118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE USE OF THERMIONICS IN THE STUDY OF ADSORPTION OF VAPOURS AND GASES. BY JOSEPH A. BECKER (Bell Telephone Laboratories, New York). Received 3rd December, 193 I. The object of this paper is to point out a relatively new but very powerful tool in the study of adsorption phenomena. This tool is thermionic emission. While it is not applicable to all surfaces and all kinds of adsorbents, it makes up for these deficiencies by the insight i t gives us into the nature of adsorption processes.The results obtained thus far from this tool have come largely as by-products of the study of thermionic emission of electrons. When the primary emphasis is placed upon the study of adsorption itself, one can confidently expect an abundant yield. This is quite apparent when one surveys the results obtained thus far. These results apply when strongly electropositive metals such as caesium, barium, or thorium or strongly electro-negative gases such as oxygen are adsorbed on surfaces of metals such as tungsten, molybdenum, or platinum. In these cases : the adsorbed particles exist either as adions (adsorbed ions) or adatoms; the ratio of adions to adatoms decreases as the surface concentration increases ; the rate of evapora- tion of adions or of adatoms varies rapidly-perhaps exponentially- with the surface concentration; as the concentration of adions plus adatoms increases the rate of evaporation of adions decreases while that for adatoms increases; the heat of adsorption or the energy required to remove an individual atom from the surface depends upon the con- centration of similar adatoms and is greatly affected by the presence of other kinds of adatoms ; the mean adsorption time or mean life of an adsorbed particle is a function not only of the temperature but also of the concentration; particles adsorbed in a given region of the surface behave like a two-dimensional gas and migrate to other regions-there is some indication that this may happen only above some critical tem- perature analogous to the melting-point in solids.The key to these results is the fact that the thermionic electron emis- sion from tungsten and similar metals varies by very large factors and in a characteristic manner when small amounts of electropositive metals are adsorbed on the surface. Fig. I illustrates this fact for caesium, barium, and thorium on tungsten a t 1100’ K. It shows the logarithm of the emission current in amperes per cm.2 vs. f, the fraction of the sur- face covered. They differ chiefly in the height at the maximum or so-called “ optimum.” This optimum occurs when the tungsten is covered with a single layer of adsorbed partic1es.l When the surface is covered with two or three layers, the emission current has the vdue characteristic of the adsorbed material in bulk.For any value off greater than zero, the current with caesium is greater than that with barium, and this in turn is greater than PhysicaZ Rev., 28, 341-361, 1926. 148 The three curves are quite similar to one another.J. A. BECKER =49 that with thorium. Between ,f = o and f = 0.8, the shape of each curve is such that its slope a t any f is proportional to (log il - log i,) where the subscripts refer to the value off. The ratio of the current a t the optimum to the current from clean tungsten, i.e. 2 depends on the temperature ; as the temperature decreases, this ratio increases. Thus, i 20 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f FRACTION OF SURFACE COV&RED.FIG. I. a 20 for caesium on tungsten log 4 = 13.3 a t I 100' K. while it is approximately 19 at 800" K. The similarity in the general shape of these curves is all the more striking since they can be obtained in three distinct ways, depending upon the method by which the material is deposited on the tungsten surface. In the case of caesium, the tungsten is exposed to caesiumvapour. Caesium atoms arrive at the tungsten surface a t a rate determined by the vapour pressure which in turn is usually determined by the tempera- ture of bulk caesium in the tube. The barium is deposited on the tungstenADSORPTION OF VAPOURS AND GASES 1 50 by flashing a primary barium-alloy filament a t a fixed temperature for a fixed time.2 Each flash deposits a small but definite amount of barium on the tungsten.Thorium can be deposited in this second way. I t can also be made to arrive a t the tungsten surface by diffusion from the interior of thoriated tungsten along the grain boundaries,* by glowing the tungsten at an activating temperature for various lengths of time, These three methods may be designated as the vapour, evaporation, and diffusion methods. The details of the use of these methods are fully described in the original articles, to which references are given. From the log i vs. f curves of Fig. I, it is possible to determine how the “work function” of the surface varies