Observation of Ultraviolet Rotational Band Contours of the DNA Base Adenine: Determination of the Transition Moment

We report the first observation and analysis of rotational band contours of the jet-cooled DNA base adenine for three vibronic bands at 36,062, 36,105, and 36,248 cm(-1). The lowest npi* and pipi* states have been labeled with their excited-state vibronic symmetry, and a strong pipi*-npi* vibronic coupling via an out-of-plane vibrational mode has been revealed. The rotational band contours have been recorded by resonant two-photon ionization (R2PI) and analyzed by a genetic algorithm (GA) based fit to obtain the optimum band parameters. The vibronic band at 36,062 cm(-1) shows dominant c-type character with transition dipole moment (TDM) components mu(a)2:mu(b)2:mu(c)2 = 0.09:0.17:0.74 and those at 36 105 and 36 248 cm(-1) show abc-hybrid character with predominantly in-plane TDM components. The band at 36,062 cm(-1) has been assigned as the n --> pi* transition, and the 36,105 cm(-1) band as the pi --> pi* transition by the symmetry analysis. The band at 36,248 cm(-1) provides evidence of the strong pipi*-npi* vibronic coupling via an out-of-plane vibrational mode.