[1] In this work, we report the OI(135.6 nm) absolute emission cross section resulting from the long-lived (180 ms) OI( 5 S ! 3 P) transition from dissociative excitation of O 2 . From the ratio of the integrated intensities of the OI(135.6 nm) and OI(130.4 nm) features and from the absolute emission cross section for the OI(130.4 nm) emission feature from electron impact dissociative excitation of O 2 at 100 eV, the absolute emission cross section for the OI(135.6 nm) feature was determined to be 6.4 Â 10 À18 cm 2 at 100 eV. Electron impactinduced optical excitation functions for optically allowed transitions at 115.2 nm and 130.4 nm and for an optically forbidden transition at 135.6 nm were also obtained over the electron impact energy range 0-600 eV. The OI(135.6 nm) emission cross section was measured in the laboratory utilizing a large collision chamber (1.5 m in diameter). Electrons were produced with an electrostatically focusing gun with a large focal length (50 cm). The OI(130.4 nm, 135.6 nm) excitation functions were put on an absolute scale as described in the text, and the OI(135.6 nm)/OI(130.4 nm) ratio was determined for the entire energy range (0-600 eV). The atomic O UV emission cross sections from dissociative excitation of O 2 can be used to model the recent Hubble Space Telescope observations of OI(130.4 nm) and OI(135.6 nm) intensities from Ganymede [Feldman et al., 2000] and Europa [Hall et al., 1995[Hall et al., , 1998].

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).

No abstract

Cross sections for the dissociative recombination of electrons and H2 + ions having low internal energies have been measured by means of the merged electron-ion beam technique. Narrow resonances associated with indirect capture into high-lying Rydberg states of the neutral molecule have been found. The use of ions with only two vibrational states significantly populated has allowed the high-resolution capability of the merged-beam technique to be clearly demonstrated.

No abstract