Knowledge Bank

The lifetime of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1$ state of $SO2$ has been measured using a photon counting technique at gas pressures up to 2 atm. At low pressures inert gases such as $N2$ and $CO2$ quench triplet $SO2$, but at a pressure of about an atmosphere, the quenching saturates and the lifetime is unaffected by further increases in the pressure of these gases. Collisions with ground state $SO2$ molecules also quench the triplet: state at low pressures, but at pressures above 250 Torr this quenching partially saturates. The residual quenching at higher $SO2$ pressures is attributed to the reaction $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}SO2+SO2\to SO3+3SO$ which was observed by Calvert and $coworkers.1$ The pressure saturation can be interpreted in terms of the theory of collision induced intersystem crossing developed by $Freed.2$ Quenching of triplet $SO2$ by $O2$ is more complicated and does not pressure saturate.