氟里昂134a的真空紫外光电离和光解离的同步辐射研究

利用同步辐射真空紫外光研究了氟里昂134a(CH₂FCF₃)的光电离和光解离过程,通过测量各离子的光电离效率曲线,得到了该分子的电离能(12.68±0.04)eV和所有碎片离子的出现势,运用Gaussian-03计算了母体和中性碎片及相应离子的电子态、对称性和能量.结合理论计算结果对离子出现势的理论值和实验值进行了比较,分析了母体离子可能的光解离通道.     

Vacuum ultraviolet (VUV) photoionization of small water clusters

Tunable vacuum ultraviolet (VUV) photoionization studies of water clusters are performed using 10-14 eV synchrotron radiation and analyzed by reflectron time-of-flight (TOF) mass spectrometry. Photoionization efficiency (PIE) curves for protonated water clusters (H2O)(n)H+ are measured with 50 meV energy resolution. The appearance energies of a series of protonated water clusters are determined from the photoionization threshold for clusters composed of up to 79 molecules. These appearance energies represent an upper limit of the adiabatic ionization energy of the corresponding parent neutral water cluster in the supersonic molecular beam. The experimental results show a sharp drop in the appearance energy for the small neutral water clusters (from 12.62 +/- 0.05 to 10.94 +/- 0.06 eV, for H2O and (H2O)4, respectively), followed by a gradual decrease for clusters up to (H2O)23 converging to a value of 10.6 eV (+/-0.2 eV). The dissociation energy to remove a water molecule from the corresponding neutral water cluster is derived through thermodynamic cycles utilizing the dissociation energies of protonated water clusters reported previously in the literature. The experimental results show a gradual decrease of the dissociation energy for removal of one water molecule for small neutral water clusters (3 相似文献   

一氯甲烷同步辐射光电离研究: 离子生成焓与键能的测定

本文报道超音速射流冷却条件下, 用同步辐射光研究CH3Cl光电离及其解离电离的动力学, 测得CH3Cl的电离能(IP)为11.28±0.01eV。通过测定CH3Cl光解离电离碎片的出现势(AP), 并结合有关已确认的热力学数据, 获得了它们的标准生成焓、离子型分子中的键能、中性分子或自由基中的键能及母体离子的解离能等热力学数据。对CH3Cl分子VUV光解离电离通道进行了分析。     

五氟乙烷的真空紫外光电离光解离

利用同步辐射真空紫外光电离结合飞行时间质谱对五氟乙烷(CHF₂CF₃)进行了光电离光解离的研究，测定了CHF₂CF₃的电离势及该分子主要碎片离子的出现势. 结果表明: CHF₂CF₃的绝热电离势为(12.25±0.10) eV, 主要碎片离子有CF₂CF_{3⁺、CHFCF3⁺、CF2CF2⁺、CF3⁺、CHF2⁺、CHF⁺和CF⁺，其出现势分别为 (13.93±0.10) eV, (15.25±0.10) eV, (15.12±0.10) eV, (13.30±0.05) eV, (13.05±0.08) eV, (19.17±0.10) eV和 (19.56±0.15) eV. 另外，用从头算分子轨道理论计算了解离电离过程中所有碎片的总能量，并给出了电子态及对称性. 根据实验结果和理论计算，分析了可能的解离通道，并得到了分子及母体离子的键解离能等重要的热力学数据.}     

Thermal vaporization of biological nanoparticles: fragment-free vacuum ultraviolet photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and beta-carotene

