State Distributions Of CO Research Articles

Photodissociation dynamics of acetaldehyde at 267 nm: A computational study of the CO‐forming channels

AbstractA recent experiment on the 267 nm photolysis of gaseous acetaldehyde reveals that the molecular products (CO + CH4) present product state distributions with computation –experiment discrepancy. This work aims to provide solution to such issue. The direct dynamics simulation of classical trajectories and the generalized multi‐center impulsive model (GMCIM) are both utilized to predict the product state distributions of CO and CH4 from 267 nm photolysis of acetaldehyde. Comparison with the experimental observation provides an assessment of the conventional approach of minimum energy path within the framework of three‐center transition state (TS) and CH3‐roaming pathways. The result shows that the high‐speed products observed in experiment are likely formed via the three‐center TS pathway. The reasons that lead to the discrepancy between computation and experiments in previous studies are also investigated. A better agreement between computational and state‐resolved experimental results can be achieved by selecting specific internal states of CO product from the computational data.

Imaging Isocyanic Acid Photodissociation at 193 nm: the NH(a1Δ)+CO(X1Σ+) Channel

The photodissociation dynamics of isocyanic acid (HNCO) has been studied by the time-sliced velocity map ion imaging technique at 193 nm. The NH(a1Δ) products were measured via (2+1) resonance enhanced multiphoton ionization. Images have been accumulated for the NH(a1Δ) rotational states in the ground and vibrational excited state (v=0 and 1). The center-of-mass translational energy distribution derived from the NH(a1Δ) images implies that the CO vibrational distributions are inverted for most of the measured 1NH(v|j) internal states. The anisotropic product angular distribution observed indicates a rapid dissociation process for the N−C bond cleavage. A bimodal rotational state distribution of CO(v) has been observed, this result implies that isocyanic acid dissociates in the S1 state in two different pathways.

Two HCl-Elimination Channels and Two CO-Formation Channels Detected with Time-Resolved Infrared Emission upon Photolysis of Acryloyl Chloride [CH2CHC(O)Cl] at 193 nm

Following photodissociation of gaseous acryloyl chloride, CH2CHC(O)Cl, at 193 nm, temporally resolved vibration-rotational emission spectra of HCl (v ≤ 7, J ≤ 35) in region 2350-3250 cm(-1) and of CO (v ≤ 4, J ≤ 67) in region 1865-2300 cm(-1) were recorded with a step-scan Fourier-transform spectrometer. The HCl emission shows a minor low-J component for v ≤ 4 with average rotational energy Erot = 9 ± 3 kJ mol(-1) and vibrational energy Evib = 28 ± 7 kJ mol(-1) and a major high-J component for v ≤ 7 with average rotational energy Erot = 36 ± 6 kJ mol(-1) and vibrational energy Evib = 49 ± 9 kJ mol(-1); the branching ratio of these two channels is ∼0.2:0.8. Using electronic structure calculations to characterize the transition states and each intrinsic reaction coordinate, we find that the minor pathway corresponds to the four-center HCl-elimination of CH2ClCHCO following a 1,3-Cl-shift of CH2CHC(O)Cl, whereas the major pathway corresponds to the direct four-center HCl-elimination of CH2CHC(O)Cl. Although several channels are expected for CO produced from the secondary dissociation of C2H3CO and H2C═C═C═O, each produced from two possible dissociation channels of CH2CHC(O)Cl, the CO emission shows a near-Boltzmann rotational distribution with average rotational energy Erot = 21 ± 4 kJ mol(-1) and average vibrational energy Evib = 10 ± 4 kJ mol(-1). Consideration of the branching fractions suggests that the CO observed with greater vibrational excitation might result from secondary decomposition of H2C═C═C═O that was produced via the minor low-J HCl-elimination channel, while the internal state distributions of CO produced from the other three channels are indistinguishable. We also introduce a method for choosing the correct point along the intrinsic reaction coordinate for a roaming HCl elimination channel to generate a Franck-Condon prediction for the HCl vibrational energy.

