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

Abstract

In a proof-of-principle experiment, we demonstrate that metastable nitric oxide molecules, NO(a 4 Π i ), generated inside a pulsed, supersonic beam, can be detected by reactive gas-phase collisions with electronically excited Li atoms in the 2 2 P 3/2 state. Since the internal energy of NO(a 4 Π i , v ≤ 4) is lower than the ionization potential of Li in the 2 2 S 1/2 electronic ground state, we observe that the product ion yield arising from autoionizing NO(a 4 Π i )+Li(2 2 S 1/2 ) collisions is a factor of 21 lower than the ion yield from NO(a 4 Π i )+Li(2 2 P 3/2 ) collisions. We also compare our findings with measurements of relative rates for collisions of metastable N 2 +Li(2 2 S 1/2 ) and metastable N 2 +Li(2 2 P 3/2 ) reactive collisions.Using this detection method, we infer densities of ≈ 600 NO(a 4 Π i ) molecules/cm 3 and ≈ 6 • 10 4 metastable N 2 molecules/cm 3 in the interaction region. Our results also allow for an estimate of the fractional population of NO(a 4 Π i , v ≥ 5) prior to the collision process. The production of NO(a 4 Π i ) in selected vibrational states using laser excitation from the X 2 Π r ground state will open possibilities for the detailed study of vibrational-state-selected NO(a 4 Π i )-Li(2 2 P 3/2 ) collisions.