Near-infrared spectroscopy of H3+ above the barrier to linearity

The first H3+ transitions above the barrier to linearity have been observed in absorption in the near infrared using a highly sensitive dual-beam, double-modulation technique with bidirectional optical multipassing. A total of 22 rovibrational transitions of H3+ have been detected and assigned to the fourth and fifth overtone and combination bands (5ν21, _{5ν25, 2ν1}+2ν22, 3ν1+ν21, ν₁+4ν22, 2ν1+3ν21, and 6ν22). These transitions, which are more than 4600 times weaker than the fundamental band, probe energy levels above 10 000 cm^-1, the regime in which H3+ has enough energy to sample linear configurations. Experimentally determined energy levels above the barrier to linearity provide a critical test of ab initio calculations in this challenging regime. The comparison between experimental energy levels and theoretical energy levels from ab initio calculations in which the adiabatic and relativistic corrections are incorporated reveals the extent of higher-order effects such as nonadiabatic and radiative corrections. We compare our results with several recent theoretical calculations.