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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 23, No. 11
Molecular Beams’ John B. Taylor RESEARCH LABORATORY, GENERAL ELECIRIC C O M P ~ NSCHENECTADY. Y, N. Y.
T
HE use of molecular Molecular beams, narrow rays of molecules formed design the apparatus to give beams in research had by a slit system and moving in one direction in an this intensity. With dimenits &st real beginning evacuated apparatus, may be used to advantage in sions and pressures as ordiin the experiments of Stern many types of research. Among those described or narily used, the beam density in Germany. This discussion suggested are determinations of molecular velocities, corresponds to an ordinary will show how the usefulness mean free paths, magnetic and electrical moments, gas a t a p r e s s u r e of a b o u t of molecular beams has not degree and heat of dissociation, vapor pressures, types 10-6 mm. A beam of this instopped with those brilliant of reflection, heats of adsorption, character of surfaces, tensity will coat a surface with first experiments, and in what and chemical reaction products and mechanisms. molecules one layer deep in directions future applications The formation and detection of the beams is deabout 1second. may lie. Before saying more scribedR o d e b u s h , to whom the a b o u t t h e s e e a r l y experiwriter is indebted for his inments, it will be instructive to describe a beam and to tel! how troduction to research in molecular beams, has recently puhbeams are produced and detected. It is important to realize lished (SO) a review of molecular beams. For further details of that the application of molecular beams in so many fields of the factors governing the production and intensity of beams, the research depends on the peculiar nature of the beam and on reader is referred to this article and especially to two articles the fact that accurate means of production and sensitive by Stern (18, 3 4 , who made a thorough investigation of metho& of meafiurement and detection have been developed. this subject before commencing his recent studies of the The term “molecule” will be used because, whether monatomic de Broglie waves. or polyatomic in the gaseous state, both species of material are Detectors properly classed as molecules. To detect these beams, whose intensity is thus often less Formation of a Beam than that of the residual gas, several methods are available. In the formation of the molecular beam, molecules of the The simplest is direct condensation of the beam on the cooled substance leave a source, 0, having a slit, &, in all directions, target, or reaction with the target to form a visible image. passing into a highly evacuated apparatus (Figure 1). A The resulting image is governed in size and intensity distribusmall group of these molecules are intercepted and made into tion by the slit areas and spacings. This method does not a unidirectional beam by one or more slits, SPand Sa. The allow an accurate measure of the intensity of the beam, but has pressure in the apparatus must be kept low enough to allow had extensive application where this factor is not so important. the passage of the beam without an appreciable number of This method is capable of great extension by use of developers scattering collisions. In the case of condensable vapors, this (10) which increase the intensity of images too weak to be simply means maintenance of the initial high vacuum, because visible. If the target can be kept clean enough, there is apthe particles leaving the source are immediately condensed on parently no reason why single atoms cannot be counted (21) the wall of the chamber surrounding the source, either at room since each should serve as a nucleus for crystal growth. When temperature or by special cooling. With more volatile accurate intensity measurements are desirable, and when the substances, high-speed pumps are required, especially on the beam is noncondensable, some form of manometer must be source chamber, to keep the pressure low enough to prevent used. This consists briefly of a chamber into which the beam scattering of the beam before it has reached the second slit. passes through a slit and which contains a device for measuring In this case a third slit, Sa, and pump may be required to the resultant increase in pressure (Figure 2). Stern (56)has very successfully modified the hot-wire manometer for this clear the path for the beam by stages. To avoid collisions in the beam itself, that is, between the purpose. The ionization gage (16) with slit