Collision energy reactant ions

It is also expected that reactive collisions may diminish the effects of collisional damping of the z-oscillation. An unreactive collision removes energy from the z-mode oscillation so that the ion contributes more signal current at its original cyclotron frequency whereas a reactive collision removes an ion from a reactant population giving a true indication of the loss from the original population. The loss rate from the reactant population for ions of z-oscillation, Az, is proportional to the density of reactant ions of amplitude Az. Thus, for very reactive ions, no change in sensitivity due to collisional relaxation is expected. 

If a reactant gas is introduced into the collision cell, ion-molecule collisions can lead to the observation of gas-phase reactions. Tandem-in-time instruments facilitate the observation of ion-molecule reactions. Reaction times can be extended over appropriate time periods, typically as long as several seconds. It is also possible to vary easily the reactant ion energy. The evolution of the reaction can be followed as a function of time, and equilibrium can be observed. This allows the determination of kinetic and thermodynamic parameters, and has allowed for example the determination of basicity and acidity scales in the gas phase. In tandem-in-space instruments, the time allowed for reaction will be short and can be varied over only a limited range. Moreover, it is difficult to achieve the very low collision energies that promote exothermic ion-molecule reactions. Nevertheless, product ion spectra arising from ion-molecule reactions can be recorded. These spectra can be an alternative to CID to characterize ions. 

The reactivity of the clusters can then be studied by various experimental techniques, including fast flow reactor kinetics in the postvaporization expansion region of a laser evaporation source [21, 22], ion flow tube reactor kinetics of ionic clusters [23, 24], ion cyclotron resonance [25, 26], guided-ion-beam [27], and ion-trap experiments [28-30]. Which of these techniques is applied depends on the charge state of reactants (neutral, cationic, anionic), on whether the clusters are size-selected before the reaction zone, on single or multiple collisions of the clusters with the reactants, on the pressure of a buffer gas if present, and on the temperature and collision energy of the reactant molecules. 

We have studied [11] collisions of state-selected H2(v) ions with He to give (see Eq. 3) HeH+ + H (chemical reaction) and He + H+ + H (collision-induced dissociation — CID) at a relative collision energy of 3.1 eV. Individual vibrational levels between 0 and 6 were studied. The cross sections were put on an absolute scale by normalizing to the earlier work of Chupka and coworkers [24], Both product channels are endothermic for H (v = 0) ions, chemical reaction by 0.81 eV and CID by 2.65 eV. Thus, one expects vibrational excitation of the reactant ions to greatly increase the cross sections for both products. This is confirmed by our results. Absolute cross sections for HeH+ products are shown in Fig. 7 and for the H + products in Fig. 8. 

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