Photoelectric method

Different test methods produce numerically different results and comparisons should only be made of results using the same test method. Tests fall into three categories gravimetric tests, which measure ability to trap and retain dust those which measure staining power of contaminants before and after filtration and those measuring the concentration of a test aerosol either side of the filter by photoelectric methods. The tests which will be met most often are as follows. 

Measurement yields both the differences between the outer potentials and the work functions (real potentials). If two phases oc an / with a common species (index i) come into contact, at equilibrium /, (< ) = (/ ), that is at(a) - <, (/ ) = ZiFApty. These quantities are mostly measured using the vibrating condenser, thermoionic, calorimetric, and photoelectric methods. 

The photoelectric method is based on the photoelectric effect. The kinetic energy of the electrons emitted during illumination of a metal with light having a frequency v obeys the Einstein equation... 

Figure 6.8. Schematic showing photoelectric method for measurement of splat diameter. (Reprinted with permission from Ref. 368.)...

Apker, L., and E. Taft Studies of alkali halides by photoelectric methods. Shockley, Hollomon, Maurer, Seitz Imperfections in nearly perfect crystals. New York Wiley Sons 1952, p.246. 

In the photoelectric method, Fq is obtained as the change in work function, (f>, of a metal when immersed in the liquid. Thus Fq is given by the dilference in work functions ... 

In a variation of the photoelectric method, Fq can be determined by measuring the emission of electrons from the liquid into the vacuum. Even when Fq is negative, electrons can penetrate this barrier and be collected in the gas phase [35,36,47]. Borghesani et al. [48] used this technique and from the time evolution of the current reaching the anode for a sample of liquid Ar at 87 K found Fq to be —0.126 eV. This is in excellent agreement with the value of —0.125 eV given by Eq. (6) (see below) for this density (2.09 x 10 cm ) using the field ionization technique. 

R. H. Muller, IEC (Anal Ed), 11, (1939) (Review on photoelectric methods in chemical analysis, including 239 refs) 2a) R. H. Muller, Ibid,... 

The change in work function which accompanies adsorption on a metal surface may be determined directly by thermionic, field-emission, and photoelectric methods. Indirect methods rely on the measurement of a C.P.D. between a reference electrode and the original and covered surfaces, respectively. 

In the photoelectric method, the measured average work function is always less than the true since patches of high work function tend to be excluded from the emission process. Thus, the nonuniform distribution of adsorbate on a patch surface may cause a slight discrepancy in the evaluation of A. Experimentally, the photoelectric method has various limitations. Photocurrents of the order of 10 A. must be measured accurately in the region of vo, and for films of work function greater than 5 v., the threshold frequency lies in the far ultraviolet—a practical disadvantage. Furthermore, the method is inapplicable at pressures in excess of 10 mm. Hg because of ionization of the gas by collision. 

A photoelectric method was also used by Baker and Rideal (76) for studying the adsorption of Ha, CO, and CaH on evaporated metal films of Ta, Fe, Ni, and Co. Photocurrents near the threshold were too small to be measured accurately, so the threshold frequencies were obtained by in-... 

Thermionic and photoelectric methods have been successfully employed in the measurement of the mobility of alkali metals on W surfaces. The migration of adsorbed gases like H2, O2, and CO over a metal surface may be followed in the F.E.M., and data concerning the mobility of these adsorbates are of particular interest, since they are frequently involved in surface reactions and other chemisorption phenomena. 

Sachtler and Dorgelo (96a,b) observed that the direction of change of resistance, when hydrogen is adsorbed on a nickel film, depends on the conditions under which the film is prepared. With films prepared by evaporation under very high vacuum (< 10 mm.), the resistance decreases, while films evaporated under conditions of lower vacuum ( 10 mm.) show an increase in resistance. Furthermore, the surface potential of the chemisorbed film, determined by the photoelectric method, was found to be positive in... 

The photoelectric method cannot be employed at pressures higher than 10" or 10 mm. Hg, on account of ionization by collision. 

The photoelectric method and the method of the contact potential show only whether, and in which direction, an electron shift has occurred at the surface, owing to an interaction of the catalyst and the adsorbed molecules. By measuring the electric resistance of transparent catalyst films, however, one can find out whether complete transfer of electrons between the electron gas of the catalyst and foreign molecules has taken place (9). 

Puthoff et al. 77) found two polaritons associated with TO phonons of species Aj with tuning ranges from 630 to 500 cm 1 and 250 to nearly 0 cm-1. The Raman scattering at these extraordinary polaritons was used to study 36) the directional dispersion of the phonons in LiNb03. Recently Winter and Claus 78> also investigated the polaritons associated with the E phonons at 582 and 154 cm-1 by both photographic and photoelectric methods. These branches were important for the assignment of the phonon spectrum 39>. 

H2. Hoffman, W. S., A rapid photoelectric method for the determination of glucose in blood and urine. ]. Biol. Chem. 120, 51-55 (1937). 

Monochromatic light from the source is linearly polarized by the initial polarizer, and then allowed to pass through the sample medium. The angle of polarization associated with the light leaving the medium is determined by rotating the analyzer polarizer to the new null position. A variety of photoelectric methods are available which can be used to automatically determine the observed angle of rotation. 

Tumerman, L. A. Photoelectric method of measuring the degree of polarization... 

The measurement of orientation angles a is experimentally more difficult than that of birefringence, and in some cases a highly sensitive photoelectric method should be used for this purpose This requirement is particularly important for the determination of characteristic orientation angles [x/g] when the measured values require careful extrapolation to both infinitely low shear rates g and zero concentration c. Figure 25 shows as an example experimental data on the dependence of orientation angle a on g for solutions of fractions of a ladder polydichlorophenylsiloxane in tetrachromethane ... 

The photoelectric method ) successfully employed in the study of FB and EB also uses the compensation principle based on Eq. (106). However, the anisotropy in solution and its compensation are recorded with the aid of the scheme shown in Fig. 86 rather than visuaUy. 

Fig. 86. Scheme of the photoelectric method for birefringence measurements. 1 H Ne laser (X = 6300 A), 2 plate X/4 ensuring circular polarization of the beam entering polarizer 3,... 

V depends linearly on the anisotropy An = 5 X/2 nl of the solution. This fact ensures a higher (not less than by an order of magnitude) sensitivity of the photoelectric method than that of the visual method in which, according to Eq. (106), the light flux affecting the eye is proportional to 5. ... 

Fig. II. Cell for the measuiement of work function by the photoelectric method. A, quartz window B, cathode C, wire to anode him D, graded glass seals E, filament F, substance to be adsorbed G, cold trap J, outlet to pumps. From R. Suhrman, Advances in Catalysis, 7 (I9SS) 308.

GraShoff [160] investigated the ktldb) dependence in a stirred tank with a hollow stirrer, in which in addition to the k a values he also determined the bubble size distribution using photoelectric method and thence obtained fee from ki a/a. The above-mentioned results [381] could be fully confirmed, as can be seen in Fig. 4.12, however, with two important restrictions ... 

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