Observation of collisional ionization electron spectra of van der Waals clusters with metastable He-*(2 S-3) atoms: An evidence for autoionization from superexcited Ar clusters

Penning ionization electron spectra (PIES) of van der Waals clusters have b een observed for the first time by means of a newly developed magnetic bott le electron spectrometer, which is 1000 times more efficient than a convent ional PIES apparatus. In the PIES of Ar clusters with He*(2 S-3), two bands of Ar 3p hole characters with different ionization energies (IE), band A ( IE=ca.15 eV) and band B (IE=ca.13.5 eV), were observed. Band A was assigned to vertical transitions in Franck-Condon regions corresponding to the band in the ultraviolet photoelectron spectra (UPS) of clusters reported by Car novale [J. Chem. Phys. 90, 1452 (1989)]. Band B, which is an extra band mis sing in the UPS of clusters, can be ascribed to the autoionization into a s table structure of Ar-n(+) via superexcited states of Ar-n. Band B can be c onsidered as an adiabatic ionization which is forbidden as a direct ionizat ion. The energy difference of ca. 1.5 eV between the vertical IE for band A and the adiabatic IE for band B was found to agree well with the reported binding energy of the trimer ion core. (C) 2000 American Institute of Physi cs. [S0021-9606(00)00712-1].