Product State Distributions Research Articles

Laser fluorescence study of the S( 1D) + CD 4 reaction: determination of the SD product internal state distribution

The dynamics of the S( 1D) + CD 4 → SD + CD 3 reaction has been studied in a photolysis-probe experiment. S( 1D) reagent was prepared by laser photolysis of CS 2, and the SD(X 2Π) product was detected by laser fluorescence excitation. The nascent rotational/fine-structure state distribution of the SD(X 2Π) product was determined. The experimentally determined rotational state distribution within each fine-structure manifold agrees well with the predictions of a statistical ‘prior’ distribution, while the F 1 to F 2 fine-structure branching ratio is higher than predicted statistically. The product state distribution is consistent with reaction through the formation and decay of an energized CD 3SD complex.

The effects of surface temperature on the gas-liquid interfacial reaction dynamics of O(3P)+squalane

OH/OD product state distributions arising from the reaction of gas-phase O(3P) atoms at the surface of the liquid hydrocarbon squalane C30H62/C30D62 have been measured. The O(3P) atoms were generated by 355 nm laser photolysis of NO2 at a low pressure above the continually refreshed liquid. It has been shown unambiguously that the hydroxyl radicals detected by laser-induced fluorescence originate from the squalane surface. The gas-phase OH/OD rotational populations are found to be partially sensitive to the liquid temperature, but do not adapt to it completely. In addition, rotational temperatures for OH/OD(v'=1) are consistently colder (by 34+/-5 K) than those for OH/OD(v'=0). This is reminiscent of, but less pronounced than, a similar effect in the well-studied homogeneous gas-phase reaction of O(3P) with smaller hydrocarbons. We conclude that the rotational distributions are composed of two different components. One originates from a direct abstraction mechanism with product characteristics similar to those in the gas phase. The other is a trapping-desorption process yielding a thermal, Boltzmann-like distribution close to the surface temperature. This conclusion is consistent with that reached previously from independent measurements of OH product velocity distributions in complementary molecular-beam scattering experiments. It is further supported by the temporal profiles of OH/OD laser-induced fluorescence signals as a function of distance from the surface observed in the current experiments. The vibrational branching ratios for (v'=1)/(v'=0) for OH and OD have been found to be (0.07+/-0.02) and (0.30+/-0.10), respectively. The detection of vibrationally excited hydroxyl radicals suggests that secondary and/or tertiary hydrogen atoms may be accessible to the attacking oxygen atoms.

Product State Distributions of Vibrationally Excited CO(v) for the CH(X2Π) and CH(A2Δ) Channels of the C2H + O(3P) Reaction

The C2H + O(3P) reaction is investigated using time-resolved Fourier transform infrared spectroscopy (TR-FTIR). The C2H and O radicals are produced by 193-nm photolysis of C2H2 and SO2 precursors. Multiple vibrationally excited products are observed from several resulting reaction processes, including products: CO, CO2, C4H2, and CH. For this choice of photolytic precursors, it is observed that the C2H + SO2, C2H2 + O, and the HCCO + O reactions contribute to the observed product signals. To evaluate the contribution of the C2H + SO2 reaction to the removal kinetics of C2H in the C2H2/SO2 system, the room-temperature rate constant for the C2H + SO2 reaction is experimentally determined to be k = (1.1 ± 0.3) × 10-11 cm3 molecule-1 s-1. The time dependencies of the CO and CO2 species are measured experimentally and simulated to determine conditions under which contributions from the several processes that give the same product can be differentiated. Analysis of the nascent vibrational distribution of the CO products from C2H + O suggests the participation of both CH(A2Δ) and CH(X2Π) products, in the ratio of ∼3:2. The surprisal parameters for the vibrational distribution of the CO products of these two channels are found to be −1.5 ± 0.2 and −1.1 ± 0.2, respectively. It is suggested that the reaction proceeds through the HCCO⧧ intermediate, in agreement with earlier studies, but the CH(A2Δ)/CH(X2Π) branching fraction may be larger than previously reported.

Rovibrational product state distribution for inelastic H+D2 collisions.

Experimental measurements of rovibrational product state distributions for the inelastic scattering process H + D2(nu=0,j)-->H + D2(nu' = 1,2,j') are presented and compared with the results of quasiclassical and quantum mechanical calculations. Agreement between theory and experiment is almost quantitative. Two subtle trends are found: the relative amount of energy in product rotational excitation decreases slightly with increasing collision energy and increases slightly with increasing product vibrational excitation. These trends are the reverse of what has been found for reactive scattering in which the opposite trends are much more pronounced.

