Ultrafast redistribution of vibrational energy after excitation of NH stretching modes in DNA oligomers

Vibrational relaxation after spectrally selective excitation within the NH stretching band of adenine–thymine base pairs in DNA oligomers was studied by subpicosecond infrared-pump/anti-Stokes Raman-probe spectroscopy. The decay of the different NH stretching vibrations populates distinct accepting modes in the NH bending range with a rise time of 0.6 ps that is close to the NH stretching decay times. The population of thymine fingerprint modes after excitation of the adenine antisymmetric NH2 stretching mode points to an ultrafast excitation transfer to the thymine NH stretching vibration before relaxation. The nonequilibrium fingerprint populations decay on a time scale of several picoseconds.