Carefully prescribed upper-limb exercise training can evoke a rapid symptomatic improvement in patients with claudication, while avoiding the physical discomfort experienced when performing lower-limb weight-bearing exercise.

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An innovative new approach has been developed for modeling the expansion of laser-generated plumes into low-pressure gases where initially the mean free path may be long enough for interpenetration of the plume and background. The model is based on a combination of multiple elastic scattering and hydrodynamic formulations. Although relatively simple in structure, it gives excellent fits to new experimental data for Si in He and Ar, and provides for the first time a detailed, coherent explanation of the observed splitting of the plume into a fast and slow component. [S0031-9007(97)03820-9] PACS numbers: 79.20.Ds, 52.50.JmPulsed laser deposition (PLD) has become an important technique for depositing a variety of materials [1,2], most notably thin films [3] and superlattices [4] of high-T c superconductors. As a consequence, a very active field of research into laser ablation phenomena underlying PLD has developed [5]. But this is not a new field since it dates back to the earliest days of the laser era when many materials were irradiated with high-powered laser pulses [6][7][8]. Thus, the work reported here has applications far beyond the PLD process itself. More recent work has provided a wealth of new diagnostics with which to study the laser ablation process [9]. It is of crucial importance to know the constitution and dynamical behavior of the plume of ablated material in order to understand how film growth can be optimized by varying the laser parameters, the targetsubstrate distance, and ambient gases introduced into the deposition chamber. In particular, the often observed but little understood phenomenon of "plume splitting" [10] into fast (ϳ vacuum speed) and background-slowed components is of great interest because the fast component may damage the growing film or otherwise affect its microstructure. Also, clustering of film constituents in the gas phase or on the surface may cause problems, but may also provide a technologically important method of producing nanostructures [5,11,12].In this paper, a new modeling approach, combining multiple scattering and hydrodynamical elements, is described and applied to recently obtained experimental data on Si ablated into He and Ar gases. The resulting model is remarkably successful in describing quantitatively the data and resolves long-standing uncertainties about the interpretation of many previous experimental observations.Silicon was selected for the target in the experiments because it is well characterized, is readily obtained as single crystals, and has been thoroughly studied in the laser-annealing regime [13]. Background gases of He and Ar were chosen because their ionization energies are high (25 and 16 eV, respectively), hence avoiding ionization, and because one is lighter and one heavier than Si. KrF laser pulses of 3.0 J͞cm 2 provided a good supply of singly ionized Si for the ion-probe detector while avoiding higher ionization states. Measurements revealed only neutral and singly ionized Si in the plume and only neutral atoms in the background. A de...

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