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

An enhanced supersonic carbon source produces carbon atoms in their C(P-3(j)) 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 piano-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 10(13) 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) cm(3) 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 similar to 10% C-2 and C-3 in helium, minimized by multiphoton dissociation, and eliminating the postablation nozzle region. (C) 1995 American Institute of Physics.