Lifetime determination of theO(So5)metastable state via 1356-Å radiation using a time-of-flight technique

The lifetime of the ${}_{}{}^5{}_{}{}^{}S_{}^o{}_{}{}^{}$ metastable state of oxygen was measured by monitoring 1356-Å photons from inflight radiative decay of the metastables in a beam. Metastables were produced by electron-impact dissociation of ${\mathrm{O}}_2$ and velocity selected using a time-of-flight (TOF) technique. The ${}_{}{}^5{}_{}{}^{}S_{}^o{}_{}{}^{}$ state was identified by passing photons from its radiative decay through a ${\mathrm{N}}_2$O gas filter, which had a high transmission for 1356-Å radiation and good discrimination against other uv photons of shorter wavelength. The use of ${\mathrm{O}}_2$ as another filter gas served as a consistency check for the absence of shorter-wavelength radiation. The metastable detector assembly also contained a surface detector that responded to metastable particles in the usual manner by Auger emission of secondary electrons. TOF spectra of the metastable dissociation fragments were measured with both these detectors at various distances from the source. The $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S_{}^o{}_{}{}^{}\right)$ lifetime obtained from a series of photon and particle measurements was found to be 180±5 μsec. This value represents a substantial improvement in accuracy over previous lifetime determinations and the only TOF study in which the $\mathrm{O}\left({}_{}{}^5{}_{}{}^{}S_{}^o{}_{}{}^{}\right)$ state was identified through radiative decay.