Ammonia dimers and trimers are studied in a femtosecond pump probe experiment. The pump laser (6.2 eV) excites the cluster into the à state which is ionized either by 4.6 eV or by 3.1 eV probe photons. Characteristic differences are explained in terms of a kinetic model involving an internal protonated neutral excited state as an intermediate.

Applying the 2CR2PI method we have obtained novel information about the spectral structures of the heterodimers C6H6 · NH3 and C6H6 · ND3 near the S1 state of benzene. For the first time the ionization threshold and the dissociation energy of the heterodimers have been determined. Preliminary results of ab initio calculations reveal that one of the two suggested isomers is characterized by hydrogen-bonded configurations with hindered rotation of ammonia about its C 3 axis. The calculated dissociation energy D 0 of this isomer very well agrees with the value of (620±100) cm-1 determined experimentally. Exciting the heteroclusters to energy levels near the S 2 state of benzene no 2CR2PI signals were obtained due to fast internal conversion.

The dynamics of ammonia clusters excited to the à state with 160 fs laser pulses of 6.2 eV was studied by pump-probe experiments with a low probe photon energy of 3.1 eV. Protonated as well as unprotonated cluster ion signals have been observed. The time evolution of both species is characteristic of the intermediate rearrangement and fragmentation processes. The observations strongly support a previously developed kinetic model for this dynamics with the signal at long delay times>6 ps reflecting the species involved in the absorption dissociation ionization (ADI) mechanism. Strong evidence is found for the formation of an internally ‘quasi protonated’ excited state and of ammoniated NH4 radicals.

The formation of protonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH3) n and (ND3) n clusters withn up to 8. The fourth harmonic (∼ 200 nm, 6.2 eV, 160 fs) of a Ti: Sapphire laser pulse is used to excite the clusters in a state corresponding to theà state of NH3 while the third harmonic (267 nm, 4.65 eV) is used for the subsequent ionisation step. Employing a combination of the optical Bloch equations for the excitation process and rate equations for the cluster dynamics we calibrate the zero time delay and carefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguished, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ionisation and protonation dynamics, accounting also for characteristic differences observed between deuterated and undeuterated species. A particularly remarkable time dependence of the homogeneous (NH3) 2 + cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH3NH2NH4.

The ionization and the fragmentation properties of Ag(NH3) n heteroclusters have been studied. The measured ion yield spectra near the threshold for complexes withn=2−20 indicate strong differences of the cluster geometry in the electronic ground and in the ionized state, respectively. For smaller clusters (n≤6) in the neutral ground state the nonsolvated Ag atom is localized near the surface of the complex. A modification of the surface structure at the transition to larger clusters (n≥7) is suggested by significant changes in the ionization and fragmentation behaviour. Preliminary results of ab initio calculations confirm the conclusion that due to the correlation interaction between the valence electron and the 4d electrons of the Ag atom no solvation effects occur.

Vibrationally resolved electronic spectra of heteroclusters C6H6-SF6 and C6H6-(SF6)2 were studied in the spectral regions near the S0-S1, 0(0)0 and 6(0)1 transitions of the benzene monomer. A nonvanishing 0(0)0 vibrational band has been observed for C6H6-SF6 with a C3v symmetry. For both clusters we have determined the ionization potentials as well as the binding energies in the electronic ground state and the ionization state. The fragmentation of larger clusters (C6H6)n(SF6)m is restricted to the loss of SF6 molecules while the emission of C6H6 molecules have not been observed.