H. Reisler

University of Southern California, Los Angeles, CA, United States

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Publications (189)439.73 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Conspectus Water is one of the most pervasive molecules on earth and other planetary bodies; it is the molecule that is searched for as the presumptive precursor to extraterrestrial life. It is also the paradigm substance illustrating ubiquitous hydrogen bonding (H-bonding) in the gas phase, liquids, crystals, and amorphous solids. Moreover, H-bonding with other molecules and between different molecules is of the utmost importance in chemistry and biology. It is no wonder, then, that for nearly a century theoreticians and experimentalists have tried to understand all aspects of H-bonding and its influence on reactivity. It is somewhat surprising, therefore, that several fundamental aspects of H-bonding that are particularly important for benchmarking theoretical models have remained unexplored experimentally. For example, even the binding strength between two gas-phase water molecules has never been determined with sufficient accuracy for comparison with high-level electronic structure calculations. Likewise, the effect of cooperativity (nonadditivity) in small H-bonded networks is not known with sufficient accuracy. An even greater challenge for both theory and experiment is the description of the dissociation dynamics of H-bonded small clusters upon acquiring vibrational excitation. This is because of the long lifetimes of many clusters, which requires running classical trajectories for many nanoseconds to achieve dissociation. In this Account, we describe recent progress and ongoing research that demonstrates how the combined and complementary efforts of theory and experiment are enlisted to determine bond dissociation energies (D0) of small dimers and cyclic trimers of water and HCl with unprecedented accuracy, describe dissociation dynamics, and assess the effects of cooperativity. The experimental techniques rely on IR excitation of H-bonded X-H stretch vibrations, measuring velocity distributions of fragments in specific rovibrational states, and determining product state distributions at the pair-correlation level. The theoretical methods are based on high-level ab initio potential energy surfaces used in quantum and classical dynamical calculations. We achieve excellent agreement on D0 between theory and experiments for all of the clusters that we have compared, as well as for cooperativity in ring trimers of water and HCl. We also show that both the long-range and the repulsive parts of the potential must be involved in bond breaking. We explain why H-bonds are so resilient and hard to break, and we propose that a common motif in the breaking of cyclic trimers is the opening of the ring following transfer of one quantum of stretch excitation to form open-chain structures that are weakly bound. However, it still takes many vibrational periods to release one monomer fragment from the open-chain structures. Our success with water and HCl dimers and trimers led us to embark on a more ambitious project: studies of mixed water and HCl small clusters. These clusters eventually lead to ionization of HCl and serve as prototypes of acid dissociation in water. Measurements and calculations of such ionizations are yet to be achieved, and we are now characterizing these systems by adding monomers one at a time. We describe our completed work on the HCl-H2O dimer and mention our recent theoretical results on larger mixed clusters.
    Accounts of Chemical Research 07/2014; · 20.83 Impact Factor
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    ABSTRACT: Rotational, vibrational and electronic states of formaldehyde and cis-hydroxymethylene products generated in the photodissociation of the hydroxymethyl radical are investigated by sliced velocity map imaging (SVMI) following excitation of the radical to its 3px and 3pz Rydberg states. SVMI of H and D photofragments is essential in these studies because it allows zooming in on low velocity regions of the images where small threshold signals can be identified. With CH2OD precursors, formaldehyde and hydroxymethylene products are examined separately by monitoring D and H, respectively. Whereas the main dissociation channels lead to formaldehyde and cis-hydroxymethylene in their ground electronic states, at higher excitation energies the kinetic energy distributions (KEDs) of the H and D photofragments exhibit additional small peaks, which are assigned as triplet states of formaldehyde and hydroxymethylene. Results obtained with deuterated isotopologs of CH2OH demonstrate that the yield of the triplet state of formaldehyde decreases upon increasing deuteration, suggesting that the conical intersection seams that govern the dynamics depend on the degree of deuteration. The rotational excitation of cis-hydroxymethylene depends on the excited Rydberg state of CH2OD, and is lower in dissociation via the 3pz state than via the lower lying 3px and 3s states. Vibrational excitation of cis-HCOD, which spans the entire allowed internal energy range, consists mostly of the CO-stretch and in-plane bend modes. When the internal energy of cis-HCOD exceeds the dissociation threshold to D + HCO, slow D and H photofragments deriving from secondary dissociation are observed. The yields of these H and D fragments are comparable, and we propose that they are generated via prior isomerization of cis-HCOD to HDCO.
