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# Vacuum-UV negative photoion spectroscopy of CH3F, CH3Cl and CH3Br

Rogers, N. J. and Simpson, M. J. and Tuckett, R. P. and Dunn, K. and Latimer, C. J. (2010) Vacuum-UV negative photoion spectroscopy of CH3F, CH3Cl and CH3Br. Physical Chemistry Chemical Physics, 12. pp. 10971-10980. ISSN 1463-9076

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URL of Published Version: http://dx.doi.org/10.1039/C0CP00234H

Identification Number/DOI: 10.1039/C0CP00234H

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH$$_3$$X (X = F,Cl,Br). The anions X$$^-$$, H$$^-$$, CX$$^-$$, CHX$$^-$$ and CH$$_2$$X$$^-$$ are observed, and their ion yields recorded in the range 8-35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + h$$v$$ $$\rightarrow$$ A$$^-$$ + B$$^+$$ (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F$$^-$$ from both SF$$_6$$ and CF$$_4$$. The cross sections for formation of X$$^-$$ + CH$$_3$$$$^+$$ are much greater than for formation of CH$$_2$$X$$^-$$ + H$$^+$$. In common with many quadrupoles, the spectra of $$m$$/$$z$$ 1 (H$$^-$$) anions show contributions from all anions, and only for CH$$_3$$Br is it possible to perform the necessary subtraction to obtain the true H$$^-$$ spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X$$^-$$ and CH$$_2$$X$$^-$$ are used to calculate upper limits to 298 K bond dissociation energies for D$$^o$$ (H$$_3$$C-X) and D$$^o$$ (XH$$_2$$C-H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F$$^-$$ from CH$$_3$$F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition.

Type of Work: Article 2010 (Publication) Colleges (2008 onwards) > College of Engineering & Physical Sciences School of Chemistry QD ChemistryQC Physics University of Birmingham The authors, Royal Society of Chemistry 768 YES

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