Photoelectron and fluorescence spectroscopy of cryogenically cooled ions

Cryogenic ion traps combined with buffer gas cooling provide ideal conditions for preparing molecular ions in their vibrational ground states. This technique is critical to obtain high-resolution photoelectron spectra of vibrationally excited anions, which would otherwise be obscured by a number of hot band transitions, but also to detect fluorescence events on extended timescales, as it increases the number of ions that can initially be prepared in the desired electronic state.

In the first part, photoelectron spectra of vibrationally excited nitrate (NO₃⁻) anions, measured with the recently developed IR-cryo-SEVI technique [1,2], will be presented. By pre-exciting the fundamental and one of the overtones of NO₃⁻ with an IR laser, new photodetachment transitions into different vibronic levels of the neutral radical can be observed. From these spectra, a clear assignment of the fundamental of NO₃ to ~1050 cm⁻¹ rather than the historically accepted frequency of 1492 cm⁻¹ can be made, settling a long-lasting controversy in the community [3]. Moreover, comparisons with spectral simulations clearly highlight the dominant role vibronic interactions play in describing this seemingly simple molecule.

In the second part, a new experimental setup for fluorescence spectroscopy of trapped molecular ions, which is under construction in our group at the Fritz Haber Institute, will be discussed. The purpose of this experiment is to study dispersed and time-resolved fluorescence in molecular ions of astrochemical relevance, in particular those showing recurrent fluorescence [4,5].

[1] M. DeWitt et al., J. Phys. Chem. A 125, 7260-7265 (2021).

[2] J. A. Lau et al., J. Phys. Chem. A 127, 3133-3147 (2023).

[3] J. F. Stanton, J. Chem. Phys. 126, 134309 (2007).

[4] A. Léger et al., Phys. Rev. Lett. 60, 921-924 (1988).

[5] Y. Ebara et al., Phys. Rev. Lett. 117, 133004 (2016).