Ultrafast laser experiments on carefully selected DNA model compounds probe the effects of base structure,base stacking,base pairing,and structural disorder on excited electronic states formed by UV absorption in single nucleic base as well as single and double DNA strands.Direct π-orbital overlap between two stacked bases in a dinucleotide or in a longer single strand creates new excited states that decay orders of magnitude more slowly than the generally subpicosecond excited states of on omeric bases.