Rotational Spectral Band Shapes in Dense Fluids

  • W. A. Steele


Although the detailed structure of infrared or Raman vibration-rotation spectra for low pressure gases is ordinarily interpreted in terms of energy levels, one can also view measurements of the rotational transitions as a method of determining quantized values of the angular momentum of an isolated molecule. In this way, one sees that spectral experiments probe the time evolution of molecular orientations even at low density. As the density increases, the rotational lines broaden and eventually merge into a continuous spectral band. At this point, it is no longer useful to describe the molecules as being in well-defined energy or angular momentum states. A more rewarding approach to the understanding of these band shapes can be found by extracting time-correlation functions from the data by Fourier transforming the band shapes. In general, a time-correlation function contains information about the evolution of some relevant molecular variable averaged over an equilibrium ensemble of molecules; for rotational spectra, the relevant variables are functions of orientation angles It should be emphasized that a Fourier transform of a spectral intensity distribution does not carry one very far toward solving the problem, which is to extract information about vibrational and rotational motions of molecules in dense phases from the data. It is only when one actually begins to model, either by a physical theory or by computer simulation, that the power of the time-correlation function approach becomes apparent.


Correlation Function Memory Function Dense Fluid Free Rotation Band Shape 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Plenum Press, New York 1980

Authors and Affiliations

  • W. A. Steele
    • 1
  1. 1.Department of Chemistry 152 Davey LaboratoryThe Pennsylvania State UniversityUniversity ParkUSA

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