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The contribution to the intensity of atomic and molecular electronic transitions beyond the Hartree—Fock, or independent‐particle model, is examined by means of ``many‐electron theory'' (MET). The contribution is of first order in the wavefunctions of the combining states.MET analysis of the off‐diagonal matrix element for the transition probability in the frozen‐core approximation yields physically interesting expressions which are applied to atomic and molecular examples. Considerable improvement in calculated atomic (e.g., Be resonance transition) and molecular (e.g., π—π* transitions) intensities are obtained when analysis shows that ground‐state ``hole pairs'' are strongly scattered into the orbital sea of the excited state.The uses of energies and operators in the calculation of electronic intensities is also examined. The relation between the expectation values of operators ∇ and r which replaces the widely used off‐diagonal hypervirial theorem in the absence of exact wavefunctions is given.

The semi-empirical free electron theory with inclusion of interelectronic and electron-nuclear molecular integrals is used to examine the spectroscopic properties of the trans-polyenes. The molecular integrals were calculated at many values ofQ, the free electron theory length parameter. Comparisons are made to equivalent LCAO-MO methods, especially in regard to molecular coulomb and exchange integrals, singlet-triplet separations and parametric dependence. The FET viriel theorem is examined.

The LCAO–MO–CI method is applied to the study of the electronic transition moment of the Lyman bands of H2 (B 1Σu+–X 1Σg+) over the range of internuclear separations 1 ≤ R ≤ 6 a.u. It is found that configuration interaction is responsible for a strong nonlinear variation of the transition moment with internuclear distance (maximum transition moment at 3.0 a.u.). Comparison with other calculations indicates that the method is applicable to more complicated molecules and permits the examination of how individual electron correlation effects contribute to the transition moment.

The characteristic features of the Rydberg transitions in a polyatomic molecule are examined with emphasis on their extravalence‐shell nature. It is made plausible that in molecular orbital perturbation theory the corresponding Hartree—Fock atomic orbital of principal quantum number n is a zero‐order approx‐imation to the Rydberg orbital. Calculations without integral approximations are carried out for the low‐lying states of H2O. The ASP—SCF—LCAO—MO treatment of the H2O ground state provides the core. The mean singlet—triplet absorption spectrum of H2O is fairly well reproduced by the calculation. The calculated oscillator strength of B1(3s)←A1, 1×10—2, and the calculated symmetry‐induced splitting of the 3p Hartree—Fock atomic orbital in C2v symmetry into B1 and A1 components, 2200 cm—1, are also in satisfactory agreement with experiment, 3×10—2 and 1500 cm—1, respectively.

The electronic absorption spectrum of NF3 was investigated for the first time. At the lowest pressures, the long wavelength limit of absorption is at about 960 A in agreement with the lowest ionization potential of the molecule (939 A). Quantitative intensity measurements showed a smooth decrease of the absorption coefficients to effectively zero at about 1800 A, with no indication of even partly resolved electronic transitions. Comparisons with the electronic spectrum of ammonia are discussed. The molecular dipole moment is used to obtain an equivalent orbital representation of the NF3 wave function.

Dipole strengths of various heteronuclear diatomic molecules are calculated for high and low energy transitions of one spectral type, 1Σ-1Σ, σ-σ. Through the use of increasingly accurate ground state wavefunctions it is possible to display the sensitivity of the transition probabilities to known approximations in the wavefunction. Selfconsistent field and configuration interaction effects are compared and contrasted.

Cyclopentanone vapor, when excited by light of wavelength 3130 Å, gives rise to four different photoproducts and fluorescence. The photochemistry of the molecule has been investigated by Srinivasan (J. Am. Chem. Soc. 81, 1546 (1959); 83, 4344, 4348 (1961)). In the present work the fluorescence spectrum and fluorescence efficiency of cyclopentanone vapor are reported for the first time. The fluorescence spectrum is identical with that obtained from liquid solutions of cyclopentanone, with a maximum at 4100 Å. Time, temperature, oxygen, and inert gases do not affect the spectrum, which contains no component ascribable to phosphorescence, maximum 4450 Å. Beer's law is obeyed.The fluorescence efficiency is also found to be independent of the variables temperature and pressure of inert gas, cyclopentanone, and oxygen. This is in marked contrast with the behavior of the photochemical yields (see work by Srinivasan). The yields of cyclobutane and ethylene tend to increase at high vibrational energy content of the excited molecule (low pressure, high temperature, short exciting wavelengths), while the yield of pentenal increases under conditions of decreasing vibrational energy content (high pressures, etc.). The fact that fluorescence stabilization does not occur in cyclopentanone can be related to the results of photolysis.

Energy factors, operators, and wave functions used in the calculation of electronic oscillator strengths are critically examined. The role of electron correlation in such calculations is emphasized. Oscillator strengths for the Mulliken bands of C 2 ( 1 Σ g + - 1 Σ u + ) have been calculated from wave functions including configuration interaction.

The spectrum of phosgene was investigated, for the first time, over the region 2000–600 A with a dispersion of about 4.15 A/mm. A number of resolved transitions were found, some with vibrational structure. The vibrational structure was in each case interpreted as a progression in a totally symmetrical frequency. A tentative interpretation of the electronic transitions is given.