Ground State Of Si Research Articles

From singlet R2Si silylene to triplet R2Si═Si ground state by DFT

AbstractIn a quest for triplet silylenes, two sets of singlet monosilylenes (R2Si), including 1,4‐di(R)tetrazol‐5‐silaylidenes (1‐5) and 1,3‐di(R)tetrazol‐5‐silaylidenes (6‐10), are converted to their corresponding triplet R2Si═Si (1′‐10′) by taking advantage of an additional electropositive silicon atom that promotes conversion of singlet to triplet, at B3LYP/6‐311++G** level of theory, where R = H, methyl, ethyl, i‐propyl, and t‐butyl. Interestingly, every triplet 1′t‐10′t (R2Si═Si) is more stable than its corresponding singlet 1′s‐10′s. Also, every singlet R2Si═Si exhibits a higher nucleophilicity (N) and appears with a narrower singlet‐triplet energy difference (ΔES‐T) and smaller band gap (ΔΕHOMO‐LUMO) than its corresponding singlet R2Si. For singlet R2Si═Si, hydrogenation energies (ΔH) and N─Si─N angle increase as the size of R increases. Both asymmetric 6‐10 and 6′‐10′ exhibit higher N than their corresponding symmetric 1‐5 and 1′‐5′, respectively. Also, a wider N1─Si─N4 bond angle is encountered for R2Si═Si compared with R2Si.

Performance of state-of-the-art force fields for atomistic simulations of silicon at high electronic temperatures

Intensive femtosecond laser pulses or ion bombardment drives Silicon (Si) into a nonequilibrium state with hot electrons and cold ions. Since ab initio molecular dynamics (MD) simulations can only deal with at most 103 atoms, an analytical interatomic potential (or force field) is necessary for performing large-scale simulations describing Si in nonequilibrium. We recently constructed a potential for Si at high electronic temperatures Te’s, which was developed from ab initio MD simulations. In this study, we analyze the performance of this potential compared to other available Te-dependent Si potentials and to some widely used ground state Si potentials, which were adapted to nonequilibrium by fitting their parameters to ab initio MD simulations. We analyze the ability for reproducing nonthermal effects like thermal phonon squeezing and ultrafast melting in bulk Si as well as the expansion due to bond softening of a thin Si film. Our results show that the available Te-dependent potentials cannot quantitatively describe the latter. A much better description is given by the potentials with parameters fitted to ab initio MD simulations. Our proposed potential gives the best description among the studied ones, since its analytical shape was optimized for the ground and the laser excited state.

Formation of silicon monoxide by radiative association: the impact of resonances

Detailed quantum chemistry calculations within the multireference configuration interaction approximation with the Davidson correction are presented using an aug-cc-pV6Z basis set, for the potential energy curves and transition dipole moments between low lying molecular states of singlet spin symmetry for the SiO molecule. The high quality molecular data are used to obtain radiative association cross sections and rate coefficients for collisions between ground state Si and O atoms. Quantal calculations are compared with semiclassical results. Using a quantum kinetic theory of radiative association in which quasibound levels are assumed to be in local thermodynamic equilibrium, we find that resonances play an important role in enhancing the rate coefficients at low temperatures by several orders of magnitude from that predicted by standard quantum scattering formulations. These new molecular formation rates may have important implications for applications in astrophysics.

Reaction dynamics of Si(PJ3)+O2→SiO(XΣ+1)+O studied by a crossed-beam laser-induced fluorescence technique

Oxidation reaction of the ground state Si atom was studied by using a crossed molecular beam technique at 13.0 kJ/mol of collision energy. The Si atomic beam was generated by laser vaporization and crossed with the oxygen molecular beam at right angle. Products at the crossing region were detected by the laser-induced fluorescence (LIF). The LIF of SiO(A 1 Pi-X 1 Sigma+) was used to determine the vibrational state distribution of the electronic ground state, SiO(X 1 Sigma+). The determined distribution was inverted with the maximum population at v"=4, and in good agreement with the recent quasiclassical trajectory calculation on the singlet potential energy surface. The agreement suggested that an abstraction mechanism is dominant at the collision energy studied here. One of the counterproducts, O(3PJ), was also observed by the vacuum ultraviolet LIF and the distribution of the spin-orbit levels were determined. The formation of O(3PJ) was consistent with the significant population of v"=7 and 8 states of SiO, which could be explained by the presence of the triplet product channel with higher exothermicity.

