Mössbauer and Electron Spectroscopy 1968-1976
This program was an excellent example of the features that made a National Laboratory program very close to unique: a chemistry program studying nuclear processes. It also was a demonstration of the changes brought about by the developing missions of the AEC, the replacement of the AEC by ERDA and then by DOE, and the natural transformations in post-World War II science over three decades: alteration of the program's focus from nuclear phenomena to chemical processes, and finally leading directly to the founding of a national facility, the National Synchrotron Light Source.
Science at Brookhaven had its roots in matters related to nuclear phenomena, and so did the AEC. Nuclear and isotopic chemistry composed a large fraction of the work conducted in the early years of the Chemistry Department. Nuclear processes are often considered independent of chemical phenomena. However, under special circumstances, such effects are easily observable: isotope effects on chemical equilibrium and chemical kinetics formed (and still do) a large and vigorous branch of chemistry. For example, refer here and here. Less well recognized, however, are the interactions of nuclei with the electron clouds surrounding them, (and, indirectly, with neighboring atoms). This program exploited these effects to study chemical processes. It is useful to review some of these effects here.
b-ray spectroscopy: Some radioactive nuclei decay by b-decay processes: they can emit b- particles (electrons), b+ particles, (positrons, anti-electrons), or they can absorb inner-shell electrons (electron capture). The study of the energy distribution of emitted electrons was called "b-ray spectrometry", and was used by nuclear chemists to determine details about the transitions between the nuclear energy levels giving rise to the observed electrons. Incidentally, even though the nuclear transitions that give rise to b-ray spectra are quantized and represent transitions between discrete, specific energy levels, b-ray spectra are continuous. They act as if the nuclear transitions were not quantized. This fact (and other worries about the lack of conservation of angular momentum) led to decades of consternation, and prompted the notion of a neutrino, a neutral particle to share the energy generated by the nuclear transition, and make the electron energy distribution appear continuous.
Electron spectroscopies: The methods used by nuclear chemists for b-ray spectroscopy were adapted to other forms of electron spectroscopy. When radioactive nuclei decay by electron capture, the captured electron's absence from the atom's inner electronic structure produces a hole in the atom's electronic structure. Trying to fill the hole, more weakly bound outer shell electrons can rattle down through an atom's energy level structure, losing excess energy by emitting an x-ray photon in the process. The x-ray can be observed directly, or utilized in an internal photoelectric process, causing ejection of an Auger Electron. The study of the energy distribution of these electrons, and other similar electrons produced directly by vacuum ultraviolet and x-ray photon absorption led to the founding of the fields of electron spectroscopy, and photoemission spectroscopy, now widely used to discover details about the bonding of atoms into molecules and the interactions among molecules in condensed phase.
We usually consider the decay of radioactive nuclei to be independent of all circumstances outside the nucleus. For instance, the half-lives of most radioactive nuclei are insensitive to the chemical environment in which the atom with a radioactive nucleus finds itself. For instance, the observed half-life of tritium, 3H is the same whether the tritium atom, (T, 3H) is bound into a gaseous diatomic tritium molecule 3H2, (T2), or whether the tritium atom is bound into a heavy water molecule, T2O and frozen as heavy ice. However, it must be noted that where electronic wavefunctions do not have a node at the nucleus, an electron bound into an atom has a nonzero probability to be inside the nucleus, and nuclear and electronic degrees of freedom can interact. Certainly, a molecule's electronic structure is influenced by its chemical environment, and, under these special conditions, so can its nuclear behavior.
The volume of an excited nucleus is different from that of a ground-state nucleus. Therefore, the probability that orbital electrons will be found inside the nucleus is different for the two nuclear states. This manifests itself as a change in the total binding energy for an atom's electrons, and, in turn as a change in the energy of the nuclear transition in different chemical compounds: the Isomer, or Chemical Shift. This effect was discovered by R.L. Mössbauer, and is called the "Mössbauer Effect". 119mSn was prepared in samples of metallic white tin and as SnO2. High-resolution electron line measurements (from internal conversion of a 23.87 keV g transition) were made. Using the value of the chemical shift observed for this transition in Sn and SnO2, , along with s-electron density measurements and calculations, it was shown that the fractional change (DR/R) in the nuclear charge radius for the 23.87 keV state of Sn is 1.8 x 10-4 larger than in the ground state.1
1. "Chemical Effect on Outer-Shell Internal Conversion in 119Sn; Interpretation of the Mössbauer Isomer Shift in Tin" J.P. Bocquet, Y.Y. Chu, G.T. Emery and M.L. Perlman, Phys. Rev. Lett. 17 809 (1966), and "Interpretation of the Mössbauer Isomer Shift in 119Sn" G.T. Emery and M.L. Perlman, Phys. Rev. B 1 3885 (1970).
The energy and energy spread (the linewidth) of electrons ejected from atoms in molecules excited by x-ray absorption reveals details about the molecules' electronic structure. Measurements of the electron linewidths where holes were generated in core levels just below the valence levels in solid Na2S2O3 and NH4NO3 showed that for S and N, linewidths depended on the particular chemical environment, so that electron linewidths could be used as a sensitive probe of chemical bonding.2
2. "Chemical Effects on Line Widths Observed in Photoelectron Spectroscopy" R.M. Friedman, J. Hudis and M.L. Perlman, Phys. Rev. Lett. 29 692 (1972).
The National Synchrotron Light Source
The reader has noticed by now that absorption of vacuum ultraviolet or x-ray photons is a ubiquitous tool in electron spectroscopy. Intense beams of such relatively high-energy photons are difficult to produce by conventional means. In the mid-1970s, R.E. Watson (BNL Physics) and M.L. Perlman (BNL Chemistry) proposed the establishment of a synchrotron for the purpose of generating such photons for a widespread user community. They held a series of workshops that led to the founding of a project that became the National Synchrotron Light Source, which has since become the synchrotron source with the largest user community in the world.3,4
3. "Synchrotron Radiation: The Light Fantastic" M.L. Perlman, E.M. Rowe and R.E. Watson, Phys. Today 27 30 (1974).
4. "Seeing with a New Light: Synchrotron Radiation" R.E. Watson and M.L. Perlman, Science 199 1295 (1978).
Last Modified: June 28, 2012