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

69th International Symposium on Molecular SpectroscopyUniversity of Illinois at Urbana-Champaign

20 June 2014FA01

Overview

• Introduction

• Spectroscopic Technique

• Results

• Future Directions/Conclusions

HeH

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 7027. From http://apod.nasa.gov/apod/ap130826.html

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 H2 and H3+, only other molecule to have

rovibrational transitions calculated with spectroscopic accuracy (~0.01 cm-1)

K. Pachucki, and J. Komasa, J. Chem. Phys 137, 204314 (2012)W. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 137, 164305, (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, 1719-1722 (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, 1664-1666 (1997)Z. Liu, and P. Davies, Phys. Rev. Lett., 79, 2779-2782 (1997)Z. Liu, and P. Davies, J. Chem. Phys, 107, 337 (1997)

Spectrum of R(1) transition recorded by Bernath and Amano (1982)

Spectroscopic MethodVelocity Modulation

(Ion-neutral discrimination)

B. M. Siller, et al. Opt. Express 19, 24822-7, (2011)

Heterodyne Detection

(Reduction of 1/f technical noise)

Cavity Enhancement(Increase signal

strength)

Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS)

NICE-OHVMS

NICE-OHVMS Spectrometer

YDFL

EOMLock-In

Amplifier

X & Y Channels

Lock-In Amplifier

X & Y Channels

80 MHz

90o Phase Shift

f = 80 kHz

nidler = npump - nsignal

40 kHz

AOM

OPO

Wave-meter

Frequency Comb

Lock Box

PZT

SlowFast

to PZT

~3 MHz

ν

3.2-3.9 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 ModesPump offset locked (~20 MHz) to nearest comb mode

AOM

HeH+ ProductionPlasma Conditions:• 1.8 Torr He• 10 mTorr H2

• 40 kHz , 170 mA discharge• Liquid N2 cooled• Signal very sensitive to H2:He ratio

HeH+ + H2 → He + H3+

Sample NICE-OHVMS Spectrum of HeH+

• P(1) Fundamental band transition• S/N ~ 140-275• Doppler Width ~800 MHz

Lamb Dip Fit

85258146.91(35) MHz85258082(60) MHz

Linecenter:Previous:

Measured Transition Frequencies

P. Bernath and T. Amano, Phys. Rev. Lett., 48, 20-22, (1982)

Transition P(2) P(1) R(0) R(2)

This Work (MHz) 83096617.69(134) 85258146.90(35) 89115533.66(138) 92275879.63(77)

Previous (MHz) 83096650(60) 85258082(60) 89115502(60) 92275875(60)

Difference(MHz) -32.69 64.80 31.04 3.89

Spectroscopic Constants

Parameter This Work (MHz) Liu & Davies (MHz) Matsushima et al. (MHz)

ν0 87268330.79(34) 87268319(33) 87268308(16)

B0 1006063.617a 1006063.3(45) 1006063.617(29)

D0 486.1956a 486.512(96) 486.1956(42)

H0 0.177809a 0.1843(11) 0.177809(99)

L0 x 104 -0.864(25) -1.331(36) -0.849b

B1 924551.41(45) 924554.8(45) 924559.4(14)

D1 475.215(46) 475.605(99) 475.489(26)

H1 0.16303(76) 0.17049(84) 0.16575(20)

L1 x 104 -9.07(34) -1.498(33) -9.61b

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, 1664-1666 (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 B1

• These new measurements plus ones currently underway should aid theorists calculating empirical potentials – (see talk FA02 next)

Acknowledgments • Advisor: Ben

McCall

• Group Members:James

HodgesCharles

MarkusGeorge

KocherilPaul Jenkins

• Funding Agencies