Supersonic Expansion Cooling

• Supersonic Expansion Cooling for Cold Molecular Beams

Molecules at ambient temperature contain thermal energy from their surroundings that manifests in vibrational, rotational, and translational motion. That motion "blurs out" much of the spectral and structural information that can be retrieved from a spectroscopy experiment due to probing a distribution of molecules of various energy states (according to the Boltzmann distribution). The recorded spectrum is generally either highly congested (vapor), or more often, all of the spectral lines smear out into broad bands nearly completely obscuring the rich spectral information underneath.

By entraining the molecule in a high pressure rare gas such as helium or argon, and allowing the mixture to expand into a vacuum, the molecule can be cooled down to a temperature of ~2 K and isolated. The difference in pressures on either side of the jet nozzle allow for a supersonic expansion of the gas mixture into the vacuum. This results in numerous collisions of the rare gas with the sample immediately at the aperture of the nozzle, leading to transfer of the sample molecule's internal energy to kinetic energy of the rare gas. After this "collisional region," the sample and rare gas are traveling at the same velocity through the vacuum chamber directed toward the probe region of the chamber, effectively isolated from every other particle in the expansion. By skimming the center of the expansion, a molecular beam of cold, isolated molecules is selected for interrogation by a tunable laser.