  Combining two-color infared pump-probe spectroscopy and anharmonic force
field calculations we characterize the anharmonic coupling patterns between
fingerprint modes and the hydrogen-bonded symmetric NH$_2$ stretching vibration
in adenine-thymine dA$_{20}$-dT$_{20}$ DNA oligomers. Specifically, it is shown
that the anharmonic coupling between the NH$_2$ bending and the CO stretching
vibration, both absorbing around 1665 cm-1, can be used to assign the NH$_2$
fundamental transition at 3215 cm-1 despite the broad background absorption of
water.
