Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 68, Number 7
Monday–Friday, June 5–9, 2023; Spokane, Washington
Session C11: V: Spectroscopy, Photoionization and CollisionsVirtual Only

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Chair: Imran Mirza, Miami University Room: Virtual Platform 
Tuesday, June 6, 2023 10:45AM  10:57AM 
C11.00001: CHARACTERISTIC FEATURES IN PHOTOIONIZATION OF Cl III Sultana N Nahar Chlorine lines have not been observed abundantly in astrophysical spectra as it is a highly reactive element and abundance is lower than those with even number of atomic number. However, they are being observed more and used for determination of physical conditions and chemical evolution processes in astronomical objects. The present study will report characteristic features of photoionization, dependent on the states, of Cl III that can impact on the line formation and hence in determination of various 
Tuesday, June 6, 2023 10:57AM  11:09AM 
C11.00002: RMatrix calculations of astrophysical opacities (RMOP) Anil Pradhan, Sultana N Nahar Abundances of elements in the Universe are uncertain. Even in the Sun 
Tuesday, June 6, 2023 11:09AM  11:21AM 
C11.00003: Capturing the isotope effects in the vibrationally resolved electronic spectra with ab initio semiclassical dynamics Eriks Kletnieks, Yannick C Alonso, Jiri Vanicek The thawed Gaussian approximation [1] has been shown to accurately reproduce the vibrationally resolved electronic spectra of molecules with weakly anharmonic potential energy surfaces [2,3,4]. Here, we demonstrate that the thawed Gaussian approximation can capture even more subtle observables, such as the isotope effects on such spectra, and we use four isotopologues of ammonia as an example. As a single trajectory semiclassical method, the thawed Gaussian approximation benefits from the onthefly ab initio implementation [2,4], in which the energy, gradient, and Hessian of the potential energy surface is evaluated only locally, avoiding the task of precomputing global surfaces. Whereas the standard global harmonic approximations fail due to the high anharmonicity of the first excited electronic state of ammonia, the experimental spectra of ammonia isotopologues (NH_{3}, NDH_{2}, ND_{2}H, ND_{3}) are very well reproduced by the semiclassical onthefly calculations. Moreover, the isotope effects  narrowing of the transition band, shift in the 00 transition, and decreasing of the peak spacing  are all captured by this semiclassical method. The activation of different normal modes upon excitation in the case of each isotopologue sheds the light on the elusiveness of the symmetric stretch progression in the spectra. 
Tuesday, June 6, 2023 11:21AM  11:33AM 
C11.00004: Efficient integrators for the variational Gaussian wavepacket dynamics with applications to tunneling and vibronic spectra Roya Moghaddasi Fereidani, Jiri Vanicek Accurate and efficient evaluation of vibronic spectra of polyatomic molecules remains a challenge of molecular and optical physics. In the timedependent approach to spectroscopy [1], the variational Gaussian wavepacket dynamics [2] is the most accurate method among singletrajectory Gaussianbased methods for solving the timedependent Schrödinger equation. In contrast to Heller’s original thawed Gaussian approximation [3], which has been successfully combined with onthefly ab initio electronic structure [4, 5], the variational method is symplectic, conserves energy exactly, and partially takes into account tunneling. However, the variational method is also much more expensive. To improve its efficiency, we symmetrically compose the secondorder symplectic integrator of Faou and Lubich [6] and obtain geometric integrators that can achieve an arbitrary even order of convergence in the time step. We demonstrate that the highorder integrators can drastically speed up convergence compared to the secondorder algorithm (we show an example, where this speedup is by a factor of 100 if a moderate accuracy of 10^{6} is required for the wavefunction). Moreover, in contrast to the popular fourthorder RungeKutta method, the proposed integrators are timereversible and conserve the norm and the symplectic structure exactly, regardless of the time step. To show that the method is not restricted to lowdimensional systems, we perform most of the analysis on a nonseparable twentydimensional model of coupled Morse oscillators. We also show that the variational method can include tunneling and, in calculation of vibronic spectra, improves accuracy over the nonvariational thawed Gaussian approximation. 
