Infrared-resonance-enhanced multiphoton ionization (REMPI) of fullerenes

Summary form only given.The gas-phase fullerenes are resonantly heated by a train of high-power subpicosecond pulses to internal energies at which they efficiently undergo thermal autoionization. Ions produced are detected in a time-of-flight mass spectrometer. When the laser is tuned from 6 to 20 /spl mu/m while recording the amount of parent ions produced by the absorption of 200-600 IR photons, almost-fragmentation-free ion spectra are observed. The resonances in the IR-REMPI spectrum of C/sub 60/ are systematically found at slightly lower frequencies than the gas-phase emission lines and show somewhat asymmetric line shapes, tailing toward the position of the gas-phase IR emission lines. The observations are explained by single-photon absorption in a micropulse, and the sequential absorptions over many micropulses can then provide the required energy for autoionization. To study the mechanism of thermal emission of electrons, the time evolution of the production of ions following IR excitation is recorded both for normal and for electronically preexcited C/sub 60/. Ion creation is observed to be unexpectedly slow, occurring at times beyond 0.1 ms after laser excitation. The ion creation process is significantly faster for electronically preexcited molecules. It is concluded that the coupling between the ground electronic state and the ionic state is via intermediate states and that reaching the first electronically excited state is a high hurdle on the way from hot molecules to ions.