No abstract

A two-laser, two-color time-resolved EPR (TREPR) technique has been applied to the investigation of the electron spin polarization in the excited triplet states of anthracene and its halogen anthracene in rigid glassy matrixes. The one-laser excitation of anthracenes with 355 nm gave the triplet TREPR spectra of the lowest excited triplet (T1) states with EEE/AAA polarization pattern. For 9-bromoanthracene and 9,10-dibromoanthracene, the reexcitation by a dye laser (442 nm) to the upper excited triplet (T n :  n > 3) states brought about the opposite phase polarization in the T1 states. The phenomena were interpreted in terms of the sublevel selective reverse intersystem crossing (RISC) from the T n (n = 2 or 3) state to the S1 state. The time profiles of the polarization were analyzed taking into account the anisotropic RISC processes. On the other hand, for anthracene and 9,10-dichloroanthracene, little effect was observed on the electron spin polarization by two-step laser irradiation.

Photoinduced effects on the electron spin polarization (ESP) in the lowest excited triplet (TI) states of porphyrins (PORs) and phthaloeyanines (PCs) have been observed with a two-color time-resolved (TR) electron paramagnetic resonance (EPR) technique in a glassy matrix at low temperatures. On single-color excitation with the wavelengths of the ground state absorptions of PORs and PCs, polarized EPR spectra due to the corresponding Tt state were observed. The polarization pattems match well with interpretation as anisotropic intersystem erossing (ISC) induced by the spinorbit coupling between the singlet excited (S 0 and the triplet states. In contrast, two-color excitation led to a change of the phase of the Tt state polarization pattern to the opposite. The observed ESP in the Tt state resulting from the excitation to the upper triplet state (T,) was interpreted in terms of anisotropic ISC between the T, and S~ states. From the analysis of the ESP, changes in the quantum yields of the reverse ISC processes were determined at different temperatures. The results could be best interpreted by the existente of thermal pathways with small activation energy in the relaxation processes.

Mechanics of hydraulic fracturing is a great branch of the application of fracture mechanics theory, since hydraulic fracturing was considered as a technique for recovering geothermal energy from hot dry rocks. A substantial amount of geothermal energy could be extracted from hot dry rocks near the earth's surface by circulating water through crack-like hydraulic fractures.Because of the low thermal conductivity of rock, relatively large fractures are necessary to gain an economically acceptable thermal energy rate.