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 meas ured 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 100 0 and 1900 K, and at pressures 0.2 bar less than or equal to p less th an 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 con centrations, numerical simulations on disilane pyrolysis were performe d 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 tempera ture and pressure dependence of the SiH2 absorption cross section at t he given wavelength. All measured concentration time-histories of SiH2 radicals formed during the thermal decomposition of disilane in the g iven experimental range could be verified. The detection limit was abo ut 1 x 10(13) cm(-3) for an absorption path length of 8 cm and the las er diagnostic was proved well suited for kinetic studies of elementary reactions involving silylene.