DELAYED IONIZATION OF IR-LASER EXCITED C-60

Using a pulsed CO2 laser we have desorbed C60 clusters from a solid ta rget into an expanding He jet, coupled to a time-of-flight mass spectr ometer. We observed self ionization of the excited clusters with a tim e delay of 120 +/-20 mus. This is large compared to previously reporte d delays of up to 15 mus after laser excitation with visible or ultrav iolet light. We suggest that the delayed ionization is caused by struc tural isomers of hot C60 clusters differing from the truncated-icosahe dron structure.