The main configurations of the VUV lamp-based photoionization mass spectrometer in Hefei and the microwave discharge flow tube reactor have been introduced in detail in our recent publications and only a brief description is presented here [27 (link),28 (link),31 (link)]. Briefly, the microwave discharge flow tube reactor is used to generate radicals and to study the self-reaction of C2H5O2 under NOx-free conditions. The flow tube mainly consists of a main tube (450 mm length, 16 mm inner diameter) and a movable coaxial inner tube (600 mm length, 4 mm inner diameter). The inner surface of the main tube and the outer surface of the inner tube are coated with halocarbon wax to minimize the wall loss of the radicals. F2 diluted to 5% in helium is discharged with a 2.45 GHz microwave discharge generator (GMS-200W, Sariem, France) to produce fluorine atoms and then to initiate the reactions. The pressure inside the flow tube is measured by a diaphragm vacuum gauge and precisely controlled by a closed-loop feedback throttle valve. In the present experiments, the pressure of the flow tube is fixed at 266 Pa. The total gas flux into the flow tube is 500 cm3 min−1 and the reactants’ initial concentrations are 3.7 × 1013 molecules·cm−3 for F atoms, 3.1 × 1014 molecules·cm−3 for C2H6 and 2.6 × 1015 molecules·cm−3 for O2, respectively.
A home-made VUV photoionization mass spectrometer is employed to online probe the intermediates and stabilized products inside the flow tube [28 (link)]. The photoionization mass spectrometer is composed of three vacuum chambers: a source chamber, an ionization chamber and a TOF chamber. The flow tube is connected with the source chamber directly, and a 1 mm diameter skimmer is adopted to sample the species inside the flow tube. Then, the species are photo-ionized with a krypton discharge lamp (PKS 106, Heraeus, Germany, hν = 10.0 and 10.6 eV) and the ions are analyzed with an orthogonal acceleration reflectron time-of-flight mass analyzer. The mass resolution of the VUV photoionization mass spectrometer is M/∆M = 2100 (FWHM, full width at half maximum). Very recently, the VUV photoionization mass spectrometer has been upgraded in some content to achieve a better detection limit, LOD < 0.001 µg/L [28 (link)]. In the kinetic experiments, the reaction time is varied by changing the distance between the inner tube and the skimmer.
The synchrotron photoionization experiments are performed at the VUV beamline at the Swiss Light Source. The detailed configurations of the synchrotron beamline and the i2PEPICO setup can be found in the literature [29 (link),30 (link)]. A 10 Hz pulsed Nd-YAG (213 nm, 16 mJ cm−2) laser is used to generate chlorine atoms through the photolysis of oxalylchloride (COCl)2, which initiates the radical reactions in a side-sampled flow reactor. The reactor is a 57.4 cm long quartz tube with a 1.27 cm outer diameter, 1.05 cm inner diameter, and a 300 µm pinhole at the halfway-point along the tube. Ethane (0.03 sccm) and oxygen (75 sccm), as well as Ar carrier gas (15 sccm), are also added into the flow reactor (6 mbar) to produce ethyl peroxy radicals and the dimeric products C2H5OOC2H5. Photoelectrons and photoions are velocity map imaged onto two position-sensitive delay-line detectors, respectively, and detected in delayed coincidences [46 (link)]. In the present experiments, due to the weak signal, only the photoionization spectrum of C2H5OOC2H5 is acquired and presented here.
