We deduce information about the dynamics of a proton transfer reaction between an acid and a base. Our probe is the fully resolved S-1<--S-0 fluorescence excitation spectrum of the 1:1 complex of 1-naphthol and ammonia in the gas phase. Analysis of this spectrum shows that the complex is planar in both electronic states, with the NH3 forming a nearly linear hydrogen bond to the hydroxy hydrogen atom of 1-naphthol. The O-H ... N heavy atom separation is R=2.86 Angstrom and the barrier to rotation of the NH3 group about its C-3 axis is V-3=39.9 cm(-1) in the S-0 state. Excitation of the complex to its S-1 state increases the acidity of 1-naphthol decreases the heavy atom separation to R=2.72 Angstrom, and increases the torsional barrier to V-3=46.5 cm(-1). Modeling these changes using the Lippincott-Schroeder potential for the hydrogen bond shows that the photoinitiated heavy atom motion produces a significant decrease in the barrier to proton transfer in the S-1 state.