Resonant crossing of a polariton branch with a second order phonon

The polariton spectrum of uniaxial K 3 Cu(CN) 4 was investigated with He-Ne-laser excitation. When the polariton branch associated with the A 1 phonon at 2075 cm -1 crosses the second order phonon at 2035 cm -1 a resonant exchange of intensities takes place and a lifting of the crossing point is observed.