Ultraviolet-Visible Absorption Spectroscopy of MoCl₅, MoOCl₄, and MoO₂Cl₂ Vapors

This article reports quantitative absorption spectra for MoCl₅, MoOCl₄, and MoO₂Cl₂ in the vapor phase from 45,500 to 15,500 cm^-1 (645 to 220 nm). Spectra are obtained in an ultrahigh purity stainless steel vacuum system by rapidly sampling a range of analyte partial pressures. The short measurement times and the differential absorbance method employed here minimize effects from uncontrolled transients such as window deposits. A detailed analysis of replicate measurements and time-resolved spectra shows generally reproducible spectral response from MoCl₅ vapors at 120 °C. The presence of HCl and MoOCl₄ impurities in the MoCl₅ vapor samples is determined to be unlikely based on the analysis of absorbance-pressure curves and spectral peak fitting. Comparison with MoOCl₄ and MoO₂Cl₂ spectra shows a unique spectral fingerprint for MoCl₅ at 28,500 cm^-1 providing a convenient means of discriminating between MoCl₅ and its oxychlorides in the visible wavelengths. Adsorption behavior of MoCl₅ on surfaces is also discussed with particular emphasis on the use of MoCl₅ as a reactant in vapor phase thin film deposition processes.