Resonance Radiation

the radiation emitted by a system of bound charges, for example, by an atom or an atomic nucleus, at a frequency that coincides with the frequency of the exciting light. It may be emitted by gases, liquids, or solids, but the clearest pattern is observed in the atomic vapors of Hg, Cd, Na, and other elements. Resonance radiation was discovered in 1905 by R. W. Wood during his research on the luminescence of Na vapors.

In order to excite resonance radiation, an atom or other system of bound charges is irradiated with light at a frequency v. Upon absorbing a quantum having the energy hv (h is Planck’s constant), an atom in its ground state (level E₀) shifts to an excited state corresponding to level E_n (the level E₂ in Figure 1). During a spontaneous transition of the atom from an excited state E_n to the ground state E₀, resonance radiation occurs—the atom emits a quantum with the frequency c, and a resonance line appears in the spectrum of the radiation. A set of resonance lines makes up the resonance spectrum of an atom. The resonance radiation of atoms and molecules is called resonant luminescence. When γ-radiation interacts with an atomic nucleus, it is possible to have resonance radiation of γ-quanta.

Resonance radiation is observed only under certain conditions, such as rarefied atomic vapors and frozen solutions. Usually an atom makes a radiationless transition from an excited state to an intermediate state (level E_l in Figure 1), and only then does a radiation transition to the ground state occur with a frequency < v. If, as a result of excitation, an atom immediately

Figure 1

has made a transition to the level E₁, a pure form of resonance radiation occurs, since there is no intermediate level in this case.

The process of resonance radiation occurs over a certain time t. The intensity of resonance radiation I varies with time according to the law I = I₀e^-t/τ, where I₀ is the initial intensity and τ is the mean lifetime of the atom in an excited state. Usually, τ ≃ 10^-8 sec. If an electron transition is forbidden by selection rules, the duration of resonance radiation may be considerably lengthened; in Hg vapor, for example, a transition occurs with τ ≃ 10^-7 sec.

Resonance radiation is always polarized, and the degree and nature of the polarization is determined by the polarization of the exciting light, the direction of the observation, the irradiated object, and whether the object contains impurities. A magnetic field has a very significant influence on the polarization of resonance radiation; during experiments, it is necessary to make allowances for the earth’s magnetic field.

In the quantum theory of resonance radiation, as in the classical theory of resonance, it is necessary to take into account the effects of decay, that is, the decay of excited electron states that are not strictly constant with respect to time. The energy of an electron in an excited state does not have a strictly defined value, and spectral lines typically have a certain width Γ. The value of Γ depends on the total probability of the electron’s transition to lower levels and on τ; the larger the value of Γ, the smaller the value of τ and, therefore, the shorter the duration of the resonance radiation.