V. Balaji

Singlet excitation in polysilanes: Ab initio calculations on oligosilane models

All-electron SCF-CI calculations with several basis sets have been performed for excited singlet states of the linear oligosilanes SinH2n+2, n = 2-5, in order to establish their nature as a function of chain length. The results suggest that the lowest σSiSi → πSiH excited state lies below the first σSiSi → σSiSi state in the shortest few oligosilanes, and that the order of the states changes as the number of silicon atoms in the chain increases. An assignment is proposed for the lowest observed two-photon allowed state of polysilanes. In the gas phase, Rydberg states are expected to dominate the low-energy region of the spectra of oligosilanes shorter than Si6H14. An increase in the SiSiSi valence angle is computed to lower the σSiSi → σSiSi* excitation energy, and torsion around a central SiSi bond from trans towards cis geometry is computed to increase it. The effect of alkyl substitution was studied using 2,2-dimethyltrisilane as a model compound. It lowers the σSiSi → σSiSi* excitation energy, primarily by destabilizing the highest σSiSi orbital by hyperconjugative interaction with the substituents. The results for the σSiSi → σSiSi* state are in agreement with simple, qualitative arguments and prior semiempirical calculations.

Photophysics and potential-energy hypersurfaces of permethylated oligosilanes

Information on the potential-energy surfaces of the ground and 1σσ* excited states of oligosilane chains, collected from experimental and theoretical sources, is discussed in terms of electronic structure and the mechanisms of bond and excitation delocalization.

Sputtering yields of condensed rare gases

Abstract Improved experimental techniques were employed to measure the initial sputtering yields of condensed rare gas targets under 0.75–5 keV ion bombardment. These sputtering yields show a strong nonlinear dependence on the total stopping power. The experimental results for condensed Ar, Kr, and Xe targets are consistent with a systematic change from a linear collision cascade to evaporation and to a gas-flow mechanism as the deposited energy density increases. For condensed Ne sputtered by heavy projectiles. the yields are larger than can be explained by any existing quantitative model. The sputtering yields for all targets are found to approach a plateau values as sputtering progresses. This is attributed to surface roughening and the initial slope is used to determine the size of the impact damage area.

New strained organic molecules: theory guides experiment

The experimental program in this laboratory for t he preparation of rare gas matrix-isolated hydrocarbons with h ighly strained carbon centers by gas-phase dehalogenation of polyhalides is guided by calculations of molecular geometry and vibrational spectra. In this communication are reported 6-31G SCF and MP2 as well as MNDO results for the highly strained molecules derived from the known stable hydrocarbons, bicyclo(l.l.l)pe ntane and (l.l.l)propellan e. Several of these improbable looking structures are calculated to be stable in low temperature inert gas matrices. The calculated ab initio SCF and MNDO symmetries, bond lengths (A), strain energies, and lowest vibrational frequencies are listed with the formulas.

Vibrational transition moment directions in medium-size molecules: Experiment and theory

Abstract Linear dichroism is observed in the infrared spectra of dimethyl ether, isobutene, 2-butyne, and n-butane dissolved in stretched sheets of ordinary and perdeuterated poly-ethylene. For each observed vibrational band the orientation factor has been determined and related to the transition-moment direction for the vibration, providing information on its symmetry species. For n-butane, the angles of the vibrational transition moment directions in the molecular framework have been derived and compared with the angles calculated at various levels of semiempirical and ab initio theory.

Calculation of Photochemical Reactivity. Oligosilanes as an Illustration

The fundamental photophysical and photochemical processes in the reactions of organic molecules are summarized with particular attention to the role of barriers, minima and funnels in the and S1 and T1 surfaces. Non-spectroscopic minima often occur at biradicaloid geometries and elementary photochemical reaction steps can be classified according to the nature of the biradicaloid minimum through which the return to the S0 state occurs. In simple cases, one active orbital per atom is involved. These steps are subdivided into two-center, cyclic multicenter, and linear multicenter processes. In complex cases, more than one active orbital participates on at least one atom. Minimal calculational requirements for a theoretical treatment are outlined.