Threshold Electron Impact Excitation of Atoms and Molecules: Detection of Triplet and Temporary Negative Ion States

A technique which utilizes SF₆ as a scavenger of low‐energy electrons is employed to study the “threshold” excitation spectra of He, N₂, HCl, H₂O, D₂O and a number of aromatic molecules. For electrons within ∼0.03 eV of threshold, it is found that the probability of exciting the $2^{3}S$ state of helium is approximately 1.5 times larger than that for the $2^{1}S$ state. For HCl all of the optically allowed transitions are observed including a Rydberg series leading to the ionization potential. Temporary negative ion resonances are observed below the first electronic state for all of the aromatic molecules studied. For benzene, in addition to the optically allowed transitions, the first (3.9 eV) and second (4.7 eV) triplet states are detected, while for naphthalene a new intense level at 5.4 eV as well as the lower triplet states are observed.