Coherent transient effects due to phase modulation of recoillessγradiation

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Abstract
Coherent transient effects due to phase modulation of recoilless γ radiation are considered both theoretically and experimentally. Transients are observed in transmission experiments when the incident radiation field interferes with the induced polarization of the Mössbauer absorber. The transient effects studied in this work resemble those observed in NMR and laser spectroscopy. They offer several new features for Mössbauer measurements. A theory for the time dependence of the recoilless radiation is derived semiclassically with use of a density-matrix formalism. Transient effects in scattering geometry are also briefly commented upon. The theory is applied to several cases of experimental interest. Time-dependent distortions affecting the line shape in conventional Mössbauer measurements are considered, e.g., the transmission minimum is delayed approximately for the lifetime of the excited nuclear state.