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/argon (FSArn). Clusters are created in a supersonic expansion and ionized by both one-color and two-color (near threshold) resonance enhanced laser ionization. Toluene/water clusters are known to fragment subsequent to ionization by loss of water molecules or by proton transfer and loss of a benzyl radical. This system is selected to test the applicability of covariance mapping techniques to investigate the fragmentation behavior of singly charged cluster ions. To explore sensitivity of the parent ion/fragment ion correlation coefficient to cluster fragmentation, correlation coefficients are measured as a function of ionization photon energy as thresholds for the various fragmentation processes are scanned. For TW: parent ions, correlation coefficients correctly reflect switching between the benzyl radical loss and water loss fragmentation channels as the photon energy is increased. For T2Wn+ cluster ions, fragmentation contributes only about 20% to the correlation coefficient-the other 80% contribution is due to neutral cluster growth. The growth-dominated correlation coefficients scale approximately with the square root of the product of the two ion signal intensities and linearly with the ionization laser intensity, and therefore are not good relative measures of correlations between ions and signals of different intensities. A normalized covariance (covariance/product of signal intensities) is introduced to eliminate this dependence. The laser intensity [similar to(signal product)(1/2)] independent component of the normalized covariance arises from ion correlation due to neutral cluster growth and the laser intensity dependent component of the normalized covariance arises from ion correlation due to cluster ion fragmentation. These findings are applied to study the cluster growth dynamics of AnAr(n) and FSArn clusters. Covariance mapping shows that the broad intensity maxima in the mass spectrum of FSArn clusters are not caused by fragmentation but can be attributed to neutral cluster growth. The observed neutral cluster distribution appears to be a superposition of three broad, overlapping, log-normal-like distributions peaking around cluster sizes n = 4, 8, 20. The difference between the overall shapes 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 equation that depends on three parameters: dimer growth rate, overall cluster growth rate, and a cluster growth cross section that scales with cluster size. (C) 1998 American Institute of Physics.