Spectroscopic characterization of diatomic molecules in the Martian
Atmosphere
R. Al Abdallah, physics department, Khalifa University, Abu-Dhabi (100059801@ku.ac.ae) M. Khalil, Department of Mechanical Engineering, Khalifa University, Abu-Dhabi, United Arab Emirates(100049371@ku.ac.ae) , M. Gacesa, Physics Department, Space and Planetary Science Center, Khalifa
University, Abu-Dhabi, United Arab Emirates( marko.gacesa@ku.ac.ae) , A. Al Ghaferi, Department of Mechanical Engineering, Khalifa University, Abu-Dhabi, United Arab Emirates (amal.alghaferi@ku.ac.ae) , N.
El-Kork, Physics Department, Space and Planetary Science Center, Khalifa University, Abu-Dhabi, United
Arab Emirates (nayla.elkork@ku.ac.ae)

Introduction:
Over the last decades, Mar’s climate has changed
significantly from a warmer, water-containing planet
into a cold, dry global desert. The escape of atmospheric gases to space likely played a role in this
change. Jeans escape of hydrogen, photochemical
escape of oxygen, and acceleration of ions above
their escape velocities by the interplanetary magnetic
fields and solar wind plasma constitute pathways for
the Martian atmospheric loss.
Processes involving diatomic molecules also play
an important role for this depletion process. For example, it was proposed that the collisions of hot oxygen atoms with other energetic neutral atoms (ENAs)
could drive the escape of light atmospheric gases,
such as He [1], H2 [2], and OH [3]. Our goal is to
have a better understanding of the phenomena involving diatomic molecules, found in the upper atmosphere of Mars, through a spectral monitoring of
their emissions and direct comparisons with synthetic
spectra. In this work, we present results related to the
electronic structure of CO and OH molecules, as a
preliminary step to the construction of their line list,
and corresponding synthetic spectra.
Computational Details:
We theoretically investigated the electronic structure of both CO and OH molecules using the highlevel ab-initio calculations. We used the highly accurate Complete Active Space Self Consistent Field
(CASSCF) method followed by the Multi-Reference
Configuration Interaction MRCI calculations including Davidson correction (+Q) [4].All calculations
were performed using MOLPRO computing package
[5].
For the OH molecule, the Oxygen atom was
treated in an all-electron scheme using the correlation consistent polarized valence triple zeta (ccpVTZ) including sp functions [6], the hydrogen atom
was treated via the augmented correlation consistent
polarized valence triple zeta (aug-cc-pVTZ) basis set
including sp functions [7].
All the calculations of the active space are done
in the C2v subgroup symmetry, and contains 6(O:
1s, 2s, 2p±1; H: 1s, 2s, 2p0) and 2(O: 2p±1; H:
2p0) and 0 molecular orbitals. The active orbitals

then are distributed into 6a1, 2b1, 2b2, 0a2 denoted
by [6,2,2,0] where 7 electrons are correlated.
For CO molecule, the basis sets used for both C and
O atoms are cc-pV5Z for s, p, and d functions. The
geometry optimization option was implemented in
the self-consistent field calculations. The active
space includes (O: 1s, 2s, 2p,3s; C: 1s, 2s, 2p,3s).
Nine molecular orbitals are considered in the active
space for which the irreducible representation includes 3a1, 2a2, 2b1 and 2b2 symmetry molecular orbitals denoted as [8,2,2,0]. All the electrons in the
active space are correlated.
The spectroscopic constants were obtained by fitting
the obtained potential energy curves to a polynomial
function.
Results and discussion:
The potential energy curves of nine (Singlet and
Triplet) electronic states of CO molecule, and eight
(Doublet and Quartet) electronic states of OH molecule were calculated in the ΛS representation. The
Doublet states of OH molecule are shown in Figure
1, while selected singlet and triplet states of CO molecule are depicted in Figure 2. For CO molecule, all
the states are converging to the same dissociation
limit, which corresponds to the atomic ground states
of carbon (3P) and oxygen (3P). Similarly, for OH
molecule, the ground and first excited state correspond to the dissociation limit of O(3P)+H(2S), while
higher excited excites dissociate to O(1D)+H(2S) and
O(1S)+H(2S).

