We have investigated the dissociative excitation of the atomic oxygen multiplet O 1(3s' 3D0→3p 3P; &lgr;7990 ¿) due to electron impact on O2 and NO. With O2 as a target, no lines of this multiplet could be identified with a cross section greater than 2.4¿10-23 cm2 at 100 eV. Similar results were obtained for the excitation of NO, yielding an upper limit cross section of 1.1¿10-23 cm2. These results are consistent with a &lgr;7990/&lgr;989 branching ratio of ~2.5¿10-5 that is at least an order of magnitude smaller than either the previous experimental upper limits or a recent theoretical calculation. This finding complicates the study of (3s' 3D0→2p4 3P; &lgr;989 ¿) radiation entrapment in the thermosphere and increases the potential importance of selective absorption of the &lgr;989-¿ multiplet by N2. Although the latter process may help to explain some aspects of the behavior of &lgr;7990-¿ emission in aurorae, it is nonetheless difficult, in the light of the present results, to account for the observed absolute intensity of this infrared cascade transition using current entrapment models.