    The journal of physical chemistry. A. 07/2014;
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    ABSTRACT: The breaking of hydrogen bonds in molecular systems has profound effects on liquids, e.g., water, biomolecules, e.g., DNA, etc. and so it is no exaggeration to assert the importance of these bonds to living systems. However, despite years of extensive research on hydrogen bonds, many of the details of how these bonds break and the corresponding energy redistribution processes remain poorly understood. Here we report extensive experimental and theoretical insights into the break-up of two or three hydrogen bonds of the dissociation of a paradigm system of a hydrogen bonded network, the ring HCl trimer. Experimental state-to-state vibrational predissociation dynamics of the trimer following vibrational excitation were studied by using velocity map imaging and resonance-enhanced multiphoton ionization, providing dissociation energies and product state distributions for the trimer's breakup into three separate monomers or into dimer + monomer. Accompanying the experiments are high-level calculations using diffusion Monte Carlo and quasi-classical simulations, whose results validate the experimental ones and further elucidate energy distributions in the products. The calculations make use a new, highly accurate potential energy surface. Simulations indicate that the dissociation mechanism requires the excitation to first relax into low frequency motions of the trimer, resulting in the breaking of a single hydrogen bond. This allows the system to explore a critical van der Waals minimum region from which dissociation occurs readily to monomer + dimer.
    The Journal of Physical Chemistry A 02/2014; · 2.77 Impact Factor
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    ABSTRACT: The photodissociation dynamics of the hydroxymethyl radical (CH2OH, CH2OD, and CD2OD) following excitation to the 3s and 3px Rydberg states is studied using time-sliced velocity map imaging of hydrogen photofragments. Dissociation takes place on the ground potential energy surface (PES) reached via conical intersections from the excited states, and formaldehyde and hydrxymethylene are identified as reaction products. The major product, formaldehyde, has a bimodal internal energy distribution. The largest fraction has high kinetic energy (KE), modest rotational excitation, and vibrational excitation mainly in the CO stretch and the CH(D)2 deformations modes (scissors, wag and rock). The minor fraction has lower KEs and a higher rovibrational excitation that is unresolved. A bimodal internal energy distribution in the formaldehyde fragment has been predicted by Yarkony [J. Chem. Phys. 2005, 122, 084316] for a conical intersection along the O-H bond coordinate. The hydroxymethylene product state distributions depend strongly on the nature of the excited state. In dissociation via the 3s state, the hydroxymethylene products have broad rovibrational state distributions and are produced at low yield. As suggested by Yarkony, they may be produced in the same dissociation events that give rise to low KE formaldehyde. In these events the bound region of the PES is sampled following the conical intersection along O-H(D). The hydroxymethylene yield is low near its threshold and increases slowly with excitation energy to the 3s state but its internal energy distribution remains broad and the contributions of the the cis and trans isomers cannot be resolved. The mechanism changes markedly when exciting to the 3px state. The hydroxymethylene products have less rotational excitation and show separate contributions of cis and trans isomers. The trans isomer is found to be a minor product relative to the higher-energy cis isomer, as predicted by Yarkony for conical intersections along the C-H coordinate. It appears that the efficiency of dissociation via conical intersections along the O-H and C-H coordinates depends on the initial excited state. While the O-H conical intersection seam (vertical cone) provides an efficient route to the ground state following excitation via the 3s or the 3px Rydberg states, conical intersections along the C-H bond coordinate (tilted cone) appear to be sampled more efficiently via 3px excitation and proceed through different dynamics. The energy separations between formaldehyde and hydroxymethylene and between the cis and trans isomers of hydroxymethylene are determined experimentally for all the investigated isotopologs and are in good agreement with theory.
    The Journal of Physical Chemistry A 07/2013; · 2.77 Impact Factor
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    ABSTRACT: This letter presents a brief overview of our recent experimental studies of state-to-state vibrational predissociation (VP) dynamics of small hydrogen bonded (H-bonded) clusters following vibrational excitation. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) are used to determine accurate bond dissociation energies (D0) of (H2O)2, (H2O)3, HCl–H2O and NH3–H2O. Pair-correlated product energy distributions from the VP of these complexes are also presented and compared to theoretical models. Further insights into mechanisms are obtained from the recent quasi-classical trajectory (QCT) calculations of Bowman and coworkers. The D0 values for (H2O)2 and (H2O)3 are in very good agreement with recent calculated values, and the results are used to estimate the contributions of cooperative interactions to the H-bonding network.