A simple, new way to introduce fragile biomolecules into the gas phase via thermal vaporization of nanoparticles is described. The general utility of this technique for the study of biomolecules is demonstrated by coupling this source to tunable synchrotron vacuum ultraviolet radiation. Fragment-free photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and beta-carotene are detected with signal-to-noise ratios exceeding 100. The 8.0 eV photoionization mass spectrum of tryptophan nanoparticles vaporized at 373 K is dominated by a single parent ion peak that exhibits a 20-fold enhancement over the methylene indole fragment ion. The degree of dissociative photoionization of tryptophan can be precisely controlled either by the thermal energy imparted into the neutral tryptophan molecule or by the energy of the ionizing photon. The results reveal how approximately 0.5 eV changes in internal energy affect both the photoionization mass spectrum of tryptophan and the appearance energy of the daughter ion fragments. This method allows the ionization energies of glycine (9.3 +/- 0.1 eV), tryptophan (7.3 +/- 0.2 eV), phenylalanine (8.6 +/- 0.1 eV), phenylalanine-glycine-glycine (9.1 +/- 0.1 eV), and beta-carotene (<7.0 eV) molecules to be determined directly from the photoionization efficiency spectra.     

Valence and inner-valence shell dissociative photoionization of CO in the 26-33 eV range. II. Molecular-frame and recoil-frame photoelectron angular distributions

Experimental and theoretical results for molecular-frame photoemission are presented for inner-valence shell photoionization of the CO molecule induced by linearly and circularly polarized light. The experimental recoil frame photoelectron angular distributions (RFPADs) obtained from dissociative photoionization measurements where the velocities of the ionic fragment and photoelectron were detected in coincidence, are compared to RFPADs computed using the multichannel Schwinger configuration interaction method. The formalism for including a finite lifetime of the predissociative ion state is presented for the case of general elliptically polarized light, to obtain the RFPAD rather than the molecular frame photoelectron angular distribution (MFPAD), which would be obtained with the assumption of instantaneous dissociation. We have considered photoionization of CO for the photon energies of 26.0 eV, 29.5 eV, and 32.5 eV. A comparison of experimental and theoretical RFPADs allows us to identify the ionic states detected in the experimental studies. In addition to previously identified states, we found evidence for the 2 (2)Δ state with an ionization potential of 25.3 eV and (2)Σ(+) states with ionization potentials near 32.5 eV. A comparison of the experimental and theoretical RFPADs permits us to estimate predissociative lifetimes of 0.25-1 ps for some of the ion states. Consideration of the MFPADs of a series of (2)Π ion states indicates the importance of inter-channel coupling at low photoelectron kinetic energy and the limitations of a single-channel analysis based on the corresponding Dyson orbitals.     

二氯甲烷同步辐射光电离的动力学研究

采用VUV同步辐射光源，在超声射流冷却条件下研究了二氯甲烷（CH2Cl2）光电离及其解离电离的动力学过程，测得CH2Cl2的电离能（IP）为11．32士0．01eV．通过对CH2Cl2光解离电离过程产生的碎片离子的出现势（AP）的测定，并结合有关中性分子的热力学数据，估算出该体系中有关离子的标准生成焓、中性分子和离子型分子中的键能及母体离子的解离能（DE）等一系列热力学数据．对CH2Cl2分子VUV光解离电离通道进行了细致的分析．     

Experimental and theoretical studies of the electronic structure and the ionization and dissociation processes of trifluoromethyl peroxynitrate

In this work, we present a complete study of the ionization and dissociation processes for trifluoromethyl peroxynitrate (CF3OONO2). CF3OONO2 was generated by UV photolysis of a mixture of (CF3CO)2O, NO2, and O2. The product was detected and characterized by the photoelectron spectroscopy (PES) and photoionization mass spectroscopy (PIMS). The geometric and electronic structures of CF3OONO2 were investigated by the combination of experiments and the density functional and ab initio calculations. It is worthwhile mentioning that drastic changes occur in the geometry of CF3OONO2 after ionization. Due to the removal of one electron from the O-N sigma bond, the COON dihedral angle changes to 180 degrees and as a result, the nonplanar structure becomes planar. And the O-N single bond length increases remarkably, with the positive charge most localized on the NO2 moiety. The experimental first vertical ionization potential is 12.39 eV. Based on the calculated bond dissociation energies, the dissociation pathway was predicted. The calculated results explain the ion intensities observed in the photoionization mass spectrum. The dissociation of O-N single bond is found to be the most favored of the possible dissociation paths for CF3OONO2+.     

