The photochemical reaction of phenol becomes ultrafast at the air-water interface.

Reactions at the interface between water and other phases play important roles in nature and in various chemical systems. Although some experimental and theoretical studies suggest that chemical reactions at water interfaces can be different from those in bulk water-for example, 'on-water catalysis' and the activation of photochemically inert fatty acids at the air-water interface upon photoexcitation-directly investigating these differences and generating molecular-level understanding has proved difficult. Here, we report on the direct probing of a photochemical reaction occurring at the air-water interface, using ultrafast phase-sensitive interface-selective nonlinear vibrational spectroscopy. The femtosecond time-resolved data obtained clearly show that the photoionization reaction of phenol proceeds 104 times faster at the water surface than in the bulk aqueous phase (upon irradiation with photons with the same energy). This finding demonstrates that photochemical reactions at water interfaces are very different from those in bulk water, reflecting distinct reaction environments at the interface.