    Chemical Physics Letters 06/2013; 575:1–11. · 2.15 Impact Factor
  • Mikhail Ryazanov, Hanna Reisler
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    ABSTRACT: Time-sliced velocity map imaging (SVMI), a high-resolution method for measuring kinetic energy distributions of products in scattering and photodissociation reactions, is challenging to implement for atomic hydrogen products. We describe an ion optics design aimed at achieving SVMI of H fragments in a broad range of kinetic energies (KE), from a fraction of an electronvolt to a few electronvolts. In order to enable consistently thin slicing for any imaged KE range, an additional electrostatic lens is introduced in the drift region for radial magnification control without affecting temporal stretching of the ion cloud. Time slices of ∼5 ns out of a cloud stretched to ⩾50 ns are used. An accelerator region with variable dimensions (using multiple electrodes) is employed for better optimization of radial and temporal space focusing characteristics at each magnification level. The implemented system was successfully tested by recording images of H fragments from the photodissociation of HBr, H2S, and the CH2OH radical, with kinetic energies ranging from <0.4 eV to >3 eV. It demonstrated KE resolution ≲1%–2%, similar to that obtained in traditional velocity map imaging followed by reconstruction, and to KE resolution achieved previously in SVMI of heavier products. We expect it to perform just as well up to at least 6 eV of kinetic energy. The tests showed that numerical simulations of the electric fields and ion trajectories in the system, used for optimization of the design and operating parameters, provide an accurate and reliable description of all aspects of system performance. This offers the advantage of selecting the best operating conditions in each measurement without the need for additional calibration experiments.
    The Journal of Chemical Physics 04/2013; 138(14). · 3.12 Impact Factor
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    ABSTRACT: We report a joint experimental-theoretical study of the predissociation dynamics of the water trimer following excitation of the hydrogen bonded OH-stretch fundamental. The bond dissociation energy (D0) for the (H2O)3→ H2O + (H2O)2 dissociation channel is determined from fitting the speed distributions of selected rovibrational states of the water monomer fragment using velocity map imaging. The experimental value, D0 = 2650 ± 150 cm(-1), is in good agreement with the previously determined theoretical value, 2726 ± 30 cm(-1), obtained using an ab initio full-dimensional potential energy surface (PES) together with Diffusion Monte Carlo calculations [Wang and Bowman, J. Chem. Phys., 2011, 135, 131101]. Comparing this value to D0 of the dimer places the contribution of non-pairwise additivity to the hydrogen bonding at 450-500 cm(-1). Quasiclassical trajectory (QCT) calculations using this PES help elucidate the reaction mechanism. The trajectories show that most often one hydrogen bond breaks first, followed by breaking and reforming of hydrogen bonds (often with different hydrogen bonds breaking) until, after many picoseconds, a water monomer is finally released. The translational energy distributions calculated by QCT for selected rotational levels of the monomer fragment agree with the experimental observations. The product translational and rotational energy distributions calculated by QCT agree also with statistical predictions. The availability of low-lying intermolecular vibrational levels in the dimer fragment is likely to facilitate energy transfer before dissociation occurs, leading to statistical-like product state distributions.
    The Journal of Physical Chemistry A 03/2013; · 2.77 Impact Factor
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    ABSTRACT: NASA's Deep Impact was a turning point in our measurements of comet properties. For the first time we obtained direct measurements of the density, thermal inertia of the surface, and, most importantly, the tensile strength of the upper layers. The very small tensile strength of only 1-10 kPa (like that of Talcum powder) tells us that comet Tempel 1 is a loose agglomerate of fluffy ice particles (Bar-Nun et al. 2007). In what follows, we describe how gases are trapped in fluffy ice particles, how they are released from them when the temperature is increased, either by overall heating or by pulsed infrared laser irradiation and finally, what happens when deeper layers release their trapped gases when the heat wave penetrates inward. In addition, it will be shown that laboratory measurements can now be carried out that address fundamental transport issues such as the release of trapped gases in such ice environments and their transport through thin and thick ice layers.
