The bandwidths and the anharmonic frequency shifts of the optical lattice phonons of crystalline CO2 are calculated as a function of temperature using three different intermolecular potentials. A previously available potential (MOSMD) which reproduces correctly the structure, energy and harmonic frequencies of the CO2 crystal provides anharmonic, calculated bandwidths about 10 times larger than the experimental values. An improved potential (PRC-1) is obtained by moving inside the OC bonds the oxygen interaction center and by increasing the molecular quadrupole. This potential predicts bandwidths and shifts of the right order of magnitude but still too large. The third potential (PRC-2) is obtained from the previous one by using a non-collinear distribution of negative charges. The negative charges are localized on exagons around the OC bond at a distance d = 0.3 Å. This potential reproduces correctly the experimental bandwidths and yields small anharmonic shifts. The temperature dependence of the bandwidths and shifts is correctly reproduced in terms of three-phonon decay processes for the T+g and for the T−g phonons. In the case of the Eg phonon four-phonon processes are required in order to fit the experimental data.
Anharmonic calculation of bandwidths and frequency shifts in crystalline CO2 / Piero Procacci;Roberto Righini;Salvatore Califano. - In: CHEMICAL PHYSICS. - ISSN 0301-0104. - STAMPA. - 116:(1987), pp. 171-186. [10.1016/0301-0104(87)80079-4]
Anharmonic calculation of bandwidths and frequency shifts in crystalline CO2
PROCACCI, PIERO;RIGHINI, ROBERTO;CALIFANO, SALVATORE
1987
Abstract
The bandwidths and the anharmonic frequency shifts of the optical lattice phonons of crystalline CO2 are calculated as a function of temperature using three different intermolecular potentials. A previously available potential (MOSMD) which reproduces correctly the structure, energy and harmonic frequencies of the CO2 crystal provides anharmonic, calculated bandwidths about 10 times larger than the experimental values. An improved potential (PRC-1) is obtained by moving inside the OC bonds the oxygen interaction center and by increasing the molecular quadrupole. This potential predicts bandwidths and shifts of the right order of magnitude but still too large. The third potential (PRC-2) is obtained from the previous one by using a non-collinear distribution of negative charges. The negative charges are localized on exagons around the OC bond at a distance d = 0.3 Å. This potential reproduces correctly the experimental bandwidths and yields small anharmonic shifts. The temperature dependence of the bandwidths and shifts is correctly reproduced in terms of three-phonon decay processes for the T+g and for the T−g phonons. In the case of the Eg phonon four-phonon processes are required in order to fit the experimental data.I documenti in FLORE sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.