In ToF-SIMS, for metal and semiconductor, electron tunneling model has been used to understand the ionization process, in which electron transfer either from the emitted particle to the the surface or the reverse. The probability of ionization has an exponential relationship with the ionization potential and electron affinity. For non-metal, this is negligible because the valence band is fully occupied. I am very curious if there is any theory explaining the ionization of fragment emitted from polymer surface.
Dear Chi,
The following two publications: (1) ToF-SIMS analyses of polystyrene and dibenzanthracene: evidence for fragmentation and metastable decay processes in molecular secondary ion emission, and (2) Secondary molecular ion emission from aliphatic polymers bombarded with low energy ions: Effects of the molecular structure
and the ion beam induced surface degradation may be helpful:
http://perso.uclouvain.be/arnaud.delcorte/articles/ss381.pdf
http://perso.uclouvain.be/arnaud.delcorte/articles/nimb100.pdf
Hoping this will be helpful,
Rafik
Thanks for your answer.
I have gone through these papers before. In fact, I am interested in knowing the analytical function which describes how the ionization probability of fragment emitted from polymer varies with its ionization potential/electron affinity. Intuitively, it should be more or less an exponential function. I am not an expert in quantum mechanics and I am just curious if anyone has solved this problem.
Hi Chi Pui,
The ion formation model due to electron tunneling was developed for sputtered atoms! By the way, the statistical model of secondary ion formation predicts the same exponential dependence on the ionization potential and electron affinity of sputtered atom…
In Tof-SIMS there are many secondary clusters in sputtered flux. This is a principal difference of TOF and a conventional SIMS. In the case of sputtered atom the ionization probability should be the same, in first approximation: proportional to exponent of the ionization potential and electron affinity.
In the case of sputtered molecules (clusters), the situation is more difficult, because we have two different processes: cluster formation (the mechanism is not defined yet) and cluster ionization. In some cases the secondary cluster ion can be formed due to a decomposition of a bigger cluster, escaping the surface (unimolecular de-composition). And so on.
Regards
Dear Dr. Kudriavtsev,
Thank you for your answer. I agree with your points. The mechanism is just so complicated that we do not know whether the cluster ionization would have an exponential dependence on the ionization potential and electron affinity of the sputtered molecule. That is probably why I could not find any work in the literature, which attempts to solve this issue.
Regards
Dear Dr. Kudriavtsev,
One more thing I would like to know. If an ion fragment is not formed from unimolecular decomposition of larger cluster, but directly formed by scission of a polymer chain. Do you think if it is possible the ionization probability of that ion fragment can be understood using electron tunneling model?
Regards,
Jeremy
Hi Jeremy,
I am not a fun of the “tunneling model”. I think that “statistical model” is more adequate to describe the secondary ion formation process.
I could add some comments about the problem, only.
- When an accelerated ion penetrates into solid and forms a cascade, the ion energy is transferred (dissipate) to atoms, then to electrons. The electron transition time is less then 1E-15 sec. So, there is a specific electron distribution in cascade area, and there are many “no-equilibrium” electrons in conducting band and holes in valence band during time of life of the cascade.
- There is a little information about ionization potential of clusters. But we can estimate a simple rule: the ionization potential (Ip) of a cluster with increase of the number of atoms in cluster tends to the value of the work function (W) of the material. Thus, the ionization probability of a “big clusters“, which is proportional to exponent of (W-Ip), tends to value 0.5 (approximately). You can see this tendency in different experimental works devoted to experimental study of the ionization probability of sputtered clusters.
(realized by professor Wucher, for example).
Dear Dr. Kudriavtsev,
Thank you for your suggestion. Your comment is very comprehensive and it definitely helps me understand more about the ionization process. Stay in touch.
Regards,
Jeremy
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