Gerald A. Segal

Theoretical Interpretation of the Optical and Electron Scattering Spectra of H2O

Energies and potential surface characteristics are assigned to the first eight excited states of the water molecule. This assignment is shown to be consistent with all data from optical spectra, electron scattering, rotational distributions of the OH fragment in photodissociation and associated data, and with semi‐empirical INDO calculations. Energies and potential surfaces are given for the lowest resonant states of H2O−. These are consistent within the explainable error of the INDO calculations, as well as with the data on dissociative attachment and associative detachment in which H2O− is an intemediate for species. Assignment‐confirming experiments are suggested.

Theoretical Interpretation of the Electron Scattering Spectrum of CO2

A theoretical study of processes observed in electron–CO2 scattering experiments is presented. Potential curves for CO2− are calculated to determine which scattering processes proceed through formation of short‐lived negative molecule–ion compound states. The four dissociative attachment processes occurring below 20‐eV incident energy are shown to result from four different electronic states of CO2−. Vibrational excitation of CO2 in the 3.0–4.5‐eV incident energy range is shown to proceed through compound‐state formation. The calculated energies of the compound states are shown to agree with the absorption spectra of CO2− trapped in crystalline solids. The inelastic scattering below 3.0 eV is shown to be nonresonant and of a direct nature. For this process the leading term in the angular dependence of the cross section is determined by dipole selection rules, which leads to strong forward peaking. For small‐angle scattering the cross section is shown to be about 10−16 − 10−17 cm2 in magnitude. Further, th...

Assignments in the electronic spectrum of water

To explain the inelastic feature at 4.5 eV in the spectrum of water and to study its spectrum in some detail, we have carried out several calculations on the excited states of water using the equations‐of‐motion method. We conclude that the calculated vertical excitation energy of 6.9 eV for the ^3B_1 state corresponds to the strong feature at 7.2 eV observed in low‐energy electron scattering spectrum. The 4.5 eV inelastic process almost certainly does not correspond to a vertical excitation of water at the ground state geometry. The other excitation energies and oscillator strengths agree well with experiment.

The electronic spectrum of NOCl: Photofragment spectroscopy, vector correlations, and abinitio calculations

The electronic absorption spectrum of NOCl in the region 620–180 nm is assigned by using vector properties of the NO photofragment and the results of ab initio calculations at the CI level. In assigning the electronic spectrum, we take into account the recoil anisotropy, rotational alignment, and Λ‐doublet populations of NO, as well as the calculated vertical excitation energies, oscillator strengths, and the nature of the orbitals involved in the transitions. In the experiments, we use expansion‐cooled samples and measure the recoil anisotropy parameters from the Doppler profiles of selected NO A 2Σ+←X 2Π rotational lines. The alignment parameters and Λ‐doublet populations are derived from the rotational spectra using different laser polarizations and excitation–detection geometries. The theoretical calculations treat all low‐lying singlet and triplet states. The calculations yield least energy paths for the excited states, with optimized rNO and ClNO angle as a function of rClN, as well as the angular d...

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.

Efficient methods for configuration interaction calculations

Abstract Advanced techniques are developed to provide efficient economic treatment of the large scale eigenvalue problem posed when configuration interaction is carried out on SCF basis sets of moderate size. When the characteristic properties of the hamiltonian matrix are examined in light of the type of solution required, partitioning of the configuration space is shown to result in an expansion of the problem about a limited core of states, where the small but cumulative interactions of vast regions of the remaining space are reduced to the form of an effective potential. With proper selection of the core, the evaluation of this potential can be readily and accurately truncated to a level involving minimum expenditure in time and effort. In particular only diagonal elements and a strip of the full CI matrix are required to achieve an accuracy of 1 – 5 kcal/mole with complete treatment for configuration spaces of order tens of thousands. In addition, a close look at current theory on the generation of matrix elements between spin symmetry adapted configurations leads to simplified expressions where the matrix elements are derived in the form of a weighted sum of molecular integrals in which the weighting coefficients represent the integrated value of the wavefunctions over spin coordinates. For typical cases of low multiplicity and limited numbers of open shells the list of unique parameters needed to generate all weights are shown to be readily stored as a program library. Actual times for matrix element generation are believed to be an order of magnitude faster than current techniques. Practical demonstration of the accuracy and efficiency of the method is provided by calculations on formaldehyde, water, and ethylene.

The theory of vibrational circular dichroism: trans-1,2-dideuteriocyclobutane and propylene oxide

Abstract A rigorous theory of vibrational rotational strengths was recently developed by Stephens. We report the first comparisons of calculations using this theory with experimental vibrational circular dichroism data. The theory is implemented using SCF MO electronic wavefunctions and a 4-31G basis set. The molecules studied are trans-l,2-dideuteriocyclobutane and propylene oxide. Encouraging agreement between theory and experiment is obtained.

Calculation of Wavefunctions for the Excited States of Polyatomic Molecules

A new method for the calculation of the wavefunctions of the excited states of polyatomic molecules is presented. The method is independent of the symmetry of the state under consideration. Sample calculations on a number of molecules have been carried out in the INDO approximation, and the results, which represent a considerable improvement over comparable configuration interaction calculations, are presented.

An efficient method for large scale configuration interaction calculations

Abstract An efficient method of handling large scale configuration interaction calculations is developed and applied to the H 2 O molecule as a test case. The method, which is based upon matrix partitioning, is shown to be capable of calculating the 1 B 1 spectrum of H 2 O to an accuracy level of 0.1 eV for each state with very moderate computational effort.

Configuration interaction calculation on the resonance states of HCl

Configuration interaction and the stabilization method is used to compute potential energy curves for the resonant states of HCl−, an important example of electron–polar molecule scattering. Resonant states that dissociate to H−+Cl and Cl−+H are found as well as those that dissociate to H+Cl+e−. These curves provide an interpretation of the known experimental observations on this system.