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Ultraviolet radiation can lead to photo-lesions and mutations in DNA, which can cause several diseases, most notably skin carcinomas. The most abundant UV-induced photo-lesions result from the formation of a cyclobutane ring between adjacent thymine bases (CPD-lesions). Although the formation of these thymine dimers has been reported in the early 1960s, until now neither the time scale of dimer formation nor the reaction mechanism has been resolved. The focus of this work is on the investigation of photophysical and photochemical processes in nucleic acids on the\npicosecond time scale.\n\nPump-probe spectroscopy allows the investigation of ultrafast, photo-induced processes. The applied spectrometer is based on a central femtosecond laser system. The emitted short light pulses (800 nm, 90 fs) are converted via nonlinear processes into the required spectral regions\n(ultraviolet pump: ~ 270 nm, mid infrared probe: 3 - 10 \xb5m). In this way the high-structure sensitivity of vibrational spectroscopy can be combined with a time resolution in the sub picosecond regime.\n\nFor the investigation of thymine dimer formation, two thymine derivatives were chosen: the 18-mer all-thymine single strand (dT)18 and the mononucleotide thymidine-5'-monophosphate. Additional experiments were performed on the all-adenine single strand poly(A) and the mononucleotide adenine-5'-monophosphate, as adenine is the complementary base of thymine in the DNA double helix.\n\nWhile in AMP virtually all excited adenine bases return to the vibrationally excited ground state via a fast, internal conversion (< 1 ps), there is an additional population of long-lived, electronic states in poly(A), with lifetimes in the 100 ps- and ns-regime. The population of these states correlates with the amount of stacked bases in poly(A). This can be explained by the formation of excimer states. The IR absorption of these states could be deduced in this paper for the first time.\n\nTo resolve the UV-induced formation of CPD-lesions, the characteristic IR absorption of thymine dimers was determined from stationary irradiation experiments on (dT)18. Afterward the formation of thymine dimers in (dT)18 was shown to occur within one picosecond (10^-12 s) by comparing the time resolved measurements on TMP and (dT)18. Long-lived electronic states (100 ps - 1 ns) do not lead to\ndimer formation. Therefore, the photoreaction can only take place if the conformation of two bases at the moment of UV absorption is already suitable for dimerization. This interpretation can be transferred to the DNA double helix, in which deviations from the ideal helix structure are necessary for dimer formation. In this work a fundamental question of photochemical reactions in DNA is resolved, which is of central importance to the understanding of the frequency of damage and mutation patterns in the genome.