Average kinetic energy release

An important example of the application of this method is seen for the case of ammonia. Referring to Figure 13, the measured average kinetic energy release of metastable (NH3)nH+ (n = 4-17) is seen to display a maximum value of... 

Figure 13. A plot of the measured average kinetic energy release < r> during the metastable unimolecular decomposition of (NH3)nH+, n = 4-17, as a function of cluster size. The technique involves use of the reflectron shown in Figure 2a. Taken with permission from ref. 2.

Equation (10.33) can also be used to obtain the average kinetic energy release ... 

This result is very different from the modified prior distribution (chapter 9). It implies that the average kinetic energy release is given by 2k T, that is, a four-dimensional translational energy distribution. When the expression for = [(s — l)/2]( ,) is substituted into Eq. (10.34), and the latter equation solved for , we finally obtain... 

The values of 5 and g depend upon the structure of the complex. If it is assumed that the cluster is made up of a central core consisting of (COj) [i-e., (C02) J+i = (C02)J(CQ2) i] surrounded by loosely held COj units, then any of the loosely held CO2 units can evaporate. Thus it is assumed that the cluster symmetry number is n — 1 while the transition state has a symmetry of 1, so that g = n-1. Because the intramolecular CO2 modes are assumed too high to contribute to the density of states at the low energies involved in the metastable ion dissociation, the total number of vibrational modes is just 5 = 5 n— 1). (With the loss of each linear CO2 unit, the cluster looses five van der Waals modes.) With these assumptions and the measured (Stace and Shukla, 1982) average kinetic energy releases, the binding energies for CO2 monomers... 

With each evaporation, the internal energy of the cluster decreases. The last dissociation will be very slow taking place in the microsecond domain. The fragment ions from these slow dissociation are the metastable ions which can be detected in a mass spectrometer. Furthermore, the average kinetic energy released in this final dissociation can be measured with considerable accuracy (see Eq. 5.30). 

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