Vibrational overtone spectroscopy of cold trapped hydroxyl anions

The second vibrational overtone transition, nu', J' = 3, 1 <- nu, J = 0, 0, in OH- confined in a 22-pole radio frequency ion trap has been measured using a laser induced reaction with molecular hydrogen. The resultant spectral line is found at 10 150.94(2) cm(-1) and the excitation rate of this overtone excitation is obtained, which sensitively probes the molecular potential surface. The Doppler broadened line profile is used to determine the translational temperature of the ions in buffer gas temperatures ranging from 9(1) to 52(1) K. The translational temperatures closely follow those of the buffer gas over this range, including an offset caused by radio frequency heating. In comparison with previous measurements of the rotational temperatures of trapped ions, we show that translational and rotational temperatures do not equilibrate at the low buffer gas temperatures.