A method of using deep learning to predict three-dimensional dose distributions for intensity-modulated radiotherapy of rectal cancer.

PURPOSE: To develop and test a three-dimensional (3D) deep learning model for predicting 3D voxel-wise dose distributions for intensity-modulated radiotherapy (IMRT). METHODS: A total of 122 postoperative rectal cancer cases treated by IMRT were considered in the study, of which 100 cases were randomly selected as the training-validating set and the remaining as the testing set. A 3D deep learning model named 3D U-Res-Net_B was constructed to predict 3D dose distributions. Eight types of 3D matrices from CT images, contoured structures, and beam configurations were fed into the independent input channel, respectively, and the 3D matrix of dose distributions was taken as the output to train the 3D model. The obtained 3D model was used to predict new 3D dose distributions. The predicted accuracy was evaluated in two aspects: (a) The dice similarity coefficients (DSCs) of different isodose volumes, the average dose difference of all voxels within the body, and 3%/5 mm global gamma passing rates of organs at risks (OARs) and planned target volume (PTV) were used to address the spatial correspondence between predicted and clinical delivered 3D dose distributions; (b) The dosimetric index (DI) including homogeneity index, conformity index, V50 , V45 for PTV and OARs between predicted and clinical truth were statistically analyzed with the paired-samples t test. The model was also compared with 3D U-Net and the same architecture model without beam configurations input (named as 3D U-Res-Net_O). RESULTS: The 3D U-Res-Net_B model predicted 3D dose distributions accurately. For the 22 testing cases, the average prediction bias ranged from -1.94% to 1.58%, and the overall mean absolute errors (MAEs) was 3.92 ± 4.16%; there was no statistically significant difference for nearly all DIs. The model had a DSCs value above 0.9 for most isodose volumes, and global 3D gamma passing rates varying from 0.81 to 0.90 for PTV and OARs, clearly outperforming 3D U-Res-Net_O and being slightly superior to 3D U-Net. CONCLUSIONS: This study developed a more general deep learning model by considering beam configurations input and achieved an accurate 3D voxel-wise dose prediction for rectal cancer treated by IMRT, a potentially easier clinical implementation for more comprehensive automatic planning.

Towards high-level theoretical studies of large biodiesel molecules: an ONIOM/RRKM/Master-equation approach to the isomerization and dissociation kinetics of methyl decanoate radicals.

The isomerization and dissociation reactions of methyl decanoate (MD) radicals were theoretically investigated by using high-level theoretical calculations based on a two-layer ONIOM method, employing the QCISD(T)/CBS method for the high layer and the M06-2X/6-311++G(d,p) method for the low layer. Temperature- and pressure-dependent rate coefficients for the involved reactions were computed by using the transition state theory and the Rice-Ramsperger-Kassel-Marcus/Master-equation method. The structure-reactivity relationships were explored for the complicated multiple-well interconnected system of ten isomeric MD radicals. Comparative studies of methyl butanoate (MB) and MD were also performed systematically. Results show that the isomerization reactions are appreciably responsible for the population distribution of MD radicals at low and intermediate temperatures, while the ß-scission reactions are dominant at higher temperatures. Although the rate constants of MB specific to methyl esters are close to those of MD in certain temperature ranges, MB is unable to simulate most of the dissociation reactions due to its short aliphatic chain. Significant differences of rate constants for isomerization reactions were observed between the calculated results and the literature data, which were estimated by analogy to alkane systems, but the rate constants of ß-scissions show generally good agreement between theory and experiment. The current work extends kinetic data for isomerization and dissociation reactions of MD radicals, and it serves as a reference for the studies of detailed combustion chemistry of practical biodiesels.

Dissociative photoionization of CF3Cl via the C2E and D2E states: competition of the C-F and C-Cl bond cleavages.