The bonding structure and dipole polarizability of two near-isoenergetic isomers of Si3

Theoretical studies were carried out on the ground state of Si$_{3}$. Two low-lying states were identified, namely, a singlet state of C$_{2v}$ symmetry with a bond angle of around 80°, and a triplet state of D$_{3h}$ symmetry. The two spin states are predicted to be very close in energy at their optimized geometries, with, for example, the ab initio methods with CCSD(T) and CASSCF(12,12) predicting the singlet state to be lower in energy by between 0.06 and 4.6 kcal/mol, respectively; whereas the DFT methods at (U)B3LYP/cc-pVQZ level predicting the opposite order with an energy difference of 1.62 kcal/mol. Walsh diagram is presented to explain the preferred spin state at the two different bond angles; and the bonding structures for the two isomers are proposed in light of the natural bond order analysis results by using the resonance hybrids model. However, unlike the energies, the two spin states are predicted to have fairly different dipole polarizability values: the mean α¯ =105.1 for the singlet and 100.4 for the triplet (in atomic units) at the (U)CCSD/Sadlej level of theory.

EQUILIBRIUM GEOMETRIES AND ELECTRONIC STRUCTURE OF SMALL SILICON MONOHYDRIDES CLUSTERS

The geometries and energies of small silicon monohydride clusters ( Si 2 H – Si 10 H ) have been systematically investigated by density functional theory (DFT) scheme with DZP++ basis sets. Several possible geometric arrangements and electronic states have been considered for each cluster. The results on Si 2 H – Si 4 H are in good accordance with previous ab initio calculation. The geometry of ground state of Si 2 H is found to be a bridged C2v structure, and Si 3 H to be a bridged C2v, while Si 4 H a non-bridged Cs symmetry with 2A′ state. The non-bridged geometries of ground state of Si 5 H – Si 10 H have been found to be corresponding to C2v(2B1), C2v(2B1), C5v(2A1), Cs(2A′′) (have two types), C1 (not symmetry), and Cs(2A′), respectively. The results on Si 5 H , Si 6 H , Si 8 H and Si 9 H are different from previous calculations. Compared silicon clusters ( Si n) with silicon monohydrides ( Si n H ) clusters, the addition of a single hydrogen atom cannot cause great changes in the ground state geometries of Si 2, Si 3, Si 4, Si 7, Si 9, and Si 10 clusters, while in the ground state geometries of Si 5, Si 6 and Si 8 clusters the change is great. The dissociation energies calculated indicates that Si 4 H , Si 7 H , and Si 10 H clusters are less stable than others.

Laser spectroscopic study of the SiAr van der Waals complex

Laser fluorescence excitation spectra of the SiAr van der Waals complex, in the vicinity of the Si D°3←3P atomic resonance transition near 220.7 nm are reported. At low resolution, a single excited-state (v′,0) progression of bands terminating in a dissociation continuum is observed. Several weaker bands associated with many of these strong bands are found in scans at higher resolution. A transition to an excited Σ−3 state which correlates with the excited Si(3D°)+Ar asymptote was assigned, and a rotational and vibrational analysis of the observed bands was carried out. The dissociation energies of the Ω=0+ components of the ground X 3Σ− and excited Σ−3 states were determined [D0″=178.8±0.4 and D0′=122.5±0.4 cm−1]. Ab initio calculations of SiAr X 3Σ− and A 3Π electronic states correlating with the ground-state Si(3s23p2 3P)+Ar asymptote were also carried out. The potential energy curves of the definite-Ω states were computed and used to estimate the dissociation energy, rotational constant, and phenomenological spin–spin interaction in the X 3Σ− state. These parameters were found to be in reasonable agreement with the experimental determinations.

