Holes in Spectral Lines

We have investigated the spontaneous emission of radiation from an atom in the presence of a static perturbation. The atom is considered to have one ground state and two excited states, one of which is nondecaying. The static perturbation couples the two excited states. Initially the atom is in the decaying state. The energy separation between the excited states is 6 and the matrix element of the static perturbation is V. The intensity of the emitted radiation is plotted in Fig. 1 as a function of the frequency for different values of V. The frequency is measured in units of y, the decay rate of the unperturbed decaying state, with the origin of the E axis at the frequency difference between the decaying state and the ground state. The energy separation of the two excited states 6 is 0. 5 ey. For V= 0 the emission line is Lorentzian, but for V0 a "hole" appears at the frequency equal to the frequency difference between the excited nondecaying state and the ground state. The position of the "hole" is independent of the static perturbation, and thus can be used to identify the nondecaying state. This effect is similar to the one discovered by Lamb' and subsequently used by McKibben, Lawrence, and Ohlsen as a nuclear-spin filter. In Lamb's case the states are coupled by a rf field and a motional electric field. Lamb has calculated the decay constants of coupled 2 P»»», 2 S&&2 &&~, and 2 S«z&&2 states in hydrogen. At a rf equal to the frequency difference between the 2 S«g «g and 2 S«z «z states, the rf-induced decay rate is zero. In our calculation this frequency corresponds to the "hole" frequency. An analysis of the resonance scattering of radiation in the presence of a static perturbation also shows "holes" in the emission spectrum when a decaying state is coupled to a nondecaying state. In order to be able to observe the "hole" in the frequency spectrum of the emitted radiation, the effect