SEMICLASSICAL MATRIX-ELEMENTS FROM PERIODIC-ORBITS

We discuss an extension of Gutzwiller's [J. Math. Phys. 8, 1979 (1967); 10, 1004 (1969); 11, 1791 (1970); 12, 343 (1971); Chaos in Classical and Quantum Mechanics (Springer, New York, 1990)] semiclassical theory for chaotic systems that allows a determination of matrix elements in terms of classical periodic orbits. Associated zeta functions are derived. The semiclassical predictions are found to be in good agreement with Fourier transforms of quantum spectra of hydrogen in a magnetic field. Expressions for off-diagonal matrix elements are derived that are extensions of the Bohr correspondence relations for integrable systems.