Tues 6 July - Thurs 8 July

We've been tiding things up in the lab for the last few days, tinkering with the system. Things seem to keep moving (vertically) and our HeNe is especially unstable, so we now have a series of pinholes in different places in the system. The lens we found that we thought was 30 cm focus was actually either an extraordinarily tight focus or a negative focus lens, so we had an adventure finding a suitable lens; we wound up going to another lab group and borrowing one of theirs. An optics lab without lenses... Anyway, once that got taken care of, we found that the CCD camera we were going to use for beam profiling disappeared. Antonen took it because he needs it for his experiments. He's going to be shooting today and tomorrow, and because of the high energy X-rays involved in his work nobody else can be in the lab when he's shooting. In other words, I can't do lab work until Monday.

On the plus side, Julien is taking Johanna and me to the C2RMF at the Louvre on Monday to do SEM and EDX on the coins I'm going to ablate. We're going to (hopefully, if time allows) do SEM of both sides of 4 coins: one nickel, the roman coin, one dime, and one KETH coin as well as element analysis. In preparation for that, Bianca helped me pick another nickel to use.


We picked this one because it exhibits the best corrosion product uniformity over the face of the coin. For that reason, I'll call it AN43U for uniform.

I've read two papers recently, both of which say something of the same thing. I'll summarize the general concepts.

Ultra-short Laser Ablation of Metals and Semiconductors: Evidence of Ultra-fast Coulomb Explosion
H. Dachraoui et al, Appl. Phys. A 83, 2006

Fast Electronic and Thermal Processes in Femtosecond Laser Ablation of Au
H. Dachraoui and Wolfgang Husinsky, Appl. Phys. Letters 89, 2006

"The natures of processes which govern the interaction of ultrashort laser radiation with materials can be a complex interplay between ultrafast processes and processes on timescales which one traditionally calls the thermal regime" ~ from the 2nd listed paper

The papers discuss Coulomb explosion as a means for ablation. Despite having such a spiffy name, the concept is quite simple. When atoms in a material are exposed to light, the electrons in said atoms are excited. If the light comes in the form of an intense pulse, the electrons may become photoexcited to very high energy levels. Depending on the duration of the pulse, one of two things may happen:
~If the pulse is longer or on the order of the electron-phonon thermal relaxation time, the electrons will transfer energy (heat) to the phonons via collision and will then fall back into their original state, or a similar state
~If the pulse is much shorter (Dachraoui states <100fs), the electrons will be knocked into the continuum band and will escape from their parent atoms. The atoms are thus ionized, and the coulomb repulsion between ions results in ejection of the ions from the lattice. This is referred to as "Coulomb Explosion". Interestingly, this can result in not only the expulsion of ions (electrons and ionized parent atoms) but also neutral particles. The neutral particles are most likely the result of electron capture between ionized atoms and electrons with similar enough velocities. The author used time of flight measurements assuming that time of flight was only dependent on the kinetic energy of the ejected particles.A typical measurement of theirs is found below. This particular measurement is for Iron.

The distribution is bimodal for both 25fs pulse energies, although the graph is not so fantastic. For the 150 mJ/cm^2 there are peaks around 1 and 3 microseconds, corresponding to energies of 3 and .2 eV, respectively. For the 400 fs pulse, only the peak around 3 microseconds is evident. This behavior suggests Coulomb Explosion as a mechanism for generating the high energy electrons and thermal effects for the lower energy electrons, as the high energy electrons are not present after the 400fs pulse.