arXiv subjects

Paul J. Dagdigian

Publications and source records attributed to Paul J. Dagdigian.

3 recordsLinked to original sources

arXiv2023

Imaging Resonance Effects in C + H$_2$ Collisions using a Zeeman Decelerator

An intriguing phenomenon in molecular collisions is the occurrence of scattering resonances, which originate from bound and quasi-bound states supported by the interaction potential at low collision energies. The resonance effects in the scattering behaviour are extraordinarily sensitive to the interaction potential, and their observation provides one of the most stringent tests for theoretical models. We present high-resolution measurements of state-resolved angular scattering distributions for inelastic collisions between Zeeman-decelerated C($^3P_1$) atoms and $\textit{para}$-H$_2$ molecules at collision energies ranging from 77 cm$^{-1}$ down to 0.5 cm$^{-1}$. Rapid variations in the angular distributions were observed that can be attributed to the consecutive reduction of contributing partial waves and effects of scattering resonances. The measurements showed excellent agreement with distributions predicted by $\textit{ab initio}$ quantum scattering calculations. However, discrepancies were found at specific collision energies, which most likely originate from an incorrectly predicted quasi-bound state. These observations provide exciting prospects for further high-precision and low-energy investigations of scattering processes that involve paramagnetic species.

physics.atom-ph↗

arXiv2023

Interaction of CH$_3$CN and CH$_3$NC with He : potential energy surfaces and low-energy scattering

Several nitrogen-bearing molecules, such as methyl cyanide (or acetonitrile, CH$_3$CN) and methyl isocyanide (CH$_3$NC) of interest here, have been observed in various astrophysical environments. The accurate modeling of their abundance requires the calculation of rate coefficients for their collisional excitation with species such as He atoms or H$_2$ molecules at low temperatures. In this work we compute new three-dimensional potential energy surfaces for the CH$_3$NC-He and CH$_3$CN-He van der Waals complexes by means of the explicitly correlated coupled cluster approach with single, double and perturbative triple excitation CCSD(T)/F12a in conjunction with the aug-cc-pVTZ basis set. We find a global minimum with $D_e= 55.10$ and 58.61 cm$^{-1}$ for CH$_3$CN-He and CH$_3$NC-He, respectively, while the dissociation energy $D_0$ of the complexes are 18.64 and 18.65 cm$^{-1}$, respectively. Low energy scattering calculations of pure rotational (de-)excitation of CH$_3$CN and CH$_3$NC by collision with He atoms are carried out with the close-coupling method and the collisional cross sections of $ortho-$ and $para-$CH$_3$NC and CH$_3$CN are computed for kinetic energies up to 100 cm$^{-1}$. While the PESs for both complexes are qualitatively similar, that of CH$_3$NC-He is more anisotropic, leading to different propensity rules for rotational excitation. For CH$_3$NC-He, we find that |$Δj$| = 1 transitions are dominant at low kinetic energy and a propensity rule that favors odd $Δj$ transitions is observed, whereas for CH$_3$CN the dominant cross sections are associated to transitions with |$Δj$| = 2. We expect that the findings of this study will be beneficial for astrophysical investigations as well as laboratory experiments.

physics.chem-ph↗

arXiv1997

A collaborative theoretical and experimental study of the structure and electronic excitation spectrum of the BAr and B(Ar)2 complexes

We report the investigation of the 3s <- 2p transition in the BAr2 cluster. In a supersonic expansion of B atoms entrained in Ar, at high beam source backing pressures we observe several features in the fluorescence excitation spectrum which cannot be assigned to the BAr diatom. Using BAr(X, B) potential energy curves which reproduce our experimental observations on this molecule and an Ar-Ar interaction potential, we employ a pairwise additive model, along with variational and diffusion Monte-Carlo treatments of the nuclear motion, to determine the lowest vibrational state of the BAr2 cluster. A subsequent simulation of the fluorescence excitation spectrum reproduces nearly quantitatively the strongest feature in our experimental spectrum not assignable to BAr. Because of the barrier in the BAr(B 2Sigma+) potential energy curve, the 3s <- 2p transition in the BAr2 cluster is predicted to have an asymmetric profile, as is found experimentally.

physics.chem-ph↗