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Cavity ring-down spectroscopy (CRDS) was used to calculate the electronic absorption spectrum of the allyl radical (CH2CHCH2) between 370 and 420 nm at 297 K. In conclusion, the absorption cross-section of the allyl radical at 402.9 nm was determined to be (2.0 plusmn 0.04) x 10(super -19) cm(super 2) molecule(super -1) through study of time-dependent absorption traces.

A deduction of the primary photodissociation mechanism of Si2H6 at 193 nm was made by photoproduct detection by laser-induced fluorescence (LIF) and time-resolved mass spectrometry techniques. Observation of the Si2H6 was conducted by the LIF method. No signal has been detected from SiH3, Si2H5, Si2H3 and Si2H4 in 193 nm photolysis. The importance of H2 elimination processes in Si2H6 produced in 193 nm photolysis was suggested by existing data combination.

A kinetic study of the reaction of the 4‐methylphenyl radical (4‐C6H4CH3) with the oxygen molecule was conducted using experimental and theoretical approaches. The absorption spectrum for the λ = 266 nm photolysis of the 4‐C6H4CH3X (X = Cl, Br)/N2/O2 mixture was measured in the wavelength range of λ = 503‐512 nm using N2 as the buffer gas at a total pressure of 40 Torr using a cavity ring‐down spectroscopy apparatus coupled with a pulsed laser photolysis system. Based on the absorbance of the product of the 4‐C6H4CH3 + O2 reaction at λ = 504 nm, the reaction rate coefficient for the 4‐C6H4CH3 + O2 reaction was determined to be k = (1.21 ± 0.10) × 10−11 cm3 molecule−1 s−1 and k = (1.18 ± 0.21) × 10−11 cm3 molecule−1 s−1 using 4‐C6H4CH3Cl and 4‐C6H4CH3Br, respectively, as the radical precursor. And there was no pressure dependence in the total pressure range of 10‐90 Torr varying partial pressure of N2 buffer gas at T = 296 ± 5 K. The geometries, vibration frequencies, and potential energy surfaces of the reactants, major products, and transition states in the 4‐C6H4CH3 + O2 reaction were determined using the CBS‐QB3 method. The k value at the high‐pressure limit was calculated to be 1.26 × 10−11 cm3 molecule−1 s−1 using the variational transition‐state theory. The calculated value of k was consistent with the experimental value, which indicated that the 4‐C6H4CH3 + O2 reaction reaches the high‐pressure limit at 10 Torr. Therefore, the oxidation of the 4‐C6H4CH3 radical is almost 10 times faster than that of the benzyl radical, which has the same chemical formula, at the high‐pressure limit. [ABSTRACT FROM AUTHOR]

A study of the reactions of Cl atoms with RI is conducted using cavity ring-down spectroscopy at a temperature of 233-313 K and at 100 Torr total pressure of N2 diluent. Visible absorption spectra of the RI-Cl adducts are recorded at 440-520 nm at 263 K.

Rate constants relevant to the thermal dissociation of Si(sub 2)H(sub 6) are calculated for the various multichannel unimolecular dissociations and chemical reactions. Quantum chemical calculations indicate that the energies of the saddlepoints for reactions R1 and R2 are lower than the energies of the product state.

Transient absorption spectra and decay profiles of hydroperoxy radical (HO2) are investigated using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. The results obtained suggest that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction.

The near-ultraviolet absorption spectrum of the phenoxyl radical (C6H5O(super .)) in 10-20 Torr of nitrogen diluent at 298 K is studied using cavity ring-down spectroscopy combined with pulsed laser photolysis. The absorption cross section of the phenoxyl radical was obtained by using a numerical fitting routine on the basis of a modeling of chemical reaction system.

Rate constants for the reactions of Si (3P, 1D, 1S) atoms with SiH4 are obtained using a combined laser photolysis/laser induced fluorescence technique. The results of the density functional theory calculations show that the reaction of Si(3P) atoms with SiH4 proceeds via a loose transition state.

