NEXAFS and ARUP Spectroscopy of an Organic Single Crystal: α-Perylene

Abstract

We have studied the applicability of Near Edge X-ray Absorption Fine Structure (NEXAFS) and Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS) on insulating organic molecular crystals, using single crystals of α-perylene as an example. The influences of sample charging were investigated. By employing suitable compensation techniques in combination with partial yield detection, the charging problem could be solved satisfactorily for NEXAFS and partially for ARUPS. The techniques to be described allowed us to obtain NEXAFS spectra with a fairly high energy resolution of δE = 0.4 eV. The spectra so obtained for α-perylene single crystals display several sharp peaks whose intensities strongly depend on the polarization angle of the incident synchrotron radiation. From the angular dependence we derive an average angle of inclination of the molecular planes relative to the (001) cleavage plane of 85°±5° which is in excellent agreement with the angle determined from a full X-ray crystal structure analysis. The various peaks are tentatively assigned to different transitions from C Is levels to π* and [sgrave]* final states; some spectral details even reflect influences by the crystal structure. It is noteworthy that useful sample thickness can be anywhere between monolayer and macroscopic dimensions.