A uniform flow-cavity ring-down spectrometer (UF-CRDS): A new setup for spectroscopy and kinetics at low temperature.

The UF-CRDS (Uniform Flow-Cavity Ring Down Spectrometer) is a new setup coupling for the first time a pulsed uniform (Laval) flow with a continuous wave CRDS in the near infrared for spectroscopy and kinetics at low temperature. This high resolution and sensitive absorption spectrometer opens a new window into the phenomena occurring within UFs. The approach extends the detection range to new electronic and rovibrational transitions within Laval flows and offers the possibility to probe numerous species which have not been investigated yet. This new tool has been designed to probe radicals and reaction intermediates but also to follow the chemistry of hydrocarbon chains and PAHs which play a crucial role in the evolution of astrophysical environments. For kinetics measurements, the UF-CRDS combines the CRESU technique (French acronym meaning reaction kinetics in uniform supersonic flows) with the SKaR (Simultaneous Kinetics and Ring-Down) approach where, as indicated by its name, the entire reaction is monitored during each intensity decay within the high finesse cavity. The setup and the approach are demonstrated with the study of the reaction between CN (v = 1) and propene at low temperature. The recorded data are finally consistent with a previous study of the same reaction for CN (v = 0) relying on the CRESU technique with laser induced fluorescence detection.

Finite slice analysis (FINA) of sliced and velocity mapped images on a Cartesian grid.

Although time-sliced imaging yields improved signal-to-noise and resolution compared with unsliced velocity mapped ion images, for finite slice widths as encountered in real experiments there is a loss of resolution and recovered intensities for the slow fragments. Recently, we reported a new approach that permits correction of these effects for an arbitrarily sliced distribution of a 3D charged particle cloud. This finite slice analysis (FinA) method utilizes basis functions that model the out-of-plane contribution of a given velocity component to the image for sequential subtraction in a spherical polar coordinate system. However, the original approach suffers from a slow processing time due to the weighting procedure needed to accurately model the out-of-plane projection of an anisotropic angular distribution. To overcome this issue we present a variant of the method in which the FinA approach is performed in a cylindrical coordinate system (Cartesian in the image plane) rather than a spherical polar coordinate system. Dubbed C-FinA, we show how this method is applied in much the same manner. We compare this variant to the polar FinA method and find that the processing time (of a 510 × 510 pixel image) in its most extreme case improves by a factor of 100. We also show that although the resulting velocity resolution is not quite as high as the polar version, this new approach shows superior resolution for fine structure in the differential cross sections. We demonstrate the method on a range of experimental and synthetic data at different effective slice widths.

Imaging multiphoton ionization and dissociation of rotationally warm CO via the B+Σ1 and EΠ1 electronic states.

Pathways for formation of C+ and O+ ions when applying (2 + 1) resonance enhanced multiphoton ionization (REMPI) of CO via the B1Σ+ and E1Π electronic states are characterized with the velocity map imaging technique. By employing an unskimmed pulsed valve, it was possible to obtain sharp images for a wide range of initial CO J-states. Most of the atomic ion production pathways could be assigned as one- or two-photon dissociation of a series of vibrational levels of the CO+ X2Σ+ and A2Π states. Large enhancements in dissociation of particular CO+ vibrational states in these progressions could be accurately assigned to accidental resonances of the REMPI laser with CO+ X2Σ+-B2Σ+ transitions.

Finite slice analysis (FINA)-A general reconstruction method for velocity mapped and time-sliced ion imaging.

Since the advent of ion imaging, one of the key issues in the field has been creating methods to reconstruct the initial 3D distribution of particles from its 2D projection. This has led to the development of a number of different numerical methods and fitting techniques to solve this fundamental issue in imaging. In recent years, slice-imaging methods have been developed that permit direct recording of the 3D distribution, i.e., a thin slice of the recoiling fragment distribution. However, in practice, most slice imaging experiments achieve a velocity slice width of around 10%-25% around the center of the distribution. This still carries significant out-of-plane elements that can blur the spectrum, lose fine resolution, and underestimate the contribution from slow recoiling products. To overcome these limitations, we developed a new numerical method to remove these out-of-plane elements from a sliced image. The finite sliced analysis method models the off-axis elements of the 3D particle distribution through the use of radial basis functions. Once applied, the method reconstructs the underlying central slice of the 3D particle distribution. The approach may be applied to arbitrarily sliced or unsliced data and has the further advantage that it neither requires nor enforces full cylindrical symmetry of the data. We demonstrate this reconstruction approach with a broad range of synthetic and experimental data that, at the same time, allows us to examine the impact of finite slicing on the recovered distributions in detail.

