Investigation of the Evaporation Thermodynamics of Erbium Trifluoride and Thulium Trifluoride by Knudsen Effusion Mass Spectrometry and Matrix Isolation Infrared Spectroscopy

Abstract

The vaporization of ErF, and TmF, was investigated in the temperature range between 1266 and 1635 K by Knudsen effusion mass spectrometry and matrix isolation infrared spectroscopy. From the ionization efficiency curves the appearance energies of all ions formed from the equilibrium vapour were determined using Vogt's deconvolution method. For the determination of vapour pressure two methods, viz. mass-loss Knudsen effusion and Knudsen effusion mass spectrometry were applied. Comparing our investigation to former ones, the phase transitions on the Clausius-Clapeyron plots of ErF, and TmF, could be identified for the first time using Knudsen effusion mass spectrometry since this latter method is usually not sensitive for small heat effects. The mean natural logarithm of equilibrium vapour pressure as a function of temperature for all condensed phasevapour regions can be expressed as follows: The vibration frequencies of the symmetric and asymmetric stretching modes of DyF,, ErF, and TmF, were measured for the first time, using matrix isolaton infrared spectroscopy. The thermochemical quantities such as enthalpy and entropy changes of sublimation and evaporation were calculated using both second and third laws of thermodynamics and the deviation was discussed in detail.The vaporization of ErF,, NdF, and TmF, has been studied by several previous investigators by both mass-loss Knudsen effusion and electron impact Knudsen effusion mass spectrometric methods.'-6 However, the solid state phase transitions and melting cannot be seen on the l n p VS. 1/T plots in the literature. In order to check the reliability of the measured thermodynamic data, the values obtained by a second law method were compared to those obtained by a third law method. The thermodynamic functions were evaluated on the basis of Jia and Zhang's calculated molecular geometry and vibration frequencies? however, the stretching frequencies were also checked by matrix isolation infrared spectroscopic measurements. The multiplicity of the basic electron state needed for the calculation of thermodynamic functions was determined by ab initio quantum chemical calculations. The ionization and appearance energies, and hence the bond dissociation energies of the neutral molecules and ions, were determined using a deconvolution method* which provided more precise data *Author for correspondence. On leave from Department of Physical Chemistry. Roland Eotvh University, H-I117 Pbminy PCter sCtAny 2, Budapest, Hungary. than the linear extrapolation approach applied by former researchers.' In the present work the effect of Knudsen cell materials (graphite and tungsten) on the spectra, the appearance energies of the ions and the vapour pressures are also discussed. EXPERIMENTALKnudsen effusion mass spectrometry For the mass spectrometric experiments, a home-made (Institute Jozef Stefan, Ljubljana, Slovenia) single focusing apparatus equipped with a Knudsen evaporator was used. The Knudsen cell, made of graphite or tungsten, was placed in a molybdenum crucible having ...