Y2O3 nanoparticles prepared by an inert gas phase condensation process were used to introduce artificial pinning centres in YBa2Cu3O7-delta thin films. The areal density of the particles was varied between 120 and 4200 particles mu m(-2) without changing the mean particle diameter of approximately 9 nm. Y2O3 particles were deposited on TiO2 terminated SrTiO3 (100) single-crystal substrates with areal densities up to 1654 particles mu m(-2) and subsequently covered with YBa2Cu3O7-delta by off-axis pulsed laser deposition (PLD). The areal density of the room temperature deposited particles is not changed by the substrate heating during PLD although their height on the substrate decreases. An influence on J(c) is demonstrated for particle densities above 1588 particles mu m(-2), indicating that the substrate decoration with Y2O3 nanoparticles from the gas phase affects the formation of artificial pinning centres in the YBa2Cu3O7-delta films and can be applied to further study of the effect of particle size and areal density on defect formation in YBa2Cu3O7-delta.

The first femtosecond pump-probe photoelectron spectroscopy experiment for neutral clusters is reported. The à state of ammonia molecules and clusters is excited with 120 fs pulses at 200 nm and probed by ionisation at 267 or 400 nm. Ions and electrons are detected in coincidence. The high repetition rate of the fs laser allows one to reduce the coincidence rate per laser pulse to less than 0.05 per pulse so that photoelectrons and photoions can be correlated unambiguously. The energy flow in the excited neutral cluster states is followed in real time, and the energy content in the ionic clusters can be related to the fragmentation patterns.

Ultrashort 155 nm pulses are used to study the dynamics of ammonia clusters excited to vibronic levels of the B̃ and C′ state. For the monomer unexpectedly long lifetimes of about 8 ps for NH3 and 65 ps for ND3 have been found while the clusters decay in the sub-ps region. In contrast to earlier observations for ammonia clusters excited to the A electronic state, for (ND3)n the lifetime of the one-photon excited vibronic levels of the B (and C′) state decreases with the cluster size.

The size dependence of the internal conversion efficiency for the optically excited electronic S2-state in benzene(NH3)n clusters has been studied. The analysis of pump-probe experiments with 160 fs laser pulses reveals a significant enhancement of the S2 to S1 conversion probability with the number n of ammonia molecules in the cluster. In contrast, the total deactivation rate of the S2-state by internal conversion of about 1013 s−1 is nearly independent of the cluster size.

The lifetime of the benzene dimer in the S2 state has been determined directly by pump-probe measurements with 160 fs laser pulses. The value of ≈ 40 fs obtained characterises the fast internal conversion to the S0 as well as to the S1 state. The probability of the S2 to S1 state conversion for the dimer is small but significant and a factor of about 2 larger than that for the benzene monomer. These results explain in a direct manner corresponding experiments with ns laser pulses where two-photon enhancement of the ion yield is observed only when the second photon has sufficient energy to ionise the S1 state.

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 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.

Vibrationally resolved electronic spectra of small heteroclusters BnSm of benzene (B) and sulfur hexafluoride (S) have been obtained by resonant two-photon ionization spectroscopy. Some spectroscopic details of the B1S1 and B1S2 complexes were studied in the spectral regions near the S0S1, 000 and 610 transitions of the benzene monomer. A nonvanishing 000 vibrational band has been observed for B1S1 with a C3v equilibrium symmetry. For this cluster we have determined the ionization potential IP=9.180 +-0.005 eV as well as the binding energies of B1S1 and B1S+1. In view of the geometry of the complexes B1Sm (m=1−4) we found that until m=3 the SF6 molecules are localized on one side of the benzene molecular plane, while for B1S4 the symmetrical arrangement with two S2 groups on each side is favoured. The fragmentation of (BnSm)+ clusters is restricted to the loss of S1 molecules while emission of B1 molecules has not been observed.

Applying mass selective two-color resonant two-photon ionization spectroscopy, we have measured the ionization and fragmentation thresholds of small benzene-(SF6)m clusters. For the resonant excitation of the electronic S1 state either the O00 vibrationless transition or the 610 vibronic transition of the heteroclusters were used. Simple theoretical model estimations were considered to interpret the experimental results. As results well defined ionization potentials for the four clusters benzene-(SF6m, n = 1–4, were obtained while the measured fragmentation threshold could be identified as the appearance potential only for the two smallest heteroclusters (m = 1,2). In this case the binding energies of the clusters in the ionic and the neutral ground states have been derived. For the larger clusters (m = 3, 4) a kinetic shift has been observed causing higher fragmentation thresholds than the true appearance potentials.