In this article we present a new approach to the already popular methods of ion imaging and velocity mapping. The novelty of this approach is that the speed and angular distributions are measured directly from the images without the need of inverse Abel transformation as in the conventional approaches. This is achieved by using delayed pulsed extraction of the ions following photodissociation and positioning of the nascent products. Delayed pulsed extraction causes a sufficient velocity spread in the ion cloud such that the time width of the ion packet at the detector is on the order of 500 ns. By using a narrow detector time gate (<40 ns) we are able to image only the center slice of the ion packet. The result is equivalent to that obtained by conventional methods using the inverse Abel transform, however, the artificial noise introduced by this transform is eliminated. The energy resolution of the new approach is at least comparable to that achieved with the velocity mapping technique.

The production of spin-polarized hydrogen atoms from the photodissociation of hydrogen chloride with circularly polarized 193-nanometer light is inferred from the measurement of the complete angular momentum distributions of ground state Cl( ² P _3/2 )and excited state Cl( ² P _1/2 )cofragments by slice imaging. The experimentally measured and ab initio predicted \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \(\mathbf{a}_{{\ }q}^{(k)}\) ( p )parameters, which describe the single-surface and multiple-surface-interference contributions to the angular momentum distributions, are in excellent agreement. For laser pulses longer than about 0.7 ns, the polarization of the electron and the proton are both 36%.

The photolysis of CH3Br is studied in the energy region of the A band between 4.94 and 5.76 eV using ion imaging. Velocity distributions for both the bromine-atom and methyl-radical photofragments are determined. Our results indicate that transitions to the 3Q0 and 3Q1 states dominate the absorption cross section and the partial cross section to each state is determined. The [Br*]/[Br] branching ratio is found to be strongly dependent on the excitation energy varying between 0.6 and 1.5. Both the bromine-atom and the methyl-radical translational energy distributions suggest that the vibrational distribution in the nascent CH3 is nonstatistical with appreciable excitation in the v2 umbrella mode. The lifetime of the A band is estimated at τ=120±40 fs.

Velocity distributions for the Cl(2P3/2) and Cl(2P1/2) photofragments produced by photolysis of Cl2 in the region between 310 and 470 nm are measured using photofragment velocity mapping. Our results indicate that at short wavelengths the absorption spectrum is dominated by the 1u(1Πu) excited electronic state which produces two ground state chlorine atoms. The 0u+(B 3Πu) state which produces a spin-orbit excited and a ground state chlorine atom becomes significant at 350 nm and dominates the spectrum beyond 400 nm. Analysis of the photofragment angular distributions indicates that the Cl(2P3/2) photofragments are aligned and the magnitude of the alignment is quantitatively determined. Nonadiabatic curve crossing between the 1u(1Πu) and the 0u+(B 3Πu) electronic states is observed and quantified below 370 nm. The measured nonadiabatic transition probability is modeled using the Landau–Zener formula and the position of the curve crossing is estimated at ∼3 eV above the zero-point of ground electronic state of Cl2.