Shock Tube Study on the Reaction of Si Atoms with CH3 with Respect to SiC Formation

The reaction kinetics of ground state Si atoms was studied behind reflected shock waves in the presence of excess CHSi+CH↔ SiCHwith

Identification of Si and SiH in catalytic chemical vapor deposition of SiH4 by laser induced fluorescence spectroscopy

Radical species produced in catalytic chemical vapor deposition (CVD), often called hot-wire CVD, processes were identified by using a laser induced fluorescence technique. Ground state Si atoms could be detected at low pressures where collisional processes in the gas phase could be ignored. The electronic temperature of Si atoms just after the formation on the catalyzer (tungsten) surfaces was 1320±490 K, when the catalyzer temperature was 2300 K. By the addition of 0.5 Pa of Ar, the electronic temperature was lowered down to 450±30 K. The absolute density of Si atoms was 3±1×109 cm−3 at 10 cm below the catalyzer when the flow rate and the pressure of SiH4 were 0.5 sccm and 4 mPa, respectively. This density is just 0.3% of that of the parent SiH4 molecules. However, since the decay rate of Si atoms is fast, it can be concluded that atomic silicon is one of the major products on the heated catalyzer surfaces. SiH radicals could also be detected, but the production rate of this species is two orders of magnitude less than that of Si atoms. It was also discovered that volatile SiH4 molecules are produced by the atomic hydrogen attack on the amorphous silicon deposited on the chamber walls.

Charge transfer between Si 3+ and helium at thermal and low energies

Cross sections and rate coefficients for selective electron capture and excitation in Si 3+–He collisions in the thermal-eV energy range are calculated using a quantum mechanical method. It is confirmed that electron capture to the ground state of Si 2+ is the dominant charge transfer mechanism and that the inverse process in which He + captures an electron in a collision with ground state Si 2+ can occur easily.

Selected ion flow drift tube studies of the kinetics of the reactions of Si +( 2P) with C 2H 2 and C 6H 6

The reaction rate coefficients, k, for the reactions of ground state Si +( 2P) with C 2H 2 and C 6H 6 have been measured as a function of reactant ion/reactant neutral centre-of-mass kinetic energy, KE CM, in a selected-ion flow drift tube (SIFDT) apparatus, operated with helium at a temperature 298 ± 2 K. The values k of the studied reactions have very pronounced negative energy dependences; k decreases by about one order of magnitude when KE CM increases from near thermal values to about 2 eV. The results are interpreted in terms of a simple model assuming that the reactions proceed via the formation of long-lived complexes. These intermediate complexes decompose back to reactants or forward to products, the unimolecular decomposition rate coefficients for these reactions being k −1 and k 2 respectively. It is found that a power law of the form k −1/ k 2 = constant ( KE CM) m closely describes each reaction. The negative energy dependence of the reaction rate coefficient of the reaction of Si +( 2P) with C 2H 2 is rationalized in terms of the negative entropy change in the reaction. The dependence of the reaction rate coefficient of the reaction of Ar + with C 6H 6 on KE CM is also reported.

Selected ion flow drift tube studies of the reactions of Si+(2P) with HCl, H2O, H2S, and NH3: Reactions which produce atomic hydrogen

The reaction rate coefficients, k, for the reactions of ground-state Si+(2P) with HCl, H2O, H2S, and NH3, have been measured as a function of reactant ion/reactant neutral center-of-mass kinetic energy, KECM, in a selected ion flow drift tube (SIFDT) apparatus, operated with helium at a temperature 298±2 K. The values k of the studied reactions have very pronounced, negative energy dependencies; the rate coefficients decrease by about 1 order of magnitude as KECM increase from near thermal values to ∼2 eV. The results are interpreted in terms of a simple model assuming the reactions to proceed via the formation of long-lived complexes. These intermediate complexes decompose back to reactants or forward to products, the unimolecular decomposition rate coefficients for these reactions being k1 and k2, respectively. It is found that a power law of the form k−1/k2=const(KECM)m closely describes each reaction.

