Osman Sorkhabi

Competing isomeric product channels in the 193 nm photodissociation of 2-chloropropene and in the unimolecular dissociation of the 2-propenyl radical

This paper presents product translational energy spectroscopy measurements of the primary photofragmentation channels of 2-chloropropene excited at 193 nm and of the unimolecular dissociation of the 2-propenyl radical. Tunable vacuum ultraviolet (VUV) photoionization of the products allows us to distinguish between the various product isomers formed in these processes. The data show evidence for three significant primary reaction channels in the dissociation of 2-chloropropene: An excited-state C–Cl fission channel producing fast Cl atoms, a C–Cl fission channel producing slow Cl atoms, and HCl elimination. A minor C–CH3 fission channel contributes as well. The measured branching of the major primary product channels is: [fast C–Cl]:[slow C–Cl]:[HCl elimination]=62%:23%:15%. The experiments also allow us to resolve selectively the product branching between the unimolecular dissociation channels of the 2-propenyl radical, a high energy C3H5 isomer; we measure how the branching ratio between the two competi...

Laser plasma spectrochemistry

An overview of laser plasma spectrochemistry is presented to demonstrate its wide range of capabilities. Laser plasmas offer the ability to perform elemental, isotopic, molecular, quantitative and qualitative sample analysis with sub-micron spatial resolution, and each feature can be measured at standoff distances. Obviously, these attributes are not all achievable at the same time, but they can be optimized for specific applications. This manuscript gives a sampling (pun intended) of the research in our group that has demonstrated each of these capabilities. Although the technology is commonly referred to as LIBS (laser-induced breakdown spectroscopy), the authors prefer to use laser plasma spectrometry to represent the underlying science.

Ultraviolet photodissociation of furan probed by tunable synchrotron radiation

The photodissociation dynamics of furan at 193 nm have been studied using photofragment translational spectroscopy with tunable vacuum ultraviolet (VUV) probe provided by synchrotron radiation on the Chemical Dynamics Beamline at the Advanced Light Source. Three primary channels are observed: HCO+C3H3, CO+C3H4, and H2CCO+C2H2. The evidence suggests that the two closed-shell channels occur on the ground-state potential energy surface (PES) following internal conversion, while the radical channel likely takes place on an excited PES. All channels exhibit a barrier for dissociation with the acetylene+ketene channel having the largest value at about 25 kcal/mol. Angular distribution measurements show anisotropy only for the radical channel. These findings are consistent with a rapid excited state dissociation for the radical channel and slow dissociation for the other two pathways. The two ground-state dissociation channels—propyne+CO and acetylene+ketene—should be important in the thermal decomposition of fu...

Photofragment translational spectroscopy with state-selective 'universal detection': the ultraviolet photodissociation of CS2

We have investigated the photodissociation of CS2 at 193 nm using the technique of photofragment translational spectroscopy. The utilization of vacuum ultraviolet synchrotron radiation for product photoionization has permitted a determination of the vibrationally resolved translational energy distribution for the CS+S(1D) channel and the translational energy distribution for the CS+S(3P) channel. A simulation of the coincident S(1D) translational energy distribution is consistent with a CS vibrational distribution of 0.02:0.17:0.19:0.46:0.15 in ν=0:1:2:3:4 and an average rotational energy of ∼1–3 kcal/mol. We find that the S(3P)/S(1D) branching ratio is 3.0±0.2, in good agreement with previous reports. Both asymptotic channels exhibit similar velocity dependent anisotropy parameters that decrease with decreasing translational energy release. The results extend earlier reports and provide further insight into the dissociation dynamics at 193 nm.

Evidence of triplet ethylene produced from photodissociation of ethylene sulfide

Tunable synchrotron radiation has been used to probe the dissociation dynamics of ethylene sulfide, providing selective determination of the translational energy distributions of both excited (1D) and ground-state (3P) sulfur atoms, with momentum-matching to the ethylene cofragments. The results suggest the presence of a channel giving S(3P) in conjunction with triplet ethylene C2H4 (3B1u), and allow for the first experimental measure of the energy of the latter species near the equilibrium geometry, in which the two methylene groups occupy perpendicular planes.

Quantum yields and energy partitioning in the ultraviolet photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402)

The photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402) has been investigated at 193 nm using photofragment translational spectroscopy with vacuum ultraviolet ionization and at 193, 233, and 266 nm using state-selected translational spectroscopy with resonance-enhanced multiphoton ionization. The product branching ratios, angular distributions, and translational energy distributions were measured at these wavelengths, providing insight into the ultraviolet photodissociation dynamics of CF2BrCF2Br. The total bromine atom quantum yields were found to be 1.9±0.1 at both 193 and 233 nm and 1.4±0.1 at 266 nm. The first C–Br bond dissociation energy was determined to be 69.3 kcal/mol from ab initio calculations. The second C–Br bond dissociation energy was determined to be 16±2 kcal/mol by modeling of the bromine quantum yield. In addition, variational Rice–Ramsperger–Kassel–Marcus theory was used to calculate the secondary dissociation rates for a range of dissociation energies above threshold. The...

Characterization of nitrogen-containing radical products from the photodissociation of trimethylamine using photoionization detection

The outcome of neutral and photoionized N(CH3)2 primary products of trimethylamine photodissociation at 193 nm is determined by combining photoionization detection with supporting G3 theoretical calculations. N(CH3)2 primary products with very little internal energy show an experimentally observed ionization onset of 9.1±0.2 eV, but do not appear at the parent ion (m/e=44). Instead, the parent ion is unstable and easily fragments to m/e=42, where the signal is observed. N(CH3)2 radicals with higher internal energies undergo H-atom loss from the neutral to give CH2NCH3, which has an observed ionization onset at parent (m/e=43) of <9.3 eV. At slightly higher ionization energies, these secondary products also appear at m/e=42 (where their appearance energy is roughly 9.8–9.9 eV, uncorrected for internal energy). Finally, N(CH3)2 radicals with the highest internal energy in this study appear to undergo H2 loss as neutrals, giving rise to a species whose parent ion has m/e=42. The ionization onset of this spec...