Transcript Slide 1

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 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 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.
Spectrum of R(1) transition recorded by
Bernath and Amano (1982)
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)
Spectroscopic Method
Velocity Modulation
(Ion-neutral
discrimination)
NICE-OHVMS
Heterodyne
Detection
(Reduction of 1/f
technical noise)
Cavity Enhancement
(Increase signal
strength)
Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy
(NICE-OHVMS)
B. M. Siller, et al. Opt. Express 19, 24822-7, (2011)
NICE-OHVMS Spectrometer
A
O
M
Frequency
Comb
Wavemeter
to PZT
40 kHz
PZT
3.2-3.9 µm
OPO
nidler = npump - nsignal
~3 MHz
EOM
Lock-In
Amplifier
80 MHz
YDFL
X&Y
Channels
ν
Fast
Lock-In
Amplifier
f = 80 kHz
X&Y
Channels
90o Phase Shift
Lock
Box
Slow
Comb Scanning
Rep. rateFrequency
tuned so that
lies within
Comb Modes signal beat
correction
applied by
A
filterkeeps
on signal Pump offset locked (~20 MHz) to
AOM
O bandpass
M frequency counter
nearest comb mode
beat within the
bandpass
Frequency
Bandpass regions (on frequency counter)
HeH+ Production
Plasma 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
Linecenter: 85258146.91(35) MHz
Previous: 85258082(60)
MHz
Measured Transition Frequencies
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
P. Bernath and T. Amano, Phys. Rev. Lett., 48, 20-22, (1982)
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 Hodges
Charles Markus
George Kocheril
Paul Jenkins
• Funding Agencies