Triphonons in crystal CO2

Abstract Experimental evidence is reported for triphonons in crystal CO 2 . A simplified theory is presented which predicts the occurrence, for sufficiently high crystal anharmonicity, of bound triphonon (TP) and biphonon+phonon (BP+P) states. Good agreement with the infrared spectrum of crystal CO 2 in the ( ω 1 + ω 2 ; 3 ω 2 ) region is found. This is, to our knowledge, the first observation of bound triphonons in molecular crystals.