spectra of dibenzo-18-crown-6 (DB18C6) complexes with alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) in a cold, 22-pole ion trap. All the comvlexes show a number of vibronically resolved UV bands in the 36 000-38 000 cm(-1) region. The Li+ and Na+ complexes each exhibit two stable conformations in the cold ion trap (as verified by IR-UV double resonance), whereas the K+, Rb+, and Cs+ complexes exist in a single conformation. We analyze the structure of the conformers with the aid of density functional theory (DFT) calculations. In the Li+ and Na+ complexes, DB18C6 distorts the ether ring to fit the cavity size to the small diameter of Li+ and Na+. In the complexes with K+, Rb+, and Cs+, DB18C6 adopts a boat-type (C-2v) open conformation. The le ion is captured in the cavity of the open conformer thanks to the optimum matching between the cavity size and the ion diameter. The Rb+ and Cs+ ions sit on top of the ether ring because they are too large to enter the cavity of the open conformer. According to time-dependent DFT calculations, complexes that are highly distorted to hold metal ions open the ether ring upon S-1-S-0 excitation, and this is confirmed by extensive low-frequency progressions in the UVPD spectra.