General discussion [Experiments on ion-pair formation in gas-phase polyatomic molecules]

A brief summary of experiments on ion-pair formation in gas-phase polyatomic molecules

CF3SF5 : a ‘super’ greenhouse gas

One molecule of the anthropogenic pollutant trifluoromethyl sulphur pentafluoride (CF$_3$SF$_5$), an adduct of the CF$_3$ and SF$_5$ free radicals, causes more global warming than one molecule of any other greenhouse gas yet detected in the Earth’s atmosphere. That is, it has the highest per molecule radiative forcing of any greenhouse pollutant, and the value of its global warming potential is only exceeded by that of SF$_6$. First, the greenhouse effect is described, the properties of a molecule that cause it to be a significant greenhouse gas, and therefore the contributions that physical chemistry can make to an improved understanding of the effect. Second, the chemistry of (CF$_3$SF$_5$), first discovered in the atmosphere in 2000, is taken as a case study. Experiments using tunable vacuum-UV radiation, electrons and small cations have determined some of the relevant physical properties of this molecule, including the strength of the (CF$_3$-SF$_5$) covalent bond. The main sink route to remove (CF$_3$SF$_5$) from the earth’s atmosphere is low-energy electron attachment in the mesosphere. Third, it is shown how such data are important inputs to determine the lifetime of this pollutant, ca. 1000 years, in the atmosphere. Finally, the generic lessons that can be learnt from the study of such long-lived greenhouse gases are described

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

Vacuum-Ultraviolet negative photoion spectroscopy of SF5Cl

Using vacuum-UV radiation from a synchrotron, gas-phase negative ions are detected by mass spectrometry following photoexcitation of SF$_5$Cl. F$^-$, Cl$^-$ and SF$_5^-$are observed, and their ion yields recorded in the range 8-30 eV. F$^-$ and Cl$^-$ show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written AB + h$v$ $\rightarrow$ C$^-$ + D$^+$ (+ neutral(s)). F$^-$ is the strongest signal, and absolute cross sections are determined by calibrating the signal intensity with that of F$^-$ from SF$_6$ and CF$_4$. Resonances are observed, and assigned to transitions to Rydberg states of SF$_5$Cl. The Cl$^-$ signal is much weaker, despite the S-Cl bond being significantly weaker than the S-F bond. Appearance energies for F$^-$ and Cl$^-$ of 12.7 ± 0.2 and 10.6 ± 0.2 eV are determined. The spectra suggest that these ions form indirectly by crossing of Rydberg states of SF$_5$Cl onto an ion-pair continuum

Advocacy : The experience of the National Institute of Adult Continuing Education

この論文は国立情報学研究所の学術雑誌公開支援事業により電子化されました

The Gospel of Mary

La figura de Maria Magdalena en els evangelis canònics ocupa un espai molt reduït, com una de les dones que acompanyen Jesús durant el seu ministeri i després apareix al peu de la creu. Ella es converteix en el focus d’atenció en una sèrie de textos cristians posteriors, i en particular en el text d’un «evangeli» el qual porta el seu nom. L’«Evangeli de Maria» és el focus principal d’aquest assaig. Es dedica una especial atenció a l’evidència que aporten els manuscrits per al text, referent a la qüestió de si es pot dir que és «gnòstic» o no i, sobretot en el debat que succeeix al final del text entre Pere i Maria, tot discutint els problemes que suposen la presentació negativa de Pere i per què precisament es produeix això. S’argumenta que l’Evangeli prové d’una data relativament primerenca, abans que les línies divisòries entre els gnòstics i els cristians s’haguessin endurit i quan tots dos s’enfronten amb hostilitat als estranys.The figure of Mary Magdalene in the canonical gospels occupies a small amount of space as one of the women accompanying Jesus during his ministry and then appearing at the cross. She becomes the focus of attention in a number of later Christian texts, and in particular in a ‘gospel’ text that bears her name. The «Gospel of Mary» is the prime focus of this essay. Attention is given to the manuscript evidence for the text, to the issue of whether it can be said to be «Gnostic», and above all to the debate occurring at the end of the text between Peter and Mary, discussing the problems of how negatively Peter is presented and precisely why. It is argued that the Gospel stems from a relatively early date, before dividing lines Christians and Gnostics had hardened and when both were facing hostility from outsiders

Why Do More Women Study the Arts but Less Women Work in the Arts?

Extract: The figures for women studying the arts versus women working in the arts, particularly in creative roles, are shocking. Lucy Kerbel as part of Tonic Theatre’s recent Advance research found that, in 2013, 59% of graduates from eight of the leading postgraduate directing programmes in the UK were women but compared that with Elizabeth Freestone’s research which found, of the top ten subsidised theatres in the UK, only 24% of directors working in those theatres were women... More information is available from: http://universitywomeninthearts.com

University Women in the Arts Mentoring: “Kicking down doors together”

Jennifer Tucket on the new UK-wide mentoring programme University Women in the Arts from Central Saint Martins