Rotational State Distributions of I2(B) from Vibrational Predissociation of I2(B)-Ne

The vibrational predissociation of triatomic, i.e., atom-diatom, van der Waals complexes in transient electronic excited state has been widely investigated. The predissociation rates or lifetimes are major concerns of the previous studies. Experimentally rotational state distributions of diatomic product are hardly investigated and few theoretical stuides on rotational state distributions have appeared in literature. In this work, choosing the frequently studied <TEX>$I_2(B)-Ne$</TEX> complex as an example, we investigate the change of rotational state distributions of <TEX>$I_2(B)-Ne$</TEX> produced from predissociation of the various initial states of <TEX>$I_2(B)-Ne$</TEX>. The present study on the rotational distributions indicates that rotational state distributions depend significantly on the predissociation energy and the van der Waals vibrational modes of <TEX>$I_2(B)-Ne$</TEX>. That is, the initial state dependency of rotational state distributions is extensively discussed.

Effect of couplings in the resonance continuum

Electronic coupling of two or more resonances via the electron scattering continuum is investigated. The effect of this coupling as a function of the resonance curves and autoionization widths is investigated, and the conditions for the maximum effect are determined. The theory is applied to two physical problems, the product state distribution produced by the dissociative recombination of electrons with HeH+ and a one-dimensional model for ion-pair production resulting from electron collisions with H+3. It is found that the coupling does not affect the product state distribution in HeH+ but produces a significant effect in the H+3 model.

State-to-state rate constants for rotational relaxation of CO in collisions with Ar: a quantum study

Integral cross-sections and thermal rate constants for the CO( v=0, j) + Ar collisions are calculated within the coupled states method on two recent ab initio potential energy surfaces. The vibrationally elastic approximation is shown to provide an excellent agreement with three-dimensional quantum results whose sensitivity to the interaction potential is also examined. It is demonstrated that at j=0, both the energy corrected sudden scaling procedure and semiclassical method are unable to reproduce oscillations in the rotational product state distributions which is in accordance with recent benchmark close coupling calculations.

Vibrational predissociation in the HCl dimer.

We present results of a combined theoretical and experimental study on the vibrational predissociation of the HCl dimer. On the theoretical side, photodissociation linewidths and product-state distributions for monomer stretch excited states with total angular momentum J=0 were computed, using the Fermi golden rule approximation. The resonances investigated include excitation of the hydrogen bond donor and acceptor stretches, as well as combinations of one of these modes with the intermolecular stretch and geared bend modes, for both even and odd permutation symmetry. Line strengths for the transitions from the J=1, K=0 ground state to excited states with J=0 were computed using quasibound states. On the experimental side, the photofragment angular distribution method was employed to obtain complete final-state distributions for the monomer stretch excited states. Three different transitions were probed, all starting from the lower tunneling component of the ground state: the (R)Q(0)(1) transition for excitation of the acceptor stretch and the (Q)R(0)(0) transition and unresolved (R)Q(0) branch for the donor stretch excitation. We find that, in contrast to the HF dimer, the excited-state alignment of the HCl dimer, resulting from excitation using a polarized laser beam, is completely lost on the time scale of the dissociation. The agreement between theory and experiment for the product-state distributions and line strengths is reasonable. The computed lifetimes are 1-2 orders of magnitude too small, which is attributed to a deficiency in the potential energy surface.

Quantum State Resolved Studies of Gas/Surface Reaction Dynamics

Laser light is an invaluable tool for detailed experimental studies of chemical reactions. The ability to prepare reagent molecules in specific quantum states and to resolve the quantum state distribution of reaction products using laser radiation has been key to the field of chemical reaction dynamics, which studies chemistry at a fundamental microscopic level. Today, state-resolved experiments are common for unimolecular and bi-molecular reactions in the gas phase, but equivalent measurements for gas/surface reactions have only recently started to appear. This contribution reviews the methodology developed in our group at the EPFL for the quantum state resolved investigation of molecule/surface reactions and gives representative results on the dissociative chemisorption of methane on single crystal nickel surfaces.

Exit interaction effect on nascent product state distribution of O(1D)+N2O-->NO+NO.

