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Molecular Beams MOLECULAR BEAMS Studies involving coherent beams of atoms and n10lecules have had a profound influence on the development of modern physics. An10ng their technological dividends are masers, lasers and atomic clocks by O. R Frisch he atomic physicist, like every­ be reached by atoms that had passed in written in 1738 by the Swiss mathema­ Tbody else, needs air for breathing, straight lines through the constrictions. tician Daniel Bernoulli. He pointed out but otherwise it is in his way. At In essence Dunoyer's apparatus was that bombardment by fast molecules normal atmospheric pressure each air the prototype of all modern molecular­ would exert a pressure on the walls of molecule is involved in nearly a billion beam devices. The three necessary ele­ a container, and that the pressure would collisions a second. Only in gases at a ments of any such device are (1) a rise in proportion to the number of much lower pressure-in what is loosely source of molecules (usually called an molecules per unit volume, or the den­ called a vacuum-do molecules attain a oven, even though it is not always sity of the gas (as Robert Boyle had ob­ reasonable degree of privacy. We mea­ heated ) with an aperture through served some 80 years earlier). Bernoulli sure such low pressures in units called which the molecules can escape into a also explained the rise of pressure with "torr," after the Italian investigator vacuum, (2) a second, collimating ap­ increasing temperature by assuming that Evangelista Torricelli, who devised a erture, which forms the beam by stop­ the molecules move faster when the gas method for obtaining fairly good vac­ ping or deflecting all the molecules that is heated. Of course Bernoulli was right uums in 1643. (Atmospheric pressure is do not pass directly through it, and (3) in every respect, but his suggestions about 760 torr.) More than 200 years a method for detecting the effects of raised at least as many questions as they later, in 1855, the German glassblower various influences on the beam. In con­ answered. Heinrich Geissler used Torricelli's meth­ trast to beams of subatomic particles, Gas molecules move as fast as rifle od to evacuate glass tubes down to which can be accelerated to speeds ap­ bullets; why, then, does it take minutes about .01 torr; this achievement led di­ proaching the speed of light, molecular for a smell to spread from one end of rectly to the discovery of cathode rays, beams normally travel at "thermal" a room to another? \lVe now know the which were eventually identified as speeds: a mile a second or so. The answer is that the molecules keep col­ beams of free electrons by J. J. Thomson generic term "molecular beam" applies liding and therefore make little head­ in 1897. At such a pressure electrons both to groups of atoms (for example way. In fact, from the rate at which a can travel many inches, but at the same neutral hydrogen, each molecule of smell does spread-to put it more gen­ pressure the mean free path of an air which consists of two hydrogen atoms ) erally, the rate at which one gas dif­ molecule between collisions is only and to single atoms (for example so­ fuses into another-it is possible to cal­ about a millimeter. To achieve a coher­ dium vapor). culate how often the molecules in a ent beam of any kind of atom or mole­ Molecular beams can be made to pass gas collide every second. Such calcula­ cule one needs a much better vacuum­ through magnetic or electric fields; they tions were not made until the middle of at least .0000l (10-") torr or better. can be directed at various obstacles, the 19th century and only then did Techniques for obtaining very low such as crystals or gas molecules; they physicists begin to regard atoms and pressures were developed rapidly dur­ can be illuminated with intense light, molecules as real and not merely as fic­ ing the first decade of this century, and bombarded with electrons or crossed tions convenient for chemists. Several in 1911 the French physicist L. Du­ by other molecular beams. From such methods were devised for computing noyer succeeded in producing the experiments much has been learned the speed of molecules and their mean world's first molecular beam; he demon­ about the basic structure of matter that free path between collisions. In general strated that in a good vacuum the so­ could not have been learned in other the results agreed and supported the dium atoms that comprised his beam ways. The invention of masers and la­ new kinetic theory of gases, which was moved in straight lines. Dunoyer's ele­ sers arose out of a