A Mass Spectrometry-Machine Learning Approach for Detecting Volatile Organic Compound Emissions for Early Fire Detection.

Mass spectrometry in parallel with real-time machine learning techniques were paired in a novel application to detect and identify chemically specific, early indicators of fires and near-fire events involving a set of selected materials: Mylar, Teflon, and poly(methyl methacrylate) (PMMA). The volatile organic compounds emitted during the thermal decomposition of each of the three materials were characterized using a quadrupole mass spectrometer which scanned the 1-200 m/z range. CO2, CH3CHO, and C6H6 were the main volatiles detected during Mylar thermal decomposition, while Teflon's thermal decomposition yielded CO2 and a set of fluorocarbon compounds including CF4, C2F4, C2F6, C3F6, CF2O, and CF3O. PMMA produced CO2 and methyl methacrylate (MMA, C5H8O2). The mass spectral peak patterns observed during the thermal decomposition of each material were unique to that material and were therefore useful as chemical signatures. It was also observed that the chemical signatures remained consistent and detectable when multiple materials were heated together. Mass spectra data sets containing the chemical signatures for each material and mixtures were collected and analyzed using a random forest panel machine learning classification. The classification was tested and demonstrated 100% accuracy for single material spectra and an average of 92.3% accuracy for mixed material spectra. This investigation presents a novel technique for the real-time, chemically specific detection of fire related VOCs through mass spectrometry which shows promise as a more rapid and accurate method for detecting fires or near-fire events.

Efficient Intermolecular Energy Exchange and Soft Ionization of Water at Nanoplatelet Interfaces.

X-ray, energetic photon, and electron irradiation can ionize and electronically excite target atoms and molecules. These excitations undergo complicated relaxation and energy-transfer processes that ultimately determine the manifold system responses to the deposited excess energy. In weakly bound gas- and solution-phase samples, intermolecular Coulomb decay (ICD) and electron-transfer-mediated decay (ETMD) can occur with neighboring atoms or molecules, leading to efficient transfer of the excess energy to the surroundings. In ionic solids such as metal oxides, intra- and interatomic Auger decay produces localized final states that lead to lattice damage and typically the removal of cations from the substrate. The relative importance of Auger-stimulated damage (ASD) versus ICD and ETMD in microsolvated nanoparticle interfaces is not known. Though ASD is generally expected, essentially no lattice damage resulting from the ionization and electronic excitation of microsolvated boehmite (AlOOH) nanoplatelets has been detected. Rather efficient energy transfer and soft ionization of interfacial water molecules has been observed. This is likely a general phenomenon at gas-oxyhydroxide nanoparticle interfaces where the density of states of the ionized chemisorbed species significantly overlaps with the core hole states of the solid.

Untangling the methane chemistry in interstellar and solar system ices toward ionizing radiation: a combined infrared and reflectron time-of-flight analysis.

Pure methane (CH4/CD4) ices were exposed to three ionizing radiation sources at 5.5 K under ultrahigh vacuum conditions to compare the complex hydrocarbon spectrum produced across several interstellar environments. These irradiation sources consisted of energetic electrons to simulate secondary electrons formed in the track of galactic cosmic rays (GCRs), Lyman α (10.2 eV; 121.6 nm) photons simulated the internal VUV field in a dense cloud, and broadband (112.7-169.8 nm; 11.0-7.3 eV) photons which mimic the interstellar ultra-violet field. The in situ chemical evolution of the ices was monitored via Fourier transform infrared spectroscopy (FTIR) and during heating via mass spectrometry utilizing a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer with a photoionization source (PI-ReTOF-MS). The FTIR analysis detected six small hydrocarbon products from the three different irradiation sources: propane [C3H8(C3D8)], ethane [C2H6(C2D6)], the ethyl radical [C2H5(C2D5)], ethylene [C2H4(C2D4)], acetylene [C2H2(C2D2)], and the methyl radical [CH3(CD3)]. The sensitive PI-ReTOF-MS analysis identified a complex array of products with different products being detected between experiments with general formulae: CnH2n+2 (n = 4-8), CnH2n (n = 3-9), CnH2n-2 (n = 3-9), CnH2n-4 (n = 4-9), and CnH2n-6 (n = 6-7) from electron irradiation and CnH2n+2 (n = 4-8), CnH2n (n = 3-10), CnH2n-2 (n = 3-11), CnH2n-4 (n = 4-11), CnH2n-6 (n = 5-11), and CnH2n-8 (n = 6-11) from broadband photolysis and Lyman α photolysis. These experiments show that even the simplest hydrocarbon can produce important complex hydrocarbons such as C3H4 and C4H6 isomers. Distinct isomers from these groups have been shown to be important reactants in the synthesis of polycyclic aromatic hydrocarbons like indene (C9H8) and naphthalene (C10H8) under interstellar conditions.

