Flash filament technique

In general, the flow rate F(t) consists of the following additive components the controlled flow rate Fd of the entering gas, the flow rate Fi which is due to parasitic leaks and/or diffusion, and the flow rate Fw resulting from possible adsorption-desorption processes on the system walls (in Section I, references are given to papers dealing with the elimination or control of the wall effects in the flash filament technique). In each of these flow rate components a particular ratio of the investigated adsorbate and of the inert gas exists and all these components contribute to the over-all mean values Fh(t) and F (t). 

IV. The Adsorption of Nitrogen on Tungsten as Deduced FROM Ion Gauge and Flash Filament Techniques... 

As noted in Section 2.1.2, measurement of the kinetics of chemisorption on clean metal surfaces generally requires ultra high vacuum techniques, in order to accomplish the experiment in a reasonable period of time. The variant of the classical adsorption method known as the flash-filament technique has been developed by several groups of workers and recently summarised by Ehrlich . 

The kinetic data obtained from the flash filament technique depend on three parameters, the absolute rate of adsorption AdNjdt) occurring on a sample of... 

Low energy (usually less than 100 eV) low intensity (10" -10 A/cm ) electron beams, causing the dissociation and desorption of adsorbed phases have been used increasingly, in recent years, as a surface studies tool (59). This approach has an advantage over flash-filament techniques in that the method does not appreciably alter the surface film being studied, provided that the surface is probed with a sufficiently low current density of electrons. Invariably, electron probe current densities used in these experiments can be a factor of a hundred lower than those used in LEED experiments, in which the processes involved are very similar. The method is applicable for the study of bulk single crystals or ribbons as well as for polycrystalline samples, unlike FEM and LEED measurements which are restricted in their application. 

An investigation of the sticking probability of CO on tantalum by Gasser and Thwaites (185) using a flash filament technique suggested value of about 0.14 for this coefficient, which was independent of temperature between 200-300°K and also of coverage up to about half a monolayer uptake. At temperatures between 800-2000°K these workers measured a CO uptake equivalent to about 300 monolayers, all of which could be desorbed as CO when the filament was flashed just above 2000°K. 

A proper judgment of the validity of these findings, as well as any extension of such work, must rest upon a detailed appreciation of the experiments involved. It is the aim of this article to review the experimental methods upon which these advances have been based—the flash filament technique, flash desorption, field emission and field ion microscopy, and the use of ultrahigh vacuum procedures. 

Rates of adsorption measured by the flash-filament technique are summarized in Fig. 3. These data are based on a knowledge of three related parameters A(dn/dt), the absolute rate of adsorption occurring on a sample of geometrical area A, p the pressure in the system, and n the instantaneous surface concentration. Procedures for determining these parameters are outlined in the next few pages. 

The chemisorption of molecular oxygen on platinum foil at low pressures has been investigated by flash-filament techniques. ... 

---