Effects of Configuration Interaction on Intensities and Phase Shifts.

In " THE PHYSICAL REVIEW ", Second Series, Volume 124 Number 6, December 15, 1961, pp. 1866-1878, the complete issue in original printed wrappers. Ownweship inscription on front cover, spine cracked, a good copy. First edition of one of the most cited articles in the Physical Review.
" One of the enduring goals of scientific work at the National Institute of Standards and Technology (NIST) has been the expression of measurements in terms related directly to natural phenomena of an invariant and absolute character. For example, the unit of time, the second, is now defined as exactly 9,192, 631, 770 periods of oscillation of the radiation associated with a specified quantum transition between states of the 133 Cs atom. This makes it possible, in principle, for any laboratory to realize the value of the second by preparing a sample of 133 Cs in conditions that make it resemble a group of identical atoms unperturbed by their immediate environment. Of course, such an ideal realization is not attainable in practice. This permanent fact of life has provided steady stream of work over the years for theoretical physicists at NIST: there is always a need for models that can provide better quantitative links between realistic and ideal situations. Ugo Fano (1912–) is believed to be the first theoret-ical physicist hired by NIST, and he has certainly been one of the most influential to date. His 1961 paper Effects of Configuration Interaction on Intensities and Phase Shifts is one of the most frequently referenced journal articles by a NIST author, having been cited over 3200 times in the scientific literature. This paper treats a subject of fundamental interest to metrology and physics: the excitation spectra of quantum-mechanical systems. Its key result, the simple formula given in Eq. (3) below, is now well known to physicists as the "Fano profile" or "Fano line shape." It addresses the challenge of expressing observed phenomena in a concise manner that can be derived from first principles. The celebrity enjoyed by this formula derives from the basic importance of the systems it describes, its wide-ranging practical utility, and the historical context in which it emerged. Ugo Fano, who did his graduate work in Italy under Fermi in the early 1930s, was hired by NBS in 1946 with a charge to build up the fundamental science base of the x-ray program. In his 19 years at NBS he provided guidance and inspiration to many of the Bureau’s physicists and chemists. After moving to the University of Chicago in 1965, he led his graduate students in the detailed analysis of noble-gas photoabsorption spectra. The analysis of these spectra was a noteworthy achievement of multichannel quantum defect theory, developed by Fano and coworkers along lines laid out by Michael Seaton. This theory had a pronounced influence on high-resolution laser spectroscopy in the 1970s and 1980s; its development is summarized in two articles in the February 1983 issue of Reports on Progress in Physics (U. Fano, Correlations of two excited electrons, Rep. Prog. Phy. 46, 97-165 (1983); M. J. Seaton, Quantum defect theory, Rep. Prog. Phy. 46, 167-257 (1983). At NIST, the Fano profile formula evokes memories
of a remarkably productive era of atomic and electron physics, one in which there was strong interplay between theory and experiment, as well as between electronic and optical spectroscopy. Many legacies of this era are visible in NIST programs today. For example, the Electron Physics Section spawned the topografiner project, work on resonance tunneling in field emission, and the development of spin-polarized electron sources and detectors—all of which are described elsewhere in this volume. The success of the SURF synchrotron source inspired to the worldwide development of synchrotron radiation as a research tool. SURF has since gone through two major upgrades and today serves as the nation’s primary standard for source-based radiometry over a wide region of the optical spectrum. Fano’s theory of spectral line shapes continues to be applied to a wide range of physical problems: his 1961 paper was
cited over 150 times in the scientific literature in 1999. Charles W. Clark in book: A Century of Excellence in Measurements, Standards, and Technology, Chapter: Effects of Configuration Interaction on Intensities and Phase Shifts, Publisher: National Institute of Standards and Technology, Editors: D. R. Lide, pp.116-119.
In 1977 Ugo Fano wrote “The paper appears to owe its success to accidental circumstances, such as the timing of its publication and some successful features of its formulation. The timing coincided with a rapid expansion of atomic and condensed matter spectroscopy, both optical and collisional. The formulation drew attention to the generality of the ingredients of the phenomena under consideration. In fact, however, the paper was a rehash of work done 25 years earlier and its context still needs extension and clarification. “It is well known that an atomic system can absorb only discrete amounts of energy as long as these amounts do not suffice to break it up; one observes then a line spectrum. A continuous spectrum is observed, instead, when the energy absorption can achieve ionization or dissociation of the system. Conspicuous discrete structures do nevertheless occur in continuous spectra when the absorbed energy runs initially into blind alleys thus allowing only a delayed break-up; the deeper the blind alley, the sharper is its influence. These spectral structures thus furnish evidence on energy migration within the absorbing system.
“In January, 1935, Emilio Segré gave me some spectroscopy papers by H.A. Beutler as a fruitful subject of study. The Beutler spectra showed unusual intensity profiles which struck me as reflecting interferences between alternative mechanisms of excitation. Fermi then taught me sequentially within a few days how to formulate my interpretation theoretically; a paper was sent to the Nuovo Cimento quickly and I dropped the matter. I
remained, however, sensitized to evidences of analogous phenomena, noticing, e.g., how they emerged through the influence of surface waves on the spectra of diffraction gratings.
“Late in 1960, R.L. Platzman called to my attention a strikingly asymmetric line profile in an unpublished spectrum of energy transfers in electron collisions by Lassettre and coworkers. This spectrum appeared analogous to those I had interpreted in 1935. My reply to Platzman provided the opportunity for a modernized formulation of the analytical treatment. He urged me to publish the new derivation; this I did, complementing it with illustrations and with fragmentary extensions and interpretations, with unexpected success. The amount of effort spent on this paper was, however, far larger than for its distant predecessor.
“By 1965 discrete structures had proved ubiquitous in the vacuum ultraviolet spectra ofmost materials. The theoretical framework I had utilized appeared, however, too restrictive. Only for isolated, sharp spectral features can we extract from spectral data well-defined quantitative information on the mechanisms that produce them. That is, we do well only when excitation energy can flow and ebb from a single blind channel before being dissipated away. Extensive efforts have been devoted to extending this analysis, over the years, by many physicists besides myself, but their
results have remained fragmentary.” ( http://garfield.library.upenn.edu/classics1977/A1977DM04900001.pdf ).