Download 2 - DORAS

where I" and I' are additional light contributions to I and I0 respectively. At 0.2 mbar
and X = 350 A, T = 0.57 and T' = 0.5. Substituting into expression above gives
which implies
S" = 0.5S' + 0.07
Substituting S' = 0.2 yields S" = 0.03 in expectedly good agreement with the values
obtained in the A1 filter absorption experiment. The further reduction in S" is likely
to the result o f enhanced attenuation o f S" by the He gas.
However the relative
contributions o f scattered light .vs. higher (2nd order) light have not been determined.
Finally an experiment was carried out to look at the photoabsorption spectrum o f
helium in the wavelength range corresponding to double-electron excitation Is2 —>
2snp (~ 200 A) [Madden and Codling 1963, Domke et al 1991], This spectrum is
shown in Figure 2.27 and shows the asymmetric nature o f the resonances which is
due to the interaction between the doubly excited state and the underlying direct
photoionisation continuum producing an interference effect in the cross section. This
series is shown in the energy region ~ 100 A to ~ 215 A in figure 2.27, where up to
the fifth member is clearly visible.
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Photon Energy (eV)
F ig u r e 2 .2 7 P h o to a b s o rp tio n s p e c tru m o f h eliu m g as in th e v ic in ity o f th e
l s 2( 1S ) - > 2 sn p ( XP ) d ou b ly e x c ite d re s o n a n c e s .