Detection of the Elusive Triazane Molecule (N3 H5 ) in the Gas Phase.

We report the detection of triazane (N3 H5 ) in the gas phase. Triazane is a higher order nitrogen hydride of ammonia (NH3 ) and hydrazine (N2 H4 ) of fundamental importance for the understanding of the stability of single-bonded chains of nitrogen atoms and a potential key intermediate in hydrogen-nitrogen chemistry. The experimental results along with electronic-structure calculations reveal that triazane presents a stable molecule with a nitrogen-nitrogen bond length that is a few picometers shorter than that of hydrazine and has a lifetime exceeding 6±2 µs at a sublimation temperature of 170 K. Triazane was synthesized through irradiation of ammonia ice with energetic electrons and was detected in the gas phase upon sublimation of the ice through soft vacuum ultraviolet (VUV) photoionization coupled with a reflectron-time-of-flight mass spectrometer. Isotopic substitution experiments exploiting [D3 ]-ammonia ice confirmed the identification through the detection of its fully deuterated counterpart [D5 ]-triazane (N3 D5 ).

Lyman α photolysis of solid nitromethane (CH3NO2) and D3-nitromethane (CD3NO2)--untangling the reaction mechanisms involved in the decomposition of model energetic materials.

Solid nitromethane (CH3NO2) along with its isotopically labelled counterpart D3-nitromethane (CD3NO2) ices were exposed to Lyman α photons to investigate the mechanism involved in the decomposition of energetic materials in the condensed phase. The chemical processes in the ices were monitored online and in situ via infrared spectroscopy complimented by temperature programmed desorption studies utilizing highly sensitive reflectron time-of-flight mass spectrometry coupled with pulsed photoionization (ReTOF-PI) at 10.49 eV. The infrared data revealed the formation of cis-methylnitrite (CH3ONO), formaldehyde (H2CO), water (H2O), carbon monoxide (CO), and carbon dioxide (CO2). Upon sublimation of the irradiated samples, three classes of higher molecular weight products, which are uniquely formed in the condensed phase, were identified via ReTOF-PI: (i) nitroso compounds [nitrosomethane (CH3NO), nitrosoethane (C2H5NO), nitrosopropane (C3H7NO)], (ii) nitrite compounds [methylnitrite (CH3ONO), ethylnitrite (C2H5ONO), propylnitrite (C3H7ONO)], and (iii) higher molecular weight molecules [CH3NONOCH3, CH3NONO2CH3, CH3OCH2NO2, ONCH2CH2NO2]. The mechanistical information obtained in the present study suggest that the decomposition of nitromethane in the condensed phase is more complex compared to the gas phase under collision-free conditions opening up not only hitherto unobserved decomposition pathways of nitromethane (hydrogen atom loss, oxygen atom loss, retro carbene insertion), but also the blocking of several initial decomposition steps due to the 'matrix cage effect'.

Synthesis of prebiotic glycerol in interstellar ices.

Contemporary mechanisms for the spontaneous formation of glycerol have not been able to explain its existence on early Earth. The exogenous origin and delivery of organic molecules to early Earth presents an alternative route to their terrestrial in situ formation since biorelevant molecules like amino acids, carboxylic acids, and alkylphosphonic acids have been recovered from carbonaceous chondrites. Reported herein is the first in situ identification of glycerol, the key building block of all cellular membranes, formed by exposure of methanol-based - interstellar model ices to ionizing radiation in the form of energetic electrons. These results provide compelling evidence that the radiation-induced formation of glycerol in low-temperature interstellar model ices is facile. Synthesized on interstellar grains and eventually incorporated into the "building material" of solar systems, biorelevant molecules such as glycerol could have been dispensed to habitable planets such as early Earth by comets and meteorites.

