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

Abstract

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

Photochemistry of CH3Cl: Dissociation and CH···Cl Hydrogen Bond Formation

“…Excitation at 193.3 nm (6.41 eV) is expected to reach the long-wavelength absorption limit of the nσ* band, 21 while at 157.6 nm (7.87 eV) it should reach a region of much larger absorption cross section, already belonging to the n3s band. 21,22 Apart from the well-known photodissociation channels forming neutral fragments, Tuckett et al 23,24 measured the cross sections for Cl  generation following photoexcitation of CH 3 Cl in the range from 8 to 35 eV and obtained the most intense peak in the region from 10.5 to 11.9 eV. Moreover, this peak is much more intense than that reported for CH 2 Cl  formation.…”

Negative‐ion/positive‐ion coincidence spectroscopy as a tool to identify anionic fragments: The case of core‐excited CHF3

“…(Some channels have no circles, but they are so intense that there is no doubt about their observation.) CF − and F 2 − anions have previously been observed in the valence region of the CF 3 Cl, CF 3 Br, and CF 3 I molecules . CF − was also observed at the core edges of CF 4 , but F 2 − was not …”

“…We attribute the observation to the increased negative ion formation at the C 1s → Rydberg excitations. In fact, anion formation in valence photoexcitation of small molecules is generally associated with Rydberg resonances . In the case of core‐excited states, spectator Auger decay is the most important decay channel in small molecules composed of light atoms.…”

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO)

“…One of the authors has both studied and reviewed extensively the formation of negative fragment ions from polyatomic molecules by mass spectrometry. 142 In many studies only the anion is detected so that the cation (plus possibly neutrals(s)) has to be inferred/guessed, usually by energetics. However, in polyatomic ion pair formation where both the anion and the cation can be produced in a range of internal vibrational states, it is highly advantageous to detect not only both ions, but also measure their translational energy distributions in order to obtain the full information about the dissociation dynamics.…”