Significantly Improved Detection of Molecular Oxygen by Two-Color Resonance-Enhanced Multiphoton Ionization.

We report a new spectroscopic detection scheme for molecular oxygen that achieves roughly two orders of magnitude higher sensitivity for fully rotationally resolved spectra than the current state of the art. Two-color (2 + 1') resonance-enhanced multiphoton ionization (REMPI) via the 3 d Rydberg complex yields state-selective spectra with signal comparable to the intense but diffuse C 3 s σ 3 Π g ← X 3 Σ g - (2 + 1) REMPI bands without significant saturation or broadening. The resulting increase in sensitivity permitted observation of the very weak 3 d π 1 Δ 2 ← X 3 Σ g - transitions and is independent of the intermediate state. This advance in ionization efficiency and quantum state-selective sensitivity for O 2 promises to aid physical and chemical studies across a wide variety of fields.