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An Ultracold Gas of Electrically Trapped Formaldehyde

Alexander Prehn

Published Jan 1, 2018• 1 citation
OpenAlex

Abstract

In this thesis, direct cooling of gaseous formaldehyde (H2CO) to the ultracold regime (T < 1 mK) is experimentally demonstrated. Employing optoelectrical Sisyphus cooling, a dissipative cooling method applicable to electrically trapped dipolar molecules, an ensemble of about 300,000 molecules with a temperature of around 400 µK is generated. More than 80% of these molecules populate a single rotational state. These results are used to implement a technique for high-resolution spectroscopy on electrically trapped molecules.

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In this thesis, direct cooling of gaseous formaldehyde (H2CO) to the ultracold regime (T < 1 mK) is experimentally demonstrated. Employing optoelectrical Sisyphus cooling, a dissipative cooling method applicable to electrically trapped dipolar molecules, an ensemble of about 300,000 molecules with a temperature of around 400 µK is generated. More than 80% of these molecules populate a single rotational state. These results are used to implement a technique for high-resolution spectroscopy on electrically trapped molecules.
Rapid progress in cooling and trapping of molecules has enabled first experiments on high-resolution spectroscopy of trapped diatomic molecules, promising unprecedented precision. Extending this work to polyatomic molecules provides unique opportunities due to more complex geometries and additional internal degrees of freedom. Here, this is achieved by combining a homogeneous-field microstructured electric trap, rotational transitions with minimal Stark broadening at a"magic" offset electric field, and optoelectrical Sisyphus cooling of molecules to the low millikelvin temperature regime. We thereby reduce Stark broadening on the J=5←4 (K=3) transition of formaldehyde at 364 GHz to well below 1 kHz, observe Doppler-limited linewidths down to 3.8 kHz, and determine the magic-field line position with an uncertainty below 100 Hz. Our approach opens a multitude of possibilities for investigating diverse polyatomic molecule species.
We present an optoelectrical cooling scheme for polar molecules based on a Sisyphus-type cooling cycle in suitably tailored electric trapping fields. Dissipation is provided by spontaneous vibrational decay in a closed level scheme found in symmetric-top rotors comprising six low-field-seeking rovibrational states. A generic trap design is presented. Suitable molecules are identified with vibrational decay rates on the order of 100 Hz. A simulation of the cooling process shows that the molecular temperature can be reduced from 1 K to 1 mK in approximately 10 s. The molecules remain electrically trapped during this time, indicating that the ultracold regime can be reached in an experimentally feasible scheme.
We present progress towards the experimental realization of optoelectrical cooling [1] which is widely applicable for producing samples of ultracold (<1 mK) polar molecules. This scheme exploits the interaction between trapped molecules and electric fields to remove energy, while a spontaneous vibrational decay removes entropy. The trap, a key element of this method, must not only provide long lifetimes, but also regions of variable homogenous electric fields, allowing the required addressing of transitions between individual rotational sublevels. We consider in detail the design of this microstructured electrical trap, where a trap depth of 1 K can be achieved. Careful patterning of the electrodes allows a suppression of trap losses by Majorana flips. [1] M. Zeppenfeld et al., Phys. Rev. A 80, 041401(R) (2009).
This thesis reports on the combination of buffer-gas cooling with electric velocity filtering to produce a high-density guided beam of slow and internally cold polar molecules. In the continuous source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electric quadrupole guide. For formaldehyde the population of individual rotational states in the guide is monitored by depletion spectroscopy, resulting in single-state populations up to 82 %. This thesis also reports on a technique to produce pulses of slow guided molecules. The pulses exhibit a narrow velocity distribution around a tunable velocity.