P. C. Cosby

Electron‐impact dissociation of nitrogen

The electron‐impact dissociation of N2 to form two nitrogen atoms is observed in a crossed beam experiment at electron energies between 18.5 and 148.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the N2 dissociation are consistent with predissociation to N(2D)+N(4S) fragments as the primary dissociation mechanism. Absolute cross sections for the electron impact dissociation are measured and compared with previous measurements. Recommended values of this cross section are given for electron‐impact energies between 10 and 200 eV.

Photofragment spectroscopy and potential curves of Ar+2

Photofragment energy distributions have been measured for the process Ar+2(2Σ+u)+hν→Ar++Ar using a 3 keV ion beam and cw lasers both coaxial and crossed with the ion beam, polarized, respectively, perpendicular and parallel to the ion beam direction. Measurements were made at 14 wavelengths between 4579 and 7993 A. Transitions to the dissociative states 2Πg and 2Σ+g are observed, as are the effects of the spin–orbit interaction in Ar+2. The experimental results are used along with theoretical calculations to determine the 2Σ+u, 2Σ+g, and 2Πg potentials. The effects of the spin–orbit interaction on the potential curves, the magnitude and wavelength dependence of the photodissociation cross section, and the angular distributions of the photofragments are considered.

Electron‐impact dissociation of oxygen

The electron‐impact dissociation of O2 to form two oxygen atoms is observed in a crossed beam experiment at electron energies between 13.5 and 198.5 eV. Detection of the correlated dissociation fragments with a time and position sensitive detector permits detection of both ground and excited state fragments, but excludes interference from dissociative ionization products. The observed translational energy releases in the O2 dissociation are consistent with production of O(1D)+O(3P) fragments following electron impact excitation to the B 3Σu−, B’ 3Σu−, and 2 3Πu states, and production of O(3P)+O(3P) fragments from excitation to the (unresolved) c 1Σu−, A’ 3Δu, and A 3Σu+ states. Absolute cross sections for the electron impact dissociation of O2 are measured.

Photodissociation and photodetachment of molecular negative ions. III. Ions formed in CO2/O2/H2O mixtures

Total photodestruction cross sections for O2−, O3−, O4−, O2−⋅H2O, CO4−, CO3−, and CO3−⋅H2O have been measured over the range from 6950 to 4579 A (1.78–2.71 eV). In most cases the photodestruction of these ions can be attributed to specific photodissociation or photodetachment processes. The ions HCO3− and HCO3−⋅H2O have also been investigated, and upper limits determined for their total photodestruction. The experiments were performed using a drift tube mass spectrometer coupled with an argon ion laser and a tunable dye laser. The cross section values vary from 2×10−20 to 1×10−17 cm2, and in most cases photodissociation is the predominant process. In CO3− and O3− evidence is found for bound, predissociating excited states.

Photodissociation Spectroscopy of CO3(

The photodissociation cross section of gas‐phase CO3 − has been measured over the wavelength range from 4579 to 6940 A, and reveals detailed structure reflecting the vibrational spacings of a predissociating excited electronic state. From an analysis of the structure, we identified three vibrational modes of the excited state having energies of 990, 1470, and 880 cm−1. The bond energy D (CO2–O−) of the ground state CO3− was determined to be 1.8±0.1 eV, and the electron affinity of CO3 was found to be 2.9±0.3 eV. By comparison with theoretical calculations, the lowest predissociating state was identified as 1 2A1. Observations regarding other excited states of CO3− are made.

Photodissociation and photodetachment of molecular negative ions. II. Ions formed in oxygen

Total photodestruction cross sections for O−2, O−3, and O−4 ions have been measured over a photon energy range of 1.93–2.71 eV using a drift tube mass spectrometer coupled with an argon ion laser and a tunable dye laser. The O−2 ion is found to photodetach at these photon energies with a cross section which varies from 1.2 to 2.2×10−18 cm2. The O−3 ion photodissociates to form O− over this energy range with a cross section which varies from 0.1 to 7.3×10−18 cm2 and exhibits structure indicative of the vibrational levels of a predissociating excited state. Structure is also observed in the O−4 photodestruction cross section which varies from 1.0 to 2.2×10−18 cm2, and in the O−2 photodetachment cross section.

Photodissociation and Photodetachment of Molecular Negative Ions. I. Ions Formed in CO2/H2O Mixtures.

A drift tube mass spectrometer and an argon ion laser have been used to study photon interactions with CO−3, CO−3⋅H2O, HCO−3, and HCO−3⋅H2O at discrete photon energies between 2.35 and 2.71 eV. CO−3 photodissociates into CO2+O− with a cross section which varies between 0.3 and 1.0×10−18 cm2 over this energy range. CO−3⋅H2O photodissociates into CO−3+H2O with a cross section near 2×10−18 cm2. HCO−3 and HCO−3⋅H2O have very small (and possibly zero) cross sections for photodestruction on this energy range.

Predissociation lifetimes of the rotational and fine structure levels of O2+(b4Σ−g, v=3,4,5)

A fast ion beam coaxial with a single mode laser has been used to measure the absorption linewidths of predissociated levels of O+2(b4Σg−, v=3,4,5) by detection of the O+ photofragments. Tuning of the absorption wavelength was accomplished by a combination of intracavity etalons and variation of the ion beam velocity (Doppler tuning). Lifetimes were determined for the b state fine structure levels of rotational levels N=31 and 33 of 16O2+(v=3), N=9–25 of 16O2+(v=4), N=14–27 of 16,18O2+(v=4), and N=5–7 of 16O2+(v=5). The lifetimes vary from 0.06 to 4 nsec. Variation with all relevant quantum numbers and with isotopic composition is discussed.

Electron‐impact dissociation of carbon monoxide

The electron‐impact dissociation of CO to form C and O atoms is observed in a crossed beam experiment at electron energies between the dissociation threshold (14 eV) and 198.5 eV. The center‐of‐mass energy released in the dissociation of individual molecules is explicitly measured using a position and time sensitive detector for the correlated neutral fragments. The observed energy release distribution is found to be highly structured, reflecting electron‐impact excitation to Rydberg states converging to CO+(X 2Σ+) which predissociate to ground state atoms. Little or no dissociation is observed from states above the first ionization limit. Total electron impact dissociation cross sections, exclusive of dissociative ionization contributions, and partial cross sections for the dissociative excitation of specific CO electronic states are presented.

N(4S0), N(2D0), and N(2P0) yields in predissociation of excited singlet states of N2

Predissociation in several singlet valence and Rydberg states of molecular nitrogen has been investigated using photofragment spectroscopy. We report here measurements of the yields of the atomic fragment products N(4S0), N(2D0), and N(2P0) from predissociation of specific rotational levels in the b’ 1Σu+ (v=9–13), c’ 1Σu+ (v=3,4), c 1Πu (v=3,4), and o 1Πu (v=3) states of N2. These states are prepared by laser excitation from the metastable a‘ 1Σg+ (v=0) level in a fast (3 keV) molecular beam. Correlated atomic fragments from single molecular dissociation events are monitored using a position‐ and time‐sensitive detector to obtain a complete and sensitive scheme for all possible N2 dissociation products. Dissociation of the N2 states is found to occur to N(2D0)+N(4S0), and N(2P0)+N(4S0) products; production of N(4S0)+N(4S0) is found not to occur from any of the states investigated here. Branching of the dissociation products between the two active limits is found to be strongly correlated with the energy ...