Degenerate and two‐color resonant four‐wave mixing of C 2 − in a molecular beam environment

In this paper, we demonstrate that degenerate and two‐color resonant four‐wave mixing spectroscopy is applicable for the sensitive and selective characterization of negative ions in a molecular beam environment. Results are shown for C $_{2}^{-}$ , which is produced by discharging a mixture of acetylene and argon prior to supersonic expansion. Substantial signal‐to‐noise ratios of ≈ 150 show that the method is generally applicable for high‐resolution optical double‐resonance spectroscopy of negative ions. A detection sensitivity for C $_{2}^{-}$ of ≈ 10 7 ions/cm 3 is estimated. Copyright © 2010 John Wiley & Sons, Ltd.