Photoionization Mass Spectrometric Study Research Articles

Visualization of the Active Sites of Zinc-Chromium Oxides and the CO/H2 Activation Mechanism in Direct Syngas Conversion.

Despite wide studies demonstrating the versatility of the metal oxide-zeolite (OXZEO) catalyst concept to tackle the selectivity challenge in syngas chemistry, the active sites of metal oxides and the mechanism of CO/H2 activation remain to be elucidated. Herein, we demonstrate experimentally the role of Cr in zinc-chromium oxides and unveil visually, for the first time, the active sites for CO activation employing scanning transmission electron microscopy-electron energy loss spectroscopy using the volumetric density of surface carbon species as a descriptor. The ZnCr2O4 spinel surface with atomic ZnOx overlayer is the most active site for C-O bond dissociation, particularly at the narrow ZnCr2O4(110) facets constrained between the (311) and (111) facets, followed by the Cr-doped wurtzite ZnO surface. In comparison, the surfaces of ZnCr2O4 with aggregated ZnOx overlayers, pure ZnO, and the stoichiometric ZnCr2O4 exhibit a significantly lower activity. In situ synchrotron-based vacuum ultraviolet photoionization mass spectrometric study on different temperature programmed surface reactions with isotopes of C18O, 13CO, and D2 validates direct CO dissociation over ZnCrn oxides in CO, forming CH2 and further to hydrocarbons if H2 is present and CH2CO intermediates in syngas. The activity of CO dissociation and hydrogenation over ZnCrn oxides correlates well with the syngas-to-light-olefins activity of ZnCrn-SAPO-18 composite catalysts as a function of the Cr/Zn ratio.

Photoionization spectroscopy of the SiH free radical in the vacuum-ultraviolet range.

The first measurement of the photoelectron spectrum of the silylidyne free radical, SiH, is reported between 7 and 10.5eV. Two main photoionizing transitions involving the neutral ground state, X+1Σ+ ← X2Π and a+3Π ← X2Π, are assigned by using ab initio calculations. The corresponding adiabatic ionization energies are derived, IEad(X+1Σ+) = 7.934(5)eV and IEad(a+3Π) = 10.205(5)eV, in good agreement with our calculated values and the previous determination by Berkowitz et al. [J. Chem. Phys. 86, 1235 (1987)] from a photoionization mass spectrometric study. The photoion yield of SiH recorded in this work exhibits a dense autoionization landscape similar to that observed in the case of the CH free radical [Gans et al., J. Chem. Phys. 144, 204307 (2016)].

On-line photoionization mass spectrometric study of lignin and lignite co-pyrolysis: Insight into the synergetic effect

Co-pyrolysis behavior of kraft lignin (KL) and lignite (LI) was studied by thermogravimetry (TG) combined with on-line photoionization time-of-flight mass spectrometry (PI-TOF-MS) to get insights into the synergetic effect on the total yield and detailed composition of volatile. Significant synergetic effect was observed between KL and LI during co-pyrolysis processes, especially near the temperature with maximum weight loss rate. The interactions of typical co-pyrolysis products were figured out via the real-time and online mass spectrometry and was evaluated by comparing the experimental value with the calculated theoretical value. The results showed that the synergetic effect was beneficial to enhance the production of volatile and decrease the total yield of char. The interaction on typical co-pyrolysis products was further discussed. The result may provide useful guidance for the practical co-conversion of biomass and low-rank coal.

The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid.