may also be used molecules composing the beam, the pressure a t the source is but is probably subject to greater difficulties. Deflectingkept a t a point where the mean free path at the source is vane manometers and thermocouples are possibilities, but always greater than the width of the first slit opening. Under none has been successfully developed as yet. When the these conditions a unidirectional beam arrives in the final ionization potential of the molecules is below about 6.5 volts, a chamber. There are practically no intercollisions and no very sensitive and accurate detector is available (Figure 3). This detector (37) is an application of the experiments of scattering by the residual gas in the apparatus. The source may be an oven containing a metal which is Langmuir and Kingdon (23) who found that such molecules evaporated through the first slit by heating, or the first slit are converted into positive ions on striking a hot tungsten may be the opening from a chamber containing a non- filament or a tungeten filament coated with oxygen. The condensable gas at low pressure, I n some instances the slit positive-ion current, as a thin filament is moved from point to may simply be a filament of the material to be evaporated or point across the beam, is a measure of its position and intena filament coated with the material, but in these cases it is sity. Especially in experiments with the alkali metals, this detector is about the only one to be considered. It is possible more difficult to obtain a beam of sufficient intensity. The beam intensity, that is, the number of molecules arriv- that its use can be extended by coating the filament of tunging a t the receiver, R , per square centimeter per second, is pro- sten or other material with the proper electronegative film. portional to the pressure a t the source and to the area of the The very great possibilities of new detectors, based on the first slit (a) and inversely proportional to the square of the influence of the beam material on the electron emission of distance (d) from source to receiver. Thus knowing the inten- tungsten, should be pointed out. For example, the experisity desired and keeping in mind the mean free-path limita- ments of Langmuir (24) on oxygen films show that beams of tions on the source and apparatus pressures, it is possible to oxygen which would deposit a single layer on tungsten in from 5 to 10 hours could be readily detected by their influence 1 Received September 15, 1931.
I,$-DUSTRIdL A N D ENGI,VEERISG CHEMISTRY
Kovember, 1931
on the cesium-tungsten electron emission. Such beams would correspond in density to a gas pressure of about lo-" mm. Beams of hydrocarbons could perhaps be detected by their carbonizing effect on tungsten filaments or on thoriated tungsten fdaments. It will now be noted that there has been produced a beam of neutral particles of much the same character as a homogeneous and well-collimated beam of electrons or positive ions, and that there is a t hand a most sensitive means of detection. The advantages of this beam are similar to those in the beam
'
PVMP
' '
PUMP
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PUMP
Figure 1-Formation of Molecular Beam
' a
Figure 2-Detection of NonCondensable Beams
I
I
v Figure 3 - D e t e c t i o n by Ionization on Hot Tungsten Filament
of charged particles.
I I Figure 4-The Stern-Gerlach Experiment
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a number of other atoms (30) and has provided a check on spectroscopic determination of the ground terms of the atoms involved, and in some cases (l4,25)ground term and Lande y factor have been determined by this method, where the spectroscopic results have been inadequate. Nuclear magnetic moments (18,31,%), especially in cases where the larger electronic moment does not interfere, may also be investigated. However, here the problem is complicated by the smallness of the moment. The existence of the different orientations of the nuclear moments in ortho- and parahydrogen is capable of further direct demonstration by the Stern-Gerlach experiment. Corresponding experiments (8,39) on organic and inorganic polar molecules in an inhomogeneous electrical field have also been carried out, though the results are rather indefinite thus far. VAPOR PRESSURES AND DEGREE O F DlssocIATIoN-since the intensity of the beam is dependent on the pressure a t the source, vapor-pressure measurements can be made. For example, the vapor pressure of lithium has been measured only with great difficulty and rather inaccurately by the usual methods. Lewis (PO), working with Stern, has applied the beam method with great success. Closely related to the vapor-pressure experiments is the determination of degrees and heats of dissociation. The molecular beam offers a t least two methods. The first and least direct involves the application of Stern's velocity apparatus. The rapidly rotated beam will be spread to an extent depending on the degree of dissociation of the molecules in the beam. This has been carried out by Zartman (40) for
