Gilbert R. Hoy

MOSSBAUER SPECTROSCOPIC STUDIES OF HEMOGLOBIN AND ITS ISOLATED SUBUNITS

Samples of 90% enriched 57Fe hemoglobin and its isolated subunits have been prepared. Mössbauer spectroscopic measurements have been made on three such samples. Sample one contained contributions of oxyhemoglobin, deoxyhemoglobin, and carbonmonoxyhemoglobin. This sample was studied from a temperature of 90 K down to 230 mK. Measurements were also made at 4.2 K using a small applied magnetic field of 1.0 T. In general, the measured quadrupole splittings and isomer shifts for each component agreed with previous measurements on single component samples in the literature, and thus demonstrated that chemically enriched hemoglobin has not been altered. The second and third samples were isolated alpha and beta subunits, respectively. We have found measurable Mössbauer spectral differences between the HbO2 sites in the alpha subunit sample and the beta subunit sample. The measured Mössbauer spectral areas indicate that the iron ion has the largest mean-square displacement at the deoxy Hb sites as compared to that at the oxy- and carbonmonoxy Hb sites. The mean-square displacement at the HbO2 sites is the smallest.

Relaxation Phenomena for Chemists

The term “relaxation,” as employed here, is used to indicate that time-dependent effects are occurring in the system under study. The system is said to be “relaxing” among its various accessible states. The term “relaxation” is a minomer here since in conventional parlance it implies that the resulting state is less tense or rigid. This is a concept that has little meaning in our context, since we will not be discussing systems that are prepared in a nonequilibrium excited state and allowed to evolve by losing energy to reach thermal equilibrium. A more specific description of the phenomena we will discuss can be given as follows. Radiating or absorbing systems often experience time-dependent interactions with their environment, and as a consequence of these external time-dependent interactions the systems will evolve in time through the states determined by the system Hamiltonian. This time evolution in which the system goes from one state to another available and accessible state, following a random “path”, is often termed relaxation. We will be interested in situations in which the external time-dependent interactions can be modeled such that the system moves randomly (or stochastically) through its accessible states. Since the radiating (or absorbing) system experiences time-dependent interactions with its environment, the resulting emission (or absorption) spectra will be different for different relaxation conditions and also from that observed in the time-independent case. It is these spectral changes and their relation to various relaxation processes and physical parameters, as observed using Mossbauer spectroscopy, that will be the subject of this chapter.

Enhanced resolution in Mössbauer spectroscopy

Using a resonant detector in Mossbauer spectroscopy can result in a spectral linewidth that is 1.46 , where is the linewidth of the excited-state nuclear level. As is well known, the minimum linewidth obtained in conventional Mossbauer experiments is 2 . The quantum mechanical calculation using a nuclear resonant detector, which predicts this result, is presented. The fundamental equations describing the system are solved by means of perturbation theory in the frequency domain. The model system is taken to consist of a source nucleus, an absorber nucleus, and the resonant-detector nucleus. As noted, the minimum linewidth obtained in a Mossbauer spectrum taken under these conditions is found to be appreciably smaller than the linewidth obtained in a conventional Mossbauer set-up. Thus the conversion-electron, resonant-detector scheme may be used to advantage in experiments requiring the highest possible energy resolution.

The Mössbauer effect in109Ag revisited

The 40-sec, 88-keV, first-excited state of109Ag provides a difficult case for the observation of the Mössbauer effect. The major difficulty is associated with the long lifetime which corresponds to a natural linewidth of only 10−17 eV. Our results indicate a 0.2% Mössbauer effect.

Stimulated emission of gamma-radiation: A proposed experiment

Abstract The purpose of this paper is to describe a method for observing stimulated emission of gamma radiation. Stimulated emission of gamma radiation has never been observed. Stimulated emission of gamma radiation will be essential for developing gamma-ray lasers. The method uses the ‘π phase-shift induced transparency’ of gamma radiation, previously called the ‘gamma echo’. The gamma-echo technique is based on a modification of the time-differential Mössbauer spectroscopic method, i.e. the source is moved ‘instantaneously’ during the lifetime of the excited state. Then a resonant absorber becomes transparent to the resonant gamma radiation. There is a type of ‘self-stimulated’ emission in which the radiation ‘absorbed’, before the phase shift, is stimulated to emit after the phase shift. To observe stimulated emission of the resonant gamma radiation, one performs the same gamma-echo experiment except the resonant absorber is replaced by another source. This secondary source is chosen to be in resonance with the original source. Under these conditions there will be no stimulated emission, in the secondary source, before the π phase shift because the incident radiation is absorbed. However, after the phase shift, the secondary source becomes transparent to the incident radiation. If one uses a matched pair of Co57 sources and the secondary source is 2mCi, there will be about 3 nuclei in the secondary source, in the first excited state of Fe57, that are available to be stimulated during the 97.8ns lifetime of the primary source radiation.

