Knowledge Bank

Laser-induced fluorescence (LIF) is observed following excitation of ground state SiCl molecules to either the $B2\Sigma +$ or the $B\prime \mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Delta$ excited electronic states. Pulsed frequency-doubled light from an excimer-pumped dye laser excites the radicals in the afterglow of a low-pressure microwave discharge in He with a trace of added $SiCl4$. The fluorescence from v' - 0-3 of the $B2\Sigma +$ state and $v\prime -0-1$ of the $B\prime \mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Delta$ state is dispersed with 2 nm resolution and the relative vibrational band strengths are measured for $v\prime \prime \le 10$ in $B2\Sigma +-X2\Pi$ emission and $v\prime \prime \le 3$ in $B\prime \mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Delta -X2\Pi$ emission. From the temporal evolution of the LIF, the fluorescence lifetimes of the B and the B' state are found to be $\le 25$ ns and $1.7\pm 0.5\mu s$, respectively. This large difference in the time behavior of the LIF allows the overlapped excitation spectra to be clearly separated with different time gates. Preliminary analysis of the helium pressure dependence of the fluorescence from the $B\prime \mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Delta$ indicates a collisional removal rate of - $6\times 10-11cm3s-1$. The dispersed fluorescence from the $B\prime \mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Delta ,v\prime -0$ state reveals a significant relaxation pathway is electronic-to-electronic collisional energy transfer to $v\prime -0.2$ in $B2\Sigma +$. The large magnitude of the collisional removal rate of the $B\prime$ state by He is very surprising and further investigations are currently underway.