Formation of complex organic molecules in methanol and methanol-carbon monoxide ices exposed to ionizing radiation--a combined FTIR and reflectron time-of-flight mass spectrometry study.

The radiation induced chemical processing of methanol and methanol-carbon monoxide ices at 5.5 K exposed to ionizing radiation in the form of energetic electrons and subsequent temperature programmed desorption is reported in this study. The endogenous formation of complex organic molecules was monitored online and in situ via infrared spectroscopy in the solid state and post irradiation with temperature programmed desorption (TPD) using highly sensitive reflectron time-of-flight (ReTOF) mass spectrometry coupled with single photoionization at 10.49 eV. Infrared spectroscopic analysis of the processed ice systems resulted in the identification of simple molecules including the hydroxymethyl radical (CH2OH), formyl radical (HCO), methane (CH4), formaldehyde (H2CO), carbon dioxide (CO2), ethylene glycol (HOCH2CH2OH), glycolaldehyde (HOCH2CHO), methyl formate (HCOOCH3), and ketene (H2CCO). In addition, ReTOF mass spectrometry of subliming molecules following temperature programmed desorption definitely identified several closed shell C/H/O bearing organics including ketene (H2CCO), acetaldehyde (CH3COH), ethanol (C2H5OH), dimethyl ether (CH3OCH3), glyoxal (HCOCOH), glycolaldehyde (HOCH2CHO), ethene-1,2-diol (HOCHCHOH), ethylene glycol (HOCH2CH2OH), methoxy methanol (CH3OCH2OH) and glycerol (CH2OHCHOHCH2OH) in the processed ice systems. Additionally, an abundant amount of molecules yet to be specifically identified were observed sublimating from the irradiated ices including isomers with the formula C3H(x=4,6,8)O, C4H(x=8,10)O, C3H(x=4,6,8)O2, C4H(x=6,8)O2, C3H(x=4,6)O3, C4H8O3, C4H(x=4,6,8)O4, C5H(x=6,8)O4 and C5H(x=6,8)O5. The last group of molecules containing four to five oxygen atoms observed sublimating from the processed ice samples include an astrobiologically important class of sugars relevant to RNA, phospholipids and energy storage. Experiments are currently being designed to elucidate their chemical structure. In addition, several reaction pathways were identified in the irradiated ices of mixed isotopes based upon the results of both in situ FTIR analysis and TPD ReTOF gas phase analysis. In general, the results of this study provide crucial information on the formation of a variety of classes of organics including alcohols, ketones, aldehydes, esters, ethers, and sugars within the bulk ices upon exposure to ionizing radiation that are relevant to the molecular clouds within the interstellar medium.

Infrared and reflectron time-of-flight mass spectroscopic study on the synthesis of glycolaldehyde in methanol (CH3OH) and methanol-carbon monoxide (CH3OH-CO) ices exposed to ionization radiation.

We present conclusive evidence on the formation of glycolaldehyde (HOCH2CHO) synthesized within astrophysically relevant ices of methanol (CH3OH) and methanol-carbon monoxide (CH3OH-CO) upon exposure to ionizing radiation at 5.5 K. The radiation induced chemical processes of the ices were monitored on line and in situ via infrared spectroscopy which was complimented by temperature programmed desorption studies post irradiation, utilizing highly sensitive reflectron time-of-flight mass spectrometry coupled with single photon fragment free photoionization (ReTOF-PI) at 10.49 eV. Specifically, glycolaldehyde was observed via the v14 band and further enhanced with the associated frequency shifts of the carbonyl stretching mode observed in irradiated isotopologue ice mixtures. Furthermore, experiments conducted with mixed isotopic ices of methanol-carbon monoxide (13CH3OH-CO, CH3(18)OH-CO, CD3OD-13CO and CH3OH-C18O) provide solid evidence of at least three competing reaction pathways involved in the formation of glycolaldehyde via non-equilibrium chemistry, which were identified as follows: (i) radical-radical recombination of HCO and CH2OH formed via decomposition of methanol--the "two methanol pathway"; (ii) via the reaction of one methanol unit (CH2OH from the decomposition of CH3OH) with one carbon monoxide unit (HCO from the hydrogenation of CO)--the "one methanol, one carbon monoxide pathway"; and (iii) formation via hydrogenation of carbon monoxide resulting in radicals of HCO and CH2OH--the "two carbon monoxide pathway". In addition, temperature programmed desorption studies revealed an increase in the amount of glycolaldehyde formed, suggesting further thermal chemistry of trapped radicals within the ice matrix. Sublimation of glycolaldehyde during the warm up was also monitored via ReTOF-PI and validated via the mutual agreement of the associated isotopic frequency shifts within the infrared band positions and the identical sublimation profiles obtained from the ReTOF spectra and infrared spectroscopy of the corresponding isotopes. In addition, an isomer of glycolaldehyde (ethene-1,2-diol) was tentatively assigned. Confirmation of the identified pathways based on infrared spectroscopy was also obtained from the observed ion signals corresponding to isotopomers of glycolaldehyde. These coupled techniques provide clear, concise evidence of the formation of a complex and astrobiologically important organic, glycolaldehyde, relevant to the icy mantles observed in the interstellar medium.

