High-resolution laser spectroscopy of diamagnetic helium in the chaotic regime

We measured a series of spectra of diamagnetic helium under fixed classical conditions. We varied the scaled energy from epsilon=-0.40 to -0.26 in ste ps of 0.02, which corresponds to a gradual increase of the number of chaoti c orbits in classical phase space. The classical dynamics of the system is studied using closed-orbit theory. The high resolution of the experiment al lows for an investigation of the evolution of the V-1(1) orbit and its repe ated traversals. Taking only this stable orbit into account, we reconstruct the most pronounced peaks in the energy spectrum recorded at epsilon=-0.36 . The accuracy of our experiment, confirmed by agreement with quantum heliu m calculations of action spectra up to scaled action 25, provides a good te sting ground for recently developed uniform semiclassical approximations in closed-orbit theory. [S1050-2947(99)07412-0].