with f. While the exact definition of the work function is quite complex, it will be sufficient for the purposes a t hand to think of it as the work an electron must do to escape from the metal surface.It is customarily expressed in equivalent volts, i.e. the potential difference in volts through which an electron would have to fall from rest in order to be able to do this amount of work. This work function W in volts is obtained by solving Richardson’s Equation in its logarithmic form, e log i = log A + 2 log T - w- 2.3kT . (1) For our purposes A may be treated as a universal constant whose value is 60 amps./cm.20K2. is numeri- cally equal to 11,600. From Fig. I and equation (I) we can determine W for any value of f. Fig. 2 is a plot of this relationship for caesium, barium, and thorium on tungsten for T = 1100 OK. The work function for tungsten is very materially reduced when its surface is more or less covered with electro- positive materials.The effectiveness of the adsorbed material, in re- ducing W is large a t first, but decreases more and more until the surface is covered with a complete layer. Additional layers increase W until a t two or three lavers, W approaches the value characteristic of the adsorbed material in bulk. A careful study of the curves reveals that the minimum value of W is approximately equal to the resonance potential of the adsorbed particles in vapour form ; and that - is proportional to (W - R) from f = o to f = 0.8, where R is again approximately equal to the resonance potential or the minimum value of W. In the attempt to explain these facts, we can learn a good deal about the nature of the adsorbed particles. It might a t first sight appear that the enhanced emission of Fig.I is to be ascribed to electrons emitted from the adsorbed particles. While this may be true for f larger than about 2, it cannot be true for the first layer. If it were true, the excess current above that from clean tungsten should be directly proportional to f and should increase steadily to the value characteristic of the adsorbed material. Instead of this, experiment shows that log i, rather than i, increases linearly with f for small values of f, and that the emission passes through a pronounced maximum. Another hypothesis which is capable of explaining these as well as If W is expressed in volts and T in OK, dW df Physical Rev., 34, 1323-1351, 1929. Langmuir, PhysicaE Rev., 22, 357-398, 1923. Clawing, Physica, 7, 193, 1927.J.A. BECKER 1.51 other observed facts is the so-called " adion grid theory " An adion is defined as an adsorbed particle whose valence electron no longer rotates about its own nucleus but has been absorbed by the underlying metal. This definition is simi1a.r to that for an ion in a compound such as caesium chloride. That both adions and adatoms may exist on one and the same surface a t the same time was shown in a previous paper.5 Of course, any particular particle may be an adatom a t one instant and an adion a t a later instant and vice versa. 4.6 4.4 4.2 4.0 3.8 v) 5 3.6 0 > Z ; 3.4 0 I- 2 3.2 3 Y g 3.0 3 3 2.8 II 2.6 2.4 2.2 2.0 I .8 1.6 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f = FRACTION OF SURFP.CE COVERED FIG. 2. These adions act like a positively charged open meshed grid placed very close to the tungsten surface.Such a grid and its negative image produce zero field a t some distance beyond the grid and an average field of 4770 = 4neN, between it and the tungsten, provided that the size of the grid is very large compared to its distance from the surface. 6 Trans. Am. Electvochem. SOC., 55, 153-175~ 1929.152 ADSORPTION OF VAPOURS AND GASES Q is the charge on the grid per cm.a of surface. N , is the number of positive ions per cm.2 If 1 is the radius of the adion in cm., then the potential difference between any point in the metal and the point outside a t some distance from the surface is 4mN,I. This means that the work an electron must do to escape has been reduced by 300.4 n . 4-77 . IO-~ON, . I volts. Hence AW = Wo - W = 1.80. IO-'N,Z . * (2) This equation can be used to determine values of N,. For any particular f, AW can be read off from Fig. 2 . The radius of the adion, I, can be found in tables obtained from X-ray data or tables based on quantum theory calculation^.^ The only unknown is N,. It is then possible to determine P, the percentage of adsorbed particles that are adions, since * (3) p = - ND N . where N is the total number of adsorbed particles. related to f, since N is very simply xr f V f=- Nl (4) where N , is the value of N for f = I. An approximate value of N , can be computed from the known or estimated values of atomic diameters, d, and the type of surface packing. From simple geometrical considera- tions, it follows that for loose packing I N l = S .