Infrared and reflectron time-of-flight mass spectroscopic analysis of methane (CH4)-carbon monoxide (CO) ices exposed to ionization radiation--toward the formation of carbonyl-bearing molecules in extraterrestrial ices.

Ice mixtures of methane and carbon monoxide were exposed to ionizing radiation in the form of energetic electrons at 5.5 K to investigate the formation of carbonyl bearing molecules in extraterrestrial ices. The radiation induced chemical processing of the mixed ices along with their isotopically labeled counterparts was probed online and in situ via infrared spectroscopy (solid state) aided with reflectron time-of-flight mass spectrometry (ReTOFMS) coupled to single photon photoionization (PI) at 10.49 eV (gas phase). Deconvolution of the carbonyl absorption feature centered at 1727 cm(-1) in the processed ices and subsequent kinetic fitting to the temporal growth of the newly formed species suggests the formation of acetaldehyde (CH3CHO) together with four key classes of carbonyl-bearing molecules: (i) alkyl aldehydes, (ii) alkyl ketones, (iii) α,ß-unsaturated ketones/aldehydes and (iv) α,ß,γ,δ-unsaturated ketones/α,ß-dicarbonyl compounds in keto-enol form. The mechanistical studies indicate that acetaldehyde acts as the key building block of higher aldehydes (i) and ketones (ii) with unsaturated ketones/aldehydes (iii) and/or α,ß-dicarbonyl compounds (iv) formed from the latter. Upon sublimation of the newly synthesized molecules, ReTOFMS together with isotopic shifts of the mass-to-charge ratios was exploited to identify eleven product classes containing molecules with up to six carbon atoms, which can be formally derived from C1-C5 hydrocarbons incorporating up to three carbon monoxide building blocks. The classes are (i) saturated aldehydes/ketones, (ii) unsaturated aldehydes/ketones, (iii) doubly unsaturated aldehydes/ketones, (iv) saturated dicarbonyls (aldehydes/ketones), (v) unsaturated dicarbonyls (aldehydes/ketones), (vi) saturated tricarbonyls (aldehydes/ketones), molecules containing (vii) one carbonyl - one alcohol (viii), two carbonyls - one alcohol, (ix) one carbonyl - two alcohol groups along with (x) alcohols and (xi) diols. Reaction pathways to synthesize these classes were derived as well. The present experiments provide clear evidence for the formation of key organic molecules--acetaldehyde, acetone, and potentially vinylalcohol--which are among the 15 carbonyl containing organic molecules detected in the interstellar medium. Despite numerous previous experimental investigations probing the effect of ionizing radiation on simple astrophysical ice representatives, our results suggest that more complex organic molecules can be formed in extraterrestrial ices than previously suggested. An outlook on further identification of individual isomers is also presented.

High-sensitivity Raman spectrometer to study pristine and irradiated interstellar ice analogs.

