Sensitive detection of metastable NO and N2 by reactive collisions with laser-excited Li

In a proof-of-principle experiment, we demonstrate that metastable nitric oxide molecules, NO(a4Πi), generated inside a pulsed, supersonic beam, can be detected by reactive gas-phase collisions with electronically excited Li atoms in the 22P3/2 state. Since the internal energy of NO(a4Πi, v ⩽ 4) is lower than the ionization potential of Li in the 22S1/2 electronic ground state, we observe that the product ion yield arising from autoionizing NO(a4Πi) + Li(22S1/2) collisions is a factor of 21 lower than the ion yield from NO(a4Πi) + Li(22P3/2) collisions. We also compare our findings with measurements of relative rates for metastable N2 + Li(22S1/2) and metastable N2 + Li(22P3/2) reactive collisions. Using this detection method, we infer densities of ≈600 NO(a4Πi) molecules/cm3 and ≈6 ⋅ 104 metastable N2 molecules/cm3 in the interaction region. Our results also allow for an estimate of the fractional population of NO(a4Πi, v ⩾ 5) prior to the collision process. The production of NO(a4Πi) in selected vibrational states using laser excitation from the X2Πr ground state will open possibilities for the detailed study of vibrational-state-selected NO(a4Πi)-Li(22P3/2) collisions.