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

Abstract

Continuous, black-body-type light emission is observed upon irradiation of gas phase C60 and C70 by 193 nm ArF excimer laser at fluences from 3 to 80 mJ/cm2 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 C60 and C70 molecules are predominantly cooled via inelastic collisions with noble gas atoms above ∼0.5 mbar ambient pressure and below ∼2800 K temperature. The quenching rates are ∼7.1 bar−1 s−1 for C60, and ∼6.4 bar−1 s−1 for C70 in Ar, and 81 bar−1 s−1 for C60 in He ambient, respectively, determined from Stern–Vollmer type relations. The inelastic quenching cross section for He gas (∼4.4×10−23 cm2) is ∼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/cm2 the fullerene temperature saturates at ∼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 C2 “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 μs for C60 and 44±4 μs for C70, respectively, at high (80 mJ/cm2) laser fluences. The measured two- and three-photon multiplicities of the excitation at low laser fluences (<15 mJ/cm2) are in good agreement with the observed cluster temperatures.