Photon emission from gas phase fullerenes excited by 193 nm laser radiation

Continuous, black-body-type light emission is observed upon irradiation of gas phase C-60 and C-70 by 193 nm ArF excimer laser at fluences from 3 to 80 mJ/cm(2) in Ar and He ambient. Cluster temperatures are estimated by calibrating the detection system against a tungsten filament and applying Wien's displacement law. Time-resolved spectroscopic measurements show that the initial internal temperature of the irradiated fullerenes (around 2800 K) decreases linearly, while the emitted light intensity decreases exponentially with time, respectively. Excited C-60 and C-70 molecules are predominantly cooled via inelastic collisions with noble gas atoms above similar to 0.5 mbar ambient pressure and below similar to 2800 K temperature. The quenching rates are similar to 7.1 bar(-1 -1) for C-60, and similar to 6.3 bar(-1) s(-1) for C-70 in Ar, and 81 bar(-1) s(-1) for C-60 in lie ambient, respectively, determined from Stern-Vollmer type relations. The inelastic quenching cross section for He gas (similar to 4.4 x 10(-23) cm(2)) is similar to 3.7 times higher than for Ar. This observation may provide further insight on the mechanisms of fullerene synthesis by coalescence of hot carbon vapor in a noble gas atmosphere. At laser fluence above 30-40 mJ/cm(2) the fullerene temperature saturates at similar to 2800 K, indicating a kind of phase-transition- "boiling" of the excited fullerenes. The cluster temperature stabilizes by other cooling mechanisms like electron and/or C-2 "evaporation" (i.e., ionization and/or fragmentation). The unperturbed (i.e., extrapolated to zero ambient pressure) lifetime of the temperature-stabilized species is 100+/-25 mu s for C-60 and 44+/-4 mu s for C-70, respectively, at high (80 mJ/cm(2)) laser fluences. The measured two-and three-photon multiplicities of the excitation at low laser fluences (<15 mJ/cm(2)) are in good agreement with the observed cluster temperatures. (C) 1997 American Institute of Physics.