High-Resolution Visible Spectroscopy of H 3 + Christopher P. Morong, Christopher F. Neese and Takeshi Oka Department of Chemistry, Department of Astronomy & Astrophysics, and the Enrico Fermi Institute University of Chicago, Chicago, IL USA High-Resolution Visible Spectroscopy of H 3 + Christopher P. Morong, Christopher F. Neese and Takeshi Oka Department of Chemistry, Department of Astronomy & Astrophysics, and the Enrico Fermi Institute University of Chicago, Chicago, IL USA Introduction Seven new rovibrational transitions of H 3 + have been observed in the visible region between 12,500-13,700 cm −1. These energy levels are above the barrier to linearity (>10,000 cm −1 ), the regime in which H 3 + has enough energy to sample linear configurations. A high-resolution, high-sensitivity spectrometer based on a Ti:Sapphire laser and incorporating velocity modulation and phase modulation with heterodyne detection a was used to observe the transitions, which are more than 16,000 times weaker than the fundamental band. Due to the abundance of strong hydrogen Rydberg transitions, both pure hydrogen and He/H 2 plasmas were used to identify the much weaker H 3 + transitions. The sparsity and weakness of the lines necessitated the use of the predicted intensities and frequencies bc to focus our search. The measured rovibrational energy levels will assist in the development of theoretical calculations of H 3 + and provide an experimental check of ab initio calculations in this region. a J. Gottfried, B. McCall, and T. Oka, J. Chem. Phys. 118, (2003). b L. Neale, S. Miller, and J. Tennyson, Astrophys. J. 464, 516 (1996). c P. Schiffels, A. Alijah, and J. Hinze, Mol. Phys. 101, 189 (2003). Direction of laser beam H3+H3+ H3+H3+ Experimental Techniques Bidirectional optical multipassing 4 passes each direction Noise subtraction balanced detector Coaddition 25 scans summed (  = 5.0 s) Velocity modulation (~19 kHz) Doppler effect (ions only) Frequency modulation (  m =500 MHz) heterodyne “beat” detection avoid 1/ f noise (e.g. laser noise) cc cc c+ mc+ m  c -  m After EOM Conclusions We have observed seven new rovibrational transitions of H 3 +. For comparison, the strongest observed line is 3.5 times weaker than the strongest line observed in the previous work by Gottfried et al. These transitions are the highest energy transitions observed to date and will assist in refining ab initio calculations. A search for a predicted transition near 13,941.6 cm -1 was performed, but two strong H 2 * transitions at 13, and 13, cm -1 are present, possibly on top of the H 3 + transition. Additional transitions between 10,000-11,000 cm -1 in the long- wavelength optics region of the Ti:Sapphire laser expect to be observed in the coming months. Higher energy transitions of H 3 + extending further into the visible will require significant improvements in experimental sensitivity. Results l -N 2 gas cell length = 1 m inner bore diameter = 18 mm -N 2 jacket T rot ~400 K outer jacket under vacuum kHz Verdi-pumped Ti:Sapphire ring laser ~12,000-14,000 cm -1 (Short-wave optics set) 1.3 W max power (cw) Iodine (~670° C) used as a reference gas Custom software was designed to allow better control of the data acquisition process Schematic Diagram Laser System Reagent gases: 500 mTorr H 2 optional 10 Torr He (to suppress H 2 * ) Burleigh Wavemeter WA-1500 Theoretical Spectrum and Observed Lines Potential Energy Surface Acknowledgements Special thanks to J. Gottfried who did the first experimental studies of H 3 + above the barrier to linearity and provided some of the figures, J. Watson for the expectation values, J. Tennyson for the intensity information, and A. Alijah for the assignments and calculations. This work was also supported by NSF Grant No. PHYS Comparison of Calculated and Experimental Energy Levels Rovibrational Transitions and Expectation Values Below the barrier to linearity ( ~ 10,000 cm -1 ), the approximately good quantum numbers v 1, v 2, l, and G are identifiable, though they deviate from integral values. Above the barrier the mixing of the rovibrational levels increases and these quantum numbers have little significance. However, color-coding the rovibrational energy levels using the good quantum number J shows that even above the barrier to linearity, the low J levels are reasonably well defined. (J. K. G. Watson 2002, personal communication) The double modulation scheme yields a second derivative Gaussian line shape in the 1f channel with near shot-noise-limited sensitivity. Anions and cations travel in opposite directions in velocity modulation which can be seen in the polarity of the line shape. Normally neutrals are not velocity modulated, however in the special case of hydrogen Rydberg (H 2 * ) transitions, the line shapes can appear as an anion or cation. The excitation from an electron impact transfers momentum to the H 2 causing it to be velocity modulated like an anion. Rydberg lines that appear as cations are believed to be caused by stimulated emission. Unfortunately the assignments of H 2 * in this region are incomplete. As can be seen in the Results section, the lines in the blue traces are mostly H 2 *, but the addition of helium suppresses all but the strongest transitions, while the H 3 + remain virtually unaffected allowing for chemical discrimination. The Black Widow Plasma Tube Intensity relative to the R(1,0) transition of the fundamental band