Radiative lifetimes of the a 3Σ+ metastable state of NO+

Radiative lifetimes for the v′=0, 1, and 2 vibrational levels of the a 3Σ+ metastable state of NO+ have been measured by counting for equal time intervals the UV photons emitted as the metastable ions decay to various ro-vibrational levels of the NO+(X 1Σ+) electronic ground state. A metastable NO+ ion population was produced inside a cylindrical radio frequency ion trap by electron bombardment of NO vapor at pressures ranging from 3.2 to 12.0×10−8 Torr. After the ions were created, the radiative decay of the a 3Σ+ population was monitored versus time by focusing a geometric-fraction of the light emitted by the decaying metastable ions onto a narrow-bandwidth (∼10 nm FWHM) interference filter in front of a photomultiplier tube operated in photon-counting mode. Several filters with differing transmittance envelopes were used during the experiment to resolve different lifetimes for the lowest, three, vibrational levels of the a 3Σ+ metastable state. Previous ‘‘indirect’’ measurements of the NO+(a 3Σ+) lifetime produced results that differed by about an order of magnitude, as have attempts to calculate the a 3Σ+ lifetime. Our experiment resolves the uncertainty associated with the radiative lifetime of the a 3Σ+ metastable state, and the improved resolution of our method, relative to previous measurements, allowed us to observe a significant vibrational-state dependence of the a 3Σ+ lifetime. Our results for the radiative lifetimes of the v′=0, 1, and 2 vibrational levels of the a 3Σ+ metastable state of NO+ are 720±70, 465−90+50, and 330−60+30 ms, respectively.