HIGH-RESOLUTION TIME-OF-FLIGHT MASS-SPECTROMETRY OF CARBON AND CARBONACEOUS CLUSTERS

We produce carbon (Cn) and carbonaceous (Cn H m) clusters by the addition of hydrogen to the helium carrier gas in a laser-vaporization cluster source, and probe the neutral cluster distributions for n≥20 by single-photon ionization in conjunction with high resolution time-of-flight mass spectrometry using an angular reflectron. By adjusting the growth conditions in the cluster source, we produce Cn distributions with wide variations in their degree of local maxima, i.e., magic numbers. We find low-growth conditions under which unstable Cn (n = 30-40 and odd «for n > 40) are produced in nearly the same abundances as the more stable clusters, particularly the even n for n > 40. Increasing the carrier gas density and residence time in the source leads to distributions in which the even clusters dominate their odd-numbered neighbors at n > 40. The carbonaceous cluster distributions produced by hydrogen addition always show a dramatic preference for the hydrogenation of clusters with odd n; the even clusters are observed predominantly as pure Cn. We perform a quantitative hydrogénation analysis of the CnHm, that yields the hydrogénation fraction, the distribution of hydrogens, and the C/H ratio for each n. The results of this analysis provide evidence for the formation of long-chain polyacetylenes, Cn H2, for even n up to n= 44 and show the dramatic even/odd alternation in hydrogenation behavior beginning near n = 40. We discuss our results for the large clusters in terms of the proposed spherical structures for even clusters and the corresponding model for particle growth via nuclei comprised of incomplete carbonaceous shells. © 1990 American Institute of Physics.