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 silic a piano-convex lens to a spot of (0.5+/-0.05) mm diameter. Ablated car bon atoms are subsequently seeded into helium or neon carrier gas yiel ding intensities up to 10(13) C atoms cm(-3) in the interaction region of a universal crossed beam apparatus. The greatly enhanced number de nsity and duty cycle shift the limit of feasible crossed beam experime nts down to rate constants as low as 10(-11)-10(-12) cm(3) s(-1). Carb on beam velocities between 3300 and 1100 m s(-1), with speed ratios ra nging from 2.8 to 7.2, are continuously tunable on-line and in situ wi thout changing carrier gases by varying the time delay between the las er pulse, the pulsed valve, and a chopper wheel located 40 mm after th e laser ablation. Neither electronically excited carbon atoms nor ions could be detected within the error limits of a quadrupole-mass spectr ometric detector. Carbon clusters are restricted to similar to 10% C-2 and C-3 in helium, minimized by multiphoton dissociation, and elimina ting the postablation nozzle region. (C) 1995 American Institute of Ph ysics.