The dissociative photoionization of CF3Cl was investigated using threshold photoelectron photoion coincidence (TPEPICO) imaging in the energy range of 12.30-18.50 eV. The coincident time-of-flight mass spectra and three-dimensional time-sliced images of the CF2Cl+ fragment were recorded at a few specific photon energies. Two fragmentation pathways were observed that led to the breakage of the C-Cl and C-F bonds, while the branching ratio elicited an energy-dependent relationship. The CF3+ fragment was dominant in the dissociation of the X2E, A2A' and B2A'' states, while CF2Cl+ became the predominant fragment and its branching ratio remained constant in the energy range associated with the C2E and D2E states. Based on the inflection point in the energy-dependent curve of the fragment branching ratios, the adiabatic ionization energy (IEa) of C2E is suggested to be 15.46 eV. Although the excess energy increased considerably from C2E to D2E, the kinetic energy release distributions (KERDs) of CF2Cl+ were similar. Moreover, the anisotropy parameters ß for the F-loss channel were positive and larger than those for the Cl-loss channel. The calculated F-loss potential energy curves of CF3Cl+ suggested that for the C2E and D2E states, the C-F bond rupture occurred via the internal conversion to the A2A' state followed by the dissociation attributed to the crossing of the barrier along the C-F coordinate. Based on the experimental and theoretical conclusions, the internal conversion is the rate-determining step in the dissociative photoionization of CF3Cl via the C2E and D2E ionic states, irrespective of whether the C-F and C-Cl bonds rupture.

Theoretical Study on Criegee Intermediate's Role in Ozonolysis of Acrylic Acid.

Criegee intermediates have raised much attention in atmospheric chemistry because of their significance in ozonolysis mechanism. The simplest Criegee intermediate, CH2OO, and its reactions with acrylic acid including cycloadditions and insertions as main entrance channels have been investigated at CCSD(T)/cc-pVTZ//M06-2X/6-31G(d,p) level. Temperature- and pressure-dependent kinetics were predicted by solving the time-dependent master equations based on Rice-Ramsperger-Kassel-Marcus theory using MESS program, with temperatures from 200 to 500 K and pressures from 0.001 to 1000 atm. Variational transition state theory (VTST) was used for barrierless pathways and conventional transition state theory (CTST) for pathways with distinct barriers. Results indicate that hydroperoxymethyl acrylate is the dominant product under atmospheric conditions. The combination of two reactants will reduce the volatility and makes a possible factor that induces formation of secondary organic aerosols, which suggests CH2OO's entangled role in ever-increasing air pollution.

Hydrogen bonding and dominant conformations of hydrated sugar analogue complexes using tetrahydrofurfuryl alcohol as the model sugar molecule.

Water molecules, which serve as both hydrogen bond donors and acceptors, have been found to influence the conformational landscape of gas-phase phenyl-ß-d-glucopyranoside. Herein, tetrahydrofurfuryl alcohol (THFA), a sugar-like molecule without chromophores (e.g. phenyl-substitution), was used as the model sugar molecule for exploring the behaviour of water molecules on the conformational landscape of a pentose sugar such as deoxyribose. We used mass selected infrared-vacuum ultraviolet (IR-VUV) (118 nm) spectroscopy to investigate the hydrated neutral THFA and its complex cation in a supersonic jet. High level density functional theory (DFT) calculations were performed to ascertain the experimental results. The results revealed that the water molecule tends to insert into the twisted conformer at a position where two stronger intermolecular hydrogen bonds were formed by breaking the weak intramolecular interactions. We found that the twisted conformer of the hydrated neutral THFA complex is more stable than the envelope conformation, while the latter is more stable for the THFA molecule. However, the conformational landscape of the hydrated THFA complex cation did not significantly change on microsolvation with water molecules. These results indicated that the dominant structural landscape of the hydrated cationic complex is the twisted configuration with a trans-hydroxymethyl group. This finding provides valuable insight into the microsolvation of gas-phase sugar molecules.

Cl-Loss dynamics in the dissociative photoionization of CF3Cl with threshold photoelectron-photoion coincidence imaging.

The dissociative photoionization of CF3Cl was investigated in the photon energy range of 12.30-18.50 eV. The low-lying electronic states of CF3Cl+ cations were prepared by the method of threshold photoelectron-photoion coincidence (TPEPICO). The threshold photoelectron spectrum and the coincident time-of-flight mass spectra at the specific photon energies were recorded. Only a CF3+ fragment was observed at lower energy, while a CF2Cl+ fragment appeared for C2E and D2E states. As Cl-loss from the ground ionic state is statistical, the total kinetic energy release distribution (KERD) is represented as a Boltzmann profile, and a 0 K appearance energy of AP0 =12.79 ± 0.02 eV is derived from the statistical modelling of the breakdown diagram from 12.60 to 12.85 eV without taking into account the kinetic shift. For the A2A1 and B2A2 states of CF3Cl+ cations, the total KERDs are bimodal, where a parallel faster dissociation appears together with the statistical distribution. At higher energies like the C2E and D2E ionic states, a bimodal distribution similar to that of the A2A1 and B2A2 states is also observed for the KERD. With the aid of the calculated Cl-loss potential energy curves, the dissociative mechanisms of internal energy-selected CF3Cl+ cations are proposed.

