ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotoionization study of the heat of formation of HCS+James J. Butler and Tomas BaerCite this: J. Am. Chem. Soc. 1982, 104, 19, 5016–5018Publication Date (Print):September 1, 1982Publication History Published online1 May 2002Published inissue 1 September 1982https://doi.org/10.1021/ja00383a002Request reuse permissionsArticle Views54Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (330 KB) Get e-Alertsclose Get e-Alerts
The breakdown diagrams and photoionization efficiency curves of isomeric chlorobutane ions were obtained by energy-selected molecular ions with the photoelectron–photoion coincidence (PEPICO) technique. The loss of HCl from 1- and 2-chlorobutane molecular ions was found to be fast, and accurate dissociation onsets were determined from the crossover energy in the breakdown diagrams and appropriately corrected for the precursor thermal energy. In spit of the low activation energy, the isochlorobutane molecular ion was found to dissociate slowly near the dissociation limit, the time-of-flight distribution having been analyzed in terms of two exponential decay rates. The dissociation reaction involved a large kinetic energy release that pointed to a 1,2-elimination leading to a stable C4H ion. AB initio calculations indicated that the reaction path involved an H-atom transfer through a barrier, which is a favorable case for a tunneling model to explain the slow decomposition rate. The kinetic energy release of chlorine radical loss from tert-chlorobutane was determined as a function of the ion internal energy. At low energies the kinetic energy release was nearly statistical but the dissociation from the excited electronic states resulted in a large and non-statistical kinetic energy release.
Single aerosol particles of ethylene glycol and oleic acid are vaporized on a heater at temperatures between 500 and 700 K, and the resulting vapor plume is ionized by a 10.5-eV vacuum ultraviolet (VUV) laser. The mass spectra are compared to those obtained by CO2 laser vaporization followed by VUV laser ionization. The relative intensities of the parent and fragment ion peaks are remarkably similar for the two modes of vaporization. A Maxwell−Boltzmann distribution of speeds accurately describes the dependence of the signal as a function of the VUV laser pulse timing. The signal levels obtained with this design are sufficient to obtain good-quality mass spectra.
Infrared laser evaporation of single aerosol particles in a vacuum followed by vacuum ultraviolet (VUV) laser ionization and time-of-flight mass spectroscopy of the resulting vapor provides a depth profile of the particle's composition. Analyzing glycerol particles coated with 60-150-nm coatings of oleic acid using either a CO2 laser or a tunable optical parametric oscillator as an evaporation laser results in mass spectra that depend on the IR laser power. Low infrared laser powers incompletely vaporize particles and preferentially probe the composition of the surface layers of the particle, but high laser powers evaporate the entire particle and produce spectra representative of the particle's total composition. In the limit of low laser power, the fraction of oleic acid in the mass spectra is as much as 50 times greater than the fraction of oleic acid in the particle, providing a surface-layer-specific characterization. The OPO laser provides even more surface specificity, producing an [oleic acid]/[glycerol] ratio as much as four times larger (for a 60-nm coating) than that obtained using the CO2 laser. The infrared laser power required to sample the core of the particle increases with the thickness of the coating and is sensitive to changes in the coating thickness on the order of 10 nm. In contrast to these intuitively appealing results, high CO2 laser powers (approximately 90 mJ/pulse) produce mass spectra that, at short delays between the CO2 and VUV lasers, show enrichment of the core material rather than the coating. Likewise, tuning the OPO to frequencies that are resonant with the core material but transparent to the coating also results in selective detection of the core. The results suggest that a shattering mechanism dominates the vaporization dynamics in these situations.