    The Science of Solar System Ices. 01/2013;
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    ABSTRACT: The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (D(0)) of (D(2)O)(2). The value obtained, 1244 ± 10 cm(-1) (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm(-1) (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H(2)O)(2). In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H(2)O)(2) and (D(2)O)(2) were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking. A minor channel is also observed in which both fragments are in the ground vibrational state and are highly rotationally excited. The theoretical calculations reveal equal probability of bending excitation in the donor and acceptor subunits, which is a result of interchange of donor and acceptor roles. The rotational distributions associated with the major channel, in which one water fragment has one quantum of bend, and the minor channel with both water fragments in the ground vibrational state are calculated and are in agreement with experiment.
    Journal of the American Chemical Society 08/2012; 134(37):15430-5. · 10.68 Impact Factor
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    ABSTRACT: The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.
    The Journal of Chemical Physics 02/2012; 136(8):084304. · 3.12 Impact Factor
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    M Ryazanov, C Rodrigo, H Reisler
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    ABSTRACT: The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ↔ CH(3)O (CD(2)OH ↔ CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).
    The Journal of Chemical Physics 02/2012; 136(8):084305. · 3.12 Impact Factor
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    ABSTRACT: Thin films composed of 400–500 monolayers (ML) of either amorphous solid water (ASW) or ASW/CO2 mixtures are grown atop a MgO(100) substrate under ultrahigh vacuum conditions. Samples are irradiated at an infrared frequency of 3424 cm–1, which lies within the broad OH stretch band of condensed water. Ablation is achieved using 10 ns pulses whose energy (<2.7 mJ) is focused to a beam waist of approximately 0.5 mm. By using a time-of-flight mass spectrometer to monitor ablated material, excellent single-shot detection is demonstrated. This capability is essential because, in general, the first infrared pulse can induce irreversible changes throughout the irradiated volume. With ASW/CO2 samples, CO2 is released preferentially. This is not surprising in light of the metastability of the samples. Indeed, repeated irradiation of the same spot can rid the sample of essentially all of the CO2 in as little as a few pulses, whereas only 10–20 ML of H2O are removed per pulse. The influence of the substrate is profound. It cools the sample efficiently because the characteristic time for heat transfer to the substrate is much less than the infrared pulse duration. This creates temperature gradients, thereby quenching processes such as explosive boiling (phase explosion) and the heterogeneous nucleation of cavities that take place at lower depths in significantly thicker samples, i.e., with sufficient inertial confinement. This efficient quenching accounts for the fact that only 10–20 ML of H2O are removed per pulse. The presence of small protonated water cluster ions in the mass spectra is interpreted as evidence for the trivial fragmentation mechanism examined assiduously by Lewis and co-workers. Mixed samples such as ASW/CO2, where species segregation plays a pivotal role, add interesting and potentially useful dimensions to the ablation phenomenon.
    The Journal of Physical Chemistry C. 12/2011; 116(1):563–569.
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    ABSTRACT: The bond dissociation energy (D(0)) of the water dimer is determined by using state-to-state vibrational predissociation measurements following excitation of the bound OH stretch fundamental of the donor unit of the dimer. Velocity map imaging and resonance-enhanced multiphoton ionization (REMPI) are used to determine pair-correlated product velocity and translational energy distributions. H(2)O fragments are detected in the ground vibrational (000) and the first excited bending (010) states by 2 + 1 REMPI via the C̃ (1)B(1) (000) ← X̃ (1)A(1) (000 and 010) transitions. The fragments' velocity and center-of-mass translational energy distributions are determined from images of selected rovibrational levels of H(2)O. An accurate value for D(0) is obtained by fitting both the structure in the images and the maximum velocity of the fragments. This value, D(0) = 1105 ± 10 cm(-1) (13.2 ± 0.12 kJ/mol), is in excellent agreement with the recent theoretical value of D(0) = 1103 ± 4 cm(-1) (13.2 ± 0.05 kJ∕mol) suggested as a benchmark by Shank et al. [J. Chem. Phys. 130, 144314 (2009)].
