UV and IR spectroscopy of cold 1,2-dimethoxybenzene complexes with alkali metal ions

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of 1,2-dimethoxybenzene (DMB) complexes with alkali metal ions, M+·DMB (M = Li, Na, K, Rb, and Cs), in a cold, 22-pole ion trap. The UVPD spectrum of the Li+ complex shows a strong origin band. For the K+·DMB, Rb+·DMB, and Cs+·DMB complexes, the origin band is very weak and low-frequency progressions are much more extensive than that of the Li+ ion. In the case of the Na+·DMB complex, spectral features are similar to those of the K+, Rb+, and Cs+ complexes, but vibronic bands are not resolved. Geometry optimization with density functional theory indicates that the metal ions are bonded to the oxygen atoms in all the M+·DMB complexes. For the Li+ complex in the S0 state, the Li+ ion is located in the same plane as the benzene ring, while the Na+, K+, Rb+, and Cs+ ions are located off the plane. In the S1 state, the Li+ complex has a structure similar to that in the S0 state, providing the strong origin band in the UV spectrum. In contrast, the other complexes show a large structural change in the out-of-plane direction upon S1–S0 excitation, which results in the extensive low-frequency progressions in the UVPD spectra. For the Na+·DMB complex, fast charge transfer occurs from Na+ to DMB after the UV excitation, making the bandwidth of the UVPD spectrum much broader than that of the other complexes and producing the photofragment DMB+ ion.