The thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH 3 OC(O)H, were elucidated to be CH 3 OH + CO, 2 CH 2 O and CH 4 + CO 2 as in part distinct from the dissociation of the radical cation (CH 3 OH +• + CO and CH 2 OH + + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH 2 O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.

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The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O=C(H)-O-C(H 2 )-CH 3 and trans (staggered) configuration of the O=C(H)-O-C(H 2 )-CH 3 dihedral angles. We observed the following ethyl formate pyrolysis products: CH 3 CH 2 OH, CH 3 CHO, C 2 H 6 , C 2 H 4 , HC(O) OH, CH 2 O, CO 2 , and CO, with HC(O)OH and C 2 H 4 pyrolyzing further, forming CO + H 2 O and C 2 H 2 + H 2 . The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH 3 CH 2 O + CHO and CH 3 CH 2 + HC(O)O, were studied computationally at the M06-2X-GD3/ aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.