Laser raman study of quasi-spin wave hydrogen tunneling modes in KH2PO4 and KD2PO4

The low frequency laser excited Raman spectra of KH2PO4 and KD2PO4 single crystals have been measured at room temperature for various orientations and a complete symmetry assignment of the observed modes has been made. In addition to the strongly damped "ferroelectric" quasi-spin wave-like proton mode of symmetry B-2 observed by Kaminov, a non-degenerate proton mode at 397 cm(-1) and a double degenerate proton mode around 80 cm(-1) of predicted symmetry properties have been found. The frequencies of all these proton modes are reduced on deuteration by a factor of ten or more, thus demonstrating their collective proton tunneling nature.