The relationship between covariance and anti-covariance mapping

Pyridine clusters are subjected to intense femtosecond laser pulses at fluences around 10(15) W cm(-2) at 620 nm, which results in multiple ionization and concomitant Coulomb explosion. A combined covariance map is considered which can be viewed as the overlay of two maps that describe the statistical interaction of the resulting time-of-flight spectra of the pyridine clusters which lead to species having large values of kinetic energy. The combined map is constructed from ones involving both positive covariance values and anti-covariance values obtained in the same experimental study. Significantly, the individual points representing positive and anti-covariance effects arising from the ionization of the clusters via femtosecond laser pulses are found to be mutually exclusive of each other as exemplified in this pyridine study. Our extension of covariance mapping, which was originally formulated by Frasinski et al. for simple molecular interactions, and is further developed in the present study, elucidates the presence of competitive reactions within cluster systems by comparison of the two maps. (C) 2002 Elsevier Science B.V. All rights reserved.