Session C11: V: Spectroscopy, Photoionization and Collisions

CHARACTERISTIC FEATURES IN PHOTOIONIZATION OF Cl III

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

quantities relevant to the object. The present study has been carried out using the state-of-the-art relativistic Breit-Pauli R-matrix method with close coupling (CC) approximation. A CC wavefunction expansion of Cl III includes 45-levels of the core ion with excitation going up to 4s from configurations $3s^23p^2$, $3s3p^3$, $3s^23p3d$, $3s^23p4s$, $3s3p^23d$, and $3p^4$. The low lying levels showed features that benchmarked those observed using Advance light Source experiment at LBNL

(Nahar et al 2023). The present report will elaborate those with other features, such as of low and high lying excited levels, of the process from a study of photoionization cross sections of 890 bound levels of Cl III with n$leq$10 and $lleq$9.   

R-Matrix calculations of astrophysical opacities (RMOP)

Abundances of elements in the Universe are uncertain. Even in the Sun

the discrepancies are up to 30-40% for common elements C, N, O and Ne.

The primary problem is the intricate interplay of atomic and plasma

physics of opacities that govern radiation transport.

In order to address this "solar problem", and discrepancies among

current theoretical models and experiments,

an extended version of the R-matrix methodology is employed to compute

iron opacity accurately, especially due to

prominent Fe ions that contribute significantly to overall opacity at

the radiative/convection zones boundary in the Sun.

In addition to atomic calculations of heretofore unprecedented

complexity using the Breit-Pauli R-Matrix method (BPRM), a new

theoretical and computational formalism has been developed to

incorporate plasma effects due to thermal, collisional, Stark,

free-free transitions and core excitations, as function of temperatures

and densities, particularly those prevailing in high-energy-density (HED)

plasmas.

    

Capturing the isotope effects in the vibrationally resolved electronic spectra with ab initio semiclassical dynamics

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 on-the-fly 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 (NH₃, NDH₂, ND₂H, ND₃) are very well reproduced by the semiclassical on-the-fly calculations. Moreover, the isotope effects - narrowing of the transition band, shift in the 0-0 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.

 

[1] E. J Heller, .J. Chem. Phys. 62, 1544 (1975)

[2] M. Wehrle, M. Sulc, and J. Vanícek, J. Chem. Phys. 140, 244114 (2014)

[3] M. Wehrle, S. Oberli, and J. Vanícek, J. Phys. Chem. A 119, 5685 (2015)

[4] T. Begušic, E. Tapavicza, J. Vanícek, J. Chem. Theory Comput. 18, 3065 (2022)

    

Efficient integrators for the variational Gaussian wavepacket dynamics with applications to tunneling and vibronic spectra

Accurate and efficient evaluation of vibronic spectra of polyatomic molecules remains a challenge of molecular and optical physics. In the time-dependent approach to spectroscopy [1], the variational Gaussian wavepacket dynamics [2] is the most accurate method among single-trajectory Gaussian-based methods for solving the time-dependent Schrödinger equation. In contrast to Heller’s original thawed Gaussian approximation [3], which has been successfully combined with on-the-fly 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 second-order 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 high-order integrators can drastically speed up convergence compared to the second-order 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 fourth-order Runge-Kutta method, the proposed integrators are time-reversible and conserve the norm and the symplectic structure exactly, regardless of the time step. To show that the method is not restricted to low-dimensional systems, we perform most of the analysis on a non-separable twenty-dimensional 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 non-variational thawed Gaussian approximation.

[1] E. J. Heller, The semiclassical way to dynamics and spectroscopy (Princeton University Press, Princeton, NJ, 2018).

[2] R. D. Coalson and M. Karplus, J. Chem. Phys. 93, 3919 (1990).

[3] E. J. Heller, J. Chem. Phys. 62, 1544 (1975).

[4] M. Wehrle, M. Šulc, and J. Vani´cek, J. Chem. Phys. 140, 244114 (2014).

[5] T. Begušic, E. Tapavicza and J. Vani´cek, J. Chem. Theory Comput. 18, 3065 (2022).

[6] E. Faou and C. Lubich, Comput. Visual. Sci. 9, 45 (2006).   

Capturing tunneling, wavepacket splitting, and single vibronic level excitation effects on vibronic spectra with Hagedorn wavepackets

Vibrationally resolved electronic spectroscopy provides important information on the structure and dynamics of polyatomic molecules. On-the-fly ab initio implementation [1,2,3] of the thawed Gaussian approximation (TGA) [4] has been successful in describing such spectra within the Condon approximation but cannot capture non-Condon effects, tunneling or wavepacket splitting. While the extended TGA [5,6] captures first-order non-Condon effects, Hagedorn’s wavepackets [7,8], i.e. superposition of states, each of which is the Gaussian multiplied by a cleverly chosen polynomial, enable the propagation of non-Condon wavepackets of arbitrary shapes. I will demonstrate that a variational implementation [9] of the Hagedorn wavepackets can capture tunneling and wavepacket splitting, as well as their effects on spectra. To demonstrate that the method is not restricted to small model systems, I will combine our recently developed algorithm for an algebraic evaluation of the overlap of Hagedorn wavepackets with on-the-fly ab initio semiclassical dynamics to compute single vibronic level fluorescence spectra of polyatomic molecules.

