Approaching the classical limit in the diamagnetic helium atom

We studied the diamagnetic helium atom under conditions of constant scaled energy over an extended range of Rydberg levels (n = 77-160). Experimental spectra are compared with extensive quantum calculations. In the studied ra nge the effective Planck constant h(e) varies a factor of 2 (from 0.01 to 0 .005). When comparing Fourier-transformed spectra in different h(e) regimes , the resulting action spectra show a large similarity, in accordance with the invariance of the classical dynamics. Prominent variations in the recur rence strength of peaks in the action spectra with h(e) could be attributed to interferences between orbits with nearly the same scaled action. In par ticular, the predicted difference in h(e) scaling of the orbit parallel to the magnetic field when compared with the off-axis orbits is confirmed expe rimentally. [S1050-2947(99)00509-0].