Tuesday, June 6, 2023 11:33AM  11:45AM 
C11.00005: Capturing tunneling, wavepacket splitting, and single vibronic level excitation effects on vibronic spectra with Hagedorn wavepackets Zhan Tong Zhang, Jiri Vanicek

Tuesday, June 6, 2023 11:45AM  11:57AM 
C11.00006: Atomic and Molecular Spectral Models of Biosignatures in Exoplanetary Atmospheres Anil Pradhan, Michael Rothman, Dax Begeny, Sultana N Nahar, Kevin Hoy As thousands of extrasolar planets are being discovered the next big step is the search for life as we might understand. Spectral analysis of atomicionicmolecular features is the primary mechanism for detection of biosignature elements H, C, N, O, P and S. We have adapted a generalpurpose software package as a new toolkit Geant4EXOP (G4EXOP), for modeling hoststar radiation transmission through planetary atmospheric layers. G4EXOP uses eclipse geometry with radiation propagated through various layers of exoplanetary atmosphere to analyze potential biosignature spectral features and abundances. G4EXOP is based on Geant4, a Monte Carlo program package enabling modeling of radiation and particle transmission through matter. Our toolkit uses atomic and molecular data to generate emission and absorption spectra to ascertain abundances from line strengths. We focus on atomicionic biosignatures (H, C, N, etc.) in emission, superimposed on and molecular absorption from common molecules H_{2}O, CO_{2}, CH_{4}, etc. Phosphorus in particular is a focus of these studies, as it is vital for DNAbased life. We calculate atomic transition data using SUPERSTRUCTURE and Rmatrix codes and report new results for P, C, O and other elements. We obtain molecular data from the database ExoMol using calculated oscillator strengths for H_{2}O and CO_{2}. Numerical experiments are carried out for different temperaturedensity regimes, and broadening mechanisms to elicit spectral features in wavelength ranges detectable from groundbased telescopes or spacebased observatories such as the James Webb Space Telescope. 
Tuesday, June 6, 2023 11:57AM  12:09PM 
C11.00007: Simple rateequation model for polarizationdependent probeabsorption in Dopplerbroadened ^{87}Rb vapor Rajni B Upadhyay, Joyee Ghosh, Vivek Venkataraman In a multilevel atomic vapor such as Rb, Zeeman pumping (within the magnetic sublevels of the "bright" hyperfine ground state) and hyperfine pumping (to another "dark" hyperfine ground state) compete to determine the absorption, even for a weak probe (intensity<< I_{sat0} , where I_{sat0} is the saturation intensity predicted by the conventional 2level model) [13]. Zeeman pumping depends on the probe polarization, whereas hyperfine pumping is independent of it. For the Doppler broadened ^{85}Rb D_{2} line, two analytical approaches derived from the full multilevel densitymatrix model [1] and rate equation model [2] have been used to evaluate these effects. However, these models have limitations, deviating considerably from the experimental data and/or valid only for linearly or circularly polarized probes. Here, we present an abinitio but simple, reduced 7level rate equation model (∼100Χ faster/less memory intensive than full multilevel models) that includes the dependence of probe absorption on its intensity, polarization, and beam diameter (via the transitrelaxation) to consider the effect of hyperfine pumping and Zeeman pumping. Our model predictions are experimentally verified for the ^{87}Rb D_{2} line, without any fitting parameters (residual error<5%), for various input probe polarizations (linear, elliptical, and circular) at probe intensities ranging from ∼0.01I_{sat0} to I_{sat0}. The interplay of hyperfine and Zeeman pumping is decided by whether the atomic transitions being probed are primarily closed or open. 
