A photoabsorption, photodissociation and photoelectron spectroscopy study of C6H6 and C6D6

External standard calibration method for high-repetition-rate shock tube kinetic studies with synchrotron-based time-of-flight mass spectrometry

The signal levels observed from mass spectrometers coupled by molecular beam sampling to shock tubes are impacted by dynamic pressures in the spectrometer due to rapid pressure changes in the shock tube. Accounting for the impact of the pressure changes is essential if absolute concentrations of species are to be measured. Obtaining such a correction for spectrometers operated with vacuum ultra violet photoionization has been challenging. We present here a new external calibration method which uses VUV-photoionization of CO2 to develop time-dependent corrections to species concentration/time profiles from which kinetic data can be extracted. The experiments were performed with the ICARE-HRRST (high repetition rate shock tube) at the DESIRS beamline of synchrotron SOLEIL. The calibration experiments were performed at temperatures and pressures behind reflected shock waves of 1376 ± 12 K and 6.6 ± 0.1 bar, respectively. Pyrolytic experiments with two aromatic species, toluene (T5 = 1362 ± 22 K, P5 = 6.6 ± 0.2 bar) and ethylbenzene (T5 = 1327 ± 18 K, P5 = 6.7 ± 0.2 bar), are analyzed to test the method. Time dependent concentrations for molecular and radical species were corrected with the new method. The resulting signals were compared with chemical kinetic simulations using a recent mechanism for pyrolytic formation of polycyclic aromatic hydrocarbons. Excellent agreement was obtained between the experimental data and simulations, without adjustment of the model, demonstrating the validity of the external calibration method.

The Auger spectrum of benzene

We present an ab initio computational study of the Auger electron spectrum of benzene. Auger electron spectroscopy exploits the Auger-Meitner effect, and although it is established as an analytic technique, the theoretical modeling of molecular Auger spectra from first principles remains challenging. Here, we use coupled-cluster theory and equation-of-motion coupled-cluster theory combined with two approaches to describe the decaying nature of core-ionized states: (i) Feshbach-Fano resonance theory and (ii) the method of complex basis functions. The spectra computed with these two approaches are in excellent agreement with each other and also agree well with experimental Auger spectra of benzene. The Auger spectrum of benzene features two well-resolved peaks at Auger electron energies above 260 eV, which correspond to final states with two electrons removed from the 1e_1g and 3e_2g highest occupied molecular orbitals. At lower Auger electron energies, the spectrum is less well resolved, and the peaks comprise multiple final states of the benzene dication. In line with theoretical considerations, singlet decay channels contribute more to the total Auger intensity than the corresponding triplet decay channels.

Efficient implementation of time-dependent auxiliary density functional theory

The random phase approximation of time-dependent auxiliary density functional theory (TDADFT) is rederived from auxiliary density perturbation theory. Our exhaustive validation of TDADFT reveals an upshift of the excitation energies by ∼0.1 eV with respect to standard time-dependent density functional theory. For the computationally efficient implementation of TDADFT, floating point operation optimized three-center electron repulsion integral recurrence relations and their double asymptotic expansions are implemented into the Davidson solver. The computational efficiency of TDADFT is benchmarked with four sets of molecules comprising alkanes, fullerenes, DNA fragments, and zeolites. The results show that TDADFT has a computational scaling between 1.3 and 1.9 with respect to the number of basis functions, which is lower than the scaling of standard time-dependent density functional theory. Due to its computational simplifications, TDADFT is particularly well suited for Born-Oppenheimer molecular dynamics simulations. As illustrative examples, we present the temperature effects on the gas-phase absorption spectra of benzene, naphthalene, and anthracene.

Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy

The unimolecular isomerisation of the prompt propargyl + propargyl "head-to-head" adduct, 1,5- hexadiyne, to fulvene and benzene by the 3,4-dimethylenecyclobut-1-ene (DMCB) intermediate (all C6H6) was studied in the high-pressure limit...

