Temperature Dependence of the Helium Metastability-Exchange Cross Section in the Range 15-115 °K

Abstract

ery close to P", and the measured deviations from 1 may reflect the influence of the longitudinal field on stimulated scattering rather than on SF.In order to see the true critical power, we resorted to mode-locked laser operation (open circles). At 1-nsec pulse duration stimulated scattering was weak and disappeared completely at large 8. An obvious increase of P"TM/P"Ts was found this time which depends on 8 in the same way as the calculated curves. Most interesting, however, the experimental values are in between the areas ascribed to electrostriction (1.00) and its superposition with the optical Kerr effect (0. 15). They are clearly away from the region of the optical Kerr effect alone (-0. 50).Hence, one might tentatively conclude that electrostriction does in fact contribute significantly to the SF in our investigation even at pulse durations as small as 1 nsec. This result would be in good agreement with the focal-zone transient times estimated above. But it should be kept in mind that various details of the SF process have been omitted in our investigation, and more work has to be done in order to clear up completely the role of the mechanisms involved. The mere fact, however, that there is a difference in the SF of TEp, and TMp, light is clear evidence for the influence of longitudinal field components on the nonlinear propagation of light; a phenomenon which, to our knowledge, has never been observed before.The author gratefully acknowledges the contribution of E. Courtens to the mathematical. formul, ation of the problem, and the technical assistance of R.

Étude par pompage optique de l'échange de métastabilité dans le néon

“…La determination a 20 % pres de la section efficace de ces collisions d'6change constitue, a notre connaissance, le premier r6sultat experimental sur ces collisions dans le neon ('). Les seuls autres atomes pour lesquels ces collisions d'6change de metastabilite aient ete 6tudi6es par pompage optique sont 1'helium [2,3,4] et le xenon [27]. Pour…”

“…C'est pourquoi l'un des int6rets des mesures de section efficace d'6change de metastabilite est l'obtention d'informations sur les potentiels interatomiques. Une telle etude a déjà ete menee avec succes dans le cas de 3He dans 1'6tat metastable 2 3S1 [2,3,4]. Nous pr6sentons ici une extension de ces methodes au cas du Ne* dans 1'etat metastable 'P2 appartenant a la configuration 2p5 3s (cet etat est nomme 1 ss en notation de Paschen, voir la figure 1).…”

“…6lectroniques et nucl6aires du metastable sont affectées par les collisions. Comme celles-ci durent un temps Te tres court, on peut supposer que le couplage hyperfin entre les variables electronique et nucl6aire n'a pas le temps d'agir pendant la duree des collisions(4).…”

The role of the Pauli principle in metastability exchange collisions

“…Another related reference is a paper by S. D. Rosner and F. M. Pipkin [7] which gives the uncorrelated part (the part obtained in [4] for distinguishable nuclei) of the density operator and also includes nuclear identity effects. The method we use here is different (we wish to calculate one-atom density operators for example) and some of the results we shall obtain do not coincide with those of reference [7].…”

Optically polarizedHe3

“…13 is evaluated from the plot of reduced linewidths Δ ν / N in the temperature range 100 K to 550 K (Colegrove et al , 1964, Fig. 7) using the correcting relation k ME =9/4× π Δ ν / N. 6 Other experimental data for the exchange rate coefficients k ME are derived from published values of linewidth Δ ν ex (Rosner and Pipkin, 1972), exchange rate 1/ τ (Dupont-Roc et al , 1971), cross section σ ex (Barbé, 1976; Zhitnikov et al , 1975), and number densities N using: $$k_{\text{ME}}=\pi \mathrm{\Delta }{\nu }_{\text{ex}}/N,k_{\text{ME}}=1/N\tau ,\text{or}{k}_{\text{ME}}={\sigma }_{\text{ex}}{\overline{v}}_{\text{rel}},$$ where ${\overline{v}}_{\text{rel}}=4\sqrt{k_{\mathrm{normalB}}T/\pi M}$ is the average relative velocity of colliding He atoms at temperature T ( k B is the Boltzmann constant, M the 3 He atomic mass). The experimental and calculated values of ME rate coefficients are in fair agreement (the expected effect of isotope mass difference through v̄ rel is of the order of symbol size).…”