A. Chutjian

Electron-impact excitation of the low-lying electronic states of formaldehyde

Electron‐impact excitation has been observed at incident electron energies of 10.1 and 20.1 eV to the first five excited electronic states of formaldehyde lying at and below the 1B2 state at 7.10 eV. These excitations include two new transitions in the energy‐loss range 5.6–6.2 eV and 6.7–7.0 eV which have been detected for the first time, either through electron‐impact excitation or photon absorption. The differential cross sections of these new excitations relative to that of the optically‐allowed 1B2 ← X 1A1 transition are given at scattering angles between 15° and 135°. These cross‐section ratios peak at large scattering angles—a characteristic of triplet ← singlet excitations. From a comparison of the observed and calculated vertical transition energies, the transitions are assigned as 3A1 ← X (5.6–6.2 eV) and 3B2 ← X (6.7–7.0 eV). The design and performance of the electron‐impact spectrometer used in the above observations is outlined and discussed.

Miniature, high-resolution, quadrupole mass-spectrometer array

A miniature quadrupole mass spectrometer array consisting of 16 rods in a 4×4 array is reported. Each rod is 25 mm in length and 2 mm in diameter. The ionizer is of a miniature Nier-type design, and the detector is a channel-type electron multiplier. Operating frequencies are 5.3, 7.1, and 12.9 MHz. The mass range demonstrated herein is 1–300 u; and the resolution of the system is 0.1–0.5 u (full width at half-maximum), or m/Δm=600. The present sensitivity is calculated and measured to be approximately 1×1012 counts/Torr s.

Electron‐impact excitation of H2O and D2O at various scattering angles and impact energies in the energy‐loss range 4.2–12 eV

The electron‐impact excitation of H2O and D2O has been studied at electron energies close to threshold and at large scattering angles in order to enhance spin and/or symmetry forbidden electronic transitions; and at energies far from threshold and at small scattering angles to enhance optically allowed transitions. The energy‐loss range covered is 4.2–12 eV. From a comparison of the present measurements and recent, accurate ab initio calculations, several new assignments of electronic transitions in both H2O and D2O have been made or suggested. Also suggested are future works which could be carried out in order to unravel the complex Rydberg spectra above 11 eV.

Absolute elastic differential electron scattering cross sections in the intermediate energy region. III - SF6 and UF6

A recently developed technique has been used to measure the ratios of elastic differential electron scattering cross sections (DCS) for SF6 and UF6 to those of He at electron impact energies of 5, 10, 15, 20, 30, 40, 50, 60, and 75 eV and at scattering angles of 20° to 135°. In order to obtain the absolute values of DCS from these ratios, He DCS of McConkey and Preston have been employed in the 20° to 90° range. At angles in the 90° to 135° range the recently determined cross sections of Srivastava and Trajmar have been utilized. From these DCS, elastic integral and momentum transfer cross sections have been obtained.

Theoretical interpretation of the optical and electron scattering spectra of polyatomic molecules. III - N2O and the discovery of resonant phenomena in the B region at 6.8 eV.

Energies and parts of potential surfaces are calculated for the first eleven excited electronic states of nitrous oxide; the states within ten electron volts of the ground state. The assignment of the energies and symmetries of these states which is carried out with the aid of semiempirical INDO calculations, is shown to be consistent with all available optical absorption spectra, electron scattering data, and with photolysis and photosensitization experiments. The weak, diffuse vibrational bands in the 6.8 eV B region have been interpreted as resonant interaction between the continuum levels of the 1 1II state and bound vibrational levels of the underlying 1Σ− state. The mixing is made electronically allowed through bending in the excited states. Supporting experimental and theoretical evidence for this assignment are given, and experiments are suggested to confirm several of the other assignments made.

Measurement of Charge Exchange and X-Ray Emission Cross Sections for Solar Wind-Comet Interactions.

X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized.

A compact, high-resolution Paul ion trap mass spectrometer with electron-impact ionization

A Paul ion trap has been developed for use as a high-resolution mass spectrometer. It is of small size (r0=10 mm), having a resolution of m/Δm=324, which is limited by the machining accuracy of the trap. It has a demonstrated mass range of 1–300 u, and a sensitivity of 2×1014 counts/Torr s, or to 500 parts per trillion detection sensitivity in a typical vacuum of 10−5 Torr. Ionization of the room-temperature gas within the trap is carried out with an electron beam traversing the trapping volume. The trap operates in a radio frequency only mode, and no dc is required. Trapping is accomplished within the well depth of the dynamic radio frequency potential, and no cooling gas is required, such as helium. This combination of factors makes the trap potentially of use for autonomous operation in harsh environments requiring low power, low weight, and low volume, such as undersea, on the surface of a planet or asteroid, or in a spacecraft.

Electron impact excitation of SF6

A study of the electron impact energy-loss spectrum of SF6 under both optical (low scattering angle, high impact energy) and non-optical conditions (high scattering angle, low impact energy) has revealed a number of electronic excitation processes. With the help of theoretical calculations, several of these transitions have been assigned and approximate cross sections associated with four features have been determined. In addition, a strong resonance at 12 eV has been observed in both elastic and vibrationally inelastic ( Delta E=0.092 eV) channels.

Line shapes for attachment of threshold electrons to SF6 and CFCl3: Threshold photoelectron (TPSA) studies of Xe, CO, and C2H2

Using the technique of threshold photoelectron spectroscopy by electron attachment (TPSA), the shapes of the threshold electron attachment cross sections for SF6 and CFCl3 are determined by direct photoionization measurements of the 2P1/2 level of Xe+. The observed TPSA line shape is deconvoluted using a Lorentizian attachment profile with full‐width at half‐maximum of 30 meV for SF6 and 25 meV for CFCl3. In addition, TPSA spectra of the ground electronic states of CO+ and C2H2+ are reported. In CO+ the intensity of the threshold spectral features are dominated by autoionization, while for the vibrational features of C2H2+ direct photoionization is more important. The two processes (autoionization and direct ionization) are characterized by different TPSA spectral line profiles.

Measurements of absolute, single charge-exchange cross sections of H+, He+ and He2+ with H2O and CO2

Absolute measurements have been made of single-electron charge-exchange cross sections of H+, He+, and He2+ in H2O and CO2 in the energy range 0.3-7.5 keV amu-1. Collisions of this type occur in the interaction of solar wind ions with cometary gases and have been observed by the Giotto spacecraft using the Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) during a close encounter with comet Halley in 1986. Increases in the He+ ion density, and in the He2+ to H+ density ratio were reported by Shelley et al. and Fuselier et al. and were explained by charge exchange. However, the lack of reliable cross sections for this process made interpretation of the data difficult. New cross sections are presented and discussed in relation to the Giotto observations.