In this work we have studied the Li/C-60, Li/C-70, (99)mTc/C-60, (TC)-T-99/C-70, Gd/C-60, Gd/C-70, Nd/C-60, Nd/C-70, Y/C-60 and Y/C-70 systems by surface ionization mass spectrometry and investigated a possibility of the formation of endohedral fullerenes. Using the ion implantation technique (introducing negatively charged fullerene into a low temperature lithium, technetium, gadolinium, neodymium or yttrium plasma column by a strong axial magnetic field), the endohedral fullerenes Li congruent to C-60, Li congruent to C-70, Li-2 congruent to C-70, Tc-99m congruent to C-60, Tc-99m congruent to C-70, Gd congruent to C-60, Nd congruent to C-60, Y congruent to C-60, Gd congruent to C-70, Nd congruent to C-70 and Y congruent to C-70 have been produced and ionization energies were measured. Their existence was demonstrated through the high sensitivity, magnetic mass spectrometer (TIMS- thermal ionization mass spectrometer). These metallofullerenes were identified as stable molecules. The reaction of the obtained endohedral fullerenes with oxygen compounds (H2O, O-2 and N2O) has also been investigated. The result showed that these metallofullerenes are very inert to all of these gas phase oxygen-rich molecules, leading support to the endohedral structure of the metallofullerenes. Observed ionization energies for Li congruent to C-60, Li congruent to C-70, Li-2 congruent to C-70, Tc-99m congruent to C-60, Tc-99m congruent to C-70, Gd congruent to C-60, Nd congruent to C-60, Y congruent to C-60, Gd congruent to C-70, Nd congruent to C-70 and Y congruent to C-70 were 5.9eV, 5.1eV, 5.3eV, 5.1eV, 5.3ev, 6.2eV, 6.5eV, 6.6eV, 6.4eV, 6.5eV and 6.7eV, respectively
