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

The first ${\mathrm{H}}_3^{+}$ 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 ${\mathrm{H}}_3^{+}$ have been detected and assigned to the fourth and fifth overtone and combination bands ${\text{(5ν}}_2^1,$ ${\text{5ν}}_2^5,$ ${\text{2ν}}_1{\text{+2ν}}_2^2,$ ${\text{3ν}}_1{\mathrm{+\nu }}_2^1,$ ${\nu }_1{\text{+4ν}}_2^2,$ ${\text{2ν}}_1{\text{+3ν}}_2^1,$ and ${\text{6ν}}_2^2\mathrm{).}$ These transitions, which are more than 4600 times weaker than the fundamental band, probe energy levels above $\text{10\hspace{0.05em}000\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}},$ the regime in which ${\mathrm{H}}_3^{+}$ 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.