We have determined the rotational state distributions of NO(v'=0,1,2) products produced from the reaction O(1D)+N2O. This is the first full characterization of the product rotational distribution of this reaction. The main part of each rotational distribution (up to j' approximately 80) has rotational temperature approximately 20,000 K and all these distributions are quite near to those predicted by the phase space theory (PST). This observation and previously reported vibrational distribution indicate that the most part of the energy partitioning of the reaction products is at least apparently statistical although the intermediate of this reaction is not so stable as to ensure the long lifetime. On the other hand, the distributions in the high rotational levels (j'=80-100) are found to decrease more sharply as j' increases than the PST predictions. The origin of the observed decrease of the distribution is discussed with quasiclassical trajectory (QCT) calculations on a five-dimensional ab initio potential energy surface (PES). The observed near-statistical distribution and the sharp decrease in the high-j' levels are well reproduced by a "half-collision" QCT calculation, where statistical distribution at the reaction intermediate is assumed. This agreement shows the rotation-translation interaction in the exit region has an effect of yielding small high-j' populations. However, a little bias of the calculated distribution toward lower rotational excitation than the observed one indicates that the combination of the statistical intermediate and the exit interaction on the current PES does not completely describe the real system. It is suggested that the reaction intermediate is generated with the distribution which is close to statistical but a little biased toward yielding high-j' products, and that the interaction in the exit region of the PES results in the sharp decrease in the high-j' levels.

Ultrasound-assisted hydrogenation of cinnamaldehyde

The hydrogenation, employing hydrogen gas, of cinnamaldehyde was performed using Pd-black and Raney Ni catalysts at 298 ± 3 K in a water-cooled (jacketed) reaction vessel. Sampling at pre-determined time intervals and GC/MS analysis yielded time-dependent product state distribution information. A kinetic modeling of the data revealed that cinnamaldehyde was both hydrogenated directly to the final product benzenepropanol, as well as a fraction being converted to the intermediate benzenepropanal, where the latter was subsequently hydrogenated to benzenepropanol. Comparing the ultrasound-assisted and blank (stirred) experiments revealed that a higher maximum relative concentration of the intermediate benzenepropanal was formed in the ultrasound experiments compared to the stirred experiment. The activity of the ultrasound experiments compared to blank were 9-fold and 20-fold greater for the Pd-black and Raney Ni catalysts, respectively. Finally, an application of the Bell–Evans–Polanyi principle to yield an estimate of the ratio of rate coefficients for benzenepropanal and benzenepropanol formation was performed by considering chemical group energy differences and surface adsorption energy differences in the first mechanistic step of hydrogenation.

Collision energy dependence of the HD(nu' = 2) product rotational distribution of the H + D2 reaction in the range 1.30-1.89 eV.

An experimental and theoretical investigation of the collision energy dependence of the HD(nu' = 2,j') rotational product state distribution for the H + D2 reaction in the collision energy range of Ecol = 1.30-1.89 eV has been carried out. Theoretical results based on time-dependent and time-independent quantum mechanical methods agree nearly perfectly with each other, and the agreement with the experiment is good at low collision energies and very good at high collision energies. This behavior is in marked contrast to a previous report on the HD(nu' = 3,j') product state rotational distribution [Pomerantz et al., J. Chem. Phys. 120, 3244 (2004)] where a systematic difference between experiment and theory was observed, especially at the highest collision energies. The reason for this different behavior is not yet understood. In addition, this study employs Doppler-free spectroscopy to resolve an ambiguity in the E, F-X resonantly enhanced multiphoton ionization transition originating from the HD(nu' = 2,j' = 1) state, which is found to be caused by an accidental blending with the transition coming from the HD(nu' = 1,j' = 14) state.

Laser control of product electronic state: desorption from alkali halides.

We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin-orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P(1/2)), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.

