Single and Double Photoionization and Photodissociation of Toluene by Soft X-rays in Circumstellar Environment

The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 $\mu$m, observed in infrared emission spectra of several objects, are attributed C-H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 $\mu$m is more intense than the 3.4 $\mu$m. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge with time-of-flight mass spectrometry. Partial ion yields of a large number of ionic fragments were extracted from single and 2D-spectra, where electron-ion coincidences have revealed the doubly charged parent-molecule and several doubly charged fragments containing seven carbon atoms with considerable abundance. \textit{Ab initio} calculations based on density functional theory were performed to elucidate the chemical structure of these stable dicationic species. The survival of the dications subjected to hard inner shell ionization suggests that they could be observed in the interstellar medium, especially in regions where PAHs are detected. The ionization and destruction of toluene induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB star. In this context, a minimum photodissociation radius and the half-life of toluene subjected to the incidence of the soft X-ray flux emitted from a companion white dwarf star were determined.Comment: 11 pages, 4 figures, accept for publication in Ap

Ion-Molecule Reactions in Unsaturated Hydrocarbons: Allene, Propyne, Diacetylene, and Vinylacetylene

Ion-molecule reactions in allene, propyne, diacetylene, and vinylacetylene (1-buten-3-yne) have been studied at near-thermal energies by the technique of ion cyclotron resonance mass spectrometry. Rate coefficients and branching ratios are reported for the reactions of C_3H^+_n (n = 1-4) with allene and propyne and for the reactions of C_4H^+_n (n = 0-5) with diacetylene and vinylacetylene. Branching ratios are also given for the reactions of C_4H^+_n, C_5H_n, and C_6H^+_n with propyne and for reactions of C_6H^+_n with diacetylene and vinylacetylene. More than 90% of the reactive channels lead to product ions having a larger carbon skeleton than the reactant ion. Evidence for ions with the same m/e ratio having differing reactivities was obtained for C_3H^+_3, C_6H^+_7, and C_7H^+_7. Ion reaction sequences in allene and propyne were followed at higher pressures (l0^(-4) torr) to investigate secondary, tertiary, and higher order processes

Employing Soft X-rays in Experimental Astrochemistry

The presence of soft x-rays is very important for the chemical evolution of interstellar medium and other astrophysical environments close to young and bright stars. Soft X-rays can penetrate deep in molecular clouds and protostellar disks and trigger chemistry in regions in which UV stellar photons do not reach. The effects of soft X-rays in astrophysical ices are also remarkable because they release secondary electrons in and on the surface of the ices, which trigger a new set or chemical reactions. In this chapter we will discuss firstly about the origin and relevance of soft X-rays in astrophysics. Next we will move to the effect of ionizing radiation in organic molecules present in astrophysical environment. We will discuss the use soft X-rays in astrochemistry laboratory studies at both gas- and solid-phase (ice). We will make a review covering our publications in this field, in particular, about the experiments employing time-of-flight spectroscopy (TOF-MS), Fourier transform infrared (FTIR) spectroscopy and photon stimulated ion desorption (PSID-TOF-MS). This study help us to understand the chemical evolution several astrophysical regions and also put constrains in the researches related with the life's origin.Comment: 34 pages, 25 figures and 2 tables. This a book chapter of "X-Ray Spectroscopy" ISBN:978-953-307-967-7; InTech Open Access Publisher (http://www.intechweb.org/). Edited by Shatendra K. Sharma. Publication date: December 201

Photoionization Mass Spectrometry and Photoelectron-Photoion Coincidence (PEPICO) Spectroscopy Studies of Select Biofuel Molecules