( 5 4 while for close packing Since close packing appears more probable than loose packing, the calculations that are to follow will be based on close packing. It should also be realised that in the derivation of equations (5a) and (5b) it is assumed that the number of particles in the monatomic layer is not affected by the size and arrangement of the underlying atoms. This may not be true, or it may be true only if the size of the adions is larger than the atomic size of the base metal, or it may be true only a t low temperatures. In any event, it will be a rather simple process to recompute the data when more reliable values of Nl are available for barium and thorium. For caesium, the value of Nl has been obtained by direct observation. Table I gives the values of P, the percentage of adsorbed particles that are ions, for various values off.The values of I and d , used in computing P, are also given. In some cases, the values of I have been obtained by interpolation. It is also desirable to know how the tendency to form adions varies as f increases. As a measure of this tendency, we may take dN,I which represents the ratio of the increment in the number of adions to a small dN Goldschmidt, Trans. Faraday SOC., 25, 253-283, 1929. ?Pading, Jour. Am. Chem. SOL, 49, 765, 1927.J. A. BECKER 0'2. 48 27 12 I 5 3 0'4. 0'6. 0.8. 1'0. ----- 40 33 28 23 23 20 17 14 9.5 7-9 6.6 5-4 TABLE I. I = radius of singly charged ion. N, = number per cm.2 when f = I. d = diameter of neutral atom.f = fraction of surface covered. 1 I 1 1 increment in the total number of adsorbed particles. ( 2 ) and (4) and the experimental fact that From equations -- dW _ - aN,(W-R) . df where a is constant : it follows that a dND = ( W - R ) . dN 1.8 x I O - ~ (7) This means that the tendency to form adions is directly proportional to the amount by which the work function of the surface exceeds the re- sonance potential of the adsorbed particles. The proportionality con- stant depends upon the nature of the adsorbed particles. On the basis of the adion grid theory, i t is easy to see why the ten- dency to form adions must decrease as f or N increases. For as N in- creases, N , will increase. As a result, the adsorption field, which helps electrons away from the surface, increases.Consequently, more elec- trons leave the underlying metal to neutralise the adions, and fewer electrons leave the adatoms and enter the underlying metal. Evaporation of Positive Ions and of Atoms. The adsorption fields greatly affect the rate of evaporation of positive ions. Since these fields are in a direction to help electrons out of the surface, they should make it more difficult for positive ions to escape. Experiments with caesium on tungsten fully confirm this prediction. In fact, they show that in a region fromf= 0.01 to 0.20, the more caesium there is on the surface, the less evaporates in any given time, and any caesium that does evaporate comes off as ions. For very small values of f, the ion evaporation rate increases with the concentration.It comes to a maximum when the surface is only about I per cent. covered. These experiments show that the forces on an adion, due to neighbouring adsorbed ions and atoms, are appreciable even when the average separa- tion is 10 atom diameters. At first sight, one might expect E,, the rate of evaporation of neutral atoms to be unaffected by the adsorption fields and consequently directly proportional tof. Actually, the evaporation rate a t a given temperature increases rapidly with the surface con- centration. As a first approximation, E, increases exponentially with f. Such evaporation curves for caesium on oxidised tungsten a t various temperatures were given in a previous p~blication.~ For caesium, barium, But this is not the case.I54 ADSORPTION OF VAPOURS AND GASES thorium and oxygen on tungsten, similar curves, though not so accurate nor so complete, have been obtained. The interpretation to be put on these curves is that while the adatom as a whole is neutral, the forces on the various parts of the atom are not zero.This is undoubtedly due to the fact that the adsorption fields vary so rapidly with the distance from the surface. In these fields, the net force on an atom is much the same as on a negatively charged particle, since the atom evaporation curves are similar to the electron evaporation curves. The technique employed to obtain such evaporation curves depends upon the adsorbed material. For caesium on tungsten,l the evapora- tion rate E at a given temperature T and a givenf, is equal to the arrival rate whenever equilibrium prevails.The arrival rate can readily be obtained from t.he saturation positive ion emissi0n.l f is obtained by suddenly decreasing the temperature of the surface t o a sufficiently low value and noting the time that elapses before the electron emission reaches its optimum value. During this time, every atom that strikes the surface sticks to it. At the optimum emission, the surface is