Cl-Loss Dynamics of Vinyl Chloride Cations in the B2A″ State: Role of the C2A' State.

The dissociative photoionization of vinyl chloride (C2H3Cl) in the 11.0-14.2 eV photon energy range was investigated using threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging. Three electronic states, namely, A2A', B2A″, and C2A', of the C2H3Cl+ cation were prepared, and their dissociation dynamics were investigated. A unique fragment ion, C2H3+, was observed within the excitation energy range. TPEPICO three-dimensional time-sliced velocity map images of C2H3+ provided the kinetic energy release distributions (KERD) and anisotropy parameters in dissociation of internal-energy-selected C2H3Cl+ cations. At 13.14 eV, the total KERD showed a bimodal distribution consisting of Boltzmann- and Gaussian-type components, indicating a competition between statistical and non-statistical dissociation mechanisms. An additional Gaussian-type component was found in the KERD at 13.65 eV, a center of which was located at a lower kinetic energy. The overall dissociative photoionization mechanisms of C2H3Cl+ in the B2A″ and C2A' states are proposed based on time-dependent density functional theory calculations of the Cl-loss potential energy curves. Our results highlight the inconsistency of previous conclusions on the dissociation mechanism of C2H3Cl+.

Dissociative ionization of the 1-propanol dimer in a supersonic expansion under tunable synchrotron VUV radiation.

Photoionization and dissociation of the 1-propanol dimer and subsequent fragmentations have been investigated by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. Besides the protonated monomer cation (C3H7OH)·H(+) (m/z = 61) and Cα-Cß bond cleavage fragment CH2O·(C3H7OH)H(+) (m/z = 91), the measured mass spectrum at an incident photon energy of 13 eV suggests a new dissociation channel resulting in the formation of the (C3H7OH)·H(+)·(C2H5OH) (m/z = 107) fragment. The appearance energies of the fragments (C3H7OH)·H(+), CH2O·(C3H7OH)H(+) and (C3H7OH)·H(+)·(C2H5OH) are measured at 10.05 ± 0.05 eV, 9.48 ± 0.05 eV, and 12.8 ± 0.1 eV, respectively, by scanning photoionization efficiency (PIE) spectra. The 1-propanol ion fragments as a function of VUV photon energy were interpreted with the aid of theoretical calculations. In addition to O-H and Cα-Cß bond cleavage, a new dissociation channel related to Cß-Cγ bond cleavage opens. In this channel, molecular rearrangement (proton transfer and hydrogen transfer after surmounting an energy barrier) gives rise to the generated complex, which then dissociates to produce the mixed propanol/ethanol proton bound cation (C3H7OH)·H(+)·(C2H5OH). This new dissociation channel has not been reported in previous studies of ethanol and acetic acid dimers. The photoionization and dissociation processes of the 1-propanol dimer are described in the photon energy range of 9-15 eV.

In situ synchrotron X-ray diffraction analysis of deformation behaviour in Ti-Ni-based thin films.

Deformation mechanisms of as-deposited and post-annealed Ti50.2Ni49.6, Ti50.3Ni46.2Cu3.5 and Ti48.5Ni40.8Cu7.5 thin films were investigated using the in situ synchrotron X-ray diffraction technique. Results showed that initial crystalline phases determined the deformation mechanisms of all the films during tensile loading. For the films dominated by monoclinic martensites (B19'), tensile stress induced the detwinning of 〈011〉 type-II twins and resulted in the preferred orientations of (002)B19' parallel to the loading direction (∥ LD) and (020)B19' perpendicular to the LD (⊥ LD). For the films dominated by austenite (B2), the austenite directly transformed into martensitic variants (B19') with preferred orientations of (002)B19' ∥ LD and (020)B19' ⊥ LD. For the Ti50.3Ni46.2Cu3.5 and Ti48.1Ni40.8Cu7.5 films, martensitic transformation temperatures decreased apparently after post-annealing because of the large thermal stress generated in the films due to the large differences in thermal expansion coefficients between the film and substrate.

Unexpected methyl migrations of ethanol dimer under synchrotron VUV radiation.

