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Townes, Charles Hard

energy frequency molecules university

(1915– ) US physicist: discovered the theory of the maser and produced the first working examples.

Townes was the son of a lawyer and he completed his education in 1939, having attended Furman University, Duke University and California Institute of Technology. He began work at the Bell Telephone Laboratories and spent the war years developing radar-assisted bomb sights. Radar uses microwave radiation, with a frequency between that of radio and infrared light. In 1947 he joined the physics department at Columbia University. He spent 1961–7 at Massachusetts Institute of Technology and then became a professor at the University of California at Berkeley.

Beginning as early as 1945, Townes had studied the absorption and emission of photons when a molecule goes from one configuration to another. This involves very precise photon frequencies because molecular energy states are discrete and precisely defined. Thus the ammonia molecule (NH3 ) may flip between two configurations rather like an umbrella blowing inside-out, but with the absorption of a specific microwave frequency, of 1.25 cm wavelength.

In 1951, Townes realized that a wave of photons could be amplified in a practical way by the spontaneous emission process first suggested by . That is, if a molecule in the higher energy state is stimulated by photons of just the correct frequency, it will fall back to its lower state and emit another photon of precisely the same frequency or energy. If there are fewer molecules in the lower energy state to absorb photons than in the upper state, a net amplification results. He also recognized that if the wave is reflected back and forth in a resonant cavity, it interacts with the molecules for some time, steadily gaining more energy or amplification and resulting in a coherent output signal consisting of a wavetrain of extremely well-defined frequency.

To obtain ammonia with many molecules in the higher energy state, Townes took advantage of molecular beam techniques, separating out a beam of molecules in a high energy state with a non-uniform electric field. This ‘population inversion’ method, giving a majority of the high-rather than low-energy molecules, then provided a working amplifier and oscillator (1954). It was called a maser ( m icrowave a mplification by s timulated e mission of r adiation). A somewhat similar idea was suggested in the Soviet Union by A Prokhorov (1916–2002) and .

Masers were soon used in atomic clocks and in sensitive receivers, for example for radio telescopes and space communications. In 1958 Townes and showed that an optical version of the maser (the laser, for ‘ l ight a mplification by s timulated e mission of r adiation’) was possible, and discussed its properties and oscillation theoretically. The first operating system was constructed, however, by (1960), and now many versions are made.

Townes, Prokhorov and Basov were awarded the 1964 Nobel Prize for physics ‘for fundamental work in the field of quantum electronics, which had led to the construction of oscillators and amplifiers based on the maser–laser principle’.

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