Reduced cluster ion energy

Figure 3.2 Cluster-surface collision process space diagram illustrating the dependence of the Cluster-surface collision-induced process on the cohesive energies of the system (R = ( (Coh) V( (Coh) " )) and reduced cluster ion energy ( ]dn /- ci/ (Coh) )- Reprinted with permission from Harbich (2000) Copyright 2000 from Springer-Verlag.

Reduced cluster ion energy Cluster ion energy divided by the number of atoms... 

Increasing the dose (the deposited energy into the irradiated medium, expressed in Grays, 1 Gy = 1 J.kg" ) leads to more reduced metal ions and then to more clusters with higher nuclearities. 

Thus, the ambient air can be directly introduced to achieve an on-line measurement in the PTR-MS operation. Due to the presence of electric field, in the reaction region, the ion energy is closely related to the reduced-field E/N, where E is the electric field and N is the number density of gas in the drift tube. In a typical PTR-MS measurement, E/N is required to set to an appropriate value normally in the range of 120-160 Td (1 Td = 10 Vcm /molecule), which may restrain the formation of the water cluster ions H30 (H20) n = 1-3) to avoid the ligand switch reaction with analyte M [6] ... 

Many related effects have been studied in detail. Dissociation energies have been derived from metastable decay rates of alkali cluster ions. The gaps between electronic shells have been probed by measuring the size dependence of the ionization energy. Reduced ionization energies are, indeed, found for systems with 9, 21, 41, etc. valence electrons. The anomalies are much less than predicted, but refined effective (spherically nonsymmetric) potentials can explain this trend as well as the observation of some additional fine structure in the size distribution and in the ionization energy. Additional information... 

H30+(H20) clusters form by termolecular reactions of the type H30 (H20) .i + H2O + X H30 (H20) + X, where X is some tiiird body. In addition to increased fragmentation of H30 (H20)n clusters at an elevated kinetic energy, the rate of formation of diese cluster ions is reduced because the rate coefficient has a strongly negative energy dependence. 

To see the effect that the E/N has on cluster survival, it is useful to calculate the collision energy between a cluster ion and a buffer gas molecule. We will focus on the H3O+(H2O) cluster, since this is likely to be the most abundant of the hydrated hydronium cluster ions in any PTR-MS instrument. For reasons discussed in Section 3.4.2 the mean velocities of H30" (H20) and H30" in a drift tube will be the same despite their different reduced mobilities and therefore, since H3O+ will be far more abundant than H30+(H20) at an E/N of 120 Td (see below), we can use the drift velocity (906 m s ) of H3O+ calculated in Section 3.4.1 for illustration purposes. Employing Equation 3.28 and including the expression for KE on in Equation 3.22, the centre-of-mass collision energy reduces to... 

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