Diffusion ofHe(2S13)in Helium Gas;

W. A. Fitzsimmons

¹

,

Neal F. Lane

²

,

G. K. Walters

³

Abstract

The existence at intermediate separations of a repulsive barrier in the interaction potentials between a He^Sj) metastable atom and a He(l 1 S 0 ) ground-state atom has been well established both experimentally and theoretically. The present investigation deals with the quantitative nature of this repulsive interaction for large internuclear separations. The diffusion coefficient, and thus the diffusion cross section, for He(2 3 Sj) atoms in helium gas has been measured over the temperature range from 1 to 300° K by means of a pulsed afterglow technique. The diffusion cross sections so determined are 82, 58, 46, and 34 (10~1 6 cm 2 ) ±5% at 4.2, 20, 77, and 300°K, respectively. For purposes of comparison, diffusion cross sections were also calculated quantum mechanically using adjustable long-range potentials of a form consistent with previous theoretical calculations. A comparison of the calculated and measured diffusion cross sections clearly indicates that the previous theoretically determined interaction potentials are much too strongly repulsive at large internuclear separations. Accordingly the parameters in the adjustable potentials were varied until agreement with the experimental diffusion cross sections was obtained. The adjusted interaction potentials were then used to calculate total and excitation transfer cross sections, and the results are shown to be in excellent agreement with previous and independent experimental measurements.

Fundamentals of metastability exchange optical pumping in helium

“…A lower bound is set by de-excitation on the cell wall, which involves diffusion of metastable atoms in the gas (with a diffusion coefficient given in [27]). …”

The role of the Pauli principle in metastability exchange collisions

“…Equation (24) (24) is obviously completely negligible and the effect of nuclear identity can be ignored, as stated in [6].…”

“…The terms in (18) [6], [15], [21][22][23][24][25][26][27] (the notation is the same as in [1] In this expression, the factor (-1)l arises from the fact that a nuclear identity interference term is involved (this factor has already been discussed for formula (A. 8) of the appendix of [ 1 ]).…”

Contribution à l'étude du pompage optique par échange de métastabilité dans 3He. - Première Partie

“…Une fois que l'on connaît ce potentiel, on peut théoriquement en déduire la section efficace d'échange de métastabilité a ech; cependant, les valeurs ainsi trouvées dépendent de façon critique des variations avec R de V(R), notamment à grande distance, et de la hauteur de la barrière de potentiel. Les [6], [7]. C'est pourquoi les calculs récents cherchent-ils à obtenir un potentiel V(R) conduisant à la fois à des valeurs de a ech et Ud en accord avec l'expérience ([8] à [11] …”