Determination of kinetic characteristics of molecule thermodesorption on the surface of oxidated molybdenum

Abstract

Adsorption and surface ionization (SI) of morphine molecules С₁₇Н₁₉NО₃ with (m/z 285) on the surface of oxidized molybdenum was studied by voltage modulation method (VMM) with a high-vacuum mass spectrometric setup using a “black chamber” with all walls cooled with liquid nitrogen. The SI mass spectra and temperature dependences of ion current of C₉H₇N⁺CH₃ radical with m/z 144 were obtained by ionization of morphine molecules on the surface of oxidized molybdenum by using stationary SI method. They correspond to the previous results obtained by ionization of the same molecules on the surface of oxidized tungsten. The new peaks in the mass spectra can be explained by the catalytic properties of molybdenum. The sublimation heat and SI coefficient of C₉H₇NCH₃ radicals with m/z 144 was determined. The rate constants K⁺ and activation energy E⁺ for radical ions C₉H₇N⁺CH₃ with m/z 144 were determined in the adsorption of morphine molecules. For the first time the rate constants K⁰ and activation energies of thermal desorption Е° for radicals C₉H₇N⁺CH₃ with m/z 144 during adsorption of morphine molecules were determined on the surface of oxidized molybdenum with a break of (C − C₁)_β bonds with the formation of ionized radicals by SI. The dependences LnΔ(t_i) =  f(t) obtained by the VMM for all temperatures of experiments were well approximated by straight lines and made it possible, from the slope of the graphs, to calculate the average lifetimes ${\overline{\tau }}_i$ due to the processes of dissociative SI of morphine molecules on the surface. The ionization potential of this radical was estimated. It was found that the heats of desorption weakly depend on the nature of the surface itself, therefore, in theoretical energy calculations, the Lenard-Johnson model can be used.