Multiphoton Ionization/Dissociation of Molecular Sulfur S2 in the UV Region.

The multiphoton ionization/dissociation dynamics of molecular sulfur (S2) in the ultraviolet range of 205-300 nm is studied using velocity map ion imaging (VMI). In this one-color experiment, molecular sulfur (S2) is generated in a pulsed discharge and then photodissociated by UV radiation. At the three-photon level, superexcited states are accessed via two different resonant states: the B3Σu- (v' = 8-11) valence states at the one-photon level and a Rydberg state at the two-photon level. Among the decay processes of these superexcited states, dissociation to electronically excited S atoms is dominant as compared to autoionization to ionic states S2+ (X2Πg) at wavelengths λ < 288 nm. The anisotropy parameter extracted from these images reflects the parallel character of these electronic transitions. In contrast, autoionization is found to be particularly efficient at S(1D) and S(1S) detection wavelengths around 288 nm. Information obtained from the kinetic energy distributions of S atoms has revealed the existence of vibrationally excited S2+ (X2Πg (v+ > 11)) that dissociates to ionic products following one-photon absorption. This work also reveals many interesting features of S2 photodynamics compared to those of electronically analogous O2.

Photoionization cross sections measurements of the excited states of lutetium and ytterbium in the near threshold region.

In this work, the threshold photoionization cross sections from the excited states of lutetium and ytterbium atoms were investigated by the laser pump-probe scheme under the condition of saturated resonant excitation. We obtained the resonance enhanced multiphoton ionization spectra of the lutetium and ytterbium atoms of the lanthanide metals in the range of 307.50-312.50 nm and 265.00-269.00 nm, respectively; the photoionization cross sections of the 5d6s(1D)6p(2D05/2) and 5d6s(3D)6p(2P01/2) states of lutetium and the 4f13(2F0)5d6s2(J = 1) states of ytterbium above threshold regions (0.4-1.6 eV) were measured, and measured values ranged from 2.3 ± 0.2 to 17.7 ± 1.5 Mb.

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.

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. A simple and robust (1 + 1' + 1'') VUV (Vacuum Ultra-Violet) REMPI (Resonance Enhanced Multi Photon Ionization) scheme is used and images are obtained by setting the VUV light polarization direction parallel or perpendicular to the scattering plane. Clear differences between the images for the two polarizations are observed, indicating strong collision induced alignment of the rotational angular momentum of scattered CO. A direct image analysis procedure as described in our previously published paper (A. G. Suits et al., J. Phys. Chem. A., 2015, 119, 5925), is employed to extract the fully quantum state resolved alignment-free differential cross sections (DCSs) and the state-to-state angle-dependent alignment moments for each final rotational state. The experimental results are compared with advanced theory, in particular with the predictions of CC QM (Close-Coupling Quantum Mechanical) and QCT (Quasi-Classical Trajectory) calculations. The agreement between experiment and theory is generally found to be quite good throughout the entire scattering angle range for all the final states probed, showing the reliability of the experiment and use of the direct extraction method, as well as the accuracy of the potential surface over the studied collision energy range. A classical kinematic apse (hard shell) model was found to be useful in interpreting the measured collision induced alignment moments.

Dissociation of internal energy-selected methyl bromide ion revealed from threshold photoelectron-photoion coincidence velocity imaging.

Dissociative photoionization of methyl bromide (CH3Br) in an excitation energy range of 10.45-16.90 eV has been investigated by using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. The coincident time-of-flight mass spectra indicate that the ground state X(2)E of CH3Br(+) is stable, and both A(2)A1 and B(2)E ionic excited states are fully dissociative to produce the unique fragment ion of CH3 (+). From TPEPICO 3D time-sliced velocity images of CH3 (+) dissociated from specific state-selected CH3Br(+) ion, kinetic energy release distribution (KERD) and angular distribution of CH3 (+) fragment ion are directly obtained. Both spin-orbit states of Br((2)P) atom can be clearly observed in fast dissociation of CH3Br(+)(A(2)A1) ion along C-Br rupture, while a KERD of Maxwell-Boltzmann profile is obtained in dissociation of CH3Br(+)(B(2)E) ion. With the aid of the re-calculated potential energy curves of CH3Br(+) including spin-orbit coupling, dissociation mechanisms of CH3Br(+) ion in A(2)A1 and B(2)E states along C-Br rupture are revealed. For CH3Br(+)(A(2)A1) ion, the CH3 (+) + Br((2)P1/2) channel is occurred via an adiabatic dissociation by vibration, while the Br((2)P3/2) formation is through vibronic coupling to the high vibrational level of X(2)E state followed by rapid dissociation. C-Br bond breaking of CH3Br(+)(B(2)E) ion can occur via slow internal conversion to the excited vibrational level of the lower electronic states and then dissociation.

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.