Ethane cation decomposition characterization by EBMI spectroscopy: gas‐phase dissociative recombination as a source of secondary products

Abstract

The decomposition products of the d(6) -ethane cation following charge-transfer ionization with Ar(+) , under conditions of varying ionization electron current, have been isolated in solid argon matrices at 18 K and examined using Fourier transform infrared spectroscopy. Gas samples containing 1 : 1600 d(6) -ethane : Ar were subjected to electron bombardment by using either a high (pin) or a low (plate) ionization density anode configuration with ionization currents between 20 and 150 μA. Under high ionization density conditions, the observed major products were d(4) -ethene (C(2) D(4) ) and d(2) -acetylene (C(2) D(2) ), with smaller yields of C(2) D(5) , C(2) D(3) , and C(2) D. The yield of each dehydrogenation product was enhanced with increased current. Analogous experiments employing the low ionization density plate anode resulted in reduced C(2) D(6) destruction and the formation of only C(2) D(4) and C(2) D(2) . The results suggest the onset of dissociative recombination processes under high ion density conditions. In this context, the results can be interpreted as a dissociative recombination of primary ion products, which gives rise to further dehydrogenation, and appearance of additional neutral radical products.

Acetone cation chemistry under varying electron density conditions: from unimolecular decomposition to dissociative recombination processes

“…These results indicate that the chemistry associated with ions formed under elevated ionization electron density conditions in our experiments includes a significant contribution from electron density‐dependent secondary processes, in accord with the conclusions of the previously published results for ethane ionization . The behavior of ethane formed from acetone decomposition, and its dehydrogenation products ethene, acetylene and CCCO, follows the pattern that we have already reported following direct ionization of ethane . Such behavior is consistent with a mechanism for dehydrogenation of the ethane involving DR steps.…”

“…Species such as 1‐propen‐2‐ol, ketene and ethane are formed most abundantly at low ionization currents and diminish as the ionization current increased, in favor of other products such as ethene, acetylene, CCCO and HCO. These results indicate that the chemistry associated with ions formed under elevated ionization electron density conditions in our experiments includes a significant contribution from electron density‐dependent secondary processes, in accord with the conclusions of the previously published results for ethane ionization . The behavior of ethane formed from acetone decomposition, and its dehydrogenation products ethene, acetylene and CCCO, follows the pattern that we have already reported following direct ionization of ethane .…”

“…Thus, the relative yield of ethane is the greatest for all species present at the ionization currents up to and including 50 μA, which of ethene and acetylene increases steadily with increased ionization current, and CCCO is only observed at the highest currents tested. The growth and decay behavior of these four species corresponds to that observed directly following ionization of ethane in our apparatus …”

“…The extent to which the reaction proceeds along these sequential steps is a function of the ionization electron density in the ionization region, which determines the probability of a cation encountering an electron before reaching the matrix. The observation of such behavior in the acetone system clearly demonstrates that the products of acetone cation decomposition are subject to the same DR processes that have been observed upon direct ionization of ethane itself …”

“…All of these products have been previously identified as arising from acetone ionization under EBMI conditions, and each of ethene, acetylene CCCO have been shown to be associated with secondary chemistry of ethane in our earlier work …”