Measured Appearance Energies Research Articles

Absolute cross sections for the dissociative electron impact ionization of the CF x ( x = 1−3) free radicals

We studied the dissociative ionization of the CF x ( x = 1−3) free radicals by electron impact and measured absolute partial cross sections and appearance energies for the formation of the various fragment ions. Fast beams (3–3.5 kV) of CF 3, CF 2 and CF were prepared by near-resonant charge transfer of CF + 3, CF + 2 and CF + with Xe under conditions where the neutral beams contained primarily ground state ions with little vibrational excitation (0.5 eV or less). Absolute cross sections for the various molecular fragment ions were measured from threshold to 200 eV with peak cross sections of up to 1.3 Å 2. The cross sections for the formation of the atomic C + and F + ions were found to be small (0.3 Å 2 or less) except for the F + cross section from CF 2. The measured appearance energies indicate that the fragment ions are formed with excess kinetic energies ranging from essentially zero to about 3 eV per fragment. The CF + cross section from CF 2 shows a second onset which corresponds to the double positive ion formation CF 2 → CF + + F +. Extensive ion trajectory calculations were carried out in order to quantify the collection efficiency of our apparatus for fragment ions formed with excess kinetic energy.

Electroionization study of ethane: ionization and appearance energies, ion-pair formations and negative ions

Threshold region ionization efficiency curves of ethane, of all singly charged fragments consisting of at least one carbon atom, and of [Formula: see text] and [Formula: see text] are presented and analysed. An energy filtered monoenergetic electron beam apparatus capable of producing very high signal-to-noise ratio data was used to study these ions. The C2H6 ionization energy is evaluated to be 11.57 ± 0.02 eV and structures due to competition with [Formula: see text] fragment production are observed at higher energy in the [Formula: see text] curve. As a result of the high apparatus sensitivity several of the measured appearance energies are lower (some by several eV) than previously reported. [Formula: see text], [Formula: see text], and [Formula: see text] ion pair formations are observed. The [Formula: see text] ion pair formation, previously reported by several studies, is shown to be obscured by the isotopic [Formula: see text] fragment signal.

Molecular beam photoionization study of S2

Higher resolution photoionization efficiency data for the formation of S+2 and S+ from S2 in the wavelength region of 600–1350 Å have been obtained using the supersonic oven beam method. The ionization energy (IE) of S2 is determined to be 9.356±0.002 eV (1325.2±0.3 Å). The measured appearance energy of 14.732±0.005 eV (841.6±0.3 Å) for the dissociative ionization process S2+hν→S++S+e−, together with the IE of S2, yields a value of 5.376±0.005 eV for the bond dissociation energy of S+2. Two Rydberg series converging to the b̃ 4Σ−g state (13.20 eV) of S+2 are observed. Window resonances resolved in the wavelength region of 670–850 Å are assigned as members of the Rydberg series converging to the c̃ 4Σ−u (17.70 eV) and 2Πu (or 2Σ−u) (18.66 eV) states of S+2.

A study of the unimolecular decompositions of the (C3H6)+2 and (c-C3H6)+2 complexes

The major product channels identified in the unimolecular decompositions of C3H+6⋅C3H6 and c-C3H+6⋅c-C3H6 in the total energy [neutral (C3H6)2 or (c-C3H6)2 heat of formation plus excitation energy] range of ∼230–450 kcal/mol are C3H+7+C3H5, C4H+7+C2H5, C4H+8+C2H4, and C5H+9+CH3. The measured appearance energy for C4H+7(9.54±0.04 eV) from (C3H6)2 is equal to the thermochemical threshold for the formation of C4H+7+C2H5 from (C3H6)2, indicating that the exit potential energy barrier for the ion–molecule reaction C3H+6+C3H6→C4H+7+C2H5 is negligible. There is evidence that the formations of C4H+7+C2H4+H from (C3H6)+2 and (c-C3H6)+2 also proceed with high probabilities when they are energetically allowed. The variations of the relative abundances for C4H+7,C4H+8, and C5H+9 from (C3H6)+2 and (c-C3H6)+2 as a function of ionizing photon energy are in qualitative agreement, suggesting that (C3H6)+2 and (c-C3H6)+2 rearrange to similar C6H+12 isomers prior to fragmentation. The fact that C6H+11 is found to be a primary ion from the unimolecular decomposition of (c-C3H6)+2 but not (C3H6)+2 supports the conclusion that the distribution of C6H+12 collision complexes involved in the C3H+6+C3H6 reactions is different from that in the cyclopropane ion–molecule reactions. Using the ionization energies (IE) of (C3H6)2(9.33±0.04 eV) and (c-C3H6)2(9.61±0.04 eV) determined in this study, the calculation of the bond dissociation energies for C3H+6⋅C3H6 and c-C3H+6⋅c-C3H6 gives 0.43 and 0.14 eV, respectively. The measured IE of C3H6 is 9.738±0.003 eV and that of c-C3H6 is 9.721±0.011 eV.

