Absorption Cross-section and Density Measurement of Dye Vapors

In the vapor phase a solvent-free spectroscopic analysis of dye molecules is applicable [1-3]. For some dye vapors laser action was achieved [1-3]. The dye vapor pressure rises approximately exponentially with temperature. Reasonable vapor pressures (p around 100 Pa = 1 mbar) are obtained for many non-ionic dyes in a temperature region between 200 and 400 °C without considerable thermal decomposition of the dye molecules [1-3]. Ionic dyes have lower vapor pressures and are more difficult to transfer to the vapor phase without decomposition [4-6]. Most dye vapor absorption and emission spectra are presented in relative units, since the number density of dye molecules in the gas phase is seldom determined. In ref. [4] the vapor pressure of some dyes was measured with an isoteniscope (a glass spring pressure gauge) [7]. If all dye molecules in a closed cell are evaporated the vapor pressure is determined by the amount of inputted dye and the absolute absorption cross-section spectrum may be obtained [8-11]. The method of flow of molecules through a calibrated orifice and dye condensation at a cooled plate [12] was sometimes applied [9-12]. In this paper the saturated dye vapor density and dye vapor pressure of dimethyl-POPOP (l,4-di-[2-(4methyl-5~phenyloxazolyl)]benzene) is determined as a function of temperature by absorption measurements. Calibration is achieved by studying the temperature dependence of the absorption for a fixed amount of inputted dye. The absolute absorption cross-section spectra of dimethyl-POPOP are measured for various vapor temperatures. In some measurements buffer gas (He) was added to the dye vapor. The dye vapor absorption cross-section spectra are compared with liquid solution spectra (solvents cylohexane, toluene, benzene, chloroform and methanol). Relevant dye-dye desorption and dye-stainless steel desorption parameters are deduced.