Aug. 5, 1999 The origin of the abundant negative and positive charges produced during low energy surface-impact of polar molecule clusters has been discovered by Scientists from the Max Planck Institute for Quantum Optics in Garching/Germany.
It is a puzzling observation, that snowballs formed from a handful of neutral molecules such as H20 or SO2 - so called molecular clusters - efficiently break up into positively and negatively charged fragments upon impact on essentially any solid surface. The amazing part of this is the fact, that each constituent molecule has far to little energy to induce ionization. Where does the energy come from, that is required to strip a tightly bound electron off the molecule? Is this one example of a collective effect in which many particles help together to cope with one greater task?
Scientists from the Max Planck Institute of Quantum Optics, who were taking a closer look at the composition of the charged fragments during their study of cluster surface collision, now have discovered a different mechanism behind this phenomenon. Their findings appear in the August 5th issue of Nature.
Based on the mass analysis of charged SO2 cluster fragments, they propose that the key to charge separation is the pickup of neutral alkali atoms during surface contact, followed by the delocalization of their outermost electron (valence electron) inside the cluster. The collision induced rapid fragmentation of the cluster finally leads to the formation of separate ionic fragments.
"It was a tremendously exciting moment, when we cross-checked our assumption on the role of alkali atoms in the charging process by treating the surface with additional cesium atoms from an oven," recalls C. R. Gebhardt, PhD student conducting the experiments. "The relevant peaks in our spectrum started rising, ... and we knew, that we understood what is going on."
The signal prior to treating the target with alkali atoms is attributed to a ubiquitous alkali presence, be it in extremely low concentration. Due to this high sensitivity for alkali adsorbates, it seems possible to base new surface analysis methods on this effect reaching a new order of magnitude in the detection limit, as well as to devise new surface cleaning methods. The possibility of increasing the charging efficiency by offering additional alkali atoms, makes the phenomenon interesting for heavy ion creation in connection with ion propulsion, cluster or aerosol analysis, heavy anion plasmas or the creation of size selected clusters for spectroscopy or solvation studies.
"This effect is so strong and robust that we nick-named it the clusterelectric effect in analogy to the well known photoelectric effect because free charge carriers are created due to the interaction of a cluster or a beam of clusters with matter. Moreover, the efficiency per incident particle is also comparable. We are positive, that it will be applied in a wide variety of situations," says Hartmut Schröder, senior scientist and group leader at the Max Planck Institute for Quantum Optics. "We have been able to detect charges created via this mechanism even from a randomly selected pebble. With all the water clusters around in our atmosphere and in space, the clusterelectric effect might also be relevant for natural electrification processes."
The new mechanism for the creation of charge carriers in the atmosphere might proceed as follows: a hydrometeor - the scientific term for various forms of atmospheric water - picks up one or several alkali atoms upon collision with e.g. an aerosol and increases its intrinsic conductivity through the formation of solvated cations and electrons. This may octivate one of the many possible cloud charging mechanisms to become effective, or the hydrometeor may directly burst into charged pieces like the clusters in the experiments.
The Garching group found in experiments with clusters of molecules like ammonia and sulfur tetrafluoride and water that impact-induced charge separation is quite common among polar molecule clusters. Moreover, it is not only induced by alkali atoms, since the scientists could observe the effect also with indium surface adsorbates. On the other hand, clusters of unpolar molecules like oxygen or nitrogen did not show the effect at all. "The microscopic mechanism seems to be the interaction of the molecular dipole moment with an easy-to-ionize particle picked up during surface collision," explains Dr. Hartmut Schröder. "The time window for this process, given by the short duration of the collision, is on the order of one picosecond. Based on this enormous time constraint, we have to accept that the process of electron delocalization takes place without a hindering barrier, in a way quite similar to field ionization."
For Karl-Ludwig Kompa, director of the Max Planck Institute for Quantum Optics, the present work represents a further step on the way to a new regime of chemistry: "We like to think of the cluster as a tiny test tube, which we examine by fragmenting it. Having this new cluster charging mechanism at hand, we now should be able to extend our investigations to higher collision energies. Using the technique of cluster-surface collisions we can expose a reactive system embedded in the cluster to extreme energy densities on a femtosecond time scale. As predicted by my colleague Raphael Levine, it then should be possible to drive strongly forbidden reactions, a field which for a long time has intrigued chemists and physicist all over the world."
This project was supported by SFB 377, Chemiefonds and the BMBF. The authors have applied for a patent.
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