Atomic-scale spatial imaging of one-dimensional chains of silver atoms allows Fermi's golden rule, a fundamental principle governing optical transitions, to be visualized. We used a scanning tunneling microscope (STM) to assemble a silver atom chain on a nickel-aluminum alloy surface. Photon emission was induced with electrons from the tip of the STM. The emission was spatially resolved with subnanometer resolution by changing the tip position along the chain. The number and positions of the emission maxima in the photon images match those of the nodes in the differential conductance images of particle-in-a-box states. This surprising correlation between the emission maxima and nodes in the density of states is a manifestation of Fermi's golden rule in real space for radiative transitions and provides an understanding of the mechanism of STM-induced light emission.
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