We report on observations of redshifted He ii 303.8 Å absorption in the high-redshift QSO HE 2347 - 4342 ( z = 2.885 , V = 16.1 ) with the Goddard High Resolution Spectrograph on board HST in its low resolution mode ( \bigtriangleup \lambda = 0.7 Å ) .
With f _ { \lambda } = 3.6 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } Å ^ { -1 } at the expected position of He ii 304 Å absorption it is the most UV-bright high redshift QSO discovered so far .

We show that the He ii opacity as a function of redshift is patchy showing spectral regions with low He ii opacity ( “ voids ” ) and regions with high He ii opacity ( blacked-out “ troughs ” ) and no detectable flux .
Combination with high-resolution optical spectra of the Ly \alpha forest using CASPEC at the 3.6 m telescope shows that the voids can be explained either exclusively by Ly \alpha forest cloud absorption with a moderate N _ { \mbox { \scriptsize He { \sc ii } } } / N _ { \mbox { \scriptsize H { \sc i } } } ratio \eta \leq 100 and turbulent line broadening or by a combination of Ly \alpha forest with \eta = 45 and thermal broadening plus a diffuse medium with \tau _ { \mbox { \scriptsize GP } } ^ { \mbox { \scriptsize He { \sc ii } } } \approx 0.3 .
Since the latter is a minimum assumption for the Ly \alpha forest , a strict upper limit to a diffuse medium is \Omega _ { \mbox { \scriptsize diff } } < 0.02 h _ { 50 } ^ { -1.5 } at z = 2.8 .

In the troughs in addition to the Ly \alpha forest opacity a continuous He ii 304 Å opacity \tau = 4.8 ^ { + \infty } _ { -2 } is required .
In case of photoionization , the troughs would require a diffuse component with a density close to \Omega \simeq 0.077 ( \eta / 45 ) ^ { -0.5 } h _ { 50 } ^ { -1.5 } , i.e .
all baryons in the universe , which is inconsistent , however , with the observed absence of such a component in the voids .
A tentative interpretation is that we observe the epoch of partial He ii reionization of the universe with patches not yet reionized .
In that case a diffuse component with \Omega _ { \mbox { \scriptsize diff } } \geq 1.3 10 ^ { -4 } h ^ { -1 } _ { 50 } would be sufficient to explain the “ trough ” opacity .
The size of the 1163–1172 Å trough is \sim 6 h _ { 50 } ^ { -1 } Mpc or \sim 2300 km s ^ { -1 } , respectively .

We also discuss partially resolved He ii absorption of a high-ionization associated absorption system .
Despite its high luminosity HE 2347 - 4342 does not show a He ii proximity effect .
A possible reason is that the strong associated system shields the He ii ionizing continuum .