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 E and the energy-loss function for charged particles -Im(l/E) 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