Many fields of science are interested in using beams of molecules with kinetic energies below 20 wave numbers: astronomers and chemists could understand chemical reaction mechanisms below 20 K, spectroscopists could make very precise measurements, and molecular physicists could chart the potential energy surfaces more accurately. Several of the already existing cold molecule source designs are presented.
This work describes the design and testing of a cold source based on the collisional cooling technique: The molecules (ammonia) are injected in the middle of a precooled buffer gas (helium at 4.2 K). The molecules are cooled by collisions with helium atoms well below their freezing point without condensation. The molecules leave the cell through a 1 millimeter hole. The beam is pulsed by either a shutter blade or a chopper (1 millisecond pulse length).
In test experiments the emerging molecules are focused and detected. The molecules are organized into a beam by an electrostatic quadrupole state selector. The operation of the state selector is analyzed in details. Two detectors are used: a microwave cavity, and a mass spectrometer. The design of the microwave cavity (Fabry-Perot) is presented. Time of flight technique is used to measure the kinetic energies.
Computer simulation is used to calculate the molecule trajectories and to predict the detector signals. The simulations revealed possible artifacts caused by the focuser. Measures to eliminate the artifacts are presented.
The experimental measurements unveiled further undesired effects: cluster formation, snow accumulation and high sensitivity to helium pressures. The cluster formation has been reduced by switching from closed cell configuration to open cell. The snow accumulation inside the cell limited the experimenting time to 20 minutes. The high sensitivity to helium pressure made pressure stabilization necessary.
In the optimal regime of the source cell, the speed distribution of the beam has the following characteristics: about 100000 molecules arrive at the detector per unit speed interval and per pulse; the useful speed interval is 55-500 m/s.