We report the first observation of the manifestation of exchange effects in recurrence spectra. Using fast-atom -laser-beam scaled-energy spectroscopy, we have measured the Rydberg-Stark recurrence spectrum of the two spin forms of helium in strong fields. Singlets and triplets are produced by excitation of metastable 1s2s 1,3 S states. We observe pronounced modulations in the triplet spectrum due to interference between hydrogenic and core-scattered combination orbits. The modulations are phase correlated with repeating hydrogenic orbits and not present in singlets or in hydrogen. [S0031-9007(98)06521-1] PACS numbers: 32.60. + i, 03.65.SqA complete theoretical treatment of nonhydrogenic atoms in external fields remains a challenge. Experimental progress has provided substantial information on Rydberg atoms in external fields [1]. Application of the semiclassical description, in the form of closed orbit theory, has been quite successful and is the basis for the field of recurrence spectroscopy [2]. Standard closed orbit theory treats nonhydrogenic effects with quantum defects. Recently, nonhydrogenic effects have been included through core-scattered waves [3] and by an explicit core potential resulting in classical core scattering [4]. These extensions have demonstrated that core scattering gives rise to combination orbits not seen in hydrogen. To date, experimental studies of Rydberg-Stark recurrence spectra have used alkali atoms which contain closed shell ionic cores and have one spin state [5,6]. The present work extends experiment to open shell ionic cores and their role in producing combination orbits. In addition, open shell cores provide polarized spin-state dependent scattering systems that permit experimental access to the influence of electron exchange.The exchange interaction is isolated experimentally by measuring recurrence spectra versus spin state. This provides a tool to examine the role of the Pauli exclusion principle in the semiclassical theory and in the dynamics of Rydberg systems, where it is not well understood. Closed orbits are sensitive to the spin-dependent exchange interaction since the Rydberg electron returns to the nucleus and the charge clouds of the electrons overlap. In helium, when the Rydberg electron samples the core, it moves as if it experiences an attraction or repulsion depending on the relative orientation of the two spins. This introduces a coupling between the spatial variables of the Rydberg electron and the spin state of the atom. How this is revealed in the Stark recurrence spectrum of the prototypical two-electron helium atom is a focus of the present Letter.Extensive data have been obtained for m 1 states at 60 different scaled energies in strong fields ͑20 , n , 30͒ up to scaled action 15. This allows a global view of the spectrum and observation of trajectory evolution as a function of scaled energy. Using this information, we introduce a new analysis technique of integrating recurrence strength of an orbit over its scaled energy range. This permits study of the...

We present part I in a two-part study of an open chaotic cavity shaped as a vase. The vase possesses an unstable periodic orbit in its neck. Trajectories passing through this orbit escape without return. For our analysis, we consider a family of trajectories launched from a point on the vase boundary. We imagine a vertical array of detectors past the unstable periodic orbit and, for each escaping trajectory, record the propagation time and the vertical detector position. We find that the escape time exhibits a complicated recursive structure. This recursive structure is explored in part I of our study. We present an approximation to the Helmholtz equation for waves escaping the vase. By choosing a set of detector points, we interpolate trajectories connecting the source to the different detector points. We use these interpolated classical trajectories to construct the solution to the wave equation at a detector point. Finally, we construct a plot of the detector position versus the escape time and compare this graph to the results of an experiment using classical ultrasound waves. We find that generally the classical trajectories organize the escaping ultrasound waves.

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We have adapted inexpensive ultrasonic rangefinders to measure ablation rates on the surface of a glacier. While ultrasonic rangers are commercially available for this purpose, our goal was to utilize rangefinders typically used in hobby robotics without significantly compromising performance. To correct for environmental factors that affect the speed of sound we use two ultrasonic rangefinders, one focused on a fixed target. Measurements of ablation correlate well with manual measurements with an uncertainty of about ±3 cm, suggesting an accuracy comparable with other non-manual methods of recording ablation. The limitations of our rangefinder include those inherent in commercially available units as well as having less acoustical power, which results in a reduced effective range of the sensor (~2 m) and difficulties in detecting surfaces lying below low-density snow. Our sensor design provides a cost-effective means of increasing the spatial coverage of ice ablation measurements.

We describe new techniques involving trapped atoms as targets for measurements of electron-impact ionization cross sections out of an excited level. By directly detecting the ions formed by electron collisions, we have obtained the first experimental ionization cross sections out of the 5(2)P excited level in rubidium at energies from 40 to 300 eV.