass spectrometer. In a third experiment, we measured the power dependence of the relative yields of H, Cl, HCl, and HCl+ produced from vinyl chloride at 193 nm. For Cl detachment, two primary processes have been confirmed. One produces an isotropic angular distribution of photofragments, while the other produces an anisotropic distribution. For H atom detachment, an isotropic angular distribution and a Boltzmann velocity distribution were found. The ratio of yields of the Cl and H fragments was found to be 4±1 for CH2CCl2 and higher than 10 for t-CHClCHCl and CCl2CClH. The H, Cl, and HCl yields were found to be first order in laser intensity, while the HCl+ yield was found to be third order. Saturation measurements of the ion yield indicate that the latter results from a 1+1+1 resonance-enhanced process involving a bound state of the parent molecule. This intermediate state may also be responsible for producing the statistical component of the Cl atom product. [ABSTRACT FROM AUTHOR]

Discusses the ultraviolet photodissociation dynamics of 2-chloro-1, 1-diffluoroethylene. Product state population having a Boltzmann-like distribution; Agreement with a simple phase-space model; Nonstatistical state distribution of HCl produced in the photodissociation of vinyl chloride and dichloroethylene.