Doubly charged ion mass spectra. 7—acetylenes

AbstractDoubly charged ion mass spectra of 20 aliphatic and 3 aromatic acetylenic compounds have been measured using a double focusing Hitachi RMU‐7L mass spectrometer. Spectra were obtained using 100 eV ionizing electron energy and 3.2 kV ion accelerating voltage. In general, the spectra of aliphatic type acetylenic compounds were dominated by fragment ions formed by extensive H loss from doubly charged molecular ions. Intense molecular ions were observed in the doubly charged ion spectra of phenyl‐substituted acetylenes. Total product ion intensities for doubly charged ion spectra of acetylenic compounds were found to be smaller, in general, than the total product ion intensity observed in the benzene doubly charged ion mass spectrum. Measured appearance energies of intense product ions ranged from 24 to 47 eV. A geometry optimized quantum mechanical self‐consistent field molecular orbital treatment was employed to compute energies and structural parameters of prominent ions in the doubly charged ion mass spectra of acetylenic compounds.

Molecular beam photoionization study of HgBr2 and HgI2

Photoionization efficiency (PIE) data for HgBr2+ and HgI2+ and their fragment ions have been obtained in the region 600–1350 Å using an oven-type supersonic beam source. The ionization energies (IE) for the X̃ 2Π3/2g states of HgBr2+ and HgI2+ were determined to be 10.560±0.003 and 9.5088±0.0022 eV, respectively. The analyses of the Rydberg series converging to the 2Π1/2g states of HgBr2+ and HgI2+ yield a value of 10.8846±0.0012 eV for the IE of the 2Π1/2g state of HgBr2+ and 10.1953±0.0025 eV for that of HgI2+. The major fragment ions from HgBr2 were identified to be HgBr+, Hg+, Br2+, and Br+ and those from HgI2 were found to be HgI+, I2+, and I+. The measured appearance energies for HgBr+ and HgI+ allow the calculation of the bond dissociation energies for HgBr+ and HgI+ to be 55±2 and 59±1 kcal/mol, respectively. Similar to the observation in the PIE curves for HgCl2+ and its fragment ions, the PIE spectra for HgBr2+, HgI2+, and their fragment ions are dominated by autoionization structures exhibiting asymmetric Beutler–Fano line profiles. The comparison of the PIE curves of HgCl2+, HgBr2+, HgI2+, and Hg+ from Hg confirms the previous conclusion that these autoionizing Rydberg series can be assigned to transitions [(5d)10σg2σu2πu4πg4] → [(5d)9σg2σu2πu4πg4 2D±5/2]np and [(5d)10σg2σu2πu4πg4] → [(5d)9σg2σu2πu4πg4 2D±3/2, ±1/2]np, n≥6, which are mainly associated with the mercury atom in HgX2, X=Cl, Br, and I.

Appearance energies fro tungsten ions field-evaporated from ionic bonding states

General formulae are derived for the standard field-ion appearance energies associated with desorption from ionic bonding states. Values for tungsten are obtained using the image-potential approximation, and are compared with the measured appearance energy for W3+ ions. The measured value is compatible with field evaporation from a primarily ionic bonding state, but a better understanding of correlation-type interactions at charged surfaces is necessary if useful information about initial bonding state is to be derived from appearance-energy measurements alone.

Energetics of the loss of H2 from [C3H7]+

AbstractThe internal energies of [C3H7]+ ions contributing to the metastable peak [C3H7]+ → [C3H5]+ + H2 are higher (by perhaps > 100 kJ mol−1) than those of the ion contributing to the threshold current in appearance energy measurements on [C3H5]+. The measured appearance energy may lead to an underestimation of the activation energy, i.e. negative ‘kinetic shift’, due to quantum, mechanical tunnelling. The distribution of energy released in the decomposition can be explained on the basis that much of the reverse activation energy and a statistical proportion of the excess energy is released as translation.