ABSTRACT

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. Ugo Fano is believed to be the first theoretical 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. Exponential decay is encountered in a number of physical systems, such as the natural transmutation of radioactive elements. An alternative mode of observation of exponential decay is the resolution of its frequency spectrum.