We discuss the novel design of a sensitive, normal-Raman spectrometer interfaced to an ultra-high vacuum chamber (5 × 10(-11) Torr) utilized to investigate the interaction of ionizing radiation with low temperature ices relevant to the solar system and interstellar medium. The design is based on a pulsed Nd:YAG laser which takes advantage of gating techniques to isolate the scattered Raman signal from the competing fluorescence signal. The setup incorporates innovations to achieve maximum sensitivity without detectable heating of the sample. Thin films of carbon dioxide (CO2) ices of 10 to 396 nm thickness were prepared and characterized using both Fourier transform infrared (FT-IR) spectroscopy and HeNe interference techniques. The ν+ and ν- Fermi resonance bands of CO2 ices were observed by Raman spectroscopy at 1385 and 1278 cm(-1), respectively, and the band areas showed a linear dependence on ice thickness. Preliminary irradiation experiments are conducted on a 450 nm thick sample of CO2 ice using energetic electrons. Both carbon monoxide (CO) and the infrared inactive molecular oxygen (O2) products are readily detected from their characteristic Raman bands at 2145 and 1545 cm(-1), respectively. Detection limits of 4 ± 3 and 6 ± 4 monolayers of CO and O2 were derived, demonstrating the unique power to detect newly formed molecules in irradiated ices in situ. The setup is universally applicable to the detection of low-abundance species, since no Raman signal enhancement is required, demonstrating Raman spectroscopy as a reliable alternative, or complement, to FT-IR spectroscopy in space science applications.

Application of Reflectron Time-of-Flight Mass Spectroscopy in the Analysis of Astrophysically Relevant Ices Exposed to Ionization Radiation: Methane (CH4) and D4-Methane (CD4) as a Case Study.

Methane ices have been detected on ice-coated interstellar grains and on the surface of Kuiper belt objects. These ices are chemically altered by ionizing radiation in the form of energetic photons and charged particles, leading to complex organic molecules. Despite decades of research, the chemical makeup of these newly synthesized molecules has not been completely understood to date. Here, we present a novel application of reflectron time-of-flight mass spectrometry coupled to soft photoionization to probe the molecular formulas of the molecules formed upon interaction of ionizing radiation with simple methane and D4-methane ices. Our study depicts clear evidence of high-molecular-weight hydrocarbons of up to C22, among them alkanes, alkenes, and alkynes/dienes, with those product classes in italics identified for the first time on line and in situ. These studies are particular timely as they provide laboratory data of methane-processed ices, which can be compared to actual data from the New Horizons mission on route to Pluto.

Formation of benzene in the interstellar medium.

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block--the aromatic benzene molecule--has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C(6)H(6)) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C(2)H + H(2)CCHCHCH(2) → C(6)H(6) + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.

Crossed molecular beam study on the ground state reaction of atomic boron [B((2)P(j))] with hydrogen cyanide [HCN(X(1)sigma(+))].

The linear boronisocyanide species, [BNC(X(1)Sigma(+))], represents the simplest triatomic molecule with three distinct, neighboring main group atoms of the second row of the periodic table of the elements: boron, carbon, and nitrogen. This makes boronisocyanide a crucial benchmark system to understand the chemical bonding and the electronic structure of small molecules, in particular when compared to the isoelectronic tricarbon molecule, [CCC(X(1)Sigma(g)(+))]. However, a clean, directed synthesis of boronisocyanide-a crucial prerequisite to study the properties of this molecule-has remained elusive so far. Here, we combine crossed molecular beam experiments of ground state boron atoms ((2)P(j)) with hydrogen cyanide with electronic structure calculations and reveal that the boronisocyanide molecule, [BNC(X(1)Sigma(+))], is formed as the exclusive product under gas phase single collision conditions. We also show that higher energy isomers such as the hitherto unnoticed, ring-strained cyclic BNC(X(3)A') structure, which is isoelectronic to the triplet, cyclic tricarbon molecule, [C(3)(X(3)A(2)')], do exist as local minima. Our studies present the first directed synthesis and observation of gas phase boronisocyanide providing a doorway for further fundamental studies on one of the simplest triatomic molecules composed solely of group III-V elements.

Crossed molecular beam study on the formation of phenylacetylene and its relevance to Titan's atmosphere.

The crossed molecular beam experiment of the deuterated ethynyl radical (C(2)D; X(2)Sigma(+)) with benzene [C(6)H(6)(X(1)A(1g))] and its fully deuterated analog [C(6)D(6)(X(1)A(1g))] was conducted at a collision energy of 58.1 kJ mol(-1). Our experimental data suggest the formation of the phenylacetylene-d(6) via indirect reactive scattering dynamics through a long-lived reaction intermediate; the reaction is initiated by a barrierless addition of the ethynyl-d(1) radical to benzene-d(6). This initial collision complex was found to decompose via a tight exit transition state located about 42 kJ mol(-1) above the separated products; here, the deuterium atom is ejected almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. The overall experimental exoergicity of the reaction is shown to be 121 +/- 10 kJ mol(-1); this compares nicely with the computed reaction energy of -111 kJ mol(-1). Even though the experiment was conducted at a collisional energy higher than equivalent temperatures typically found in the atmosphere of Titan (94 K and higher), the reaction may proceed in Titan's atmosphere as it involves no entrance barrier, all transition states involved are below the energy of the separated reactants, and the reaction is exoergic. Further, the phenylacetylene was found to be the sole reaction product.

