A QUANTITATIVE RING DYE-LASER ABSORPTION DIAGNOSTIC FOR FREE SIH2 ((X)OVER-TILDE(1)A(1)) RADICALS AT HIGH-TEMPERATURES

The silylene radical (SiH2) was detected in reacting high temperature mixtures of disilane (Si2H6) highly diluted in argon by time-resolved ring dye laser absorption measurements at lambda = 579.35 nm. The measured absorption was caused by a single line transition in the A-X (0,2,0)-(0,0,0) band of SiH2 which was exclusively initiated by the narrow bandwidth laser source. The absorption profiles were measured in the reaction zone behind reflected shock waves at temperatures between 1000 and 1900 K, and at pressures 0.2 bar less than or equal to p less than or equal to 1.3 bar. Several calibration experiments were performed at temperatures above 1500 K and low initial Si2H6 concentrations. To establish the connection between measured absorption and the SiH2 concentrations, numerical simulations on disilane pyrolysis were performed based on a reaction kinetic model. From both the measured absorption and the calculated SiH2 concentrations the silylene absorption cross section was determined to be sigma(SiH2) = 1.3 x 10(-17) cm(2), at the reference conditions nu(0) = 17260.82 cm(-1), T-0 = 1757 K and p(0) = 0.3 bar. A spectroscopic model was introduced to describe the temperature and pressure dependence of the SiH2 absorption cross section at the given wavelength. All measured concentration time-histories of SiH2 radicals formed during the thermal decomposition of disilane in the given experimental range could be verified. The detection limit was about 1 x 10(13) cm(-3) for an absorption path length of 8 cm and the laser diagnostic was proved well suited for kinetic studies of elementary reactions involving silylene.