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The role of secondary processes in kinetics of triplet states of a hydrogen molecule in an ECR discharge

An electronic state sensitive semiempirical collision-radiative model of hydrogen plasma of ECR-discharge is used to analyze the applicability of emission of triplet states of molecular hydrogen for plasma diagnostics. It is shown that secondary processes make the greatest contribution to the kinetics of population-depopulation of triplet states a3Σg+,c3Πu,d3Πu,e3Σu+,g3Σg+,h3Σg+ , i3Πg and r3Πg. The secondary processes give the smallest contribution to the excitation and deactivation of triplet states f3Σu+иk3Πu . Thus a simplified coronal model (electron impact excitation followed by radiative decay) can be used to process the intensities of the dipole allowed f3Σu+→a3Σg+,g3Σg+ and k3Πu→a3Σg+ transitions. The complicated collision-radiative model should be used for other transitions.


Introduction
Interest in the hydrogen-containing low-temperature plasma of the ECR-discharge is associated with the problem of creating the gas-discharge sources of negative atomic hydrogen ions H - [1]. Studies [2,3] show that the dominant role in the formation of Hin the ECR-discharge is played by processes involving vibrationally excited hydrogen molecules in the ground electronic state H 2 (X 1  g + ,v). An important stage in creating and optimizing the parameters of gas-discharge sources His the development of non-perturbing and rapid methods for the diagnosis of H 2 (X 1  g Increasingly widespread in the diagnostics of the vibrational temperature of the ground state in hydrogen plasma are experimental-calculation techniques that are based on a combination of emission spectroscopy and collision-radiative plasma models [4][5][6][7]. A simplified coronal model is often used to process results of emission spectroscopy [8]. For example, in processing the emission spectra of the Fulcher system of the hydrogen molecule H 2 (d 3 П u a 3  + g ) it is assumed that the radiating state of the d 3 П u state is excited by the electron impact, and deactivated by radiative decay. The presence of additional (secondary) collision-radiative processes may violate this assumption. Neglecting of secondary processes in the simplified models can lead to overestimated values of question on the role of secondary processes in the mechanisms of excitation of the triplet states of molecules is still not fully understood.
In the present paper this problem is investigated by modeling the processes of population and deexcitation of the triplet states of a hydrogen molecule in an ECR-discharge. It is a continuation of [8], in which studies of the distribution function for the rotational and vibrational levels of a hydrogen molecule in triplet state d 3 П u in the ECR-discharge in hydrogen were performed by emission spectroscopy.
Results of study the problem for nonequilibrium DC and microwave discharges are presented in [7,9]. From the point of view of the problem under consideration, the ECR-discharge is of particular interest, since the electron energy distribution function (EEDF) in this discharge differs significantly from the EEDF in other discharges in that it is enriched by electrons in the high-energy region compared to other discharges. This can lead to a change in the ratio of the channels in the triplet states kinetics.

Experimental conditions
The study of the processes of population and deexcitation of the triplet states of a hydrogen molecule is performed for the conditions described in [1]. ECR discharge was created in a chamber of cylindrical geometry (diameter D = 100 mm) made of stainless steel in a stream of hydrogen in the pressure range 0.001-0.01 Torr. The plasma was isolated from the chamber walls by a Pyrex cylinder. Microwave power in the range 200 -1200 W at 2.45 GHz in continuous generation mode was supplied to four dipolar sources. The wall temperature of the T W chamber was kept at room temperature. The value of the translational temperature T g measured on the discharge axis in the region of dipolar sources measured by the emission spectroscopy method was 600 ± 650 K and decreased to 430 ± 50 K at a distance of 90 mm from them [8]. The EEDF measured by the probe method is bimaxwellian and enriched by electrons in the high-energy region [1]

Semiempirical level-to-level collision-radiative model of hydrogen plasma in ECR-discharge
In the model developed in [7] and used in this paper, the number of electronically excited states of a hydrogen molecule is significantly increased, compared with those described in [4][5][6], and the kinetic scheme is extended in the part of the collisional-radiative processes of interest for emission spectroscopy of hydrogen plasma. It additionally takes into account processes involving hydrogen molecules in singlet ( 1 To determine the level-rate coefficients of the processes describing the collisions of electrons with heavy particles, we used the EEDF, obtained as a result of probe measurements for the experimental conditions from [1]. A list of collision-radiative processes with a corresponding database (or the total concentration of particles N), D, P H and T W are the initial data in the model. kinetics.