Molecular beam

A molecular beam is produced by allowing a gas at higher pressure to expand through a small orifice into a chamber at lower pressure to form a beam of particles (atoms, free radicals, molecules or ions) moving at approximately equal velocities, with very few collisions between the particles. Molecular beams are useful for fabricating thin films in molecular beam epitaxy and artificial structures such as quantum wells, quantum wires, and quantum dots. Molecular beams have also been applied as crossed molecular beams. The molecules in the molecular beam can be manipulated by electrical fields and magnetic fields.[1] Molecules can be decelerated in a Stark decelerator or in a Zeeman slower.

History

The first to study atomic beam experiments was Louis Dunoyer de Segonzac 1911, but were simple experiments to confirm that atoms travelled in straight lines when not acted on by external forces.[2]

In 1921, Hartmut Kallmann and Fritz Reiche wrote[3] about the deflection of beams of polar molecules in an inhomogeneous electric field, with an ultimate aim of measuring their dipole moments. Seeing the page proofs for the Kallman and Reiche work prompted Otto Stern at the University of Hamburg and University of Frankfurt am Main to rush publication of his work with Walther Gerlach on what later became known as the Stern–Gerlach experiment. (Stern's paper references the preprint, but the Kallman and Reiche work would go largely unnoticed.[4])

When the 1922 Stern-Gerlach paper appeared is caused a sensation: they claimed to have experimentally demonstrated "space quantization": clear evidence of quantum effects at a time when classical models were still considered viable.[4]: 50  The initial quantum explanation of the measurement -- as an observation of orbital angular momentum -- was not correct. Five years of intense work on quantum theory was needed before it was realized that the experiment was in fact the first demonstration quantum electron spin[2] Stern's group would go on to create pioneering experiments with atomic beams, and later with molecular beams. The advances of Stern and collaborators led to decisive discoveries including: the discovery of space quantization; de Broglie matter waves; anomalous magnetic moments of the proton and neutron; recoil of an atom of emission of a photon; and the limitation of scattering cross-sections for molecular collisions imposed by the uncertainty principle[2]

The first to report on the relationship between dipole moments and deflection in a molecular beam (using binary salts such as KCl) was Erwin Wrede in 1927.[5][4]

In 1939 Isidor Rabi invented a molecular beam magnetic resonance method in which two magnets placed one after the other create an inhomogeneous magnetic field.[6] The method was used to measure the magnetic moment of several lithium isotopes with molecular beams of LiCl, LiF and dilithium.[7][8] This method is a predecessor of NMR. The invention of the maser in 1957 by James P. Gordon, Herbert J. Zeiger and Charles H. Townes was made possible by a molecular beam of ammonia and a special electrostatic quadrupole focuser.[9]

The study of molecular beam led to the development of molecular-beam epitaxy in the 1960s.

See also

References

  1. ^ van de Meerakker, Sebastiaan Y. T.; Bethlem, Hendrick L.; Vanhaecke, Nicolas; Meijer, Gerard (2012-03-27). "Manipulation and Control of Molecular Beams". Chemical Reviews. 112 (9). American Chemical Society (ACS): 4828–4878. doi:10.1021/cr200349r. hdl:2066/103491. ISSN 0009-2665. PMID 22449067.
  2. ^ a b c Ramsey, N. F. (1988). "Molecular beams: our legacy from Otto Stern". Zeitschrift für Physik D. 10 (2–3): 121–125. Bibcode:1988ZPhyD..10..121R. doi:10.1007/BF01384845. ISSN 0178-7683. S2CID 120812185.
  3. ^ "Kallmann, H.; Reiche, F. (1921). "Über den Durchgang bewegter Moleküle durch inhomogene Kraftfelder". Zeitschrift für Physik (in German). 6 (1). Springer Science and Business Media LLC: 352–375. Bibcode:1921ZPhy....6..352K. doi:10.1007/bf01327996. ISSN 1434-6001. S2CID 119947742.
  4. ^ a b c Friedrich, Bretislav; Schmidt-Böcking, Horst (2021), Friedrich, Bretislav; Schmidt-Böcking, Horst (eds.), "Otto Stern's Molecular Beam Method and Its Impact on Quantum Physics", Molecular Beams in Physics and Chemistry, Cham: Springer International Publishing, pp. 37–88, Bibcode:2021mbpc.book...37F, doi:10.1007/978-3-030-63963-1_5, ISBN 978-3-030-63962-4
  5. ^ "Wrede, Erwin (1927). "Über die Ablenkung von Molekularstrahlen elektrischer Dipolmoleküle im inhomogenen elektrischen Feld". Zeitschrift für Physik (in German). 44 (4–5). Springer Science and Business Media LLC: 261–268. Bibcode:1927ZPhy...44..261W. doi:10.1007/bf01391193. ISSN 1434-6001. S2CID 120815653.
  6. ^ Kellogg, J. B. M.; Millman, S. (1946-07-01). "The Molecular Beam Magnetic Resonance Method. The Radiofrequency Spectra of Atoms and Molecules". Reviews of Modern Physics. 18 (3): 323–352. Bibcode:1946RvMP...18..323K. doi:10.1103/RevModPhys.18.323. ISSN 0034-6861.
  7. ^ Rabi, I. I.; Millman, S.; Kusch, P.; Zacharias, J. R. (1939-03-15). "The Molecular Beam Resonance Method for Measuring Nuclear Magnetic Moments. The Magnetic Moments of 3Li6, 3Li7 and 9F19". Physical Review. 55 (6). American Physical Society (APS): 526–535. doi:10.1103/physrev.55.526. ISSN 0031-899X.
  8. ^ The Rabi molecular-beam method - The Feynman Lectures on Physics
  9. ^ Gordon, J. P.; Zeiger, H. J.; Townes, C. H. (1954-07-01). "Molecular Microwave Oscillator and New Hyperfine Structure in the Microwave Spectrum of NH3". Physical Review. 95 (1). American Physical Society (APS): 282–284. Bibcode:1954PhRv...95..282G. doi:10.1103/physrev.95.282. ISSN 0031-899X.