The Compton Research Group at the University of Tennessee

Dipole Bound Negative Anions 

Any molecule that has a dipole moment greater than about 2.5 Debye can bind an extra electron.  That means that if there is enough charge separation in a molecule such that one end of the molecule has a partial negative charge and the other end has a partial positive charge an excess electron can be attached to the end that has the partial positive charge.  We have been able to make about thirty dipole-bound anions, a few of which have been previously studied by others, but for most of them this is the first time they have been observed.  These molecules include those with low dipole moments of about 2.8 D such as acetone and cyclohexanone to those with large dipole moments of 4 and 5 Debye such as acetonitrile.

These dipole-bound anions are created by transferring an electron from an atom to the molecule of interest.  The donor atoms used are atoms that have just one electron in their outermost electron shell – the alkali metals.  The energy of this electron can be fine-tuned by excitation with a laser to a higher shell which is called a Rydberg state.  Any of the alkali metals (sodium, potassium, etc.) would work, but we use rubidium because it is easiest to excite the electron to high Rydberg states using the lasers that are in the visible part of the spectrum.  After the atoms have been excited to a particularly high Rydberg state the molecule under study (such as acetone) is pulsed into the interaction region and the electron is transferred.  The resulting negative ion is then extracted into a time of flight mass spectrometer by applying a positive electric field that pushes the molecule in the direction of the detector.  

Since every molecule is different we would expect different molecules to accept electrons that have been excited to different energies.  In fact this turns out to be the case.  Molecules with low dipole moments accept electrons that have been excited to very high Rydberg states (such as n=50) whereas molecules with high dipole moments accept electrons that have been excited to very low Rydberg states (such as n=10).  Accepting an electron from a high Rydberg state corresponds to a low electron affinity and from a low Rydberg state corresponds to a high electron affinity.  However, even though there is a good correlation between dipole moment and electron affinity, molecules with similar dipole moments do not follow this rule exactly.  There are other factors that play a role, such as polarizibility, shape, etc. that must be examined.

One of the main goals of our research is to obtain accurate electron affinities for each of these molecules so that we can better understand what exactly is going on in the electron transfer reaction.  We can estimate the electron affinity from the Rydberg state that gives the maximum anion yield, but that is not an exact measurement.  One method to measure the electron affinity that was developed in the process of our research is increasing the electric field used to push out the ions into my mass spectrometer until it was large enough to field ionize the dipole-bound electron.  That means that as the electric field increases eventually there is an electric field in which the anion cannot exist.  This has been measured for a number of the molecules we have studied and the results are promising.  However, it is difficult to calculate the electron affinities exactly because of tunneling effects and the amount of time that the molecule spends in the electric field.  Perhaps the most promising method under development is the detachment of the excess electron from the dipole-bound anion using another laser.  The electron then has energy that is equal to the energy of the laser less the electron affinity of the anion.  These electrons will travel down the time of flight tube at a velocity dictated by their energies.  If certain velocities are calibrated then the exact energies of the electrons and therefore the electron affinity of the anion can be exactly calculated.