The valence threshold photoionization of oleic acid has been studied using synchrotron VUV radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. An oleic acid aerosol beam was impacted on a copper thermodesorber, heated to 130 °C, to evaporate the particles quantitatively. Upon threshold photoionization, oleic acid produces the intact parent ion first, followed by dehydration at higher energies. Starting at ca. 10 eV, a large number of fragment ions slowly rise suggesting several fragmentation coordinates with quasi-degenerate activation energies. However, water loss is the dominant low-energy dissociation channel, and it is shown to be closely related to the unsaturated carbon chain. In the lowest-barrier process, one of the four allylic hydrogen atoms is transferred to the carboxyl group to form the leaving water molecule and a cyclic ketone fragment ion. A statistical model to analyze the breakdown diagram and measured rate constants yields a 0 K appearance energy of 9.77 eV, which can be compared with the density functional theory result of 9.19 eV. Alternative H-transfer steps yielding a terminal C=O group are ruled out based on energetics and kinetics arguments. Some of the previous photoionization mass spectrometric studies also reported 2 amu and 26 amu loss fragment ions, corresponding to hydrogen and acetylene loss. We could not identify such peaks in the mass spectrum of oleic acid.

On-line photoionization mass spectrometric study on the mouth retention of gaseous mainstream cigarette smoke

Measurement of the retention of cigarette smoke components in the human mouth is important for evaluating smoking-related diseases.

A VUV photoionization mass spectrometric study on the OH-initiated photooxidation of isoprene with synchrotron radiation

The gas-phase organic compounds resulting from OH-initiated photooxidation of isoprene have been investigated on-line by VUV photoionization mass spectrometry based on synchrotron radiation for the first time. The photoionization efficiency curves of the corresponding gaseous products as well as the chosen standards have been deduced by gating the interested peaks in the photoionization mass spectra while scanning the photon energy simultaneously, which permits the identification of the pivotal gaseous products of the photooxidation of isoprene, such as formaldehyde (10.84 eV), formic acid (11.38 eV), acetone (9.68 eV), glyoxal (9.84 eV), acetic acid (10.75 eV), methacrolein (9.91 eV), and methyl vinyl ketone (9.66 eV). Proposed reaction mechanisms leading to the formation of these key products were discussed, which were completely consistent with the previous works of different groups. The capability of synchrotron radiation photoionization mass spectrometry to directly identify the chemical composition of the gaseous products in a simulation chamber has been demonstrated, and its potential application in related studies of atmospheric oxidation of ambient volatile organic compounds is anticipated.

A photoionization mass spectrometric and threshold photoelectron-photoion coincidence study of vinylbromide (C 2H 3Br) in the vacuum ultraviolet range of 6–21 eV

The dissociative photoionization of vinyl bromide (C 2H 3Br) has been investigated by photoionization mass spectrometry (PIMS) and time-of-flight threshold photoelectron-photoion coincidence (TOF-TPEPICO) spectrometry using synchrotron radiation. The photoionization efficiency curves of the three most abundant ions, i.e. C 2H 3Br +, C 2 H 3 + and C 2 H 2 + , are measured and analyzed in detail. A C 2 H 3 + / Br - photoion-pair formation process has been detected for the first time. Some arguments are provided in favor of the electronic excitation of the C 2 H 3 + fragment and of the isomerization of the C 2 H 2 + ions. The breakdown diagram for these ions in the 9.8–21 eV photon energy range, derived from TOF-TPEPICO mass spectra, is discussed. The involvement of the successive C 2H 3Br + ionic states and the role of autoionization of C 2H 3Br Rydberg states are emphasized. For the two minor HBr + and Br + fragment ions only the breakdown diagrams are measured and appearance energies are derived.

Photoionization mass spectrometric studies of N-methyl formamide and N,N′-dimethyl formamide in the 7–18 eV photon energy range

A photoionization mass spectrometric study of N-methyl formamide and N,N′-dimethyl formamide has been made using synchrotron radiation over the photon energy range 8–20 eV. Photoion yield curves were measured for the parent ion in both species and for six fragment ions in N-methyl formamide. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made on the basis of ion appearance energies in conjunction with thermochemical data and the results of electron impact mass spectral studies. Our results illuminate aspects of the effects of methylation of formamide on ionization energies and dissociative ionization channels, as well as on the relative order of analogous molecular orbitals. The principal dissociative ionization process in both compounds involves HCO-loss. This neutral product may be formed in an electronic excited state. A comparison between the ionization properties, in particular the heats of formation of the cations, of the methyl derivatives of formamide and acetamide, shows that the ionization energies of the latter require re-investigation.

