Intensity distributions in R and P branches of (0–0) band of the A1Π→ X1Σ+ electronic transition of the BH molecule and determination of gas temperature in non-equilibrium plasmas

Abstract

Relative transition probabilities of spontaneous emission in the R and P branches of the A 1 Π→ X 1 Σ + (0–0) band of BH molecule have been obtained for the first time. It was observed that measured ratios of the line strengths are in agreement with corresponding ratios of Hönl–London factors. Thus, the non-adiabatic effect of perturbations is negligibly small and Hönl–London formulas may be used for derivation of rovibronic population densities from measured line intensities of the (0–0) R branch of BH. The (0–0) P branch is less prospective because too much overlapping by other BH emissions cannot be avoided with the moderate resolution spectrometers usually used in plasma spectroscopy. General considerations are illustrated on the example of a low-pressure plasma of a planar microwave discharge in a H 2–Ar–B 2H 6 (64:33:3) gas mixture (total pressure 1– 2.5 mbar , power 1.2– 2.4 kW ). The values of the rotational temperatures obtained directly for the excited electronic-vibrational states d 3Π u −, v′=2 (T d2 rot ( H 2)) and A 1Π +, v′=0 (T A0 rot ( BH)) and those calculated in the framework of excitation-deactivation models for the ground X 1Σ u +, v=0 vibronic states ( T X0 rot(H 2) and T X0 rot(BH)) have been analysed. It has been observed that T X0 rot(H 2) and T X0 rot(BH) are in excellent agreement, while compared to these values T A0 rot(BH) is noticeably higher and T d2 rot(H 2) is about two times lower. Thus, the simultaneous measurements of intensity distributions in the rotational structure of BH and H 2 emission bands can be used as a prospective method for determination of gas temperature in non-equilibrium boron and hydrogen containing plasmas.