Collision-induced non-adiabatic transitions between the ion-pair states of molecular iodine: A challenge for experiment and theory

The ion-pair states of molecular iodine provide a unique system for studying the efficiency, selectivity, and mechanisms of collision-induced non-adiabatic transitions. Non-adiabatic transitions between the first-tier ion-pair states in collisions with molecular partners and rare gases are analyzed and discussed. The qualitative features of the rate constants and product state distributions under single collision conditions are summarized and interpreted in terms of appropriate theoretical approaches. Two mechanisms for the non-adiabatic transitions are clearly identified. The first, operative for collisions involving molecular partners possessing permanent or transition electrostatic moments, is highly selective. It connects the initially prepared level in the E 0+g electronic state with the near-resonant vibronic level of the D 0+u state with a minimum change of the total angular momentum. In an extreme quasi-resonant case when the gap between initial and final rovibronic level is less than 1 cm−1, this mechanism has a giant cross section, 40 times that of a gas kinetic collision. An electrostatic model, which includes the coupling of the giant E–D transition dipole moment with a moment of the colliding partner and the semiclassical Born approximation, provides a plausible interpretation of this mechanism. A second mechanism is shown to govern collisions with rare gas atoms. It results in population of several ion-pair states and broad distributions over rovibronic levels. This mechanism is successfully interpreted by quantum scattering calculations based on the diatomics-in-molecule diabatic potential energy surfaces and coupling matrix elements. The calculations provide good agreement with experimental measurements and reveal different mechanisms for the population of different electronic states. Unexplained features of the non-adiabatic dynamics and directions of future work are outlined.

3 Optical studies of ion–molecule reactions

This review outlines the advances that have been made in recent years in the study of elementary gas-phase processes involving molecular ions by employing optical techniques and spectroscopic methodologies. Two particularly fruitful areas of experimental study have been the analysis of VIS/UV emissions from ion–molecule reactions to determine product state distributions and experiments in which resonance enhanced multiphoton ionisation (REMPI) is used to study the effect of ion reagent vibrational energy on reactivity. Studies in which product state distributions in ion–molecule reactions are determined by either analysis of IR emissions or laser-induced fluorescence (LIF) remain rare.

Photodissociation dynamics of the Kr-HBr cluster: the effect of the rare gas atom substitution.

The ultraviolet photolysis dynamics of Kr-HBr(v=0) is investigated by means of wave packet calculations, focusing on the fragmentation pathway Kr-HBr+ variant Planck's over 2pi omega-->H+Kr-Br. Photolysis is simulated by starting from two different cluster initial states, namely the ground van der Waals (vdW) and an excited vdW bending state, associated with the Kr-H-Br and Kr-Br-H isomers, respectively. The results show that, for the two initial states of the cluster, the Kr-Br product yield is lower than that of Ar-Br radicals found in previous studies on Ar-HBr photolysis. Despite this decrease, the Kr-Br yield is found to be still rather high, in particular for the initial excited vdW state of Kr-HBr(v=0). In addition, the Kr-Br product state distributions exhibit a remarkably higher excitation (mainly rotational) than the corresponding Ar-Br distributions. The lower yield and higher excitation of Kr-Br as compared to Ar-Br, are attributed to a larger share of the energy available for the radical going to internal excitation in the case of the Kr-Br product. The different partition of the energy available for Kr-Br also causes significant deviations in the photolysis behavior of Kr-HBr when compared to that of Ar-HBr, in the case of the initial excited vdW state of both clusters. A common feature of the photodissociation of Kr-HBr and Ar-HBr is the manifestation of quantum interference effects in the Kr-Br and Ar-Br rotational state distributions, in the form of pronounced structures of supernumerary rotational rainbows.

On product state distributions in triatomic unimolecular reactions: IV. Selfconsistency of the statisticodynamical approach

In a recent series of papers, we built up the so-called ‘statisticodynamical approach’ (SDA), a dynamically corrected transition state theory of rotational state distributions for elementary unimolecular dissociations governed by short-range forces. The dynamical correction was semi-empirical, requiring in part the use of the quasi-classical trajectory (QCT) method. The goal of the present note is the development of an iterative analytical procedure corroborating the QCT predictions and strengthening the selfconsistency of SDA.

Dynamics of the gas–liquid interfacial reaction of O(3P) atoms with hydrocarbons

We describe an experimental approach to the determination of the nascent internal state distribution of gas-phase products of a gas–liquid interfacial reaction. The system chosen for study is O(3P) atoms with the surface of liquid deuterated squalane, a partially branched long-chain saturated hydrocarbon, C30D62. The nascent OD products are detected by laser-induced fluorescence. Both OD (v′=0) and (v′=1) were observed in significant yield. The rotational distributions in both vibrational levels are essentially the same, and are characteristic of a Boltzmann distribution at a temperature close to that of the liquid surface. This contrasts with the distributions in the corresponding homogeneous gas-phase reactions. We propose a preliminary interpretation in terms of a dominant trapping-desorption mechanism, in which the OD molecules are retained at the surface sufficiently long to cause rotational equilibration but not complete vibrational relaxation. The significant yield of vibrationally excited OD also suggests that the surface is not composed entirely of –CD3 endgroups, but that secondary and/or tertiary units along the backbone are exposed.

