Luminescence of solutions of complex organic compounds irradiated by a high-power electron beam

Abstract

UDC 535.37 The high radiation stability of organic scintillators and their ability to fluoresece under the action of ionizing radiation have been the basis for their application as the active substances in scintillation counters. It has subsequently turned out that many of them can also serve as active media of lasers. Efforts were Undertaken to induce lasing by exciting solutions with an electron beam [I], however these efforts were not successful. A study was also made of the mechanism of formation of excited molecules in solutions subject to irradiation by electrons (see, e.g., review articles [2, 3]), but this effort has not been taken to conclusion at the present time. In connection with this it would be of interest to study the luminescence of different classes of complex molecules under conditions of irradiation by a high-power electron beam and to examine those phenomena which promote or hinder generation of laser radiation by electron-beam pumping. Experiments were carried out on a setup which has been previously described [4]. The electron accelerator provided a current density up to 2 kA/cm 2 at electron energies of ~150 keY, duration of current pulse at half-width (HWFM) of %1 nsec, and beam cross-section immediately after the foil of 20 x 2 mm=. The electron beam was directed into a stainlesssteel cuvette through a Lavsan or polyimide film of thickness 10-15 ~m perpendicular to the direction of observation. The fluorescence spectra were recorded using a WP-4 optical spectrum analyzer. Their temporal characteristics were recorded by the use of an FEK-IISPU coaxial photoelement and a $7-19 oscilloscope. In contrast with optical excitation (OE), in electron excitation (EE), excitation energy is acquired by the solvent molecules as well as by the molecules of the dissolved substance. In this case there can take place transfer of excitation energy to the complex molecules from excited molecules of solvent as well as quenching of the luminescent states of the complex molecules by ions and radicals of the solvent, giving rise to dynamic quenching of fluorescence [5]. A correlation exists between the radiation stability of the solvent molecules and the intensity of the dynamic quenching. Depending on which of these proceses predominates, one distinguishes "effective," "intermediate," and "poor" solvents [6]. For the same concentration of dissolved substance the intensity of fluorescence in different solvents can differ by as much as two orders of magnitude. It is well known that the most "effective" solvents are toluene, xylene, and benzene, and that the "poor" ones include water, the alcohols, and the ethers. In the present article we consider the luminescence properties of molecules of various classes: paraquaterphenyl, 1,4-di(benzoxazol-2'-yl)benzene (BoPBo), 1,4-di(phenylethynyl)benzene (DPEB), anthracene, pyrene, fluorenone, coumarians, phthalimides, rhodamine 6G, oxazine-17, 2,2-difluoro-4-( 4-diethylaminostyrylnaphtho[l,2, e])-l,3,2-dioxab orine (DOB), and also a number of compelxes of Eu s+ As the solvent we primarily used toluene, however in a number of cases we used acetone and benzonitrile, which we discovered to be "intermediate" solvents, and also benzene, o-xylene, and n-hexane. Figure 1 shows fluorescence spectra of a number of complex organic compounds dissolved in toluene at a concentration of ~10 -3 mole/liter for which intense fluorescence was found to take place upon bombardment by an electron beam. It should be noted that the intensity of fluorescence of these compounds did not decrease with increase of the number of pulses of irradiation (measurements were made for up to i03 pulses), which confirms their radiation stability. The intensity of fluorescence of the solutions of some compounds, e.g., coumarin7, oxazine-17, and DOB, even exceed the intensity of luminescence of known effective scintillators. A comparison of the above spectra with spectra obtained by photoexcitation shows that they coincide within the limits of experimental error, in contrast with chemiluminescent excitation [7]. Some cases constitute an exception. The greatest difference is observed for anthracene (Fig. ib), where in the case of EE one maximum is completely absent