Unimolecular Reaction Mechanism of an Imidazolin‐2‐ylidene: An iPEPICO Study on the Complex Dissociation of an Arduengo‐Type Carbene

The photoionization and dissociative photoionization of Im(iPr)₂, 1,3‐diisopropylimidazolin‐2‐ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone‐pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IE_ad=7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate‐determining step as hydrogen‐atom migration from the isopropyl group to the carbene carbon center forming a resonance‐stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene‐loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance‐stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate‐sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.     

CH3I同步辐射光电离及生成焓、键能和质子亲合势的测定

在超声射流冷却条件下用VUV同步辐射研究了CH3I分子的光电离过程。测定了CH3I光电离及解离电离产生的CH3I^+, CH3^+和I^+的出现势, 结合已确认的热力学数据, 估算出体系中有关离子的标准生成焓、分子和分子离子的键能、自由基的质子亲合势及母体离子的解离能等数据。对CH3I分子VUV光解离电离通道进行了分析。     

Valence Photoionization of Thymine: Ionization Energies,Vibrational Structure,and Fragmentation Pathways from the Slow to the Ultrafast

The photoionization of thymine has been studied by using vacuum ultraviolet radiation and imaging photoelectron photoion coincidence spectroscopy after aerosol flash vaporization and bulk evaporation. The two evaporation techniques have been evaluated by comparison of the photoelectron spectra and breakdown diagrams. The adiabatic ionization energies for the first four electronic states were determined to be 8.922±0.008, 9.851±0.008, 10.30±0.02, and 10.82±0.01 eV. Vibrational features have been assigned for the first three electronic states with the help of Franck–Condon factor calculations based on density functional theory and wave function theory vibrational analysis within the harmonic approximation. The breakdown diagram of thymine, as supported by composite method ab initio calculations, suggests that the main fragment ions are formed in sequential HNCO-, CO-, and H-loss dissociation steps from the thymine parent ion, with the first step corresponding to a retro-Diels–Alder reaction. The dissociation rate constants were extracted from the photoion time-of-flight distributions and used together with the breakdown curves to construct a statistical model to determine 0 K appearance energies of 11.15±0.16 and 11.95±0.09 eV for the m/z 83 and 55 fragment ions, respectively. These results have allowed us to revise previously proposed fragmentation mechanisms and to propose a model for the final, nonstatistical H-loss step in the breakdown diagram, yielding the m/z 54 fragment ion at an appearance energy of 13.24 eV.     

New determination of the adiabatic ionization potential of the BaOH radical from laser photoionization-molecular beam experiments and ab initio calculations

The adiabatic ionization potential of the BaOH radical, as generated in a laser vaporization-supersonic expansion source has been determined by laser photoionization experiments to be (4.55 ± 0.03) eV. This value supports the three lowest out of seven previous experimental estimates, the former ranging from 4.35 to 4.62 eV. The present result is compared to ab initio calculations, as performed using both quantum chemistry at different levels of theory and density functional theory, and trying several effective core potentials and their accompanying basis sets for Ba. The most satisfactory agreement is obtained for either the adiabatic or vertical ionization potentials that derive from post-Hartree-Fock [MP2 and CCSD(T)] treatments of electron correlation, along with consideration of relativistic effects and extensive basis sets for Ba, in both BaOH and BaOH(+). Such conclusions extend to the results of related calculations on the Ba-OH dissociation energies of BaOH and BaOH(+), which were performed to help in calibrating the present computational study. Bonding in BaOH/BaOH(+), as well as possible sources of discrepancy with previous experimental determinations of the BaOH adiabatic ionization potential are discussed.     

