Construction of a subpicosecond double-beam laser photolysis system utilizing a femtosecond Ti:sapphire oscillator and three Ti:sapphire amplifiers (a regenerative amplifier and two double passed linear amplifiers), and measurements of the transient absorption spectra by a pump-probe method

Abstract

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 tested for the practical availability are satisfactory enough, the transient absorption spectrum thus obtained exhibits an artificial time-dependent spectral change owing to the group velocity dispersion of the probing light pulse. Hence, its wavelength-dependent arrival time to the sample cell is determined utilizing the optical Kerr effect induced in carbon tetrachloride and the true transient absorption spectrum (the corrected spectrum) at a given delay-line setting is calculated by a microcomputer using a great number of uncorrected transient absorption spectra obtained at different delay-line settings. Interestingly, the corrected singlet-singlet absorption band (band C with a lifetime of 0.8 ps) obtained for the lowest excited singlet state of 9-nitroanthracene in cyclohexane really shifts with time. This can be interpreted in terms of the wide wavelength-range superposition of band C and an absorption band which increases and then decreases with rise and decay times 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 excited triplet state of 9-nitroanthracene. By 400 nm biphotonic excitation of the neat solvents (cyclohexane and n-heptane), furthermore, population of their higher excited singlet states with a very short lifetime of 0.4 ps can be seen.