The photoionization dynamics of methyl iodide (CH3I): a joint photoelectron and mass spectrometric investigation

Threshold photoelectron (TPES) and photoionization mass spectrometric (PIMS) studies of CH3I in the 8–20 eV photon energy range are presented. The interpretation and assignments are supported by ab initio calculations. The TPES study shows many new discrete features in the Jahn–Teller split ground 2E (2A′–2A″) state of CH3I+. A new continuous band starting at about 11.7 eV is detected. These observations are essentially correlated with autoionizing transitions. This interpretation is supported by constant ion state (CIS) spectroscopy. A large enhancement of the transitions to the 2A and 2E ionic states is ascribed to large autoionizing contributions. Based on the present calculations, the weak to very weak bands in the 18.0–23.0 eV photon energy range are mainly assigned to 2a−11 ionization and to double excitations corresponding essentially to the 2e−2 4a11 and 3a−112e−14a11 configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH3I+ leading to CH+2, CH+3, I+ and CH2I+ from the threshold up to 20 eV photon energy. The experimental data are compared to ab initio calculated dissociation energies. The threshold of appearance of CH+3, I+ and CH2I+ fragments is concentrated in the 12.2–12.7 eV photon energy range. All three exit channels are correlated with the ground state of CH3I+ via non-adiabatic transitions. All three fragment ions have to appear through predissociation of the ionic 2E state and autoionizing dissociation from the (2E3/2)6p Rydberg state. This interpretation is strongly supported by the photoabsorption spectrum measured recently in the same photon energy range. At higher energies, besides direct or predissociation of the 2A1 and 2E states of CH3I+, autoionization is also suggested to contribute to the fragmentation in all decay channels.

Halogenated silanes, radicals, and cations: Theoretical predictions on ionization energies, structures and potential energy surfaces of cations, proton affinities, and enthalpies of formation

Quantum chemistry study has been carried out on the structure and energetics of halogenated silanes, radicals, and cations (SiH x X y 0,+1, X = F, Cl, Br; x + y = 1–4). The geometries are optimized at B3LYP/6-31+G(2df,p) level. The adiabatic ionization energiess (IE as), relative energetics of cations, proton affinities (PAs) of silanes, and the enthalpies of formation are predicted using G3(CC) model chemistry. Non-classical ion complex structures are found for hydrogenated cations and transition states connecting classical and non-classical structures are also located. The most stable cations for silylene and silyl radicals have their classical divalent and trivalent structures, and those for silanes have non-classical structures except for SiH 3Br + and SiH 2Br 2 +. The non-classical structures for halosilane cations imply difficulty in experimentally measurement of the adiabatic ionization energies using photoionization or photoelectron studies. For SiH 3X, SiH 2X 2, and SiHX 3, the G3(CC) adiabatic IE as to classical ionic structures closest to their neutrals agree better with the photoelectron spectroscopic measurements. The transition states between classical and non-classical structures also hamper the photoionization determination of the appearance energies for silylene cations from silanes. The G3(CC) results for SiH x 0,+1 agree excellently with the photoionization mass spectrometric study, and the results for fluorinated and chlorinated species also agree with the previous theoretical predictions at correlation levels from BAC-MP4 to CCSD(T)/CBS. The predicted enthalpy differences between SiH 2Cl +, SiHCl 2 +, and SiCl 3 + are also in accordance with previous kinetics study. The G3(CC) results show large discrepancies to the collision-induced charge transfer and/or dissociation reactions involving SiF x + and SiCl x + ions, for which the G3(CC) enthalpies of formation are also significantly differed from the previous theoretical predictions, especially on SiF x + ( x = 2–4). The G3(CC) IE a and PA of SiF 4 are significantly different from previous experimental and theoretical studies; however, they are supported by current benchmark calculations at level of CCSD(T)/CBS + core-valence correction.

