How Can Molecular Beams Be Slowed and Neutralized for MOT Trapping?

[Malamala]

Hello! If I have a beam of positively (single) charged molecular ions (they can be either bunched on continuous), and I want to end up with a decelerated (close to stationary) ensemble of neutral molecules (e.g. to trap them in a MOT trap), is there a way to reduce the velocity of the charged ions using simple electrodes (I know quite well the mean energy of the ions and the spread) and then somehow add one electron to these ions in the region of interest and then trap the resulting neutral molecules? Can someone point me towards some readings that did something similar? Thank you!

 

[Twigg]

My understanding is that neutralizing a beam of cations is much harder than ionizing a beam of neutrals, as you need a source of electrons for the reaction to occur. However, I think you could REMPI a beam of molecular anions to produce neutral molecules. Slowing the anionic molecules is trivial using a quick high voltage pulse to the trap electrode voltages, as you mentioned. However, it seems that producing a high throughput beam of anions is highly non-trivial, as discussed https://www.researchgate.net/publication/327023879_Loading_of_a_continuous_anion_beam_into_a_Penning_trap_with_a_view_to_laser_cooling.

I think it's fair to say that, even if you started with stationary cations, any reaction that would neutralize your molecular ions would also impart such a large momentum that it would inevitably kick the molecule out of the capture range of a dipole or MOT force.

 

[Malamala]

My understanding is that neutralizing a beam of cations is much harder than ionizing a beam of neutrals, as you need a source of electrons for the reaction to occur. However, I think you could REMPI a beam of molecular anions to produce neutral molecules. Slowing the anionic molecules is trivial using a quick high voltage pulse to the trap electrode voltages, as you mentioned. However, it seems that producing a high throughput beam of anions is highly non-trivial, as discussed https://www.researchgate.net/publication/327023879_Loading_of_a_continuous_anion_beam_into_a_Penning_trap_with_a_view_to_laser_cooling.

I think it's fair to say that, even if you started with stationary cations, any reaction that would neutralize your molecular ions would also impart such a large momentum that it would inevitably kick the molecule out of the capture range of a dipole or MOT force.

Thanks a lot for this! So in general, how do people decelerate and trap neutral molecules? I read some stuff about Stark deceleration, but that requires a dipole moment. The external electric field field does induce a dipole moment in the molecules, but for example if the ground state is a $\Sigma$ state, you can't fully polarize the molecule, so you are not necessarily in the linear Stark regime (right?). Can Stark acceleration work in this case, too?

 

[Twigg]

Decelerating neutral molecules is very much an active area of research. Stark decelerators are one pathway, but they have a reputation for being very technically challenging. Coherent optical forces are another pathway. Notably, bichromatic force deflection and molasses was achieved in molecules (SrOH and Sr2, respectively) in the last few years. Another important technology here is cryogenic buffer gas beams (CBGB's) which I believe we've discussed in past threads. The point of the CBGB is to reduce the initial velocity of the molecular beam from the supersonic regime to the diffusive regime. Lower initial velocity means less slowing force is required. There are also some groups that have used Zeeman slowers for molecules; however, this is not a great solution because for a general molecule the branching ratios will ruin your state purity (you'll need repump lasers for days!).