Most of our knowledge of the electronic structure of atoms and molecules is derived from excitation energies and transition probabilities. These observable quantities are related to the electronic wave functions by integrals over unmeasured variables. Another observable more directly related to the wave function than energy or transition probability is the single-electron momentum density, the probability that an electron in a well-defined orbital has a given value of momentum. Over the last twenty years a technique has been developed for measuring momentum densities in atoms and molecules. The technique, (e,2e) spectroscopy, is based on electron-impact ionization with complete determination of the momentum of both incoming and outgoing electrons. The conditions necessary to extract momentum-density information from the ionization experiments are examined and related to general theories of electron scattering. Different experimental arrangements are reviewed and momentum-density results from selected examples are discussed.

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The six lowest excited states of benzene have been investigated in the gas-phase free molecule at low pressure by electron-impact spectroscopy. Incident electron energies of 13.6 and 20.0 eV and scattering angles from 9° to 80° were used. Three singlet–singlet transitions at 5.0, 6.2, and 6.9 eV were identified. These transitions agree with the results of optical absorption and higher-energy electron-impact experiments. In addition, three triplet states were observed at 3.9, 4.7, and 5.6 eV. The positions of the first two triplet states agreed with optical data on solid benzene and threshold electron-impact experiments. The third triplet state at 5.6 eV was assigned on the basis of the relative intensity of the transition at various scattering angles. The ratio of the intensity of this transition to the allowed singlet–singlet transition at 6.9 eV was in a constant proportion to the corresponding ratio for the first triplet state (3.9 eV) at all scattering angles. The spacings of the first and second and second and third triplet states in benzene were determined to be 0.80 and 0.85 eV. The difference in the spacing is not significant with respect to experimental error.

Abstract. The direct excitation cross section for the atomic oxygen 3p ___> 3S0 transition (X1304•) has been measured at four incident energies between 13.4 and 40 eV. These measurements were made to add sufficient detail to previous direct excitation measurements to allow the near-threshold cross section to be determined. Previous optical measurements of the 1304fk emission cross section disagree by up to a factor of 2 in this important energy region. We have also compared the atomic hydrogen e + H(ls) --• H(2s + 2p) cross section, used previously as a secondary standard for our direct excitation measurements of atomic oxygen cross sections, to recent measurements of the total e + H --• Lyman alpha cross section and have found agreement at the _+ 15% level, making a revision of our previously reported atomic oxygen cross sections unnecessary. Previous MeasurementsAny measurement of a cross section for production of excited states of an unstable species such as atomic oxygen is a difficult experimental problem. Not only is it necessary to produce the species in reasonable density, but its abundance must be determined, either absolutely or with reference to another species also present which has an observable transition whose cross section is well enough known to be used as a secondary

The 300–180 nm (4.1–6.9 eV) optical absorption and 4–10 eV (310–124 nm) electron energy loss spectra of 1,3-cyclopentadiene, 1,3-cyclohexadiene, and 1,3-cycloheptadiene were measured. Three valence and several Rydberg transitions were observed in each molecule. The two strong, optically allowed valence transitions are interpreted as the NV1(B2) and NV3(A+1) transitions (states). The locations of the unobserved A−1 states in cyclic dienes is discussed. Correlations are drawn between the three valence excited states observed here in each cis-diene and those previously reported for trans-butadiene.

We report the absolute differential and integral cross sections of the forbidden atomic oxygen 3P → 1D( 1.97 eV), and 3P → 1S( 4.19 eV), transitions for incident electron impact energies of 4.0 to 30 eV. The 3P → 1D cross section was measured over the entire energy range while the 3P → 1S transition was measured above 7 eV. The differential cross section (DCS) for the 3P → 1D transition is strongly backward peaked at all incident energies, in agreement with theoretical calculations. The DCS for the 3P → 1S transition varies considerably in shape over the energy range 7–30 eV. At the lower energies, the transition shows an axially forbidden shape (no intensitiy in the forward or backward direction) in agreement with the Thomas and Nesbet (1975) calculations. At higher energies (20–30 eV), the DCS has a remarkable drop near 70°. The agreement between the DCS measured here and the previous measurements of Shyn and Sharp (1986) and Shyn et al. (1986) varies from fair to poor at different incident energies. The integral cross section (ICS) for the 3P → 1D transition agrees within experimental error with the measurements of Shyn and Sharp (1986) and the theoretical calculations of Henry et al. (1971) at 30 eV but is larger than the previous measurements by a factor of approximately 1.3 down to 15 eV and a factor of 2 near the 5 eV peak. The low‐energy peak in the ICS is shown to be between 5 and 7 eV incident energy and the 4 eV experimental measurement is definitely below the peak. No pronouced peak is observed for the 3P → 1S transition. The ICS is in good agreement with the calculations at 30 eV but is larger than the calculations by a factor of approximately 1.4 at lower energies down to 7 eV. The present results are systematically smaller in magnitude than those of Shyn et al. (1986) over the entire energy range studied. The difference varies from a factor of 1.3 at 10 eV to 2.5 at 30 eV. There is no pronounced peak in the 3P → 1S ICS, in agreement with the theoretical calculations.

The daytime photoelectron energy spectrum has been measured at altitudes above 154 km by the photoelectron spectrometer experiment on the Atmosphere Explorer‐E satellite. Much higher energy resolution spectra than previous AE‐C results have been obtained. Below 200 km, well resolved peaks are observed in the spectrum corresponding to production of O+ and N2+ in various electronic states from photoionization of N2 and O by solar He II (304A) radiation. Vibration‐rotation excitation of N2 by inelastic electron scattering is observed at 2.5 eV. The features due to nitrogen disappear from the spectrum at altitudes above 200 km.