Photodissociation of iodine monochloride has been investigated at 235–248 nm by studying Doppler profiles of (2+1) resonance enhanced multiphoton ionization spectra of fragment chlorine atoms and two-photon laser induced fluorescence spectra of iodine atoms. At 235.3–237.8 nm, a measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] is 0.68±0.10. At 248 nm, the [I*(2P1/2)]/[I(2P3/2)] ratio is 0.71±0.27. From the measured Doppler profiles and the ab initio calculation, the photoprepared states leading to Cl and Cl* production are assigned to the 3Π0+ (and possibly 3∑-0+) and the 1Π1 states, respectively. Nonadiabatic couplings between the potential curves have been discussed. [ABSTRACT FROM AUTHOR]

The reactions O(1D)+HCl→OH+Cl (1a) and OCl+H (1b), O(1D)+DCl→OD+Cl (2a) and OCl+D (2b), and O(1D)+Cl2→OCl+Cl (3) are studied at an average collision energy of 7.6, 7.7, and 8.8 kcal/mol for (1), (2), and (3), respectively. H, D, and Cl atoms are detected by the resonance-enhanced multiphoton ionization technique. The average kinetic energies released to the products are estimated from Doppler profile measurements of the product atoms. The relative yields [OCl+H]/[OH+Cl] and [OCl+D]/[OD+Cl] are directly measured, and a strong isotope effect (H/D) on the relative yields is found. The fine-structure branding ratios [Cl(2P1/2]/[Cl(2P3/2)] of the reaction products are also measured. The results suggest that nonadiabatic couplings take place at the exit channels of the reactions (1a) and (2a), while the reaction (3) is totally adiabatic. [ABSTRACT FROM AUTHOR]

The H, D, and Cl atoms from the photodissociation of HCl and DCl at 157 and 193 nm were detected by laser-induced fluorescence (LIF) in the vacuum ultraviolet region. The Doppler profiles of the H and D resonance lines at 121.6 nm in the LIF spectra indicate that the absorption of HCl and DCl at 157 nm is a mixture of perpendicular and parallel transition (A 1Π, a 3Π0+←X 1Σ+), while that at 193 nm results almost solely from the A 1Π←X 1Σ+ transition. The fine-structure branching ratios, are measured for the chlorine atom by LIF. Combined with the previous results, the branching ratios [Cl*(2P1/2)]/[Cl(2P3/2)] are 0.81±0.09 at 157 nm, 0.50±0.05 at 193 nm for HCl, and 0.23±0.05 at 193 nm for DCl. These results suggest that the nonadiabatic couplings during the breakup of HCl (DCl) in the excited states play an important role in determination of the fine-structure branching ratios. [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]

Photofragment chlorine atoms in the 2Pj states from Cl2 at 266–500 nm are measured by a resonance-enhanced multiphoton ionization technique. The measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] increases monotonically with dissociation wavelengths from (9.5±1.8)×10-3 at 308 nm to (4.7±0.5)×10-1 at 475 nm. At 375 nm≤λ≤475 nm, Doppler profiles of the chlorine atoms indicate adiabatic formation of two Cl(2P3/2) from the 1Π(1u) state of Cl2 and Cl(2P3/2)+Cl*(2P1/2) from the B 3Π(0+u) state. On the other hand, at a shorter wavelength (308 nm), Cl*(2P1/2) is generated from the 1Π(1u) state through nonadiabatic transitions during the dissociation. The relative contribution of the 1Π(1u)←X 1Σ(0+g) and B 3Π(0+u)←X 1Σ(0+g) transitions in the photoabsorption is estimated from the obtained branching ratios at 375 nm≤λ≤475 nm. [ABSTRACT FROM AUTHOR]

The Doppler profiles of the O(1D) products from the photodissociation of O2 at 157.6 nm and NO2 and N2O at 205.47 nm are detected by a resonance enhanced multiphoton ionization technique. The translation energy and angular distributions are deduced for the O(1D) atoms. Present results indicate that (a) O2 (B 3Σ-u) has a short dissociative lifetime when irradiated at 157.6 nm, (b) NO(2Π) photofragment from NO2 at 205.47 nm is mostly vibrationally excited, and (c) most of the available energy is released as kinetic energy when N2O is photodissociated at 205.47 nm. [ABSTRACT FROM AUTHOR]

Hydrocarbons (CnH2n+2, n=3, 4, and 6) and methyl halides (CH3X, X=Cl, Br, I) are photodissociated at 157 nm. The hydrogen atom photofragments are detected by a resonance-enhanced multiphoton ionization technique. The Doppler profiles of the hydrogen atoms from hydrocarbons are well represented by a Gaussian profile, while those from methyl halides by a mixture of a Gaussian and a non-Gaussian profiles. These Doppler profiles are interpreted assuming that (a) hot ethyl photofragments from hydrocarbons result in the formation of hydrogen atoms and (b) methyl halides undergo both direct and indirect photolytic scissions of the C—H bonds at 157 nm. [ABSTRACT FROM AUTHOR]

