Photodissociation of $CF_3I$ at 277 nm: The photodissociation of $CF_3I$ cooled in a supersonic molecular beam has been investigated at 277 nor by state-selective photofragment imaging. Fragmented iodine atoms of two spin-orbit states are state-selectively ionized and projected onto a two-dimensional position-sensitive detector, to obtain their speed and angular distributions. The anisotropy parameter for an excited iodine atom $I^*(^2P1/2)$, $β(I^*)$, is found to be 1.83 and is consistent with a dissociation lifetime in the order of 150 - 350 fs from rotational correlation function. Contrary to earlier reports, the parallel-like distribution for the ground state iodine atom $I(^2P3/2)$ at this wavelength, shows a more favorable curve-crossing dissociation path (68%) from $^3Q_0$ to $^1Q_1$ and a less favorable direct dissociation path (32%) from $^3Q_1$. The recoil energy distribution of I is found to be broader than that of $I^*$ and is correlated with a variety of energy disposal channels by an e symmetry vibration at the crossing point. The results are compared with previous works, and the strong photon energy dependence of the energy partitioning in $CF_3+I^*$ channel and curve crossing are interpreted in terms of the final state interaction and curve crossing probability, respectively.
Photodissociation of t-Butyl Iodide at 277 and 304nm: The photo-dissociation of $t-C_4H_9I$ has been studied at 277 and 304 nm in a supersonic molecular beam. The fragments (I and $t-C_4H_9I$ radical) are selectively ionized by resonance-enhanced multiphoton ionization and then projected onto a two-dimensional position-sensitive-detector to obtain their translational energy and angular distributions. The energy distribution is found to consist of three components - one Boltzmann and two Gaussian distributions. Their anisotropy parameters range from 0.5 to 1.7 and display the parallel transition characteristic, where the greater the kinetic energy of the component the stronger its anisotropy. From present and previous works, these three components can be interpreted in terms of three independent reaction paths on an excited potential energy surface: (1) the prompt dissociation along the C-I stretching mode, (2) the repulsive mode along the C-I stretching, coupled with some bending motions, (3) the indirect dissociation, probably along a bounded reaction coordinate for the high-$E_t$ and the low-$E_t$ Gaussian, and the Boltzmann, respectively. Relative quantum yields for $I(^2P_3/2)$ at 277 and 304 nm have been determined and found to be 0.93± 0.03 and 0.92±0.04, respectively. Experiments have shown that $t-C_4H_9I$ has the highest curve-crossing probability from $^3Q_0$ state to $^1Q_1$ among alkyl iodides. The extensive vibrational coupling between two states in the proximity of a crossing point supports this interpretation.
Photodissociation of IBr Near 267 nm: The photodissociation dynamics of IBr has been investigated near 267 mn by two-dimensional photofragment imaging technique. Speed and angular distributions for $Br(^2P_3/2$) and $Br^*(2P_1/2)$ fragments are obtained from the center-stripe analysis of their images. Based on the speed distributions, three dissociation channels are observed and identified with the formations of $Br(^2P_3/2)+I(^2P_3/2)$, $Br^*(2P_1/2)_I(^2P_3/2)$, and $Br(^2P_3/2)+I^*(^2P_1/2)$. Although there is a difference in contribution, these three paths are found to have the mixed character of parallel and perpendicular transition judging from the values of their anisotropy parameters. Relative quantum yields for the three channels are determined and then resolved into transition types from their anisotropies. Combined with the recent work at 304 mn [Jung, Griffiths, and El-Sayed, J. Chem. Phys. 103, 6999 (1995)1 our results present the complete assignment of the photoexcitation leading to the three channels in the UV band region: predominantly the perpendicular transition to $^1Π_1(2341)$ state, mainly the parallel $^3Π_0+(2341)$ transition, and the mixed one to the parallel $^3Π_0+(2341)$ and/or $^3∑^-_0+(2422)$ and perpendicular $^3Π_1+(2341)$ states for $I+Br^*$, $I^*+Br$, and I+Br channels, respectively. From the change in the relative quantum yield according to excitation photon energy, nonadiabatic couplings and an order of vertical energy between excited states have been discussed.
'2+1' REMPI of Iodine Atom: Photoelectron imaging technique is applied to study on '2+1' REMPI of iodine atoms produced from the A-band photolysis of $CH_3I$ in the wavelength region 277-313 nm. The technique provides a chance to obtain the relative branching ratios among possible ion states and their angular distributions of ejected photoelectrons, simultaneously. The branching ratios reveal a propensity toward core-preservation ionization. State-selective formation of $I^+(^3P_j)$ and $I^+(^1D_2)$ ions is possible at several resonant wavelengths. The photoelectron angular distributions show contributions from outgoing electrons with a maximum
angular momentum, $l_max=2$.
