Absolute dielectronic recombination ( DR ) rates for lithium-like Ni ^ { 25 + } ( 1 s ^ { 2 } 2 s ) ions were measured at high-energy resolution at the Heidelberg heavy-ion storage ring TSR .
We studied the center-of-mass energy range 0–130 eV which covers all \Delta n =0 core excitations .
The influence of external crossed electric ( 0–300 V/cm ) and magnetic ( 41.8–80.1 mT ) fields was investigated .
For the measurement at near-zero electric field resonance energies and strengths are given for Rydberg levels up to n = 32 ; also Maxwellian plasma rate coefficients for the \Delta n =0 DR at electron temperatures between 0.5 and 200 eV are provided .
For increasing electric field strength we find that for both the 2 p _ { 1 / 2 } and the 2 p _ { 3 / 2 } series of Ni ^ { 24 + } ( 1 s ^ { 2 } 2 p _ { j } n \ell ) Rydberg resonances with n > 30 the DR rate coefficient increases approximately linearly by up to a factor of 1.5 .
The relative increase due to the applied electric field for Ni ^ { 25 + } is remarkably lower than that found in previous measurements with lighter isoelectronic Si ^ { 11 + } , Cl ^ { 14 + } and also Ti ^ { 19 + } ions , [ T. Bartsch et al , Phys .
Rev .
Lett . 79 , 2233 ( 1997 ) ; 82 , 3779 ( 1999 ) and to be published ] and in contrast to the results for lighter ions no clear dependence of the electric field enhancement on the magnetic field strength is found .
The Maxwellian plasma rate coefficients for \Delta n =0 DR of Ni ^ { 25 + } are enhanced by at most 11 \% in the presence of the strongest experimentally applied fields .