- The vibronic spectrum of the adenine-thymine (A-T) base pair was obtained by one-color resonant two-photon ionization (R2PI) spectroscopy in a free jet of thermally evaporated A and T under conditions favorable for formation of small clusters. The onset of the spectrum at 35,064 cm-1 exhibits a large red shift relative to the pi-pi* origin of 9H-adenine at 36,105 cm-1. The IR-UV spectrum was assigned to cluster structures with HNH...O=C/N...HN hydrogen bonding by comparison with the IR spectra of A and T monomers and with ab initio calculated vibrational spectra of the most stable A-T isomers. The Watson-Crick A-T base pair is not the most stable base-pair structure at different levels of ab initio theory, and its vibrational spectrum is not in agreement with the observed experimental spectrum. Experiments with methylated A and T were performed to further support the structural assignment.