They are moving in one direction with
no scattering. This fact a t once allows the carrying out of experiments which change the direction of the beam or scatter it in any way. A number of other experiments are made pos-
sible because of the accuracy with which the intensity and location of the beam may be measured. Effects due to the walls of the containing vessel are avoided. This becomes important in studies of the nature of chemical reactions. Applications
Dunnoyer (6) was probably the first to demonstrate a beam of the type just described, though the rectilinear motions of molecules had undoubtedly (22) been observed many times before. The first important applications were the classical experiments of Stern and Gerlach ( I S , $3) in 1921, when a beam of silver atoms (Figures 4, 5, 6) was caused to split into separate paths by the action of an inhomogeneous field as a direct proof of spatial quantization and a determination of the magnetic moment of the silver atom. It should benoted that the flat pole pieces suggested and developed by Rabi (27) have certain advantages over the original pole pieces (see Figure 4) used for producing the inhomogeneous field. Just previous to this, Stern (32) had used a molecular beam in a direct determination of molecular velocities. He observed the spread in the beam caused by rapidly rotating the apparatus (Figure 7). Since these experiments of Stern's, the use of molecular beams has increased steadily. The relation of the Stern-Gerlach experiment to spectroscopy and the Zeeman effect can be pictured simply as follows: In the Zeeman effect, just as in the Stern-Gerlach effect, a magnetic field causes the magnetic vectors of the atoms, originally oriented in all directions, to assume only definite positions with respect t o the field (spatial quantization). In the Zeeman effect this results in a splitting of spectral lines into certain components. In the Stern-Gerlach effect there results a splitting or deflection of the beam into distinct paths. The magnetic-moment experiment has been carried out on
Figure 5-SternGerlach Effect with Beam of Lithium Atoms 'Image condensed on cooled target)
Figure 6-Image of Figure 5 More Accurately Recorded, Using Tungsten-Filament Detector
Figure 7-Stern's Velocity Determination Atoms
Figure 8 - S e p a r a t i o n of Atoms and Molecules i n an I n h o m o g e n e o u s Magnetic Field
the case of bismuth atoms and molecules. This method is capable of wide application. The second method is possible when the atomic or molecular form of the material possesses a magnetic moment. For example, a beam of alkali metal atoms passing through a magnetic field will be deflected to either side of the central position (Figure 8). However, if there are molecules of the alkali metal present in the beam, these will be undeflected. Thus a direct counting of the relative numbers of atoms and molecules is possible. Lewis (26) i n Stern's laboratory has carried this out for sodium, potassium, and lithium. By varying the oven pressure and temperature the equilibrium is changed, and the heat of dissociation can be measured. Leu (25) has done a similar experiment with bismuth by observing the relative times of appearance of the central bismuth-molecule image and the side bismuth-atom images on a cooled target. Thus in one experiment, if it were
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INDUSTRIAL AND EHGINEERING CHEMISTRY
necessary, the spectroscopic ground state, vapor pressure, degree, and heat of dissociation could be determined. COLLISION RmiI-Mean free paths or effective collision radii can be measured by noting the decrease in beam intensity as the residual gas pressure is varied. This has been already carried out (1) but is capable of far greater refinement and extension. I n connection with the mean free-path experiments, selective scattering (depending on the angle of scattering as a beam passes through another gas) can also be investigated by use of a molecular beam. Such experiments would give information as to the shape and kind of interaction of the molecules.
