CONSTRUCTION OF A SUBPICOSECOND DOUBLE-BEAM LASER PHOTOLYSIS SYSTEM UTILIZING A FEMTOSECOND TI-SAPPHIRE OSCILLATOR AND 3 TI-SAPPHIRE AMPLIFIERS (A REGENERATIVE AMPLIFIER AND 2 DOUBLE PASSED LINEAR-AMPLIFIERS),AND MEASUREMENTS OF THE TRANSIENT ABSORPTION-SPECTRA BY A PUMP-PROBE METHOD

A double-beam Ti:sapphire laser photolysis system has been constructed for measurements of the subpicosecond transient absorption spectra by a pump-probe method. Although the spectroscopic characteristics teste d for the practical availability are satisfactory enough, the transien t absorption spectrum thus obtained exhibits an artificial time-depend ent spectral change owing to the group velocity dispersion of the prob ing light pulse. Hence, its wavelength-dependent arrival time to the s ample cell is determined utilizing the optical Kerr effect induced in carbon tetrachloride and the true transient absorption spectrum (the c orrected spectrum) at a given delay-line setting is calculated by a mi crocomputer using a great number of uncorrected transient absorption s pectra obtained at different delay-line settings. Interestingly, the c orrected singlet-singlet absorption band (band C with a lifetime of 0. 8 ps) obtained for the lowest excited singlet state of 9-nitroanthrace ne in cyclohexane really shifts with time. This can be interpreted in terms of the wide wavelength-range superposition of band C and an abso rption band which increases and then decreases with rise and decay tim es of 0.8 and 1.8 ps, respectively; the latter band is ascribed to the absorption of 9-nitrite (or its precursor) or that of a higher excite d tripler state of 9-nitroanthracene. By 100 nm biphotonic excitation of the neat solvents (cyclohexane and n-heptane), furthermore, populat ion of their higher excited singlet states with a very short lifetime of 0.4 ps can be seen. (C) 1997 American Institute of Physics.