FT-ICR Studies of Laser Desorbed Carbon Clusters.

Abstract

Atomic and molecular clusters are being recognized as playing an important role in the thin-film deposition process and phase-change phenomena. Furthermore, small clusters are the most adequate system for the verification of quantum molecular dynamics calculations such as the interference of light and matter. Hence, experimental treatments of such atomic and molecular clusters are now desired. In order to examine such clusters, we have implemented a Fourier transform ion cyclotron resonance (FT-ICR) spectrometer directly connected to a laser-vaporization supersonicexpansion cluster beam source.The heart of the FT-ICR spectrometer was made of ICR cell cylinder centered in a strong homogeneous magnetic field of a 6 Tesla superconducting magnet. The atomic cluster beam was generated outside of magnetic field by the laser vaporization of a solid sample disk, followed by cooling with supersonic expansion of pulsed helium gas. The ionized cluster was carried by helium gas and directly injected to the magnetic field. By measuring the ion-cyclotron frequency, which was inversely proportional to the ion mass, a very highresolution mass spectrum can be obtained.The high mass-resolution was demonstrated for positive mass spectra of silicon, carbon, and metal-carbon binary clusters and negative mass spectra of metal-carbon binary clusters. For bare carbon positive clusters, we found the special condition where the odd-numbered clusters were observed in the range of C 30 to C 50 and the continuous change to C 60 -dominant condition and 'normal' even-numbered distribution.An example of mass spectrum measured by the directinjection FT-ICR apparatus is shown in Figure A Copyright © 1999 by ASME 2Magnetic field f :Ion cyclotron resonance frequency m :Mass of cluster ion q :Charge of cluster ion r :Radius of cyclotron motion v : Velocity INTRODUCTIONAtomic and molecular clusters are being recognized as playing an important role in the thin-film deposition process and phase-change phenomena. Furthermore, small clusters are the most adequate system for the verification of quantum molecular dynamics calculations such as the interference of light and matter, since a cluster is the unique small atomic system with a physically clear boundary condition. Hence, experimental treatments of such atomic and molecular clusters are now desired.Studies of clusters have another possibility leading to the discovery of new materials such as fullerene. Fullerene was first discovered in the mass-spectroscopic study by Kroto et al. (1985). Then, in 1990 the macroscopic generation by arcdischarge technique [Krätschmer et al. (1990), Haufler et al. (1991)] was introduced. Furthermore, higher fullerene [Kikuchi et al. (1992)], metal-containing fullerene , Kikuchi et al. (1993), Takata et al. (1995] and nanotube [Iijima (1991), Thess et al. (1996)] were also generated by the similar technique. Even though the generation of fullerene is now possible by the arc-discharge technique or laser-oven technique ), Wakabayashi et al. (1997], the formation ...