D. N. Trubnikov

Ion-exchanged binuclear Ca2OX clusters, X = 1–4, as active sites of selective oxidation over MOR and FAU zeolites

A new series of calcium oxide clusters Ca2OX (X = 1–4) at cationic positions of mordenite (MOR) and faujasite (FAU) is studied via the isolated cluster approach. Active oxide framework fragments are represented via 8‐membered window (8R) in MOR, and two 6R and 4R windows (6R+4R) possessing one common SiOSi moiety in FAU. Structural similarities between the Ca2OX(8R) and Ca2OX(6R+4R) moieties are considered up to X = 4. High oxidation possibilities of the Ca2O2(nR) and Ca2O3(nR) systems are demonstrated relative to CO, whose oxidation over the Ca‐exchanged zeolite forms is well studied experimentally. Relevance of the oxide cluster models with respect to trapping and desorption of singlet dioxygen is discussed.

Convergence of electric field and electric field gradient versus atomic basis sets in all-siliceous and Mg substituted phillipsites

Electrostatic potential (EP), electric field (EF), and electric field gradient (EFG) values are calculated in periodic models of magnesium substituted phillipsite (MgPHI) zeolite forms using periodic DFT (PDFT) hybrid B3LYP level with fourteen different basis sets. Relative root mean square differences between the EP, EF, or EFG values estimated between different basis sets are evaluated in several spatial domains available for adsorbate molecules in the zeolite. In these areas, the EF increase in MgPHI is evaluated relative to all‐siliceous PHI types. The EP is interpreted in terms of framework ionicity for MgPHI and all‐siliceous PHI models. Angular SiOSi dependence of the EFG asymmetry at 17O atoms in all‐siliceous zeolites is discussed.

Explicitly correlated similarity transformed equation-of-motion coupled-cluster method

Similarity transformed equation-of-motion method, based on linearly approximated explicitly correlated coupled-cluster singles and doubles [CCSD(F12)] model, has been formulated and implemented. An extension of similarity transformation operator is introduced in order to treat short-range correlation effects for excited states. Additionally, effective reduction of the number of active virtuals can be obtained by such modification. Numerical tests for sets of valence and Rydberg excited states of several molecules are conducted. Statistical measures of errors in excitation energies show that explicitly correlated results are accurate up to 0.1 e.V already at a double-ζ level compared to those in the complete basis set limit, if the excitation energy is not too close to an ionization threshold. An example of long-range charge transfer excitation is also considered and highly accurate results are obtained.

Effective generation of characteristic K-rays from large laser-excited SF6 clusters in the presence of an Ar carrier gas

The parameters of characteristic sulfur Kα-rays generated from SF6 clusters that are surrounded by argon atoms and are irradiated by intense laser radiation have been analyzed. It has been found that the formation of large SF6 clusters under the optimum experimental parameters is accompanied by the high-efficiency generation of the characteristic X rays, and the flux density of the characteristic X-ray photons is 100 photons/(mrad2 pulse) at a laser-pulse energy of 5 mJ. It has been shown that the third-harmonic generation process can be used to characterize the spatial sizes of the gas-cluster jet and the region of the cluster plasma.

Study of PbTe(Ga) evaporation using a nanosecond pulsed laser

Abstract Vapor composition and temperature, and time-of-flight (TOF) distributions of the vaporized particles have been studied for vacuum evaporation of PbTe(Ga) by pulses of a Nd 3+ :YAG laser. A quadrupole mass spectrometer and the optical multichannel analyser (emission spectroscopy) have been used for this purpose. It has been observed that the composition of the evaporated substance, ion-neutral particle ratio and TOF distributions depended strongly on the laser pulse energy density w in the region of w = 0.47–4.8 J/cm 2 . Estimations of the kinetic energies of the ions have been carried out. The laser plasma temperature changed from 8000 to 4500 K at w = 4.8 J/cm 2 for a space of 25 mm from the target surface along the normal.

Electric multipole moments calculation with explicitly correlated coupled-cluster wavefunctions

A method of calculation of expectation values of dipole and quadrupole moments with wavefunctions, corresponding to linearly approximated explicitly correlated coupled-cluster singles and doubles [CCSD(F12)] model has been formulated and implemented. As a part of algorithm, explicitly correlated version of Λ equations has also been derived and implemented. Numerical tests, conducted for sets of molecules, show that explicitly correlated results for expectation values of dipole moment are accurate up to 0.01 a.u. already at a double-ζ level compared to those in the complete basis set limit. The corresponding results for quadrupole moments at double-ζ level are accurate up to 0.1 a.u., while for the triple-ζ bases errors do not exceed 0.01 a.u.

