Detection and Identification of Triplet States of H2O by Electron Impact

It has been previously (1-4) that the angular behavior of the differential cross section for exciting molecular electronic states by low-energy electron impact is a useful tool in distinguishing transitions which are optically spin forbidden from those which are spin allowed. We have used this method to investigate water vapor and have detected and identified at least two singlet→triplet transitions. One is a broad feature having a maximum intensity at about 4.5 eV excitation energy and the other has a sharp minimum at 9.81 eV. The first one had been detected previously (5-10) and the triplet state character of the upper state involved has been implied (5). The second one had not previously been detected. We show that the differential cross section for both of these excitation processes have an angular dependence characteristic of singlet→triplet transitions.