The spectroscopy and relaxation dynamics of three-phonon bound states in crystal CO2

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Abstract
The high‐resolution Fourier‐transform infrared spectrum of a single CO2 crystal has been studied in the (ω1+ω2);3ω2 Fermi resonance region. The spectrum shows three types of vibrational excitation, one where the three ω2 vibrons propagate freely (P+P+P) into the crystal, a second with two (out of three) coupled on the same molecule (BP+P), and the last with all three ω2 modes linked on one molecule and travelling as a single excitation (TP). Fermi resonance may split off the three‐phonon continuum (ω2+ω2+ω2) both the biphonon+phonon (BP+P) band and the sharp triphonon (TP) peak. A theoretical analysis of triphonons following a Green‐function approach is proposed. The temperature dependence of triphonon bandwidths has been measured in the temperature range 20–200 K. This gives information on the relaxation mechanisms of bound crystal states. Our results emphasize the role of dephasing through bending mode in the vibrational relaxation of the upper triphonon. The lower triphonon has a behavior with temperature which may be explained as due only to dephasing by lattice phonons. Triphonon depopulation is treated theoretically in a highly simplified form to enlighten the main channels available for decay.