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 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.

We report on results of ionization and fragmentation studies of small silver-ammonia heteroclusters. By comparison of the experimental results with ab-initio calculations we have obtained first information about the structure and binding energies of these complexes.

The dynamics of intracluster fragmentation in highly excited benzene(NH 3 ) n heteroclusters has been studied and compared with the ultrafast process of internal conversion. In pump-probe experiments with 170 fs laser pulses the clusters were excited to 6.2 eV and probed with 3.1 eV photons. The time constants for the dominant process of internal conversion as well as the parallel channel of fragmentation by NH 2 loss have been determined by fitting the measured time dependence with corresponding theoretical curves. No significant differences were obtained comparing the internal conversion in benzene(NH 3 ) n and benzene(ND 3 ) n complexes.

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.

The dynamic off benzene(NH3)n heteroclusters is studied with 170 fs laser pulses in a molecular beam. The clusters were excited to about 6.2 eV, i.e. above the S2 state of bare benzene. By varying the probe energy (3.1 and 4.65 eV) we examined the dynamics of internal conversion and their dependence on the cluster size. The obtained time constants of internal conversion are in the order of 100 fs. The ultrafast dynamics were not observed before and show that earlier nanosecond studies were only revealing the long-time behaviour after further relaxation and fragmentation of the vibrationally highly excited complexes.

Silver dimers formed in a seeded supersonic argon beam are examined with two laser spectroscopic methods. Excitation fluorescence spectra of the A-X system excited with a narrow band cw dye laser yield accurate constants of the X 1Σg+ and the A 1Σu+ state. From isotope selective resonant two-photon ionization spectra of the B-X and the E-X system excited by a pulsed dye laser and monitored with a time-of-flight mass spectrometer, improved vibrational constants and rotational constants of the B 1Πu and the E 1Πu state are derived. Rotational constants of the C and the D state could be determined from the spectral separations between bandhead and band origin in partly rotationally resolved bands. Autoionizing Rydberg states of Ag2 are stepwise excited with two pulsed dye lasers in two resonant steps. From the convergence limits of different Rydberg series converging towards different vibrational levels v+ in the X 2Σg+ ground state of Ag2+ the rotational constants of the ion ground state and the adiabatic ionization potential IP(Ag2+)=61 747±4 cm−1 could be accurately determined.

Sub-Doppler excitation spectra of the A 1Σu←X 1Σg system of Ag2 molecules in a collimated supersonic Ar/Ag2 beam have been measured with a cw dye laser. Accurate molecular constants of both states are derived from 740 analyzed rotational lines.