Study of He clusters by means of a compact time-of-flight detector for atom scattering

The design of a time-of-flight detector for energy resolved He atom scattering experiments is discussed in detail. The project has aimed at realizing a high efficiency detector combining an ionization region with length of ~4 mm and a quadrupole mass filter with mass resolution M/M~10 for helium. The detector performances have been tested using a supersonic helium atom source cooled down to ~20 K. Measurements performed at the helium mass show that the requirements concerning the mass resolution, the sensitivity, and the response time of the detection system have been fulfilled. Helium clusters have been detected within the supersonic beam tuning the filter on mass 8 a.m.u. An intriguing behavior of the dimer ion velocity is pointed out. Moreover, the evolution of the helium and the dimer ion signal with the source pressure is investigated at different source temperatures. The data are interpreted suggesting that clusters of increasing size condense within the beam.