The ionization potentials of eithty‐nine molecules have been determined by photoionization measurements. The results are tabulated and compared with those obtained by spectroscopic and electron impact methods. Uncertainties in interpretation and several discrepancies are discussed.

Ionization Potentials of Some Molecules

General expressions are derived for calculating Franck—Condon factors for most transitions of polyatomic molecules (excluding transitions between linear and bent configurations) in the harmonic oscillator approximation. The derivation employs the method of generating functions and also linear transformation of normal mode coordinates between initial and final states. Input data required in the general case are geometries, frequencies, and vibrational force fields for the initial and final states. Explicit algebraic expressions are presented for the transition between linear, symmetric, triatomic initial and final states. The results are used to show that fragmentation cannot result from direct vertical ionization to the ground electronic states of CS2+ and CO2+ or to the excited 2IIu state of CO2+. An attempt was made to fit the experimental photoionization data for C2H2 and C2D2. This resulted in an estimated increase of 0.05 A in the equilibrium C–C bond distance and no change in the C–H bond distance i...

Franck—Condon Factors for Polyatomic Molecules

An electrostatic lens system which compensates for chromatic aberration has been tested in an electron spectrometer. The results indicate that this lens is suitable for comparisons of peak intensities in electron‐impact spectra. Relative intensities in vibrational progressions that belong to a single electronic transition have been studied in N2, CO, and NH3 and found to be nearly independent of the scattering angle. Electron‐impact spectra have been reported for helium, nitrogen, oxygen, argon, nitric oxide, nitrous oxide, ammonia, water vapor, carbon dioxide, ethylene, acetylene, and benzene at electron kinetic energies between 33 and 100 eV. Spectral regions of special interest are encountered in CO2 and C6H6. At excitation energies of 7–10 eV in CO2 a change in intensity distribution, attributed to transition from an electric‐quadrupole to an electric‐dipole spectrum, is observed as the kinetic energy is raised. In the case of C6H6 a change in the spectrum with angle is encountered which strongly sugg...

HIGH-RESOLUTION STUDY OF ELECTRON-IMPACT SPECTRA AT KINETIC ENERGIES BETWEEN 33 AND 100 eV AND SCATTERING ANGLES TO 16--.

This paper is devoted to a critical review of photoabsorption cross sections for molecules of aeronomic and astrophysical interest at wavelengths less than 3000 A. A discussion of the relative merits of various experimental techniques is given along with possible systematic and random errors that may be associated with them. The problems in data analysis associated with finite spectral bandwidths are reviewed, with special emphasis on the interpretation of published absorption cross sections. This review does not contain a complete set of cross-section-versus-wavelength values for each molecule; the prepared figures are used to compare the results of several determinations or to point out where difficulties of interpretation might arise. However, references to all papers believed to contain the more reliable data are given.

Critical review of ultraviolet photoabsorption cross sections for molecules of astrophysical and aeronomic interest.

Absorption coefficients of oxygen were measured at several hundred wavelengths in the region 1050–1900A with a one‐meter vacuum monochromator and a phosphor‐coated photomultiplier as detector. The absorption intensity of the Schumann‐Runge continuum was found to be somewhat less than previous results, and the intensity distribution was less symmetrical. The 19–0 and 20–0 bands of the Schumann‐Runge progression were observed and the dissociation limit of the B3Σu− state was 57140±60 cm−1. Several other bands and continua were found at shorter wavelengths.

Absorption Coefficients of Oxygen in the Vacuum Ultraviolet

The far ultraviolet absorption spectrum of oxygen has been photographed under high dispersion in the region 1300-650A. The bands are explained as going to the various excited states of $\mathrm{O}_{2}^{}{}_{}{}^{+}$ as limits and are attributed to the removal of $v\ensuremath{\pi}$, $\ensuremath{\omega}\ensuremath{\pi}$ and $x\ensuremath{\sigma}$ electrons, respectively, from the normal state of ${\mathrm{O}}_{2}$. The results strongly support Mulliken's assignment of the visible $\mathrm{O}_{2}^{}{}_{}{}^{+}$ bands to the transition $^{4}{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}\ensuremath{\rightarrow}^{4}\ensuremath{\Pi}_{u}$. The distances of the $^{4}\ensuremath{\Pi}_{u}$ and $^{4}{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}$ states of $\mathrm{O}_{2}^{}{}_{}{}^{+}$ from the ground state of ${\mathrm{O}}_{2}$ are given as 16.1 and 18.2 volts, respectively, with possible errors of about a tenth of a volt. The appearance and nonappearance of strong electronic series going to known ionization potentials are also discussed.

The Far Ultraviolet Absorption Spectrum of Oxygen

An absorption spectrum of acetylene in the region 1520A to 1050A has been found. It appears at very low pressures. The analysis was made by obtaining the spectrum of acetylene made from about 50 percent heavy water. The small isotopic shifts of many bands revealed them to be vibrationless electronic transitions. These are arranged into two Rydberg series going to the same limit which corresponds to an ionization potential of 11.35 volts. The upper levels of the two series are designated as $\mathrm{np}\ensuremath{\Sigma}$ and $\mathrm{np}\ensuremath{\Pi}$ levels. The excitation and ionization are from the CC bond and only vibrations affecting this bond appear. The character of the vibrations present is deduced from the magnitude of the isotopic shifts. Strong predissociation occurs around 1520A. This yields a value of about 187 Cal/mole for the strength of the triple CC bond. A similar set of bands appears in ethylene in the region 1750A to 1200A. The bands are more diffuse than those in acetylene, and only one strong Rydberg series was discovered. The limit of the series corresponds to an ionization potential of 10.41 volts. The vibrational analysis is similar to that in acetylene. The spectrum of ethane is extremely diffuse and no analysis of it could be made. It is concluded that, as in methane, all the upper states are unstable.

W. C. Price

Papers

The Far Ultraviolet Absorption Spectrum of Oxygen

The Absorption Spectra of Acetylene, Ethylene and Ethane in the Far Ultraviolet

A Source of the Lyman Continuum for Use with Spectrographs of High Dispersion

The Ultraviolet Absorption of Acetylene and Ethylene