Tuesday, June 6, 2023 12:09PM  12:21PM 
C11.00008: Theoretical Study of Coincident Ionization Crosssections of Atoms and Molecules by Twisted Electron Impact Nikita Dhankhar, Rakesh Choubisa The electron impact ionization process drives various physical processes. The geometry and kinematics of the (e, 2e) and (e, 3e) (single and double ionization) processes help one to study the dynamics of the collision process and also to probe the structure of the target atom, molecule, thin film, or surface. The generation of electron vortex (or twisted) beams (EVBs) has attracted much attention from both experimental and theoretical sides. In contrast to conventional (planewave) electron beams, twisted electron beams possess a nonzero projection of the orbital angular momentum (OAM) m onto their propagation direction. The intrinsic angular momentum of the electron vortex beam influences the role of the beam in the ionization process. To explore the influence of the kinematics of EVBs on the ionization processes, we theoretically studied the (e, 2e) and (e, 3e) processes on different atomic and molecular targets. We investigated the angular profiles of the triple differential crosssections (TDCS) and fivefold differential crosssections (FDCS) for different atomic and molecular targets for an incident twisted electron beam within the first Born approximation (FBA). We also present the average overimpact parameter crosssection ((TDCS)_{av}) for the (e, 2e) processes. In the coplanar asymmetric geometry, we studied the twisted electron (e, 2e) process on noble gas atoms, H_{2},_{ }H_{2}O, CH_{4}, and NH_{3} molecules. We also investigated the angular profiles in the coplanar geometry for θvariable and constant θ_{12} mode for the (e, 3e) process on the He atom. We examined the angular profiles of the TDCS for the atomic and molecular targets for the OAM number m and the opening angle θ_{p} of the twisted electron beam for the coplanar asymmetric geometry. The theoretical investigations on the different atomic and molecular targets revealed that the twisted electron parameters m and θ_{p }strongly influence the angular distribution of TDCS and FDCS in the given coplanar geometries. 
Tuesday, June 6, 2023 12:21PM  12:33PM 
C11.00009: Biosignature Line Ratios of [P II] in Exoplanetary and Nebular Environments Kevin Hoy, Sultana N Nahar, Anil Pradhan Being the backbone element of DNA, phosphorus is a key component in the search for life in the Universe. To aid in its detection, we present line emissivity ratios for the five lowestlying forbidden [P II] transitions among levels 3s^{2}3p^{2} (^{3}P_{0}, ^{3}P_{1}, ^{3}P_{2}, ^{1}D_{2}, ^{1}S_{0}) within wavelengths range of 0.4470 µm where several lie within the spectroscopic domain of observation of James Webb Space Telescope. These line ratios have been obtained using a new collisionalradiativerecombination (CRR) model that incorporates a new scheme for the total levelspecific (e+ion) recombination rate coefficients from the unified method of Nahar and Pradhan. CRR implements electron impact excitation (EIE) rate coefficients calculated from the accurate BreitPauli RMatrix method. We will report its effect compared to that obtained using only electron impact excitation and spontaneous radiative decay rates. We find that (e + ion) recombination has a significant impact on all line ratios, and represents a major improvement in the physical accuracy of emission line models. 
Tuesday, June 6, 2023 12:33PM  12:45PM 
C11.00010: In search of a new SF_{6 }alternative gas: crosssections, transport coefficients and breakdown characteristics Nidhi Sinha, MiYoung Song, Hyonu Chang, Heechol Choi, HyunJae Jang, YeonHo Oh, KiDong Song SF_{6} is one of the popular choices for gasinsulated applications. However, its global warming potential is very high and hence there is a need for an ecofriendly alternative [1]. A suitable alternative will have high ionization energy and high electronegativity. Further, to restrict the excitations, the orbital band gap needs to be high. Also, the liquefaction temperature should be low enough to maintain its gaseous state. To check the environmental impact, the gas should be nontoxic and its global warming potential (GWP) should be low. Further, the electron attachment capacity should be high. The dielectric strength and breakdown voltage should obviously be high to suit the insulation applications. 
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