Vacuum-UV negative photoion spectroscopy of CH₄

Abstract

_________________________________________________Using synchrotron radiation in the range 12−35 eV, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of methane. Ion yields for H − , CH − and CH 2 − are recorded, the spectra of CH − and CH 2 − for the first time. All ions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F − from SF 6 and CF 4 .Following normalisation to total vacuum-UV absorption cross sections, quantum yields for anion production are reported. There is a major discrepancy in the H − cross section with an earlier measurement, which remains unresolved. The anions arise from both direct and indirect ion-pair mechanisms. increasing with increasing v in the ν 1 (a 1 ) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions.___________________________________________________

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

“…The experimental AE 298 (F − ) is 11.05±0.05 eV, and this anion gives rise to peak 1 centred at 11.7 eV. This peak occurs below the onset of ionisation for SF 5 CF 3 , 12.9 eV [42], so the presence of photoelectrons from reaction (19) is not relevant. The energy of peak 1 is close to peaks observed in the SF 5 CF 3 photoabsorption and total fluorescence yield spectra at 11.4 eV [39,61].…”

“…Examples where this has caused severe problems include the weak H − signal detected from CH 3 Y molecules (Y = F, Cl, Br), which mimic the much stronger X − spectra [20]. By contrast, H − detected from CH 4 is an example where this is not an issue because the H − signal is dominant [19].…”

“…In a series of experiments performed over the last five years, we have exploited the increased sensitivity of modern mass spectrometers and the wide tunability and availability of synchrotron sources over the energy range 10−30 eV to study anion formation from a wide range of polyatomic molecules. The systems studied include CH 4 , CF 4 , SF 6 , CH 3 X (X = F,Cl,Br), CF 3 Y (Y = Cl,Br,I), SF 5 Z (Z = Cl,CF 3 ), CH x F y (x+y = 4), CH a Cl b (a+b = 4), CF c Cl d (c+d = 4), C m H n and C m F n (m = 1,2,3) [17][18][19][20][21][22]. For common molecules studied, a much wider range of anions are observed than those observed by Mitsuke et al, and we have developed a generic methodology to determine absolute cross sections and quantum yields for anion formation; this has never been done before for such a wide range of molecules.…”

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

“…As in our previous studies on SF 5 CF 3 , 21 the CF 3 X series (X = Cl,Br,I), 22 and CH 4 , 23 we can write that:…”

Vacuum Ultraviolet Negative Photoion Spectroscopy of SF5Cl

“…The signals are first normalised to photon flux, ring current, gas pressure, ionization gauge sensitivity, and relative mass sensitivity of the QMS to detection of the different anions; full details are given elsewhere. 23 These signals are then put onto an absolute scale by determining the F − intensity in the experiment from SF 6 2,20 We estimate that the cross sections are accurate to a factor of ca. This suggests that SF 5 − likely results from the two-step process of dissociative electron attachment.…”