High-Precision Sub-Doppler Infrared Spectroscopy of HeH + Adam J. Perry, James N. Hodges, Charles Markus, G. Stephen Kocheril, Paul A. Jenkins II, and Benjamin J. McCall 69 th International Symposium on Molecular Spectroscopy University of Illinois at Urbana-Champaign 20 June 2014 FA01

Overview Introduction Spectroscopic Technique Results Future Directions/Conclusions He H

HeH + Background HeH + is one of the first molecules formed in the early universe Thought to be present in many astronomical environments –Planetary nebulea –Dense clouds –Supernovae No unequivocal detection has been made Hubble Space Telescope image of the planetary nebulae NGC From S. Lepp, Astrophys. Space Sci. 285, 737 (2003) S. Lepp, P. C. Stancil, and A. Dalgarno, J. Phys. B 35, R57 (2002) Dabrowski and G. Herzberg, Top. N. Y. Acad. Sci. 2 38, 14 (1977) J. H. Black, Astrophys. J. 222, 125 (1978) W. Roberge and A. Dalgarno, Astrophys. J. 255, 489 (1982)

Theoretical Investigations Simplest 2-electron system/heteronuclear molecule Excellent benchmark for ab initio calculations with QED and relativistic corrections Great for studying isotope effects –Breakdown of Born-Oppenheimer Approx. –See talk FA02 next Along with H 2 and H 3 +, only other molecule to have rovibrational transitions calculated with spectroscopic accuracy (~0.01 cm -1 ) K. Pachucki, and J. Komasa, J. Chem. Phys 137, (2012) W. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 137, , (2012)

Previous Experimental Work First rovibrational spectrum in 1979 by Tolliver et al. –Doppler-tuned ion beam –P(12) and P(13) lines P(4)-R(4) observed by Bernath and Amano (1982) –30-60 MHz uncertainty P(5)-P(6) and R(5)-R(7) measured by Crofton et al. (1989) Rotational work by Liu, D. et al., Matsushima et al., and Liu, Z. et al. D. Tolliver, G. Kyrala, and W. Wing, Phys. Rev. Lett., 19, (1979) P. Bernath and T. Amano, Phys. Rev. Lett., 48, 20-22, (1982) D. Liu, W. Ho, and T. Oka, J. Chem. Phys, 87, 2442, (1987) M. Crofton, R. Altman, N. Haese, J. Chem. Phys., 91, 5882 (1989) F. Matsushima, T. Oka, and K. Takagi, Phys. Rev. Lett. 78, (1997) Z. Liu, and P. Davies, Phys. Rev. Lett., 79, (1997) Z. Liu, and P. Davies, J. Chem. Phys, 107, 337 (1997) Spectrum of R(1) transition recorded by Bernath and Amano (1982)

Spectroscopic Method Velocity Modulation (Ion-neutral discrimination) Velocity Modulation (Ion-neutral discrimination) B. M. Siller, et al. Opt. Express 19, , (2011) Heterodyne Detection (Reduction of 1/f technical noise) Heterodyne Detection (Reduction of 1/f technical noise) Cavity Enhancement (Increase signal strength) Cavity Enhancement (Increase signal strength) Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS) NICE-OHVMS

NICE-OHVMS Spectrometer YDFL EOM Lock-In Amplifier X & Y Channels Lock-In Amplifier X & Y Channels 80 MHz 90 o Phase Shift f = 80 kHz idler  pump  signal 40 kHz AOMAOM AOMAOM OPO Wave- meter Frequency Comb Lock Box PZT Slow Fast to PZT ~3 MHz ν µm

Frequency correction applied by AOM keeps signal beat within the bandpass Rep. rate tuned so that signal beat lies within bandpass filter on frequency counter Bandpass regions (on frequency counter) Comb Scanning Frequency Comb Modes Pump offset locked (~20 MHz) to nearest comb mode AOMAOM AOMAOM

HeH + Production Plasma Conditions: 1.8 Torr He 10 mTorr H 2 40 kHz, 170 mA discharge Liquid N 2 cooled Signal very sensitive to H 2 :He ratio HeH + + H 2 → He + H 3 +

Sample NICE-OHVMS Spectrum of HeH + P(1) Fundamental band transition S/N ~ Doppler Width ~800 MHz

Lamb Dip Fit (35) MHz (60) MHz Linecenter: Previous:

Measured Transition Frequencies P. Bernath and T. Amano, Phys. Rev. Lett., 48, 20-22, (1982) TransitionP(2)P(1)R(0)R(2) This Work (MHz) (134) (35) (138) (77) Previous (MHz) (60) (60) (60) (60) Difference (MHz)

Spectroscopic Constants ParameterThis Work (MHz)Liu & Davies (MHz)Matsushima et al. (MHz) ν0ν (34) (33) (16) B0B a (45) (29) D0D a (96) (42) H0H a (11) (99) L 0 x (25)-1.331(36) b B1B (45) (45) (14) D1D (46) (99) (26) H1H (76) (84) (20) L 1 x (34)-1.498(33) b a. Fixed at value of Matsushima et al. b. Fixed to ab initio values F. Matsushima, T. Oka, and K. Takagi, Phys. Rev. Lett. 78, (1997) Z. Liu, and P. Davies, J. Chem. Phys, 107, 337 (1997)

Conclusions HeH + studied with NICE-OHVMS Measured 4 fundamental band transitions of HeH + with precision of ~1 MHz Improved precision on the band origin and B 1 These new measurements plus ones currently underway should aid theorists calculating empirical potentials –(see talk FA02 next)

Acknowledgments Advisor: Ben McCall Group Members: James Hodges Charles Markus George Kocheril Paul Jenkins Funding Agencies