Gas-phase generation and electronic structure investigation of chlorosulfanyl thiocyanate, ClSSCN

The chlorosulfanyl thiocyanate molecule, ClSSCN, was generated in the gas phase through heterogeneous reaction of SCl2 on the surface of finely powdered AgSCN for the first time. The reaction products were detected and characterized in situ by ultraviolet photoelectron and photoionization mass spectrometry. The molecular geometry and electronic structures of ClSSCN were investigated by a combination of PES experiment and theoretical calculations with the density functional theory and ab initio methods. It was found that the outermost electrons of ClSSCN reside in the Cl-S antibonding pi orbital, predominantly localized on the sulfur atom, and the experimental first vertical ionization potential of ClSSCN is 10.20 eV. The dominant fragment SSCN+ in the mass spectrum indicates that the ClSSCN cation prefers the dissociation of the Cl-S bond.     

Dissociative excitation of SiH4, SiD4, Si2H6 and GeH4 by 0–100 eV electron impact

We have measured emission cross sections of various electronically excited fragments produced by electron-impact dissociation of SiH₄, SiD₄, Si₂H₆ and GeH₄. At low impact energy (10–20 eV), the measured appearance potentials are correlated to specific dissociation processes. Below 22 eV superexcited states of SiH₄ play a dominant role in the formation of neutral excited fragments. In agreement with the results obtained on alkanes, the cross sections for fragment emission from Si₂H₆ are lower than those for SiH₄. On the other hand, the comparison of cross sections at 100 eV for fragment emission, dissociation and ionization on going from CH₄ to SiH₄ and GeH₄ shows an increase of the probability for production of neutral ground-state fragments at the cost of excited or ionic fragments. Both effects can be explained by a growing probability for internal conversion among the decay channels of superexcited states with increasing number of atoms or electrons in the parent molecule. For each molecule, the H Balmer-emission cross sections at 100 eV are proportional to n^−b, where n 3 is the principal quantum number of the upper state of H and 3 < b < 5 is a parameter characteristic of the parent molecule. Finally, a detailed analysis of the isotopic effect between SiH₄ and SiD₄ on both fragment emission and ionization cross sections from 0 to 100 eV gives strong evidence of the competition between dissociation and autoionization in the decay of superexcited states.     

二茂铁的同步辐射光电离

The vacuum ultraviolet photoionization of ferrocene has been studied by using synchrotron radiation and a time-of-flight（TOF）mass spectrometer. The photoionization TOF mass spectrum and photoionization efficiency （PIE）curves of some ions were measured. VUV absorption by ferrocene results in Fe（C5H5）2+,FeC5H5+,Fe+,FeC3H3+,FeC3H+,C10H9+,C10H8+ and C5H6+. The ionization potential（IP）of ferrocene is determined to be （6.78±0.05）eV. The appearance potential（AP）of the fragment FeC5H5+ was measured to be（13.40±0.10）eV. In addition,theoretical calculations with the density functional method B3LYP and the basis set 6-31G（d）have been carried out. The calculation result shows that the ionization potential of ferrocene is 6.16 eV,which is smaller than that from the experiment because the ionization potential from calculation is adiabatic value and the experimental result is vertical value. Due to the limited available computational cost,the case of the electron spin S=1/2 for Fe is only considered,which may lead to some low precision in calculation. So the calculation result is just as references. The appearance potential of FeC5H5+ is 12.17 eV,which is also smaller than the experimental value. According to the experimental and calculation results,the bond energies of D0（FeC5H5+-C5H5）,D0（Fe+-C5H5）,D0（C5H5-Fe+-C5H5）have been evaluated and the possible channels of dissociation photoionization have been analyzed. Sequential elimination of C5H5 ligands is a major dissociation channel,but concerted elimination of two C5H5 ligands also takes place.     

Study on the atmospheric photochemical reaction of CF3 radicals using ultraviolet photoelectron and photoionization mass spectrometer

A study of the atmospheric photochemical reaction of CF₃ radical with CO and O₂ was performed by using a homemade ultraviolet photoelectron spectrometer-photoionization mass spectrometer (PES-PIMS). The electronic structures and mechanism of ionization and dissociation of CF₃OC(O)OOC(O)-OCF₃ were investigated. It was indicated that the two bands on the photoelectron spectrum of CF₃OC(O)OOC(O)OCF₃ are the result of ionization of an electron from a lone pair of oxygen and a fluorine lone pair of CF₃ group. The outermost electrons reside in the oxygen lone pair. The experimental and theoretical first vertical ionization energy is 13.21 and 13.178 eV, respectively, with the PES and OVGF method. They are in good agreement. The photo ionization and dissociation processes were discussed with the help of theoretical calculations and PES-PIMS experiment. After ionization, the parent ions prefer the dissociation of the C—O bond and giving the fragments CF₃OCO⁺ and CF ₃ ⁺ . It demonstrated that the ultraviolet photoelectron and photoionization mass spectrometer could be applied widely in the study of atmospheric photochemical reaction.     

