RECURRENCES ASSOCIATED WITH A CLASSICAL ORBIT IN THE NODE OF A QUANTUM WAVE-FUNCTION

Absorption spectra of atoms in magnetic fields reveal recurrences: man ifestations of classical orbits (or quantum wave packets) that go out from the atom and later return. A formula from closed-orbit theory ass erts that if the orbit Lies on a node of the outgoing wave function, t hen the strength of the recurrence is zero. New quantum calculations, however, show that the recurrence strength is nonzero, though small. W e derive a semiclassical formula for the recurrence strength associate d with a classical orbit at a node of the quantum wave function. This formula is compared to the quantum mechanical calculation. Compared to other orbits, the recurrence is about 100 times weaker, and obeys a d ifferent classical scaling law.