  DNA autoionization is a fundamental process wherein UV-photoexcited
nucleobases dissipate energy by charge transfer to the environment without
undergoing chemical damage. Here, single-wall carbon nanotubes (SWNT) are
explored as a photoluminescent reporter for studying the mechanism and rates of
DNA autoionization. Two-color photoluminescence spectroscopy allows separate
photoexcitation of the DNA and the SWNTs in the UV and visible range,
respectively. A strong SWNT photoluminescence quenching is observed when the UV
pump is resonant with the DNA absorption, consistent with charge transfer from
the excited states of the DNA to the SWNT. Semiempirical calculations of the
DNA-SWNT electronic structure, combined with a Green's function theory for
charge transfer, show a 20 fs autoionization rate, dominated by the hole
transfer. Rate-equation analysis of the spectroscopy data confirms that the
quenching rate is limited by the thermalization of the free charge carriers
transferred to the nanotube reservoir. The developed approach has a great
potential for monitoring DNA excitation, autoionization, and chemical damage
both {\it in vivo} and {\it in vitro}.
