Vibrational Relaxation in SiH₄, SiH₃F, SiH₂F₂, and SIF₄

Abstract

A time resolved optoacoustic technique was used to measure the vibrational relaxation times of SiH_nF_4-n, (n=4, 3, 2, and 0) diluted in Ar. Fundamental vibration-rotation bands of these molecules were excited by using a line tunable CO₂ laser. It was found that several vibration-rotation lines of SiH₄(v₄), SiH₃F(v₄), SiH₂F₂(v₂) and SiF₄(v₃) coincided with the CO₂ laser lines, as shown in Fig.1. The vibrational relaxation times obtained in the prese nt work are summarized in Table 1 together with the existing experimental data on CH₄, and its fluorine substituents. It is noted that the self-relaxation rate of SiH₂F₂ is slower than that of CH₂F₂ in spite of its lower frequency of the lowest vibrational normal mode. This observation is in conflict with the prediction of simple adiabatic theories for V-T energy transfer. A theory for V-R, T energy transfer proposed by Nikitin and Moore was applied to explain this anomaly. Predictions of this V-R, T theory were found to be consistent with experimentally observed variation of the relaxation rates along with the number of hydrogen atoms, n, in SiH_nF_4-n, and CH_nF_4-n, . Slower vibrational relaxation of SiH₂F₂ than that of CH₂F₂ is caused by its larger effective reduced mass which is defined by equation ( 4 ). Not only the amount of energy transfered but also the effective reduced mass have large effect on the vibrational relaxation rates of these molecules.