UV and IR Spectroscopy of Cryogenically Cooled, Lanthanide-Containing Ions in the Gas Phase

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⁺) are substantially more stable than other states of lower-spin multiplicity. The TD-DFT calculations suggest that UV absorption of the EuOH⁺ and EuCl⁺ ions arises from Eu(4f) → Eu(5d,6p) transitions, whereas 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 gas-phase spectroscopy for large molecules.