Coherent transient effects due to phase modulation of recoillessγradiation

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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 M\"ossbauer absorber. The transient effects studied in this work resemble those observed in NMR and laser spectroscopy. They offer several new features for M\"ossbauer 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 M\"ossbauer measurements are considered, e.g., the transmission minimum is delayed approximately for the lifetime of the excited nuclear state.The time dependence of the oscillatory transient excited by a rapid sweep through the resonance is derived. Transient experiments with sinusoidal phase modulation allow metrological calibration of mechanical displacements. The separation between adjacent zeros of the interference oscillations corresponds to a displacement of the source by half a wavelength of the {γ} radiation. As compared with conventional M\"ossbauer measurements, more efficient separation of the source and absorber parameters can be achieved in transient experiments. In addition, the linewidths and line positions are obtained in terms of the modulation frequency which simplifies the calibration of the energy scale.If the phase of the incident radiation is rapidly switched, a simultaneous change is observed in the transmission intensity. An exact expansion shows that, with a thick absorber and an appropriate center shift between the source and absorber line positions, this intensity change can be as large as eight times the maximum conventional resonance absorption. Distortions due to nonideal phase modulation are considered as well. Most measurements were performed with use of the 93-keV M\"ossbauer resonance of ⁶⁷Zn in ZnO. Comparative results are also presented from ⁵⁷Fe experiments with sinusoidal phase modulation. In transient experiments with ⁶⁷Zn, mechanical displacements of the order of 10^-13 m were resolved.As a further application the recoilless fractions of both the ⁶⁷Ga:ZnO source (fₛ) and of the enriched ⁶⁷ZnO powder absorber (fₐ) were determined and found to be equal: fₛ=0.0212(16) and fₐ=0.0213(18). In phase-switching experiments, filtering of the driving voltage was used to prevent excitation of mechanical resonances of the piezoelectric drive. With a relatively thick absorber and zero center shift, the observed change in the transmitted intensity was three times the conventional resonance absorption. Good agreement between the theoretical calculations and the experimental data was found.