Combined experimental and theoretical study on photoionization cross sections of benzonitrile and o/m/p-cyanotoluene

Photoionization cross sections (PICSs) for the products of the reaction from CN with toluene, including benzonitrile and o/m/p-cyanotoluene, were obtained at photon energies ranging from ionization thresholds to 14 eV by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Theoretical calculations based on the frozen-core Hartree-Fock approximation and Franck-Condon simulations were carried out to cross-verify the measured PICS. The results show that the photoionization cross sections of benzonitrile and cyanotoluene isomers are similar. The generalized charge decomposition analysis was used to investigate the components of the highest occupied molecular orbital (HOMO) and HOMO-1. It was found that the HOMO and HOMO-1 of benzonitrile and cyanotoluene isomers are dominated by the features of the benzene ring, indicating that the substitution of CN and methyl has a minor influence on the PICS of the studied molecules. The reported PICS on benzonitrile and cyanotoluene isomers in the present work could contribute to the near-threshold PIMS experiments and determine the ionization and dissociation rates in interstellar space for these crucial species. The theoretical analysis on characteristics of molecular orbitals provides clues to estimating the PICS of similar substituted aromatic compounds.

Atmospheric-Pressure Pyrolysis Study of Chlorobenzene Using Synchrotron Radiation Photoionization Mass Spectrometry

The pyrolysis of chlorobenzene (C₆H₅Cl) at 760 Torr was studied in the temperature range of 873-1223 K. The pyrolysis products including intermediates and chlorinated aromatics were detected and quantified via synchrotron radiation photoionization mass spectrometry. Furthermore, the photoionization cross sections of chlorobenzene were experimentally measured. On the basis of the experimental results, the decomposition pathways of chlorobenzene were discussed as well as the generation and consumption pathways of the main products. Benzene is the main product of chlorobenzene pyrolysis. Chlorobiphenyl (C₁₂H₉Cl), dichlorobiphenyl (C₁₂H₈Cl₂), and chlorotriphenylene (C₁₈H₁₁Cl) predominated in trace chlorinated aromatic products. Chlorobenzene decomposed initially to form two radicals [chlorophenyl (·C₆H₄Cl) and phenyl (·C₆H₅)] and the important intermediate o-benzyne (o-C₆H₄). The propagation processes of chlorinated aromatics, including polychlorinated naphthalenes and polychlorinated biphenyls, were mainly triggered by chlorobenzene, chlorophenyl, and benzene via the even-numbered-carbon growth mechanism. Besides, the small-molecule products such as acetylene (C₂H₂), 1,3,5-hexatriyne (C₆H₂), and diacetylene (C₄H₂) were formed via the bond cleavage of o-benzyne (o-C₆H₄).

Determination of absolute photoionization cross-sections of some aromatic hydrocarbons

RATIONALE

Aromatic hydrocarbons play an important role in the formation and growth of polycyclic aromatic hydrocarbon (PAH) and soot particles. Measurements of their absolute photoionization cross-sections (PICSs), that benefit the quantitative investigation of mass spectrometry, are still lacking, however.

METHODS

PICSs of some aromatic hydrocarbons were measured with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Nitric oxide and benzene were chosen as standard references for PICS calibration, since their photoionization cross-sections are well documented in the literature. Binary liquid mixtures of the investigated molecules and their specific solvents were used in the measurements.

RESULTS

The investigated aromatics include naphthalene, phenanthrene, 1-methylnaphthalene, indene, 2-/3-/4-methylphenylacetylene, 2-methylindene, diphenylacetylene, 1-/2-ethynylnaphthalene and acenaphthylene. Photo-induced fragments from the molecules were also observed with increasing photon energy.

CONCLUSIONS

Based on our measurements and literature data, PICSs of most aromatic molecules have very similar values beyond their ionization energies. However, molecules that contain the phenylacetylene structure have PICSs higher than other aromatics.

