Localization of electrons in the sugar/phosphate backbone in DNA investigated via resonant Auger decay spectra

In order to elucidate the localized nature of electrons in sugar/phosphate backbone in DNA molecules, resonant Auger decay spectra excited by soft x-rays around the inner-shell ionization thresholds have been measured for single-strand DNA. The systems investigated are thin films of DNA as well as related phosphorus compounds such as nucleotide (adenosine triphosphate, ATP), sodium phosphate, and indium phosphide. For ATP and DNA, it was observed that the resonant excitations from P $1s$ to valence unoccupied ${\pi }^{*}$ orbitals are followed by spectator-type Auger decays where the excited electrons remain in valence orbitals during the core-hole decays. It was also found that the energy of the P $K{L}_{2,3}{L}_{2,3}$ $(2p^{-1}\bullet {\pi }^{*})$ spectator Auger peak shifts linearly with the photon energy due to the resonant Auger Raman scattering. Most of the decay channel at the core-to-valence resonant excitation is spectator-type Auger decay in DNA, which is quite different from the Auger decay processes in metallic and semiconducting materials. We conclude that the excited electrons in valence unoccupied states around the phosphates in DNA molecules are strongly localized, resulting in the insulating properties in a one-dimensional direction along sugar/phosphate backbone.

Figure 1

X-ray photoemission spectra excited by $2400\mathrm{eV}$ photons for DNA molecule; (a) wide scan, (b) carbon $1s$, (c) nitrogen $1s$, (d) oxygen $1s$, and (e) phosphorus $1s$ regions.

Figure 2

X-ray absorption spectra for (a) ATP, (b) DNA, (c) sodium phosphate, and (d) InP(100). The spectra were measured by total electron yield.

Figure 3

(a) Phosphorus $K{L}_{2,3}{L}_{2,3}$ resonant Auger decay spectra for DNA molecule. Peak $A$ with constant kinetic energy is due to normal Auger decay, whose initial state is $2p^{-2}$. Peak $B$ is originating from spectator Auger decay of which initial state is $2p^{-1}\bullet {\pi }^{*}$. Peak $C$ is due to the C $1s$ photoelectrons. (b) X-ray absorption spectra around the phosphorus $K$ edge for DNA molecule. (c) Kinetic energies of P $K{L}_{2,3}{L}_{2,3}$ Auger peak as a function of the photon energy.

Figure 4

(a) Phosphorus $K{L}_{2,3}{L}_{2,3}$ resonant Auger decay spectra for InP(100). (b) X-ray absorption spectra around the phosphorus $K$ edge for InP(100).