High-level theoretical calculations, consisting of the determinations of the structures of the neutral and ionic species of C2H5OOC2H5, the corresponding AIE, the Franck-Condon factors involved in the ionization and the reaction potential energy surfaces, have been performed. Briefly, the structures of the dimeric product C2H5OOC2H5 and its cation C2H5OOC2H5+ have been fully optimized at the explicitly correlated coupled cluster single-double and perturbative triple excitations approach, CCSD(T)-F12, in conjunction with the aug-cc-pVTZ basis set as implemented in the MOLPRO 2015 program [44 ]. Subsequently, the AIE of C2H5OOC2H5 has been theoretically calculated at the same level of theory. The Franck-Condon factors for the ionization transitions are calculated at the M062X/aug-cc-pVTZ level of theory using the time-independent adiabatic Hessian Franck-Condon model in the Gaussian 16 package. The reaction potential energy surfaces (PES) for the C2H5O2 self-reaction, as well as the optimization of the structures of the reactants, intermediates, transition state and products are also calculated at the M06-2X/aug-cc-pVTZ level of theory with the Gaussian 16 package [45 ].
A home-made VUV photoionization mass spectrometer is employed to online probe the intermediates and stabilized products inside the flow tube [28 (link)]. The photoionization mass spectrometer is composed of three vacuum chambers: a source chamber, an ionization chamber and a TOF chamber. The flow tube is connected with the source chamber directly, and a 1 mm diameter skimmer is adopted to sample the species inside the flow tube. Then, the species are photo-ionized with a krypton discharge lamp (PKS 106, Heraeus, Germany, hν = 10.0 and 10.6 eV) and the ions are analyzed with an orthogonal acceleration reflectron time-of-flight mass analyzer. The mass resolution of the VUV photoionization mass spectrometer is M/∆M = 2100 (FWHM, full width at half maximum). Very recently, the VUV photoionization mass spectrometer has been upgraded in some content to achieve a better detection limit, LOD < 0.001 µg/L [28 (link)]. In the kinetic experiments, the reaction time is varied by changing the distance between the inner tube and the skimmer.
The synchrotron photoionization experiments are performed at the VUV beamline at the Swiss Light Source. The detailed configurations of the synchrotron beamline and the i2PEPICO setup can be found in the literature [29 (link),30 (link)]. A 10 Hz pulsed Nd-YAG (213 nm, 16 mJ cm−2) laser is used to generate chlorine atoms through the photolysis of oxalylchloride (COCl)2, which initiates the radical reactions in a side-sampled flow reactor. The reactor is a 57.4 cm long quartz tube with a 1.27 cm outer diameter, 1.05 cm inner diameter, and a 300 µm pinhole at the halfway-point along the tube. Ethane (0.03 sccm) and oxygen (75 sccm), as well as Ar carrier gas (15 sccm), are also added into the flow reactor (6 mbar) to produce ethyl peroxy radicals and the dimeric products C2H5OOC2H5. Photoelectrons and photoions are velocity map imaged onto two position-sensitive delay-line detectors, respectively, and detected in delayed coincidences [46 (link)]. In the present experiments, due to the weak signal, only the photoionization spectrum of C2H5OOC2H5 is acquired and presented here.
High-level theoretical calculations, consisting of the determinations of the structures of the neutral and ionic species of C2H5OOC2H5, the corresponding AIE, the Franck-Condon factors involved in the ionization and the reaction potential energy surfaces, have been performed. Briefly, the structures of the dimeric product C2H5OOC2H5 and its cation C2H5OOC2H5+ have been fully optimized at the explicitly correlated coupled cluster single-double and perturbative triple excitations approach, CCSD(T)-F12, in conjunction with the aug-cc-pVTZ basis set as implemented in the MOLPRO 2015 program [44 ]. Subsequently, the AIE of C2H5OOC2H5 has been theoretically calculated at the same level of theory. The Franck-Condon factors for the ionization transitions are calculated at the M062X/aug-cc-pVTZ level of theory using the time-independent adiabatic Hessian Franck-Condon model in the Gaussian 16 package. The reaction potential energy surfaces (PES) for the C2H5O2 self-reaction, as well as the optimization of the structures of the reactants, intermediates, transition state and products are also calculated at the M06-2X/aug-cc-pVTZ level of theory with the Gaussian 16 package [45 ].
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