Modulation of Mössbauer radiation by coherent ultrasonic excitation in crystals

Abstract

For the first time experimental observations are made of the oscillation in the intensity of the unshifted line and in the intensities of the US satellites when the y-resonance radiation is modulated by the coherent US excitation. It is stated that the coherent US can be excited in the investigated sample if the US amplitude is transmitted homogeneously from the generator to the sample.A special sample-unit is described which provides a homogeneous transmission, and i t is shown hat the lifetime of the US phonons is not substantial and does not affect the oscillation of the absorption line intensities. Es werden erstmalig Oszillationen der Intensitat der nichtverschobenen Linie und der Intensitaten der US-Satelliten beobachtet, wenn die y-Resonanzstrahlung durch koharente US-Anregung moduliert wird. Es wird festgestellt, daa koharenter US in der untersuchten Probe angeregt werden kann, wenn die US-Amplitude homogen vom Generator zur Probe transmittiert wird. Eine spezielle Probeneinheit wird beschrieben, die eine homogene Transmission ermoglicht, and es wird gezeigt, daB die Lebensdauer der US-Phononen nicht wesentlich ist und nicht die IntensitatsOszillation der Absorptionslinie beeinfluat.After the discovery of the Mossbauer effect, determined primarily by the character of the thermal motion of the nuclei in solids [l], experiments were performed to study the changes in the dynamics of the nuclear motion due to a n external ultrasonic (US) influence [2 to 51.The theoretical preniises for the understanding and analysis of such a n influence were set forth long ago by Shapiro [B], and later a more detailed theory was proposed by Abragarn a ) The width of the US generation band is AQ < T (where r is the width of the resonance level) -the so-called coherent regime.b) The case AQ > r -incoherent excitation.I n the latter case the US band is supposed to broaden due to relaxation processes [ 5 ] . I n both cases the central and satellite line intensities depending on the US amplitude are determined by different laws. I n case a ) the intensities of the components in the spectrum are determined by where J, are the unmodified Bessel functions of the first kind, k is the wave vector of the y-quantum, u is the ultrasonic wave amplitude. For n = 0, this formula expresses the central line intensity, and for n = 1 , 2 , 3, and so on, it expresses the n-th satellite intensity. W , = J;(?CU) ,(1)

Application of the Mössbauer effect to the study of subnanometer harmonic displacements in thin solids

“…Actually the spectra of powder absorbers [27] and thin films, for example, stainless steel foil, experiencing mechanical vibrations, [28,[30][31][32]37] are quite different. Usually these absorbers are glued on the surface of the transducer, fed by the oscillating voltage.…”

“…Usually these absorbers are glued on the surface of the transducer, fed by the oscillating voltage. The transducers, made from piezo-crystal (for example, quartz) [23,24,[26][27][28][29][30][31][32][33][34] or piezo-polymerfilm (for example, a polyvinylidene fluoride -PVDF), also produce different spectra since the conversion factor of the PVDF drive is more than ten times larger than that of quartz [38]. Meanwhile, the Rayleigh distribution has only one parameter, which is a variance of the displacement amplitude a 2 , specifying also the values m 2 and m. However, in general the distribution of amplitudes and phases of the nuclear vibrations should depend on the construction of the absorber-transducer.…”

“…[32], it was assumed that some part of nuclei in the absorber vibrate coherently with the same amplitude, while another part of nuclei also participate in the coherent vibration, but for them the mean square displacement value changes from nucleus to nucleus. We tried to fit experimental data for SS absorber to the model [32], where experimental spectra are compared with theoretical predictions assuming a mixture (a simple sum with different weights) of the coherent and incoherent models. However, this method did not allow to obtain good fitting.…”

“…Mössbauer sidebands, produced from a single parent line by absorber vibration, were observed in many different samples [23,24,[26][27][28][29][30][31][32][33][34]. However, the intensity of the sidebands has not been yet satisfactory explained [30][31][32].…”

“…However, the intensity of the sidebands has not been yet satisfactory explained [30][31][32]. There are two models of coherent and incoherent vibrations of nuclei in the absorber [27][28][29][30][31][32].…”