| Abstrakt: |
The commonly studied photon-gas interactions are those in which the photon is absorbed and an excited, dissociated or ionized molecule is produced. These processes have cross-sections of order 1 Mb. It is often possible to determine these cross-sections by measuring the attenuation of a photon beam, though observation of the collision products is desirable in many instances. There are other processes in which the photon energy is essentially unchanged. We refer to these as scattering processes and distinguished three types: (i) Resonance scattering, (ii) Rayleigh scattering and (iii) Raman scattering. Resonance scattering is characterized by extremely large ‘cross-sections’ and the principal experimental difficulties lie in ensuring that multiple scattering does not over-complicate the analysis. Cross-sections for Rayleigh scattering are of order 1 barn and it is impracticable to determine them by attenuation techniques because of the masking effect of contaminant gases. The direct measurement of the scattered photon flux can only be made if the stray scattered light is reduced to an unusually low value. The same considerations apply with even more force to the measurement of Raman scattering cross-sections which will be considerably smaller. Some measurements of Resonance and Rayleigh scattering of Lyman-α are described. |
