Infrared absorption of silane, ammonia, acetylene and diborane in the range of the CO₂ laser emission lines: Measurements and modelling

Abstract

Absorption spectra of the gases SiH₄, NH₃, C₂H₂ and of SiH₄/Ar and SiH₄/B₂H₆ mixtures have been measured in the spectral range of the CO₂ laser from 9.2 to 10.8 µm. In agreement with literature, silane shows the highest absorption (absorption coefficient ∝ = 3.3 × 10⁻² Pa⁻¹ m⁻¹). The deviation of the measured absorption behaviour of silane from literature, as far as the pressure dependence is concerned, can be explained by the enhanced spectral energy density in our experiment. This is confirmed by a rate-equation model involving the basic mechanisms of V-V and V-T energy transfer between vibrationally excited silane molecules. In contrast to silane, the absorption coefficient ∝ of NH₃ at the 10P(20) laser line is 4.5 × 10⁻⁴ Pa⁻¹ m⁻¹ atp = 20 kPa and has its maximum of 4.5 × 10⁻³ Pa⁻¹ m⁻¹ at the 10R(6) laser line. For C₂H₂ and B₂H₆, ∝ is even less ( ≤ 2.1 Ò 10⁻⁵ Pa⁻¹ m⁻¹ for C₂H₂).