A comparison between conventional and ultrasound-mediated heterogeneous catalysis: hydrogenation of 3-buten-1-ol aqueous solutions

A power flow scheme applicable to probe-type ultrasound reactors is presented, that has been deduced from both theoretical estimates and experimental measurements employing a thermal insulated vessel. Under typical conditions for water at 1 atm pressure, 77% of the electrical power is converted into mechanical motion of the probe, that in turn is dissipated to both acoustic power (∼12%) and cavitational heating (∼88%). Approximately 92% of the mechanical power of the probe was converted into heat, with the remaining power presumably converted into audible acoustic and/or mechanical motion. In a second type of experiment performed here, heterogeneous catalysis experiments have been performed at 298 K in an isothermal (i.e., jacketed) reaction vessel comparing chemistry in conventional (e.g., thermal) versus ultrasound-assisted systems. Both product state distribution and reaction rate measurements have been performed for the hydrogenation (using hydrogen gas) of aqueous 3-buten-1-ol solutions employing Pd-black powder. Products from the heterogeneous catalysis include isomerization to cis- and trans-2-buten-1-ol, as well as hydrogenation to 1-butanol. A reaction scheme involving surface-bound alkyl-radical species, consistent with previous published work, is proposed to explain product formation. Based on the observed differences in cis- to trans-2-buten-1-ol ratios in conventional versus ultrasound experiments, employing untreated and prereduced catalysts, it has been determined that ultrasound creates catalyst site(s) enhancing the cis-to- trans 2-buten-1-ol ratio from 0.25 to 0.55. In addition, comparing the total isomerization to hydrogenation ratio ( cis- plus trans-2-buten-1-ol to 1-butanol ratio), for ultrasound-assisted and conventional catalysis, reveal a ∼5-fold enhancement in isomerization relative to the more energetically favored hydrogenation due to the application of ultrasound. Finally, the product formation rates for 1-butanol, as well as isomerization plus hydrogenation, revealed that conventional and ultrasound experiments showed both a nonlinear dependence with applied ultrasound power and no differences between untreated and prereduced catalysts. The observed reaction rate enhancements were 1:36:183 for the conventional, 90 W ultrasound, and 190 W ultrasound experiments, respectively.

Exit channel dynamics in the ultraviolet photodissociation of the NO dimer: (NO)2→NO(A 2Σ+)+NO(X 2Π)

The correlated angular and product rotational state distributions obtained in the 221.67 nm photodissociation of (NO)2 yielding NO(A 2Σ+)+NO(X 2Π) have been examined in the molecular beam using the velocity map ion imaging technique. The translational energy and angular distributions of selected rotational states of NO(A 2Σ+) products in N=0, 5, 6 for which the maximum energies available to the NO(X 2Π) products are 202.5, 142.5, and 118.5 cm−1, respectively, have been measured. The recoil anisotropy parameter of the photofragments, βeff, is 1.2±0.1, less than that previously measured at 213 nm (1.36±0.05). The correlated product state distributions near dissociation threshold agree with the predictions of phase space theory. These experimental results, as well as those obtained previously at 213 nm, are compared to statistical calculations, including v⋅J correlations. Application of the β-ET correlation model to the 213 nm results indicates that [NO(A,N),NO(X,J)] pairs with high NO(X,J) rotational levels are produced preferentially via planar dissociation, in contrast to the statistical expectation of the v⋅J correlation, which reveals no preference for planar dissociation. A mechanism involving vibrational predissociation with restricted intramolecular vibrational energy redistribution can explain both the observed scalar and vector properties. Specifically, the low frequency torsional (out-of-plane) mode does not couple efficiently to the other modes, especially at higher excess energies when the dissociation is rapid. On the other hand, the long-range attraction between NO(A) and NO(X), which is revealed both in the photodissociation dynamics of the dimer and in the quenching of NO(A) by NO(X), encourages long-range mode couplings and can explain the largely statistical rotational state distributions observed near threshold. From images obtained near threshold, the bond energy of the NO dimer in the ground state is determined to be 710±10 cm−1, in good agreement with previous results.