A high-intensity, pulsed supersonic carbon source with C(3P j) kinetic energies of 0.08–0.7 eV for crossed beam experiments

Abstract

An enhanced supersonic carbon source produces carbon atoms in their C(3Pj) electronic ground states via laser ablation of graphite at 266 nm. The 30 Hz (40±2) mJ output of a Nd-YAG laser is focused onto a rotating graphite rod with a 1000 mm focal length UV-grade fused silica plano-convex lens to a spot of (0.5±0.05) mm diameter. Ablated carbon atoms are subsequently seeded into helium or neon carrier gas yielding intensities up to 1013 C atoms cm−3 in the interaction region of a universal crossed beam apparatus. The greatly enhanced number density and duty cycle shift the limit of feasible crossed beam experiments down to rate constants as low as 10−11–10−12 cm3 s−1. Carbon beam velocities between 3300 and 1100 m s−1, with speed ratios ranging from 2.8 to 7.2, are continuously tunable on-line and in situ without changing carrier gases by varying the time delay between the laser pulse, the pulsed valve, and a chopper wheel located 40 mm after the laser ablation. Neither electronically excited carbon atoms nor ions could be detected within the error limits of a quadrupole-mass spectrometric detector. Carbon clusters are restricted to ∼10% C2 and C3 in helium, minimized by multiphoton dissociation, and eliminating the postablation nozzle region.