Doppler profiles of O(2p 3Pj) (j=2,1,0) produced from the photodissociation of O2 in the Herzberg I system (A3Σ+u←X3Σ-g) at 226 nm are measured with a resonance-enhanced multiphoton ionization technique. The anisotropy parameter for the angular distribution of the photofragments is found to be 1.6±0.4, in quantitative agreement with theory and indicating that the Herzberg I system borrows most of its intensity from a parallel band. The branching ratios to the j levels of O(2p 3Pj) (j=2,1,0) are found to be (2.5±0.3):1:(0.26±0.03) and are compared with the values expected for the limits of adiabatic and diabatic dissociation. [ABSTRACT FROM AUTHOR]

Alkyl chlorides (R=CH3, C2H5, C3H7, and C4H9), chloromethanes (CHnCl4-n), and chlorofluoromethanes (CFnCl4–n) are photodissociated at 157 and 193 nm. The chlorine atom photofragments are detected by a resonance enhanced multiphoton ionization technique. The branching ratios of the Cl photofragments [Cl*(3p2P1/2)]/[Cl(3p2P3/2)] are almost identical (0.23±0.03) for alkyl monochlorides at 157 and 193 nm and for CH2Cl2 and CHCl3 at 157 nm, while the ratios are rather small (0.10±0.02) for CH2Cl2 and CHCl3 at 193 nm. No discernible isotope effects on the branching ratios were observed when D atoms were substituted for H atoms in chloromethanes. For CCl4, CF3Cl, CF2Cl2, CFCl3 at 157 and 193 nm, the ratios are small (<0.05) irrespective of the number of Cl atoms in the parent molecules. The Doppler profiles of the chlorine photofragments have been obtained and it is found that (a) CF3Cl undergoes a perpendicular optical transition at 157 nm and (b) for halomethanes containing more than two Cl atoms, the Cl photofragment has a Boltzmann distribution in translational energy and an isotropic angular distribution. [ABSTRACT FROM AUTHOR]

The Doppler profiles of the hydrogen atom photofragments from the photodissociation of HCl and HBr at 157 and 243 nm are observed to determine the symmetries of the photoexcited states by using a resonance enhanced multiphoton ionization technique. The phototransitions are perpendicular for HCl at 157 nm and 193 nm and for HBr at 157 nm and 243 nm. Isotope effects on the branching ratios of [Cl*(2P1/2)]/[Cl(2P3/2)] are observed for the photodissociation of HCl and DCl at 157 and 193 nm. Our results indicate that in the photodissociation processes of HCl (A 1Π) the dynamical curve crossings during the breakup of the molecule play important roles in the determination of the branching ratio. [ABSTRACT FROM AUTHOR]

The O(2p 3Pj) (j=2, 1, and 0) fragments produced in the 157 nm photodissociation of CO2 were detected by resonance-enhanced multiphoton ionization in a molecular beam. The Doppler profiles and fine-structure branching ratios were measured for the oxygen-atom photofragment in the 3Pj states. The Doppler profiles were analyzed to give an anisotropy parameter of β=2.0±0.2 and an internal energy equivalent to 3.9±0.3 vibrational quanta of CO. The fine-structure populations were found to be 0.70±0.05, 0.16±0.03, and 0.14±0.03 (with error bars of ±σ) for j=2, 1, and 0, respectively. A mechanism is proposed in which complex on the 1B2 surface undergoes intersystem crossing to the 3B2 surface. A phase-space model with a constraint on the impact parameter is shown to be consistent with the observed energy release. The nonstatistical fine-structure population could be caused by long-range interactions on the triplet surface. In a bulb experiment, O(3P) was produced by quenching of O(1D). The fine-structure populations of the resulting O(3Pj) were 0.64±0.04, 0.25±0.04, and 0.11±0.04. This state distribution is consistent with a long-lived complex which decays to give statistical products. [ABSTRACT FROM AUTHOR]

O(2p3Pj) atoms produced from the reaction between H(1s2S) and O2(X3Σ-g) at collision energies (Ecoll) of 2.5 and 2.1 eV are detected by a resonance enhanced multiphoton ionization technique. From the Doppler profile measurements of O(3Pj), the average kinetic energy released to the products is estimated. The results suggest that the average fraction of the kinetic energy disposal at Ecoll =2.1 eV is larger than that at Ecoll =2.5 eV. The obtained branching ratios of O(3Pj) j=2, 1, and 0 at Ecoll =2.5 eV are (0.70±0.02), (0.21±0.02), and (0.09±0.02), and those at Ecoll =2.1 eV are (0.63±0.02), (0.25±0.04), and (0.12±0.02), respectively, with error bars of ±σ. The results suggest that the nonadiabatic couplings at the last stage of the reaction are important for the determination of the ratios. [ABSTRACT FROM AUTHOR]