HighResolution Visible Spectroscopy of H3

1
High-Resolution Visible
Spectroscopy of H3
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 60637 USA

• Introduction
• Seven new rovibrational transitions of H3 have
  been observed in the visible region between
  12,500-13,700 cm-1. These energy levels are
  above the barrier to linearity (gt10,000 cm-1),
  the regime in which H3 has enough energy to
  sample linear configurations. A high-resolution,
  high-sensitivity spectrometer based on a
  TiSapphire laser and incorporating velocity
  modulation and phase modulation with heterodyne
  detectiona 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/H2 plasmas were used to identify the much
  weaker H3 transitions. The sparsity and
  weakness of the lines necessitated the use of the
  predicted intensities and frequenciesbc to focus
  our search. The measured rovibrational energy
  levels will assist in the development of
  theoretical calculations of H3 and provide an
  experimental check of ab initio calculations in
  this region.
• a J. Gottfried, B. McCall, and T. Oka, J. Chem.
  Phys. 118, 10890 (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).

Rovibrational Transitions and Expectation
Values Below the barrier to linearity ( 10,000
cm-1), the approximately good quantum numbers v1,
v2, 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)

• 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 (wm500 MHz)
• heterodyne beat detection
• avoid 1/f noise (e.g. laser noise)

Potential Energy Surface
6?22
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 (H2)
transitions, the line shapes can appear as an
anion or cation. The excitation from an electron
impact transfers momentum to the H2 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 H2 in this region are incomplete.
As can be seen in the Results section, the lines
in the blue traces are mostly H2, but the
addition of helium suppresses all but the
strongest transitions, while the H3 remain
virtually unaffected allowing for chemical
discrimination.
Results
Laser System

• Verdi-pumped TiSapphire 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
The Black Widow Plasma Tube
Intensity relative to the R(1,0) transition of
the fundamental band

• cell length 1 m
• inner bore diameter 18 mm
• l-N2 jacket
• Trot 400 K
• outer jacket under vacuum
• 500 mA _at_ 19 kHz

Comparison of Calculated and Experimental Energy
Levels
Conclusions We have observed seven new
rovibrational transitions of H3. 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 H2 transitions at 13,943.24 and
13,945.45 cm-1 are present, possibly on top of
the H3 transition. Additional transitions
between 10,000-11,000 cm-1 in the long-wavelength
optics region of the TiSapphire laser expect to
be observed in the coming months. Higher energy
transitions of H3 extending further into the
visible will require significant improvements in
experimental sensitivity.

• Reagent gases
• 500 mTorr H2
• optional 10 Torr He (to suppress H2)

Acknowledgements Special thanks to J. Gottfried
who did the first experimental studies of H3
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-0354200.