UC San Diego

Transient species play a central role in governing the chemical dynamics of reactions, but are difficult to study due to their short lifetime and high reactivity. By using kinematically complete techniques such as photoelectron-photofragment coincidence (PPC) spectroscopy, characteristics such as energetics and dissociative pathways for transient species can be studied. One major limitation of such a technique is limited knowledge of the internal energy of the precursor ion prior to photoexcitation. While for some species, this information is less critical and can be inferred from combining theoretical calculations with experimental observations, other species are much more ambiguous.

One example is the tert-butoxy radical (CH3)3CO which is a reaction intermediate in the combustion of tert-butyl alcohol, a candidate for alternative fuels. Due to the lack of α hydrogens with respect to the radical center leading to a low reactivity with O2 as well as a large barrier to isomerization, unimolecular decomposition as the primary gas phase reaction pathway for this radical. Due to the method of ion selection being by mass, isomers of the tert-butoxy radical were taken into consideration for determining the dissociation dynamics observed. Ab initio calculations found that the dissociation of highly vibrationally excited tert-butoxy to methyl radical + acetone products is energetically very similar to theoretical ab initio calculations for the dissociative dynamics of its carbanion isomer ((CH3)2C(CH2)OH¯) to methyl radical + propen-2-ol. Utilizing Franck-Condon simulations the observed dissociation dynamics was assigned to the photoexcitation of highly vibrationally excited tert-butoxide anion precursor.

While efforts to generate cold precursor ions to photoelectron photofragment coincidence spectroscopy have been carried out through the installation of an electrostatic ion beam trap, studies on tert-butoxy radical have shown that high frequency vibrations are not sufficiently cooled through supersonic expansion alone. To address this issue, the source for the PPC spectrometer has been redesigned to include a Wiley McLaren style ion extraction coupled with a cryogenic octopole accumulation trap (COAT) to allow for the preparation of both hot and cold of ions for PPC spectroscopy. The demonstration of the effectiveness of COAT is demonstrated through the measurement of the dissociation dynamics of ozonide (O3−). Photoexcitation of O3ˉ at 3.20 eV results in a stable channel and two dissociative pathways, Oˉ + O2 and O + O2ˉ. The O + O2ˉ channel in particular is observed as an autodetachment channel where an electron in O2ˉ autodetaches when v > 3. At hv = 3.20 eV, the product O2 (v > 3) is only energetically accessible from vibrationally excited O3ˉ making this channel particularly sensitive to vibrational excitation of the precursor ion, O3ˉ. Utilizing different trapping times and temperatures for COAT, the intensity of the autodetachment channel demonstrates the ability to generate not only vibrationally cold ions, but hot ions as well. Additionally, it was found that the autodetached electrons from O2ˉ(v″ = 4) exhibits resolved features consistent with bend (ν2), asymmetric stretch (ν3) and a stretching combination band (ν1 + ν3) in the intermediate electronic state.