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Geometric arrangement of the tip and surface used in our simulation and definition of the effective probing diameter. In simulation, only the tip-apex region can receive photoelectrons; most of the tip is considered nonconductive. With the work function of the metal surface set at and the emission probability as , this plot shows the relative extent of photoelectrons capable of reaching the bare tip region as a function of their emission positions. According to this plot, the effective probing diameter is defined as the region producing 90% of the photocurrent collected by the tip. The dip of photocurrent appearing at is due to the finite grid numbers used to describe the tip apex.
Dependences of on parameters , , and . (a) Probing diameter vs separation between tip and surface is examined for several values of . (b) Probing diameter vs voltage bias at . The relevant dimension appearing in both cases clearly demonstrates how distant photoelectrons affect .
Radiation from a synchrotron incident on a metal surface generates a wide spectrum of excited electrons, but not all leave the surface like those in group A. A metallic tip placed near a surface eliminates a constraint posted by a critical angle of total reflection and allows excited electrons marked as groups B and C to reach the tip via tunneling under the effective work functions and .
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