Molecular Generator

a device in which coherent electromagnetic oscillations are generated by means of induced quantum transitions of molecules from their initial energy state to a state with a lower intrinsic energy. The first quantum generator, which was developed in 1954 by N. G. Basov and A. M. Prokhorov (USSR) and independently by C. Townes, J. Gordon, and H. Zeiger (USA), was a molecular generator. Both versions of this molecular generator ran on molecules of ammonia, NH³, and generated electromagnetic oscillations with a frequency of 24,840 megahertz (MHz) and a wavelength λ = 1.24cm.

Two principal conditions must be met in order to stimulate the generation of coherent vibrations: the number of particles in the initial state in the working cavity of the instrument must be greater than the number in a state with a lower intrinsic energy (population inversion), and a link must be provided among particles that radiate at different instants (positive feedback). In a molecular generator the first condition is satisfied by electrostatic sorting of the molecular beam, and feedback is provided by a cavity resonator tuned to a frequency equal to that of the radiation that accompanies the molecule’s transition from the initial energy state to the final state. The molecular beam is formed when the molecules fly from the source into a vacuum through narrow apertures or pores.

Electrostatic sorting of molecules by energy states in a molecular generator is based on the fact that, upon passage through a nonuniform electric field, molecules that have an electric dipole moment (particularly NH₃ molecules) are deflected differently from a rectilinear path, depending on their energy. In the first molecular generator the sorting system was a quadrupole capacitor consisting of four parallel rods of a special shape, connected in pairs to a high-voltage rectifier. The electric field of such a capacitor is extremely nonuniform. This causes bending of the trajectory of the NH₃ molecules moving along its axis. The properties of NH₃ molecules are such that those that are in the upper of the pair of energy states used are deflected toward the axis of the capacitor and enter the cavity resonator. Molecules in the lower state are cast aside and do not enter the resonator. A separated beam contains molecules that are in the upper energy state. Upon entering the resonator, such molecules radiate under the influence of the electromagnetic field of the resonator (induced radiation). The radiated photons remain within the resonator, amplifying its field and increasing the probability of induced radiation for molecules that pass by later. If the intensity of the beam of active molecules is such that the probability of induced radiation of a photon is greater than the probability of absorption of a photon in the resonator walls, then the process of self-excitation arises—the intensity of the resonator’s electromagnetic field increases rapidly at the transition frequency because of the intrinsic energy of the molecules in the beam. This increase ceases when the field in the resonator reaches the value at which the probability of forced emission becomes so great that precisely half the molecules in the beam succeed in emitting a photon during traversal of the resonator. In the process the probability of absorption for the beam as a whole becomes equal to that of induced emission. The power generated by a molecular generator using an NH₃ beam is 10⁻⁸ watt, and the frequency stability is 10^∐7-10^∐11.

Molecular generators using a number of other dipole molecules that operate in the centimeter and millimeter bands, as well as quantum generators using a hydrogen-atom beam operating at a wavelength of 21 cm, have been developed since then. These devices, like quantum amplifiers in the radio band, are sometimes called masers. There are several design varieties of the molecular generator that differ in the arrangement of the sorting systems, the number of resonators, and other factors. Quantum generators in which the population inversion of the molecular levels is achieved not by sorting but rather by other methods, such as exposure to an auxiliary electromagnetic field (pumping) and electric discharge, are also molecular generators. Quantum generators in the optical band (lasers), which use molecular gases as their working substances, may also be classified as molecular generators in this sense.