We present a novel means of analyzing velocity-map images of angular momentum polarization in inelastic scattering. In this approach, linear combinations of angular distributions obtained by integrating select regions of images for two probe laser polarizations directly yield the alignment-free differential cross sections and the differential alignment moments. No fitting is needed in the analysis. The method relies on the fact that the angular distribution for out-of-plane scattering is encoded in the distribution along the relative velocity vector, and this may be recovered quantitatively owing to the redundancy of the in-plane and out-of-plane scattering for the horizontal polarization case.
angular distributions. For small changes in the rotational angular momentum quantum number (j), the ND 3 is predominantly forward scattered, but the scattering shifts to the sideways and backward directions as ∆j increases. For scattering into a given ݆Ԣ ᇱ േ state, crosssections for collisions that conserve the +/-symmetry associated with the ND 3 inversion vibration are larger and generally more forward scattered than the corresponding symmetry-changing processes.
A joint theoretical and experimental study of state-to-state rotationally inelastic polarization dependent differential cross sections (PDDCSs) for CO (v = 0, j = 0, 1, 2) molecules colliding with helium is reported for collision energies of 513 and 840 cm(-1). In a crossed molecular beam experiment, velocity map imaging (VMI) with state-selective detection by (2 + 1) and (1 + 1') resonance enhanced multiphoton ionization (REMPI) is used to probe rotational excitation of CO due to scattering. By taking account of the known fractions of the j = 0, 1, and 2 states of CO in the rotationally cold molecular beam (Trot ≈ 3 K), close-coupling theory based on high-quality ab initio potential energy surfaces for the CO-He interaction is used to simulate the differential cross sections for the mixed initial states. With polarization-sensitive 1 + 1' REMPI detection and a direct analysis procedure described by Suits et al. ( J. Phys, Chem. A 2015 , 119 , 5925 ), alignment moments are extracted from the images and the latter are compared with images simulated by theory using the calculated DCS and alignment moments. In general, good agreement of theory with the experimental results is found, indicating the reliability of the experiment in reproducing state-to-state differential and polarization-dependent differential cross sections.
The inelastic scattering of HO by He as a function of collision energy in the range 381 cm to 763 cm at an energy interval of approximately 100 cm has been investigated in a crossed beam experiment using velocity map imaging. Change in collision energy was achieved by varying the collision angle between the HO and He beam. We measured the state-to-state differential cross section (DCS) of scattered HO products for the final rotational states J = 1, 1, 2 and 4. Rotational excitation of HO is probed by (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectroscopy. DCS measurements over a wide range of collision energies allowed us to probe the HO-He potential energy surface (PES) with greater detail than in previous work. We found that a classical approximation of rotational rainbows can predict the collision energy dependence of the DCS. Close-coupling quantum mechanical calculations were used to produce DCS and partial cross sections. The forward-backward ratio (FBR), is introduced here to compare the experimental and theoretical DCS. Both theory and experiments suggest that an increase in the collision energy is accompanied with more forward scattering.
Molecular oxygen (O2) is extremely important for a wide variety of processes on and outside Earth. Indeed, O2–He collisions are crucial to model O2 abundance in space or to create ultracold O2 molecules. A crossed molecular beam experiment to probe rotational excitation of O2 due to helium collisions at energies of 660 cm–1 is reported. Velocity map imaging was combined with state-selective detection of O2(X3Σg–) by (2+1) resonance-enhanced multiphoton ionization. The obtained raw O2+ images were corrected from density to flux and the differential cross sections (DCS) were then extracted for six O2 final states. Exact quantum mechanical calculations were also performed. A very good agreement between experimental and theoretical DCSs was found by using an initial O2 beam population ratio of 80% for the first rotational state and 20% for the first excited state. The agreement demonstrates our ability to model inelastic processes between O2 molecules and rare gas both theoretically and experimentally.
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