    The Journal of Chemical Physics 06/2011; 134(21):211101. · 3.12 Impact Factor
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    ABSTRACT: The state-to-state vibrational predissociation dynamics of the (H_2O)_2 dimer were studied by resonance-enhanced multiphoton ionization (REMPI) and velocity-map imaging (VMI) to obtain pair-correlated product energy distributions. The 2+1 REMPI spectra of the H_2O photofragments were recorded via the tilde{C}^1B_1 (000)
    06/2011;
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    ABSTRACT: The overtone-induced vibrational predissociation of the hydroxymethyl radical is achieved following excitation of the radical to the third O-H stretch overtone. The excited O-H stretch is also the bond that breaks; i.e. overtone excitation is in the reaction coordinate. The production of H atoms takes place via tunneling through the barrier to the H + formaldehyde channel. H-atom photofragment yield spectra in the region of the third overtone reveal two mixed bands with contributions from the third OH overtone and a combination band comprised of two quanta of OH stretch and one quantum of CH asymmetric stretch. Using velocity map imaging, sliced images of H-atom products are obtained with kinetic energy resolution sufficient to reveal the vibrational structure in the formaldehyde co-fragment. As expected, most of the formaldehyde molecules are born without vibrational excitation but some exhibit excitation in other modes, such as wagging and CO stretch. The rotational contours of the vibrational bands are well described by temperatures in the range 100-150 K. Slice imaging allows scanning the pump laser while monitoring H fragments in selected kinetic energy ranges, and in this way it is demonstrated that all the observed vibrational levels of formaldehyde have their parentage in the hydroxymethyl radical. The barrier to isomerization to methoxy is comparable to the barrier to direct dissociation and the role of isomerization is investigated by using partially deuterated radicals.
    05/2011; -1.
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    ABSTRACT: The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded HCl-H(2)O dimer was studied following excitation of the dimer's HCl stretch by detecting the H(2)O fragment. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, H(2)O fragments were detected by 2 + 1 REMPI via the C (1)B(1) (000) ← X (1)A(1) (000) transition. REMPI spectra clearly show H(2)O from dissociation produced in the ground vibrational state. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational states of H(2)O and were converted to rotational state distributions of the HCl cofragment. The distributions were consistent with the previously measured dissociation energy of D(0) = 1334 ± 10 cm(-1) and show a clear preference for rotational levels in the HCl fragment that minimize translational energy release. The usefulness of 2 + 1 REMPI detection of water fragments is discussed.
    The Journal of Physical Chemistry A 03/2011; 115(25):6903-9. · 2.77 Impact Factor
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    ABSTRACT: The 365 nm pulsed laser photolysis of nitrosyl chloride adsorbed on a rough MgO(100) surface at 90 K has been studied. Mass spectrometric detection was used to record time-of-flight (TOF) spectra by monitoring Cl+ and NO+. These ions can derive from parent ClNO, which fragments completely in the mass spectrometer, as well as from Cl and NO photofragments. The TOF distributions are considerably slower than for the corresponding gas phase photodissociation process. NO was also detected state selectively by using resonance enhanced multiphoton ionization (REMPI), and a channel corresponding to direct adsorbate photolysis was identified. The state selective detection of NO molecules that emerge from the surface following photolysis shows unambiguously that their rotational degrees of freedom reflect the surface temperature (Trot = 100−140 K), even at low coverages. At similar photolysis wavelengths, gas phase ClNO photodissociation is known to produce highly rotationally excited NO with a distinctive non-statistical distribution peaked at J″ = 46.5. Our studies suggest that, contrary to the gas-phase photolysis results, Cl and NO are not ejected rapidly following photolysis of surface-bound species on a repulsive potential energy surface. We postulate that ClNO grows in islands, with MgO defect sites serving as nucleation centers. Photofragment rotational and translational excitations are quenched efficiently due to strong attractive interactions that equilibrate NO to the surface temperature. Desorption of intact ClNO may also take place, but following (i.e., not during) the photolysis pulse. Such desorbed species can contribute to the TOF spectra, but not the REMPI spectra.