Resonance-state selective photodissociation of OCS (2 1Σ+): Rotational and vibrational distributions of CO fragments

The rotational and vibrational state distributions of the CO fragments produced through the photodissociation of OCS in the vacuum ultraviolet (VUV) region (150–155 nm), OCS (2 1Σ+)→CO (X 1Σ+)+S(1S), are derived for the three lowest quasi-bound vibrational resonances (v*=0−2) in the 2 1Σ+ state. The rotational state distributions of the CO fragments in the vCO=0 and 1 vibrational states are determined, respectively, by the analysis of the rotational structures in the laser-induced fluorescence (LIF) spectra of the A1Π–X 1Σ+(0,0) and (1,1) transitions of CO. The rotational temperatures of CO in the vCO=0 state are low (∼100 K) for all the three resonances, while those in the vCO=1 state are substantially higher, i.e., 2210, 940, and 810 K for v*=0, 1, and 2, respectively. The vibrational state distributions of CO are derived from the Doppler spectroscopy of the counterpart S(1S) fragments. From the analysis of the observed Doppler profiles, it is found for all the three lowest vibrational resonances of OCS that the vibrational distributions are represented well by the Boltzmann-type distribution with a vibrational temperature of around 7000 K. On the basis of these new findings, the energy partitioning in the photodissociation process through these three vibrational resonances in the 2 1Σ+ state is discussed.

Photochemistry of phosgene in the solid phase: State-resolved dissociation dynamics

The translational, rotational, and vibrational state distributions of CO (g) resulting from the single photon photodissociation of Cl2CO in the condensed phase at ∼90 K have been determined by time-of-flight (TOF) distribution measurement and resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The TOF distribution of CO (g) is bimodal. Internal state characterization of the slow channel reveals a completely thermalized origin, with a rotational temperature of Trot=88±5 K, which is equal to the translational temperature as well as the substrate temperature. We believe these slow CO molecules originate from photodissociation below the topmost surface of the molecular film and achieve thermal equilibrium with the substrate before escaping into the gas phase. Internal state characterization of the fast channel shows, on the other hand, an energetic origin: at hν=5.0 eV, the rotational distribution, with an overall flux-weighted mean rotational energy of 〈Erot〉=0.12±0.01 eV, is non-Boltzmann and can be approximated by a bimodal distribution with rotational temperatures of 210±40 K at low J″(s) and 2200±300 K at high J″(s); the relative vibrational population is Nν=1/Nν=0=0.33±0.05. Both rotational and translational distributions of fast CO show positive correlation with photon energy. These CO molecules must be promptly ejected into the gas phase, carrying nascent energetic information from the photodissociation reaction on the surface of the molecular film. For electronic excitation events that result in photodissociation, 74% of the excess excitation energy is distributed in the translational and internal motions of products (CO and Cl); only 26% of the available energy is converted to motions of surrounding molecules.

Dynamics of CO formation in the reaction O(3P)+C2H3

The internal state distributions of CO( ν , J ) formed in the reaction of oxygen atoms with vinyl radicals were measured using an arrested relaxation FTIR apparatus. In spite of the deep potential energy minimum corresponding to vinoxy radical, the expected intermediate in the reaction, the CO is born with a strongly non-statistical vibrational distribution. Ab initio calculations on the important stationary points of the C 2 H 3 O potential energy hypersurface provide some insight into the reaction dynamics. In particular, the barrier for H-atom migration in the process: vinoxy → acetyl, is calculated to be 42 kcal mol −1 above the vinoxy potential minimum; this lies some 80 kcal mol −1 below the reagent energy. Since this is the only substantial barrier on the reactive potential surface, the dynamics of the reaction are better described as following a direct, rather than a complex-forming mechanism.

