We measure UV and IR spectra in the gas phase for EuOH+, EuCl+, and TbO+ ions, which are produced by an electrospray ionization source and cooled to ~10 K in a cold, 22-pole ion trap. The UV photodissociation (UVPD) spectra of these ions show a number of sharp, well-resolved bands in the 30000–38000 cm–1 region, although a definite assignment of the spectra is difficult because of a high degree of congestion. We also measure an IR spectrum of the EuOH+ ion in the 3500–3800 cm–1 region by IR-UV double resonance spectroscopy, which reveals an OH stretching band at 3732 cm–1. We perform density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of these ions in order to examine the nature of the transitions. The DFT results indicate that the states of highest spin multiplicity (octet for EuOH+ and EuCl+, and septet for TbO+) is substantially more stable than other states of lower spin multiplicity. The TD-DFT calculations suggest that the UV absorption of the EuOH+ and EuCl+ ions arises from Eu(4f)→Eu(5d, 6p) transitions, whereas the electronic transitions of the TbO+ ion are mainly due to the electron promotion of O(2p)→Tb(4f, 6s). The UVPD results of the lanthanide-containing ions in this study suggest the possibility of using lanthanide ions as “conformation reporters” for the gas-phase spectroscopy for large molecules.

This work was partly supported by JSPS KAKENHI Grant Number 16H04098 and by the Swiss National Science Foundation under grant number 200020_152804.