UV photodissociation of cyanoacetylene: a combined ion imaging and theoretical investigation.

The photodissociation of cyanoacetylene, one of the key minor constituents in Titan's atmosphere, was studied in a molecular beam under collisionless conditions using direct current slice ion imaging at 121.6, 193.3, and 243.2 nm. The experimental results were augmented by high-level theoretical calculations of stationary points on the ground-state and second excited singlet potential surfaces, and by statistical calculations of the dissociation rates and product branching on the ground-state surface. Results at 121.6 and 243.2 nm are nearly identical, suggesting that the 243.2 nm photodissociation is the result of a two-photon process. The translational energy distributions show only a modest fraction of the available energy in translation and are consistent with barrierless dissociation from the ground state. The results at 193.3 nm are quite distinct, showing up to half of the available energy in translation, implying dissociation with an exit barrier. The 193 nm result is ascribed to dissociation on the S(1) potential energy surface. The theoretical calculations show significant rates for H loss on the ground state at 193 nm and significant branching to CN + CCH at 157 nm and higher.

Production of O2 Herzberg states in the deep UV photodissociation of ozone.

High-resolution imaging experiments combined with new electronic structure and dynamics calculations strongly indicate that the O((3)P)+O(2) products with very low kinetic energy release (E(tr)<0.2 eV) formed in the deep UV (226 nm) photodissociation of ozone reflect excitation of the Herzberg states of O(2): A(')(3)Delta(u)(v=0,1,2) and A (3)Sigma(u)(+)(v=0,1). This interpretation contradicts the earlier assignment to very high (v> or =26) vibrational states of O(2)((3)Sigma(g)(-)).

H elimination and metastable lifetimes in the UV photoexcitation of diacetylene.

We present an experimental investigation of the UV photochemistry of diacetylene under collisionless conditions. The H loss channel is studied using DC slice ion imaging with two-color reduced-Doppler detection at 243 nm and 212 nm. The photochemistry is further studied deep in the vacuum UV, that is, at Lyman-alpha (121.6 nm). Translational energy distributions for the H + C(4)H product arising from dissociation of C(4)H(2) after excitation at 243, 212, and 121.6 nm show an isotropic angular distribution and characteristic translational energy profile suggesting statistical dissociation from the ground state or possibly from a low-lying triplet state. From these distributions, a two-photon dissociation process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a one-photon dissociation process prevails. The results are interpreted with the aid of ab initio calculations on the reaction pathways and statistical calculations of the dissociation rates and product branching. In a second series of experiments, nanosecond time-resolved phototionization measurements yield a direct determination of the lifetime of metastable triplet diacetylene under collisionless conditions, as well as its dependence on excitation energy. The observed submicrosecond lifetimes suggest that reactions of metastable diacetylene are likely to be less important in Titan's atmosphere than previously believed.

The role of intermediate state polarization in determination of vector properties of the ground state using multiphoton excitation.

We present a general theory for calculating the vector and geometrical properties of the multiphoton excitation of an arbitrary atomic or molecular system. The results are applied to study the influence of the polarization of the two-photon excited state, which is usually neglected, on the intensity of (2 + 1) resonant multiphoton ionization in atoms. Two examples of specific atomic systems of practical importance are presented: oxygen and chlorine. For some cases, the effect of the polarization of the pre-ionized state can be significant and must be properly treated.

The roaming atom: straying from the reaction path in formaldehyde decomposition.

We present a combined experimental and theoretical investigation of formaldehyde (H2CO) dissociation to H2 and CO at energies just above the threshold for competing H elimination. High-resolution state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H2. The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H2. Quasi-classical trajectory calculations performed on a global potential energy surface for H2CO suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.

"Heavy electron" photoelectron spectroscopy: rotationally resolved ion pair imaging of CH3+.

