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A method of two-quantum excitation of molecules at Doppler-broadened transitions in a standing-wave radiation field is considered. It is shown that it is possible to increase the excitation selectivity of molecules with overlapping absorption lines when using spectra of two-photon absorption. Estimations of two-quantum transition probabilities for molecular gases are given.

The paper reviews the state-of-the-art in production of supernarrow resonances in the optical band by the method of saturated absorption and their application in spectroscopy. The influence of collisions, transit and other effects on the resonance width is discussed. Presented are the results of investigations of elastic scattering at collisions, hyperfine structure of vibrational-rotational transitions in molecules, second-order Doppler effects by using supernarrow resonances. Various physical applications of supernarrow optical resonances are reported.

The production of a time scale based on the use of an oscillation period of a highly stable laser is first reported. The new time standard allows to transfer frequency characteristics of a highly stable laser in the frequency range from 0 to 1014 Hz with no losses in accuracy. Radio-frequency oscillators were synchronized with the aid of fast-response systems of phase offset lock at division of laser frequencies.

Investigations are reported on the frequency characteristics of the cw high pressure CO 2 laser and its use for high precision spectroscopy. The problem of using a cw tunable CO 2 laser with a mixture of a few isotopes ( 12 C 16 O 2 , 12 C 18 O 2 , 12 C 16 O 18 O and others) in a cw high pressure tunable CO 2 laser is discussed. Broadening of the tuning range has given us the opportunity to find seven new absorption lines of SF 6 near the transition P(20) of the 00 0 1–10 0 0 band of the CO 2 laser.