Absolute calibration of an atomic helium beam time-of-flight apparatus by flight path variation

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Abstract
Virtually all inelastic helium atom scattering experiments utilize time-of-flight (TOF) analysis to energy analyze the scattered beam. To date, the demands upon the accuracy of these TOF measurements have not been particularly great (typically a few percent), and rather rudimentary calibration techniques have sufficed. For certain experiments, e.g., for time-resolved measurements of specular scattering to determine surface-bound states of helium or to observe surface-step interference, this is no longer the case. We describe an in situ calibration technique based upon a variation of the TOF flight path that allows us to calibrate our path length and time origin to within a few millimeters and microseconds, respectively (<2×10−3). A comparison with ‘‘zero flight time’’ laser timing measurements shows that finite drawout/transit times through the beam detector can introduce nonnegligible systematic errors into the laser calibration technique, even in the present favorable instance (light beam mass, drawout along the beam axis). We demonstrate the utility of surface diffraction as either a consistency check or to accurately determine unknown reciprocal lattice vectors.