The optical excitation function of the O I 3 S o → 3 P transition (130.4 nm), produced by electron-impact excitation of atomic oxygen, has been measured over an extended energy range from threshold to 1.0 keV. Measurements were obtained in a crossed-beam experiment using both magnetically confined and electrostatically focused electrons in collision with atomic oxygen produced by a microwave discharge source. A 0.2 m vacuum ultraviolet monochromator system was used to measure the emitted O I radiation at 130.4 nm. The relative O I (130.4 nm) emission intensity corresponding to the 3 S o → 3 P transition was then put on the absolute scale by normalization to the O I (130.4 nm) cross section produced by dissociative excitation of O 2 at 30 eV (Kanik et al 2000).
A pulsed crossed-beam method incorporating time-of-flight spectroscopy, previously developed in this laboratory, has been used to obtain relative cross sections σ n for n = 1-to 6-fold ionization of ground-state Pb atoms by electrons at energies ranging from near threshold to 3000 eV. These results greatly extend the energy range of previous measurements which, in any case, exhibit serious discrepancies. Rather than rely on any previous measurements on Pb, the present data have been normalized to the previously measured cross section σ 1 for electron impact ionization of Ga at 200 eV. At our high-energy limit of 3000 eV, our measured cross sections σ n decrease by much less than an order of magnitude with increasing n and σ 2 and σ 3 are also converging rapidly at high velocities, indicating large contributions from Auger processes. Observed near-threshold structure in the σ 1 cross section curve is consistent with known autoionization transitions, and while there is also some indication of structure in cross section curves for multiple ionization, the large number of possible levels and experimental uncertainties preclude a detailed analysis. Recent theoretical predictions of multiple ionization based on approximate scaling procedures have been shown to be only partially successful in describing the present data.
In this Letter, we report for the first time, the ratio o• tn• O • (1•.• •)/O I (1•0.4 •) •o•ut• •i•io• cross sections from electron-impact dissociative excitation of 02 at 100 eV using facilities located at the University of Colorado, Laboratory for Atmospheric and Space Physics (LASP). The O I (135.6 nm) emission glow produced in the laboratory from the long-lived transition O I gap •_ 5SO was measured in a large collision chamber (lm diameter) with a long focal length (50 cm) electrostatic electron gun. The ratio of the absolute emission cross sections of the O I (135.6 nm) and O I (130.4 nm) features resulting from electronimpact dissociative excitation of 02 at 100 eV was found to be 2.3+0.6. From this ratio, the absolute emission cross section for the O I (135.6 nm) feature at 100 eV electron-impact energy was determined to be (6.4+2.2) x 10 -xs cm 2. The O I (135.6 nm) cross section reported here can be used to better understand the recent Hubble Space Telescope observations of the O I (130.4 nm) and O I (135.6 nm) emission intensities from Ganymede [Feldman et al., 2000] and Europa [Hall et al. , 1995].
The dissociation of molecular hydrogen ions with energies up to 100 keV in H targets has been studied. An Si barrier detector is used to distinguish between the production of one or more fast fragment ions or neutrals and coincidence techniques involving the slow ion or photon produced in the collision allow the different dissociation channels to be quantified. The dissociative excitation and ionization channels are found to be about an order of magnitude weaker in H than those induced by collisions with electrons. Comparison with Born calculations shows that the vibrational states of H2+ play an important role in the dissociation process.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.