[en] The addition of foreign gases (e.g. Ar, He, and CH₃Cl) is shown to strongly affect the MPI current resulting from excitation of NO through the C²π (v = 0) intermediate state. The NO excitation spectrum is not affected by a large excess of Ar. Quantitative and specific determination of NO is demonstrated to 6 x 10⁶ Torr in a bulk mixture. Foreign gas effects are interpreted as arising from secondary ionization of neutrals by energetic electrons. The linear dependence of the signal on NO pressure and the quadratic dependence on laser power suggest that the technique can be used to determine reaction rates involving selected states of NO. (orig.)