A preliminary investigation of the spectral signatures of excited electronic states of OH in the Martian atmosphere

Mars has changed from a warmer, water-containing planet into a cold and arid environment. Collisions of superthermal oxygen and other atoms with surrounding gases may lead to the escape of light atmospheric molecules, such as D1, OH², He3, and H₂4, from the Martian atmosphere. Such processes have probably contributed to the thinning of its atmosphere and the transformation of Mars' climate.

OH molecules can be produced in the Martian atmosphere by the photodissociation of water vapor and in several chemical reactions, such as the reactions of thermal molecular hydrogen and energetic oxygen atoms O + H₂ → H + OH ⁵. Emission and absorption spectra of OH molecules within the Martian atmosphere can lead to a better understanding of these processes. For example, they can help monitor the variation of its abundance with altitude ⁶. In general, astronomical spectra of specific molecules can be better interpreted through a detailed identification of their line list.

In this work, we present an extensive line list for the B²S⁺ - X²P and D²S^- - X²P electronic transitions of OH, including line intensities, line positions with the relevant quantum numbers of the upper and lower states, e/f parity and oscillator strength, calculated using PGOPHER⁷ program. The line intensities are found based on calculated ab-initio Transition Dipole Moment Function and potential energy curves obtained using the quantum computational chemistry program MOLPRO⁸, using CASSCF method followed by MRCI including Davidson correction term (+Q). LEVEL⁹ program is used to compute Transition Dipole Moment Matrix Elements in Hund's case (b) using the procedure of Numerov-Cooley¹⁰ which are then transformed to Hund's case (a) as required by PGOPHER.

 

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⁵ M. Gacesa, N. Lewkow and V. Kharchenko, "Non-thermal production and escape of OH from the upper atmosphere of Mars". Icarus, 284, pp.90-96 (2017).

⁶ S. Raghuram, A. Bhardwaj, & M. Dharwan, “Model for Nitric oxide and its dayglow emission in the Martian upper atmosphere using NGIMS/MAVEN measured neutral and ion densities”. Icarus, 382, 115010 (2022).

⁷ CM. Western, PGOPHER: a program for simulating rotational, vibrational and electronic spectra. J Quant Spectrosc Radiat Transf., 186:221–42 (2017).

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⁹ RJ. LeRoy, "LEVEL: A computer program for solving the radial Schrödinger equation for bound and quasibound levels." J Quant Spectrosc Radiat Transf., 186:167–78 (2017).

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