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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

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 (>10,000 cm−1), the regime in which H3

+ 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 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).

Direction of laser beam

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+ m

c - mAfter EOM

ConclusionsWe 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 Ti:Sapphire 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.

15

22

21 2

41 2

22

. ( )( 10 )

6 0 (3,3) 9.47 12502.341 12502.606(10)

4 0 (3,3) 17.1 12657.560 12658.335(10)

4 0 (1,0) 6.39 12897.016 12897.888(10)

6 0 (1,0) 10.5 130

n

n

t

t

RelativeBand Assignment Theor Observed cmIntensity

P

R

R

R

55.237 13056.013(10)

(1,0) 6.27 13596.985 13597.367(10)

(3,3) 7.05 13604.842 13606.093(10)

(1,0) 6.99 13674.236 13676.446(10)

Q

R

Q

Results

2

2

+2

+ +2 3

2

H H

H H H H

e e

l-N2

gas cell length = 1 m

inner bore diameter = 18 mm

-N2 jacket

Trot ~400 K

outer jacket under vacuum

500 mA @ 19 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 H2

optional 10 Torr He (to suppress H2

*)

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 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.

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 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)

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.

The Black Widow Plasma Tube

622

Intensity relative to the R(1,0) transition of the fundamental band

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