Deperturbation and assignment of the vibronically tangled spectrum of pyrene in a biphenyl matrix

We apply the technique of the Green's function to the vibronically tangled spectrum of pyrene in a biphenyl matrix. We can deperturb the spectrum mathematically and readily assign the resulting weighted density of states. The analysis is reasonably successful and agrees with ground-state data and estimates of the excited-state frequencies previously made by others.     

Observation of localized modes in TbO x using the Mößbauer effect

The resonant absorption of the 58 keV Mößbauer line under excitation of one phonon states in TbO _x was investigated. Using an ultra centrifuge, theγ energy was shifted by ±25meV. Narrow absorption lines at 14.1 meV, 16.0 meV and presumably at 12.3 meV are interpreted as being caused by localized modes of the terbium ion.     

Effective cross sections and rate constants for predissociation of ClO in the earth's atmosphere

Line-by-line absorption cross sections have been computed at 220° and 300°K for the $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{?A}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, v″ = 0 to v″ = 3 progressions of ClO. When the 300°K cross sections are convolved with a Gaussian function of 2 Å bandwidth, a maximum of 10.6 mb is observed at 277.4 nm, corresponding to the band head of the v″ = 0 to v′ = 11 transition. To calculate effective rate constants for predissociation of the $\text{A}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ state of ClO at altitudes ranging from 20 to 50km, the absorption cross sections were combined with estimates of the solar flux that penetrates to each altitude. The rate of predissociation is calculated to be much greater than the rate of direct photodissociation, but the former is still small compared to the loss rate of ClO caused by reactions with NO and O. Therefore, the contribution of predissociation to the destruction of ClO accounts for at most 2% of the total ClO loss. Further, calculations with a simplified chemical model indicate that the effect of ClO predissociation is to lower the rate of ozone depletion at all altitudes by a maximum of only 1%.     

Valence-shell momentum distributions and ionization energies of carbon disulfide

Valence-shell binding energy spectra and momentum distributions of CS₂ have been measured using non-coplanar symmetric binary (e,2e) spectroscopy. The present measurements are compared with previously published binding energy spectra calculated using the many body 2ph-TDA Green's function (GF) method and the symmetry-adapted cluster configuration-interaction (SAC CI) method. The measured and the calculated binding energy spectra both show extensive population splittings particularly above 20 eV, confirming a significant breakdown of independent particle ionization picture. A relatively strong-outer valence many-body state at 17.0 eV is shown to be satellite of the (2π₀)^?1 state, in accord with earlier conclusions of photoelectron studies. Momentum distributions measured at several carefully chosen binding energies are compared with the corresponding molecular orbital momentum distributions calculated using small and extended gaussian basis sets. The good qualitative agreement between momentum distributions measured in the inner-valence region wth theoretical 4σ_m and 5σ_g orbital momentum distributions confirms the qualitative predictions of satellite parentages by GF and SAC CI calculations. Momentum and position density contour maps of individual orbitals are used to interpret the shapes and atomic characters of the experimental momentum distributions. Momentum densities of the valence orbitals of CS₂ are compared with those of the respective valence isoelectronic species CO₂     

Improved gain on the Cl2(D′3 П g −A′3 П u ) transition at 258 nm by halogen donor mixing

The temporal development of the small-signal gain on the Cl₂(D–A) transition at 258 nm has been investigated by means of an amplified spontaneous emission (ASE) technique. For electron beam pumped He/Cl₂ mixtures, the gain appears only at the end of the pumping pulse, whereas for He/Cl₂/CCl₄ mixtures the temporal gain profile coincides with the fluorescence pulse, and the maximum gain coefficient increases by about a factor of two. The observed effects are due to the mixing of both halogen donors and can be explained by considering the quenching of theD-state by electrons.     

Theoretical study of the6Σ+,6Π, and4Σ+ van der Waals states of NO

Summary Potential energy curves for the weakly bound⁶⁺,⁶, and⁴⁺ states of NO are presented at various levels of correlation treatment. The binding energies for the van der Waals minima vary from about 30 cm^–1 for the⁶⁺ state to about 20 cm^–1 for the⁴⁺ and⁶ states. We investigate the importance of constraining the wave function to dissociate to a spherically symmetric O atom where the oxygen 2p orbitals are equivalent. For high levels of correlation treatment, we find that these restrictions have little effect on the potential, while greatly increasing the length of the CI expansion.     

Full configuration interaction benchmark calculations for transition moments

Full configuration-interaction (FCI) calculations have been performed for the ã ¹A₁–b¹B₁and ã ¹A₁–(2)¹A₁transitions in CH₂ and for selected dipole and quadrupole transitions in BeO. The FCI transition moments are compared to those obtained from correlation treatments that truncate the n-particle expansion. The state-averaged MCSCF/SOCI and FCI results agree well, even for BeO, where the CASSCF level nonorthogonal transition moment differs from the state-averaged CASSCF transition moment.     

Photoabsorption in H2O: Stieltjes-Tchebycheff calculations in the time-dependent Hartree-Fock approximation

Total and partial-channel photoabsorption cross-section calculations in H₂O are reported employing the time-dependent Hartree-Fock approximation and Stieltjes-Tchebycheff imaging procedures, and comparisons are made with the corresponding results of recent (e, 2e), (e, e + ion), and synchrotron-radiation dipole oscillator-strength studies.     

Computation of electronic transition moments: the length versus the velocity representation

Summary We have compared transition moments (TMs) obtained using the length and velocity representations for transitions from the ground state of H₂ to the lowest two¹ _u and two¹ _u ⁺ Rydberg states, theA ¹–X ¹⁺ transition in BH, and theA ¹ _u –X ¹ _g ⁺ transition in C₂. For H₂, the TMs in the length and velocity representations agree well even in cases where the one-particle basis is incomplete and the TM has not converged. For BH and C₂ the TM in the length representation converges rapidly with improvements in the one-particle basis set and is insensitive to inner-shell correlation. In contrast, in the velocity representation convergence with improvements in the one-particle basis is much slower, especially for C₂, and the TMs are significantly changed by inner-shell correlation. Thus the difference between the TMs in the length and velocity representations would not appear to be a viable diagnostic of TM convergence.