  A method is presented for determining the initial conditions of classical
orbits from the quantum spectra of the diamagnetic hydrogen atom. Each
classical trajectory which is closed at the nucleus produces a sinusoidal
fluctuation in the photoabsorption spectrum. The amplitude of each orbit's
contribution appears in the Fourier transform of a spectrum computed at
constant scaled energy. For a given initial state, closed-orbit theory gives
the dependence of this recurrence amplitude on the initial angle of an orbit.
By comparing the recurrence amplitudes for different initial states, the
initial conditions of closed classical orbits are determined from quantum
spectra.
