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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3457788]