Influence of hydrodynamic behavior of laser ablation plume on cluster formation

We conducted fast-photography of the ablation plume and mass analysis of generated clusters. Time-resolved observation of the ablation plumes showed that the behavior of the plume drastically changes depending on the background helium gas pressure. Particularly when the pressure was more than 100 Pa, a strong light emission from the front edge of the plume was observed, indicating that the strong compression of the plume vapor occurs and the kinetic energy of the drifting plume may be converted into the thermal energy of the compressed vapor. We found that this plume compression process could be enhanced by introducing an ellipsoidal cavity, which is due probably to converging waves in the background gas towards the focal point of the ellipsoid. A TOF mass spectroscopy indicated that clusters having a mass corresponding to Al, AlO, Al2, and Al2O were generated in the laser ablation cavity and transported downstream. However, large aluminum clusters consisting of more than a few tens of atoms were not observed in the present experimental condition.