Figure 6

N K-edge NEXAFS spectra of the A _n T ₁₅ brushes after rinsing with only H ₂ O (upper panels) or with H ₂ O-NaOH (lower panels) followed by hybridization with an A ₁₅ target. Left panels: Spectra acquired at an X-ray incidence angle of 55˚. The positions of the most pronounced, π*-like absorption resonances of thymine (T) and adenine (A) are marked by the thin vertical blue (T) or red (A) dotted lines. Right panels: Difference of the spectra acquired at X-ray incidence angles of 90° and 20°. Adenine and thymine contributions are indicated by the red or blue fill (respectively), with an upward or downward direction indicating a preferentially horizontal or vertical orientation (respectively) of the DNA strand relative to the surface. Horizontal dashed lines correspond to zero for the individual difference spectra.