Unusual behavior in magnesium-copper cluster matter produced by helium droplet mediated deposition.

We demonstrate the ability to produce core-shell nanoclusters of materials that typically undergo intermetallic reactions using helium droplet mediated deposition. Composite structures of magnesium and copper were produced by sequential condensation of metal vapors inside the 0.4 K helium droplet baths and then gently deposited onto a substrate for analysis. Upon deposition, the individual clusters, with diameters ∼5 nm, form a cluster material which was subsequently characterized using scanning and transmission electron microscopies. Results of this analysis reveal the following about the deposited cluster material: it is in the un-alloyed chemical state, it maintains a stable core-shell 5 nm structure at sub-monolayer quantities, and it aggregates into unreacted structures of ∼75 nm during further deposition. Surprisingly, high angle annular dark field scanning transmission electron microscopy images revealed that the copper appears to displace the magnesium at the core of the composite cluster despite magnesium being the initially condensed species within the droplet. This phenomenon was studied further using preliminary density functional theory which revealed that copper atoms, when added sequentially to magnesium clusters, penetrate into the magnesium cores.

Rotational and vibrational dynamics of ethylene in helium nanodroplets.

Rotationally resolved infrared spectra are reported for the asymmetric C-H stretching fundamental bands of C(2)H(4) in helium nanodroplets, as well as two weak combination bands. The J=2 rotor levels are strongly shifted from the energies estimated from a rigid rotor calculation and can be accounted for with two centrifugal distortion constants. The excited states of the three bands with B(3u) symmetry are strongly coupled in the gas phase and exhibit lifetimes >100 ps in helium, with the upper member of the polyad exhibiting the shortest lifetime. In contrast, the nu(9) band (B(2u) symmetry) exhibits very broad, homogeneously broadened line profiles (full width at half maximum approximately 0.5 cm(-1)) corresponding to an excited state lifetime of approximately 10 ps. This short lifetime is presumed to be due to an efficient, solvent mediated vibration-to-vibration relaxation process. In addition, the absence of transitions to the 2(21) and 2(20) rotor levels in the nu(9) band suggests they form rotational resonances with the elementary modes of helium, resulting in very short excited state lifetimes of less than 2 ps.

Sharp spectral lines observed in gamma-ray ionized parahydrogen crystals.

We have observed sharp infrared spectral lines ( Deltanu approximately 60 MHz FWHM) in gamma-ray ionized para- H2 crystals. The lines are assigned to the Q1(0) transition of H2 near H+3 and H-, which becomes optically active and Stark shifted by the Coulomb fields of the ions. A simple model calculation gives a spectrum which agrees approximately with the observation. The spectrum stays the same over many days, demonstrating the stability of the ionized system. The remarkable sharpness and reproducibility of the line positions indicate uniformity of the local structure surrounding the charges.

Markets for medical care and medical education: an integrated long-run structural approach.

This paper investigates the relationship between the market for medical care and the market for medical education. Within a standard supply and demand framework, the capacity of medical schools is considered as an endogenous element within a complete structural system of the market for physicians' services. The parameters of the model are estimated using data from the postwar period. The estimated system is then simulated both to compare it to alternative time-series forecasting approaches and to examine the nature of earnings cycles in medicine. The results of this exercise support the applicability of traditional economics to this sector of the economy.