Infrared absorption spectroscopy of few hundred H 3 ϩ ions trapped in a 22-pole ion trap is presented using chemical probing as a sensitive detection technique down to the single ion level. By exciting selected overtone transitions of the (v 1 ϭ0,v 2 l ϭ3 1 )←(0,0 0 ) vibrational band using an external cavity diode laser an accurate diagnostics measurement of the effective translational and rotational temperatures of the trapped ions was performed. The absolute accuracy of the measured

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... n frequencies was improved by a factor of four compared to previous plasma spectroscopy measurements using velocity modulation ͓Ventrudo et al., J. Chem. Phys. 100, 6263 ͑1994͔͒. The observed buffer gas cooling conditions in the ion trap indicate how to cool trapped H 3 ϩ ions into the lowest ortho and para rotational states. Future experiments will utilize such an internally cold ion ensemble for state-selected dissociative recombination experiments at the heavy ion storage ring Test Storage Ring ͑TSR͒.