A vacuum ultraviolet photoionization mass spectrometric study of acetone

The photoionization and dissociative photoionization of acetone have been studied at the photon energy range of 8-20 eV. Photoionization efficiency spectra for ions CH3COCH3+, CH3+, C2H3+, C3H3+, C3H5+, CH(2-)CO+, CH3CO+, C3H4O+, and CH3COCH2+ have been measured. In addition, the energetics of the dissociative photoionization has been examined by ab initio Gaussian-3 (G3) calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. With the help of G3 results, the dissociation channels for the formation of the fragment ions CH3CO+, CH2CO+, CH3+, C3H3+, and CH3COCH2+ have been established. The G3 results are in fair to excellent agreement with the experimental data.     

Detection of chlorobenzene derivatives using vacuum ultraviolet ionization time-of-flight mass spectrometry.

Vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) has been applied for the detection of chlorobenzene, o-dichlorobenzene, and o-chlorophenol as surrogates for polychlorinated dibenzo-p-dioxine/furans (PCDD/F). The photoionization mass spectra of these compounds appear to be fragmentation free in the ionization processes by the VUV-SPI at 10.2 eV (121.6 nm). Quantum chemical calculations support no fragmentation in the photoionization of chlorobenzene derivatives at around 10 eV. The absolute photoionization cross-sections of chlorobenzene, o-dichlorobenzene, and o-chlorophenol were estimated at 10.2 eV. The photoionization cross-section is an important parameter in the detection of chlorobenzene derivatives by the single-photon ionization technique. The detection limit for chlorobenzene is on the order of tenth parts-per-billion volume (ppbv) in the present experimental setup.     

Single and double photoionizations of methanal (formaldehyde)

Single and double photoionization spectra of formaldehyde have been measured at 40.81 and 48.37 eV photon energy and the spectrum of the doubly charged cation has been interpreted using high-level electronic structure calculations. The adiabatic double-ionization energy is determined as 31.7+/-0.25 eV and the vertical ionization energy is 33 eV. The five lowest excited electronic states are identified and located. The potential-energy surfaces of the accessible states explain the lack of stable H2CO2+ dications and the lack of vibrational structure. The experimental double-ionization spectrum can be decomposed into two distinct contributions, one from direct photoionization and the second from indirect double photoionization by an inner-valence shell Auger effect.     

Photoabsorption and S 2p photoionization of the SF6 molecule: resonances in the excitation energy range of 200-280 eV

Photoabsorption and S 2p photoionization of the SF(6) molecule have been studied experimentally and theoretically in the excitation energy range up to 100 eV above the S 2p ionization potentials. In addition to the well-known 2t(2g) and 4e(g) shape resonances, the spin-orbit-resolved S 2p photoionization cross sections display two weak resonances between 200 and 210 eV, a wide resonance around 217 eV, a Fano-type resonance around 240 eV, and a second wide resonance around 260 eV. Calculations based on time-dependent density functional theory allow us to assign the 217-eV and 260-eV features to the shape resonances in S 2p photoionization. The Fano resonance is caused by the interference between the direct S 2p photoionization channel and the resonant channel that results from the participator decay of the S 2s(-1)6t(1u) excited state. The weak resonances below 210-eV photon energy, not predicted by theory, are tentatively suggested to originate from the coupling between S 2p shake-up photoionization and S 2p single-hole photoionization. The experimental and calculated angular anisotropy parameters for S 2p photoionization are in good agreement.