MONTE CARLO CALCULATION OF THE TRANSLATIONAL RELAXATION OF SUPERTHERMAL H ATOMS IN THERMAL H₂GAS

Abstract

A simple and reliable method to study the translational relaxation of "hot" H atoms following their production by chemical mechanisms is proposed. The problem is relevant to interstellar medium, shocks, photospheres, and atmospheric entry problems. It is shown that the thermalization of H atoms can be conveniently studied by the Monte Carlo method, including the thermal distribution of background molecules, and sets the basis for further investigations. The transport cross section is determined by the inversion of transport data. The collision density of H atoms in H 2 gas is also calculated and discussed in the context of simple theories. The application of the results to astrophysical problems is outlined, including numerical results for the reaction H + H 2 O → H 2 + OH. An approximate analytical formula for the reaction probability during H atom thermalization is proposed.

The kinetics of energetic O−ions in oxygen discharge plasmas

“…The type of collision was stochastically determined using the probability of the jth collisional process N i g i σ ij (g i )/ν, where σ ij is the cross section of the jth process in collisions between an ion and molecule of the ith species. The equations for the ion velocity after collisions are given, for instance, in [14,23]. We used the random number generator presented in [24].…”

“…This effect has been well studied for superthermal atoms. The role of superthermal atoms in chemical reactions has been studied for the reactions of energetic O atoms with other atmospheric species [8,9], for reactions of superthermal N atoms with O 2 to produce NO molecules (see, for instance [10,11]), for chain reactions in H 2 :F 2 mixtures [12,13] and for the abstraction of H atoms from H 2 O molecules in collisions with energetic H atoms [14]. In [15][16][17][18], the mechanisms of superthermal atom production were considered in a high-voltage nanosecond discharge initiated in combustible mixtures.…”

Kinetic modelling of atom production and thermalization in CCRF discharges in H2

“…The neutral H atom solver is based on the same MC solver applied to ions in the PIC/MCC model, which allows us to include accurately the effect of the thermal gas velocity distribution in the ion kinetics [24,45], and neutral atoms [17,46]. This method, as shown in [45] amounts to using the Neumann rejection technique [47] to select the collision pair for an ion-neutral collision with the correct probability given by the kinetic theory and thereby leading to the correct distribution of collision times also for slow particles in the thermal velocity field of neutrals.…”

“…As reported in table 3, the expression for the momentum transfer cross section of H-H 2 collision of [17] is used.…”

“…This issue is best addressed by a Monte Carlo (MC) approach, which avoids most simplifications necessary in other approaches. MC models of fast atoms have been developed for different systems and, in particular, MC models of H atom thermalization have been formulated for uniform gases in the context of chemical kinetics [17,18]. Many MC models for different atomic species in electric discharges have been produced in the past [19][20][21][22][23].…”