Internal (SiH)X groups, X = 1–4, in microcrystalline hydrogenated silicon and their IR spectra on the basis of periodic DFT modelling

Vibrational Si–H frequencies were calculated on the basis of density functional theory (DFT) using periodic boundary conditions for N-Si voids, N < 8, in microcrystalline hydrogenated silicon (MHS) and (100), (110), and (111) slabs of 8, 5, and 8 layers, respectively, with the dangling bonds being saturated with hydrogen atoms. The slabs are considered as the models of inter-grain boundaries (IGB) in MHS. The N-Si voids of different shapes have been obtained via random deleting N silicon atoms. It was shown that the high stretching modes (HSM) of Si–H vibrations, which are usually assigned to SiHX, appear also due to (SiH)X groups, X = 2–4, in the N-Si voids. No such (SiH)X groups were formed with X > 1 at the IGB. The low stretching modes (LSM) are thus assigned to Si–H groups presented at both N-Si voids and IGB. Similar relative stability of the voids is obtained with two different DFT approaches, i.e., B3LYP with atomic basis set and Perdew-Burke-Ernzerhof (PBE) with plane wave basis set. This result allows a simple interpretation of usually small IHSM/(ILSM + IHSM) intensity ratio as a consequence of minor concentration of any voids in device quality MHS.

A method of calculation of the thermodynamic properties of Ar3

Abstract Calculation of the complete energy spectrum of the quantum system Ar 3 is extremely difficult. The reason is the strong nonlinearity that entangles the modes of internal motion even at the lowest energy levels of the system. One can say that vibrations of the trimer are never small and that its vibrational modes are never normal. For our purposes the exact (pointwise) density of states can be replaced by the smoothed density of states. To decrease the errors, the volume Ω( E ) should be converted into the convolution of two sixfold integrals for energies less than − 1. For energy E >− 1 the conformation can represent a relatively stable trimer or it can eventually turn into a “dimer-monomer at infinity” configuration. To distinguish between these two cases, we studied the evolution of the initial state with time by the three-body trajectories method. The method is applicable to the calculation of thermodynamic properties in the low-temperature region 2 T 〈30 K and to the treatment of chemical equilibrium for trimers of rare gases.

Explicitly correlated coupled-cluster theory for static polarizabilities

A method of calculation of static polarizabilities with wavefunctions, corresponding to linearly approximated explicitly correlated coupled-cluster singles and doubles [CCSD(F12)] model, has been formulated and implemented. For the proper description of the response of system on applied electric field, modified ansatz is introduced for geminal part of cluster operators. Such extension of CCSD(F12) model provides balanced description of both perturbed and unperturbed wave functions, what leads to the increase of the accuracy of target polarizabilities. As a part of algorithm, explicitly correlated version of coupled-perturbed CCSD equations has also been derived and implemented. Numerical tests conducted for the set of eight molecules show good agreement between static polarizabilities, calculated with developed explicitly correlated approach and corresponding complete basis set results in regular CCSD already at triple-ζ level.

Efficient x-ray line production from laser excited CF2Cl2 clusters. Mixed cluster formation and control of the x-ray line yield

We report on effective x-ray generation under excitation of clusters of polyatomic CF2Cl2 molecules by femtosecond laser radiation of moderate intensity I ≈ 5 × 1015 W cm−2 (E ≈ 5 mJ). The maximal laser energy conversion efficiency to chlorine x-ray lines (E = 2.6–2.8 keV) reached 2 × 10−5 when the CF2Cl2 clusters were formed in the presence of the carrier gas helium. It was demonstrated that the spectrum of laser-induced x-rays can be used to detect mixed clusters, appearing in a 3% mixture of CF2Cl2 with Ar. We proposed adding a light thermalizing gas to the mixture, i.e., using a ternary mixture CF2Cl2–Ar–He, to control the fraction of components that the mixed clusters are composed of. We have given the first demonstration of a dual-energy x-ray source with a tunable line amplitude that is controlled by the helium fraction in the mixture.