A simple resonance enhanced laser ionization scheme for CO via the A1Π state

Imaging rotational energy transfer: comparative stereodynamics in CO + N2 and CO + CO inelastic scattering

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.

Dynamics and vector correlations of vacuum ultraviolet (VUV) photodissociation of CO2 at 155 nm

In this work, the CO₂ Vacuum Ultraviolet (VUV) photodissociation dynamics of the dominant O(¹D) channel near 155 nm have been studied using Velocity Map Imaging (VMI) technique. Correlations among the transition dipole moment of the parent molecule, recoil velocity vector and rotational angular momentum vector of the photofragments were extracted from the anisotropic angular distributions of the images. The vector correlations extracted indicated a picture of photodissociation mainly via the excited 2¹A' (A) state. The transition dipole moment  lies in the bending molecular plane, and the j⃑ is pointing perpendicular to the plane, while the μ-v vectors angle is between 41°-45°. In addition, a clear trend was observed. As the product CO rotational state j increases, the spatial anisotropy parameter (β ≡ 2β20(20)) decreases. This j-dependent attenuation of spatial anisotropy parameter can be explained mainly with the consideration of non-axial recoil effect. These results are in good agreement with both theoretical work and previous experimental work.

High-Resolution Imaging of C + He Collisions using Zeeman Deceleration and Vacuum-Ultraviolet Detection

High-resolution measurements of angular scattering distributions provide a sensitive test for theoretical descriptions of collision processes. Crossed beam experiments employing a decelerator and velocity map imaging have proven successful to probe collision cross sections with extraordinary resolution. However, a prerequisite to exploit these possibilities is the availability of a near-threshold state-selective ionization scheme to detect the collision products, which for many species is either absent or inefficient. We present the first implementation of recoil-free vacuum ultraviolet (VUV) based detection in scattering experiments involving a decelerator and velocity map imaging. This allowed for high-resolution measurements of state-resolved angular scattering distributions for inelastic collisions between Zeeman-decelerated carbon C(³P₁) atoms and helium atoms. We fully resolved diffraction oscillations in the angular distributions, which showed excellent agreement with the distributions predicted by quantum scattering calculations. Our approach offers exciting prospects to investigate a large range of scattering processes with unprecedented precision.

Molecular square dancing in CO-CO collisions

Knowledge of rotational energy transfer (RET) involving carbon monoxide (CO) molecules is crucial for the interpretation of astrophysical data. As of now, our nearly perfect understanding of atom-molecule scattering shows that RET usually occurs by only a simple "bump" between partners. To advance molecular dynamics to the next step in complexity, we studied molecule-molecule scattering in great detail for collision between two CO molecules. Using advanced imaging methods and quasi-classical and fully quantum theory, we found that a synchronous movement can occur during CO-CO collisions, whereby a bump is followed by a move similar to a "do-si-do" in square dancing. This resulted in little angular deflection but high RET to both partners, a very unusual combination. The associated conditions suggest that this process can occur in other molecule-molecule systems.

Imaging inelastic scattering of CO with argon: polarization dependent differential cross sections

Rotationally inelastic scattering of carbon monoxide (CO) with argon at a collision energy of 700 cm⁻¹ has been investigated by measuring polarization dependent differential scattering cross sections (PDDCSs) for rotationally excited CO molecules using a crossed molecular beam apparatus coupled with velocity-map ion imaging.

Imaging multiphoton ionization and dissociation of rotationally warm CO via the B+Σ1 and EΠ1 electronic states

Pathways for formation of C⁺ and O⁺ ions when applying (2 + 1) resonance enhanced multiphoton ionization (REMPI) of CO via the B¹Σ⁺ and E¹Π electronic states are characterized with the velocity map imaging technique. By employing an unskimmed pulsed valve, it was possible to obtain sharp images for a wide range of initial CO J-states. Most of the atomic ion production pathways could be assigned as one- or two-photon dissociation of a series of vibrational levels of the CO⁺ X²Σ⁺ and A²Π states. Large enhancements in dissociation of particular CO⁺ vibrational states in these progressions could be accurately assigned to accidental resonances of the REMPI laser with CO⁺ X²Σ⁺-B²Σ⁺ transitions.