This thesis is the culmination of numerous experiments, performed on two different continents, investigating the spectroscopic and thermodynamic properties of several biofuels and fuel additives. It will start with an introduction about the motivation behind these experiments. The second chapter will outline the experimental details of the apparatus at the Advanced Light Source (ALS) in Berkeley, CA, followed by the Swiss Light Source (SLS) in Villigen, Switzerland. Third, the theoretical concepts and data analysis methods will be discussed in detail. Chapter 4 will be the start of the newly obtained data. It presents some photoionization mass spectrometry studies on γ-valerolactone (GVL). The photoionization cross section of GVL and some suspected combustion products are measured. In addition, several dissociative ions are determined. Theoretical calculations are used to support all alignments. This data is vital to product identification and branching fraction calculations in time- and energy-resolved photoionization experiments. The fifth chapter will investigate the chlorine-initiated reactions of methyl butyrate (MB) and ethyl butyrate (EB) in the presence of oxygen. Ethyl crotonate is found to be a major product in the EB reaction. Formaldehyde and acetaldehyde are both formed in the EB and MB reactions. Both products are suspected to be produced by the RO2-HO2 radical-radical reaction and ­ab initio calculations are performed to analyze the reaction mechanisms. Finally, chapter six will discuss the photoelectron-photoion coincidence (PEPICO) spectroscopy of two biofuel additives (GVL and mesitylene). This technique is used to measure various thermodynamic quantities, such as heats of formation, entropies of activation, and bond strengths. In addition, dissociation rates will also be obtained with the use of two statistical models, Rice-Ramsperger-Kassel-Marcus (RRKM) theory and the Simplified Statistical Adiabatic Channel Model (SSACM)

The Investigation and Characterization of the Reaction of 2-Methylfuran and 2-Methyl-3-Buten-2-Ol with O(3P) and the Photodissociation of Xylyl Bromide Isomers

This thesis has studied the oxidation behavior of different biofuels or additives, 2-methyl-3-buten-2-ol and 2-methylfuran, in combustion experiments at the Chemical Dynamics Beamline held at the Advanced Light Source of the Lawrence Berkley National Laboratory. The oxidation of these fuels were initiated through O(3P) and the combustion experiments were analyzed using a multiplexed chemical kinetics photoionization mass spectrometer with tunable synchrotron radiation. Products of the different reactions were identified using kinetic profiles and further characterized using the photoionization spectra. The amount of each species was calculated using branching fractions. Additionally, the unimolecular dissociation of the xylyl bromide isomers was studied using imaging and double imaging photoelectron photoion coincidence spectroscopy to obtain accurate thermochemical data. These experiments were conducted using the Swiss Light Source held at the Paul Scherrer Institute in Villigen, Switzerland. The importance of biofuels, fuel additives, and aromatic hydrocarbons is discussed in detail in Chapter 1 of this thesis. Further, the specific experimental components of the beamlines used at the ALS and the SLS are thoroughly explained in Chapter 2. The theory behind the experiments and the computational methods to analyze the substantial experimental findings from both experimental apparatuses are explained in Chapter 3. The two combustion systems, 2-methyl-3-buten-2-ol and 2-methylfuran with O(3P) are presented in Chapter 4 and 5. Lastly, the photodissociation dynamics of the xylyl bromide isomers is presented in Chapter 6, where a specific program, miniPEPICO, is used to determine the accurate appearance energy of the daughter ion and to calculate thermochemical data

Imaging Molecular Structure through Femtosecond Photoelectron Diffraction on Aligned and Oriented Gas-Phase Molecules

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray Free-Electron Laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and dissociating, laseraligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.Comment: 24 pages, 10 figures, Faraday Discussions 17

The Investigation of the Low Temperature Combustion of Mesitylene and Tert-Amyl Methyl Ether by Synchrotron Photoionization Mass Spectrometry

This thesis describes the combustion experiments performed at the Chemical Dynamics Beamline of the Advanced Light Source (ALS) located at the Lawrence Berkeley National Laboratories (LBNL). The need for renewable fuel sources, the need for the study of their combustion in the contexts of homogenous charge combustion ignition (HCCI) engines can be found in Chapter 1. The components of the experimental set-up used throughout this thesis—time-of-flight mass spectrometer, Excimer laser, vacuum pumps and other components—and the components of ALS responsible in synchrotron radiation generation and processing—the linear accelerator, the booster ring, insertion devices, gas filters and monochromator are explained in Chapter 2. The data analysis method and the computational method used in the data analysis are expounded in Chapter 3. There are two combustion systems being investigated in this thesis. Mesitylene, an aromatic fuel additive, is proposed to be jet fuel, and its combustion is described in Chapter 4. Semi-biorenewable tert-amyl methyl ether (TAME) is studied in Chapter 5. Thermodynamic calculations, proposed mechanism, product identification and branching fractions are included in the analysis of the combustion of all these molecules

Molecular beam photoionization studies of molecules and their van der Waals clusters

