Knudsen cell mass spectrometric study of the Cs2IOH(g) molecule thermodynamics

• The pronounced ionic character leads to only dissociative ionization processes.
• Ions formed are same as those coming from pure dimmers.
• De-convolution of the ions origin needs accurate thermodynamic values for the pure gas phase.
• Mass spectrometric interpretation has to be performed gradually and as a function of suitable condensed compositions.
• Thermal functions have to be fully estimated.

The gas phase of the CsI + CsOH system is analyzed by high temperature Knudsen cell mass spectrometry in order to confirm the existence of the Cs2IOH(g) complex molecule. The mass spectrometric analysis is quite complex since such molecules undergo dissociative ionization into fragment ions that mix with the same ions from dimers of the pure compounds in the same vapor phase. Varying the chemical conditions for vaporization by using different CsI + CsOH mixture contents showed that the ionization of the Cs2IOH(g) molecule led to five different fragment ions, Cs2OH+, Cs2I+, Cs+, CsOH+ and CsI+. This complex ionization pattern was studied in relation with previous assessed values for the vaporization of CsOH and CsI pure compounds in which monomer and dimer molecules are predominant. The equilibrium constant for the reaction CsI(g) + CsOH(g) = Cs2IOH(g) was determined and, after modeling the structure of the Cs2IOH molecule, the enthalpy of formation was determined using the third law of thermodynamics, as follows: ΔfH°(Cs2IOH, g, 298.15 K) = −578 ± 14.7 kJ · mole−1.