DYNAMICS OF NEUTRAL CLUSTER GROWTH AND CLUSTER ION FRAGMENTATION FOR TOLUENE WATER, ANILINE/ARGON, AND 4-FLUOROSTYRENE/ARGON CLUSTERS - COVARIANCE MAPPING OF THE MASS-SPECTRAL DATA/

Neutral cluster growth and ionic cluster fragmentation are studied for toluene/water (TWn), aniline/ argon (AnAr(n)), and 4-fluorostyrene/ar gon (FSArn). Clusters are created in a supersonic expansion and ionize d by both one-color and two-color (near threshold) resonance enhanced laser ionization. Toluene/water clusters are known to fragment subsequ ent to ionization by loss of water molecules or by proton transfer and loss of a benzyl radical. This system is selected to test the applica bility of covariance mapping techniques to investigate the fragmentati on behavior of singly charged cluster ions. To explore sensitivity of the parent ion/fragment ion correlation coefficient to cluster fragmen tation, correlation coefficients are measured as a function of ionizat ion photon energy as thresholds for the various fragmentation processe s are scanned. For TW: parent ions, correlation coefficients correctly reflect switching between the benzyl radical loss and water loss frag mentation channels as the photon energy is increased. For T2Wn+ cluste r ions, fragmentation contributes only about 20% to the correlation co efficient-the other 80% contribution is due to neutral cluster growth. The growth-dominated correlation coefficients scale approximately wit h the square root of the product of the two ion signal intensities and linearly with the ionization laser intensity, and therefore are not g ood relative measures of correlations between ions and signals of diff erent intensities. A normalized covariance (covariance/product of sign al intensities) is introduced to eliminate this dependence. The laser intensity [similar to(signal product)(1/2)] independent component of t he normalized covariance arises from ion correlation due to neutral cl uster growth and the laser intensity dependent component of the normal ized covariance arises from ion correlation due to cluster ion fragmen tation. These findings are applied to study the cluster growth dynamic s of AnAr(n) and FSArn clusters. Covariance mapping shows that the bro ad intensity maxima in the mass spectrum of FSArn clusters are not cau sed by fragmentation but can be attributed to neutral cluster growth. The observed neutral cluster distribution appears to be a superpositio n of three broad, overlapping, log-normal-like distributions peaking a round cluster sizes n = 4, 8, 20. The difference between the overall s hapes of the AnAr(n) and FSArn mass distributions appears to be due to faster dimer and cluster growth kinetics for the FSArn cluster system . The growth kinetics for the latter two cluster systems can be fully explained and modeled by a simple closed form algebraic kinetic equati on that depends on three parameters: dimer growth rate, overall cluste r growth rate, and a cluster growth cross section that scales with clu ster size. (C) 1998 American Institute of Physics.