Selected ion flow drift tube studies of the reaction of Si + ( 2P) with C 2H 4. Observation of the ternary reaction with two channels: collisional stabilization and collisional dissociation

A selected ion flow drift tube study on the reaction of ground state Si +( 2P) with C 2H 4 has been carried out in the pressure range from 0.14 to 0.52 Torr for average relative centre-of-mass kinetic energies, KE CM, from near thermal to ≈ 2 eV. Two product ions have been observed, SiC 2H 3 + and SiC 2H 4 +. The apparent binary (“effective”) rate coefficient, k eff, and the product distribution have been determined as functions of KE CM. The reaction rate coefficients of the binary ( k BIN) and ternary ( k 3) channels have been determined by two procedures, from the pressure dependence of k eff (indicated by index “PRESSURE”) and by using the observed product distribution (indicated by index “PROD”). For KE CM < 0.1 eV k 3 obtained by both methods, k 3-PRESSURE and k 3-PROD, are equal and their dependence on KE CM can be expressed in the form: k 3-PROD = k 3-PRESSURE ∝ ( KE CM) −0.5. It was observed that dependence of the reaction rate coefficient of the binary channel can be expressed also in the form k BIN-PRESSURE ∝ ( KE CM) −0.5. This similarity in the dependence on KE CM may indicate that both binary and ternary channels are proceeding via the same rate-determining process prior to separation into two channels. The situation is different for KE CM > 0.1 eV. Here k 3-PROD decreased more rapidly with KE CM than k 3-PRESSURE, indicating that in the collisions of He atoms with the excited intermediate collision complex the dissociation of (SiC 2H 4) +∗ to SiC 2H 3 + takes place. Here we report the first observation of a ternary reaction having two product channels—collisional stabilization (product (SiC 2H 4 +) and “collision induced dissociation” (product SiC 2H 3) with probabilities α 1 and α 2 (where α 2 = 1 - α 1), respectively. α 1 and α 2 have been determined from k 3-PROD and k 3-PRESSURE. For KE CM < 0.1 eV the stabilization channel is dominant ( α 1 ⪢ α 2), for KE CM ≈ 0.2 eV both channels have the same probability ( α 1 = α 2), and for KE CM > 0.2 eV the “collision induced dissociation” of the intermediate complex, producing SiC 2H 3 +, is dominant ( α 2 > α 1).

Si3O4-: vibrationally resolved photoelectron spectrum and ab initio calculations

In this communication we report a vibrationally resolved photoelectron spectrum of a relatively complex cluster, Si{sub 3}O{sub 4}{sup -}, for which we observe a single vibrational progression with a frequency of 800(50) cm{sup -}. We use quantum mechanical calculations to determine the structure of the cluster, which is found to be D{sub 2d} for the neutral ground state Si{sub 3}O{sub 4} and C{sub 2v} for Si{sub 3}O{sub 4}{sup -}. The agreement between the theoretical and experimental vertical and adiabatic detachment energies of the predicted Si{sub 3}O{sub 4}{sup -} geometry and the frequency and symmetry of the vibrations of Si{sub 3}O{sub 4}, suggest that the predicted structure is correct. 19 refs., 2 figs., 1 tab.

Theoretical vibration-rotation spectrum of Si2C

The theoretical potential energy function has been generated and used in the evaluation of the vibration-rotation energy levels of the 1 A 1 electric ground state of Si 2 C. For the bent equilibrium structure, the rotational constants and the vibrational band origins up to 1200 cm -1 are given. The height of the barrier to linearity has been calculated to be 310 cm -1 . The shallow potential energy function along the bending coordinate results in large amplitude bending modes

Guided ion beam studies of the reaction of silicon(1+) (2P) with methylsilane: reaction mechanisms and thermochemistry of organosilicon species

Reaction of ground-state Si + ( 2 P) with methylsilane (SiH 3 CH 3 ) is studied from thermal to 10-eV kinefic energy by using guided ion beam mass spectrometry. The major products at thermal energies are SiCH 3 + , Si 2 HCH 3 + , and, above 1 eV, SiH 2 CH 3 + . Labeling experiments involving 30 Si + provide additional mechanistic information that SiCH 3 + formed via three different mechanisms

Gas-phase reactions of silyl cation (2P) with small hydrocarbon molecules: formation of silicon-carbon bonds