While methyl transfer is well known to occur in the enzyme- and metal-catalyzed reactions, the methyl transfer in the metal-free organic molecules induced by the photon ionization has been less concerned. Herein, vacuum ultraviolet single photon ionization and dissociation of ethanol dimer are investigated with synchrotron radiation photoionization mass spectroscopy and theoretical methods. Besides the protonated clusters cation (C2H5OH)⋅H(+) (m/z = 47) and the ß-carbon-carbon bond cleavage fragment CH2O⋅(C2H5OH)H(+) (m/z = 77), the measured mass spectra revealed that a new fragment (C2H5OH)⋅(CH3)(+) (m/z = 61) appeared at the photon energy of 12.1 and 15.0 eV, where the neutral dimer could be vertically ionized to higher ionic state. Thereafter, the generated carbonium ions are followed by a Wagner-Meerwein rearrangement and then dissociate to produce this new fragment, which is considered to generate after surmounting a few barriers including intra- and inter-molecular methyl migrations by the aid of theoretical calculations. The appearance energy of this new fragment is measured as 11.55 ± 0.05 eV by scanning photoionization efficiency curve. While the signal intensity of fragment m/z = 61 starts to increase, the fragments m/z = 47 and 77 tend to slowly incline around 11.55 eV photon energy. This suggests that the additional fragment channels other than (C2H5OH)⋅H(+) and CH2O⋅(C2H5OH)H(+) have also been opened, which consume some dimer cations. The present report provides a clear description of the photoionization and dissociation processes of the ethanol dimer in the range of the photon energy 12-15 eV.

Site-selective dissociation processes of cationic ethanol conformers: the role of hyperconjugation.

In present report, we explored hyperconjugation effects on the site- and bond-selective dissociation processes of cationic ethanol conformers by the use of theoretical methods (including configuration optimizations, natural bond orbital (NBO) analysis, and density of states (DOS) calculations, etc.) and the tunable synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry. The dissociative mechanism of ethanol cations, in which hyperconjugative interactions and charge-transfer processes were involved, was proposed. The results reveal Cα-H and C-C bonds are selectively weakened, which arise as a result of the hyperconjugative interactions σCα-H → p in the trans-conformer and σC-C → p in gauche-conformer after being ionized. As a result, the selective bond cleavages would occur and different fragments were observed.

Dissociation of internal energy-selected methyl bromide ion revealed from threshold photoelectron-photoion coincidence velocity imaging.

Dissociative photoionization of methyl bromide (CH3Br) in an excitation energy range of 10.45-16.90 eV has been investigated by using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. The coincident time-of-flight mass spectra indicate that the ground state X(2)E of CH3Br(+) is stable, and both A(2)A1 and B(2)E ionic excited states are fully dissociative to produce the unique fragment ion of CH3 (+). From TPEPICO 3D time-sliced velocity images of CH3 (+) dissociated from specific state-selected CH3Br(+) ion, kinetic energy release distribution (KERD) and angular distribution of CH3 (+) fragment ion are directly obtained. Both spin-orbit states of Br((2)P) atom can be clearly observed in fast dissociation of CH3Br(+)(A(2)A1) ion along C-Br rupture, while a KERD of Maxwell-Boltzmann profile is obtained in dissociation of CH3Br(+)(B(2)E) ion. With the aid of the re-calculated potential energy curves of CH3Br(+) including spin-orbit coupling, dissociation mechanisms of CH3Br(+) ion in A(2)A1 and B(2)E states along C-Br rupture are revealed. For CH3Br(+)(A(2)A1) ion, the CH3 (+) + Br((2)P1/2) channel is occurred via an adiabatic dissociation by vibration, while the Br((2)P3/2) formation is through vibronic coupling to the high vibrational level of X(2)E state followed by rapid dissociation. C-Br bond breaking of CH3Br(+)(B(2)E) ion can occur via slow internal conversion to the excited vibrational level of the lower electronic states and then dissociation.

Dissociative photoionization of ß-pinene: an experimental and theoretical study.

