No abstract

Laser-induced fluorescence (LIF) measurements of OH(X 2Π) production following electron impact on water have been carried out over an energy range from threshold to 300 eV. Data have been made absolute using existing measurements of OH(X) production via dissociative attachment. The cross section reaches a maximum value of 2.1×10−16 cm2 at an energy of 75 eV. Dipole-allowed processes are shown to dominate the production of OH(X) particularly via the A(1B1) repulsive state, though significant population of higher rotational levels is demonstrated, indicating the relevance of other channels as in photodissociation.

We have observed the metastable species O(1S0) following the dissociation of molecules by pulsed electron impact. A crossed beam apparatus was used to obtain time-of-flight spectra of the fragments. We have employed a novel detector which consists of a layer of freshly deposited Xe. O(1S0) atoms impinging on the surface quickly form XeO* excimers which rapidly decay (<1 μs) producing easily detected photons. Our present results indicate that this method is sensitive (with high quantum efficiency) solely to the 1S state of oxygen. Other metastables (such as 5S and 1D) and ground state oxygen atoms have not been detected. Low resolution optical spectra of the XeO emission reveal bands at 375, 550, and 725 nm. Time-of-flight and fragment kinetic energy spectra for both target gases at various electron impact energies are presented together with excitation functions from threshold to 1000 eV. These have been made absolute using a Bethe–Born calibration technique for N2O. O2 data are calibrated relative to N2O. Maximum cross sections for O(1S) production are 2.25×10−17 cm2 at 45 eV and 2.1×10−18 cm2 at 80 eV for N2O and O2 targets, respectively.

[1] Electron capture and ionization cross sections for protons and nitrogen ions incident on N 2 are measured in the energy range 10-100 keV using time of flight (TOF) coincidence counting techniques. In the case of proton impact the formation of N 2 + ions dominates for both electron capture and ionization channels at all energies, whereas for N + ions, the N 2 + formation dominates for electron capture and the dissociative processes for ionization channels. The energy distribution of the fragment products at 20 and 100 keV have also been measured for the first time using the TOF method. These cross sections are useful in the simulation of energetic ions and atoms interacting with Titan's N 2 -rich atmosphere. Titan resides primarily within Saturn's magnetosphere where H + and N + ions are the major ions present along its orbit. It is found that the neutralization of these ions by charge exchange does not occur efficiently above Titan's exobase, so energetic particles with large gyroradii penetrate primarily as ions. The ionization rate and energy deposition in Titan's atmosphere by the energetic H + ions observed by the Voyager spacecraft are explained with the help of the present measurements.