We applied the velocity map imaging technique under high-resolution conditions to study ion pair products of the vacuum ultraviolet photodissociation of methyl chloride. We obtained rotationally resolved kinetic energy release spectra that directly provide vibrational frequencies and rotational constants of the fundamental carbocation, CH3+. The technique is analogous to photoelectron spectroscopy, with the chloride anion playing the role of a "heavy electron." The approach shows promise as a general probe of ionic species not amenable to study by traditional methods.

The ultraviolet photochemistry of phenylacetylene and the enthalpy of formation of 1,3,5-hexatriyne.

The ultraviolet photochemistry of phenylacetylene was studied in a molecular beam at 193 nm. The only primary photofragments observed were HCCH (acetylene) and C(6)H(4). Some of the C(6)H(4) molecules were found to decompose to 1,3,5-hexatriyne and molecular hydrogen. An enthalpy of formation of DeltaH(f) < or = 160 +/- 4 kcal mol(-1) was determined for 1,3,5-hexatriyne from the energetic threshold for this process. This experimentally determined value agrees well with our ab initio calculations performed at the G2 level of theory. Angular distribution measurements for the HCCH + C(6)H(4) channel yielded an isotropic distribution and were attributed to a long-lived intermediate and ground-state dissociation. An exhaustive search yielded no evidence for the phenyl + ethynyl or the atomic hydrogen elimination channels even though these were observed in the pyrolytic studies of phenylacetylene [Hofmann, J.; Zimmermann, G.; Guthier, K.; Hebgen, P.; Homann, K. H. Liebigs Ann. 1995, 631, 1995. Guthier, K.; Hebgen, P.; Hofmann, K. H.; Zimmermann, G. Liebigs Ann. 1995, 637, 1995].

Photodissociation of ethylene sulfide at 193 nm: a photofragment translational spectroscopy study with VUV synchrotron radiation and ab initio calculations.

Photodissociation of ethylene sulfide at 193 nm has been studied using photofragment translational spectroscopy and ab initio theoretical calculations. Tunable synchrotron radiation was used as a universal but selective probe of the reaction products to reveal new aspects of the photodissociation dynamics. The channel giving S + C2H4 was found to be dominated by production of ground-state sulfur atoms (S(3P):S(1D) = 1.44:1), mostly through a spin-forbidden process. The results also suggest the presence of a channel giving S(3P) in conjunction with triplet ethylene C2H4 (3B(1u)) and allow insight into the energy of the latter species near its equilibrium geometry, in which the two methylene groups occupy perpendicular planes. In addition, a channel leading to the production of H2S with C2H2 also has been observed. Our experimental results are supported and elaborated by theoretical calculations.

A combined experimental and theoretical study on the formation of interstellar C3H isomers.

The reaction of ground-state carbon atoms with acetylene was studied under single-collision conditions in crossed beam experiments to investigate the chemical dynamics of forming cyclic and linear C3H isomers (c-C3H and l-C3H, respectively) in interstellar environments via an atom-neutral reaction. Combined state-of-the-art ab initio calculations and experimental identification of the carbon-hydrogen exchange channel to both isomers classify this reaction as an important alternative to ion-molecule encounters to synthesize C3H radicals in the interstellar medium. These findings strongly correlate with astronomical observations and explain a higher [c-C3H]/[l-C3H] ratio in the dark cloud TMC-1 than in the carbon star IRC+10216.

Dynamics of Carbonium Ions Solvated by Molecular Hydrogen: CH5+(H2)n (n = 1, 2, 3).

The dynamics of the carbonium ion (CH(5)(+)), a highly reactive intermediate with no equilibrium structure, was studied by measuring the infrared spectra for internally cold CH(5)(+)(H(2))n(n = 1, 2, 3) stored in an ion trap. First-principle molecular dynamics methods were used to directly simulate the internal motion for these ionic complexes. The combined experimental and theoretical efforts substantiated the anticipated scrambling motion in the CH(5)(+) core and revealed the effect of the solvent molecular hydrogen in slowing down the scrambling. The results indicate the feasibility of using solvent molecules to stabilize the floppy CH(5)(+) ion in order to make it amenable to spectroscopic study.

The "Ozone Deficit" Problem: O2(X, v ge 26) + O(3P) from 226-nm Ozone Photodissociation.

