Optical and dielectric properties of DNA in the extreme ultraviolet

Abstract

Optical constants of dry films of deoxyribonucleic acid (sodium salt of calf thymus DNA) have been determined in the 2–82 eV region of photon energy. The extinction coefficient k was obtained by transmission measurements on thin films on metal mesh screens. The refractive index n in the nonabsorbing region 2–4 eV was obtained from critical-angle measurements on films deposited on a sapphire semicylinder. To determine the value of n above 4 eV, a Kramers-Kronig analysis was performed using the measured values of k. From these optical constants the complex dielectric function ε and the energy-loss function for charged particles −Im(1/ε) were derived. The results exhibit, in addition to the well-established absorptions around 4.7 and 6.5 eV, a strong absorption at 13.8 eV and a prominent peak in the energy-loss function at 21.6 eV which exhibits a large degree of collective behavior.