Formation of the phenyl radical [C6H5(X(2)A1)] under single collision conditions: a crossed molecular beam and ab initio study.

Reactions of dicarbon molecules (C(2)) with C(4)H(6) isomers such as 1,3-butadiene represent a potential, but hitherto unnoticed, route to synthesize the first aromatic C(6) ring in hydrocarbon flames and in the interstellar medium where concentrations of dicarbon transient species are significant. Here, crossed molecular beams experiments of dicarbon molecules in their X(1)Sigma(g)(+) electronic ground state and in the first electronically excited a(3)Pi(u) state have been conducted with 1,3-butadiene and two partially deuterated counterparts (1,1,4,4-D4-1,3-butadiene and 2,3-D2-1,3-butadiene) at two collision energies of 12.7 and 33.7 kJ mol(-1). Combining these scattering experiments with electronic structure and RRKM calculations on the singlet and triplet C(6)H(6) surfaces, our investigation reveals that the aromatic phenyl radical is formed predominantly on the triplet surface via indirect scattering dynamics through a long-lived reaction intermediate. Initiated by a barrierless addition of triplet dicarbon to one of the terminal carbon atoms of 1,3-butadiene, the collision complex undergoes trans-cis isomerization followed by ring closure and hydrogen migration prior to hydrogen atom elimination, ultimately forming the phenyl radical. The latter step emits the hydrogen atom almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. On the singlet surface, smaller contributions of phenyl radical could not be excluded; experiments with partially deuterated 1,3-butadiene indicate the formation of the thermodynamically less stable acyclic H(2)CCHCCCCH(2) isomer. This study presents the very first experimental evidence, contemplated by theoretical studies, that under single collision conditions an aromatic hydrocarbon molecule can be formed in a bimolecular gas-phase reaction via reaction of two acyclic molecules involving cyclization processes at collision energies highly relevant to combustion flames.

Vacuum ultraviolet excitation spectroscopy of the autoionizing Rydberg states of atomic sulfur in the 73 350-84 950 cm(-1) frequency range.

The photoionization efficiency (PIE) spectra of metastable sulfur (S) atoms in the 1 D and 1 S states have been recorded in the 73 350-84 950 cm(-1) frequency range by using a velocity-mapped ion imaging apparatus that uses a tunable vacuum ultraviolet laser as the ionization source. The S(1 D) and S(1 S) atoms are produced by the 193 nm photodissociation of CS2. The observed PIE spectra of S(1 D) and S(1 S) shows 35 autoionizing resonances with little or no contribution from direct photoionization into the S+(4S 3/2)+e(-) ionization continuum. Velocity-mapped ion images of the S+ at the individual autoionizing Rydberg resonances are used to distinguish whether the lower state of the resonance originates from the 1 D, 1 S, or 3P states. The analysis and assignment of the Rydberg peaks revealed 22 new Rydberg states that were not previously known. The autoionization lifetimes tau of the Rydberg states are derived from the linewidths by fitting the lines with the Fano formula. Deviations from the scaling law of tau(n*) proportional to, n*3, where n* is the effective quantum number of the Rydberg state, are observed. This observation is ascribed to perturbations by nearby triplet Rydberg states, which shorten the autoionization lifetimes of the singlet Rydberg levels.

A vacuum ultraviolet laser photoionization and pulsed field ionization study of nascent S(3P2,1,0) and S(1D2) formed in the 193.3 nm photodissociation of CS2.

