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