We present fine structure and recombination lines observations of the ionized gas toward a sample of 18 sources located within 300 pc of the center of the Galaxy ( hereafter Galactic center , GC ) .
The sources were selected as molecular clouds located far from thermal continuum sources .
The fine structure lines from [ N ii ] and [ S iii ] have been detected in 16 sources .
In 10 sources we have even detected the [ O iii ] 88 \mu m line .
Several techniques have been used to determine lower and upper limits to the extinction toward each source to correct the observed line fluxes .
The derived electron densities of the ionized gas vary from \sim 100 to \leq 30 cm ^ { -3 } .
For some sources we were able to derive N , S and Ne abundances .
We found that they are similar to those measured in the H ii regions in the 5-kpc ring and in the nuclei of starburst galaxies .
The fine structure lines ratios measured for all the sources can be explained by photo-ionization with an effective temperature of the ionizing radiation of 32000-37000 K and an ionization parameter , U , of -1 > \log U > -3 .
The highest excitation is found in the Radio Arc region but it does not decrease smoothly with distance .
There must be more ionizing sources distributed over the Galactic center than the known clusters of massive stars .
Most of the clouds are located far ( up to 45 pc for M-0.96+0.13 ) from the prominent continuum complexes ( Sgr C , B … ) .
However , it is possible that the clouds are ionized by escaped photons from those complexes .
The comparison of the effective temperatures of the ionizing radiation to the measured Lyman continuum photons emission rate imply that the clouds are indeed ionized by distant sources .
The excitation ratios , effective temperature and ionization parameter measured in the GC are similar to those found in some low excitation starburst galaxies .
The [ Ne iii ] / [ Ne ii ] line ratios measured in the GC sources are consistent with the results of the Thornely et al .
( 2000 ) model for a short burst of massive star formation less than 8 Myr ago .
We have also found that the [ Ne ii ] 13 \mu m  to far-infrared continuum ratio measured for the GC sources is similar to that of external galaxies , supporting the idea by Sturm et al .
( 2002 ) that the far-infrared continuum in Active Galaxies is dominated by dust heated by stellar radiation rather than by the AGN .