Figure 1: Calculated doublet electronic states of OH
molecule

Figure 2: Calculated singlet and triplet states of
CO molecule

5.21 %. It is important to note here that our computed theoretical values agree closely to 100 % with the
experimental ones for the ground X2 and first excited state (1)2. Similarly, the values of Be, Te, ωe
and De vary as 0.17% <ΔBe/Be< 0.74%, 0.17%
<ΔBe/Be< 0.74%, 1.11% <ΔTe/Te< 2.43%, 0.06%
and <Δωe/ωe< 1.26%.
We could not compare our values with previous
work in the literature for the highest excited states
(2)4, (3)4, (1)4, and (1)4, as up to our
knowledge, they have been calculated here for the
first time.
The spectroscopic constants of the ground and selected excited states of CO molecule are shown in
Table 2.

Spectroscopic constants, such as the electronic
energy relative with respect to the ground state energy Te, the harmonic frequency e, the internuclear
distance Re, the rotational constants Be, and the dipole moment at equilibrium μe can used to characterize the accuracy of potential energy curves.
We present some spectroscopic constant for the
bound investigated molecular states of OH molecule,
with a comparison of previously obtained results
from the literature, in Table.1

a] Present work
[b]
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Molecular Physics, 114(14), 2204-2216.
[c]
Srivastava, S., & Sathyamurthy, N. (2014). Ab Initio Potential Energy Curves for the
Ground and Low-Lying Excited States of OH and OH–and a Study of Rotational Fine Structure in
Photodetachment. The Journal of Physical Chemistry A, 118(33), 6343-6350.
[d]
Qin, X., & Zhang, S. D. (2014). Low-lying electronic states of the OH radical:
Potential energy curves, dipole moment functions, and transition probabilities. Journal of the Korean
Physical Society, 65(12), 2017-2022.
[e]
Martin, J. M. (1998). Spectroscopic quality ab initio potential curves for CH, NH, OH
and HF. A convergence study. Chemical physics letters, 292(4-6), 411-420.
[f]
Huber, K. P., & Herzberg, G. (1979). Molecular Structure and Molecular Spectra. IV.
Constants of Diatomic Molecules. Van Rostrand-Reinhold, New York.
[g]
Chu, S. I., Yoshimine, M., & Liu, B. (1974). Ab initio study of the X 2Π and A 2Σ+
states of OH. I. Potential curves and properties. The Journal of Chemical Physics, 61(12), 5389-5395.

Table 1: Spectroscopic constant for the ground
and excited electronic states of OH molecule
The obtained results show very good agreement
with previously published work. More precisely, for
OH molecule, the relative error of the internuclear
distance at equilibrium (Re) varies as: 0 < ΔRe/Re <

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Table 2: Spectroscopic constant for the ground
and excited electronic states of CO molecule
As is the case for OH molecule, the CO molecule
results agree well with previously published work,
especially those of experimental nature. For example,
for the ground state, the relative error of Re with
experimental work is ΔRe/Re =0.23%, while that for
the A1Π state is ΔRe/Re =0.24%. In general, the relative errors vary as: 0.024% <ΔRe/Re< 9.43%, 0.21
%<Δωe/ωe< 10%, 1.03%<Δωe/ωe< 9.71.

Conclusion
Ab-initio investigations of the lowest electronic
states of CO and OH molecule were carried out
via CASSCF/MRCI+Q method. The results show
excellent agreement with the available theoretical
and the experimental results for the majority of the
calculated states. Our future work includes line lists
calculations and the construction of synthetic emission spectra. We also plan to conduct a comparison
with the recorded HOPE (EMUS) spectral data of
atmospheric molecules, in an effort to maximize the
scientific output of the Emirates Mars Mission.
Aknowledgement
N.El-Kork and M.Gacesa are partly supported by the internal grant (8474000336-KU-SPSC). Financial support
for this work is provided by Khalifa University of Science
and Technology under Award No. CIRA-2019-054, and
the ASPIRE AARE award, under grant number 00032900001.Support from the Scientific Computing Department
at Khalifa University and computational resources at the
Almesbar HPC cluster are gratefully acknowledged.

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