Two-photon%20Precision%20Spectroscopy%20of%20H2

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Two-photon Precision Spectroscopy of H2
Jean-Philippe Karr Albane Douillet Vu-Quang Tran,
PhD Laurent Hilico
Vladimir Korobov
Rachidi Osseni, post doc Jofre Pedregosa, post
doc Franck Bielsa, PhD Tristan Valenzuela, post
doc
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• outline

• Motivations
• Experimental status
• Theoretical progress

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mp/me measurement
Why ?

• Fundamental constant determination

mp/me
codata
1836.152 672 45 (75) 4.1 10-10
Fine structure constant a 1/137,03

• e- g-2 measurement, G. Gabrielse 2008

a-1 137.035 999 084 (51) (3.7 10-10)
a-1 137.035 999 037 (91) (6.6 10-10)

• h/MRb measurement, F. Biraben 2010

h/mRb 7. 10-10 ? 4.2 10-10
mx/my lt 10-10 ? 3.5 10-10
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mp/me measurement
Why ?

• Fundamental constant time-variations

Astrophysics and spectroscopy H2, HD,
NH3, CO, HCO, HCN
Dt 1010 years
red shifts analysis
Laboratory physics
SF6spectroscopy
Proposals on CaH, MgH, SrH, , GeBr
Ultra narrow lines Low polarisability Low linear
Zeeman effect
HD, H2
at 10-16

• QED test on simple molecules

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mp/me measurement
How ?
mP and me in atomic units are determined
separately through RF measurements in Penning
traps.
Accuracy

• Electron mass

Larmor to cyclotron frequency ratio
C, O, Si

• Proton mass cyclotron frequencies, using
  12C4.
• R.S. Van Dyck, Jr. et al., in Trapped Charged
  Particles and Fundamental Physics
• AIP Conf. Proc. 457, pp. 101-110 (1999).

8.9 10-11
mp 1.007 276 466 812 (90) me 0.000 548 579
909 46 (22)
4.0 10-10
Codata 2011
Mp/me 1836.152 672 45 (75)
4.1 10-10
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mp/me Direct optical determination by H2
spectroscopy
Method

• Doppler-free Two-photon
• spectroscopy
• 21 REMPD
• Trapped ions
• High precision calculations

32.6 THz ( 9.1 µm ) (1091 cm-1) Dn 1600 Hz
Energy (atomic units)
expected
Internuclear distance (atomic unit)
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What do we know on H2 ?
Lundeen group, H2 Rydberg states
L
Jefferts group, Hyperfine or Zeeman spectroscopy
from R.E. Moss, Molecular Physics, 80, 1541 1993.
Project challenges
v

• state selected H2 ion production
• H2 trapping
• REMPD lasers
• High precision calculations

nexp mp/me
Carrington group, Southampton
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Two-photon transition probabilities
How to choose v?v ?
v0 ? v1 transitions
9.1 µm
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Two-photon transition probabilities
How to choose L?L ?
Total nuclear spin I(-1)L
L0, v0 ? L0, v1 l9.128µm
L3, v0 ? L3, v1 l9.205µm
L2, v0 ? L2, v1 l9.166µm
Close to a CO2 laser emission line Quantum
Cascade Laser available
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Experimental setup
Hyperbolic Paul trap
Quantum cascade laser (QCL)
Optical cavity
248 nm KrF excimer Pulsed Laser
2 mm
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IR laser source
HCOOH formic or methanoic acid
MHz
HITRAN
v, L
12
IR source
QCL / CO2 beat note
Free QCL
Quantum cascade Laser
5 MHz
O.I.
2 mm
QCL / CO2 beat note
HCOOH stabilized CO2 laser
lt 200 Hz
Band width 6 MHz
RBW 10 kHz VBW 1 kHz
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IR source

• Results
• optical power 54 mW
• linewidth dn 3kHz
• high finesse cavity (1000)
• Faraday optical isolator at 9.2 µm

F. Bielsa al., Optics Letters 32, 1641-1643
(2007) L. Hilico, Rev. Sc. Instr. 82, 096106
(2011)
G2ph0.3 s-1 p polarization G2ph0.07 s-1 s
polarization

• HCOOH stabilized CO2 laser

Absolute frequency measurement
32 708 391 980.5 (1.0) kHz
F. Bielsa al. J. Mol. Spectrosc. 247, 41-46
(2008)
LPL, Villetaneuse, France
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The ion trap

• 2p x 14 MHz
• DC -10 / 10 VAC 150 V

r0 4.2 mm z0 3 mm
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H2 creation electron impact
T300K
G. Werth Al. Z phys D 28, (1993).
v0 12 v1 19
Rotational distribution
L2 12
Vibrational distribution
Result 0.07 x 0.12 x 0.6 0.5
Hyperfine structure J3/2 40 J5/2 60
Very small !!
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Photodissociation at 248 nm
2
3
1
UV
1- ion creation ( 500) 1,0
s 2- 1 to 30 UV pulses (20 mJ) 0,3
s 3- extraction, time of flight and counting
0.32 mJ
signal
1 adjustable parameter ion cloud size
experiments 0.85 mm num. simulations
0.83 mm
1.10 mJ
3.25 mJ
11.2 mJ
34.0 mJ
114 mJ
Laser pulse number n
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Photodissociation at 248 nm
Results
J.-Ph. Karr al., Applied Phys. B (2011)

• v0 v1 population difference

L2, J5/2
30 pulses at 34 mJ, pv0 - pv1 33
2.4 30 pulses at 114 mJ, pv0 -
pv1 86 6.2 drawback
ion losses

• Photodissociation yield

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Can we perform H2 REMPD spectroscopy ?
Two-photon transitions Photodissociation Trap
losses
Ion number fluctuations
Present experiment signal to noise ratio 0.27

• H2 v0,L2 population
• G2ph

Improvements
SNR 30
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Experimental developments

• State selected H2 ion creation
  increase v0 v1 population difference

H2 v0, L0, 1, 2 à 300 K
H2 X1Sg, v0, L2
3 hn
H2 C 1Pu-, v0, L2
31 REMPI
hn
mJ 303 nm 10 ns
H2 X Sg, v0, L e-
V. Mac Koy
Anderson, Al, Chem. Phys. Lett. 105, 22 (1984)
H2 branching ratios
v0 v1 L2, J5/2 pop. diff. 0.8 x 1 x 0.6
0.48
L
v 0 1 1 0.1
Photo-electron yield

• 0 0.005
• 1
• 4 0.01

OHalloran, J. Chem. Phys. 87, 3288 (1987)
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Experimental developments

• A linear trap for tighter focussing

waist 3
G2ph x 81

• H2 sympathetic cooling by laser cooled Be ions

T 300 K
7 kHz
Second order Doppler effect
T 20 mK
negligible
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