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The transition probabilities and the shifts of the crossing and anticrossing points of a two-level system (spin-1/2 atom) under the simultaneous influence of a static magnetic field and three orthogonal oscillating rf fields are calculated by using the resolvent formalism in which the rf fields are quantized. A new interference effect, which can be called "crossing of the third kind" in the rf transition, is predicted, and can be interpreted as the interference between different-order transition processes such as single-and two-photon processes, single-and three-photon processes, and so on. Furthermore, for coexistence of the single-and two-photon processes and of the single-and three-photon processes, we can obtain expressions of the transition probabilities (or effective dipole-coupling coefficients) similar to the results obtained in our previous paper, where the interference between single-and twophoton processes were treated by using the time-dependent Schrodinger equation.

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Absorption of intracavity radiation by CO2 molecules in an unexcited region has detrimental effects on output power characteristics, especially above a certain level of output power, i.e., the threshold level. These absorption effects have been investigated experimentally in detail. The threshold level depends both on CO2 concentration of the laser gas and on the mirror transmission of the optical cavity. It is found from these dependences that the absorption effect becomes serious above ∼0.2 W/cm3, which is the density of optical power absorbed by the unexcited CO2 molecules. The influence of self-absorption on the laser efficiency and the oscillating line of 10.4-μm band can be explained well by a usual theory for laser oscillation. Furthermore, a novel method for monitoring the concentration of CO2 molecules is demonstrated.

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