Dissociative Photoionization Dynamics Of Nitrogen Dioxide And Dimethoxymethane

Photoionization and photodissociation of gas-phase molecules are widely found in interstellar chemistry,atmospheric chemistry,and plasma processes.The study of related topics can not only obtain important physicochemical parameters of molecules,such as Ionization energy(IE),fragment ion appearance potential(AP)and Bond energy(BE),but also provide important clues to simulate the evolution of interstellar chemistry,prevent and control It also provides important clues to model interstellar chemical evolution,prevent atmospheric pollution,and influence combustion dynamics.Combination with tunable synchrotron vacuum ultraviolet(VUV)photoionization,the photoelectron-photoion coincidence(PEPICO)technique allows the application of single-photon ionization for the study of ionization-dissociation kinetics of quantum state selection.The ion-focusing electric field allows us to simultaneously detect photoelectrons and photoions,record threshold photoelectron spectra,time-of-flight mass spectra of electron-ion coincidence,and velocity-focused images of ion fragments,and then obtain kinetic information such as channel branching ratios of products,advective energies and angular distributions of dissociated fragments,and finally elucidate the ionization-dissociation mechanisms and related reaction rates of the corresponding quantum states.In addition,we have developed a new high time-resolved and high-energy-resolved fast switching ion velocity focusing imaging technique.The main innovative research results are as follows.(1)The dissociation mechanism of NO2+ions in a3B2 and b3A2 states.To clarify the contentions about dissociative photoionization mechanism of nitrogen dioxide via the a3B2 and b3A2 ionic states,a new threshold photoelectron-photoion coincidence(TPEPICO)velocity imaging has been conducted in the 12.8-14.0 eV energy range at the Hefei Light Source.Fine vibrational-resolved threshold photoelectron spectrum agrees well with the previous measurements.The ro-vibrational distributions of NO+,as the unique fragment ion in the dissociation of NO2+in specific vibronic levels of a3B2 and b3A2 states,are derived from the recorded TPEPICO velocity images.A "cold"vibrational(v+=0)and "hot" rotational population is observed at the a3B2(0,3,0)and(0,4,0)vibronic levels,while the dissociation of NO2+in b3A2(0,0,0)leads to the NO+fragment with both hot vibrational and rotational populations.With the aid of the quantum chemical calculations at the time-dependent B3LYP level,minimum energy paths on the potential energy surfaces of the a3B2 and b3A2 states clarify their adiabatic dissociation mechanisms near the thresholds,and propose reliable explanations for the observed internal energy distributions of fragment ions.Additionally,this study provides valuable insights into the application of the classical "impulsive" model on an overall slow dissociation process.(2)The mechanisms of thermal decomposition of dimethoxymethane.Pyrolysis and low-temperature oxidation of dimethoxymethane(methylal,MeOCH2OMe)play an important role in the ignition of blended diesel fuels,but the underlying mechanisms are still debated.In these kinetic models,bimolecular hydrogen ion or unimolecular C-O bond fission are considered as the primary initial steps,while MeOCH2OMe isomerization is sometimes disregarded.In this work,we investigate the pyrolysis of MeOCH2OMe combining imaging photoelectron photoion coincidence spectroscopy with vacuum ultraviolet(VUV)synchrotron radiation and CBS-QB3 theoretical calculations to unveil reaction paths and energetics.In the mass spectrum of MeOCH2OMe,pyrolysis products and radical intermediates were observed at m/z 15(CH3),28(CO),29(HCO),30(H2CO),31(CH2OH),32(CH3OH),45(CH3OCH2),and 75(MeOCH2OMe-H).Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH2OMe,the other mass spectral peaks are attributed to ionization of the neutral MeOCH2OMe pyrolysis products.The m/z 31 peak was assigned to the methoxy radical in the previous studies.However,our photoion mass-selected threshold photoelectron spectrum(ms-TPES)confirms that it originates from dissociative photoionization of the primary pyrolysis fragment methanol.Based on the experimental and computational results,a thermal decomposition mechanism of MeOCH2OMe is proposed.Here,H-migration precedes the production of hydroxymethylene(CH3OCH)and methanol,while dimethyl ether and formaldehyde are probably formed in multi-step processes,too.The sequential dissociation of CH3OCH and of dimethyl ether yields enhanced m/z 15,28 and 29 signals at high temperature.Rate constants have been calculated to confirm the dominant role of MeOCH2OMe isomerization and to help improve predictive combustion models.(3)Development of high time resolution and high energy resolution fast switching ion velocity focusing image technology.To address the shortcomings of the conventional time-sliced ion velocity focusing imaging technique in time resolution(i.e.,mass resolution)and the weakness of 2D velocity focusing images in energy resolution.We propose a new fast switching imaging technique that can rapidly achieve time-sliced ion velocity-focused imaging and high time-resolved two-dimensional velocity-focused image detection without changing the original ion-focusing electric field configuration,thus satisfying the limit requirements of advective energy measurement and mass resolution for different dissociated fragment ions.