Femtosecond relaxation of photoexcited p-nitroaniline in water: solvation, internal conversion and cooling

Summary form only given. Para-nitroaniline (PNA) is the smallest organic molecule which exhibits strong photoexcited electron transfer from the NH/sub 2/ group to the NO/sub 2/ group. This charge transfer results in pronounced solvatochromic and nonlinear optical molecular properties. Therefore this molecule is of great interest both for chemists and engineers developing new nonlinear optical materials. However the photophysics of PNA is not fully understood yet. For example, fluorescence has not been observed due to very fast quenching processes in the first excited electronic state. Even a recent subpicosecond study, failed to detect the earliest (before 300 fs) spectral evolution. We observe for the first time and in spectroscopic detail all early stages of relaxation of PNA in water, by measuring transient absorption spectra with the pump-supercontinuum probe technique, after excitation at 400 nm with 50 fs pulses. The transient spectra are recorded in the range 300-700 nm with 20 fs time resolution.