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The nature of the intermolecular bonding in the indole-water complex is investigated by highly resolved UV-REMPI spectra with rotational fine structure. First, we determine the coordinates of the amino hydrogen atom by isotope substituting it with deuterium for the S 0 electronic ground and S 1 excited state of indole. In a second step, we calculate the rotational fine structure resulting from possible cluster structures and compare it with the experimental results. A fit routine adapts the spectroscopic constants within the condition of a planar complex which is a reasonable assumption for the observed rotational a,b-type band structure. The best agreement between simulation and experiment is found when the water molecule oxygen is located 2.93(5) Å away from the amino hydrogen of indole. This clearly demonstrates that in the indole-water complex a hydrogen bond is formed between the hydrogen donor indole and the oxygen atom of the water molecule. Upon electronic excitation to S 1 this bond length is nearly constant.

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