Off-resonance energy absorption in a linear Paul trap due to mass selective resonant quenching

Abstract

Linear Paul traps (LPT) are used in many experimental studies such as mass spectrometry, atom-ion collisions, and ion-molecule reactions. Mass selective resonant quenching (MSRQ) is implemented in LPT either to identify a charged particle's mass or to remove unwanted ions from a controlled experimental environment. In the latter case, MSRQ can introduce undesired heating to co-trapped ions of different mass, whose secular motion is off resonance with the quenching ac field, which we call off-resonance energy absorption (OREA). We present simulations and experimental evidence that show that the OREA increases exponentially with the number of ions loaded into the trap and with the amplitude of the off-resonance external ac field.

Measurement of the low-energyNa+−Natotal collision rate in an ion-neutral hybrid trap

“…For a detailed discussion of our group's simulations, see Refs. [20,56]. The most important factor in predicting the ion cloud's thermalized mean secular energy (from which one can assign a temperature, assuming a MB speed distribution) is the size of the MOT when the LPT is loaded via PI from a MOT [3,20].…”

“…To remove the undesired NaJ ions, we add to the driving rf voltage a small mass selective resonant quenching (MSRQ) [53,56,61,62] voltage with amplitude Vrad = 0.625 ± 0.005 V at a frequency 157 ± 1 kHz, which corresponds to the measured second harmonic secular frequency 2 [cor/{2rr)] for NaJ. The MSRQ signal resonantly drives the secular motion of the co-trapped NaJ until the molecular ion's energy exceeds the LPT's trap depth.…”

“…The MSRQ signal resonantly drives the secular motion of the co-trapped NaJ until the molecular ion's energy exceeds the LPT's trap depth. As a result, the added MSRQ field continuously quenches the NaJ population with little to no off-resonant heating of the trapped Na+ [56].…”

Sympathetic cooling of molecular ions with ultracold atoms

“…In this regime, the ions are created and then immediately ejected from the trap with minimal effect on the trapped ions. The Smith group at UCONN has also implemented secular frequency excitation to remove unwanted Na + 2 ions, produced through PAI, from their experiments [53]. It may also be possible to use an intercombination MOT, in e.g.…”

“…In our experience, we have found that these ions can typically be ejected from the trap by changing the Mathieu a and q parameters to eject the unwanted ions. Secular excitation has been used in other experiments [53] to remove unwanted ions, but it often has the undesirable effect that the excited ions heat and eject the other ions in the trap. Finally, it is also possible to load molecular ions into the trap by first trapping an atomic ion of interest, usually produced by ionization of a neutral atomic gas, and then leaking in neutral molecular gas to react with the atomic ions and produce the desired molecular ions.…”