A new ion-molecule reaction apparatus, which consists of a photoionization source, a tandem mass spectrometer, and a radio frequency (rf) octopole reaction cell, is described. Using a quadru- pole mass filter to reject H(,3)('+) background ions formed at the photo- ionization region, absolute total cross sections for the reaction H(,2)('+)((upsilon)(,0)') + H(,2)((upsilon)(,0)'' = 0) (--->) H(,3)('+) + H, have been measured as a function of the vibrational state of reactant H(,2)('+), where (upsilon)(,0)' = 0-4, over the center-of-mass collision energy range of 0.04-15eV;Absolute spin-orbit-state-selected total cross sections for charge transfer reactions Ar('+)(('2)P(,3/2)) + N(,2)((')X, v = 0) and Ar('+)(('2)P(,1/2)) + N(,2)((')X, v = 0), (sigma)(,3/2) and (sigma)(,1/2), respectively, at the E(,c.m.) range of 0.25-115.3 eV have also been measured using the same apparatus;With the triple-quadrupole double-octopole photoionization tan- dem mass spectrometer, absolute state-selected total cross sections, (sigma)(,v'), v' = 0 and 1, for reaction N(,2)('+)((')X, v' = 0,1) + Ar(('1)S(,0)) (--->) N(,2)((')X,v) + Ar('+)(('2)P(,3/2,1/2)) (reaction (1)) over E(,c.m.) range of 1.2-140 eV have been;measured. These measurements, together with the relative values for s(,v'), v' = 0-2, and spin-orbit-state distributions of product Ar('+) ions determined using the crossed ion-neutral beam photoionization apparatus, allow the determination of the absolute values for (sigma)(,2) and partial state-to-state cross sections, (sigma)(,v'(--->)J), v' = 0-2, for reaction (1);Experimental state-to-state cross sections obtained in this study are consistent with those for the reaction, Ar('+)(('2)P(,3/2)) + N(,2)((')X, v = 0) (--->) Ar(('1)S(,0)) + N(,2)('+)((')X, v'), from the consideration of microscopic reversibility;Using the triple-quadrupole double-octopole apparatus, absolute fine-structure transition cross sections for the reactions Ar('+)(('2)P(,3/2));+ Ar and Ar('+)(('2)P(,3/2)) + N(,2)((')X, v = 0) at laboratory collision energies of 25-200 eV have been measured; *USDOE Report IS-T-1261. This work was performed under Contract W-7405-Eng-82 with the U.S. Department of Energy.