Search results for " molecule"
showing 10 items of 1523 documents
SOFT X-RAY IRRADIATION OF PURE CARBON MONOXIDE INTERSTELLAR ICE ANALOGUES
2012
There is an increasing evidence for the existence of large organic molecules in the interstellar and circumstellar medium. Very few among such species are readily formed in conventional gas-phase chemistry under typical conditions of interstellar clouds. Attention has therefore focused on interstellar ices as a potential source of these relatively complex species. Laboratory experiments show that irradiation of interstellar ice analogues by fast particles or ultraviolet radiation can induce significant chemical complexity. However, stars are sources of intense X-rays at almost every stage of their formation and evolution. Such radiation may thus provide chemical changes in regions where ult…
Convergence of Nuclear Magnetic Shieldings in the Kohn-Sham Limit for Several Small Molecules.
2015
Convergence patterns and limiting values of isotropic nuclear magnetic shieldings were studied for several small molecules (N2, CO, CO2, NH3, CH4, C2H2, C2H4, C2H6, and C6H6) in the Kohn-Sham limit. Individual results of calculations using dedicated families of Jensen's basis sets (pcS-n and pcJ-n) were fitted toward the complete basis set limit (CBS) using a simple two-parameter formula. Several density functionals were used; calculated vibrational corrections (ZPV) applied; and, for comparison purposes, similar calculations performed using RHF, MP2, SOPPA, SOPPA(CCSD), and CCSD(T) methods and additionally, the aug-cc-pVTZ-J basis set. Finally, the CBS estimated results were critically com…
Laser spatial profile effects in measurements of impulsive molecular alignment
2005
0953-4075; We use a non-intrusive technique based on a polarization scheme to probe the periodic alignment of the CO2 and N2 molecules induced by a strong laser pulse linearly polarized. A weak probe field experiences the birefringence resulting from the alignment of the molecules. By comparing the probe depolarization signal with the numerical simulation of the time-dependent Schrödinger equation, it is possible to quantify the alignment of the molecular sample. The modelling takes into account the spatial profile of the pump intensity. It allows from the alignment signal to determine the onset of saturation due to ionization, despite the intrinsic saturation of the alignment arising for a…
Nucleon Pairing and Seniority
2007
Until now we have been dealing with particle and hole aspects of nuclear structure. In this second part of the book we go farther away from a closed major shell. Still near the beginning or end of a major shell we encounter vibrational, spherical open-shell nuclei that cannot be described in terms of a few particles or holes. Farther towards the middle of the shell the spherical shape will give way to permanent deformation signalled by rotational bands analogous to those of diatomic molecules. Microscopic description of such nuclei requires a deformed mean field as the starting point. Deformed nuclei will not be considered in any detail in this book.
Perturbative treatment of spin-orbit coupling within spin-free exact two-component theory.
2014
This work deals with the perturbative treatment of spin-orbit-coupling (SOC) effects within the spin-free exact two-component theory in its one-electron variant (SFX2C-1e). We investigate two schemes for constructing the SFX2C-1e SOC matrix: the SFX2C-1e+SOC [der] scheme defines the SOC matrix elements based on SFX2C-1e analytic-derivative theory, hereby treating the SOC integrals as the perturbation; the SFX2C-1e+SOC [fd] scheme takes the difference between the X2C-1e and SFX2C-1e Hamiltonian matrices as the SOC perturbation. Furthermore, a mean-field approach in the SFX2C-1e framework is formulated and implemented to efficiently include two-electron SOC effects. Systematic approximations …
Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules
1997
Using molecular dynamics computer simulations, we investigate the dynamics of the rotational degrees of freedom in a supercooled system composed of rigid, diatomic molecules. The interaction between the molecules is given by the sum of interaction-site potentials of the Lennard-Jones type. In agreement with mode-coupling theory (MCT), we find that the relaxation times of the orientational time correlation functions C_1^(s), C_2^(s) and C_1 show at low temperatures a power-law with the same critical temperature T_c, and which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT we find, however, that for these correlators the time-tempera…
Quantum mechanical study of the proton exchange in the ortho-para H2 conversion reaction at low temperature
2011
Ortho-para H2 conversion reactions mediated by the exchange of a H+ proton have been investigated at very low energy for the first time by means of a time independent quantum mechanical (TIQM) approach. State-to-state probabilities and cross sections for H+ + H 2 (v = 0, j = 0,1) processes have been calculated for a collision energy, Ec, ranging between 10-6 eV and 0.1 eV. Differential cross sections (DCSs) for H+ + H2 (v = 0, j = 1) → H+ + H2 (v′ = 0, j′ = 0) for very low energies only start to develop a proper global minimum around the sideways scattering direction ( ≈ 90°) at Ec = 10-3 eV. Rate coefficients, a crucial information required for astrophysical models, are provided between 10…
Ionization of atoms by slow heavy particles, including dark matter
2016
Atoms and molecules can become ionized during the scattering of a slow, heavy particle off a bound electron. Such an interaction involving leptophilic weakly interacting massive particles (WIMPs) is a promising possible explanation for the anomalous 9 sigma annual modulation in the DAMA dark matter direct detection experiment [R. Bernabei et al., Eur. Phys. J. C 73, 2648 (2013)]. We demonstrate the applicability of the Born approximation for such an interaction by showing its equivalence to the semiclassical adiabatic treatment of atomic ionization by slow-moving WIMPs. Conventional wisdom has it that the ionization probability for such a process should be exponentially small. We show, howe…
Full triples contribution in coupled-cluster and equation-of-motion coupled-cluster methods for atoms and molecules in strong magnetic fields
2020
Coupled-cluster as well as equation-of-motion coupled-cluster methods play an important role whenever high accuracy is warranted. Concerning excitation energies, consideration of triple excitations is typically required to reach an accuracy better than 0.1–0.3 eV. In the context of strong magnetic fields such accuracy is needed for the prediction of spectra of strongly magnetized White Dwarfs. In addition it turns out that in order to correctly model the behavior of energies with respect to the magnetic field strength, triple excitations are required. Due to avoided crossings which are extremely often encountered in the context of strong magnetic fields, double-excitation character can be t…
Spin-restricted open-shell coupled-cluster theory for excited states
2000
Using a linear-response approach, the recently introduced spin-restricted coupled-cluster (SR-CC) theory is extended to the treatment of excited states of high-spin open-shell molecules. Explicit equations are given within the usual singles and doubles approximation and our implementation (within an existing spin–orbital code) is described. It is shown that in SR-CC theory, due to spin constraints, the spin-expectation value for the excited states calculated as corresponding energy derivatives always corresponds to the exact value. In addition, the SR-CC singles and doubles (SR-CCSD) approach is extended to include also the so-called pseudotriple excitations (best described as double excita…