Energy-dependent dynamics of large-DeltaE collisions: highly vibrationally excited azulene (E=20 390 and 38 580 cm(-1)) with CO2

We report the energy dependence of strong collisions of CO(2) with highly vibrationally excited azulene for two initial energies, E=20 390 and 38 580 cm(-1). These studies show that both the distribution of transferred energy and the energy transfer rates are sensitive to the azulene energy. Highly excited azulene was prepared in separate studies by absorption of pulsed excitation at lambda=532 or 266 nm, followed by rapid radiationless decay from S(1) or S(4) to vibrationally excited levels of the ground electronic state. The appearance of scattered CO(2) (00(0)0) molecules with E(rot)>1000 cm(-1) was monitored by high-resolution transient IR absorption at lambda=4.3 mum. The average rotational and translational energies of the scattered CO(2) molecules double when the azulene energy is increased by a factor of 2. The rate of energy transfer in strong collisions increases by nearly a factor of 4 when the azulene energy is doubled. The energy transfer probability distribution function for DeltaE>3000 cm(-1) at each initial energy is an exponential decay with curvature that correlates with the energy dependence of the state density, in excellent agreement with predictions from GRETCHEN, a model based on Fermi's golden rule to describe collisional quenching of highly excited molecules.     

Enhanced photoluminescence from group 14 metalloles in aggregated and solid solutions

The unusual photoluminescence characteristics of a series of six group 14 metalloles (1,1-dimethyl-2,3,4,5-tetraphenylmetalloles and 1,1-diphenyl-2,3,4,5-tetraphenylmetalloles) containing silicon, germanium, or tin have been investigated. Although the compounds are weakly luminescent in dilute fluid solution at room temperature, they undergo a substantial enhancement of photoluminescence when forced to aggregate, as in mixed solvent systems. The compounds also exhibit considerable emission when incorporated into rigid room-temperature glasses of sucrose octaacetate. Absorption and emission characteristics of the compounds, including luminescence quantum yields, in fluid solution, solution-phase aggregates, and room-temperature glasses are reported. Quantum yields increase by as much as 2 orders of magnitude in the aggregates and glasses, compared to fluid solution. Experimental evidence supports the conclusion that the aggregation-induced enhancement of luminescence results from restricted intramolecular rotations in the packed metalloles. The unusual aggregation-induced enhancement of these compounds makes them potentially useful for the fabrication of a variety of electrooptical devices and sensors. In addition, the X-ray crystal structure of hexaphenylgermole is reported.     

Element incorporation in vapour grown III–V compounds

Some recent developments and ideas in the field of element incorporation in vapour grown III–V compounds are highlighted in this paper. The study is based mainly on GaAs and GaP. Specific areas of consideration are as follows. The role of equilibrium thermodynamics in predicting element incorporation. Here, theoretical developments are discussed and the difficulties in testing models in vapour growth in contrast to melt growth are noted. The residual impurity problem is considered, particularly the effects of vapour pressure parameters on the properties of epitaxial layers. The role of surfaces in relation to growth rate and the orientation-dependence of element incorporation are discussed. In connection with the field of defect chemistry, the effect of III–V ratio on element incorporation is debated. Also in the context of defects, the role of ionised As vacancies in GaAs is considered as a controlling parameter in element incorporation. This development allows one to account for the well-known insensitivity of doping to the intrinsic electron concentration.