Theoretical study of the low-lying electronic states of magnesium sulfide cation including spin–orbit interaction

doi:10.1016/j.jqsrt.2017.06.036

Journal of Quantitative Spectroscopy and Radiative Transfer

Volume 201, November 2017, Pages 104-114

https://doi.org/10.1016/j.jqsrt.2017.06.036 Get rights and content

Highlights

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High level computations with the inclusion of spin–orbit coupling are employed.
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Geometrical structures and spectroscopic properties are predicted.
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Radiative transition probabilities and lifetimes are reported.
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The photoelectron spectrum has been simulated.
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Capable of supporting further experimental and theoretical researches.

Abstract

Highly correlated ab initio calculations have been performed for an accurate determination of electronic structures and spectroscopic features for the low-lying electronic states of the MgS⁺ cation. The potential energy curves for the four Λ-S states correlating to the lowest dissociation asymptote are studied for the first time. Four Λ-S states split into nine Ω states through the spin–orbit coupling effect. Accurate spectroscopic constants are deduced for all bound states. The spin–orbit couplings and the transition dipole moments, as well as the PECs, are utilized to calculate Franck–Condon factors and radiative lifetimes of the vibrational levels. To verify our computational accuracy, analogous calculations for the ground state of MgS are also carried out, and our derived results are in reasonable agreement with available experimental data. In addition, photoelectron spectrum of MgS has been simulated. The predictive results are anticipated to serve as guidelines for further researches such as assisting laboratorial detections and analyzing observed spectrum.

Introduction

Alkaline earth chalcogenides have attracted a wide range of interests from scientific researches to technological applications based on their specific characteristics of wide band gap, high-pressure behaviors and low dielectric constants. Owing to their promising potential uses for various electrical and optical devices, researches on structural, electronic, spectroscopic and transport properties of the species have been subjects to both experimental and theoretical investigations.

Extensive research efforts have been made for the alkaline earth monoxides (MO, M = Be, Mg, Ca, Sr and Ba) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Transitions originated from the A¹Σ⁺ and A′¹Π states to the ground X¹Σ⁺ state were recorded in different spectral regions. Isotopic species of MO with M = ²⁴Mg, ²⁵Mg, ²⁶Mg, ⁸⁶Sr, ⁸⁷Sr, ⁸⁸Sr, ¹³⁵Ba and ¹³⁷Ba were studied experimentally. For CaO, the measurements cover only one isotopic species, ⁴⁰Ca¹⁶O. Perturbations were analyzed for the low-lying electronic states of the MO radicals involving the a³Π, b³Σ⁺, A¹Σ⁺ and A′¹Π states. According to the global least-squares procedures, which include infrared, microwave and millimeter-wave data, the vibrational and rotational constants describing interactions among these states were determined.

Studies on electronic structure, spectroscopy and stability of the lowest electronic states have been performed on the alkaline earth monoxide ions (MO⁺, M = Be, Mg, Ca, Sr and Ba) [13], [14], [15], [16], [17]. The bonding nature in the ground states of MO⁺ are mostly due to M²⁺O⁻ ionic interactions. Besides, with the increment of the atomic number of the metal atom, bonding between the two atoms in MO⁺ becomes increasingly ionic, while the ionization energy decreases.

The alkaline earth sulfides (MS, M = Be, Mg, Ca, Sr and Ba) [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31] have been widely investigated experimentally. Spectroscopic observations were mainly focused on the A¹Σ⁺−X¹Σ⁺ and B¹Σ⁺−X¹Σ⁺ bands by the high-resolution Fourier transform spectrophotometer, dye laser spectroscopy and optical stark spectroscopy. Rotational analyses yielded molecular parameters of the ground, first and second excited vibrational states. Spectroscopic constants of individual vibrational levels and equilibrium molecular constants were determined. Besides, the A¹Σ⁺ state was found to be extensively perturbed by two low-lying electronic states, A′¹Π and a³Π_i.

As for the alkaline earth sulfide ions MS⁺, molecular geometries and spectroscopic information for these systems are lacking. To our knowledge, the only systematical research was carried out for BeS⁺ [18] from a theoretical perspective. No literature associating with experimental research of the MS⁺ family has been reported. The absence of essential information on the geometric structure prevents us from understanding relevant characteristics of the molecular ion. Therefore, we are motivated to carry out investigations for MS⁺. In this work, we have performed the first high-level ab initio computations to investigate molecular constants, spectroscopic parameters and transition properties for the MgS⁺ species.

The aims of the present work are (1) to fill the knowledge gap on molecular structure, spectroscopy and metastability of this experimentally unknown species; (2) to model the system with accurate geometric and spectroscopic parameters; (3) to gain insight into the variation of the ionic character of the cation; (4) to motivate experimental studies on MgS⁺ and its derivatives, and (5) to serve as a guideline for assisting spectroscopic detections and astronomical observations.

However, we believe it is possible to perform laboratorial observations for MgS⁺. Production of the cation sample is a prerequisite to perform spectroscopic detections. It can be achieved by generating MgS molecule and then ionizating the neutral species to MgS⁺. The MgS molecule can be formed by reacting Mg vapor with sulfur vapor [21] or with OCS [22], where the Mg vapor can be produced either from a laser ablation source [22] or by heating Mg metal to its melting point [2]. Following the reactions that formed MgS, a discharge [2] as well as a resonantly enhanced two-photon excitation [17] can be used for ionization.

This paper is organized as follows. In Section 2, it begins by describing theoretical methods and basis sets used in this study. In Section 3, we present a wealth of computation results together with corresponding discussions, including equilibrium geometric parameters, potential energy curves (PECs), spectroscopic constants, vibrational energy levels, permanent dipole moments (PDMs), transition dipole moments (TDMs), Franck-–Condon factors (FCFs), Einstein coefficients and radiative lifetimes. Subsequently, our computational results for neutral MgS are presented and are compared with literature data. In addition, the single photo-ionization spectrum of MgS is simulated.

Section snippets

Computational details

The energy scans of all the PECs are calculated using the Hartree–Fock method [32], [33] to generate an initial guess of the molecular orbitals (MOs). The MOs obtained are further optimized using the state-averaged complete active space self-consistent field (SA-CASSCF) method [34], [35]. The final wave function is then generated with the multireference configuration interaction (MRCI) method [36], [37] on a reference set from the CASSCF configurations. Size non-consistency errors are minimized

Results and discussions

In this section, we report molecular structures and transitions properties of the low-lying electronic states of MgS⁺ as well as the ground state of MgS. Firstly, we will focus on the PECs and structural constants of MgS⁺. Secondly, we present vibrational energy levels, spectroscopic parameters, dipole moments and transition properties for this cation. Subsequently, we report computation results on neutral MgS, and finally the simulated photoelectron spectrum of MgS is presented.

Conclusions

In the present work, we have performed ab initio computations for the MgS⁺ cation on the MRCI+Q level of theory. Four low-lying electronic states that correlate with the lowest dissociation asymptote are studied. The spin–orbit coupling splits these bound states into nine Ω states. By constructing accurate PECs, geometric parameters, spectroscopic constants as well as vibrational energy levels of the cation have been derived. Moreover, transition properties such as transition dipole moments,

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 11304023), the Educational Commission of Hubei Province of China(Grant No. Q20151307) and by the Yangtze Youth Talents Fund of Yangtze University(Grant No. 2015cqr21).

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