Taken from Mossbauer Spectroscopy at Knox College:  http://nthome.knox.edu/student/ferazo/physics/moss

 

 

Mössbauer Spectroscopy of Iron Porphyrins

 

 

 

Knox College

Hughes Fellowship Summer Research

 

 

 

 

Fernando J. Erazo

Prof. Charles Schulz, mentor

 

 

 

August 22, 1999

 

 

Introduction:  

 

Mössbauer Spectroscopy is the measurement of recoilless resonant gamma ray absorption by certain nuclei.  It was Rudolph Mössbauer who first discovered the existence of recoilless nuclear resonance fluorescence in 1957 (Greenwood, pp. 1).  After him, the nuclear emission or absorption of a photon without energy lost to recoil is known as the Mössbauer effect.  The measurements provided by this spectroscopy method are extremely precise and have a great resolution, allowing scientist to detect otherwise unobservable interactions between the nucleus and orbital electrons – called hyperfine interactions. 

 

Although in theory the Mössbauer effect is present in all excited-state to ground-state nuclear transitions, it has only been detected in a total of 88 gamma ray transitions in 72 isotopes of 42 different elements (Greenwood, pp. 1).  This is partly because of limitations in current instrumentation but mostly because in the majority of transitions the Mössbauer line width is too small to be measured. The most commonly studied isotope in Mössbauer spectroscopy is 57Fe, specifically, this isotope’s first excited state decay.

 

It is not a coincidence that 57Fe has been so largely documented and studied by using the Mössbauer method.  In Mössbauer spectroscopy, 57Fe has a series of physical characteristics that qualify it as a very approachable isotope.  Also, an important factor qualifying iron for such extensive Mössbauer analysis is this element’s presence in very important biological molecules.  A perfect example is hemoglobin.

 

In our summer study of Mössbauer spectroscopy, we found the spectra of various tetraethylporphyrins and tetrapropylporphyrins.  Porphyrins are planar atomic structures with an iron atom in the center; in this case they are compounded with very long chains of amino acids to form large protein molecules – such as hemoglobin.  A computer analysis of the obtained spectra returns precise information about the hyperfine interactions around the iron nucleus in these proteins.  The result is a better understanding of the environment surrounding the iron nucleus in such proteins.  Thanks to this information scientists can learn more about the types of chemical reactions implicated in fundamental biological processes.

 

The Mössbauer effect:

 

It is now widely known that atoms and nuclei have excited states that consist of different electron, proton, and, neutron configurations.  These excited states are sometimes accessible from the ground or low-lying states through the absorption of gamma rays (photons).   The excited states in nuclei, however, have very narrow energy widths.  Therefore, in order to observe a good resonant absorption of gamma rays by nuclei we need a highly precise and sharply defined incident gamma ray energy.  For example, a 57Fe nucleus on its first excited state can decay by emitting a photon with an energy of 14.4KeV.  Since the energy levels in the nucleus have a very narrow width (long lifetime), only a 14.4KeV photon can be absorbed by another such nucleus.  A photon with an energy of 14KeV is completely useless to the 57Fe nucleus (Wiarda).  

 

            The next obvious question is how to obtain gamma rays of the precise resonant energy needed to excite the desired nuclei.  Obviously, a perfect source is an identical nucleus that is currently in the excited state; when it decays it will emit a photon of precisely the needed energy.  The gamma ray source - excited nuclei - can be obtained from decay products of radioactive isotopes.  In our study of iron Porphyrins we use radioactive 57Co isotopes because when they decay into 57Fe they sometimes emit 14.4KeV photons.

 

            The challenge does not stop there.  From a mechanical perspective, if an excited 57Fe nucleus decays to its ground state it won’t necessarily emit a photon of 14.4KeV.  The energy of that photon is not really 14.4KeV, but lower.  Since the nucleus recoils while emitting the particle, this recoil takes up some of the energy of the emitted photon.  The energy lost to recoil, though relatively very small, is sufficient so that at normal temperatures the photon cannot be absorbed by another 57Fe nucleus.

 

            In 1958, Mössbauer discovered that a photon could indeed be emitted with the full transition energy.  If encrusted in a lattice – a lattice is the quantum mechanical perspective of a solid – a nucleus can emit its photon with negligible loss of energy due to recoil.  In the lattice the nucleus is bound by elastic forces and the effective recoil mass is the entire solid.  Similarly, in the absorbing nucleus – also embedded in a solid – we can have a fraction of recoilless absorptions.  “Mössbauer’s important contribution was the discovery that a nucleus which is embedded in a solid can sometimes emit its gamma ray with negligible energy loss to recoil” (Lang 4).

           

            The Mössbauer Spectrum:

 

            One of Mössbauer Spectroscopy’s main features is that it has made it possible to resolve the elusive hyperfine interactions in the sample nuclei.  This is allowed by the sharpness of the Mössbauer line, and, the ability to determine the energies of the emitted gamma rays from the source with respect to the absorber with great accuracy.  The hyperfine interactions represent associations and relations among a nuclear property and appropriate electronic or atomic configurations.  These hyperfine relations are of great importance because they provide information regarding electron- and spin-density distributions (Gonser 21). 