molecular-beam ex­ firmly established during the latter half gantly simple apparatus consisted of an periment, and a beam of cesium or hy­ of the 19th century by James Clerk evacuated glass tube with two con­ drogen atoms in an "atomic clock" is the Maxwell of Britain, Ludwig Boltzmann strictions [see top illustration on page most accurate timekeeper ever built. of Austria and other theorists. Nonethe­ 60]. When he heated some sodium in less, the evidence for the kinetic theory one of the end compartments, a dark de­ The Stern-Gerlach Experiment remained indirect. posit of sodium appeared in the com­ The first direct measurement of the partment at the opposite end. The de­ The idea that gas molecules are in speed of gas molecules was made in posit covered only the area that could rapid motion goes back to a treatise 1920 by Otto Stern, who was then 58 © 1965 SCIENTIFIC AMERICAN, INC working at the University of Frankfurt; it was also the first measurement in which a molecular beam was employed. Instead of an oven Stern used a hot platinum wire plated with silver and positioned in the middle of an evacuated bell jar. Silver atoms evaporated from the wire in all directions, but only those that passed through a narrow slit were able to reach a glass detector plate, on which they left a deposit in the form of a narrow line. As long as the apparatus remained stationary the speed of the atoms did not matter; fast or slow, they all followed the same straight path. But Stern had mounted the main parts of his apparatus-the silver-plated wire, the slit and the glass plate-on a vertical axis that could be made to rotate at some 1,500 turns per minute inside the bell jar [see bottom illustra­ tion on next page J. At that time, with vacuum techniques still in their infancy, this was quite a tour de force. When the apparatus was rotating, the deposit was formed in a slightly different place, because now the glass plate was moving TYPICAL MOLECULAR·BEAM APPARATUS, photographed in the laboratory of Nor­ as the atoms were heading for it. From man F. Ramsey at Harvard University, is used for studies of nuclear magnetic resonance. the amount of shift Stern was able to A beam of hydrogen molecules enters the apparatus at the far end, passes through several estimate the average speed of the atoms magnetic fields and is detected at the near end (see schematic diagram at bottom of page 62). at 580 meters per second; this was with­ in a few percent of the figure predicted by the kinetic theory of gases. More­ over, he observed that the deposit formed during rotation was a bit fuzzy; this showed that not an the silver atoms had the same speed, which Maxwell had foreseen. About a year later Stern published, together with his colleague Walther Gerlach, the result of another molecu­ lar-beam experiment. This experiment had a profound influence on the devel­ opment of quantum mechanics; it sup­ ported the rather incredible conclusion, drawn from spectroscopic observations, that the orbit of an electron moving about the nucleus of an atom in a mag­ netic field must always be at right angles to the lines of force in the field, al­ though the electron can orbit in one direction or the other. Such "spatial quantization" was completely at odds with classical physics, according, to which a magnetic field would merely make the electron's orbit precess, or wobble, around a field line rather like a spinning top, without imposing any limit on its orientation. In thinking about the Stern-Gerlach experiment it is helpful to treat the atom as though it were a small bar magnet with a magnetic moment /1-. SOURCE·END VIEW of the molecular·beam resonance apparatus shown above reveals the {The magnetic moment of an orbiting end of the first, or A-field, deflecting magnet. The beam of hydrogen molecules passes charge e, traveling with the speed v on between the two semicircular·shaped pole pieces at the center. The source equipment and a circle of radius r, is given by the equa- the bulkheads separating the various chambers have been removed for easy viewing. 59 © 1965 SCIENTIFIC AMERICAN, INC SOURCE COLLIMATOR OBSERVATION CHAMBER ergy could have only the two extreme CHAMBER CHAMBER values -/�H and +I-'-H and nothing in between. For nearly a decade before the Stern­ J Gerlach experiment the classical con­ cept of randomly oriented electron or­ bits had been in conflict with the Zeeman effect, the fact (first noted by the Dutch physicist Pieter Zeeman in 1896) that the lines in a spectrum are often split into two or more lines when TO PUMP the emitting atom is placed in a mag­ netic field.
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