Uv continuum emission and diagnostics of hydrogen-containing nonequilibrium plasmas

Abstract

The emission of the radiative dissociation continuum of the hydrogen molecule ${(a}^{3}{\ensuremath{\Sigma}}_{g}^{+}\ensuremath{\rightarrow}{b}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ electronic transition) is proposed to be used as a source of information for the spectroscopic diagnostics of nonequilibrium plasmas. The detailed analysis of excitation-deactivation kinetics, rate constants of various collisional and radiative transitions, and fitting procedures made it possible to develop two methods of diagnostics of (1) the ground ${X}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ state vibrational temperature ${T}_{\mathrm{vib}}$ from the relative intensity distribution, and (2) the rate of electron impact dissociation $(d[{\mathrm{H}}_{2}]{/dt)}_{\mathrm{diss}}$ from the absolute intensity of the continuum. The known method of determination of ${T}_{\mathrm{vib}}$ from relative intensities of Fulcher-$\ensuremath{\alpha}$ bands was corrected and simplified due to the revision of $\stackrel{\ensuremath{\rightarrow}}{d}a$ transition probabilities and cross sections of $d\ensuremath{\leftarrow}X$ electron impact excitation. General considerations are illustrated with examples of experiments in pure hydrogen capillary-arc and ${\mathrm{H}}_{2}+\mathrm{A}\mathrm{r}$ microwave discharges. In pure ${\mathrm{H}}_{2}$ plasma the values of ${T}_{\mathrm{vib}}$ obtained by two independent methods are in rather good accordance ${(T}_{\mathrm{vib}}=3000--5000 \mathrm{K}).$ In the ${\mathrm{H}}_{2}+\mathrm{A}\mathrm{r}$ microwave plasma it was observed that the shape of the continuum depends on the ratio of the mixture components. Absorption measurements of the population of the ${3s}^{2}{3p}^{5}4s$ levels of Ar together with certain computer simulations showed that the ${\mathrm{Ar}}^{\mathrm{*}}{\ensuremath{\rightarrow}\mathrm{}\mathrm{H}}_{2}$ excitation transfer plays a significant role. In our typical conditions (power flux: $4 {\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2},$ pressure $p=0.5 \mathrm{mbar}, {\mathrm{H}}_{2}:\mathrm{A}\mathrm{r}=1:1)$ the following values were obtained for the microwave discharge: $(d[{\mathrm{H}}_{2}]{/dt)}_{\mathrm{diss}}\ensuremath{\approx}2.5--5\ifmmode\times\else\texttimes\fi{}{10}^{17} {\mathrm{cm}}^{\ensuremath{-}3}{\mathrm{s}}^{\ensuremath{-}1}.$ The contribution of the excitation transfer is about 10--30 % of the total population of the ${a}^{3}{\ensuremath{\Sigma}}_{g}^{+}$ state.