1+1′ resonant multiphoton ionisation of OH radicals via the A2σ+ state: insights from direct comparison with A-X laser-induced fluorescence detection

A 1+1 ' resonance-enhanced multiphoton ionisation (REMPI) scheme for OH X-2 pi radicals is characterised for a broad range of intermediate A(2)sigma(+) (v = 1, J, F-i) levels. The intensities of OH A-X (1,0) transitions detected by subsequent fixed-frequency VUV ionisation are compared with those obtained by near simultaneous laser-induced fluorescence (LIF) measurements. The ratios of the 1+1 ' REMPI to LIF signals are used to derive enhancement factors which reflect the VUV absorption to the OH A(3)pi, 3d, v = 0 Rydberg state and/or the fast autoionisation process that yields OH+ ions. The determination of the enhancement factors permits 1+1 ' REMPI to be utilised as a quantitative state-specific probe of OH X-2 pi radicals.