Atmospheric Pressure Photoionization Mass Spectrometry of Polyisobutylene Derivatives

Atmospheric pressure photoionization mass spectrometric (APPI-MS) study on three types of polyisobutylene derivatives is reported. Two of the polyisobutylenes investigated were polyisobutylene with dihydroxy and diolefinic end-groups derived from aromatic moieties [dicumyl chloride, 1,4-bis(2-chloro-2-propyl)benzene], and the third contained no aromatic moieties with a monohydroxy end-group. All three polyisobutylene derivatives (PIBs) had an average molecular weight (M(n)) of approximately 2000 g/mol, with a polydispersity lower than 1.2. In the positive ion APPI mode, protonated PIB molecules were formed, but the molecular weights obtained were considerably lower than those expected, indicating fragmentation of the PIB chains. In the negative APPI mode, using solvents such as tetrahydrofuran and toluene as dopants, no signal was obtained. However, in chlorinated solvents, such as CCl(4), CHCl(3), and CH(2)Cl(2), in the presence of toluene dopant, PIB adducts with chloride ions were formed with relatively high signal intensity. In the case of CH(2)Cl(2), no dopant (toluene) was necessary to generate chlorinated adduct ions, albeit increasing the toluene concentration in the flow increased the PIB signal intensity. The effect of the toluene concentration on PIB signal intensity was studied and models that include (1) photoionization of toluene, (2) formation of chloride ions from the chlorinated solvents by dissociative electron capture, (3) formation of chlorinated adduct ions and charge recombination reactions between the toluene radical cation, (4) chloride ions, and (5) chlorinated adduct ions are proposed based on the experimental results.

About the photoionization of methyl bromide (CH 3Br). Photoelectron and photoionization mass spectrometric investigation

The threshold photoelectron (TPES) and the photoionization mass spectrometric study of CH 3Br in the 8–20 eV photon energy range is presented. The interpretation and assignments are supported by ab initio calculations. The TPES shows several new discrete features in the Jahn–Teller split ground state X ˜ 2 E ( 2 A ′ – 2 A ″ ) of CH 3Br +. An additional continuous band starts at about 11.8 eV. These observations are both correlated with direct ionization and autoionizing transitions. This is supported by constant ion state (CIS) spectroscopy. A large enhancement of the transitions to the A ˜ 2 A and B ˜ 2 E states is ascribed to important autoionizing contributions. Based on the present calculations, the weak to very weak bands in the 17.5–22.0 eV photon energy range were mainly assigned to 2 a 1 - 1 ionization and to double excitations described essentially by the 2 e - 2 4 a 1 1 and 1 e - 1 2 e - 1 4 a 1 1 configurations. The photoionization mass spectrometric study allowed us to investigate in detail the ionization and dissociation of CH 3Br + leading to CH 2 + , CH 3 + , Br + and CH 2Br + from threshold up to 20 eV photon energy. The experimental data are compared to ab initio dissociation energies. At the onset, the CH 3 + and CH 2Br + fragment ion production is correlated with the ground state of CH 3Br + and both fragment ions have to appear through dissociative autoionization from the ( 3 a 1 1 / 1 e 3 ) 6 s or 5s Rydberg state. This interpretation is supported by the photoabsorption spectrum measured recently in the same photon energy range. At higher energies, beside a likely direct (pre)dissociation of the A ˜ 2 A 1 and B ˜ 2 E states of CH 3Br +, autoionization also contributes to the fragmentation in all decay channels. Avoided crossings in a manifold of 2A′ states are likely to be involved. This is supported by ab initio calculations. For CH 3 + the photoion-pair process is analyzed and detailed assignments are proposed on the basis of our latest VUV photoabsorption spectroscopic data.

A photoionization mass spectrometric study of acetylene and ethylene in the VUV spectral region

The molecular and dissociative photoionization of C2H2 and C2H4 has been studied using synchrotron radiation within the energy range 11–30eV. Positive and negative photoion mass spectra have been obtained. Appearance thresholds for the formation of fragment ions have been determined and are compared with the thermochemical values. Above threshold it is shown that superexcited states, involving intravalence transitions and shape resonances which can decay through ion pair formation, play an important role in the photodissociative ionization process. The positive photoion yield curves display significant hitherto unobserved structures, which are unrelated to those present in the photoelectron spectra.