The unimolecular dissociation of HCO: I. Oscillations of pure CO stretching resonance widths

The unimolecular dissociation of the formyl radical HCO in the electronic ground state is investigated using a completely new ab initio potential energy surface. The dynamics calculations are performed in the time-independent picture by employing a variant of the log-derivative Kohn variational principle. The full resonance spectrum up to energies more than 2 eV above the vibrational ground state is explored. The three fundamental frequencies (in cm−1) for the H–CO and CO stretches, and the bending mode are 2446 (2435), 1844 (1868), and 1081 (1087), where the numbers in parentheses are the measured values of Sappey and Crosley obtained from dispersed fluorescence excitation spectra [J. Chem. Phys. 93, 7601 (1990)]. In the present work we primarily emphasize the dissociation of the pure CO stretching resonances (0v20) and their decay mechanisms. The excitation energies, dissociation rates, and final vibrational–rotational state distributions of CO agree well with recent experimental data obtained from stimulated emission pumping. Similarities with and differences from previous time-independent and time-dependent calculations employing the widely used Bowman–Bittman–Harding potential energy surface are also discussed. Most intriguing are the pronounced oscillations of the dissociation rates for vibrational states v2≥7 which are discussed in the framework of internal vibrational energy redistribution.

Investigation of Acetyl Chloride Photodissociation by Photofragment Imaging

The photofragment ion imaging technique is used to measure the velocity distributions of atomic chlorine, methyl, and carbon monoxide fragments generated in the photodissociation of acetyl chloride at 236 nm. The fragments are selectively ionized by (2 + 1) multiphoton ionization and projected onto a two-dimensional position-sensitive detector to obtain speed and angular distributions. The Cl images display anisotropic angular distributions, characteristic of a prompt, impulsive dissociation of the C-Cl bond. A fraction of the CH[sub 3]CO, produced as a primary photoproduct, subsequently decomposes to form CH[sub 3] and CO. The CH[sub 3] and CO images are isotropic, suggesting that rotation of the acetyl radical intermediate occurs prior to the secondary dissociation. The internal state distribution of CO is probed using (2 + 1) multiphoton ionization via the B[sup 1][Sigma][sup +] state near 230 nm. The rotational state distribution of CO extends to J[double prime] = 30, while no vibrational excitation is observed. The transition state structure of the CH[sub 3]CO intermediate, leading to dissociation into CH[sub 3] and CO, is computed via ab initio quantum mechanical methods. 42 refs., 7 figs., 4 tabs.

The Renner-Teller-induced predissociation of HCO(Ã 2A′): Wavepacket calculations using new ab initio potential energy surfaces

We present first results for the Renner-Teller-induced predissociation of HCO(à 2A′) using new ab initio potential energy surfaces for the à (2A′) and the X̃(2A') states. The quantum mechanical dynamics calculations are performed in the time-dependent representation with the CO bond distance being fixed. The linewidths and final rotational state distribution of CO following the decay of various highly excited bending states of HCO (à 2A′) agree with recent experimental data. The dissociation mechanism is discussed in terms of the topology of the X̃ (2A')-state potential energy surface.

The interaction of CO with Ni(111): Rainbows and rotational trapping

Angularly resolved rotational state distributions of CO scattered and desorbed from a clean, single-crystal Ni(111) surface were measured using (2+1) resonance-enhanced multiphoton ionization. Molecules scattered from the surface displayed highly non-Boltzmann rotational distributions that varied with incident translational energy and detection angle, but not with surface temperature. A rotational rainbow was seen in the scattering distribution and interpreted as arising from the interaction of the weakly attractive O end of the CO molecule with the Ni(111) surface. Up to total rotational-to-translational energy conversion was seen at incident translational energies of 0.26–0.45 eV. This energetic cutoff was the result of rotational trapping and was caused by the strongly attractive interaction of the C end of the molecule with the surface. The rotational state distributions of molecules desorbed from the Ni(111) surface were well fit by Boltzmann distributions each with a temperature which is 0.82±0.08 of the surface temperature.

