Sensitive detection of metastable NO and \ce{N2} by reactive collisions with laser-excited Li

In a proof-of-principle experiment, we demonstrate that metastable nitric oxide molecules, NO(a$^4\Pi_i$), 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$^4\Pi_i$, $v \leq 4$) 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$^4\Pi_i$)+Li($2^2$S$_{1/2}$) collisions is a factor of 21 lower than the ion yield from NO(a$^4\Pi_i$)+Li($2^2$P$_{3/2}$) collisions. We also compare our findings with measurements of relative rates for collisions of metastable \ce{N2}+Li($2^2$S$_{1/2}$) and metastable \ce{N2}+Li($2^2$P$_{3/2}$) reactive collisions. Using this detection method, we infer densities of $\approx 600$ NO(a$^4\Pi_i$) molecules/cm$^3$ and $\approx 6 \cdot 10^{4}$ metastable \ce{N2} molecules/cm$^3$ in the interaction region. Our results also allow for an estimate of the fractional population of NO(a$^4\Pi_i$, $v \geq 5$) prior to the collision process. The production of NO(a$^4\Pi_i$) in selected vibrational states using laser excitation from the X$^2\Pi_r$ ground state will open possibilities for the detailed study of vibrational-state-selected NO(a$^4\Pi_i$)-Li($2^2$P$_{3/2}$) collisions.