Vol. 23, No. 11
simply inserting a shutter in the path of the beam. The possibility is noted of two or more beams impinging on the same area. Perhaps most interesting of all is the possibility of determining the result of the reaction on the surface, when a new molecular species is ejected into the gas phase. An example of how this might be done is contained in the discussion of oxygen films on tungsten by Langmuir and Villars (54). As the result of oxygen impinging on a surface or as the result of oxygen being formed on the surface, it may be wished to decide whether the oxygen remains on the surface, or, if it leaves, whether it is in the form of atoms or molecules. A tungsten filament would be used as a detector. Oxygen molecules leaving the surface would probably not affect the cesium-tungsten electron emission of the adjacent cold filament, whereas oxygen atoms would. By raising the temperature of the tungsten filament, the molecules could be measured as well. _-- ---The activating and decomposing effects of hot platinum, tantalum, and tungsten surfaces on beams of acetone have Figure 11-Specular Reflecbeen studied recently by Rice and Byck (28). tion a n d Diffraction of Beam from Crystal Surface Accompanying condensation, adsorption, and diffuse reGrating F i e u r e 9-Measurement of evaporation, there mag be diffuse reflection and specular Reflected Intensities reflection. Specular reflection will be characterized by the location and sharpness of the specular beam. I n addition, there may be wavelike reflection. This has led to a confirmation of the de Broglie wave theory for neutral atoms (Figures 11 and 12). Stern, Knauer, and Estermann (9, 11, 19, 34) have shown that helium and hydrogen are diffracted from the surface lattices of sodium chloride and lithium fluoride crystals. Johnson (17) has shown the same for atomic hydrogen. Since beams of neutral particles do not Figure IO-The Cosine DisFigure 12-Example of Specut r i h u t i o n of Reflection of penetrate the surface (as do electrons and x-rays), it is possible lar and Diffracted Beams of Alkali Metals from Sodium Helium , Measured by Stern’s that this type of experiment offers the most definite way of Chloride and Lithium FluorManometer ide investigating surface layers. Thus it is possible to determine not only the equilibrium between the components of a chemiSurface Phenomena cal reaction and a solid surface, and how tightly they are held Perhaps one of the most important uses of beams in the to the surface, but also how they are spaced with reference to future will be in the study of surfaces and phenomena oc- the lattice beneath. curring on surfaces. Several processes may occur when a Reactions occurring and products formed by electrical disbeam of molecules strikes a surface. In the first place, there charges through gases can be studied by examining the beam may be condensation followed by reevaporation. The inter- formed by placing a slit in the wall of the discharge tube or vening phenomenon is known as adsorption. A measurement vessel. The particles in the beam could be examined as to (Figures 9 and 10) of the angular intensity (88) of the leaving velocity, magnetic properties, state of excitation (for example, particles will reveal the diffuse distribution given by the cosine metastable atoms), chemical action on various solid targets, law. If complete equilibrium with the surface does not exist, degree of association, etc. An interesting application of beams has been made by von the accommodation coefficient may be measured by a modifiHippel (15)and Funk ( I d ) who shot electrons through beams of cation of this experiment. If some molecules remain on the surface, there is the pos- narrow cross section in order to study the efficiency of ionizasibility of surface migration. This may be studied, as in the tion as the voltage of the electrons was changed. The use of experiments of Semenoff (d), Estermann (7), and Cockcroft (3). such a beam insured that the electrons could make but one If the molecules remain more or less fixed on the surface, it is ionizing collision as they passed through the gas. Finally there is the possibility of beams composed of atoms possible to determine this amount, that is, the surface concentration. Conversely, the rate of evaporation may be having nearly the same velocity. This is attained in several measured, and heats of adsorption calculated, together with ways. Mechanical sorters of the notched-disk type have been their dependence on the amount of surface already covered. described by Stern (34) and Rodebush (2’9) and have led to The detector in these experiments may be any of those men- experiments by Lammert (20), Eldridge (6), and Costa, Smyth, and Compton (4, in which the Maxwell distribution tioned before, depending on the type of beam. Here, therefore, is a tool which gives vital information re- law was checked. A second method involves the deflected garding the components of reactions occurring on surfaces. beams in a magnetic-moment experiment. These beams have The method has several advantages over the usual method, a velocity spread, and any section of these gives a new beam of where the surface is exposed to gases in bulk. I n fact, many nearly uniform velocities. A third method uses the diffracted experiments in this field can be made only with the aid of beams in the de Broglie experiment as a velocity sorter. beams. For example, the use of a beam insures that activat- This has already been done by Stern (9) to sharpen his ing collisions will be with the surface and not with the hot gas. diffracted beams from a second crystal. The use of such The beam pressure may be varied easily and, in particular, beams of uniform velocity should be valuable in almost all of changed accurately to a very low intensity. The rate of the experiments just described. For example, the various arrival, condensation, or reaction may be made as slow as velocity components of a high-temperature beam might be desired. The area of surface treated may be localized. The singled out in relation to their parts in the mechanism of a arrival of molecules may be stopped at any point of time by surface or molecular collision.
