15min:

J. GOTTFRIED, Department of Chemistry and the Enrico Fermi Institute, University of Chicago, Chicago, IL 60637; B. J. MCCALL, Departments of Chemistry and Astronomy, University of California, Berkeley, CA 94720; T. OKA, Department of Chemistry, Department of Astronomy & Astrophysics, and the Enrico Fermi Institute, University of Chicago, Chicago, IL 60637.

Twenty transitions from the fourth overtone and combination bands of H₃⁺ (5 ₂¹, 5 ₂³, 5 ₂⁵, 2 ₁+2 ₂²,...) have been observed. These transitions, which are more than 4600 times weaker than the fundamental band, occur in the near-infrared region and probe energy levels above the barrier to linearity (\verb1>110,000 cm^-1), the regime in which H₃⁺ has enough energy to sample linear configurations. The detection of these transitions required the development of a high-resolution, high-sensitivity ( I/I 1×10^-7) spectrometer based on a Ti:Sapphire laser and incorporating velocity modulation, heterodyne detection, noise subtraction, and multi-passing. Both pure hydrogen and He/H₂ plasmas were used to discriminate between H₃⁺ and Rydberg transitions of H₂. The primary motivation for continuing the study of vibrational states beyond those spectroscopically probed to date\footnoteC.~M.~Lindsay and B.~J.~McCall, J.~Mol.~Spectrosc. \underline\textbf210, 60 (2001). is to assist in the development of theoretical calculations of H₃⁺. The measured rovibrational energy levels provide an experimental check of ab initio calculations in this region, which present a unique challenge to theorists\footnoteL.~Neale, S.~Miller, J.~Tennyson, Astrophys. J. \underline\textbf464, 516, (1996)..