Highly vibrationally excited O(2)(X(3)sigmag(-), v >/= 26) has been observed from the photodissociation of ozone (O(3)), and the quantum yield for this reaction has been determined for excitation at 226 nanometers. This observation may help to address the "ozone deficit" problem, or why the previously predicted stratospheric O(3) concentration is less than that observed. Recent kinetic studies have suggested that O(2)(X(3)sigmag(-), v >/= 26) can react rapidly with O(2) to form O(3) + O and have led to speculation that, if produced in the photodissociation of O(3), this species might be involved in resolving the discrepancy. The sequence O(3) + hv --> O(2)(X(3)sigmag(-), v >/= 26) + O; O(2)(X(3)sigmag(-), v >/= 26) + O(2) --> O(3) + O (where hv is a photon) would be an autocatalytic mechanism for production of odd oxygen. A two-dimensional atmospheric model has been used to evaluate the importance of this new mechanism. The new mechanism can completely account for the tropical O(3) deficit at an altitude of 43 kilometers, but it does not completely account for the deficit at higher altitudes. The mechanism also provides for isotopic fractionation and may contribute to an explanation for the anomalously high concentration of heavy O(3) in the stratosphere.

Phagocytosis of lipase-aggregated low density lipoprotein promotes macrophage foam cell formation. Sequential morphological and biochemical events.

Macrophages internalize aggregated low density lipoprotein (LDL) by LDL receptor-dependent phagocytosis. To investigate this model of foam cell formation, we have used human and mouse macrophages to characterize biochemically and morphologically the fate of ingested phospholipase C-modified low density lipoprotein (PLC-LDL). When LDL was digested with phospholipase C, it lost phospholipid and aggregated. Human monocyte-derived macrophages rapidly ingested and degraded 125I-PLC-LDL. The degraded PLC-LDL released free cholesterol, measured either as free sterol mass or by the stimulation of [14C]oleate incorporation into cellular cholesteryl ester. Esterification was blocked by chloroquine, a weak base that inhibits lysosomal degradation. Macrophages exposed to PLC-LDL exhibited a 30-fold to a 50-fold increase in esterified sterol: by light microscopy, cytoplasmic inclusions were abundant. The inclusions were stained with oil red O, indicating that they were neutral lipid droplets. By electron microscopy, mouse peritoneal macrophages incubated with PLC-LDL contained numerous membrane-bounded vacuoles and cytoplasmic inclusions that were not surrounded by a limiting membrane. Pulse-chase experiments demonstrated that vacuoles filled with particulate material appeared first. Subsequently, the macrophages exhibited vacuoles containing multivesicular bodies. Last, inclusions that were homogeneously electron-dense and that lacked a tripartite membrane accumulated in the cytoplasm of the cells. These results are consonant with the following model of foam cell formation. Cultured macrophages rapidly ingest PLC-LDL that is initially localized in phagosomes. The aggregated lipoprotein subsequently is digested in secondary lysosomes, thus releasing free cholesterol that is reesterified, forming cytoplasmic cholesteryl ester droplets lacking a tripartite membrane.

Phagocytosis of aggregated lipoprotein by macrophages: low density lipoprotein receptor-dependent foam-cell formation.

Low density lipoprotein (LDL) modified by incubation with phospholipase C (PLC-LDL) aggregates in solution and is rapidly taken up and degraded by human and mouse macrophages, producing foam cells in vitro. Human, mouse, and rabbit macrophages degraded 125I-labeled PLC-LDL (125I-PLC-LDL) more rapidly than native 125I-labeled LDL (125I-LDL), while nonphagocytic cells such as human fibroblasts and bovine aortic endothelial cells degraded 125I-PLC-LDL more slowly than 125I-LDL. This suggested the mechanism for internalization of PLC-LDL was phagocytosis. When examined by electron microscopy, mouse peritoneal macrophages appeared to be phagocytosing PLC-LDL. The uptake and degradation of 125I-PLC-LDL by human macrophages was inhibited greater than 80% by the monoclonal antibody C7 (IgG2b) produced by hybridoma C7, which blocks the ligand binding domain of the LDL receptor. Similarly, methylation of 125I-LDL (125I-MeLDL) prior to treatment with phospholipase C decreased its subsequent uptake and degradation by human macrophages by greater than 90%. The uptake and degradation of phospholipase C-modified 125I-MeLDL by macrophages could be restored by incubation of the methylated lipoprotein with apoprotein E, a ligand recognized by the LDL receptor. These results indicate that macrophages internalize PLC-LDL by LDL receptor-dependent phagocytosis.