Speed ratios v/Av greater than 1000 and temperatures less than 1 mK have been achieved for the first time in a gas expansion into vacuum. The pulsed-beam source used in these experiments had an open time of 150 /xsec, a nozzle diameter of 0.125 mm, and was operated with pure He gas at 294 K and pressures between 34 and 136 atm. Speed distributions for the collimated He beam were measured by time of flight. It is possible that such temperatures may be low enough to probe the low-energy limiting behavior predicted by Wigner in 1948 for the single-collision quantum dynamics of atoms and molecules. 34.40.+n, Several recent experiments 1 " 6 and related calculations 7 " 9 have suggested that the microscopic collision dynamics of atomic and molecular systems may exhibit fundamentally interesting and unique features in the limit of extremely low kinetic energies. Not only are quantum resonances 10 of several different types generally expected to become prominent at very low energies, but as the kinetic energy approaches zero the dynamics must become dominated by the fundamental quantum threshold singularities predicted by Wigner in 1948. 11,12 According to the Wigner threshold laws for a two-channel system not subject to Coulomb forces at large separations, the cross section for collisional relaxation of a molecular excited state becomes infinite in the s wave, zero collision velocity limit, while the corresponding microscopic rate coefficient becomes a finite constant. Although the threshold laws are thought to be rigorous, very little is known at present about their range of validity for heavy-particle systems. Two recent calculations, 8,9 both motivated by questions concerning the interpretation of vibrational relaxation data for the h + He system, 1,2 ' 4 showed the threshold regime for a model of that system to be limited to temperatures less than 1 mK. The implications for molecular dynamics and spectroscopy experiments will be very important if it proves to be possible to access the Wigner threshold regime experimentally, since the threshold laws provide a ubiquitous mechanism for relaxation of gas-phase molecules to arbitrarily low temperatures. The question is whether such low temperatures or kinetic energies are accessible in the gas phase. As a first step toward answering that question, we report here the results from a novel approach to the generation of ultracold He beams by means of extreme supersonic expansions. The purpose of these experiments is to learn whether submillikelvin translational temperatures in the gas phase can actually be achieved and measured in a nominally pure beam of He, which is the most favorable case. If so, this would offer hope for our performing similar experiments with dilute mixtures of a second species in He.The behavior of He in an extreme supersonic expansion is unusual because the weakly attractive 4 He-4 He interaction can support at most one vibrational state, which must lie very close in energy to that of the separated stationary atoms. The question of whether that sta...

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Objective: To evaluate the prevalence, symptoms, spontaneous course, and behavioral consequences of visual height intolerance in children in comparison to adults. Methods:Survey in three primary schools on the frequency of visual height intolerance in 455 children aged 8 to 10 years. Individual structured interviews of 90 susceptible children and 54 of their parents. Results:The prevalence of visual height intolerance in children amounted to 34%; there was no gender preponderance. The condition began at a mean age of 5.9 years and rarely showed a tendency to generalize with respect to trigger stimuli. Triggers and symptoms were similar to those in adults. Avoidance behavior was reported by less than one third of the children, and there was only minimal subjective impairment of life quality. The condition was not perceived to be a disease or an individual weakness. Frequency and severity had already spontaneously improved in nearly one half of the children by the time of the survey. Conclusion:More than one third of prepubertal girls and boys exhibited susceptibility for visual height intolerance which -in contrast to the adult-onset type of the condition -appeared to take a benign spontaneous course.

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