covered with a monatomic layer. The observed time, multiplied by the arrival rate, gives the number of atoms that were added before a monatomic layer was reached. The difference between this number and N,, the number corresponding to a monatomic layer, gives the number N which were present a t the prevailing T and arrival rate. In this way, a single point on an E vs.f curve is obtained. To get other points, the process is repeated for other surface temperatures and for other arrival rates. on tungsten, the process is as follows : A definite amount of barium or thorium is deposited on the tungsten. The electron emission from this surface is then determined at a testing temperature so low that the amount on the surface is not thereby altered. From a log i vs. f curve similar to Fig. I , f is obtained. The surface is then heated for a short time to a temperature at which evaporation takes place. The emission is determined a t the testing temperature. From this, a new value of f i s obtained. The decrease inf, divided by the time in seconds and multiplied by N,, gives E in atoms per cm.2 per second for the mean value off.If this process is continued, E can be determined for other values off, and a complete E vs. f curve obtained. The whole process is then repeated a t another temperature at which evaporation takes place. These evaporation ciirves allow us to draw conclusions as to the way in which the “ heat of adsorption ” or work necessary to remove an in- dividual atom or ion depends upon the concentration. If this heat or work did not depend upon f, then the rate of evaporation should be directly proportional to f. Since the rate decreases with increasing f for ions, but increases more rapidly than proportional to f for atoms, we conclude that as the surface concentration increases, the heat of adsorp- tion increases for ions, but decreases for atoms. If a complete family of accurate E vs.f curves at various values of T were available, numerical values of the heat of adsorption could be For barium or thorium obtained for various values off from the slope of a plot of log E vs. 2- T’ Another interesting quantity that can readily be obtained from a family of E vs. f curves is r , the mean life of an adatom, or the average 8 See also Andrews, Physical Rev., 33, 454, 1929.J. A. BECKER I55 number of seconds an atom spends on the surface under various con- ditions. It can be shown that T is very simply related to E andf, namely that t, is the time required to deposit one layer if every atom sticks and the arrival rate is equal to E. Table II., taken from a paper on (‘The Life History of Adsorbed Atoms and Ions,” gives values of T for various values off and T.The large values of 7 are rather surprising. This table also emphasises the dependence of r on f as well as on T. TABLE II.-MEAN LENGTH OF LIFE OF CAESIUM ATOMS. On a Tungsten Surface Partially Covered with Oxygen (Cs on OW) Nl = 4.0 x 1014 atoms/sq. cm. Fraction of Surface Covered: f. 0.8 0.8 0.8 0.8 0.80 0.85 0.90 0.95 I ‘00 0’2 0’4 0-6 0.8 1’0 1’2 Temperature To K. Xme to Form One Layer: f l sec. 830 I33 60 22 830 350 I35 59 28 I33 I33 I33 I33 I33 I33 Mean Life, r sec. 665 106 :: 2: 665 300 I21 27 53 79 I 06 I33 I 60 Surface Migration. The thermionic emission characteristics are useful in studyin, sur- face migration. This tech- nique apparently is well adapted for a quantitative investigation of the factors involved in the movement of adsorbed atoms.Briefly this technique is exemplified by depositing barium on one side only of a flat tungsten ribbon. The thermionic cur- rent emitted from each side of the ribbon is measured under standard testing con- ditions and is used to deter- mine the amount of barium on each side. The ribbon is then flashed a t a temperature BARIUM SOURCE I TUNGSTEN RIBBON I PLATE 2 FIG. 3. PLATE 1156 ADSORPTION OF VAPOURS AND GASES between 900" K. and I 100" K. for a short period of time. The emissions are redetermined under standard testing conditions. This flashing and testing is repeated until the emissions from both sides are not altered by flashing. Fig. 3 is a simplified diagram of the tube used, while Fig. 4 is an idealised curve of the result.The first part of Fig. 4 shows how the logarithm of the current from the front side (il) and from the back side (iz) changes as barium is deposited on the front side of the tungsten ribbon. The deposition is discontinued when f = 0.80 on one side. Subsidiary tests show that during this deposition and testing no barium reaches the back side of the ribbon. For the second part of Fig. 4, the tungsten ribbon is flashed a t about 1000' K. Periodically the flashing is interrupted, while i, and i, are determined under the standard testing conditions. As a result of this flashing, the emission from the front side decreases rapidly a t first and then more and more slowly; while the I2 0 emission from