Chemical Physics ELSEVIER Chemical Physics 218 (1997) 57-69 Simulation of the Sill (A2A --> X 21-I) emission spectrum in a silane glow discharge and derivation of an improved set of molecular constants S. Stamou, D. Mataras, D. Rapakoulias Laboratory of Plasma Chemistry, Department of Chemical Engineering. University of Patras, P.O. Box 1407, 26500, Patras, Greece Received 25 November 1996 Abstract An improved simulation of the rotational intensity distribution of the 0-0 emission band of the A2A-X 21I transition of Sill, that is experimentally obtained by optical emission spectroscopy from a silane rf glow-discharge, is reported. The improvements consist in the use of a new term value formula, a recent set of molecular constants and an extended least squares fitting analysis for the micro-optimization of the set of constants. Thus, the rotational temperature of Sill is determined by utilizing a sufficient spectral resolution together with an experimentally determined instrument function of the optical system. The optimum fit of the emission spectrum is obtained for a rotational temperature TRO T = 2840 + 50 K which is significantly higher compared to those previously reported. Furthermore, an abnormal behaviour of the observed splittings compared to the theoretical calculations is observed, while in two cases experimental measurements of A-doublets were performed. 1. Introduction Electronically excited Sill is the main emitting fragment, resulting from electron collisional dissocia- tive excitation [1], in silane containing rf discharges which are commonly used for thin film microelec- tronics. Furthermore, these diatomic hydrides are of considerable interest in spectroscopy and astro- physics. The occurrence of Sill in the stellar atmo- sphere [2,3] combined with its resemblance to the CH radical, make Sill an interesting object for exper- imental and theoretical studies. The electronic structure of Sill has been the subject of several experimental [4-11] and theoreti- cal investigations [12-15] concerning mainly the A2A-X21-I transition, which is analogous to the 4135 .~ band of CH. However, precise information concerning all the energy levels of the Sill radical is not available. For instance, very few attempts to detect Sill by laser magnetic resonance (LMR) have been reported [16,17], in contrast to the CH radical which has been extensively studied by LMR spec- troscopy [18-20]. Thus, compared to CH, there are still aspects of the electronic structure of Sill that need to be clarified. The intensity distribution in the rotational fine structure of electronic transitions, such as A2A - X 2II, is often used for the measurement of the rotational temperature of the emitting species in vari- ous plasma sources [21]. Generally, the measurement 0301-0104//97]$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S030 1-0104(97)00052-9