π phase-shift induced transparency of resonant gamma radiation

The so-called gamma-echo effect has been observed experimentally and analyzed using the semiclassical optical theory. Here the effect is reinterpreted using a new 1D quantum mechanical model. This leads to a different interpretation of the effect as a π phase-shift induced transparency. In the basic time-differential Mossbauer spectroscopic technique the forward-scattered recoil-free radiation is observed, in delayed coincidence, after passing through a nuclear-resonant absorber. The effect in question is produced most efficiently when the source of recoil-free radiation is moved abruptly causing a π phase shift of the source radiation during its radiative lifetime. Using the 1D model the effect is seen to arise from the constructive interference between the source radiation at a later time, and the radiation coming from the absorber excited at an earlier time. The exact form of the source modulation and the nuclear-resonant thickness of the resonant absorber determines the shape of the time-differential resonant gamma ray transmission spectrum. Numerical results are given using the familiar 57 Fe recoil-free resonant transition. The π phase-shift-induced transparency allows the resonant gamma radiation, incident on the resonant absorber, to be transmitted through the absorber without appreciable attenuation.

Nuclear resonant scattering using synchrotron radiation

A one-dimensional quantum model for nuclear resonant scattering using synchrotron radiation has been developed. This model gives a clear physical interpretation of the most prominent features of the coherent forward scattering process namely, the “speed-up” and “dynamical beat” effects. The form of the solution, for the time-dependent forward scattered intensity of the resonant radiation from the resonant medium after synchrotron radiation excitation, is a finite series. This unique solution can be interpreted in terms of a summation over all multiple forward scattering paths the radiation takes in reaching the detector. The resonant medium is represented by a linear chain of N effective resonant nuclei. The analysis starts from a coupled set of quantum mechanical equations for the relevant amplitudes in frequency space. Transformation to the time domain gives an analytical expression for the forward scattered intensity. The contribution of every order of the multiple scattering processes from the N effective nuclei appears naturally. The expression gives a clear physical understanding of all relevant aspects of resonant forward nuclear scattering. Furthermore, the present formalism allows the consideration of incoherent processes. This permits the study of processes in which there is gamma emission with recoil or emission of internal-conversion electrons.

Time-integrated energy domain measurements with synchrotron radiation

A new time integrated method for the study of resonant nuclear scattering of synchrotron radiation in the forward direction or in Bragg directions is introduced. This method gives in principle similar information as the well known time differential method. A brief comparison of both methods is presented. The idea is to excite coherently the nuclei incorporated in two absorbers, one moving with respect to the other. The fields radiated by the nuclei from both absorbers interfere and each time the nuclear energy in one absorber matches, by Doppler modulation, the nuclear energy of the other, an extremum in the time integrated intensity is observed. The results of the first experiments at the Advanced Photon Source at the Argonne National Laboratory will be presented.

Novel experimental schemes for observing the Mössbauer effect in long‐lived nuclear levels

The development of gamma ray lasers (GRASER) will depend on the utilization of long‐lived, recoilless, nuclear, gamma transitions, i.e., the Mossbauer effect. The lifetimes required from practical considerations must be on the order of seconds at least. It is not clear that the Mossbauer effect has ever been observed in such longlived states. We propose some experimental techniques to unambiguously observe the effect of about 40 seconds. The techniques proposed are: coincidence Mossbauer spectroscopy; conversion electron Mossbauer spectroscopy; and gravitational line sweeping.

Resonant-Detector Mössbauer Spectroscopic Studies of Sn Doped SiO2 Analysed Using Quantum Mechanical Theory

It has already been established that, by using a resonant detector in Mossbauer spec-troscopy, the minimum spectral linewidth is 1.46Γ. Here Γ is the linewidth of the Mossbauer excited-state nuclear level. It is well known that the minimum linewidth obtained in conventional Mossbauer experiments is 2Γ. The quantum mechanical calculation using a nuclear-resonant detector, which predicts this result, is summarized. The fundamental equations describing the system are solved in the frequency domain and applied to the experimental results. The experimental results using the resonant-detector Mossbauer technique and an Sn-doped SiO2 sample are presented. The best fit to the data is obtained using the resonant-detector quantum mechanical theory.