| Summary: | <p>In this work, we are interested in the diagnostics in electronic temperature of a plasma purely photoionized, based on the intensity ration of lines emitted by ions helium-like, which have an atomic number <em>Z </em>relatively small. We considered the three lines corresponding to the transitions starting from the excited levels 1s<em>2l</em> towards the fundamental level 1s<sup>2 1</sup>S<sub>0</sub>, like appropriate lines. More precisely, the line of resonance w<em> </em>due to the transition 1s2p <sup>1</sup>P<sub>1</sub> --- 1s<sup>2</sup> <sup>1 </sup>S<sub>0</sub>, the line of intercombinaison (x,y) 1s2p <sup>3 </sup>P<sub>2,1 </sub>--- 1s<sup>2</sup> <sup>1 </sup>S<sub>0</sub> as well as prohibited line z<em> </em>due to the transition 1s<sup>2</sup> <sup>3 </sup>S<sub>1</sub> --- 1s2 <sup>1 </sup>S<sub>0</sub>. These lines appear clearly in the spectra of astrophysical plasmas. As helium-like ion, we chose two, the oxygen O<sup>6+</sup> (Z=8) and neon Ne<sup>8+ </sup>(Z=10). We carried out calculations of the ration of lines intensity G=(z+x+y)/w<em> </em>of O<sup>6+</sup> and Ne<sup>8+ </sup>according to the electronic temperature in the range going from 10<sup>5</sup> to 10<sup>7</sup> K. We will see that, like it was shown by Gabriel and Jordan in 1969 [1], this intensity ration can be very sensitive to the temperature electronic and practically independent of the electronic density. Consequently, the ration G<em> </em>can be used to determine in a reliable way the electronic temperature of plasma observed [2].<em></em></p>
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