Collisional Cooling (CC) of Neutrals -- an experimental technique

Figure: Collisional Cooling System.

Over the past 25 years we have developed and characterized a simple methodology for the study of gas phase molecules at temperatures far below their freezing points [1]. This is accomplished by injecting small amounts of warm, spectroscopically active gas into cells filled with cold He or H2. Because at typical spectroscopic pressures the warm gasses collisionally cool to the temperature of the background gas in far fewer collisions than required to reach a wall (where they condense), these collisionally cooled (CC) molecules are attractive subjects for a wide range of studies.

As an example, Figure: Collisional Cooling System shows the layout of one of the collisional cooling systems. The entire apparatus is in a vacuum chamber maintained at a pressure below 10-5 Torr. Shields at 77 K (maintained by liquid nitrogen) and at 4 K (maintained by liquid helium) surround the region which contains the collisionally cooled cell. This cell is connected to a helium pot which is filled via a continuous fill capillary from the helium reservoir and pumped via an external pumping line. The cell is made of oxygen-free high conductivity copper and has 5 mil mylar windows. All removable flanges are sealed with indium. The temperature of the cell is varied by pumping on the pot through a vacuum regulator valve, which accurately controls the pressure and thus the temperature of the liquid helium. Since in this system the cell is isolated from the helium reservoir, temperatures above 4.3 K are also readily maintained. This technique allows the straightforward variation and control of the temperature, which is read directly with calibrated germanium resistance thermometers. The cell pressure is measured by a capacitance manometer at room temperature, with a correction for the effects of thermal transpiration.

At the side of the experimental chamber, warm spectroscopically active gas is injected via a vacuum insulated tube. Provisions exist for electrically heating this tube, but under normal conditions this has not been necessary for the relatively volatile gases used. The collisionally cooled cell is first filled with a static pressure of helium or hydrogen in equilibrium with the wall temperature. The spectroscopically active gas then cools as it collides with the cold buffer gas. The quantitative relations that make this a useful and general technique are: (1) the injected gas cools very rapidly as it collides with the buffer gas, requiring far fewer than 100 collisions to closely approach the temperature of the background; (2) at typical pressures, about 10000 collisions are required for the gas to reach the walls and to condense; and (3) the very large absorption coefficients characteristic of very low temperature spectroscopy make possible large dilution ratios that ensure the concentration of the spectroscopic gas is so small as to not perturb the temperature of the buffer gas. In order to avoid the corrections to lineshape that are associated with large absorption, the flow rate is typically adjusted to produce an absorption of 1% - 10%. This corresponds to dilution ratios at temperatures around 4 K (depending upon the absorption coefficient of the gas) of ~ 10-4 - 10-6.

Collisional cooling has now been used by a number of laboratories in a rather wide variety of applications. In an experiment which combined a collisionally cooled system built by Dan Willey's group at Allegheny College with the infrared lasers of George Flynn's group at Columbia, Willey et al. have extended the technique into the infrared and shown that both the translational temperature and rotational temperature are consistent with the measured wall temperature at the somewhat higher pressures and injector flow rates appropriate for infrared studies [7]. In this experiment differential rotational relaxation of the initially warm NH3 in collision with cold He produces a significant population inversion on the (J,K = 4,3) transition.

References

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