Optical excitation has been combined with the scanning tunneling microscope and a laser-induced change in the tunneling gap width of about 0.25 Å, shows a resonance effect with the electronic absorption of the sample, a monolayer of the J-aggregating dye 1,1-diethyl-2,2-cyanine bromide on a silver substrate. The strength of the effect is reduced by spacing the J aggregate away from the silver substrate using a carboxylic acid functionalized alkylthiol self-assembled monolayer. We propose that the origin of the resonant response is the nonradiative decay of the molecular excited state into the underlying metal substrate that causes local heating and thermal expansion.

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