Internal state distributions of CO from HNCO photodissociation

The internal state distributions of CO produced by photodissociation of HNCO at 1930 (230.1 nm) and at 10 200 cm−1 (193.3 nm) in excess of the dissociation energy are determined from multiphoton ionization spectra of the CO fragment measured under collision-free conditions. The rotational state distribution of the CO produced at the lower photolysis energy is characterized by a temperature of (491±23) K. The rotational state distribution of CO produced by photodissociation at the higher photon energy in not well characterized by a temperature. This latter distribution has maximum population near J=37, extends beyond J=65, and accounts for ∼20% of the available energy in excess of that necessary to rupture the HN–CO bond. An impulsive dissociation model assuming that dissociation occurs from an excited HNCO complex containing a nonlinear N–C–O configuration accounts for the average CO rotational excitation while phase-space theory does not agree with the observed distributions. Fitting a semiclassical model to the data using a logically constructed potential energy surface and a ground-state-dependent function provides a useful parametrization for the impulsive dissociation. Although not absolute, this analysis suggests that the dissociation occurs directly on a repulsive excited state potential energy surface.

Rotational state distribution of CO photofragments from triplet ketene

The nascent rotational state distribution of CO(v″=0,J″) following excimer laser photolysis of ketene at 351 nm has been determined under collisionless conditions in a flow cell. At this low excitation energy dissociation can only take place on the triplet potential surface leading to CH2(X̃ 3B1) and CO(X̃ 1Σ+). The available energy permits only the vibrational ground state of CO to be populated. The observed rotational distribution of CO(v″=0,J″) deviates drastically from a phase space theory statistical distribution as well as from a thermal one. A Boltzmann plot of this distribution exhibits a population inversion for J″<13. The nonstatistical behavior is attributed to a barrier along the dissociation path. The fragments are repelled too rapidly for energy to be randomized between them. Thus the photofragmentation dynamics of triplet ketene contrasts markedly with dissociation on the singlet surface which has no barrier and gives a statistical CO rotational state distribution. An impulsive model calculation for the ab initio transition state geometry is in surprisingly good agreement with the experimental energy partitioning among the fragment degrees of freedom. This suggests that the CCO bond angle is strongly bent at the top of the barrier and that the barrier height is a substantial fraction of available energy.

Photodissociation studies of isocyanic acid: heat of formation and product branching ratios

The heat of formation (..delta..H/sub f/(298 K)) of HNCO is determined to be -24.9/sub -2.8//sup +0.7/ kcal/mol (based on ..delta..H/sub f/(NH) = 85.2 kcal/mol). This value is obtained by measuring the threshold for the production of NH(a/sup 1/..delta..) and by determining the energy contents of the NH fragment and the CO cofragment produced by photolysis of HNCO at wavelengths near the threshold. Saturated laser-induced fluorescence is used to determine the internal state distribution of NH(a/sup 1/..delta..), and multiphoton ionization is used to measure the internal state distribution of CO. An upper limit for the branching ratio of NCO/NH production from photodissociation of HNCO at 193 nm is determined from an analysis of kinetic experiments to be 0.10. To clarify the mechanism of photodissociation, HNCO fluorescence-excitation and NH(a/sup 1/..delta..) action spectra are also measured. They imply that two excited states of HNCO are present where only one had previously been considered.

Photofragmentation dynamics of ketene at 308 nm: Initial vibrational and rotational state distributions of CO product by vacuum UV laser-induced fluorescence

The vibration-rotation state distribution of CO(v,J) from CH2CO flash photolysis at 308 nm has been determined by pulsed VUV laser-induced fluorescence. The excited ketene molecule appears to internally convert to vibrationally excited ground singlet state and to dissociate on that surface, CH2CO+308 nm→CH2(1A1)+CO+6.7 kcal/mol. There are no energetic CO molecules which would indicate formation of the lower energy CH2 triplet state. The observed rotational distributions of CO(v=0) and CO(v=1) are roughly matched by rotational temperatures of 1300 and 550 K, respectively; the data are accurately matched by phase space theory (PST). PST assumes that all energetically accessible product states have equal probability; it contains no adjustable parameters. PST predicts the ratio of v=1 to v=0 populations to be 0.013 compared to an observed value of 0.09±0.05. A Franck–Condon calculation gives too much vibrational excitation, 0.22±0.07. The separate statistical ensemble (SSE) model of Wittig which decouples overall ketene rotation from the other internal degrees of freedom gives 0.082 in agreement with experiment. This type of statistical distribution is likely to characterize bond fissions for which the barrier is just the dissociation limit itself.