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Abstract The hydrated electron, e– (aq), has attracted much attention as a central species in radiation chemistry. However, much less is known about e– (aq) at the water/air surface, despite its fundamental role in electron transfer processes at interfaces. Using time-resolved electronic sum-frequency generation spectroscopy, the electronic spectrum of e– (aq) at the water/air interface and its dynamics are measured here, following photo-oxidation of the phenoxide anion. The spectral maximum agrees with that for bulk e– (aq) and shows that the orbital density resides predominantly within the aqueous phase, in agreement with supporting calculations. In contrast, the chemistry of the interfacial hydrated electron differs from that in bulk water, with e– (aq) diffusing into the bulk and leaving the phenoxyl radical at the surface. Our work resolves long-standing questions about e– (aq) at the water/air interface and highlights its potential role in chemistry at the ubiquitous aqueous interface.

The transfer of electronic energy through a photosystem can harm the photosynthetic apparatus when not balanced with CO2 fixation. Here, the authors show that CO2 modulates electronic energy transfer in cyanobacteria by binding to and enhancing the activity of the light-harvesting complex.