We investigated the photoionization and dissociation photoionization of the ß-pinene molecular using time-of-flight mass spectrometry with a tunable vacuum ultraviolet source in the region from 8.00eV to 15.50eV. The experimental ionization energy (IE) value is 8.60eV using electron impact as the ionization source which is not in good agreement with theoretical value (8.41 eV) with a G3MP2 method. We obtained the accurate IE of ß-pinene (8.45 ± 0.03eV) derived from the efficiency spectrum which is in good agreement with the theoretical value (8.38eV) of the CBS-QB3 method. We elucidated the dissociation pathways of primary fragment ions from the ß-pinene cation on the basis of experimental observations in combination with theoretical calculations. Most of the dissociation pathways occur via a rearrangement reaction prior to dissociation. We also determined the structures of the transition states and intermediates for those isomerization processes.

Site-selective ionization of ethanol dimer under the tunable synchrotron VUV radiation and its subsequent fragmentation.

Site-selective ionization of ethanol dimer and the subsequent fragmentation were studied by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. With photoionization efficiency spectra measurements and theoretical calculations, the detailed mechanisms of the ionization-dissociation processes of ethanol dimer under VUV irradiation were explored. In 9.49-10.89 eV photon energy range, it was found that the ejection of the highest occupied molecular orbital (HOMO) electron from hydrogen bond donor induces a rapid barrierless proton-transfer process followed by two competitive dissociation channels, generating (C2H5OH)[middle dot]H(+) and CH2O[middle dot](C2H5OH)H(+), respectively. The latter comes from a carbon-carbon bond cleavage in the donor. While the photon energy is 10.9-11.58 eV, the electron of HOMO-1 of the hydrogen bond acceptor, is removed. Besides the dissociation channel to produce C2H5OH and C2H5OH(+), a new channel to generate (C2H5OH)[middle dot]CH2OH(+) is opened, where the cleavage of the carbon-carbon bond occurs in the acceptor. When the photon energy increases to 11.58 eV, the electron from HOMO-2 is ejected.

Dissociation limit and dissociation dynamic of CF4(+): application of threshold photoelectron-photoion coincidence velocity imaging.

Dissociation of internal energy selected CF4(+) ions in an excitation energy range of 15.40-19.60 eV has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Only CF3(+) fragment ions are observed in coincident mass spectra, indicating all the X(2)T1, A(2)T2, and B(2)E ionic states of CF4(+) are fully dissociative. Both kinetic energy released distribution (KERD) and angular distribution in dissociation of CF4(+) ions have been derived from three-dimensional TPEPICO time-sliced images. A parallel distribution of CF3(+) fragments along the polarization vector of photon is observed for dissociation of CF4(+) ions in all the low-lying electronic states. With the aid of F-loss potential energy curves, dissociation mechanisms of CF4(+) ions in these electronic states have been proposed. CF4(+) ions in both X(2)T1 and A(2)T2 states directly dissociate to CF3(+) and F fragments along the repulsive C-F coordinate, while a two-step dissociation mechanism is suggested for B(2)E state: CF4(+)(B(2)E) ion first converts to the lower A(2)T2 state via internal conversion, then dissociates to CF3(+) and F fragments along the steep A(2)T2 potential energy surface. In addition, an adiabatic appearance potential of AP0(CF3(+)∕CF4) has also been established to be 14.71 ± 0.02 eV, which is very consistent with the recent calculated values.

Experimental and theoretical study on the photoionization of styrene.

This study employed a vacuum ultraviolet synchrotron radiation source and reflectron time-of-flight mass spectrometry (TOF-MS) to investigate the photoionization and dissociation of styrene. By analyzing the photoionization mass spectrum and efficiency curve alongside G3B3 theoretical calculations, we determined the ionization energy of the molecular ion, appearance energy of fragment ions, and relevant dissociation pathways. The major ion peaks observed in the photoionization mass spectra of styrene correspond to C8 H8 + , C8 H7 + and C6 H6 + . The ionization energy of styrene is measured as 8.46 ± 0.03 eV, whereas the appearance energies of C8 H7 + and C6 H6 + are found to be 12.42 ± 0.03 and 12.22 ± 0.03 eV, respectively, in agreement with theoretical values. The main channel for the photodissociation of styrene molecular ions is the formation of benzene ions, whereas the dissociation channel that loses hydrogen atoms is the secondary channel. Based on the experimental results and empirical formulas, the required dissociation energies (Ed ) of C8 H7 + , C8 H6 + and C6 H6 + are calculated to be (3.96 ± 0.06), (4.00 ± 0.06) and (3.76 ± 0.06) eV, respectively. Combined with related thermochemical parameters, the standard enthalpies of formations of C8 H8 + , C8 H7 + , C8 H6 + and C6 H6 + are determined to be 964.2, 1346.3, 1350.2 and 1327.0 kJ/mol, respectively. Based on the theoretical study, the kinetic factors controlling the styrene dissociation reaction process are determined by using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. This provides a reference for further research on the atmospheric photooxidation reaction mechanism of styrene in atmospheric and interstellar environments.

