Cross sections for projectile AT-shell ionization were measured for 0.75-3.5-MeV/nucleon C 5+ and O 74 " projectiles in collisions with H2 and He targets. The experimental results agree with plane-wave Born-approximation calculations which take into account the interaction between projectile and target electrons. We demonstrate that for energies where the target electrons have sufficient kinetic energy in the projectile frame to ionize the projectile electron, the electron-electron interaction can lead to a significant increase in the total ionization cross section.

Several structural and dynamic properties of unstretched and stretched ultrahigh molecular weight polyethylene (UHMWPE) films, as well as UHMWPE films doped with pyrene (Py/UHMWPE) and either irradiated with UV−vis (eV range) photons or bombarded with 4.5 MeV protons, have been investigated. X-ray diffraction patterns of the three-dimensional orthorhombic crystal structure of a uniaxially stretched UHMWPE film, prepared by the gel technique, have been completely indexed for the first time. A minor monoclinic crystalline component, whose importance depends on the draw ratio, is also present. Linear dichroism spectra of dopant pyrene molecules indicate that their long molecular axes are oriented to a smaller extent in highly stretched UHMWPE films than in polyethylene films of much lower crystallinity and draw ratios. When UHMWPE films were stretched, the heat of melting was unchanged, but the melting temperature increased. Stretched films bombarded with 4.5 MeV protons exhibited lower heats of melting and melting temperatures than unstretched bombarded films. When Py/UHMWPE films were bombarded with 4.5 MeV protons or irradiated with >300 nm photons under oxygen-free conditions, 1-pyrenyl groups were attached covalently to polymer chains, yielding Py-UHMWPE. Diffusion of N,N-dimethylaniline (DMA) in to and out of Py-UHMWPE films was followed in real time by fluorescence intensity changes. In unstretched films, at least two parallel diffusion pathways appear to operate. The activation energies for diffusion of DMA in unstretched Py-UHMWPE are larger than those in stretched Py-UHMWPE or in other types of polyethylene films. The results are discussed in terms of the structural differences between unstretched and stretched UHMWPE and other polyethylene types.

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We demonstrate the attachment of pyrene molecules to chains of polyethylene (PE) films using 1-4.5 MeVrange protons (H + ). Despite the enormous energy available for reaction, the attachment is exceedingly selective; MeV-range protons lead to photochemistry as selective as that initiated by eV-range photons (>300 nm). The effects of proton kinetic energy, proton dose, polymer crystallinity, and initial pyrene concentration within the films on the selectivity of the attachment process are analyzed on the basis of UV/vis absorption spectroscopy, steady state and dynamic fluorescence measurements, and differential scanning calorimetry. Attachment selectivity is independent of proton kinetic energy but increases with decreasing proton dose. The efficiency of pyrenyl attachment increases with decreasing polymer crystallinity. In polyethylene of 42 % crystallinity, the efficiency of attachment is only slightly dependent on pyrene dopant concentration (in the range 10 -4 -10 -2 mol/kg) at low doses but increases significantly with concentration at higher doses. Time-resolved fluorescence studies on films subjected to high doses indicate either a drastic change in local environments around attached 1-pyrenyl groups or that the attached groups have suffered secondary transformations. Extensive cross-linking of proton-bombarded PE samples, indicative of C-H and C-C bond scission, was observed in both the absence and presence of pyrene. At all proton energies, films containing 10 -2 mol/kg pyrene and exposed to 10 13 H + /cm 2 , the highest fluence employed, exhibited a pronounced, broad emission band centered at ∼470 nm, attributed to excimer. Dissolution of these samples led to loss of the excimer band, indicating that the aggregated pyrenyl groups are attached to neighboring polyethylene chains. A mechanism for attachment, based on the addition of preformed carbon-centered polymer radicals or cations to ground-state pyrene molecules, is proposed. It is contrasted with the mechanism for attachment initiated by eV-range photons.

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