Precise Measurements of Total Cross Sections for the Scattering of Potassium Atoms by Rare Gases.(Part II)

Abstract

The molecular beam apparatus used for the measurements of total collision cross sections of potassium and rare gases is shown in detail. A new type of the scattering chamber with a variable length is used to eliminate the error of the effective scattering length. To measure the pressure of the rare gases in the scattering chamber an ionization gauge is connected directly to the scattering chamber. The sensitivities of the ionization gauge for the rare gases are calibrated by a McLeod gauge. To eliminate the mercury drag effect in the calibration, the cut-off position of the McLeod gauge is cooled with dry ice. The magnitudes of the effect are in good agreement with those calculated by Takaishi's theory. Effective total cross sections for the scattering of potassium by He, Ne, Ar, Kr, and Xe have been measured in the velocity range, 400-1200 m/sec, of potassium beam. Corrections for the angular resolution are calculated by a general method derived by the present authors. The correction values are compared with those calculated with Busch's formula. To estimate true total cross sections, the effective total cross sections are corrected for the thermal motion of the scattering gases by means of the method of Berkling et al. The velocity dependences of the total cross sections in the potassium-argon, potassium-krypton, and potassium-xenon systems are shown to be explained by the Van der Waals potential, —C/γ5, where s=6. Glory undulations are observed for the potassium-xenon system. For neon the velocity dependence can not be explained by s=6. For helium the correction factors for the thermal motion of the scattering gas are large and the value of s can not be determined unambiguously. To estimate the Van der Waals constants from the total cross sections, the SLL approximation formula is used and the values of 231, 337, and 515×10-60erg·cm6 are obtained for CK-Ar, CK-Kr, and CK-Xe, respectively. These values are in good agreement with the theoretical ones of Dalgarno and Davison. For the potassium-xenon system the potential parameters of L-J(12, 6)are estimated from the glory undulations, giving 2.3×10-14 erg and 4.7Åforεandγm, respectively.