Interplay of Open-Shell Spin-Coupling and Jahn-Teller Distortion in Benzene Radical Cation Probed by X-ray Spectroscopy

We report a theoretical investigation and elucidation of the X-ray absorption spectra of neutral benzene and of the benzene cation. The generation of the cation by multiphoton ultraviolet (UV) ionization and the measurement of the carbon K-edge spectra of both species using a table-top high-harmonic generation source are described in the companion experimental paper [Epshtein, M.; et al. J. Phys. Chem. A http://dx.doi.org/10.1021/acs.jpca.0c08736]. We show that the 1s_C → π transition serves as a sensitive signature of the transient cation formation, as it occurs outside of the spectral window of the parent neutral species. Moreover, the presence of the unpaired (spectator) electron in the π-subshell of the cation and the high symmetry of the system result in significant differences relative to neutral benzene in the spectral features associated with the 1s_C → π* transitions. High-level calculations using equation-of-motion coupled-cluster theory provide the interpretation of the experimental spectra and insight into the electronic structure of benzene and its cation. The prominent split structure of the 1s_C → π* band of the cation is attributed to the interplay between the coupling of the core → π* excitation with the unpaired electron in the π-subshell and the Jahn-Teller distortion. The calculations attribute most of the splitting (∼1-1.2 eV) to the spin coupling, which is visible already at the Franck-Condon structure, and we estimate the additional splitting due to structural relaxation to be around ∼0.1-0.2 eV. These results suggest that X-ray absorption with increased resolution might be able to disentangle electronic and structural aspects of the Jahn-Teller effect in the benzene cation.

Dipole oscillator strength distributions, sum rules, mean excitation energies, and isotropic van der Waals coefficients for benzene, pyridazine, pyrimidine, pyrazine, s-triazine, toluene, hexafluorobenzene, and nitrobenzene

Experimental, theoretical, and additive-model photoabsorption cross sections combined with constraints provided by the Kuhn-Reiche-Thomas sum rule and the high-energy behavior of the dipole oscillator strength density are used to construct dipole oscillator strength distributions for benzene, pyridazine (1,2-diazine), pyrimidine (1,3-diazine), pyrazine (1,4-diazine), s-triazine (1,3,5-triazine), toluene (methylbenzene), hexafluorobenzene, and nitrobenzene. The distributions are used to predict dipole sum rules S(k) for -6 ≤ k ≤ 2, mean excitation energies I(k) for -2 ≤ k ≤ 2, and isotropic van der Waals C₆ coefficients. A popular combination rule for estimating C₆ coefficients for unlike interactions from the C₆ coefficients of the like interactions is found to be accurate to better than 1% for 606 of 628 cases (96.4%) in the test set.

The longer timescale excited state dynamics of isolated benzene

The excited state photodynamics of isolated benzene have been studied in the nanosecond range by two-step photoionization through various vibrations of the lowest singlet state, with imaging photoelectron spectroscopy detection. Photoelectron spectra are measured as a function of pump-probe delay time, and their time evolution is successfully compared to a biexponential decay function without regard to a particular kinetic model. The only reasonable kinetic model with only two exponentials is the one that involves an intersystem crossing from S₁ to T₁, although that model has previously been called into question by high-resolution studies that failed to find any singlet-triplet perturbations in Zeeman studies of the S₁ spectrum. That contradiction remains unresolved.

Selective identification of cyclopentaring-fused PAHs and side-substituted PAHs in a low pressure premixed sooting flame by photoelectron photoion coincidence spectroscopy

Selective on-line identification of polycyclic aromatic hydrocarbons (PAHs) formed in a low-pressure methane sooting flame, carried out using the double imaging Photoelectron Photoion Coincidence Spectroscopy method (i²PEPICO).

Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan's surface ices and on airless bodies

Titan's equatorial dunes represent the most monumental surface structures in our Solar System, but the chemical composition of their dark organics remains a fundamental, unsolved enigma, with solid acetylene detected near the dunes implicated as a key feedstock. Here, we reveal in laboratory simulation experiments that aromatics such as benzene, naphthalene, and phenanthrene-prospective building blocks of the organic dune material-can be efficiently synthesized via galactic cosmic ray exposure of low-temperature acetylene ices on Titan's surface, hence challenging conventional wisdom that aromatic hydrocarbons are formed solely in Titan's atmosphere. These processes are also of critical importance in unraveling the origin and chemical composition of the dark surfaces of airless bodies in the outer Solar System, where hydrocarbon precipitation from the atmosphere cannot occur. This finding notably advances our understanding of the distribution of carbon throughout our Solar System such as on Kuiper belt objects like Makemake.

