= Photoelectric effect
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The <photoelectric effect> is the release of <Electrons> from matter following <photon absorption>. Here the focus is emission from a metal surface into <vacuum>: a <photon> supplies <energy>, the <work function> pays the escape cost, and the remainder can become the emitted <Electron>'s <kinetic energy>. A <photocurrent> counts collected <electric charge>; a <stopping potential> probes the upper <kinetic energy> limit. These are different measurements.
In 1905, <Albert Einstein> proposed that an individual light quantum could transfer its whole <energy> to one <Electron>. His §8 allowed losses before escape, distinguished the fastest emitted <Electrons> from the rest, and predicted a straight <stopping potential> versus <frequency> relation with a material-independent slope. He separately predicted that changing <light intensity> changes the number emitted without changing their <velocity> distribution under otherwise identical conditions. He also discussed incomplete transfer and limits to the account.<Einstein's Miraculous Year/On a Heuristic Point of View Concerning the Production and Transformation of Light by Albert Einstein>[Einstein's Miraculous Year; §8, printed pp. 193–196; full 21-page English translation of the 1905 paper]{ref}
The useful modern starting model is a clean, homogeneous metal with independent single-<photon> events, negligible initial thermal broadening, negligible lowering of the surface barrier by an applied <electric field>, and negligible <electric fields> from the emitted <Electrons> themselves. Specify the surface and its <work function>; it is not enough to name the bulk material. The <Einstein photoelectric equation> describes the upper edge of the emission <energy> distribution. It neither predicts that every absorbed <photon> liberates an <Electron> nor supplies the whole <photocurrent> curve. Its appeal is that a complicated surface process still exposes the universal <Planck constant> through a simple slope.
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