Study of Excitation Transfer in a Flowing Helium Afterglow Pumped with a Tuneable Dye Laser. I. Measurement of the Rate Coefficient for Selected Quenching Reactions Involving He (53P)

A system is described for the measurement of excited state reaction times in the nanosecond range. A flowing afterglow produces large populations of chemically unstable species and a pulsed, tuneable dye laser is used to selectively pump these into the reacting excited state. The transient fluorescence from the populations of reactants and end products is used to determine reaction lifetimes and yields. This radiation is collected with a photon counting system which logs arrival times of spectrally dispersed photons with 10 nsec resolution. With this system the rate coefficient for the reaction $\mathrm{He}{\text{(5}}^3\mathrm{P)+}\mathrm{He}\to {\mathrm{He}}_2^{+}\mathrm{+e}$ has been measured to be ${\text{8× 10}}^{\text{−11}}\hspace{0.17em}{\mathrm{cm}}^3/\sec .$ Upper limits on the reactions $\mathrm{He}{\text{(5}}^3\mathrm{P)+}\mathrm{He}\to \mathrm{He}{\text{(5}}^3\mathrm{D)+}\mathrm{He}$ and $\mathrm{He}{\text{(5}}^3\mathrm{P)+e\to }\mathrm{He}{\text{(5}}^3\mathrm{D)+e}$ were determined to be ${\text{8× 10}}^{\text{−15}}\hspace{0.17em}{\mathrm{cm}}^3/\sec$ and ${\text{8× 10}}^{\text{−9}}\hspace{0.17em}{\mathrm{cm}}^3/\sec ,$ respectively.