Few-femtosecond passage of conical intersections in the benzene cation

Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system-the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.

Photoionization Efficiencies of Five Polycyclic Aromatic Hydrocarbons

We have measured photoionization-efficiency curves for pyrene, fluoranthene, chrysene, perylene, and coronene in the photon energy range of 7.5-10.2 eV and derived their photoionization cross-section curves in this energy range. All measurements were performed using tunable vacuum ultraviolet (VUV) radiation generated at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The VUV radiation was used for photoionization, and detection was performed using a time-of-flight mass spectrometer. We measured the photoionization efficiency of 2,5-dimethylfuran simultaneously with those of pyrene, fluoranthene, chrysene, perylene, and coronene to obtain references of the photon flux during each measurement from the known photoionization cross-section curve of 2,5-dimethylfuran.

XUV-induced reactions in benzene on sub-10 fs timescale: nonadiabatic relaxation and proton migration

Short XUV pulses produce excited cationic states of benzene. Their dynamics are probed by few-cycle VIS/NIR pulses. Very fast τ ≈ 20 fs nonadiabatic processes dominate the relaxation. In the CH₃⁺ fragmentation channel a non-trivial transient behaviour is observed.

On the absolute photoionization cross section and dissociative photoionization of cyclopropenylidene

We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C₃H₂, and the heat of formation of the C₃H radical and ion derived by the dissociative ionization of the carbene.

The design of double electrostatic-lens optics for resonance enhanced multiphoton ionization and photoelectron imaging experiments

Compared to single ion/electron-optics for velocity-map imaging, a double-focusing lens assembly designed not only allows for mapping velocity imaging of photoelectrons but also allows for investigating the vibrational structure of the intermediate states of neutral species in resonance enhanced multiphoton ionization (REMPI) spectra. In this presentation, in order to record REMPI and photoelectron spectra separately, we have constructed a compact photoelectron velocity-map imaging (VMI) apparatus combined with an opposite linear Wiley-Mclaren time-of-flight mass spectrometer (TOFMS). A mass resolution (m/Δm) of ∼1300 for TOFMS and electron energy resolution (ΔE/E) of 2.4% for VMI have been achieved upon three-photon ionization of Xe atom at 258.00 nm laser wavelength. As a benchmark, in combination of one-color (1 + 1) REMPI and photoelectron imaging of benzene via 6(1) and 6(1)1(1) vibronic levels in the S1 state, the vibrational structures of the cation and photoelectron angular anisotropy are unraveled. In addition, two-color (1 + 1') REMPI and photoelectron imaging of aniline was used to complete the accurate measurement of ionization potential (62,271 ± 3 cm(-1)). The results suggest that the apparatus is a powerful tool for studying photoionization dynamics in the photoelectron imaging using vibrational-state selected excitation to the intermediate states of neutrals based on REMPI technique.

Absolute photoionization cross-section of the propargyl radical

Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, σ(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of σ(propargyl)(ion)(10.213 eV)=(26.1±4.2) Mb and σ(propargyl)(ion)(10.413 eV)=(23.4±3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of σ(propargyl)(ion)(10.213 eV)=(23.6±3.6) Mb and σ(propargyl)(ion)(10.413 eV)=(25.1±3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.

Determination of absolute photoionization cross-sections of alkanes and cyclo-alkanes

Absolute photoionization and dissociative photoionization cross-sections of eleven n-alkanes (n-pentane, n-hexane, n-heptane, n-nonane, n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane and n-hexadecane), three cyclo-alkanes (cyclopentane, methylcyclohexane and trans-decahydronaphthalene) and iso-octane were measured for photon energies from the ionization thresholds to 11.5 eV. The measurements were performed with the binary-liquid-mixture method utilizing the photoionization cross-sections of benzene as a calibration standard. The ionization energies of n-alkanes and cyclo-alkanes were also calculated at the B3P86/6-31 + +G(d,p) level and by the G3B3 method.

