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Time-sliced velocity map imaging (VMI) has extensively been applied in photodissociation dynamics studies, thanks to its unique advantages, such as high energy resolution and no requirement of inverse Abel or Hankel transformations. However, its time resolution is generally insufficient for distinguishing adjacent m/z ions with a certain kinetic energy due to the overlapping of time-of-flight distributions. Herein, we have made a novel and convenient switch design for the common ion optics in three-dimensional (3D) VMI. By simply introducing two additional resistors out of the vacuum chamber, the strength ratio of the extraction and acceleration fields is easily changed from 3D VMI to two-dimensional (2D) VMI under optimized conditions, as well as a significant extension of free drift length, leading to a higher time resolution while maintaining the high energy resolution. As a result, 2D and 3D VMI can be quickly switched without breaking the vacuum and replacing the electrostatic plates. [ABSTRACT FROM AUTHOR]

Pyrolysis and low-temperature oxidation of dimethoxymethane (methylal, MeOCH 2 OMe) play an important role in the ignition of blended diesel fuels, but the underlying mechanisms are still debated. In these kinetic models, bimolecular hydrogen abstraction or unimolecular C–O bond fission are considered as the primary initial steps, while MeOCH 2 OMe isomerization is sometimes disregarded. In this work, we investigate the pyrolysis of MeOCH 2 OMe 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 MeOCH 2 OMe, pyrolysis products and radical intermediates were observed at m/z 15 (CH 3), 28 (CO), 29 (HCO), 30 (H 2 CO), 31 (CH 2 OH), 32 (CH 3 OH), 45 (CH 3 OCH 2), and 75 (H-loss from methylal). Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH 2 OMe, the other mass spectral peaks are attributed to ionization of the neutral MeOCH 2 OMe 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 MeOCH 2 OMe is proposed. Here, H-migration precedes the production of methoxymethylene (CH 3 OCH) and methanol, while dimethyl ether and formaldehyde are probably formed in multi-step processes, too. The sequential dissociation of CH 3 OCH 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 MeOCH 2 OMe isomerization and to help improve predictive combustion models. [ABSTRACT FROM AUTHOR]

Nitrates are a potential class of substances as cetane improvers of diesel fuel and are critical raw materials for some military explosives. Thus, an in-depth investigation of the thermal decomposition mechanism of nitrates is important for further improving their effects. Herein, a combination of experimental and computational studies was performed for the pyrolysis of n -pentyl nitrate in the temperature range of 400‒900 K. Using synchrotron radiation vacuum ultraviolet photoionization, some intermediates and products, such as ethylene, formaldehyde, propene, ethenol, pentanal, NO 2 , and HONO, were identified according to their photoionization efficiency curves. Quantum chemical calculations at the CBS-QB3 level were conducted to elucidate the overall reaction mechanism of n -pentyl nitrate pyrolysis. Along the direct bond fission of n -pentyl nitrate, primary products, C 5 H 11 O• radical and NO 2 , and butyl radical and CH 2 ONO 2 , can further dissociate into many products, in which formaldehyde is the most dominant. In addition, two hydrogen-atom migration channels followed by decomposition are verified for the decomposition of n -pentyl nitrate according to the observation of trans -HONO, pentanal, and 1-pentene. These evidences highlight that both bond fission and hydrogen migration play comparable roles as potential driving forces in the pyrolysis of n -pentyl nitrate at low pressure. This thermal decomposition mechanism of n- pentyl nitrate provides some useful clues for developing combustion models of long chain nitrates in future. [ABSTRACT FROM AUTHOR]

[Display omitted] • A well-resolved vibrational structure is observed in the X2B 2 state of CF 2 Cl 2 TPES. • Franck-Condon factor simulation greatly agrees with the experimental spectrum. • A corrected CF 2 Cl 2 adiabatic ionization energy of 11.565 ± 0.010 eV is obtained. In this work, we performed a joint study of threshold photoelectron spectroscopy and density functional calculations on photoionization of CF 2 Cl 2. Using the optimized geometries and vibrational frequencies of the CF 2 Cl 2 neutral and cation in ground state at the ωB97XD/cc-pVTZ level, the Franck-Condon simulation was performed for the X2B 2 band. Based on the great agreements between the experimental and simulated spectra, the observed vibrational progressions of the CF 2 Cl 2 (X1A 1) → CF 2 Cl 2 +(X2B 2) transition are well assigned, leading to a corrected adiabatic ionization energy of 11.565 ± 0.010 eV. The enthalpies of formation and C-Cl bond energies of CF 2 Cl 2 neutral and cation are then calculated, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • 2D PESs along N–NO bond length and N–N–O bond angle of N 2 O+ in the A2Σ+ and B2Π states are calculated at the CASPT2 level. • Validated a novel adiabatic decomposition path of the A2Aʹ(A2Σ+) state in bent geometry to rotationally excited NO+(X1Σ+) and N(2D). • The respective contributions of the Aʹ and A ″ components split from the B2Π state are clarified separately. Two-dimensional multistate potential energy surfaces along N–NO bond length and N–N–O bond angle of N 2 O+ in the A2Σ+ and B2Π states were calculated at the CASPT2/cc-pVQZ level. In comparison to the known decomposition mechanisms in linear structure, a new N–NO bond fission pathway was proposed in bent geometry for the A2Aʹ(A2Σ+) state with a lower barrier, leading to rotationally excited NO+(X1Σ+) and N(2D) fragments. Likewise, the respective contributions of the Aʹ and A ″ components split from the B2Π state were clarified. Considering avoided crossing and the coupling of spin states, ro-vibrational distributions of the NO+ fragment ion observed in experiments are elucidated. [ABSTRACT FROM AUTHOR]

