The photoluminescence ͑PL͒ from 7-and 15-nm silica (SiO 2 ) nanoparticles induced both by ArF laser light ͓ exc ϭ193 nm (6.4 eV), L ϭ15 ns͔ and by Nd:YAG ͑yttrium-aluminum-garnet͒ laser light ͓ exc ϭ266 nm (4.66 eV), L ϭ8 ns͔ was studied. The laser light intensity dependencies of the PL yields reveal the two-photon ͑TP͒ process of the PL excitation in the case of ArF laser light. The PL results from the radiative relaxation of self-trapped excitons ͑STEthe blue band͒, also from the surface hydrogen-related species ͑the green band͒, and the bulk nonbridging oxygen hole centers ͑NBOHC'sthe red band͒ excited by a radiationless relaxation of TP-produced free excitons ͑FE's͒. The main point is focused on the effect of the nanoparticle surface condition on the FE dynamics. The dynamics includes either an elastic scattering or quenching by the nanoparticle boundary, the laser heating of FE's up to energies in excess of the STE barrier, the FE energy transfer to the surface and bulk NBOHC's and hydrogen-related centers, the saturation of the FE density, and the biexciton process in the formation of Frenkel defects with their subsequent transformation into NBOHC's.

Potential energy surfaces of various primary and secondary products from the photodissociation of propyne and allene, including the C 3 H n (n ) 1-3) species, have been investigated at the CCSD(T)/6-311+G-(3df,2p)//B3LYP/6-311G(d,p) level of theory. The calculated heats of the reactions and the activation barriers for H 2 elimination from C 3 H n (n ) 2-4) have been employed to analyze the experimental translational energy distribution for different photodissociation channels. The electronic spectra of propyne and various isomers of C 3 H 2 have been calculated by using the CASSCF, MRCI, and EOM-CCSD methods with the ANO(2+) basis set. The calculations suggest that the photodissociation of propyne at 193 nm involves a Franck-Condon transition to the 1 E excited state. After internal conversion into the vibrationally excited ground electronic state, propyne can either dissociate to produce HCCCH + H 2 or isomerize to allene which, in turn, undergoes the H 2 elimination giving H 2 CCC. The HCCCH produced from propyne can have sufficiently high internal energy to rearrange to H 2 CCC. In both mechanisms, the formation of C 3 + H 2 from propyne and allene goes via the same intermediate, which explains the identical rotational distribution of the C 3 products in experiment. The H 2 elimination is a minor channel of propyne photodissociation and the major channel is elimination of the acetylenic hydrogen atom. The rearrangement mechanism of C 3 H 2 in the ground electronic state also has been studied. Automerization of H 2 CCC can take place either via a cyclopropyne transition state (the barrier is 37.5 kcal/mol, ref 18) or through isomerization to cyclopropenylidene and backward via TS6 (the barrier is 41.7 kcal/mol). Isomerization of triplet propargylene to cyclo-C 3 H 2 occurs by the ring closure via the triplet-singlet seam of crossing MSX1, and the activation energy is predicted to be about 41 kcal/mol. Cyclopropenylidene can undergo automerization by the 1,2-H shift via TS10 with the barrier of 32.4 kcal/mol. The direct triplet HCCCH f H 2 CCC isomerization proceeds by the 1,3-hydrogen shift via MSX2 and TS8 or TS9 with a high activation energy of 78-81 kcal/mol. The singlet propargylene can also rearrange to cyclo-C 3 H 2 via TS7 (barrier 37.4 kcal/mol) and to H 2 CCC via TS8 or TS9. The calculated PES for the ground and excited states have allowed us to explain the experimentally observed automerizations and isomerizations of C 3 H 2 isomers and to assign their UV absorption spectra.