DELAYED IONIZATION OF IR-LASER EXCITED C-60

Using a pulsed CO2 laser we have desorbed C60 clusters from a solid target into an expanding He jet, coupled to a time-of-flight mass spectrometer. We observed self ionization of the excited clusters with a time delay of 120 +/-20 mus. This is large compared to previously reported delays of up to 15 mus after laser excitation with visible or ultraviolet light. We suggest that the delayed ionization is caused by structural isomers of hot C60 clusters differing from the truncated-icosahedron structure.