Although laser vaporization of aerosol particles plays an important role in aerosol mass spectrometry, relatively little is known about disposal of excess energy in the degrees of freedom of the gas-phase species. A two-laser scheme, in which an infrared laser vaporizes aerosol particles prior to ionization of the vapor plume by a vacuum-ultraviolet laser, permits the determination of the internal energy of the neutral molecules created in laser vaporization. In this work, the fragmentation patterns of vacuum UV (VUV) photoionization mass spectra of ethylene glycol, in conjunction with photoelectron-photoion coincidence (PEPICO) measurements, determines the internal energy of gas-phase molecules created in the CO2 laser vaporization of neat ethylene glycol particles and ethanol particles mixed with trace ethylene glycol. The internal energy ranges from 1300 to 10250 cm-1 for CO2 laser powers between 25 and 112 mJ/pulse. For neat ethylene glycol, the rate with which the internal and kinetic energy grows with laser power increases sharply above 65 mJ/pulse, consistent with a change in vaporization mechanism from thermal to explosive. Monitoring the total ion signal as a function of the delay between the CO2 and VUV lasers provides an estimate of the relative kinetic energy of the vaporized molecules. At high laser fluences, the estimated translational energy is greater than the corresponding internal energy, indicative of vibrational cooling in the vapor plume. Ethanol particles containing 1.0% ethylene glycol produce similar results, with the transition in heating rate occurring at a lower temperature. The simplicity of the fragmentation pattern in these spectra and the broad range of temperatures that can be measured in this fashion make ethylene glycol an excellent "chemical thermometer" for reactions initiated by the laser heating of aerosol particles.
Most laser-based aerosol mass spectrometers rely on a single ultraviolet laser to both ablate and ionize the aerosol particle. This technique produces complex and fragmented mass spectra, especially for organic compounds. The approach presented here achieves a more robust and quantitative analysis using a CO2 laser to evaporate the aerosol particle and a vacuum ultraviolet laser to ionize the vapor plume. Vacuum ultraviolet laser ionization produces little fragmentation in the mass spectra, making the identification of an aerosol particle's constituents more straightforward. An analysis of simple, three-component mixtures of aniline, benzyl alcohol, and m-nitrotoluene shows that the technique also provides a quantitative analysis for all the components of the mixture. Furthermore, the detection of predominantly parent ion signal from anthracene particles demonstrates the utility of the technique in the analysis of lower vapor pressure, solid-phase aerosols. Finally, we discuss the potential and limitations of this technique in analyzing organic atmospheric aerosols.
A synchrotron radiation based aerosol time-of-flight mass spectrometer using tunable vacuum-ultraviolet (VUV) light is described for real-time analysis of organic compounds in ultrafine and large aerosol particles. Particles are sampled from atmospheric pressure and are focused through an aerodynamic lens assembly into the mass spectrometer. As the particles enter the source region, they impinge on a cartridge heater and are vaporized. The particle vapor expands back into the source region and is softly ionized with tunable, quasicontinuous VUV light generated with synchrotron radiation. The radiation can be tuned to an energy close to the ionization energy of the sample molecules, thus minimizing the complications resulting from ion fragmentation. Photoionization efficiency scans (photon scans) can be readily collected, which permit measurement of the molecule's ionization energy and fragmentation onsets. Four high molecular weight, low vapor pressure organic compounds of importance in atmospheric aerosols are analyzed and their ionization energies measured with uncertainties of +/-60 meV. These are oleic acid (8.68 eV), linoleic acid (8.52 eV), linolenic acid (8.49 eV), and cholesterol (8.69 eV).
The vapor deposition of oleic acid onto silica surfaces at 25% relative humidity and temperatures ranging from 60 to 80 °C is found to proceed in three stages: (I) rapid formation of monolayer-high islands of approximately 100 nm diameter on timescales of a few minutes; (II) relatively little growth over timescales of tens to hundreds of minutes; and (III) a linear increase in apparent thickness as a function of time, characterized by the formation of multilayer islands on time scales of thousands of minutes. The rate of growth in region III is faster at higher temperatures. This growth process is analyzed in the context of the transition from two-dimensional to three-dimensional island growth at submonolayer coverage observed for metal vapor deposition on oxide surfaces. At a relative humidity of 94%, with several layers of water molecules present on the silica surface, the oleic acid wets the surface rather than forming discrete islands.
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