Microwave MASER

More recently the invention of microwave masers and of lasers operating at optical frequencies has transformed the field of optical physics in a remarkable way. In solid state physics, Franken s demonstration of the frequency doubling of ruby laser light immediately opened up a new field of non-linear optics. In atomic and molecular physics the applications of laser radiation were initially less rapid and spectacular. However, following the development of narrow-bandwidth tunable dye lasers in 1970, the interest in this field of research has experienced an explosive growth which as yet shows no signs of slowing down. 

This illustrates why resonant cavities for amplification by stimulated emission in the visible region (lasers) need to have very much greater 0 values than those for the amplification of microwaves (masers). 

The word laser is an acronym derived from light amplification by the stimulated emission of radiation . If the light concerned is in the microwave region then the alternative acronym maser is often used. Although the first such device to be constructed was the ammonia maser in 1954 it is the lasers made subsequently which operate in the infrared, visible or ultraviolet regions of the spectrum which have made a greater impact. 

By 1954 Townes, with the help of graduate students Herbert Zeiger and James Gordon, developed the maser, an acronym for microwave amplification by stimulated emission of radiation. The maser had... 

The presence of the potential barrier in the ammonia molecule results in splitting of the vibrational energy levels, as shown in Fig. 5. The separation between die two components of the first level is equal to 23.87 GHz 0.79 cm-1). The corresponding absorption line is easily observable in the microwave spectrum of ammonia. In fact, this transition was utilized in the MASER,f the forerunner of die LASER. 

Maser The microwave equivalence of a laser. A stimulated transition in a molecule that relaxes a population inversion in an excited state... 

Maser The microwave equivalent of a laser that represents some of the strongest transition intensities in the radio region of the electromagnetic spectrum, e.g. the water maser at 22.235 GHz. 

The term MASER stands for Microwave Amplification by Stimulated Emission Radiation... 

Amplification by Stimulated Emission of Radiation . (Similar devices producing coherent beams of microwave radiation are known as masers) A typical arrangement for a pulsed ruby laser is depicted in Figure 8.5. 

Since htjkT is small, the ratio of the two transition probabilities is small and Amn Bmn p (vam). This condition is obtained in the microwave region and is utilized in the construction of masers (microwave amplification by stimulated emission of radiation). 

The first stimulated-emission amplification device was produced at microwave frequencies by Townes and co-workers in 1954. (This was the maser—microwave amplification ) In 1958 Schawlow and Townes de-... 

Maser amplifiers are used where the requirement for a very low noise amplifier outweighs the technological problems of cooling 10 low temperatures. They have been used in passive and active radiostronomical work, in satellite communications, and us preamplifiers for microwave spectrometry The ammonia and the atomic hydrogen masers have been studied as frequency standards and have heen used in accurate tests of special relativity. 

D.C. MASER Microwave Amplification by Simulated Emission of Radiation... 

Inversion splitting of the vibrational spectrum of ammonia has been used to create the first molecular microwave amplifier (maser) [86, 87]. The inversion population in the ammonia maser is achieved by transmission of the molecular beam through a non-homogeneous electric field. Ammonia molecules in symmetric and antisymmetric states interact with the electric field in different ways and they are therefore separated in this field. They are then directed to the resonator. 

Hydrogen masers are used as a high-precision frequency standard its frequency of 1,420,405,752 Hz (long-term stability = 1 part in 5 x 1016 in 5 years ) also corresponds to the most intense microwave source in the universe, due to the energy difference between the (S = 1/2,7 = +1/2) and (S = 1/3, 7=1/2) states the energy separation is due to hyperfine coupling in hydrogen. 

The first successful application of molecular beam electric resonance to the study of a short-lived free radical was achieved by Meerts and Dymanus [142] in their study of OH. They were also able to report spectra of OD, SH and SD. Their electric resonance instrument was conventional except for a specially designed free radical source, in which OH radicals were produced by mixing H atoms, formed from a microwave discharge in H2, with N02 (or H2S in the case of SH radicals). In table 8.24 we present a complete A-doublet data set for OH, including the sets determined by Meerts and Dymanus, with J = 3/2 to 11/2 for the 2n3/2 state, and 1/2 to 9/2 for the 2ni/2 state. Notice that, for the lowest rotational level (7 = 3/2 in 2n3/2), the accuracy of the data is higher. These transitions were observed by ter Meulen and Dymanus [143], not by electric resonance methods, but by beam maser spectroscopy, with the intention of providing particularly accurate data for astronomical purposes. This is the moment for a small diversion into the world of beam maser spectroscopy. It has been applied to a large number of polyatomic molecules, but apparently OH is the only diatomic molecule to be studied by this method. 

Figure 8.46. Principles of a molecular beam maser spectrometer. Population inversion of the A -doublet levels is produced in the quadrupole field, leading to enhanced stimulated emission in the microwave cavity.

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