A 1+1 ionization scheme for sensitive detection of the OH radical

We introduce a new laser detection scheme for the OH radical that employs 1+1 X²Π ⇋ D²Σ^- resonance enhanced excitation to a high-lying Rydberg state followed by field ionization. We place an upper limit of 8 ns on the lifetime of the D²Σ^- state. We also correct published values for T₀ for the D²Σ^- state, provide a new value for the energy of the 6s¹11p^{1 3}P₁⁰ state of Hg (81 807.6 cm^-1), and measure the rotational temperature of OH radicals produced in the photolysis of formic acid at 212.6 nm to be 300 ± 50 K. Our REMPI probe might be suited to studies that require both sensitive detection of the OH radical combined with accurate kinetic energy information.