Quantum diamond sensors of multiple physical quantities, especially magnetic field and temperature, have great application prospects in materials science, biophysics, and engineering. Here, we demonstrate joint quantum sensing of vector magnetic field and temperature based on ensembles of nitrogen-vacancy (NV) centers in the diamond. Combining the intrinsic C 3 ν symmetry of diamond crystals and multiple-frequency excitation technique, we can directly measure frequency-modulated optically detected magnetic resonance (ODMR) spectra for three different NV orientations and then obtain the Zeeman splittings and the center frequency movement of the electron spin. An artificially generated underlying external magnetic field is used to eliminate degeneracy and facilitate coordinate transformations. The six-channel phase-locked amplification technology enables simultaneous detection of vector magnetic field and temperature with a sensitivity of about 0.87 nT/Hz1/2 and 0.32 mK/Hz1/2; meanwhile, a 23 dB isolation between magnetic and temperature signals is kept. This technology realizes the simultaneous measurement of arbitrary vector magnetic field and temperature, which can be used to study the temperature-varying magnetic phase transition. [ABSTRACT FROM AUTHOR]

[Display omitted] • Distinct vibrational structure was observed in the B2B 1 band of TPES of CF 2 Cl 2. • Franck-Condon factor simulations were performed for the B2B 1 and C2A 1 bands. • A precise AIE towards the B2B 1 ionic state was determined to be 13.150 ± 0.005 eV. Threshold photoelectron spectroscopy and density functional theory calculations were performed on dichlorodifluoromethane (CF 2 Cl 2) in the photon energy range of 11.70–13.90 eV. Using optimized geometries and vibrational frequencies of the CF 2 Cl 2 neutral and the corresponding cation in the B2B 1 and C2A 1 states at the ωB97XD/aug-cc-pVTZ level of theory, the Franck-Condon factor simulation was conducted for these two bands. According to the great agreement between experimental and simulated spectra, the observed vibrational progressions of the B2B 1 band was reliably assigned, where the 0–0 transition band indicated a precise adiabatic ionization energy (AIE) towards the B2B 1 ionic state of 13.150 ± 0.005 eV. The three symmetric vibrational frequencies of B2B 1 were determined to be ν 1 + = 1170 cm−1, ν 2 + = 645 cm−1, and ν 4 + = 242 cm−1, respectively. Similarly, the AIE towards the C2A 1 ionic state was suggested to be 13.340 eV, based on its plausible Franck-Condon simulation. [ABSTRACT FROM AUTHOR]