 

            The advantage of the Mössbauer method to detect the hyperfine interactions arises from the very narrow line width of the nuclear transitions involved in this effect.  The absorption of resonant gamma rays is extremely sensitive to the smallest variations of the radiation energy.  Therefore, the otherwise undetected interactions between the nucleus and the orbiting electrons manifest themselves very markedly when analyzed under Mössbauer Spectroscopy (Danon 66). 

           

            There are three main hyperfine interactions:

·        The nuclear isomer shift – electric monopole interaction.

·         The nuclear quadrupole splitting – electric quadrupole interaction.

·         The nuclear Zeeman effect – magnetic dipole interaction.

 

First, the isomer shift occurs due to the electrostatic interaction between the electron charge density at the nucleus and the charge distribution of the nucleus itself over its radius.  The chemical environment that surrounds both the emitting and the absorbing elements can change the electron density at their nuclei.  When that occurs, electrostatic interaction also changes the levels of the nuclear ground and excited states – therefore altering their transition energies.  In our Mössbauer resonant measurements, the difference between these transition energies is called the isomer shift.  The isomer shift is identified in the spectrum as the shift from zero-velocity of the center of the Mössbauer line.

 

The quadrupole splitting arises from the interaction between the nuclear electric quadrupole moment and the electric field gradient (EFG) at the nucleus.   The determinacy of such an electric quadrupole interaction by Mössbauer Spectroscopy is one its most useful features.  The quadrupole splitting measurements help scientist determine deformities in the nuclear charge distribution.  The sign of the quadrupole moment depends solely on the charge distribution deformation: a negative quadrupole moment indicates a nucleus that is flat along the spin axis, while a positive quadrupole moment indicates a nucleus is elongated (Greenwood et al, 54).  The quadrupole interactions resolved by using Mössbauer spectroscopy are of fundamental importance in chemistry and solid state physics since they provide a means to derive the electric field gradient at the nucleus.  In the case of 57Fe, there is a considerable uncertainty about the absolute value of the quadrupole moment of its first excited state.  Therefore, the inaccuracy of its derived EFG is fairly high.  A perfect example of a spectrum that represents a quadropole splitting follows this page. 

 

The third hyperfine interaction is the Zeeman effect.  Also called the magnetic dipole interaction, this property is present only when the nucleus is under a magnetic field.  The applied magnetic field can vary in strength and can be originated from within the atom itself or any other external source.  When the field originates from the atom’s own orbiting electrons, it is called internal field.  The result of the Zeeman effect is the splitting of the nuclear state with spin I  (I >0) into (2 I + 1)  (Gonser, 24).  After this page, a spectrum of a sample with a large magnetic interaction has been included.

 

Besides the three main interactions briefly described above, Mössbauer spectroscopy can also determine perturbations or combined effects.  Sometimes, the splitting of a nuclear state can involve nuclear magnetic dipole and electric quadrupole interactions simultaneously.  In this case, the interpretation of the obtained spectrum becomes much harder.  Relaxation effects can also be investigated when the hyperfine interactions present involved time dependant features.  Lastly, relativistic effects may also be observed, these arise from the high frequency vibrations of the atoms in a solid, the average square velocity of these oscillations causes a shift of the resonance line – also called the second-order Doppler effect (Gonser, 32).

 

The apparatus:

 

The most basic Mössbauer spectrometer is in itself a very simple experimental setup: all we need is a source emitting a good fraction of recoil-free gamma rays, an absorber, and a proportional counter aligned together in the obvious way.  However, in order to observe the nuclear resonant absorption of gammas, we must instead look for conditions where this absorption does not occur.  Thanks to the very narrow resonance line width, it is sufficient to Doppler shift the source or the absorber to sweep over the resonance.  The velocity ranges of the Doppler shift applied depend on the width of the resonance.  In our study of iron Porphyrins, the velocity used was 6 mm/sec.  It most be made clear that the purpose of the Doppler shift applied is solely to sweep over the resonance and not at all to compensate for the fraction of the gamma rays with energy lost to recoil (Danon, 44).  For our summer research, we chose to Doppler shift the incoming gamma rays by using a velocity transducer on the 57Co source.  Also called a velocity sweep device, the transducer consists of a multi-channel analyzer that allows the inspection of the entire velocity spectrum simultaneously. 

 

Besides the Doppler shift velocity, there are more variables a scientist can control in a Mössbauer experiment.  For example: the magnetic field applied to the absorber, and, its temperature.  Towards that end, there are commercially available cryostats with various temperatures ranges designed specifically to work in a Mössbauer setup.  Similarly, superconductive magnets that apply very high fields to the sample in a Mössbauer experiment are also available.  All the samples that we analyzed this summer were studied in a spectrometer capable of delivering a magnetic field of up to 90.000 gauss and a constant temperature as low as 4.2 Kelvin.

