Stimulated emission of microwaves

The ODMR technique has a distinct advantage in sensitivity over conventional EPR spectroscopy for measurements made on biological molecules. To begin with, in ODMR, the absorption or stimulated emission of microwave quanta is not detected directly, as is the case with EPR. Rather, the microwave quanta are converted to optical photons which are the detected entities. The quantum up-conversion by a factor of about 10 in energy results in greatly increased sensitivity over conventional EPR the actual attainable sensitivity depends on various factors such as phosphorescence quantum yield, the light collection efficiency, the decay characteristics of the triplet state, and other factors discussed later. 

An example of a quantitatively-analysed experimental result for these constants is shown in Fig. 7.29 in mixed crystals of naphthalene-dg 0.1% quinoxaline, the ESR transition T. To for the field direction Bo Xquinoxaiine and at a temperature T = 1.8 K is an absorption signal in the stationary state (Fig. 7.29a), while the transition I To) T-) in the stationary state exhibits stimulated emission of microwaves (Fig. 7.29b). After the end of the UV excitation at t = 0, the absorption line temporarily becomes an emission tine and vice versa. The interpretation of these results is simple (Fig. 7.29d) due to the negligible spin-lattice relaxation at T= 1.8 K, the three Zeeman components decay after the end of the U V excitation independently of one another, each with its own lifetime tj = into the So ground state. Since the difference of the populations of the three states is directly proportional to the intensity of the ESR signals, their time dependence can be used to determine the individual lifetimes of the Zeeman components involved. In the case of the particular orientation Boll, the state is To) = IT ), and one obtains directly from the measurements, e.g. the decay constant feo = kx and thus the lifetime of the zero-field constant Tx) of quinoxaline. 

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. 

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... 

Acronym for Microwave Amplification by Stimulated Emission of 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). 

Atomic filter A sort of device with an ultra-high Q obtained by using the vibrations of atoms. These systems employ the microwave amplification by stimulated emission of radiation (maser) of substances such as cesium or ruby to stabilize the primary oscillator of extremely precise devices that work at high frequency values [i]. 

The central problem of the maser is to obtain a suitable excess population in the upper state, thereby stimulating the emission of microwave radiation having a single wavelength and frequency. Such radiation is said to be coherent. In practice, masing action is accomplished in various ways. Good low noise amplifiers at microwave frequencies have been made using ruby masers. These amplifiers have found application in radio astronomy and space communication. 

A laser (acronym for light amplification by stimulated emission of radiation) amplifies light in a different region of the electromagnetic spectrum by the same method that the maser amplifies microwaves. 

As shown by Fig. 8 we can see collisions in which stimulated emission of up to three microwave photons occurs using very low microwave powers. It is interesting to note that laser assisted collisions typically require optical intensities of MW/cm2 to be visible at all [Falcone 1977], In Fig. 8 it appears that the one photon assisted collision disappears only to reappear at higher microwave fields. This unexpected feature becomes more apparent if we plot the resonance signal vs the microwave field, as shown in Fig. 9. Also shown by the lines in Fig. 9 are the results of a model describing collisions in which from zero to three photons are emitted. 

In 1951, Purcell, Pound and Ramsey [21] did some NMR experiments with inverted populations of the nuclear spin systems in LiF and noted that the spin systems were at negative absolute temperatures and that they were intrinsic amplifiers rather than absorbers. The first suggestions to use systems with inverted populations as practical amplifiers or oscillators were made independently in the early 1950 s by Townes [15,22], Weber [15] and Bassov and Prokhorov [15]. The first such amplifier was a molecular beam apparatus operating on the NH3 inversion states and built by Gordon, Zeiger and Townes [22] and was called a microwave amplifier by stimulated emission of radiation (MASER). 

During recent years, the techniques of spectroscopy have been greatly enhanced by the introduction of lasers. The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The development of this field began in 1953 with the introduction by the American physicist Charles H. Townes of the maser, which stands for Microwave Amjplification by Stimulated Emission of Radiation. 

Microwaves sources include electron beams (e.g.. magnetron, klystron), semiconductors (e.g, Gunn diode, transistor) and masers (microwave amplification by stimulated emission of radiation) [88]. The depth of penetration d of microwaves into a dielectric is a function of the lo.ss factor S in the dielectric material and the frequency / of operation (or vacuum wavelength aq) [89] and is given by... 

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... 

The idea of amplification of EM radiation by stimulated emission was first realized for microwaves. The MASER (microwave amplification by stimulated emission of radiation) was developed in the Soviet Union in the early part of the 1950s, particularly by N. Basov and A. Prokhorov. At the same time, it was developed by C. H. Townes, A. L. Schawlow, and others in the United States. Originally, the laser was called light-maser, but the name was changed to LASER (light amplification by stimulated emission of radiation). 

Lasers. The term laser is an acronym for light amplification by stimulated emission of radiation. The possibility of lasers was postulated by Albert Einstein, and a microwave laser was developed in the 1950 s. Some credit American physicist Gordon Gould with the invention of the first laser using light however the ruby laser invented by American physicist Theodore Maiman in 1960 is considered to be the first laser to use light. 

Maser (Charles Hard Townes) The maser (microwave amplification by stimulated emission of radiation) is a laser for microwaves. Discovered later, the laser patterned its name the acronym maser. ... 

Lymph A clear, colorless fluid which circulates through the vessels of the lymphatic system. Maser Microwave amplification by stimulated emission of radiation. 

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