Heterogeneous single-metal-site Ir catalyst (Ir 1 /AC, 5 wt%) was prepared from the in-situ atomic dispersion of corresponding supported nanoparticles on activated carbon (Ir/AC), by the thermal treatment with CO and CH 3 I. The physical coordinated structure of those single-metal-site Ir catalysts were determined by HAADF-STEM and EXAFS. The key species, I radicals for this process were creatively detected by photoelectron photoion coincidence spectroscopy with vacuum ultraviolet synchrotron radiation in swiss light source. Our findings provide an alternative approach to prepare the supported atomically dispersed single-metal-site catalyst, which have the analogically homogeneous structure and show a comparable catalytic activity as homogeneous catalyst for methanol carbonylation. More importantly, 5 wt% Ir 1 /AC catalyst, deactivated by metal atoms agglomeration can be regenerated repeatedly with the thermal treatment of CO and CH 3 I to regain their initial activity for heterogeneous methanol carbonylation. General speaking, we provide a method of preparation and regeneration of supported single-metal-site catalyst, which has broad application, especially for the regeneration of supported sinter and agglomerated catalyst. • Single-metal-site Ir catalyst is prepared from atomic dispersion of supported NPs. • Coordinative structure of single-metal-site Ir catalyst is well determined. • The key species I radicals are creatively detected by PEPICO spectroscopy. • Atomic dispersion strategy can regenerate agglomerated Ir 1 /AC catalyst. • A method to preparing and regenerating supported single-metal-site catalyst. Heterogeneous single-metal-site catalysts (HSMSCs) have been attracting increasing interest because of their unique traits. However, deactivation induced by metal agglomeration restrains the application especially when they have high metal loading. Herein, we present a method of CO/CH 3 I thermal treatment to prepare and regenerate Ir HSMSCs on activated carbon (AC) via atomically dispersive strategy of nanoparticles (NPs). The intermediate key species during this process, I radicals, have been detected in situ by photoelectron photoion coincidence spectroscopy with vacuum ultraviolet synchrotron radiation (PEPICO-VUV). Our findings imply a strong relationship between I radicals' as well as CO molecule's roles in the breakage of NPs metal–metal bond. NPs with size larger than about 5 nm are difficult to be dispersed due to strong Ir–Ir bond together with the kinetic stability. Besides, 5 wt% Ir HSMSCs deactivated by metal agglomeration can be regenerated repeatedly and regain their initial activity for heterogeneous methanol carbonylation. [ABSTRACT FROM AUTHOR]

The low temperature oxidation of alkenes exhibits some distinct reaction pathways that may form different kinds of keto-hydroperoxides to start the chain-branching by further decomposition. This is mainly due to the existence of the double bond that the OH radical could add to it. This work explores that phenomenon by investigating the low temperature oxidation of three alkenes (1-hexene, 1-heptene and 1-decene) in a jet-stirred reactor (JSR) at atmospheric pressure. The synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) experiments combined with hydrogen-deuterium (H-D) exchange reactions were used to evaluate the chain branching pathways. Alkenyl keto-hydroperoxide (AnKHP) and hydroxyl keto-hydroperoxide (HyKHP) are identified. They represented two different chain branching reaction networks that are initiated from the H-abstraction of the primary and secondary C Hs and via OH addition to the double bond. The experimental estimated and model predicted ratios of AnKHP to HyKHP are compared, and the result indicates that the literature model underpredicts the chain branching reactions via the formation of AnKHP, but overpredicts the chain branching reactions via HyKHP. [ABSTRACT FROM AUTHOR]

Comprehensive analysis of the low-temperature oxidation intermediates, including the reactive intermediates and isomers, is crucial to develop the low-temperature oxidation models of the fuels. In this aim, the complementary analysis by different analytic methods is needed. Furthermore, the cross check by different analytic methods increases the fidelity of the experiment data. In this work, we developed a jet-stirred reactor (JSR) system that coupled to the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) analysis. The JSR was designed to couple the SVUV-PIMS and GC, and the temperature homogeneity and mixing of the JSR was examined by a CFD simulation. The SVUV-PIMS system was developed to have a mass resolution of ca. 5000 at m/z 100. The GC system included a flame ionization detector (FID), thermal conductivity detector (TCD), and a mass spectrometer (MS). This newly developed setup was validated by repeating the oxidation data of n -heptane in the literature. A good agreement was observed between the two datasets. The low-temperature oxidation of n -butane was then studied to show the advantage of this setup, i.e., the simultaneous and comprehensive measurement of the low-temperature oxidation products. Specifically, the SVUV-PIMS enables the separation of the C/H/O composition of the intermediates, the measurement of formaldehyde and the peroxide intermediate, while the GC analysis gives a better separation of the oxygenated isomers and the quantification of the species that the SVUV-PIMS has difficult in quantifying. The experimental method developed in this work is valuable to study the fuel low-temperature oxidation chemistry, to reveal the species pool, and to provide reliable mole fraction data for the validation and development of combustion models. [ABSTRACT FROM AUTHOR]

[Display omitted] • Single photon double ionization of substituted benzenes using synchrotron radiation. • Resonance-like enhancement for halogenated benzenes, but not for toluene. • The molecular orbital plays a pivotal role in single photon double ionization. We investigate single photon double ionization (spDI) of substituted benzenes (toluene, chlorobenzene and fluorobenzene) in photon energy of 20–40 eV. We observe that a resonance-like enhancement of spDI, which deviates from the prediction of the "knock-out" mechanism, appears at a specific photon energy near the threshold for halogenated benzenes, but not for toluene. The enhanced spDI occurs if the de Broglie wavelength of the two-electron pseudoparticle matches the size of the conjugated p-π orbital of halogenated benzene. Our study shows that the molecular spDI is not a local event and the molecular orbital plays a pivotal role in double ionization. [ABSTRACT FROM AUTHOR]