FORMATION OF $N_{2}{^{+}}$ BY THE ELECTRON IMPACT DISSOCIATION OF $N_{2}O$ COOLED IN A SUPERSONIC MOLECULAR BEAM

Author Institution:If a molecular beam of rotationally cold neutral molecules formed in a supersonic expansion is bombarded with a beam of electrons, it is possible to both ionize and electronically excite the parent molecule. Because of the difference in mass between the electron and the molecule, there is very little angular momentum transfer and therefore there is the possibility of producing a rotationally cold beam of electronically exited ions. We have prepared a rotationally cold beam of electronically exicted $N_{2}O^{+}$ ions by bambarding a skimmed supersonic molecular beam of $N_{2}O$ with 500 eV electrons. The fluorescence spectrum of the product $N_{2}O^{+}$ has been studied in the 2000-6000{\AA} region and an intense band system belonging to the $A^{2}\Sigma^{+} \rightarrow X^{2}\Pi$ electronic transition was observed. The rotational state distribution of the $N_{2}O^{+}$ corresponded to a temperature estimated to be 10-15 K. In addition to the $N_{2}O^{+}$ fluorescence we also observed the 0-0 band of the first negative system of $N_{2}^{+}(B^{2}\Sigma^{+}_{u} \rightarrow X^{2}\Sigma^{+}_{g})$. This emission is thought to be due to the product of the dissociation of $N_{2}O^{+}$. The rotational state distribution of the product $N_{2}^{+}$ is characterized by a Boltzmann temperature of $160 \pm 80$ K. The rotational temperature of $N_{2}^{+}$ prepared by direct electron bombardment of a cooled beam of $N_{2}$ was found to be $20 \pm 5$K. Our inability to observe bands originating in the $v^{\prime\prime}=1$ level of the $N_{2}^{+}$ produced by dissociation of $N_{2}O^{+}$ means that the population of the $v^{\prime\prime}=1$ level was less than 2% that of the $v^{\prime\prime}=0$ level