C I‘ Y
A number of expcriiirciits usiug beanis have iiot bee11 nieiitioned, especially those in which the beam is really not a beam Imt.9. vapor strenm having many intercollisions. Such streams may have a greater intensity, but the benefids of no collisions iletween molecules are entirely lost. In discussing molecular beams as a new research tool, the nature of these beams which niakes their use advantageous has been particularly emphasized. All of the uses made of tlieni so far and other possible uses arise from the peculiar nature of the beams. It has been seen that many problems can be attacked only by the use of beams. I n deciding whether or not beams can be usefully applied to a problem in research, one Irns only to think what the presence of any one of the ingre{lientsof t.he research in the form of a beam will accomplish. Literature Cited '11 B ~ W mysir. . z., ai, 578 (19201. c21 Chariton end Semenail, Z. Physik. 26,287 (19241. 131 . . Cockccrotf. Proc. ROY. So'. (Londonl. A l U . 293 41928). :4) Corlu. Smyth. and Comptoi, Phyr. Rca.. do, 347 (19271. ( 5 ) Dunnoyer. Le Radium, 8 , 142 (19111: Compl. rend., 169. SDP (1911) (01 Eldridge. PhyS. Rci.. 30, 931 (1957J. (7) Estermaon. 2. Physik, 33,340 (1925). (8) E ~ ~ e r m m Z. o , pkyrik. Chem.. B1,101 (19281. (9) Estermnnn. Frisch. and Stern. Phys. Rm.(abs.1. 38, 581 (18311 $10) Estermann and Stern. Z.physik. Chem., 106,399 (1923).
(111 Estsmann andStcm, 2.Physik. 61, 95 (1030). (121 Funk. Ann. Physik, [SI 4. 149 (19301. (131 Gerlach end Stern. Z.Physik. 9,349 (19221. (141 Guthrieiind Copley. Phys. Rm.. 88,360 (1931). (16) Hippel, voll, Ann. Phyrik. [41 81, 1035 (1928). (1% Johnson, Phyr. Rm.,31, 108 (1928). (171 Johnson. Ibid., 37. 847 (19311. (181 Knauci and Stern. Z. Phyrik. 39. 764 (1928). (19) Knauer and Stern, Ihid.. 63, 779 (19291. (201 Zammert, Ibcd., 66, 244 (19291. (211 Langrnuir, P ~ LNol. . Acod. Sci., 3, 141 (19141. (221 Langmuir, Phyi. Rm..8, 149 (19161. (231 Langmuir and Kiogdon, Proc. Ray. Soc., 41,380 (192D. (241 Laogmulr and Villar~.J . Am. Cham. Soc.. 63, 486 (1931) (251 Leu, Z. Physik. IS, 408 (19281. (261 Lewis. Ibid., 69. 786 (19311. (271 Rabi. Ibid., S I , 190 (19291. (281 Rice snd Byck, Proc. Roy. .Soot. (London],Alsa, 60 (1931). (291 Rodebush. Proc. Nal. Acad. Sri.. 13. 50 119i71. (30) Rodebush, RES.Mod. Phys..3, 392 i19311. (311 Rodebush and Nichols. J . Am. Cham. Soc., 6% 3884 (19301 l3?1 Stern. E . Physik. 3.49,417 (1920). (331 Stern,Zbzd., 1,249(1921). I341 Stern. lkid.. 39,751 (1026). (381 Stern, I b i d . . 68, 766 (19281. (36) Tsyiar. Ibid.. 61, 840 119291. G7) T ~ y l w {bid.. . S7,2M 11829). (381 Taylor. Phyr. Reo.. 85. 375 (19301. (391 Wrede, 2.Phyiik. 44,281 (19271. (401 Zartmao, Phyr. Rea., 3'l. 583 (19311.
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