The primary X-ray diagnostic lines in He-like ions are mainly excited by electron impact from the ground level to the n = 2 levels , but at high temperatures n > 2 levels are also excited .
In order to describe the atomic processes more completely collision strengths are computed for O VII including for the first time all of the following : ( i ) relativistic fine structure , ( ii ) levels up to the n = 4 , and ( iii ) radiation damping of autoionizing resonances .
The calculations are carried out using the Breit-Pauli R-matrix ( BPRM ) method with a 31-level eigenfunction expansion .
Resonance structures in collision strengths are delineated in detail up to the n = 4 thresholds .
For highly charged He-like ions radiation damping of autoionizing resonances is known to be significant .
We investigate this effect in detail and find that while resonances are discernibly damped radiatively as the series limit n \rightarrow \infty is approached from below , the overall effect on effective cross sections and rate coefficients is found to be very small .
Collision strengths for the principal lines important in X-ray plasma diagnostics , w , x , y and z , corresponding to the 4 transitions to the ground level 1 s ^ { 2 } ( ^ { 1 } S _ { 0 } ) \longleftarrow 1 s 2 p ( ^ { 1 } P ^ { o } _ { 1 } ) , 1 s 2 p ( ^ { 3 } P ^ { o } _ { 2 } ) , 1 s 2 p% ( ^ { 3 } P ^ { o } _ { 1 } ) , 1 s 2 s ( ^ { 3 } S _ { 1 } ) , are explicitly shown .
It is found that the effective collision strength of the forbidden z-line is up to a factor of 4 higher at T < 10 ^ { 6 } K than previous values .
This is likely to be of considerable importance in the diagnostics of photoionized astrophysical plasmas .
Significant differences are also found with previous works for several other transitions .
This work is carried out as part of the Iron Project-RmaX Network .