Kumasaburo Kodera

Scattering of potassium atomic beams from various cleaved surfaces

Abstract Scattering of potassium atomic beams from cleaved surfaces of LiF, KCl and mica were studied. The observed spatial distributions follow the cosine distribution in most cases. At large incident angles the distributions from LiF and KCl deviate from the cosine and show apparent backscatterings. A simple model is proposed to interpret the backscattering in terms of macroscopic roughness of cleaved surfaces. The sticking coefficient S ( t ) is defined and the mass balance between the incident and outgoing atoms is examined. Mass balance holds within the limits of errors on the surfaces of LiF and KCl at temperatures above 400 K. On a mica surface deposition of potassium atoms is observed even at temperatures higher than room temperature. It is shown from the change of the sticking coefficient that the deposition does not proceed linearly in the initial period.

Measurements of Velocity Distribution in Potassium Molecular Beams by Time-of-Flight Method

Velocity distribution in potassium molecular beams is measured by time-of-flight (TOF) method using a surface-ionization detector, which has high sensitivity and signal-to-noise ratio. The residence time of potassium on tungsten filament surface has been confirmed before hand to be far less than TOF. To avoid troublesome computation for the dynamic response function of the electronic system a sampling method is employed, and the sampled tons are accumulated into a coulombmeter. Experimental intensity vs. time curves are compared with those by calculations. The calculations are carried out on the assumption of the Maxwellian velocity distributions in molecular beams with a gate function determined for the experimental system. As was the case in previous works, considerable deficiency of low velocity molecules is observed but general agreement with the theory is considered to be reasonable.

Measurement of Mean Residence Times of Potassium on Clean and Contaminated Tungsten Surfaces

The mean residence time () of potassium atoms, ions or their intermediate states on clean and contaminated tungsten surfaces was studied by means of a pulsed molecular beam method. Taking into consideration the shape of the gate function, which can be determined from the experiment, and assuming the transient adatoms (M) and adions (M+) are in equilibrium, we estimated by a new method of analysis from the measured intensity function of desorbed ions. Using an Arrhenius-type expression =;0exp (/kT) and taking in to account the temperature dependence of the ionization coefficient β, we found the experimental desorption energies () to be 2.30 eV for a clean surface and 1.85 eV for a contaminated surface. The corresponding pre-ex-ponential factors (0) were found to be 3.24×10-14 secand 2.2×10-12 sec, respectively.

Kinetics of the Deposition of Potassium Atoms on a Mica Surface

In the study of the process of the deposition of potassium atoms on a mica surface by means of molecular beams, the phenomenon that the sticking coefficient changes with the exposure time and has a minimum at room temperature was observed. To interpret this phenomenon, simultaneous rate equations of adsorption were proposed and it was found that the sticking coefficient is expressed by S=1-\frac{N}{I}∑i=1∞\frac{\theta_{i}}{\tau_{i}}. The sticking coefficient curve having a minimum as observed in the experiment can be produced from the solutions of the equations related to the BET model. If the surface migration of adatoms is taken into account, the minimum of the theoretical sticking coefficient curves becomes less deep. If b≡N/Iτ2>1 the sticking coefficient curve monotonically tends to zero as observed in the experiment at higher temperature.