Radiofrequency resonance

Shaw, Determining the Potential of Radiofrequency Resonance Absorption Detection of Explosives Hidden in Airline Baggage , SWR1-15-4226-F (1975), (AD A022 111 /9ST) 58) P. 

The radiofrequency resonances observed arise from transitions between the nuclear hyperftne components of the J" = 28 rotational level. The 23Na nuclei have spin I =h = 3/2, so that the total nuclear spin / can take the values 3, 2, 1 and 0. A rotational level with J even combines with 1 = 2 and 0, so that for 1 = 2, the total angular momentum F takes all integral values from 26 to 30 when I = 0 we have F =... 

Lamb and Retherford [81] proposed to make a beam of atomic hydrogen, and to excite it by electron bombardment. Most of the atoms in the 22S level, but not those in the 22P, levels, would live. long enough to reach a detector. A radiofrequency field of the correct frequency in the path of the atoms would induce transitions from 22 to one of the 22P levels. Decay of the latter would result in a reduction in the number of atoms detected, which would indicate radiofrequency resonance. We shall give here a brief account of the work and quote the results. Details of the experiments are given in a series of six papers [82], [83], [80], [84], [131], [30] and a review [79]. 

Lamb and Skinner [86] were able to measure the interval 2in He4- by a method which, as in the experiment on hydrogen, depended on the metastability of the 22 level, but in which the method of detection of radiofrequency resonance was different. In the case of hydrogen, the method of detection depended on the possibility of distinguishing metastable atoms from atoms in the ground state through their power of liberating electrons from tungsten. Helium ions can also liberate electrons from metals, but the relative ejection efficiency of metastable and non-metastable ions is not known. Lamb and Skinner... 

We have explained (section 12.5) how the steady magnetic field mixes the Mj = 0 states of lS0 and ZSV By virtue of the fast decay rate of XS0, the state Sl9 Mj = 0 will therefore be less populated than ZSV Mj = 1. Under these conditions, there will be a net transfer of positronium atoms from Mj = 1 to Mj = 0 at radiofrequency resonance. 

In the experiment of Deutsch and Brown, the positronium was deteoted through the annihilation y-rays. Radiofrequency resonance was indicated by an increase in the ratio of two-photon to three-photon processes. The value of ATT calculated by means of equation 12.7 was (2-032 0 003)x 106 Mc/s. More recent experiments [133], [90], give... 

Figure 11.53. Radiofrequency resonance arising from the J = 13/2, F = 9/27 = 11/2, F = 111 transition in the ground state ofNj [99].

Electron concentrations have been measured by radiofrequency resonant circuits " in which the flame is placed inside a coil or condenser which is part of a resonant circuit. Electron concentrations have also been obtained by cyclotron resonance/ microwave scattering/ microwave absorp-tion/ microwave interferometry/ and microwave cavity reso-nance. " All of these methods for measuring electron concentration have the advantage of not perturbing the flame, but suffer from the disadvantage of poor spatial resolution. Also, measurements can be made only over a relatively limited range of concentrations—typically, one or two orders of magnitude. 

When Brossel and Kastler proposed the intermarriage of optical spectroscopy and radiofrequency resonance nearly four decades ago, they triggered off the development of numerous novel spectroscopic methods— a development that continues even today. The first generation of optical pumping and optical double resonance experiments not only yielded a wealth of very precise data on atoms, molecules, condensed matter, and nuclei alike, but they also initiated the detailed study of the interaction of atoms with radiation, particularly under the aspects of coherence, and of the modification of atomic states through radiation fields. 

Principles of the technique. In section 7.4 we discussed in detail the spontaneous decay of excited atoms by the simultaneous emission of two electric dipole photons. Consideration of arguments similar to those used in Chapter 9 in the derivation of the Einstein relations show that, corresponding to this spontaneous decay process, there must also exist stimulated transitions involving the simultaneous emission or absorption of two photons. Such multiple-quantum transitions were observed experimentally in radiofrequency resonance experiments by Brossel et al. (1954) and Kusch (1954), and a theory of this effect for magnetic dipole... 

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