Direct detection of isoprene photooxidation products by using synchrotron radiation photoionization mass spectrometry.

We report the combination of a vacuum ultraviolet photoionization mass spectrometer, operating on the basis of synchrotron radiation, with an environmental reaction smog chamber for the first time. The gas- and pseudo-particle-phase products of OH-initiated isoprene photooxidation reactions were measured on-line and off-line, respectively, by mass spectrometry. It was observed that aldehydes, methacrolein, methyl vinyl ketone, methelglyoxal, formic acid, and similar compounds are the predominant gas-phase photooxidation products, whereas some multifunctional carbonyls and acids mainly exist in the particle phase. This finding is reasonably consistent with results of studies conducted in other laboratories using different methods. The results indicate that synchrotron radiation photoionization mass spectrometry coupled with a smog chamber is a potentially powerful tool for the study of the mechanism of atmospheric oxidations and the formation of secondary organic aerosols.

Measurements of secondary organic aerosol formed from OH-initiated photo-oxidation of isoprene using online photoionization aerosol mass spectrometry.

Isoprene is a significant source of atmospheric organic aerosol; however, the secondary organic aerosol (SOA) formation and involved chemical reaction pathways have remained to be elucidated. Recent works have shown that the photo-oxidation of isoprene leads to form SOA. In this study, the chemical composition of SOA from the OH-initiated photo-oxidation of isoprene, in the absence of seed aerosols, was investigated through the controlled laboratory chamber experiments. Thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) was used in conjunction with the environmental chamber to study SOA formation. The mass spectra obtained at different photon energies and the photoionization efficiency (PIE) spectra of the SOA products can be obtained in real time. Aided by the ionization energies (IE) either from the ab initio calculations or the literatures, a number of SOA products were proposed. In addition to methacrolein, methyl vinyl ketone, and 3-methyl-furan, carbonyls, hydroxycarbonyls, nitrates, hydroxynitrates, and other oxygenated compounds in SOA formed in laboratory photo-oxiadation experiments were identified, some of them were investigated for the first time. Detailed chemical identification of SOA is crucial for understanding the photo-oxidation mechanisms of VOCs and the eventual formation of SOA. Possible reaction mechanisms will be discussed.

Direct experimental evidence for dissociative photoionization of oxygen molecule via 2Σ(u)(-) ionic "optical dark" state.

Direct experimental evidence for dissociative photoionization of oxygen molecule via the (2)Σ(u)(-) ionic optical dark state is presented by an investigation using the method of threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging. Besides vibrational progress of the B(2)Σ(g)(-) state, several weak vibrational bands of the (2)Σ(u)(-) ionic optical dark state are observed concomitantly in an excitation energy range of 20.2-21.1 eV. Only O(+) fragments are detected in the whole excitation energy range; therefore, all vibrational bands are completely predissociative. TPEPICO three-dimensional time-sliced velocity images of O(+) fragments dissociated from vibrational state-selected O(2)(+)((2)Σ(u)(-),v(+)) ions are recorded. For the (2)Σ(u)(-)(v(+)=0-3) vibrational states, only the lowest dissociation channel of O(+)((4)S) + O((3)P) is observed. Once the photon energy is slightly increased to the (2)Σ(u)(-)(v(+)=4) level, a new concentric doughnut appears in the image, indicating that the second dissociation channel of O(+)((4)S) + O((1)D) is identified indeed. With the aid of potential energy curves, the dissociative mechanism of O(2)(+) in the (2)Σ(u)(-)(v(+)) state is proposed.

Competitive fragmentation pathways of acetic acid dimer explored by synchrotron VUV photoionization mass spectrometry and electronic structure calculations.

In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH(3)COOH)(n)·H(+), the feature related to the fragment ions (CH(3)COOH)H(+)·COO (105 amu) via ß-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH(3)COOH)·H(+) and (CH(3)COOH)H(+)·COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH(3)COOH)H(+)·COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH(3)COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH(3)COOH)·CH(3)CO(+). Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.