    Canadian Journal of Chemistry 02/2011; 72(3):737-744. · 0.96 Impact Factor
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    ABSTRACT: We report molecular dynamics simulations of the unimolecular dissociation of energetic C2H4OH radicals using a full-dimensional potential energy surface (PES) fit to density functional theory (DFT) and coupled cluster (CCSD) calculations. Quasiclassical trajectories are initiated on the surface using microcanonical sampling at a total energy of 85.0 kcal/mol (44.1 kcal/mol relative to the zero-point energy). The trajectories reveal a roaming channel leading to the formation of water. The transition state (TS) corresponding to direct water production is energetically inaccessible. However, the roaming pathway finds a lower-energy path via frustrated dissociation to hydroxyl that makes water + vinyl production feasible. The trajectory calculations suggest that the roaming pathway constitutes a minor (a few percent) channel of the overall C2H4OH dissociation. The mechanism of the roaming reaction is analyzed in terms of the geomteric proximity of the TSs leading to OH loss and internal abstraction as well as the partitioning of vibrational energy into different modes.Keywords (keywords): roaming mechanism; reaction dynamics; radical dissociation; potential energy surfaces; quasiclassical trajectories
    The Journal of Physical Chemistry Letters. 10/2010; 1(20).
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    ABSTRACT: The state-to-state vibrational predissociation dynamics of the hydrogen-bonded HCl-H(2)O dimer were studied following excitation of the HCl stretch of the dimer. Velocity-map imaging and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, HCl fragments were detected by 2 + 1 REMPI via the f (3)Delta(2) <-- X (1)Sigma(+) and V (1)Sigma(+) <-- X (1)Sigma(+) transitions. REMPI spectra clearly show HCl from dissociation produced in the ground vibrational state with J'' up to 11. The fragments' center-of-mass translational energy distributions were determined from images of selected rotational states of HCl and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well when using a dimer dissociation energy of D(0) = 1334 +/- 10 cm(-1). The rotational distributions in the water cofragment pair-correlated with specific rotational states of HCl appear nonstatistical when compared to predictions of the statistical phase space theory. A detailed analysis of pair-correlated state distributions was complicated by the large number of water rotational states available, but the data show that the water rotational populations increase with decreasing translational energy.
    The Journal of Physical Chemistry A 09/2010; 114(36):9774-81. · 2.77 Impact Factor
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    ABSTRACT: The unimolecular decomposition of expansion-cooled isocyanic acid (HNCO) via channels (1) 3NH + CO, (2) H + NCO, and (3) 1NH + CO [where 3NH and 1NH denote NH(X3σ−) and NH(a1 Δ), respectively] has been investigated following photoexcitation to the S1(1A ) state in two energy regimes: (i) in the region of the 1NH + CO threshold (41 700–45 500 cm−1; 240–220 nm), and (ii) ˜ 3200 cm−1 above D0(1NH + CO), at around 46000 cm−1 (217.6 nm). Several complementary experiments are presented: NCO, 3NH and 1NH photofragment yield spectra and relative 1NH/3NH branching ratios are obtained by laser induced fluorescence (LIF); photo-fragment ion imaging is used to record CO angular recoil distributions, and 1NH rotational distributions correlated with specific CO (v, J) levels, HNCO excited to S1 undergoes complex dynamics reflecting simultaneous decomposition on several potential energy surfaces, and including internal conversion (IC) and intersystem crossing (ISC). In energy region (i), a progressive loss of structure in the 3NH yield spectrum is observed above the opening of channel (3), and is interpreted as the imprint of short-time dynamics characteristic of the ISC step. State selectivity in the photodissociation is revealed by comparing the photofragment yield spectra of the three channels. In region (ii), product state distributions for channel (3) exhibit clear dynamical signatures, as expected for dissociation on S1. At low excess energies channel (2) derives from dissociation on S0, but the respective roles of S0 and S1 at higher energies are not well established yet. The results are discussed in terms of vibronic levels of mixed electronic character coupled directly or via radiationless decay to the various continua. The competition between the different processes depends sensitively on photolysis energy and excitation conditions.
    Berichte der Bunsengesellschaft für physikalische Chemie. 06/2010; 101(3):469 - 477.

Publication Stats

2k Citations
439.73 Total Impact Points

Institutions

  • 1978–2013
    • University of Southern California
      • Department of Chemistry
      Los Angeles, CA, United States
  • 2010
    • University of Sussex
      • Department of Chemistry
      Brighton, ENG, United Kingdom
  • 1979–2010
    • University of California, Los Angeles
      • Department of Chemistry and Biochemistry
      Los Angeles, CA, United States
  • 2007
    • Hebrew University of Jerusalem
      Yerushalayim, Jerusalem District, Israel