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Physical Review A

Femtosecond laser detection of Stark-decelerated and trapped methylfluoride molecules

Congsen Meng, Aernout P. P. van der Poel, Cunfeng Cheng, Hendrick L. Bethlem

Published Aug 7, 2015• 17 citations
Publisher

Abstract

We demonstrate deceleration and trapping of methylfluoride $({\mathrm{CH}}_{3}\mathrm{F})$ molecules in the low-field-seeking component of the $J=1,K=1$ state using a combination of a conventional Stark decelerator and a traveling wave decelerator. The methylfluoride molecules are detected by nonresonant multiphoton ionization using a femtosecond laser. Subsequent mass and velocity selection of the produced ions enables us to eliminate most background signal resulting from thermal gas in our vacuum chamber. This detection method can be applied to virtually any molecule, thereby enhancing the scope of molecules that can be Stark decelerated. Methylfluoride is so far the heaviest and most complex molecule that has been decelerated to rest. Typically we trap $2\ifmmode\times\else\texttimes\fi{}{10}^{4} {\text{CH}}_{3}\text{F}$ molecules at a peak density of $4.5\ifmmode\times\else\texttimes\fi{}{10}^{7} {\text{cm}}^{\ensuremath{-}3}$ and a temperature of 40 mK.

DOI
10.1103/physreva.92.023404

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We demonstrate deceleration and trapping of methylfluoride (CH3F)({\mathrm{CH}}_{3}\mathrm{F})(CH3​F) molecules in the low-field-seeking component of the J=1,K=1J=1,K=1J=1,K=1 state using a combination of a conventional Stark decelerator and a traveling wave decelerator. The methylfluoride molecules are detected by nonresonant multiphoton ionization using a femtosecond laser. Subsequent mass and velocity selection of the produced ions enables us to eliminate most background signal resulting from thermal gas in our vacuum chamber. This detection method can be applied to virtually any molecule, thereby enhancing the scope of molecules that can be Stark decelerated. Methylfluoride is so far the heaviest and most complex molecule that has been decelerated to rest. Typically we trap 2\ifmmode×\else\texttimes\fi104CH3F2\ifmmode\times\else\texttimes\fi{}{10}^{4} {\text{CH}}_{3}\text{F}2\ifmmode×\else\texttimes\fi104CH3​F molecules at a peak density of 4.5\ifmmode×\else\texttimes\fi107cm\ensuremath−34.5\ifmmode\times\else\texttimes\fi{}{10}^{7} {\text{cm}}^{\ensuremath{-}3}4.5\ifmmode×\else\texttimes\fi107cm\ensuremath−3 and a temperature of 40 mK.
Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually, thereby greatly enhancing the scope of Stark decelerated molecules. For comparison, we detect decelerated and trapped ammonia molecules using two different schemes: (i) nonresonant multiphoton ionization using intense femtosecond (fs) pulses in the near infrared, and (ii) resonance-enhanced multiphoton ionization using nanosecond (ns) pulses from a tunable UV laser. The observed number of ions per shot for both schemes is similar. The fs laser detection scheme suffers from an increased background, which can be effectively eliminated by subsequent mass and velocity selection. To determine the detection volume of the ns laser detection scheme, we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below ∼100 μm. It is concluded that the detection volume for the ns laser detection scheme is 1.5–2 times larger than that of the fs laser detection scheme.
We report on the electrostatic trapping of neutral SrF molecules. The molecules are captured from a cryogenic buffer-gas beam source into the moving traps of a 4.5-m-long traveling-wave Stark decelerator. The SrF molecules in X^{2}Σ^{+}(v=0,N=1) state are brought to rest as the velocity of the moving traps is gradually reduced from 190 m/s to zero. The molecules are held for up to 50 ms in multiple electric traps of the decelerator. The trapped packets have a volume (FWHM) of 1 mm^{3} and a velocity spread of 5(1) m/s, which corresponds to a temperature of 60(20) mK. Our result demonstrates a factor 3 increase in the molecular mass that has been Stark decelerated and trapped. Heavy molecules (mass>100 amu) offer a highly increased sensitivity to probe physics beyond the standard model. This work significantly extends the species of neutral molecules of which slow beams can be created for collision studies, precision measurement, and trapping experiments.
We demonstrate the deceleration of heavy polar molecules in low-field-seeking states by combining a cryogenic source and a traveling-wave Stark decelerator. The cryogenic source provides a high-intensity beam with low speed and temperature, and the traveling-wave decelerator provides large deceleration forces and high phase-space acceptance. We prove these techniques using YbF molecules and find the experimental data to be in excellent agreement with numerical simulations. These methods extend the scope of Stark deceleration to a very wide range of molecules.
