Excited-state proton transfer in 1-naphthol(NH3)n clusters:: Wavelength-dependence of the picosecond pump-probe spectra

The threshold size and timescale for excited-state proton transfer in 1-naphthol/ammonia clusters, 1-NpOH(NH3)(n), were investigated by picosecond time-resolved pump-probe photoionisation, using different wavelengths for both the pump and probe laser. For the cluster with four ammonia molecules surrounding the naphthol moiety, 1-NpOH(NH3)(4), a transient with a time constant of around 60 ps is observed. This transient can be assigned to the motion of the system along a complicated reaction coordinate that includes proton transfer in the excited state as well as intramolecular vibrational-energy redistribution and solvent reorganisation. At sufficiently high excess energies in the n=4 ion, a transient at the mass of the 1-NpOH(NH3)(3) cluster is observed, and shown to be caused by loss of an ammonia unit in the n=4 cluster ion. This fragmentation process is suppressed by lowering the probe wavelength and avoiding multiphoton absorption, thus reducing the amount of excess energy deposited in the [1-NpOH(NH3)(4)](+) ion.