Photolysis of diatomic molecules as a source of atoms in planetary exospheres

Abstract

We calculated the cross sections of photolysis of OH, LiO, NaO, KO, HCl, LiCl, NaCl, KCl, HF, LiF, NaF, and KF molecules using quantum chemistry methods. The maximal values for photolysis cross sections of alkali metal monoxides are on the order of 10−18 cm2. The lifetimes of photolysis for quiet Sun at 1 astronomical unit are estimated as 2.0 × 105, 28, 5, 14, 2.1 × 105, 225, 42, 52, 2 × 106, 35 400, 486, and 30 400 s for OH, LiO, NaO, KO, HCl, LiCl, NaCl, KCl, HF, LiF, NaF, and KF, respectively. We performed a comparison between values of photolysis lifetimes obtained in this work and in previous studies. Based on such a comparison, our estimations of photolysis lifetimes of OH, HCl, and HF have an accuracy of about a factor of 2. We determined typical kinetic energies of main peaks of photolysis-generated metal atoms. Impact-produced LiO, NaO, KO, NaCl, and KCl molecules are destroyed in the lunar and Hermean exospheres almost completely during the first ballistic flight, while other considered molecules are more stable against destruction by photolysis.

Is either direct photolysis or photocatalysed H-shift of peroxyl radicals a competitive pathway in the troposphere?

“…As in our previous calculations [33][34][35], the variations observed between the photolysis cross-sections of the five studied peroxyl radicals mainly arise from differences in the transition dipole moments. The cross-sections are thus roughly proportional to the squares of the transition dipole moments.…”

“…Cross-sections can then be calculated in the framework of the adiabatic and Franck-Condon approximations. For diatomic molecules, assuming that the vibrational wave function of the initial state can be treated using the harmonic oscillator approximation, we obtain [33][34][35]39]…”

“…Here, K is the Macdonald function and ε is the energy which is counted from the predissociation limit. Expressions (2.4) and (2.5) are obtained by representing the potential energy curve of the unbound excited state as an analytical function UðRÞ ¼ A Á expðÀBRÞ, where A and B are constant parameters [33][34][35]40]. m is the reduced mass of the molecule, ω is the harmonic frequency of molecular vibration in the ground electronic state, R 0 is the equilibrium position (or equilibrium bond length) of the molecule, E if is the energy of dissociation to the electronic state f (i.e.…”

“…The photolysis cross-section as a function of photon energy σ(E ) can be calculated as [ 30 – 34 ] where is the vibronic wave function of the molecule in the final excited state, is the vibronic wave function of the molecule in the initial state, E is the energy difference between the initial and final states and d is the dipole moment operator of the molecule, r represents all electronic coordinates and R all nuclear coordinates. For photolysis reactions, the relevant final excited states are usually unbound (i.e.…”

Volatiles and Refractories in Surface-Bounded Exospheres in the Inner Solar System

“…LiOH is the main Li-containing impactproduced compound at temperatures of quenching of chemical reactions <3,700 K, typical for collisions of meteoroids exceeding 3 cm in radius. Photolysis lifetimes of LiO and LiCl at 1 AU for quiet Sun are equal to 28 and 225 s, respectively, whereas typical velocities of Li atoms produced upon LiO and LiCl photolysis are calculated as 2.6 and 3.8 km/s, respectively (Valiev et al 2020). The LiOH photolysis lifetime at 1 AU for quiet Sun is estimated as 900 s, and the typical energy of Li atoms produced upon LiOH photolysis is estimated as 1.8 eV (Berezhnoy 2013).…”