ABSTRACT
State-to-state rotational energy transfer in collisions of ground ro-vibrational state 13CO molecules with N2 molecules has been studied using the crossed molecular beam method under kinematically equivalent conditions used for 13CO + CO rotationally inelastic scattering described in a previously published report (Sun et al., Science, 2020, 369, 307-309). The collisionally excited 13CO molecule products are detected by the same (1 + 1' + 1'') VUV (Vacuum Ultra-Violet) resonance enhanced multiphoton ionization scheme coupled with velocity map ion imaging. We present differential cross sections and scattering angle resolved rotational angular momentum alignment moments extracted from experimentally measured 13CO + N2 scattering images and compare them with theoretical predictions from quasi-classical trajectories (QCT) on a newly calculated 13CO-N2 potential energy surface (PES). Good agreement between experiment and theory is found, which confirms the accuracy of the 13CO-N2 potential energy surface for the 1460 cm-1 collision energy studied by experiment. Experimental results for 13CO + N2 are compared with those for 13CO + CO collisions. The angle-resolved product rotational angular momentum alignment moments for the two scattering systems are very similar, which indicates that the collision induced alignment dynamics observed for both systems are dominated by a hard-shell nature. However, compared to the 13CO + CO measurements, the primary rainbow maximum in the DCSs for 13CO + N2 is peaked consistently at more backward scattering angles and the secondary maximum becomes much less obvious, implying that the 13CO-N2 PES is less anisotropic. In addition, a forward scattering component with high rotational excitation seen for 13CO + CO does not appear for 13CO-N2 in the experiment and is not predicted by QCT theory. Some of these differences in collision dynamics behaviour can be predicted by a comparison between the properties of the PESs for the two systems. More specific behaviour is also predicted from analysis of the dependence on the relative collision geometry of 13CO + N2 trajectories compared to 13CO + CO trajectories, which shows the special 'do-si-do' pathway invoked for 13CO + CO is not effective for 13CO + N2 collisions.
ABSTRACT
Photofragment imaging is shown to provide a sensitive method for detection of the O2 A'3Δu Herzberg III state using a one-laser dissociation/O(1D) resonance enhanced multiphoton ionization (REMPI) scheme with a focused nanosecond dye laser beam tuned to 203.8 or 205.2 nm, combined with velocity map imaging of the atomic oxygen photofragment. O2 populated in the Herzberg states is generated by photodesorption at 250 nm of solid O2 ice held at 15 K and by an electric discharge in a pulsed molecular beam of pure O2. Ice photo-desorption results in Herzberg state products with higher translational, vibrational and rotational energy spreads, yielding the same signal as the discharge source but with lower velocity resolution. A clear signal with parallel character (ß â¼ 0.9) assigned to dissociation of O2 A'3Δu(v = 0, 1 Ω = 1) was observed when using a pulsed electric discharge source under specific 'cold' conditions with O(1D) detection, driving one-photon dissociation around 205 nm. No products corresponding to O2 A'3Δu state dissociation were observed for 225.625 or 200.32 nm dissociation with O(3P2) detection, which implies that the O2 A'3Δu state dissociates exclusively to the third (O1D + O1D) dissociation limit. Dissociation is suggested to take place through the 21Πg upper state to the O1D + O1D limit where spin-orbit coupling of the A'3Δu state with the 11Πu state accesses the allowed parallel 1Πu â 1Πg transition. While the absence of a parallel-type photodissociation signal from the c1Σ-u state may be expected, the A3Σ+u should spin-orbit couple through the same pathway as the A'3Δu state. The fact that no clear A3Σ+u signal is observed suggests a faster deactivation process compared to the A'3Δu state in the discharge and ice desorption process.
