The chemical bonding of hard and elastic amorphous carbon nitride (a-CN x) thin films was examined using solid-state 13 C NMR spectroscopy. The films were deposited by DC magnetron sputtering in a pure nitrogen discharge on Si͑001͒ substrates at 300°C. Nanoindentation tests reveal a recovery of 80%, a hardness of 5 GPa, and an elastic modulus of 47 GPa. This combination of low modulus and high strength means the material can be regarded as hard and elastic; the material gives when pressed on and recovers its shape when the load is released. The 13 C NMR results conclusively demonstrate that hard and elastic a-CN x has an sp 2 carbon bonded structure and that sp 3 hybridized carbons are absent. Our results stand in contrast with earlier work that proposed that the interesting mechanical properties of hard and elastic a-CN x were due, in part, to sp 3 bonded carbon.
The phenomenon of resonant photoemission happens when, in addition to a direct photoemission channel, a second indirect channel opens up as the absorption threshold of a core level is crossed. A massive increase in emission cross section can occur, but the nature of the process remains clouded. Using novel magnetic linear dichroism in photoelectron spectroscopy experiments and theoretical calculations, we can now clearly demonstrate that temporal matching of the processes as well as energy matching is a requirement for true "resonant photoemission." [S0031-9007(98)06819-7] PACS numbers: 75.70. -i, 75.50. -y, 79.60. -i The photoemission of 4f and 5p electrons from rareearth metals and their compounds is strongly enhanced when the photon has just enough energy to excite a 4d electron to an unoccupied 4f level, leading to a process called "resonant photoemission." (See Fig. 1.) In a generic picture, the indirect channel of the resonant photoemission is interpreted as due to a process where a 4d electron in the initial state is first excited to the unoccupied 4f level, forming a tightly coupled, bound intermediate state, 4d core hole plus 4f electrons. Then a decay via autoionization occurs, producing a final state identical to that obtained by a direct photoemission process for the ejected electron [1]. The transition rate is greatly enhanced if the excited state decay is by a (super)-Coster-Kronig [(s)CK] process [2,3]. The key question is whether these processes are coherent or incoherent: Is it truly resonant photoemission or merely the incoherent addition of a second emission channel? Should the overall intensity be treated as a squaring of the sum of the amplitudes (coherent) or summing of the squares of the amplitudes (incoherent)? A true resonant photoemission process should be coherent, involving interference terms between the direct photoemission and indirect photoemission channels. Possibly, incoherence would give rise to the loss of photoemission characteristics in the process, with a domination of Auger-like properties.To this problem we have applied the new photoelectron spectroscopy technique of magnetic linear dichroism in angular distributions (MLDAD) [4][5][6][7]. This technique is related to but distinct from the techniques of magnetic x ray circular dichroism (MXCD) in photoelectron spectroscopy and x ray absorption [8][9][10][11][12][13]. The key is that while strong MXCD effects in ferromagnets can be observed with photoemission and absorption, the large MLDAD effect in ferromagnets is solely a photoemission, not an absorption-driven, process. This is because the chirality which gives rise to magnetic sensitivity is due to the vectorial configuration in MLDAD as opposed to the FIG. 1. (a) Schematic diagram of the direct and indirect channels in Gd 4f resonant photoemission. Time estimates are based on Refs. [2,3]. (b) Same for Gd 5p emission. (c) Comparison of coherent and incoherent additions of channel contributions. A0 (AI) is the direct (indirect) amplitude. (d) The photoabsorption of Gd͞...
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