About the photoionization of methyl chloride: A threshold photoelectron spectroscopic and photoionization mass spectrometric investigation

The threshold photoelectron spectroscopic (TPES) and the photoionization mass spectrometric (PIMS) study of methyl chloride (CH 3Cl) in the 8–20 eV photon energy range is presented. The interpretation and assignments are supported by ab initio calculations. The detailed analysis of the TPES shows numerous features in the Jahn–Teller split ground X 2 E ( 2 A ′– 2 A ′′ ) state of CH 3Cl + and a new continuous band peaking at 13.4 eV. These were assigned to both direct ionization and autoionizing transitions. The transitions to the A ̃ 2 A and to the B 2 E states undergo a large enhancement ascribed to important autoionizing contributions. Based on the present calculations, the weak to very weak signals in the 19–26 eV photon energy range were mainly assigned to 2a 1 −1 ionization and to double excitations described essentially by the 2e −2, 4a 1 1 and 1e −12e −14a 1 1 configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH 3Cl + into CH 2 +, CH 3 +, CHCl + and CH 2Cl + from threshold up to 20 eV photon energy. At the onset, the CH 3 +, CHCl + and CH 2Cl + fragment ion production is correlated to the ground state of CH 3Cl + and all fragment ions have to appear through dissociative autoionization from the 2e → 3p Rydberg state. This interpretation is supported by the photoabsorption spectrum (PAS) measured recently in the same photon energy range. At higher energies, beside direct dissociation of the A ̃ 2 A 1 and B 2 E states of CH 3Cl + autoionization also contributes to the fragmentation in all decay channels. For CH 3 + the photoion-pair process is analyzed and detailed assignments are proposed on the basis of recent PAS data.

Gaussian-2 and Gaussian-3 Study of the Energetics and Structures of Cl2On and Cl2On+, n = 1−7

The structures and energetics of the dichlorine oxides, Cl{sub 2}O{sub n}, n = 1 {minus} 7, and their cations have been investigated theoretically at the Gaussian-2 (G2) and the Gaussian-3 (G3) levels of theory. The G2 and G3 heats of formation obtained for both neutral and cationic species allow the interpretation of assigned appearance energies and ionization energies reported in a recent photoionization mass spectrometric study on these chlorine oxides. The calculations predict that Cl{sub 2}O{sub 6}{sup +} dissociates spontaneously to ClO{sub 2}{sup +} and ClO{sub 4}, in agreement with the nonobservation of Cl{sub 2}O{sub 6}{sup +} in the experimental study by Ruehl et al. For systems with n {le} 4, the calculated and experimental thermochemical properties (such as heats of formation and ionization energies) are in good to excellent agreement with each other. However, this agreement deteriorates as the size of the molecular system increases or as n exceeds 4. Thus it appears that the G3 method may also suffer from an unfavorable accumulation of component small errors, as has been found for the G2 method and its variants.

A Vacuum Ultraviolet Photoionization Mass Spectrometric Study of Propylene Oxide in the Photon Energy Region of 10−40 eV

The photoionization and dissociative photoionizations of propylene oxide have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions C3H6O+, C2H3O+, CH3O+, CH2O+, CHO+, C2H4+, C2H3+, C2H2+, CH3+, and CH2+ were obtained. Theoretically, the energetics of the dissociative photoionizations were examined by ab initio Gaussian-2 calculations. The computational results are useful in our attempt to establish the dissociation channels near the ionization thresholds. The dissociation channels we propose include simple bond cleavage reactions as well as reactions involving intermediates and transition structures.

