Laser photolysis and spectroscopy: a new technique for the study of rapid reactions in the nanosecond time range

Abstract

We have developed a new, very rapid, spectroscopic recording technique with the aid of which photographic absorption spectra of transient intermediates with lifetimes in the nanosecond time range can be obtained. The technique is a hundred times faster than present flash spectrographic instrumentation and provides time-resolved absorption spectra over a wide spectral range in a single experiment. Frequency-doubling is used to obtain both a 347 nm laser pulse suitable for excitation and a 694 nm laser pulse from a single Q -switched ruby laser pulse. The 694 nm pulse is converted into a continuum for absorption spectroscopy by bringing it to a sharp focus in a suitable gas, so as to cause a laser-induced breakdown spark. The excitation pulse has a duration of 30 ns. The duration of the continuum pulse varies with the gas used. In 1 atm of oxygen it has a duration of 30 ns, is synchronized to within 10 ns with the excitation pulse and has an intensity adequate for single-shot flash spectroscopy. The laser-induced spark in 1 atm of xenon has a duration of several μs and provides an excellent background continuum, of essentially constant intensity, for kinetic spectroscopy over periods up to 1 μs, using an image-converter camera to provide time resolution. We have applied the laser photolysis technique to observing, for the first time, absorption spectra arising from the lowest excited singlet states of several aromatic hydrocarbons. The time-resolved spectra obtained show the decay of the new excited singlet absorption bands and the concomitant build-up of triplet-triplet absorption bands during the first microsecond following light absorption, thus depicting graphically the non-radiative process of intersystem crossing from the lowest excited singlet state to the triplet manifold. The new bands also serve to locate the energies of higher excited singlet levels which, in many cases, are inaccessible from the ground state. The new technique should find wide application to solid, liquid and gaseous systems and should contribute to the understanding of photochemical primary processes in the time range 10 -9 to 10 -6 s. Eventual extension of the technique to the 10 -12 s range appears possible by using mode-locked lasers.