I Ion Ablation Target Experiment: Estimating Energy Spread

[kelly0303]

Hello! For the experiment I am working on we are using as the ion source an ablation target. We have a 1 inch coin of the material of interest (ytterbium currently) and we send laser pulses to it (we are using a 532 Nd:YAG laser). Can someone point me towards a paper (or tell me from their experience) about what is the energy spread of the produced ions (i assume as a function of laser power - or of any parameters that might be of interest)? I would like an order of magnitude estimation not a precise value necessarily. Thank you!

 

[Twigg]

As a zero-th order approximation, I would use the boiling point for your ytterbium target, which google tells me is $1196^\circ \mathrm{C}$. Here's a numerical modelling paper for aluminum that backs up my assumption; however, they focus on the surface temperature of the solid target rather than the temperature of the gaseous/plasma plume. If you were really curious you could always do Doppler spectroscopy on the $^1S_0 \rightarrow ^1P_1$ line.

 

[kelly0303]

As a zero-th order approximation, I would use the boiling point for your ytterbium target, which google tells me is $1196^\circ \mathrm{C}$. Here's a numerical modelling paper for aluminum that backs up my assumption; however, they focus on the surface temperature of the solid target rather than the temperature of the gaseous/plasma plume. If you were really curious you could always do Doppler spectroscopy on the $^1S_0 \rightarrow ^1P_1$ line.

Thank you for this! But is that at thermal equilibrium? I assumed that the process is fast (and violent) such that thermal equilibrium is not reached fast enough (in our case we have a voltage difference to extract and accelerate the ions from the ion source). Also the energy spread would be on the order of (please let me know if this is wrong) $kT = 0.1$eV. For some reason I thought that would be a lot bigger (I might be wrong but I remember reading about a plasma ion source with energy spreads on the order of hundreds of eV).

 

[Twigg]

But is that at thermal equilibrium? I assumed that the process is fast (and violent) such that thermal equilibrium is not reached fast enough (in our case we have a voltage difference to extract and accelerate the ions from the ion source).

I agree with you on this. However, without making a heroic effort of numerical modelling I don't think you'll be able to calculate the temperature of the plume. I was unable to find an existing paper that does this work.

For some reason I thought that would be a lot bigger (I might be wrong but I remember reading about a plasma ion source with energy spreads on the order of hundreds of eV).

I'm no specialist, but my understanding is that plasma sources come in many shapes and sizes (or temperatures). For example, the Earth's ionosphere is a plasma that ranges in temperature from 200 to 500K (source). On the other hand, the Earth's magnetosphere ranges from 6000K to 35100K (source).

Also, I believe that the laser-ablated plasma will have a high rate of elastic collisions with the ablated neutral gas. I couldn't tell you if the ions were in thermal equilibrium with the neutrals or not (need the densities, initial temperatures, and the elastic collisional cross section to know), but it's something to think about.

If you were really curious you could always do Doppler spectroscopy on the 1S0→1P1 line.

I'm a dummy. That's the line for neutral ytterbium atoms. For the singly-ionized ytterbium ion, it would of course be $^2S_{1/2} \rightarrow ^2P_{1/2}$.

 

[hutchphd]

How about this reference?
https://link.springer.com/content/pdf/10.1007/s10751-020-01708-0.pdf

 

[kelly0303]

I agree with you on this. However, without making a heroic effort of numerical modelling I don't think you'll be able to calculate the temperature of the plume. I was unable to find an existing paper that does this work.I'm no specialist, but my understanding is that plasma sources come in many shapes and sizes (or temperatures). For example, the Earth's ionosphere is a plasma that ranges in temperature from 200 to 500K (source). On the other hand, the Earth's magnetosphere ranges from 6000K to 35100K (source).

Also, I believe that the laser-ablated plasma will have a high rate of elastic collisions with the ablated neutral gas. I couldn't tell you if the ions were in thermal equilibrium with the neutrals or not (need the densities, initial temperatures, and the elastic collisional cross section to know), but it's something to think about.I'm a dummy. That's the line for neutral ytterbium atoms. For the singly-ionized ytterbium ion, it would of course be $^2S_{1/2} \rightarrow ^2P_{1/2}$.

Thank you! I'll have to go into more details through the references you suggested!

At this point, I mainly need an order of magnitude estimate (more like is it below 1eV or above 100eV?), so the thermal equilibrium seems reasonable. It is just much smaller than I imagined. I know that in some cases (such as Isolde - it is not laser ablation there, tho, but I assume the energies are similar?) they take the ions, put them in an ion trap filled with He gas at room temperature and then use the resulting, cooled ions for the experiments. Of course bunching itself helps a lot with the duty cycle when using pulsed lasers, but if one goes from 1500K to 300K, the Doppler linewidth is reduced by only a factor of $\sqrt{5}$, which is not a lot given that those ion bunchers can be very expensive, hence I thought that the Doppler reduction is a lot more significant. Am I missing something?

 

[kelly0303]

How about this reference?
https://link.springer.com/content/pdf/10.1007/s10751-020-01708-0.pdf

Thank you! That looks very promising, I'll take a look at it.