Stefan Truppe MEASUREMENT OF THE LOWEST MILLIMETER- WAVE TRANSITION FREQUENCY OF THE CH RADICAL

Motivation – many good reasons to improve the lab frequencies 1 A. J. de Nijs, W. Ubachs, H. L. Bethlem, PRA, 86, , (2012) 2 M. Gerin et al., A&A 521, L16 (2010), S. L. Qin et al., A&A 521, L14 (2010) 3 S. Muller et al., arxiv: (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 (PKS ) using ALMA. 3

The CH molecule – level structure Shorthand: (J p, F) Λ-doubling frequencies are known to Hz-level accuracy 1,2 Aim: improve the lowest mm- wave transition between J=1/2 and J=3/2 (<1kHz) 1 S. Truppe et al., Nature Communications 4, 2600 (2013) 2 S. Truppe et al., Journal of Molecular Spectroscopy 300, 70 (2014)

Click Production: 248 nm photodissociation of bromoform (CHBr 3, 2x10 9 /sr/pulse, 10Hz) Detection: Laser-induced fluorescence on X 2 Π (v=0) – A 2 Δ(v=0) transition near 430nm 430 nm (CW, 5mW, doubled Ti:Sapph) 248 nm (20ns, 220mJ) CHBr 3 Ar 4 bar Supersonic expansion (10Hz) Time resolved laser induced fluorescence CH – production and detection Skimmer

CH - optical spectrum and TOF! Optical spectrum: excitation on the R 22ff (1/2) line of the A-X transition. Time-of-flight profile of the molecular pulse (T~0.4K, v= m/s). Source produces cold 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 R 22 (1/2) of A-X and scan the mm-waves. Increase of the J=3/2 population: lock the laser to R 11 (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.

Results 1 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 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