Vacuum-UV negative photoion spectroscopy of gas-phase polyatomic molecules

This Review describes recent experiments to detect anions following vacuum-UV photoexcitation of gas-phase polyatomic molecules. Using synchrotron radiation in the range 10-35 eV at a resolution down to 0.02 eV, negative ions formed are detected by mass spectrometry. The molecules studied in detail include CF\(_4\), SF\(_6\) and CH\(_4\); the CF\(_3\)X series where X = Cl,Br,I; the CH\(_3\)Y series where Y = F,Cl,Br; and SF\(_5\)Z where Z = CF\(_3\),Cl. Spectra and raw data only are reported for other members of the CH\(_x\)F\(_y\), CH\(_x\)Cl\(_y\) including CCl\(_4\), and CF\(_x\)Cl\(_y\) series where (\(x\)+\(y\)) = 4; and saturated and unsaturated members of the C\(_m\)H\(_n\) and C\(_m\)F\(_n\) series up to m = 3. Anions detected range from atomic species such as H-, F- and Cl- through to heavier polyatomics such as SF\(_5^-\), CF\(_3^-\) and CH\(_2\)Cl\(^-\). The majority of anions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written as ABC + h\(v\) \(\rightarrow\) D\(^-\) + E\(^+\) + neutral(s). In a few cases, the anion signal increases much more rapidly than a linear dependence with pressure, suggesting that anions now form via a multi-step process such as dissociative electron attachment. Cross sections for ion-pair formation can be put on to an absolute scale by calibrating the signal strength with those of F\(^-\) from SF\(_6\) and CF\(_4\), although there are difficulties associated with the determination of H\(^-\) cross sections from hydrogen-containing molecules unless this anion is dominant. Following normalisation to total vacuum-UV absorption cross sections (where data are available), quantum yields for anion production are obtained. Cross sections in the range ca. 10\(^{-23}\) to 10\(^{-19}\) cm\(^2\) , and quantum yields in the range ca. 10\(^{-6}\) to 10\(^{-3}\) are reported. The Review describes the two ion-pair mechanisms of indirect and direct formation and their differing characteristics, and the properties needed for anion formation by dissociative electron attachment. From this huge quantity of data, attempts are made to rationalise the circumstances needed for favourable formation of anions, and which anions have the largest cross section for their formation. Since most anions form indirectly via predissociation of an initially-excited Rydberg state of the parent molecule by an ion-pair continuum, it appears that the dynamics of this curve crossing is the dominant process which determines which anions are formed preferentially. The thermochemistry of the different exit channels and the microscopic properties of the anion formed do not appear to be especially significant. Finally, for the reaction ABC + h\(v\) \(\rightarrow\) A\(^-\) + BC\(^+\) , the appearance energy of A\(^-\) can be used to determine an upper limit to the bond dissociation energy of AB (to A + BC), or an upper limit to that of ABC\(^+\) (to A + BC\(^+\)). Where known, the data are in excellent agreement with literature values.