| Summary: | A new high resolution dipole (e,e) method is described for the
measurement of absolute optical oscillator strengths (cross sections) for
electronic excitation of free atoms and molecules throughout the discrete
region of the valence shell spectrum. The technique, utilizing the virtual
photon field of a fast electron inelastically scattered at negligible
momentum transfer, avoids many of the difficulties and errors associated
with the various direct optical techniques which have traditionally been
used for absolute optical oscillator strength measurements. In particular,
the method is free of the bandwidth (line saturation) effects which can
seriously limit the accuracy of photoabsorption cross section
measurements for discrete transitions of narrow linewidth obtained using
the Beer-Lambert law (formula removed)). Since these perturbing “line
saturation” effects are not widely appreciated and are only usually
considered in the context of peak heights a detailed new analysis of this
problem is presented considering the integrated cross section (oscillator
strength) over the profile of each discrete peak.
Using a low resolution dipole (e,e) spectrometer (~1 eV FWHM),
absolute optical oscillator strengths for the photoabsorption of the five
noble gases He, Ne, Ar, Kr and Xe have been measured up to 180, 250,
500, 380 and 398 eV respectively. The absolute scales for the
measurements of helium and neon were obtained by TRK sum rule
normalization and it was not necessary to make the difficult
determinations of photon flux or target density required in conventional
absolute cross section determinations. Single point continuum
normalization to absolute optical data was employed for the
measurements of argon, krypton and xenon due to the closely space in
the subshells of these targets which cause problems in the extrapolation
procedures required for TRK sum—rule normalization. The newly
developed high resolution dipole (e,e) method (0.048 eV FWHM) has
then been used to obtain the absolute optical oscillator strengths for the
valence discrete excitations of the above five noble gases with the
absolute scale normalized to the low resolution dipole (e,e)
measurements in the smooth ionization continuum region. The
measured dipole oscillator strengths for helium excitation (1 ¹S—n ¹P,
n=2-7) are in excellent quantitative agreement with the calculations
reported by Schiff and Pekeris (Phys. Rev. 134, A368 (1964)) and by
Fernley et al. (J. Phys. B 20, 6457 (1987)). High resolution absolute
optical oscillator strengths are also reported for the autoionizing
resonances, corresponding to the double excitation of two valence
electrons and/or single excitation of a inner valence electron, of the
above five noble gases.
High resolution absolute optical oscillator strengths (0.048 eV
FWHM) for discrete and continuum transitions for the photoabsorption of
five diatomic gases (H₂, N₂, 0₂, CO and NO) throughout the va1ence shell
region are reported. The absolute scales were obtained by normalization
in the smooth continuum to TRK sum rule normalized data determined
using the low resolution dipole (e,e) spectrometer.
(For full abstract, view file)
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