A Vacuum Ultraviolet Photoionization Mass Spectrometric Study of Ethylene Oxide in the Photon Energy Region of 10−40 eV

The photoionization and dissociative photoionizations of ethylene oxide have been studied both experimentally and theoretically. In experiments, photoionization efficiency spectra for ions C2H4O+, C2H3O+, C2H2O+, C2HO+, CH3O+, CH2O+, CHO+, C2H4+, C2H3+, C2H2+, C2H+, CH4+, CH3+, CH2+, CH+, and C+ have been obtained. In addition, the energetics of the dissociative photoionizations have been examined by ab initio Gaussian-2 calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. The dissociation channels established may be broadly divided into two types: simple bond cleavage reactions and those involving transition structures and activation energies.

Photoionization mass spectrometric study of neutral species from pulsed laser ablation of SnO 2

We report the application of vacuum ultraviolet (VUV) photoionization mass spectrometry (PIMS) to probe neutral species generated in the 532 nm laser ablation of sintered SnO 2 targets. The major (>90%) Sn containing species are of composition (SnO) x ( x=1,2,3), with near-natural abundance isotopic distributions. The translational energy distribution was determined for each product and compared to a Maxwellian velocity distribution. The utility of VUV PIMS as a universal probe of neutral species produced in laser ablation is discussed.

Primary products of the reactions of fluorine atoms with CH3OH, CH3SH and CH3OCH3 studied with ultraviolet photoelectron spectroscopy

Abstract He I photoelectron spectra have been recorded for the reactions F+CH 3 OH, F+CH 3 SH and F+CH 3 OCH 3 and bands associated with the primary reaction products are identified. The known first photoelectron band of CH 2 OH, with an adiabatic ionization energy (IE) of (7.56±0.01) eV, and a sharp band with an adiabatic IE of (10.72±0.01) eV have been observed early in the F+CH 3 OH reaction. This latter feature is assigned to the first photoelectron band of CH 3 O. The corresponding band of CD 3 O has been observed in the F+CD 3 OH and F+CD 3 OD reactions and is found to be unchanged in shape and position from the first band of CH 3 O. Similarly, the first photoelectron bands of CH 3 S and CH 2 SH have been identified in spectra recorded for the F+CH 3 SH reaction. The CH 3 S first photoelectron band is sharp and has an adiabatic IE of (9.25±0.02) eV whereas the CH 2 SH first band is broad and has an adiabatic IE of (7.52±0.03) eV. These values are in excellent agreement with ionization energies obtained from recent photoionization mass spectrometric studies and molecular orbital calculations on these short-lived molecules. In addition, a broad band observed at low reaction times in the F+CH 3 OCH 3 reaction, with an adiabatic IE of (7.04±0.02) eV, is assigned to the first ionization of the CH 2 OCH 3 radical. The first band of the CD 2 OCD 3 isotopomer has also been identified in the corresponding F+CD 3 OCD 3 reaction. A comparison is made with the results obtained for the F+CH 3 SCH 3 reaction in which the first photoelectron band of CH 2 SCH 3 was recorded.

The H–NCS bond energy, ΔH○f (HNCS), ΔH○f (NCS), and IP(NCS) from photoionization mass spectrometric studies of HNCS, NCS, and (NCS)2

The species HNCS and NCS have been studied by photoionization mass spectroscopy. The adiabatic ionization potential (IP) of HNCS is ≤9.92 eV; the shape of the ion yield curve near threshold implies a large change in geometry, perhaps to linear HNCS+. Various appearance potentials (AP) from HNCS are determined by a more objective procedure, from which ΔH○f(HNCS)≳29.6 kcal/mol (and very likely ≥31.0±0.7 kcal/mol) is deduced. The adiabatic IP of NCS is 10.689±0.005 eV. This quantity, together with AP(NCS+/HNCS), yields D0(H–NCS)≤97.2±0.2 kcal/mol. However, a measurement of the appearance potential of NCS+ from (NCS)2 results in a still lower value, D0(H–NCS)≤93.3±1.1 kcal/mol, and ΔHf0○(NCS) ≤ 72.7 ± 0.8 kcal/mol. Approximate values are obtained for ΔHf0○(HNCS+) and for the proton affinity of NCS.