The observation and analysis of 46 lines of the infrared ν₂ band of the molecular ion are described. The data include 16 new observations as well as the 30 previously published lines. The spectra are observed in absorption in a hydrogen discharge using tunable monochromatic sources, either a laser difference-frequency system or a diode laser. Two modulation techniques, either discharge amplitude modulation or Doppler velocity modulation, are used to increase the sensitivity. Ground state combination differences between the observed lines are in good agreement with ab initio calculations, and scaled ab initio rotational term values are used to relate the different K-energy stacks. Padé-type expressions are found useful in representing the line frequencies, and 44 lines are fitted by 23 parameters with a standard deviation of 0.014 cm⁻¹. The two lines omitted appear to be perturbed by a rovibrational interaction between the ν₂ and ν₁ states that should become more important for higher values of J.