No evidence for Evans' holes in the A, B, C vibrational structure of potassium dihydrogen arsenate.

We have used inelastic neutron scattering (INS) spectroscopy to study the "A, B, C" region of the hydrogen bond in potassium dihydrogen arsenate. The broad spectral feature observed in infrared spectroscopy is shown to be associated with a ridge of constant intensity in the INS, which follows the recoil line for a unit-mass particle. The onset energy of the ridge is unclear but, we believe, is associated with the optical "C" feature at 1610 cm(-1), and which we assign to nu(O-H). The "B" and "A" optical bands were both demonstrated to be two-quantum events and are, thus, not associated with the fundamental nu(O-H). They are readily assigned to the harmonic overtone delta(OH)(0-2) and the combination gamma(OH)(0-1)+delta(OH)(0-1), which both sit astride the ridge and there is no evidence for Evans' holes. The other overtone, gamma(OH)(0-2), has been assigned to a very weak feature, observed for the first time at 1900 cm(-1). A simple model was used to describe the intensity distributions.