Picosecond transient spectroscopy of molecules in solution

The transient absorption spectra of a number of compounds in solution at room temperature have been measured with picosecond-scale time resolution and unprecedented accuracy and reliability. It is found that the spectra of benzophenone, xanthone, 9-fluorenone, p-benzoquinone, acridine, quinoxaline, t-stilbene all show significant evolution following excitation with the 353 nm third harmonic of a mode-locked Nd:glass laser. These spectral changes provide evidence for previously unrecognized photophysical behavior in the systems studied, including vibrationally nonthermalized triplet states in benzophenone which persist for tens of picoseconds, and formerly unobserved singlet absorptions in acridine, quinoxaline, fluorenone, and benzoquinone. The inadequacy of single wavelength probing techniques in deducing the dynamics of complex molecular systems such as these is demonstrated by the large extent to which spectra of different states overlap. In many cases a reinterpretation of previous single wavelength results is required because of the presence of vibrationally and electronically unrelaxed species. The apparatus used here to obtain the high quality visible transient spectra is described and its performance characterized.