Samples of cold and ultracold polar molecules have the potential to revolutionize physical chemistry, precision measurement, and few-body quantum physics. This thesis describes experimental results for the production of cold samples of OH radicals by Stark deceleration of a supersonic beam. Since Stark deceleration cannot increase the phase space density of the sample, the initial production stage of the OH molecule is critical. The first set of experiments describes a general methodology for the production of OH beams with maximal phase space density, as well as the subsequent coupling to a Stark decelerator. Additionally, we describe the redesign of our electrostatic trap, optimized for future collision experiments of OH with co-trapped ultracold Rb atoms. The new design resulted in a 15-fold increase in total number of trapped molecules over the previous design when tested with ND3 molecules. The second set of experiments focuses on laser-based detection of OH molecules at the exit of the decelerator based on laser induced fluorescence (LIF) and resonance enhanced multiphoton ionizaton (REMPI). The latter method uses vacuum-ultraviolet light at 118nm produced by third-harmonic generation in Xe/Ar gas mixtures as the ionizing step. The sensitivity of this latter technique is limited by the attainable photon flux of the ionizing radiation at 118nm. We present detailed measurements of the conversion efficiencies as well as absolute photon fluxes. Strategies to overcome these limitations as well as prospects for detection of OH molecules in a trap are discussed.
The growing field of cold molecule physics has demonstrated many methods of quantum control for precision measurement and molecular interaction studies. One particular method is Stark deceleration, which uses electric field gradients to produce packets of slow polar molecules. Since Stark deceleration does not increase phase-space density, the efficiency of the deceleration process is critical in determining the final cold molecule density. This dissertation describes three different Stark deceleration techniques. The first type of deceleration is pulsed-pin deceleration. We use this technique to produce slow OH molecules that can be trapped for cold collision studies. A two photon state-selective detection system, which includes a 118 nm photon, is used to determine the density distribution of OH in the trap. We explore the advantages and limitations to creating 118 nm light. The second and third types of Stark deceleration are pulsed-ring and traveling-wave methods of running a ring-geometry decelerator. Ring-geometry decelerators present improved deceleration efficiencies due to the cylindrical symmetry of the electrodes and true three-dimensional confinement. The pulsed-ring mode of the ring decelerator uses commercial high-voltage switches to apply discrete pulses to the ring electrodes. The continuous operation mode of the ring decelerator uses varying sine-wave voltages to create a traveling Stark potential well and has proven to be incredibly challenging due to the analog high voltage requirements of the system. In order to bring ND3 seeded in krypton (∼415 m/s) down to rest in the laboratory, sine wave voltages with amplitudes up to ±10 kV, currents up to 500 mA, and a bandwidth of 30 kHz down to DC are required. We describe a high-voltage amplifier that can be used for traveling-wave deceleration. The traveling-wave Stark deceleration mode is then compared to the pulsed-ring Stark deceleration mode and cases where one mode is superior to another are discussed.
A near-resonant, red-detuning laser-assisted Stark deceleration scheme is proposed to slow CaF in its high-field-seeking rovibronic ground state. The assisting Gaussian laser beam can confine CaF molecules transversely owing to the optical Stark effect. Simulations suggest that the present scheme is superior to previous Stark decelerators. Under typical experimental conditions, when the assisting laser frequency is red-detuned to the molecular transition ( λ ∼606.3 nm) by 5.0 GHz and the laser power is about 5.6 W, the proposed decelerator can achieve a total number at the order of 10 4 CaF molecules with a number density at the order of 10 8 cm −3 . The equivalent temperature of the obtained cold CaF molecules is 2.3 mK. Additionally, the desired assisting laser power can be as low as about 1.2 W if keeping the red-detuning value to be 1.0 GHz, which further suggests its experimental feasibility.