$\emph{Absolute primary H atom quantum yield measurements in the 193.3 nm and 121.6 nm photodissociation of acetylene}$
Absolute quantum yields for primary H atom formation ($φ_H$) were measured under collision-free conditions for the room-temperature gas-phase dissociation of acetylene ($C_2H_2$) after photoexcitation at 193.3 nm and at the H-atom Lyman-α wavelength (121.6 nm) by a pulsed laser photolysis (LP)-laser-induced fluorescence (LIF) ‘pump-and-probe’ technique. Using HCl and $CH_4$ photolysis at 193.3 and 121.6 nm, respectively, as a reference, values of $φ_H$ (193:3 nm) = 0.94±0.12 and $φ_H$ (121.6 nm) = 1.04±0.16 were obtained which demonstrate that for both photolysis wavelengths the H + $C_2H$ product channel dominates the primary $C_2H_2$ photochemistry.
$\emph{Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of $SO_2$ at 222.4 nm}$
The dynamics of formation of oxygen atoms after UV photoexcitation of $SO_2$ in the gas-phase was studied by pulsed laser photolysis (LP)-laser-induced fluorescence ‘pump-and-probe’ technique in a flow reactor. $SO_2$ at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and $O(^3P_j)$ photofragments were detected under collision-free conditions by vacuum ultraviolet (VUV) laser-induced fluorescence (LIF). The use of narrow-band probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent $O(^3P_{j=2,1,0})$ fine-structure state distribution: $n_{j=2}/n_{j=1}/n_{j=0}$ = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing $NO_2$ photolysis as a reference, a value of $φ_{o(^3p)}$=0.13 ± 0.05 for the absolute $O(^3P)$ atom quantum yield was determined. The measured $O(^3P)$ quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited $SO_2(C^1B_2)$ (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.
$\emph{Photodissociation dynamics of methyl iodide using two-photon resonant four-wave mixing spectroscopy}$
Two-photon resonant four-wave mixing spectroscopy has been newly applied to detect photofragments formed by photodissociation. The photodissociation of methyl iodide has been studied at 266 nm by monitoring the atomic iodine nascent via two-photon degenerate four-wave mixing spectroscopy. The ground state $I(^2P_{3/2})$ was detected via the 5p $^2P_{3/2}$ → 6p $^2D_{5/2}$ two photon transition (304.67 nm), and the spin-orbit excited state $I^*(^2P_{1/2})$ via the 5p $^2P_{1/2}$ → 6p $^2D_{1/2}$ transition (304.02 nm). The signal line profiles were well characterized with Gaussian shaped functions. A cubic dependence of the signal intensity on the input laser was observed. The line center saturation intensities have been determined to be 3863 MW/㎠ for I and 1969 MW/㎠ for $I^*$. The signal ratios between I and $I^*$ has varied with the input laser intensity. In this study we have found that the $I^*$ /I branching ratio has been extracted based on the various signal intensities. Two-photon resonant four-wave mixing scheme is very useful for the measurement of the internal state distributions of the photofragments, which are formed in photodissociation.
$\emph{The Dynamics of $Br(^2P_j)$ Formation in the Photodissociation of Vinyl and Perfluorovinyl Bromides}$
The photodissociation dynamics of vinyl bromide and perfluorovinyl bromide have been investigated at 234 nm using a photofragment ion imaging technique coupled with a state-selective [2+1] resonance-enhanced multiphoton ionization scheme. The nascent Br atoms stem from the primary C-Br bond dissociation leading to the formation of $C_2H_3$(\~{X})$ and $Br(^2P_j;j=1/2,3/2)$. The obtained translational energy distributions have been well fitted by a single Boltzmann and three Gaussian functions. Boltzmann component has not been observed in the perfluorovinyl bromide. The repulsive $^3A'(n,σ^*)$ state has been considered as the origin of the highest Gaussian components. Middle translational energy components with Gaussian shapes are produced from the $^1A'(π,σ^*)$ and/or $^3A'(π,σ^*)$ which are very close in energy. Low energy Gaussian components are produced via predissociation from the $^3A'(π,π^*)$ state. The assignments have also been supported by the recoil anisotropy corresponding to the individual components. It is suggested that intersystem crossing from the triplet states to the ground state has been attributed to the Boltzmann component and the fluorination reduces the probability of this electronic relaxation process.
$\emph{The photodissociation dynamics of cycloalkyl bromides, $c-C_nH_{2n-1}Br$ (n=3,4,5,6), at 234 nm}$
The photodissociation dynamics of bromocycloalkanes respective to their ring number (c-CnH2n-1Br; n=3-6) have been investigated at 234 nm. The nascent Br and $Br^*$ atom were state-selectively detected via a [2+1] resonance-enhanced multiphoton ionization (REMPI) scheme combined with photofragment ion imaging technique. The relative quantum yields were measured by REMPI with time-of-flight mass spectrometry (TOF-MS). The translational energy distributions and recoil anisotropies were extracted by photofragment 2D ion-imaging technique. In $c-C_3H_5Br$ and $c-C_4H_7Br$, the obtained translational energy distributions have been well fitted by a single Gaussian function implying that the bromine fragments are produced via direct dissociation on the repulsive state. On the other hands, two Gaussian functions are required to fit the distributions in $c-C_5H_9Br$ and $c-C_6H_11Br$. It can be inferred that they are originated from different conformational structures corresponding to the axial and equatorial position of bromine atom, respectively. The soft radical limit of the impulsive model have been applied to the related energy partitioning.