The photoionization efficiency (PIE) and pulsed field ionization-photoion (PFI-PI) spectra for sulfur atoms S(3P2,1,0) and S(1D2) resulting from the 193.3 nm photodissociation of CS2 have been measured using tunable vacuum ultraviolet (vuv) laser radiation in the frequency range of 82 750-83 570 cm(-1). The PIE spectrum of S(3P2,1,0) near their ionization threshold exhibits steplike structures. On the basis of the velocity-mapped ion-imaging measurements, four strong autoionizing peaks observed in the PIE measurement in this frequency range have been identified to originate from vuv excitation of S(1D2). The PFI-PI measurement reveals over 120 previously unidentified new Rydberg lines. They have been assigned as Rydberg states [3p3(4S composite function nd3 D composite function (n=17-64)] converging to the ground ionic state S+(4S composite function) formed by vuv excitations of S(3P2,1,0). The converging limits of these Rydberg series have provided more accurate values, 82 985.43+/-0.05, 83 162.94+/-0.05, and 83 559.04+/-0.05 cm(-1) for the respective ionization energies of S(3P0), S(3P1), and S(3P2) to form S+(4S composite function). The relative intensities of the PFI-PI bands for S(3P0), S(3P1), and S(3P2) have been used to determine the branching ratios for these fine structure states, S(3P0):S(3P1):S(3P2)=1.00:1.54:3.55, produced by photodissociation of CS2 at 193.3 nm.

High-resolution Rydberg tagging time-of-flight measurements of atomic photofragments by single-photon vacuum ultraviolet laser excitation.

By coupling a comprehensive tunable vacuum ultraviolet (VUV) laser system to a velocity-mapped ion imaging apparatus, we show that high-resolution high-n Rydberg tagging time-of-flight (TOF) measurements of nascent atomic photofragments formed by laser photodissociation can be made using single-photon VUV laser photoexcitation. To illustrate this single-photon Rydberg tagging TOF method, we present here the results of the VUV laser high-n Rydberg tagging TOF measurements of O((3)P(2)) and S((3)P(2)) formed in the photodissociation of SO(2) and CS(2) at 193.3 and 202.3 nm, respectively. These results are compared to those obtained by employing the VUV laser photoionization time-sliced velocity-mapped ion imaging technique. The fact that the kinetic energy resolutions achieved in the VUV laser high-n Rydberg tagging TOF measurements of O and S atoms are found to be higher than those observed in the VUV laser photoionization, time-sliced velocity-mapped ion imaging studies show that the single-photon VUV laser high-n Rydberg tagging TOF method is useful and complementary to state-of-the-art time-sliced velocity-mapped ion imaging measurements of heavier atomic photofragments, such as O and S atoms. Furthermore, the general agreement observed between the VUV laser high-n Rydberg tagging TOF and velocity-mapped ion imaging experiments supports the conclusion that the lifetimes of the tagged Rydberg states of O and S atoms are sufficiently long to allow the reliable determination of state-resolved UV photodissociation cross sections of SO(2) and CS(2) by using the VUV laser high-n Rydberg tagging TOF method.

A photodissociation study of CH2BrCl in the A-band using the time-sliced ion velocity imaging method.

Employing a high-resolution (velocity resolution deltanu/nu<1.5%) time-sliced ion velocity imaging apparatus, we have examined the photodissociation of CH2BrCl in the photon energy range of 448.6-618.5 kJ/mol (193.3-266.6 nm). Precise translational and angular distributions for the dominant Br(2P32) and Br(2P12) channels have been determined from the ion images observed for Br(2P32) and Br(2P12). In confirmation with the previous studies, the kinetic-energy distributions for the Br(2P12) channel are found to fit well with one Gaussian function, whereas the kinetic- energy distributions for the Br(2P32) channel exhibit bimodal structures and can be decomposed into a slow and a fast Gaussian component. The observed kinetic-energy distributions are consistent with the conclusion that the formation of the Br(2P32) and Br(2P12) channels takes place on a repulsive potential-energy surface, resulting in a significant fraction (0.40-0.47) of available energy to appear as translational energy for the photo fragments. On the basis of the detailed kinetic-energy distributions and anisotropy parameters obtained in the present study, together with the specific features and relative absorption cross sections of the excited 2A', 1A", 3A', 4A', and 2A" states estimated in previous studies, we have rationalized the dissociation pathways of CH2BrCl in the A-band, leading to the formation of the Br(2P32) and Br(2P12) channels. The analysis of the ion images observed at 235 nm for Cl(2P(32,12)) provides strong evidence that the formation of Cl mainly arises from the secondary photodissociation process CH2Cl + hnu --> CH2 + Cl.