Stefan Truppe MEASUREMENT OF THE LOWEST MILLIMETER- WAVE TRANSITION FREQUENCY OF THE CH RADICAL.
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Transcript of Stefan Truppe MEASUREMENT OF THE LOWEST MILLIMETER- WAVE TRANSITION FREQUENCY OF THE CH RADICAL.
Stefan Truppe
MEASUREMENT OF THE LOWEST MILLIMETER-WAVE TRANSITION FREQUENCY OF THE CH RADICAL
Motivation – many good reasons to improve the lab frequencies
1A. J. de Nijs, W. Ubachs, H. L. Bethlem, PRA, 86, 032501, (2012)2M. Gerin et al., A&A 521, L16 (2010), S. L. Qin et al., A&A 521, L14 (2010)
3S. Muller et al., arxiv:1309.3301 (2014)
• Study stellar atmospheres and interstellar gas clouds.
• Essential role in combustion processes.
• Tracer for molecular hydrogen.
• Basic constituent of interstellar chemistry.
• Highly sensitive to possible variations in the electron-to-proton mass ratio, μ and the fine-structure constant, α:1
– A natural solution to fine tuning.– Probe physics beyond the Standard Model (sting theories, dark energy).
• High resolution mm-wave spectra in our own and nearby galaxies using Herschel.2
• First detection of CH at z=0.89 (PKS1830-211) using ALMA.3
The CH molecule – level structure
Shorthand: (Jp, F)
Λ-doubling frequencies are known to Hz-level accuracy1,2
Aim: improve the lowest mm-wave transition between J=1/2
and J=3/2 (<1kHz)
1S. Truppe et al., Nature Communications 4, 2600 (2013)2S. Truppe et al., Journal of Molecular Spectroscopy 300, 70 (2014)
Click
Production: 248 nm photodissociation of bromoform (CHBr3, 2x109/sr/pulse, 10Hz)
Detection: Laser-induced fluorescence on X2Π (v=0) – A2Δ(v=0) transition near 430nm
430 nm(CW, 5mW, doubled Ti:Sapph)
248 nm(20ns, 220mJ)
CHBr3
Ar 4 bar
Supersonic expansion
(10Hz)
Time resolvedlaser induced fluorescence
CH – production and detection
Skimmer
CH - optical spectrum and TOF!
Optical spectrum: excitation on the R22ff (1/2) line of the A-X transition.
Time-of-flight profile of the molecular pulse (T~0.4K, v=400-2000m/s).
Source produces cold molecules @ 0.4K >90% of the molecules in J=1/2
The experiment - hardware
• Laser-mm-wave-double-resonance technique
T. Amano, The Astrophysical Journal 531, L161 (2000)
The experiment – the measurement
• Laser-mm-wave-double-resonance technique.
• Depletion of the J=1/2 population: lock the laser to R22(1/2) of A-X and scan the mm-waves.
• Increase of the J=3/2 population: lock the laser to R11(3/2) of A-X and scan the mm-waves.
• 7 µW of radiation in a Gaussian beam (waist 5 mm).
(3/2+,2)-(1/2-,1)
The experiment – what’s the line shape?
• Model the experiment:– Gaussian intensity distribution– Wavefront curvature of the mm-
wave beam– Doppler broadening due to range of
transverse velocities
Expect: Gaussian line shape with a FWHM of 58 kHz for molecules with a speed of 567m/s (Ar as carrier gas).
We measure: 62 ± 2 kHz
(3/2+,2)-(1/2-,1)
The experiment – systematic checks
• Residual Doppler shift:– change the carrier gas– linear dependence extrapolate to zero velocity
• Systematic shifts due to the Zeeman effect:– Use the molecules to measure the
residual field (13 nT along z, x and y are at least 10x smaller)
– 13 nT leads to a symmetric splitting of 120 Hz only.
• dc Stark effect, motional Stark effect, ac Stark effect, collisions, blackbody radiation and second-order Doppler are negligible.
Results1
• Repeat the measurement at least 4 times for each carrier gas.
• Repeat the Doppler measurements 4 times.
• Take the weighted mean as the final frequency.
• Repeat everything for (3/2+,1)-(1/2-,1) to double check.
• Together with Λ-doublet freqeuncies we get all 6 hyperfine lines.
1 S. Truppe, et al., The Astrophysical Journal 780, 71 (2014)
Discussion & Outlook
• Improved the absolute accuracy of the lowest mm-wave transition to almost 1ppb.
• The new frequencies are 50-150 times more precise than the previous best values and differ from them by up to 3.6 standard deviations.
• Uncertainty in lab frequencies no longer hinder the search for varying constants.
• Allows also more accurate velocity determinations in astrophysical measurements.
• In a Ramsey experiment we could easily reach 1Hz accuracy on 1THz frequency measurement.
Thanks
Rich Hendricks
Mike Tarbutt Ed Hinds