High-resolution pulsed field ionization photoelectron–photoion coincidence study of CH4: Accurate 0 K dissociation threshold for CH3+

Abstract

The formation of methyl cation (CH 3 ϩ ) from methane (CH 4 ) has been investigated in high resolution using the newly perfected pulsed field ionization photoelectron-photoion coincidence ͑PFI-PEPICO͒ scheme. The PFI-PEPICO data reveal that fragmentation of CH 4 in high-n Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. The crossover point of the breakdown curves is found to depend strongly on the Stark field in the ion source and thus traditional simulation procedures based on such a feature for ion dissociation energy determination are not appropriate in PFI-PEPICO studies. We show that for a prompt dissociation process, the disappearance energy of the parent molecule provides an accurate measure of the 0 K ion dissociation threshold, as that for CH 3 ϩ from CH 4 is 14.323Ϯ0.001 eV.

High-resolution energy-selected study of the reaction NH3+→NH2++H: Accurate thermochemistry for the NH2/NH2+ and NH3/NH3+ systems

“…In this experiment, the ALS gap (112 ns) is narrower than that (144 ns) used in the PFI-PEPICO studies. [13][14][15] Thus, the contamination due to a finite dispersion of hot electrons into the dark gap in the present study is higher than that in the latter studies. The hot electrons occur at the dark gap are probably responsible for the majority of background coincidences, manifesting as stable, cold parent ions in the PFI-PEPICO TOF spectra detected at hν≥AE(NH 2 + ), e.g., at hν = 15.7800 and 15.8279 eV (see Fig.…”

“…These spectra have been background corrected using procedures as described in previous studies. [13][14][15] At hν=15.7129 eV, which is below the AE(NH 2 + ), only the parent ion TOF peak at 20.34 µs was observed. As the hν is increased, the relative abundance for the daughter ion at 19.78 µs increases correspondingly.…”

“…Similar to previous PFI-PEPICO studies, [13][14][15] , the TOF peak shapes for the parent NH 3 + and daughter NH 2 + ions are analyzed to consist of the contribution of a narrow component due to the cold NH 3 beam sample and a broad component arising from the thermal NH 3 background. As shown in Fig.…”

“…The PFI-PEPICO resolution achieved is ≈1.0 meV (FWHM). [13][14][15] The NH 3 sample (obtained from Aldrich, purity = 99.99%) is introduced into the PI/PEX region as a skimmed neat NH 3 supersonic beam (stagnation pressure = 600 Torr and stagnation temperature = 298 K). We estimate that NH 3 at the PI/PEX region consists of ≈85% cold beam sample and ≈15%…”

Vacuum-UV negative photoion spectroscopy of CH4

“…There have been many studies of the dissociation products following ionisation of CH 4, revealing the fragmentation dynamics of an energy-selected CH 4 + cation [7,9,[17][18][19], and detailed Lyman-α photofragmentation studies have also been undertaken [2,20]. Total photoabsorption cross sections have been measured in the vacuum-UV range of 10-30 eV many times [21][22][23][24][25], with cross sections ranging from ca.…”