- The vibronic spectra of laser desorbed and jet cooled guanine (G) adenine (A), and cytosine (C) consist of bands from four, two and two major tautomers respectively, as revealed by UV-UV and IR-UV double resonance spectroscopy. The vibronic spectrum of adenine around 277 nm consists of weak nπ* and strong ππ* transitions, based on IR-UV and deuteration experiments. Precise ionization potentials of G and A were determined with 2-color, 2-photon ionization. We also measured vibronic and IR spectra of several base pairs. GC exhibits a HNH ... OH/NH ... N/C=O ... HNH bonding similar to the Watson-Crick GC base pair but with C as enol tautomer. One GG isomer exhibits non-symmetric hydrogen bonding with HNH ... N/NH ... N/C=O ... HNH interactions. A second observed GG isomer has a symmetrical hydrogen bond arrangement with C=O ... NH/NH ... O=C bonding. Two CC isomers were observed with symmetrical C=O ... NH/NH ... O=C bonding and nonsymmetrical C=O ... HNH/NH ... N interaction, respectively. Guanosine (Gs), 2-DeoxyGs und 3-DeoxyGs each exhibit only one isomer in the investigated wavelength range around 290 nm with a strong intramolecular sugar(5-OH) ... enolguanine(3-N) hydrogen bond.