Ammonia maser

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. 

Over the next few years, Townes, with the help of graduate students Herbert Zeiger and Janies Gordon, calculated the precise size of the necessaiy cavity and built a device that used ammonia as an active medium. Success came in 1954 with the completion of the first maser, an acronym for microwave amplification by stimulated emission of radiation. 

Hie possibility that a particle with energy Jess than the barrier height can penetrate is a quantum-mechanical phenomenon known as the tunnel effect. A number of examples are known in physics and chemistry. The problem illustrated here with a rectangular barrier was used by Eyring to estimate the rates of chemical reactions. ft forms the basis of what is known as the absolute reaction-rate theory. Another, more recent example is the inversion of the ammonia molecule, which was exploited in the ammonia maser - the fbiemnner of the laser (see Section 9.4,1). 

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. 

However, one of the electrical engineers at the University of California, Jack Welch, was willing to work with me, and I could use his radio antenna. So I had a student, Albert Cheung, take a look, and he looked at these dark clouds in space, and sure enough, there was ammonia. Well, since we found ammonia, we thought, we ought to look for water too, just to try this out. So the student looked, and there was water radiation. In fact it was very intense - hey, it had to be a maser, maser amplification in interstellar space And OH had already seemed to indicate something similar. 

Fig, I Schematic representation of an ammonia beam maser. lAJicr Gordon. 

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. 

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. 

From the preceding discussions it is evident that at least four different temperatures have to be considered which under laboratory conditions are all equal the excitation temperature Tex of the molecule, defined by the relative populations of the levels, the kinetic temperature Tk, corresponding to the Maxwellian velocity distribution of the gas particles, the radiation temperature Traa, assuming a (in some cases diluted) black body radiation distribution, and the grain temperature 7, . With no thermodynamic equilibrium established, as is common in interstellar space, none of these temperatures are equal. These non-equilibium conditions are likely to be caused in part by the delicate balance between the various mechanisms of excitation and de-excitation of molecular energy levels by particle collisions and radiative transitions, and in part by the molecule formation process itself. Table 7 summarizes some of the known distribution anomalies. The non-equilibrium between para- and ortho-ammonia, the very low temperature of formaldehyde, and the interstellar OH and H20 masers are some of the more spectacular examples. 

Ammonia was the first molecule for which the effect of the molecular inversion was studied experimentally and theoretically. Inversion in ammonia was subsequently found to be so important that this molecule played an important role in the history of molecular spectroscopy. Let us recall, for example that microwave spectroscopy started its era with the measurements " of the frequencies of transitions between the energy levels in the ground vibronic state of NH3 split by the inversion effect. Furthermore, the first proposal and realization of a molecular beam maser in 1955 was based on the inversion splittings of the energy levels in NH3. The Nobel Prize which Townes, Basov and Prochorov were awarded in 1964 clearly shows how important this discovery was. Another example of the role which the inversion of ammonia played in the extension of human knowledge is the discovery of NH3 in the interstellar space by Cheung and his co-workers in 1968, by measuring the... 

Most molecular species can be expected to possess a nuclear structure, although for some, e.g., hydrogen bonded species or ammonia, the issue is not so obvious. The ammonia-maser transition, for example, is thought to be a transition between strange molecular (pure) states that do not admit a nuclear structure. 

The transition between the two stationary states and is the ammonia-maser transition with transition frequency v = (E — E )/h = 23,870,110,000 s . The very existence of the ammonia-maser transition suggests that the states I. and I, of ammonia do indeed exist in reality and not just in the quantum-mechanical formalism. 

Rusaan physicist best known for the development of the maser, the precursor of the laser. In 1955, while working as a research student with Aleksandr Prokhorov (1916-2000) at the SovietAcademy of Sciences, he devised a microwave amplifier based on ammonia molecules. The two scientists shared the 1964 Nobel Prize with American Charles Townes (1915- ), who independently developed a maser. 

Charles Townes was the first person to take advantage of the stimulated emission process to be used in the form of an amplifier by conceiving and constructing the first maser (an acronym for microwave amplification by stimulated emission of radiation). The maser produced a pure beam of microwaves that were anticipated to be useful for communications in a similar way to that of a klystron or a traveling-wave tube. The first maser was produced in ammonia vapor, and the inversion occurred between... 

Hence, the Einstein coefficients for absorption, spontaneous emission, and stimulated emission are all simply related. The factor that enters in the spontaneous emission coefficient (Eq. 8.35) has had historical importance in the development of lasers, since it implies that spontaneous emission competes more effectively with stimulated emission at higher frequencies. High-frequency lasers have therefore been more difficult to construct. This is one of the reasons why X-ray lasers have only recently been built, and why the first laser was an ammonia maser operating on a microwave umbrella-inversion vibration rather than a visible laser. 

In 1951, Charles H. Townes had an idea for stimulated emission of microwave radiation. He and his coworkers at Columbia University used a transition of the ammonia molecule that occurred in the microwave region of its spectrum in order to create a maser (micro-wave amplification by stimulated emission of radiation). On producing ammonia molecules in the upper state involved in the transition, the application of microwave radiation at the transition frequency caused excited state molecules to emit a photon and drop to the ground state. Since the emitted photons were at the same frequency as that of the incident radiation, the process led to an amplification of the applied radiation. 

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