Off-axis position

Substrates may be positioned either at a position on the anode (ground) or at an off-axis position to avoid bombardment by secondary electrons accelerated away from the cathode. 

The external geometric differential delay (see below) of an off axis source is exactly balanced within a Fizeau interferometer, resulting in fringes with the same phase on top of each source in the field. The position of a source may differ from the position of zero OPD in a Michelson interferometer depending on how dissimilar entrance and exit pupils are. The fringe contrast of off-axis sources also depend on the temporal degree of coherence of the detected light. 

The position of the focal spot between the two PMs can be varied using an external motorized stage. After reflection on each PM, the beam is collimated by a second identical off-axis parabola and sent as a parallel beam in the experimental chamber. The entire setup is maintained under clean vacuum down to at least 10 6 mbar. 

Knowledge of the composition of samples in an off-axis deposited phase spread is important. Calibration runs were used to determine the thickness vs position for single components deposited in our system at a given power setting. The drop-off in thickness as a function of distance from the gun is approximately exponential and can be modeled accurately to about 5 at%. We find that the deposition rates are additive for the three guns. RBS measurements have been made on a number of samples to validate our assumptions. In addition, we routinely use RBS measurements as a double check on the specific compositions that we identify as having superior properties. 

One important aspect of the electron diffractometer, is that any ED pattern can be captured before scanning by a CCD camera that is placed off-axis in relation with the ED pattern (can be any commercial CCD, even a webcam see fig.l). A dedicated software corrects for any type of optical distortions that may be produced due to the position of viewing angle of the CCD in relation with the ED pattern. This has the big advantage that the user that may observe and define interactively the reflections or the area of the ED pattern that will be scanned, allowing at the same time the main beam to be blanked, in order to avoid radiation damage. 

Table 1 consists of a 30s subset centred on each of the atoms supplemented by 28s subsets, obtained by deleting the two most diffuse functions from the 30s set, centred on the eight off axis atom centre, oa(ac), positions defined by (8) with Til = 5. 

Since hydrogen possesses no inner-core electrons, the central proton in a three-center, two-electron bond will exert only a simple electrostatic repulsion for the metal cores. Close approach of the metal atoms, which would maximize M-M overlap, will be resisted by this electrostatic repulsion. Thus, it can be energetically favorable for the proton to move off the M-M axis simultaneously with the closer approach of the metal atoms. However, a very large off-axis excursion will reduce M-H overlap. Thus, the nuclear repulsion, the M-M overlap, and the M-H overlap may strike a delicate balance leading to an off-axis hydrogen position. 

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