The photoinduced H-atom-transfer reaction in indole(NH3) n clusters has been analyzed by femtosecond time-resolved photoelectron−photoion coincidence spectroscopy. The different contributions to the measured time-dependent ion and electron signals resulting from ionization by one and two probe photons can be discriminated and analyzed separately. In particular, the distinctively different dynamical behavior observed for clusters with small (n = 1−3) and larger (n ≥ 4) numbers of ammonia molecules is elucidated. For the small clusters an ultrafast process with a time constant of about 150 fs is identified and attributed to internal conversion from the initially excited ππ* state to the πσ* state. In contrast, for the larger clusters (n ≥ 4) such an initial ultrafast process is not observable probably for Franck−Condon reasons, while a structural rearrangement mechanism after the H transfer on a time scale of 10 ps is clearly recognized.

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The hydrogen atom transfer reaction in indole (NH3) n clusters excited at 263 nm to the S1(ππ*) state is studied in pump−probe experiments with femtosecond laser pulses. For small clusters the reaction is characterized by two successive processes on distinct time scales:  the time constant of the primary process is in the sub-ps region, whereas the secondary decay time growing with the cluster size reflects a relaxation process within 25 to 150 ps. A preliminary model of the H-transfer reaction is discussed.

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The femtochemistry of the indole molecule which represents the chromophore of the amino acid trypophan is of particular interest because of its biological relevance. Recent studies (e.g. [1]) of indole in aqueous solution have shown that the photoexcitation leads to a fast charge separation process characterized by the formation of a solvated electron. The microscopic mechanism of the solvation process can be elucidated by gas phase studies of indole-solvent clusters with polar molecules like water or ammonia. Recent ab initio investigations of indolewater clusters have stressed the crucial role of the nonadiabatic coupling between the optically excited ππ* and a low-lying dark πσ* state for the electron transfer process [2]. In first pump-probe experiments on indole (NH3)n clusters with ns laser pulses a H atom transfer reaction has been stated [3].

The spectral distribution of the virgin (internal) recombination radiation, as calculated on the basis of a real situation in the active region of a GaAs p–n homojunction in Fabry‐Perot structure, is compared with the radiation spectra emerging from edge and area faces. The peak shift to the low energy side as found in any case yields 20 meV in the edge and 3 meV in the area emitting situation. The spectral half‐width is reduced by 30 and 7%, respectively. A superposition of both external spectra results in two clearly separated peaks, each of them a ghost peak of the virgin emission. Similar experimental results are published previously.