Stefan Truppe MM-Wave Spectroscopy and Determination of the Radiative branching ratios of 11 BH for Laser Cooling Experiments

Why should we cool molecules? L. D. Carr et al., New Journal of Physics 11 (2009) Physics beyond the Standard Model – Electric dipole moment of the electron – Variations of fundamental constants High resolution spectroscopy and quantum control Dipolar quantum gas, novel phases of matter Quantum information/simulation Cold and ultracold chemistry

rotational angular momentum parity How to cool molecules – laser cooling? Large number of states makes direct laser cooling challenging

Pick the right molecule and it might work! Criteria for molecule selection: Favourable vibrational branching ratios 1 Rotational transitions limited by selection rules 2 Simple, well understood hyperfine structure Available sources? Convenient transition with reasonable linewidth Mass Dipole moment 1 M. D. Di Rosa, The European Journal D 31, 395 (2004) 2 B. K. Stuhl et al., Physical Review Letters 101, (2008) 3 E. S. Shuman et al., Nature 467, 820 (2010)

A simple toy model molecule BH First successful laser cooling of SrF in Yale Since then: 2D MOT of JILA Chirped laser slowing & cooling of Imperial SrF Yale Open challenges: molecules are hotter than expected (~1 mK) high phase-space density reach the ultracold 1µK Solution: use the Q(1) line of a 1 Σ- 1 Π transition (BH, AlH, NH, ScF, CS, AlF, BeO) Advantages:simple level structure upper state magnetic g-factor ~ 1, whereas lower state g-factor ~ 0  model system for Zeeman slower and MOT parit y J

Crucial: vibrational branching ratios or what is the number of re-pump lasers we need? v’’ – v’rel. A coeff. [ref. 1] rel. A coeff. [ref. 2] 0 – 0 433nm – 1 480nm – 2 536nm – 3 603nm Theory: [1] J. Mol. Spec. 145, 200 (1991) [2] Chem. Phys. 115, 15 (1987) v’ = 0 v’’ = 1 v’’ = 2 v’’ = 3 v’’ = 0

Better to be safe and measure the branching ratios First: we need a source! Supersonic expansion Photodissociation (193nm, 20ns, 1x4mm2) of 1% diborane (B 2 H 6 ) in 4 bar of Ar 10 9 molecules/sr/pulse, 0.4K, v~580 m/s using Ar B 2 H 6 + argon gas in X 1 Σ-A 1 Π λ ≈ 433nm linewidth γ ≈ 1.2 MHz T D ≈ 30 μK 11 BH mass ≈ 12 amu 6000 photons to stop

The experiment v’’ = 0 v’’ = 1v’’ = 2 v’ = 0 v’’ = 1 v’’ = 2 v’’ = 3

The results v’’ – v’rel. A coeff. [ref. 1] rel. A coeff. [ref. 2] measurement 0 – 0 433nm (19) 0 – 1 480nm (18) 0 – 2 536nm (15) 0 – 3 603nm < 9.46 x [1] J. Mol. Spec. 145, 200 (1991) [2] Chem. Phys. 115, 15 (1987) 2 lasers: each molecule scatters 1000 photons (remove 77m/s)  slow a cryogenic buffer gas beam into a trap 3 lasers: each molecule scatters > photons  Zeeman slower and MOT

Discussion Additional fluorescence which is not induced by the probe laser (metastables, ions)? Use the filters in random order and orientation. Variation in molecular flux. Important: measure the transmittance of the dominant 433nm fluorescence through the filters. Measured the branching ratios for R(0) and Q(1)  agree within the errors. Measure the transmittance through lenses and beamsplitter. Calibrate the signal PMT using a lamp, grating spectrometer and calibrated silicon photodiode. No signal of the spin-forbidden A 1 Π  a 3 Π near 788 nm (<10 -4 ).

What about the hyperfine structure? Hyperfine structure: H=A B (I B ·J)+A H (I H ·J) (A-state) J=1 level is split into three components labelled by F 1 (F 1 =I B +J) Each F 1 level is split into two labelled by F (F= F 1 +I H ) 3 MHz 300 MHz

What about the hyperfine structure - measurements Hyperfine structure of the A state: extract constants A B and A H from LIF spectrum.

What about the hyperfine structure - measurements Hyperfine structure of the X state: electric quadrupole moment of the Boron nucleus with the local electric field gradient (eq B Q B ) interaction of the magnetic dipole moment of the boron nucleus with the magnetic filed generated by the rotation of the molecule (c B )

Take home message BH is a great candidate for laser cooling experiments (light, stable (dangerous) source, only three laser frequencies necessary) Ideal system to use a Zeeman slower to load a MOT and cool the molecules to the recoil temperature of 4µK We populate J=1 using resonant mm-waves We need to broaden our laser to 3 MHz and modulate the polarization to destabilize dark Zeeman levels We improved/measured for the first time spectroscopic constants

Thanks Rich HendricksMike Tarbutt Ed Hinds Darren Holland Ben Sauer Ed Hinds