- The vibronic spectrum of the adenine ± thymine (A ± T) base pair was obtained byone-color resonant two-photon ionization (R2PI) spectroscopyin a free jet of thermallyevaporated 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 ±* origin of 9H-adenine at 36 105 cm 1 . The IR ± UV spectrum was assigned to cluster structures with HNH ¥¥¥ OC/N ¥¥¥ HN hydrogen bonding bycomparison 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.