[1] M. Wehrle, M. Sulc, and J. Vanicek, J. Chem. Phys. 140, 244114 (2014)

[2] M. Wehrle, S. Oberli, and J. Vanicek, J. Phys. Chem. A 119, 5685 (2015)

[3] T. Begusic, E. Tapavicza, and J. Vanicek, JCTC 18, 3065 (2022)

[4] E. J. Heller, J. Chem. Phys. 62, 1544 (1975)

[5] A. Patoz, T. Begusic, and J. Vanícek, J. Phys. Chem. Lett. 9, 2367 (2018)

[6] A. Prlj, T. Begusic, Z. T. Zhang, G. C. Fish, M. Wehrle, T. Zimmermann, S. Choi, J. Roulet, J.-E. Moser, and J. Vanicek, JCTC 16, 2617 (2020)

[7] G. A. Hagedorn, Ann. Physics 269, 77 (1998)

[8] C. Lasser and C. Lubich, Acta Numerica 29, 229 (2020)

[9] E. Faou, V. Gradinaru, C. Lubich, SIAM J. Sci. Compt. 31, 3027 (2009)   

Atomic and Molecular Spectral Models of Biosignatures in Exoplanetary Atmospheres

As thousands of extra-solar planets are being discovered the next big step is the search for life as we might understand. Spectral analysis of atomic-ionic-molecular features is the primary mechanism for detection of biosignature elements H, C, N, O, P and S. We have adapted a general-purpose software package as a new toolkit Geant4-EXOP (G4-EXOP), for modeling host-star radiation transmission through planetary atmospheric layers. G4-EXOP uses eclipse geometry with radiation propagated through various layers of exoplanetary atmosphere to analyze potential biosignature spectral features and abundances. G4-EXOP 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 atomic-ionic biosignatures (H, C, N, etc.) in emission, superimposed on and molecular absorption from common molecules H₂O, CO₂, CH₄, etc. Phosphorus in particular is a focus of these studies, as it is vital for DNA-based life. We calculate atomic transition data using SUPERSTRUCTURE and R-matrix 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 H₂O and CO₂. Numerical experiments are carried out for different temperature-density regimes, and broadening mechanisms to elicit spectral features in wavelength ranges detectable from ground-based telescopes or space-based observatories such as the James Webb Space Telescope.   

Simple rate-equation model for polarization-dependent probe-absorption in Doppler-broadened 87Rb vapor

In a multi-level atomic vapor such as Rb, Zeeman pumping (within the magnetic sub-levels 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 2-level model) [1-3]. Zeeman pumping depends on the probe polarization, whereas hyperfine pumping is independent of it. For the Doppler broadened ⁸⁵Rb D₂ line, two analytical approaches derived from the full multi-level density-matrix 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 ab-initio but simple, reduced 7-level rate equation model (∼100Χ faster/less memory intensive than full multi-level models) that includes the dependence of probe absorption on its intensity, polarization, and beam diameter (via the transit-relaxation) to consider the effect of hyperfine pumping and Zeeman pumping. Our model predictions are experimentally verified for the ⁸⁷Rb D₂ 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.

[1] Shin S R and Noh H R 2011 Optics Communications 284 1243–1246.

[2] Shin S R and Noh H R 2009 Journal of the Physical Society of Japan 78 084302–084302

[3] Bala R et al. 2022 Journal of Physics B: Atomic, Molecular and Optical Physics 55-165003   

Theoretical Study of Coincident Ionization Cross-sections of Atoms and Molecules by Twisted Electron Impact

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 (plane-wave) 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 cross-sections (TDCS) and five-fold differential cross-sections (FDCS) for different atomic and molecular targets for an incident twisted electron beam within the first Born approximation (FBA). We also present the average over-impact parameter cross-section ((TDCS)_av) for the (e, 2e) processes. In the coplanar asymmetric geometry, we studied the twisted electron (e, 2e) process on noble gas atoms, H₂, H₂O, CH₄, and NH₃ molecules. We also investigated the angular profiles in the coplanar geometry for θ-variable and constant θ₁₂ 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.   

Biosignature Line Ratios of [P II] in Exoplanetary and Nebular Environments

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 lowest-lying forbidden [P II] transitions among levels 3s²3p² (³P₀, ³P₁, ³P₂, ¹D₂, ¹S₀) within wavelengths range of  0.44-70 µm where several lie within the spectroscopic domain of observation of James Webb Space Telescope. These line ratios have been obtained using a new collisional-radiative-recombination (CRR) model that incorporates a new scheme for the total level-specific (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 Breit-Pauli R-Matrix 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.   

In search of a new SF6 alternative gas: cross-sections, transport coefficients and breakdown characteristics

SF₆ is one of the popular choices for gas-insulated applications. However, its global warming potential is very high and hence there is a need for an eco-friendly 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 non-toxic 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.

Our group is working on the search for eco-friendly gases for insulation applications, investigating the physiochemical properties, decomposition mechanism, and dielectric strength of the specie. In the talk, we will briefly present the target parameters of C₃F₆O which we propose as the new alternative gas. We will then discuss the main theme of our work where the electron impact cross sections and the transport coefficients are obtained. Based on these coefficients the dielectric strength and the breakdown voltage of the new gas are predicted. GAUSSIAN09 [2] is used for our quantum mechanical calculations of target parameters. The cross-section is obtained using R-matrix method via QUANTEMOL-EC software [3]. For the transport coefficients, BOLSIG+ code is utilized [4]. The breakdown voltage curve is obtained by fitting these coefficients [5].

[1] Tian S et al. 2020 AIP Advances, 10(5), 050702

[2] Frisch M J et al. 2009 Gaussian 09, Revision D. 01, Gaussian, Inc., Wallingford CT. See also: URL: http://www. gaussian. com.

[3] Cooper B et al. 2019 Atoms, 7(4), 97

[4] Hagelaar, G.J.M. and Pitchford, L.C., 2005 Plasma sources science and technology, 14(4), 722

[5] Radmilovic-Radjenovic M et al. 2021 Atmosphere, 12(4), 438