Reactions of ground-state Si + ( 2 P) ions have been investigated with the hydrocarbon molecules CH 4 , C 2 H 6 , C 2 H 4 , C 2 H 2 , CH 2 CCH 2 , CH 3 CCH, and C 4 H 2 proceeding in a helium bath gas at 0.35 Torr and a temperature of 296±2 K with the selected-ion flow tube (SIFT) technique. A range in the nature and degree of reactivity was observed. Methane reacts slowly to form an adduct ion. Results of quantum chemical calculations performed at the UMP4(SDTQ)/6-31G**//UHF/6-31G** level are presented which provide insight into the possible structure of this adduct ion. Adduct formation also is a strong feature in the reactions of Si + with C 2 H 2 and C 2 H 4 in which it competes with condensation resulting in H atom elimination. The reaction with C 2 H 6 predominantly leads to Si―C bond formation in the product ions together with elimination of CH 3 , CH 4 , or H 2 . For these three reaction channels possible mechanisms are proposed involving C―C and C―H bond insertion. The reactions with allene and propyne are rapid and have similar product distributions

Laser-induced fluorescence spectroscopic study of reactive plasma. The absolute density and spatial distribution of Si atoms in silane discharege.

We used LIF spectroscopy to determine the absolute density and the spatial distribution of Siatoms in a dc and an rf glow-discharge in SiH4-He-Ar. The absolute sensitivity of the fluorescence detection system was calibrated by using Rayleigh scattering by Ar gas. The fluorescence quantum efficiency and the polarization degree of emitted light were measured to estimate the effect of collisions of Si atoms and buffer gas. The spatial distribution of the ground-state Si atom in the dc discharge is compared with that of the excited-state atoms. The distribution of Si atoms in the rf discharge is investigated for two types of electrode configurations.

CAS SCF/CI calculations of low-lying states and potential energy surfaces of Si 3

Complete active space MC SCF and restricted first-order CI calculations of low-lying states of Si 3 ( 1A 1, 3B 2, 1B 2, 3B 1, 1B 1, 3A 2, 1A 2) and potential energy surfaces of the 1A 1 and 3B 2 states are calculated. MRSDCI calculations are carried out for the 1A 1 and 3A' 1(D 3h) states. The ground state of Si 3 is found to be the 1A 1 state with the 3A' 2 (equilateral triangle) state 3.6 kcal mole above the singlet state in a TZ + ld basis set. A second shallow minimum (obtuse triangle, apex angle 160°) and a barrier are found in the potential energy surface of the 3B 2 state. The equilibrium geometry of the 1A 1 state is an isosceles triangle, apex angle 80° while that of the 3B 2 ( 3A' 22) state is an equilateral triangle. The equilateral and isosceles triangular structures are more stable than the corresponding linear structures.

Calculation of the spin-polarized electronic structure of an interstitial iron impurity in silicon.

We apply our self-consistent, all-electron, spin-polarized Green's-function method within an impurity-centered, dynamic basis set to study the interstitial iron impurity in silicon. We use two different formulations of the interelectron interactions: the local-spin-density (LSD) formalism and the self-interaction-corrected (SIC) local-spin-density (SIC-LSD) formalism. We find that the SIC-LSD approach is needed to obtain the correct high-spin ground state of Si:${\mathrm{Fe}}^{+}$. We propose a quantitative explanation to the observed donor ionization energy and the high-spin ground states for Si:${\mathrm{Fe}}^{+}$ within the SIC-LSD approach. For both Si:${\mathrm{Fe}}^{0}$ and Si:${\mathrm{Fe}}^{+}$, this approach leads to a hyperfine field, contact spin density, and ionization energy in better agreement with experiments than the simple LSD approach. The apparent dichotomy between the covalently delocalized nature of Si:Fe as suggested on the one hand by its reduced hyperfine field (relative to the free atom) and extended spin density and by the occurrence of two closely spaced, stable charge states (within 0.4 eV) and on the other hand by the atomically localized picture (suggested, for example, by the stability of a high-spin, ground-state configuration) is resolved. We find a large reduction in the hyperfine field and contact spin density due to the covalent hybridization between the impurity 3d orbitals and the tails of the delocalized ${\mathrm{sp}}^{3}$ hybrid orbitals of the surrounding silicon atoms. Using the calculated results, we discuss (i) the underlying mechanism for the stability and plurality of charged states, (ii) the covalent reduction in the hyperfine field, (iii) the remarkable constancy of the impurity M\"ossbauer isomer shift for different charged states, (iv) comparison with the multiple charged states in ionic crystals, and (v) some related speculation about the mechanism of (${\mathrm{Fe}}^{2+}$/${\mathrm{Fe}}^{3+}$) oxidation-reduction ionizations in heme proteins and electron-transporting biological systems.