Electronic Resource
College Park, Md.
:
American Institute of Physics (AIP)
The Journal of Chemical Physics
90 (1989), S. 7369-7375
ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Within a normal-mode picture of the molecular internal degrees of freedom, and accepting the ergodic assumption, methods of calculating the rate of infrared radiative cooling of hot polyatomic molecules are considered. The fundamental validity of the thermodynamic approach through definition of an internal temperature is discussed and supported. The Boltzmann equation is shown to be a remarkably accurate approximation to the thermodynamic equations; a thermodynamic equation derived from steepest-descents integration is also presented which is more accurate for very small molecules. Both of these equations are compared with exact statistical counts for small-molecule model systems, and give excellent results. The thermodynamic approach is virtually equal in accuracy to direct statistical counting, and offers advantages in many situations. Detailed consideration shows no general theoretical basis guaranteeing that the energy of the cooling molecule declines exponentially in time, but several special cases are identified in which such behavior is approached. An illustrative calculation of the cooling of benzene from 2700 to 400 K is described. The cooling rate constant declines gradually from 2.2 to 0.45 s−1, but is nearly constant over the cooling interval from 1000 to 400 K.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.456216
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