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Abstract

Estimates are reported for the internal energy distribution and the average internal energy deposited in the molecular ion of tungsten hexacarbonyl, W(CO)math image˙, when it is generated by charge exchange of the corresponding doubly charged ion. Charge exchange was performed in the collision energy range from nominal zero to 40 eV using a triple quadrupole mass spectrometer with a variety of polyatomic targets including alkanes, aromatics and tungsten hexacarbonyl itself. At all energies, dissociative charge exchange was accompanied by collision-induced dissociation and this meant that the energetics of charge exchange had to be measured by extrapolation to zero collision energy and zero pressure. Under these conditions, the average internal energy deposited into the charge-exchanged product W(CO)math image˙ was 1.0, 0.9, 1.3 and 2.3 eV for the targets isobutane, n-pentane, n-heptane and toluene, respectively. Data for the symmetrical charge exchange reaction

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demonstrated that partitioning of internal energy between the two products occurred equally. Hence, assuming equipartitioning over all degrees of freedom (at threshold) in the other cases also, the total internal energies deposited in the target molecules were determined to be 1.1, 1.1, 2.3 and 2.7 eV, respectively. Comparison of these values with the reaction exothermicities revealed that ∼2 eV (1.2–2.6 eV) of additional energy is liberated on charge exchange in each case. This is suggested to appear as translational energy of separation of the two charged products. Charge exchange in this system is suggested to occur by a Landau-Zener curve-crossing mechanism and the estimated values of the coulombic energies correspond to electron transfer at internuclear distances of the order of 7 ± 2 Å.