Generalized oscillator strengths of carbon disulfide calculated by multireference configuration interaction

Theoretical Study of Valence Shell Excitation by Electron Impact in CCl4

This paper reports a theoretical study of valence shell excitation in CCl₄ by high-energy electron impact. Generalized oscillator strengths are calculated for the molecule at the equation-of-motion coupled-cluster singles and doubles level. To elucidate the influence of nuclear dynamics on electron excitation cross-sections, the effects of molecular vibration are included in the calculation. Based on a comparison with recent experimental data, several reassignments of spectral features are made, and it is found that excitations from the Cl 3p nonbonding orbitals to σ* antibonding orbitals, 7a₁ and 8t₂, play dominant roles below the excitation energy of ∼9 eV. Furthermore, the calculations reveal that distortion of the molecular structure due to the asymmetric stretching vibration significantly affects the valence excitations at small momentum transfers, where contributions from dipole transitions are dominant. It indicates that vibrational effects have a considerable influence on Cl formation in the photolysis of CCl₄.

Extracting Inelastic Scattering Cross Sections for Finite and Aperiodic Materials from Electronic Dynamics Simulations

Explicit time-dependent electronic structure theory methods are increasingly prevalent in the areas of condensed matter physics and quantum chemistry, with the broad-band optical absorptivity of molecular and small condensed-phase systems nowadays routinely studied with such approaches. In this paper, it is demonstrated that electronic dynamics simulations can similarly be employed to study cross sections for the scattering-induced electronic excitations probed in nonresonant inelastic X-ray scattering and momentum-resolved electron energy loss spectroscopies. A method is put forth for evaluating the electronic dynamic structure factor, which involves the application of a momentum boost-type perturbation and transformation of the resulting reciprocal space density fluctuations into the frequency domain. Good agreement is first demonstrated between the dynamic structure factor extracted from these electronic dynamics simulations and the corresponding transition matrix elements from linear response theory. The method is then applied to some extended (quasi)one-dimensional systems, for which the wave vector becomes a good quantum number in the thermodynamic limit. Finally, the dispersion of many-body excitations in a series of hydrogen-terminated graphene flakes (and twisted bilayers thereof) is investigated to highlight the utility of the presented approach for capturing morphology-dependent effects in the inelastic scattering cross sections of nanostructured and/or noncrystalline materials.

Comparison among several vibronic coupling methods

A comparison of four approaches to account the vibronic coupling in photoabsorption is performed. The methods considered are nuclear ensemble (NE), direct vibronic coupling (DVC), adiabatic Hessian (AH), and vertical gradient (VG). The case study is the symmetry-forbidden [Formula: see text] [Formula: see text]A[Formula: see text] [Formula: see text] [Formula: see text] [Formula: see text]A[Formula: see text] (n [Formula: see text] [Formula: see text]) transition in formaldehyde. Being forbidden in the equilibrium geometry, this transition is entirely induced by vibronic coupling and constitutes an appropriate case to study the performance of different methods. From DVC, it is found that mode 1 (C=O out-of-plane bending) is the most inducing, followed by mode 6 (in-plane C-H asymmetric stretching) and finally by mode 2 (in-plane C-H asymmetric bending). We were able to correlate 17 out of 20 structures obtained from NE with these modes, showing that these two methods, although different in principle, give comparable results. The simulated spectra were obtained for all methods and compared, and each one has its own advantage. In what concerns the transition studied, NE gives the best description of the spectrum, DVC is the only one that easily gives an absolute value for OOS, and AH and VG are the computationally less expensive methods. From the latter two, VG is the less demanding on computational grounds, since it does not require the excited state Hessian.

Generalized Oscillator Strengths for the Valence Shell Excitations in Carbon Tetrachloride Studied by Fast Electron Impact

A joint experimental and theoretical investigation of the valence shell excitations of carbon tetrachloride has been performed by fast electron scattering and time dependent density functional theory calculations. At a collision energy of 1.5 keV and an energy resolution of about 70 meV, the dipole-forbidden transition of a₁σ* ← 2t₁ has been clearly observed at large momentum transfers, and its excitation energy of 6.15 eV and line width of 0.72 eV have been determined. Two new features are also recognized at 9.97 and 10.26 eV. The generalized oscillator strengths of the excited states at 5-11.3 eV have been determined from the measured spectra. The calculated generalized oscillator strength of the a₁σ* ← 2t₁ transition with the vibronic effect shows better agreement with the experiment, and the vibronic effect also accounts for its nonzero intensity at zero squared momentum transfer. The optical oscillator strengths of the valence shell excitations have also been obtained by extrapolating the generalized oscillator strengths to the limit of zero squared momentum transfer. The integral cross sections have been systematically determined from the threshold to 5000 eV by means of the BE-scaling method. The present oscillator strengths and cross sections provide the fundamental data of carbon tetrachloride and have important applications in photochemical modeling for atmospheric physics.

Electron-molecule collisions with explicit rovibrational resolution at MRCI level and using even tempered basis sets

A method for calculating the generalized oscillator strengths (GOSs) and differential cross section (DCS) with vibration and rotation resolution is presented. The importance of accounting for the rotational contribution is to be emphasized since it has not previously been considered in GOS calculations. Although largely neglected due to its small effect on various properties, the rotational resolution proved to be fundamental in the study of certain phenomena, such as the interference between rotational states in a molecule. As the general goal of this work is to obtain theoretical values comparable to high resolution experiments, special care was taken on the calculation of the electronic part of the scattering amplitude, particularly in what concerns the choice of the atomic basis set. Accordingly, even-tempered basis sets have proved to lead to good results. The helium atom was taken as a model system for this aspect of the problem. Then, GOS and DCS, for explicit vibrational and rotational transitions, were calculated for hydrogen and nitrogen molecules. For higher accuracy, a non-Franck-Condon approach was used to obtain transitions involving vibrational states. The resultant values have shown good agreement with the available experimental data.

Ultrafast Extreme Ultraviolet Photoelectron Spectroscopy of Nonadiabatic Photodissociation of CS2 from 1B2 (1Σu+) State: Product Formation via an Intermediate Electronic State

We studied nonadiabatic dissociation of CS₂ from the ¹B₂ (¹Σ_u⁺) state using ultrafast extreme ultraviolet photoelectron spectroscopy. A deep UV (200 nm) laser using the filamentation four-wave mixing method and an extreme UV (21.7 eV) laser using the high-order harmonic generation method were employed to achieve the pump-probe laser cross-correlation time of 48 fs. Spectra measured with a high signal-to-noise ratio revealed clear dynamical features of vibrational wave packet motion in the ¹B₂ state; its electronic decay to lower electronic state(s) within 630 fs; and dissociation into S(¹D₂), S(³P_J), and CS fragments within 300 fs. The results suggest that both singlet and triplet dissociation occur via intermediate electronic state(s) produced by electronic relaxation from the ¹B₂ (¹Σ_u⁺) state.