Determination of absolute photoionization cross-sections of aromatics and aromatic derivatives

The absolute photoionization cross-sections of aromatics and aromatic derivatives including toluene, ethylbenzene, n-propylbenzene, o-xylene, m-xylene, p-xylene, 1,3,5-trimethylbenzene, styrene, phenylacetylene, indene, indane, 1-methylnaphthalene, benzyl alcohol and benzaldehyde were measured at the photon energy range from ionization thresholds to 11.7 eV. The experiments were performed by tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Benzene was chosen as a calibration standard, since its photoionization cross-section is well known. Binary liquid mixtures of the investigated molecules and benzene were used in the measurements. Photo-induced fragments from the molecules were also observed, and their photoionization cross-sections are also presented.

Determination of the ionization potentials of security-relevant substances with single photon ionization mass spectrometry using synchrotron radiation

Several ionization potentials (IPs) of security relevant substances were determined with single photon ionization time of flight mass spectrometry (SPI-TOFMS) using monochromatized synchrotron radiation from the "Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung" (BESSY). In detail, the IPs of nine explosives and related compounds, seven narcotics and narcotics precursors, and one chemical warfare agent (CWA) precursor were determined, whereas six IPs already known from the literature were verified correctly. From seven other substances, including one CWA precursor, the IP could not be determined as the molecule ion peak could not be detected. For these substances the appearance energy (AE) of a main fragment was determined. The analyzed security-relevant substances showed IPs significantly below the IPs of common matrix compounds such as nitrogen and oxygen. Therefore, it is possible to find photon energies in between, whereby the molecules of interest can be detected with SPI in very low concentrations due to the shielding of the matrix. All determined IPs except the one of the explosive EGDN were below 10.5 eV. Hence, laser-generated 118 nm photons can be applied for detecting almost all security-relevant substances by, e.g., SPI-TOFMS.

Electronic structure of the benzene dimer cation

The benzene and benzene dimer cations are studied using the equation-of-motion coupled-cluster model with single and double substitutions for ionized systems. The ten lowest electronic states of the dimer at t-shaped, sandwich, and displaced sandwich configurations are described and cataloged based on the character of the constituent fragment molecular orbitals. The character of the states, bonding patterns, and important features of the electronic spectrum are explained using qualitative dimer molecular orbital linear combination of fragment molecular orbital framework. Relaxed ground state geometries are obtained for all isomers. Calculations reveal that the lowest energy structure of the cation has a displaced sandwich structure and a binding energy of 20 kcal/mol, while the t-shaped isomer is 6 kcal/mol higher. The calculated electronic spectra agree well with experimental gas phase action spectra and femtosecond transient absorption in liquid benzene. Both sandwich and t-shaped structures feature intense charge resonance bands, whose location is very sensitive to the interfragment distance. Change in the electronic state ordering was observed between sigma and piu states, which correlate to the B and C bands of the monomer, suggesting a reassignment of the local excitation peaks in the gas phase experimental spectrum.

Vacuum ultraviolet photoionization mass spectra and cross-sections for volatile organic compounds at 10.5 eV

Vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) has been applied to the detection of volatile organic compounds (VOCs), including aromatic, chlorinated, and oxygenated compounds. Photoionization mass spectra of 23 VOCs were measured using SPI-TOFMS at 10.5 eV (118 nm). The limits of detection of VOCs using SPI-TOFMS at 10.5 eV were estimated to be a few ppbv. The mass spectra of 20 VOCs exhibit only the parent ion and its isotopes' signals. The ionization processes of the VOCs were discussed on the basis of the reaction enthalpies predicted by the quantum chemical calculations. Absolute photoionization cross-sections for 23 VOCs, including 12 newly measured VOCs, at 10.5 eV were determined in comparison to the reported absolute photoionization cross-section of NO.