A new photoionization apparatus incorporating a supersonic^molecular beam, a quadrupole mass spectrometer and a 3-meter^vacuum ultraviolet monochromator has been used to study the^nitric oxide, carbon disulfide, carbonyl sulfide and acetylene^molecules along with their van der Waals clusters. The improved^ion intensity and higher resolution of this apparatus over earlier^similar instruments has allowed us to obtain new and more^accurate information regarding these simple molecules and,^moreover, has demonstrated this technique to be an excellent^method of studying clusters. Using a wavelength resolution of^0.14 (ANGSTROM) (FWHM), photoionization efficiency curves were obtained^for NO, CS(,2), OCS, and C(,2)H(,2) in the respective threshold regions and^ionization energy determinations accurate to within 3 meV were^made. The autoionization structures resolved have led to new^interpretations and assignments to Rydberg levels. Photoionization^efficiency data for the clusters were obtained under lower resolution,^typically 1.4 (ANGSTROM) (FWHM) and led to binding energy determinations of^the cluster ions. Other thermochemical values were deducted from^fragment appearance energies and sharp rises in ionization^efficiencies. From the (CS(,2))(,2) fragment curves, relative reaction^probabilities for the formation of the various product channels as a^function of the Rydberg level n were determined for CS(,2)(\u27*)(V,n) (.) CS(,2)^and the branching ratios were measured for the reaction CS(,2)(\u27*)(V,n) +^CS(,2). The shifts observed in the autoionization peaks of the triatomic^molecules CS(,2) and OCS due to clustering was investigated and a^model is proposed which accounts for these characteristics.^Ionization of the acetylene dimer and trimer resulted in clusters^which could rearrange to form ions of a more stable configuration.Comparisons made between the fragmentation patterns of (C(,2)H(,2))(,3)and the C(,6)H(,6) isomers benzene, 2,4-hexadiyne and 1,3-hexadiynealong with the fragment appearance energies strongly support theconclusion that the acetylene trimer ions rearrange to some commonprecursors as other C(,6)H(,6)(\u27+) isomers prior to dissociation. Energydependence measurements of the acetylene dimer fragmentation;processs yielding C(,4)H(,3)(\u27+) and C(,4)H(,2)(\u27+) were found to be consistent with predictions based on the quasi-equilibrium theory;*USDOE Report IS-T-1009. This work was performed under Contract W-7405-Eng-82 with the U.S. Department of Energy

ELEPHANT AND ANCHORS ‒ PHOTOELECTRON PHOTOION COINCIDENCE SPECTROSCOPY OF SMALL OXYGENATED MOLECULES

The dissociative photoionization reactions of two small, oxygenated organics, namely 1,3-dioxolane and methyl vinyl ketone, were studied by photoelectron photoion coincidence (PEPICO) spectroscopy. Experiments involving 1,3-dioxolane were carried out in the photon energy range of 9.5‒13.5 eV. The statistical thermodynamics model shows that a total of six dissociation channels are involved in the formation of three fragment ions, namely C3H5O2+ (m/z 73), C2H5O+ (m/z 45) and C2H4O+ (m/z 44), with two channels contributing to the formation of each. By comparing the results of ab initio quantum chemical calculations to the experimentally derived appearance energies of the fragment ions, the most likely mechanisms for these unimolecular dissociation reactions are proposed, including a description of the relevant parts of the potential energy surface.In the case of methyl vinyl ketone, an important atmospheric intermediate in the oxidation of isoprene, between 9.5‒13.8 eV four main fragment ions were detected at m/z 55, 43, 42, and 27 aside from the parent ion at m/z 70. The m/z 55 fragment ion (C2H3CO+) is formed from ionized MVK by direct methyl loss, while breaking the C–C bond on the other side of the carbonyl group results in the acetyl cation (CH3CO+, m/z 43) and the vinyl radical. The m/z 42 fragment ion is formed via a CO loss from the molecular ion after a methyl shift. The lightest fragment ion, the vinyl cation (C2H3+ at m/z 27), is produced in two different reactions: acetyl radical loss from the molecular ion and CO-loss from C2H3CO+. Their contributions to the m/z 27 signal are quantified based on the acetyl and vinyl fragment thermochemical anchors and quantum-chemical calculations. Based on the experimentally derived appearance energy of the m/z 43 fragment ion, a new, experimentally derived heat of formation is proposed for gaseous methyl vinyl ketone (ΔfH0K = −94.2 ± 4.8 kJ mol−1; ΔfH298K = −110.4 ± 4.8 kJ mol−1), together with cationic heats of formation and bond dissociation energies