Buffer gas cooling of ions inside a radio-frequency multipole trap is often used in molecular spectroscopy to reduce spectral congestion and aid in analysis. However, exactly how internal temperature relates buffer gas temperature is not well-established, especially at low trap temperatures. The efficacy of thermalizing simple ions and ionic clusters in a cryogenic octopole trap (measured with a Si diode at the endcap) has been investigated. Ammonium and the ammonium-water complex, weakly bound to various atomic and molecular messengers (or “tags”), have been used as "molecular thermometers" to be probed by Cryogenic Ion Vibrational Predissociation spectroscopy. The rotationally-resolved vibrational bands are compared to a simulation based on a Boltzmann distribution of rotational energy level population. This comparison of experimental and simulated spectra allows the internal temperature of trapped ions to be evaluated across different cationic systems and tag species.  The results show that, in all cases, the tagged ion temperature is higher than the trap temperature, with wide variability for more strongly binding tags.

Probing the internal temperature of cold trapped ion-molecule clusters

255th National Meeting (2018)

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