Tacheng Hsieh

Fragmentation of SO+2 prepared in state selected vibrational levels

The threshold photoelectron (TPE) and TPE‐coincident photoion (CPI) mass spectra of SO2 have been determined over the energy range 15.844 to 16.674 eV with a resolution of about 0.018 eV FWHM. This is sufficient to prepare most individual vibronic states in populations exceeding 85%. In the 2B2 state, excitation of the stretching mode up to ν1=5 is observed, while in the same state the ν2 bending mode may be excited concurrently with not more than one quantum. In this energy range SO+ formation sets on at 15.930±0.005 eV, suggesting that the onset at 0° K should be 15.965 eV±0.005 eV. S+ is produced over a very narrow energy range only; the onset is at 16.334 eV, coincident with the (310 2B2 state of SO2+; S+ disappears at 16.674 eV. This is interpreted in terms of competition between the assymetric stretch mode and the bending mode for intermodal energy transfer from the symmetric stretch mode, with excitation of the bending mode required to give S+.

The unimolecular decomposition rates of energy selected methylnitrite and deuterated methylnitrite ions

The fragmentation of methylnitrite ion (CH3ONO+) involves dissociation from noninterconverting electronic states characterized by the formation of CH3O+ (m/z=31) and NO+ (m/z=30) ions. The lifetimes of the precursors of these ions have been determined from ion time‐of‐flight curves obtained by threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The deuterated methylnitrite ion (CD3ONO+) was also studied to examine the kinetic isotope effects on ion lifetimes. The rate constants for the reactions leading to the formation of CH3O+ and CD3O+ were found to be independent of internal energy over ∼0.2 and 0.9 eV, respectively. The isotope effect increases with increasing internal energy, contrary to that expected from the quasiequilibrium theory (QET) of mass spectra. We suggest that methylnitrite ion fragmenting to CH3O+ and NO+ occurs from noncompeting noninterconverting electronic states involving a surface crossing.

Competition between isomerization and fragmentation of gaseous ions. II. Nitromethane and methylnitrite ions

Major fragmentation pathways of the nitromethane and methylnitrite ions have been examined. Ionization and fragmentation onsets for these processes have been determined and breakdown graphs have been constructed for both compounds using a threshold photoelectron–photoion coincident (TPEPICO) mass spectrometer. At onset, fragmentation of the nitromethane ion to NO+ is preceded by isomerization to the energetically favored methylnitrite ion; the barrier to isomerization is 0.64 eV. Symmetry considerations indicate that the nitromethane ion in its 2A2 (π) state correlates with the 2A″(π) state of the methylnitrite ion through which dissociation to the NO+ fragment occurs.

Ion fragmentation from noninterconverting electronic states

The photoionization and fragmentation of CH3ONO+ were studied using threshold photoelectron‐coincident photoion mass spectrometry, photoelectron spectroscopy, and triple sector mass spectrometry.(AIP)

Effect of energetic electrons on breakdown graphs determined by threshold photoelectron-photoion coincidence (TPEPICO) mass spectrometry using steradiancy analyzers

In TPEPICO mass spectrometry, a threshold photoelectron detector is employed to transmit zero kinetic energy electrons. The steradiancy analyzer normally employed allows a small but significant number of non-zero kinetic energy electrons to be transmitted as well. A method has been developed to estimate the contribution to measured breakdown graphs of energetic electrons transmitted through the detector. This technique consists of convoluting the known instrument function and the HeI photoelectron spectrum (PES) with various postulated breakdown curves until the experimental and calculated breakdown curves match. This method has been successfully applied to propane and the C4H8 isomers, but it cannot be applied when the HeI PES does not represent the internal energy distribution in the energy range being examined. This is the case for CF3I; for such cases it is possible to construct a variable-energy PES if the HeI PES, the real breakdown graph and the instrument function are known.

Carbon skeletal rearrangement of the propane ion

The elimination of methane from the propane molecular ion has been investigated using propane‐2‐13C. Breakdown graphs obtained by threshold photoelectron‐coincident photoion mass spectrometry indicate that approximately 11% of all methane loss includes the central carbon and that this amount is independent of internal energy over nearly 1.2 eV. These results are interpreted in terms of a potential energy surface and rate constant curves. We conclude that isomerization of the propane molecular ion does not precede fragmentation to ethylene ion and methane, but that fragmentation occurs by a concerted stepwise cleavage involving a tight transition state.

Transmission characteristics of collimated hole threshold photoelectron detectors

Abstract The threshold photoelectron-coincidence photoion mass spectrometer is a powerful tool for the study of state-selected ions. Threshold photoelectron detectors used in this apparatus transmit small but significant numbers of electrons at energies up to two orders of magnitude higher than the energy resolution of this detector. It is frequently important to know the fraction of the total coincidence count which results from non-threshold electrons, and to correct for them. This requires measurement of the transmission characteristics of the threshold photoelectron detector over a wide range of energies. We have developed a simple experimental method for checking the transmission performance of such a detector, as well as a mathematical formalism for calculating the transmission function in closed form.