the back side increases very rapidly at first and then more and more slowly.When the total time of flashing has continued €or one or two hours, the emissions from the front and back sides are equal and do not change appreciably with further flashing. The value of this emission corresponds to f = 0.40 whereas originally f for the front side was 0.80 and for the back side was 0.00. Furthermore at any stage of the flashing the sum of the f values for the front and back sides is 0.80. In other words, the material that was originally on the front side redistributes itself until it is uniform ; the back side gains what the front side loses. In actual practice, the current observed on plate I is nearly equal to il ; but the current to plate 2 consists not only of iz but also of elec- trons which come from plate I as a result of reflection or secondary emission.The result of this is that the measured current to plate 2 is never less than I per cent. of the current to plate I. When iz is muchJ. A. BECKER I5 7 smaller than i, this is serious, but as i, approaches i, the errors become negligible. Furthermore the difficulty can be greatly lessened by in- serting appropriate shields in the tube. Very likely if the currents were measured by means of a properly designed Faraday cage arrange- ment, it could be eliminated entirely. Diffusion. Still another application of thermionics to surface phenomena is in the study of diffusion from the surface into the interior and vice versa. An example of this is the diffusion of thorium to the surface of thoriated t u n g ~ t e n . ~ Another example is the diffusion of barium and oxygen in the case of oxide coated filaments.2 As far as they go these studies suggest the following picture: There is a certain amount of work in- volved when an atom is transferred from the interior of a body to its silrface.This work depends upon the surface concentration; i t may also depend upon the temperature. For each volume concentration there exists a surface concentration in equilibrium with it. If the surface concentration is much less than the equilibrium amount, every atom that reaches the surface stays there. If the surface concentration ex- ceeds the equilibrium value, its rate of decrease is rapid a t first but de- creases more and more until equilibrium is established. Oxygen on Tungsten. While most of the results described relate to electropositive materials, similar results appear probable for electronegative substances such as -6 -8 AMOUNT OF OXYGEN FIG.5. oxygen. varies with the concentration of adsorbed oxygen. Fig. 5 shows how the logarithm of the emission from tungsten The absolute valueADSORPTION OF SATURATED VAPOURS of the amount of oxygen is not known. One might venture to gtless that the maximum amount in this figure corresponds to half a layer. From the fact that the emission decreases it follows that oxygen forms negative adions. Since the magnitude of the slope decreases asfincreases, the tendency to form adions decreases as the oxygen concentration increases. that the rate of evaporation of adsorbed oxygen increases very rapidly with the surface concentration. Hence the heat of adsorption must decrease as the concentration increases. Other experiments show that oxygen migrates very rapidly a t 1400' K. Experiment shows Summary. Thermionic emission can be very useful in the study of adsorption phenomena. The primary reason is that very minute amounts of electro- positive elements, such as caesium, barium, or thorium, or electronegative gases, such as oxygen, change the thermionic emission from surfaces of tungsten, platinum, molybdenum, etc., by very large factors and in a characteristic manner. They do this by changing the work function of the surface. This effect, as well as other surface effects, can be best ex- plained by the adion grid theory : The adsorbed particles can exist on the surface either as adions (adsorbed ions) or as adatoms; the adions act like a positively charged, open meshed grid placed very close to the surface. (I) That the ratio of adions to adatoms decreases as the surface concentration increases (Table I.) ; (2) that the work required to remove an adion from the surface increases while the work to remove an adatom decreases as the surface concentration increases ; (3) the mean life of an adsorbed particle depends on the surface concentration as well as on the temperature {Table 11.) ; (4) the rate of diffusion from the surface into the interior depends upon the temperature and on the amount by which the surface concentra- tion exceeds its equilibrium value. Thermionic experiments show the existence of surface migration and can be used to make a quantitative study of this phenomenon. The techniques involved in these various ex- periments are described and references given to previous publications. From this theory and the experimental facts it follows :