K. Thomas Lorenz

Ion imaging measurement of collision-induced rotational alignment in Ar-NO scattering

Collision-induced rotational alignment of NO X 2Π1/2(v=0,j=8.5) is measured for rotationally inelastic scattering of NO X 2Π1/2(v=0,j=0.5) with Ar at 65 meV collision energy. The experiments are performed by velocity-mapped ion imaging with polarized 1+1′ REMPI probing of the scattered NO products. It is shown that the azimuthal information intrinsic to imaging detection allows the measurement of additional alignment moments not previously reported. The measured alignment shows only qualitative agreement with the predictions of the kinematic apse conservation model.

Direct measurement of the binding energy of the NO dimer

The binding energy of the NO dimer has been measured directly using velocity-mapped ion imaging. NO dimer is photodissociated to produce NO(X) and NO(A), and the NO(A) is then nonresonantly ionized to NO+. The threshold for production of NO+ ions is measured at 44 893±2 cm−1, which corresponds to a binding energy of 696±4 cm−1.

Measurement of bipolar moments for photofragment angular correlations in ion imaging experiments

A general numerical method is given to extract angular correlations from photodissociation experiments with ion imaging detection. The angular correlations among the transition dipole moment of the parent molecule, μ, the photoproduct recoil velocity, v, and its angular momentum, j, are parametrized analytically using the semiclassical bipolar moment scheme due to Dixon. The method is a forward-convolution scheme which allows quantitative extraction of all measurable bipolar moments and can be applied in experiments with both linearly and circularly polarized probe light. It avoids the cylindrical symmetry limitations of the inverse Abel transform method, traditionally used for extracting photoproduct recoil anisotropy and speed distribution from imaging data. The method presented here also takes into account the possibility of multiple photodissociation channels. The features of the method are illustrated in a two-color 1+1′ REMPI-ion imaging study of the NO photoproduct trajectories resulting from the 6...

ION IMAGING THE RECOIL ENERGY DISTRIBUTION FOLLOWING VIBRATIONAL PREDISSOCIATION OF TRIPLET STATE PYRAZINE : AR VAN DER WAALS CLUSTERS

Abstract The ion imaging technique has been used to determine the recoil energy distribution of triplet pyrazine fragments following vibrational predissociation of pyrazine–Ar van der Waals clusters containing ∼4000 cm−1 of vibrational energy. The 2-dimensional image of the isotropic recoil distribution was converted to a radial velocity distribution using an analytical inverse Abel transform. The recoil probability distribution is found to be a monotonically decreasing function of energy, with an average recoil of ∼95 cm−1. Information theory is used to interpret the experimental results.

Rotational state-to-state differential cross sections for the HCl–Ar collision system using velocity-mapped ion imaging

Rotational state-resolved differential cross sections (DCSs) for the j-changing collisions of HCl by Ar are presented. A new crossed molecular beam velocity-mapped imaging apparatus is used to measure the full (θ=0–180°) DCS for j=0→j′=1, 2, ..., 6 rotational energy transfer at a center of mass energy of ∽538 cm−1. The j=0 state accounts for over 97% of the initial HCl rotational state population, and the scattering products are state-selectively ionized via (2+1) resonance enhanced multi-photon ionization through the E state, allowing for the direct extraction of state-to-state DCSs in the center of mass frame. The angular distributions for the experimental DCSs become increasingly backscattered as Δj increases, but do so non-monotonically, as j′=3 is more forward scattered than j′=2. Images for the even Δj 0→2 and 0→4 are similar, and those for the odd Δj 0→1 and 0→3 also have similarities. The calculated cross sections, based upon the HCl–Ar H6(4,3,0) potential of Hutson [J. Phys. Chem., 1992, 96, 4237], agree qualitatively with the experimental cross sections. However, there are significant differences between the theoretical and experimental results, where many of the principal features in the calculated DCSs lie 10–30° more backscattered than the same features in the experimental DCSs. These results may suggest that an adjustment to the repulsive region of the H6(4,3,0) potential is required.