Collisional energy transfer probabilities of highly excited molecules fromkinetically controlled selective ionization (KCSI). I. The KCSI technique:Experimental approach for the determination of P(E-',E) in the quasicontinuous energy range

The method of kinetically controlled selective ionization (KCSI) for invest igating collisional energy transfer in highly vibrationally excited molecul es is presented in detail. In this first paper of a series the focus is on the key concepts and the technical realization of KCSI experiments to provi de a common basis for following reports on our available results of KCSI st udies on the vibrational relaxation of a variety of larger molecules. The K CSI technique directly monitors the energetic position and shape of the pop ulation distributions g(E,t) during the relaxation process by means of an e nergy selective two photon ionization process via an electronic intermediat e state. Such measurements allow-for the first time-to extract complete and accurate experimental sets of transition probability distributions P(E',E) even at quasicontinuous densities of states. Basic energy transfer quantit ies are already obtained from a straightforward analysis of the arrival tim e and width of the KCSI curves. A master equation formalism is outlined whi ch is the basis of a data inversion providing a complete evaluation of the experimental information content. Various examples of characteristic KCSI d ata on collisional deactivation of highly vibrationally excited molecular p opulations are used to discuss important aspects of the quality and the gen eral character of P(E-',E) parameters deduced from such measurements. The c onditions for a successful modeling of these data are very tightly bound, a nd the resulting energy transfer parameters <Delta E(E)(n)> are therefore o f high precision. In Paper II [J. Chem. Phys. 112, 4090 (2000), following a rticle] we give a full account of the toluene KCSI experiments. We will dea l with our completed studies on azulene, azulene-d(8), pyrazine and pyridin e in follow-up publications of this series. (C) 2000 American Institute of Physics. [S0021-9606(00)01504-X].