 

The largest part of our Mössbauer project this summer was the construction of a spectrometer that utilizes a closed cycle cryostat.  The advantages of this type of spectrometer are primary financial: all it requires to run is a cylinder of helium gas and a 250V electrical outlet!  That contrasts sharply with the needs of our current spectrometer, which uses up to 250 liters of liquid helium and about 100 liters of nitrogen to run at 4.2K and attain superconductivity.  On this aspect, the close cycle cryostat spectrometer is much more versatile and easy to setup.  Another advantage of the closed cycle cryostat is that it has a variable range of temperatures at which the Mössbauer spectrum can be taken: from about 20 Kelvin up to room temperature.  This versatility, of course, comes at a price: the closed cycle cryostat’s lower temperature limit falls short of the 4.2 Kelvin that can be achieved on our current cryostat.  Also, the magnetic field applied on the closed cycle cryostat’s sample is barely 400-600 gauss.  We attained this field by using 16 individual Neodymium magnets.  Thus, the field is variable – just reduce/add more magnets – but it has to be measured with a probe, therefore introducing some uncertainty.  The superconductive magnet used on our current spectrometer, on the other hand, has a variable strength field of up to 90000 gauss regulated from a control board.

 

Special considerations had to made while designing the closed cycle cryostat stand to be used on the new spectrometer.  Vibration is a primordial issue in Mössbauer spectroscopy that must be avoided at all cost.  To that end, the frame of the stand was made of hardened-steel framing and it was reinforced with aluminum plates.  To insulate the system from outside vibrations, a set of four air-cushion vibration isolators were used.  It is important to realize how critical vibration isolation is in a Mössbauer experiment.  In our research we only took spectra for 57Fe nuclides.  Because of the 57Fe very narrow line width and relatively high gamma ray energy, vibration can be a serious problem.  It is no exaggeration that the following rule of thumb is applied to a 57Fe spectrometer: “If one touches the equipment lightly with the fingertips and notices any traces of vibration, the experiment will fail (Frauenfelder, 40)!”  The vibration problem is even more marked for nuclides that have a narrower line width, for example 67Zn.  This isotope has a resonance width of about 10-10 eV.  Such a narrow width corresponds to a Doppler velocity of about 10-5 cm/sec.  “That velocity is so small that is barely faster than the speed at which human fingernails grow!”  If vibration exists on a spectrometer used for 67Zn, “an amplitude of a few times 10-8 cm can be sufficient to destroy the resonance (Frauenfelder, 33)!”  A sketch of the Mössbauer Spectrometer we built follows this page.

 

Data analysis:

 

A computer program analyzed all the spectra we obtained for the metalloproteins we studied.  Expect for the simplest of the Mössbauer spectra where no more than one interaction appears at a time, the aid of a fitting program is required to make sense of a spectrum and learn the adequate parameters.  One of the projects that we also undertook this summer was the translation of parts of this program from FORTRAN into C++.  The versatility and modularity of C++ allows us to build a larger program that can use all the modules and fitting programs into a single user-friendlier suite with a helpful graphical user interface.  Specifically, the new segment we added to the Mössbauer Spectra Analyzer program was a Hamiltonian fit module.  Previously, the FORTRAN version of this program used an Omega tensor method to approximate model fit parameters of the analyzed spectra.  Thanks to the module we added to the analyzer we obtained better fits for the spectra of the Porphyrins we studied, included after this page are two examples of such fits for the spectra of Fe (III) Tetraethyl Porphyrin Chloride and Fe (III) Tetrapropyl Porphyrin Chloride.

 

Conclusion:

 

Mössbauer Spectroscopy by itself is a huge field that offers an infinite number of possible applications.  The ten weeks spent this summer have served me very well as an advanced and practical introduction to this singular scientific method.  After being introduced into this field I have decided to continue the exploration of Mössbauer Spectroscopy by taking an independent studies this term on that topic.  This time I will concentrate my efforts on a particular subject within the Mössbauer ground.
References:

1.       Danon, J. “Lectures on the Mössbauer Effect.” Gordon and Breach Science Publishers, New York 1968.

 

2.       Frauenfelder, Hans. “The Mössbauer Effect.” Frontiers in Physics, A Lecture Note and Reprint Series, W.A. Benjamin, Inc. Publishers, New York 1962.

 

3.       Gonser, U. Ed. “Mössbauer Spectroscopy.”  Topics in Applied Physics. From a strange effect to Mössbauer Spectroscopy, Gonser.  Springer-Verlag Publishing, New York 1975.

 

4.       Greenwood, N. N. and Gibb T. C. “Mössbauer Spectroscopy.” Chapman and Hall Ltd. Publishing, London 1971.

 

5.       Wiarda, Dorothea. “Taking a Mössbauer Measurement.” Aug. 1999 http://www.public.usit.net/wiarda/scientific/Moessbauer.html