Field dissociation

The conductivity also increases in solutions of weak electrolytes. This second Wien effect (or field dissociation effect) is a result of the effect of the electric field on the dissociation equilibria in weak electrolytes. For example, from a kinetic point of view, the equilibrium between a weak acid HA, its anion A" and the oxonium ion H30+ has a dynamic character ... 

A compound ion may dissociate in a high applied electric field into a neutral atom and an ion. This dissociation process, generally known as field dissociation, was theoretically treated by Hiskes123 in 1961 as an atomic tunneling phenomenon. It is similar to field ionization of an atom... 

The reason that a compound ion can be field dissociated can be easily understood from a potential energy diagram as shown in Fig. 2.23. When r is in the same direction as F, the potential energy curve with respect to the center of mass, V(rn) is reduced by the field. Thus the potential barrier width is now finite, and the vibrating particles can dissociate from one another by quantum mechanical tunneling effect. Rigorously speaking, it... 

Fig. 2.23 When r , which points from the neutral atom to the ion, lines up in the same direction as the applied field F, the potential energy of the system is reduced on one side by the field. Thus the compound ion, in one of its vibrational states, can dissociate by particle tunneling. If r is anti-parallel to F, then the potential energy bends upward, and field dissociation becomes impossible. The direction of r is denoted by the arrow of the He - Rh2+ bond.

Fig. 2.24 Portion of a time-of-flight spectrum in pulsed-laser stimulated field evaporation of a Rh tip in 2 x 10 8 Torr of 4He. Besides the formation of 4HeRh2+, the Rh2+ line now shows a low energy peak of 51 eV additional energy deficit (shaded). Rh2+ ions in this secondary peak are produced by field dissociation of 4HeRh2+ in the field dissociation zone which is about 150 A in width and is centered at —220 A above the tip surface.

Fig. 2.25 Model proposed to explain the data shown in Fig. 2.24. A4HeRh2+ ion, in its as-desorbed orientation, cannot field dissociate as explained in Fig. 2.23. It can dissociate only when it is rotated by an angle greater than 90° from its as-desorbed orientation. The maximum dissociation rate occurs when the rotation angle is 180°. If it is not dissociated during the angular rotation between 90 and 270° then it has to wait for the right orientation again. By that time, the ion is too far away from the surface and the field is too low for field dissociation to be possible. Thus a well-defined field dissociation zone exists. From the additional energy deficit of the secondary Rh2+ peaks and the field distribution above the surface, the dissociation time and the 180° rotation time of the compound ion is calculated to be 790 21 fs.

Fig. 2.26 When 4He is replaced with 3He, the secondary Rh2+ peak disappears even though 3HeRh2+ ions are still formed. At first glance, this strong isotope effect is most surprising since one would expect 3HeRh2+ to field dissociate more easily than 4HeRh2+ because of its smaller reduced mass. This peculiar isotope effect is the result of a center of mass transformation in the applied field, as can be understood from the Schroedinger equation of eq. (2.63), already explained in...

HeRh2+ is more difficult to field dissociate than 4HeRh2+ is because of a center of mass transformation. This transformation, as seen in eq. (2.62), changes the potential of the ion in the applied field. 

Fig. 2.28 Particle penetration probability in field dissociation of 4HeRh + and 3HeRh2+ from a vibrational state 300 K and 500 K above the bottom of the potential energy curve. At the same field, the particle barrier penetration probability for 3HeRh2+ is three to four orders of magnitude smaller than that for 4HeRh2+, in good agreement with the experiment.

In general, ion reaction rates can be observed directly in a time-of-flight spectrometer with a time resolution comparable to that of the system, which is about 10-9 to 10 los. The rate measurement can achieve a much better time resolution by using an ion reaction time amplification method. With this method, very fast ion reactions can be measured with a time resolution much better than the time resolution of the system. It is with this method69 that the field dissociation reaction of 4HeRh2+ was measured with a time resolution of about 20 femtoseconds when the time resolution of the system was still only 1 ns. 

Whether or not an ionic species is desorbed directly from the surface or produced by field dissociation above the emitter surface can easily be determined with a precision measurement of the ion kinetic energy distribution. 

At high field strengths a conductance Increase Is observed both In solution of strong and weak electrolytes. The phenomena were discovered by M. Wien (6- ) and are known as the first and the second Wien effect, respectively. The first Wien effect Is completely explained as an Increase In Ionic mobility which Is a consequency of the Inability of the fast moving Ions to build up an Ionic atmosphere (8). This mobility Increase may also be observed In solution of weak electrolytes but since the second Wien effect Is a much more pronounced effect we must Invoke another explanation, l.e. an Increase In free charge-carriers. The second Wien effect Is therefore a shift in Ionic equilibrium towards free ions upon the application of an electric field and is therefore also known as the Field Dissociation Effect (FDE). Only the smallness of the field dissociation effect safeguards the use of conductance techniques for the study of Ionization equilibria. 

As the equilibrium constant Is mostly derived from conductance data the shift In Ionization with field can also be determined from the measurement of the conductance Increase upon application of a high electric field. Conductance Increase and field dissociation are generally related as ... 

Before proceeding to a complete calculation relating the conductance Increase to the amplitude of the field dissociation effect and Its temporal behavior we will give a short description of the actual circuit used for the field modulation technique (Figure 1) with reference to the previous discussion. 

Figure 10.33. Tandem mass spectrometer system employing electric field dissociation, designed to enable the study of microwave spectra of molecular ions involving vibration-rotation levels lying close to the dissociation limit.

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