Physical and Chemical Phenomena in Extreme Supersonic Gas Expansions

Extreme supersonic gas expansions through a room temperature nozzle were carried out using a pulsed valve technique, with the product of the pressure behind the nozzle and the nozzle diameter up to 1500 torr cdotcm. That the extraordinarily large quantum ^4He-^4He elastic scattering cross section at very low energies is responsible for the substantial cooling of pure extreme ^4He expansions was directly confirmed experimentally by comparison with ^3He expansions. The lowest translational temperatures achieved for pure ^4He, ^3 He, and normal H_2 expansions were 0.6 mK, 26 mK, and 43 mK, respectively. Expansions of dilute mixtures of various second species with ^4He were also investigated. It was found, in particular, that the seeded normal H_2 was able to cool down translationally to 3.9 mK. In addition, real gas enthalpy of ^3He at high pressures was obtained from the measurements of its expansion beam velocities. The long-standing question of whether ^4He dimer exists has been resolved by detection of this species in extreme ^4He expansions and its binding energy was found to be of an order of 1 mK, consistent with recent theoretical predictions. We have calculated the influence of retardation on the properties of ^4He dimer and have found that its binding energy and size are affected by 11% and 5%, respectively. Thus, ^4He dimer may provide an opportunity to study retardation effect never before seen in molecules. Finally, by measuring the transmission of ^4He dimer beams, relative to that of ^4He atom beams, through a set of sub-micron size holes as a function of the size of the holes, we have determined that the size of ^4He dimer is 6.22 +/- 1.03 nm, which is enormous compared to that of ordinary diatomic molecules.