CO Rotational Distribution Research Articles

Nascent Rotational Distribution of CO2(0000,J) States from Collisions with Highly Vibrationally Excited Na2

Nascent Rotational Distribution of CO₂(00⁰0,J) States from Collisions with Highly Vibrationally Excited Na₂

Multiple Dissociation Pathways in HNCO Decomposition Governed by Potential Energy Surface Topography.

The exquisite features of molecular photochemistry are key to any complete understanding of the chemical processes governed by potential energy surfaces (PESs). It is well established that multiple dissociation pathways relate to nonadiabatic transitions between multiple coupled PESs. However, little detail is known about how the single PES determines reaction outcomes. Here we perform detailed experiments on HNCO photodissociation, acquiring the state-specific correlations of the NH (a1Δ) and CO (X1Σ+) products. The experiments reveal a trimodal CO rotational distribution. Dynamics simulations based on a full-dimensional machine-learning-based PES of HNCO unveil three dissociation pathways exclusively occurring on the S1 excited electronic state. One pathway, following the minimum energy path (MEP) via the transition state, contributes to mild rotational excitation in CO, while the other two pathways deviating substantially from the MEP account for relatively cold and hot CO rotational state populations. These peculiar dynamics are unambiguously governed by the S1 state PES topography, i.e., a narrow acceptance cone in the vicinity of the transition state region. The dynamical picture shown in this work will serve as a textbook example illustrating the importance of the PES topography in molecular photochemistry.

Photodissociation dynamics of OCS near 214 nm using ion imaging.

The OCS photodissociation dynamics of the dominant S((1)D2) channel near 214 nm have been studied using velocity map ion imaging. We report a CO vibrational branching ratio of 0.79:0.21 for v = 0:v = 1, indicating substantially higher vibrational excitation than that observed at slightly longer wavelengths. The CO rotational distribution is bimodal for both v = 0 and v = 1, although the bimodality is less pronounced than at longer wavelengths. Vector correlations, including rotational alignment, indicate that absorption to both the 2(1)A' (A) and 1(1)A″ (B) states is important in the lower-j part of the rotational distribution, while only 2(1)A' state absorption contributes to the upper part; this conclusion is consistent with work at longer wavelengths. Classical trajectory calculations including surface hopping reproduce the measured CO rotational distributions and their dependence on wavelength well, though they underestimate the v = 1 population. The calculations indicate that the higher-j peak in the rotational distribution arises from molecules that begin on the 2(1)A' state but make nonadiabatic transitions to the 1(1)A' (X) state during the dissociation, while the lower-j peak arises from direct photodissociation on either the 2(1)A' or the 1(1)A″ states, as found in previous work.

Imaging the Pair-Correlated HNCO Photodissociation: The NH(a1Δ) + CO(X1Σ+) Channel

The NH(a(1)Δ) + CO(X(1)Σ(+)) product channel for the photodissociation of HNCO at 201 nm was investigated using the sliced velocity map ion imaging technique with the detection of NH(a(1)Δ) products via (2 + 1) resonance enhanced multiphoton ionization (REMPI). Images were measured for the NH(a(1)Δ) rotational states in the ground and vibrational excited states (v = 0 and 1). Correlation between the NH(a(1)Δ) and CO rovibrational state distributions were determined from these images. Experimental results show that the vibrational distribution of the CO fragment in the NH(a(1)Δ) + CO(X(1)Σ(+)) channel peaks at v = 1. The negative anisotropy parameter measured for the NH(a(1)Δ) (v = 0 and 1|j) products indicates a direct dissociation process for the N-C bond cleavage in the S1 state. A bimodal CO rotational distribution was observed, suggesting that HNCO dissociates in the S1 state in two distinctive pathways.

Roads leading to roam. Role of triple fragmentation and of conical intersections in photochemical reactions: experiments and theory on methyl formate

The exploration of alternative roads that open to molecules with sufficient energy to yield different products permits prediction and eventually control of the outcomes of chemical reactions. Advanced imaging techniques for monitoring laser-induced photodissociation are here combined with dynamical simulations, involving ample sets of classical trajectories generated on a quantum chemical potential energy surface. Methyl formate, HCOOCH3, is photodissociated at energies near the triple fragmentation threshold into H, CO and OCH3. Images of velocity and rotational distributions of CO exhibit signatures of alternative routes, such as those recently designated as transition-state vs. roaming-mediated. Furthermore, a demonstration of the triple fragmentation route is given, and also confirmed by H-atom product imaging and FTIR time-resolved spectra of the intermediate HCO radical. In addition, the relevance of nonadiabatic transitions promoted by a conical intersection is clarified by simulations as the privileged "reactivity funnel" of organic photochemistry, whereby the outcomes of molecular photoexcitation are delivered to electronic ground states.

Three-State Trajectory Surface Hopping Studies of the Photodissociation Dynamics of Formaldehyde on ab Initio Potential Energy Surfaces

Full-dimensional, three-state, surface hopping calculations of the photodissociation dynamics of formaldehyde are reported on ab initio potential energy surfaces (PESs) for electronic states S(1), T(1), and S(0). This is the first such study initiated on S(1) with ab initio-calculated spin-orbit couplings among the three states. We employ previous PESs for S(0) and T(1), and a new PES for S(1), which we describe here, as well as new spin-orbit couplings. The time-dependent electronic state populations and the branching ratio of radical products produced from S(0) and T(1) states and that of total radical products and molecular products at three total energies are calculated. Details of the surface hopping dynamics are described, and a novel pathway for isomerization on T(1) via S(0) is reported. Final translational energy distributions of H + HCO products from S(0) and T(1) are also reported as well as the translational energy distribution and final rovibrational distributions of H(2) products from the molecular channel. The present results are compared to previous trajectory calculations initiated from the global minimum of S(0). The roaming pathway leading to low rotational distribution of CO and high vibrational population of H(2) is observed in the present calculations.

State-selected imaging of HCCO radical photodissociation dynamics

We present a dc sliced ion imaging study of HCCO radical photodissociation to CH and CO at 230 nm. The measurements were made using a two-color reduced Doppler probe strategy. The CO rotational distribution was consistent with a Boltzmann distribution at 3500 K. Using the dc slice ion imaging approach, we obtained CO images for various rotational levels of CO (v=0). The results are largely consistent with earlier work, albeit with a significant 0.9 eV peak seen previously in the translational energy distributions absent in our state-selected imaging study.

State-correlated DC slice imaging of formaldehyde photodissociation: roaming atoms and multichannel branching

High-resolution slice imaging methods allow for detection of single product quantum states with sufficient velocity resolution to infer the full correlated product state distribution of the undetected fragment. This is a level of detail not available in previous studies of formaldehyde photodissociation, and in this application it reveals startling new aspects of unimolecular decomposition. The CO rotational distributions from near ultraviolet dissociation of formaldehyde are bimodal, and the imaging experiments allow us to decompose these into two dynamically distinct components: the conventional molecular dissociation over a high exit barrier, and a novel ‘roaming atom’ reaction in which frustrated radical dissociation events lead to intramolecular H abstraction, bypassing the transition state entirely. In probing the dynamics of the conventional molecular dissociation over the barrier, we use the complete vH2-jCO correlation to model the exit channel dynamics in new detail. Furthermore, these state-correlated measurements provide insight into radical–radical reactions and the underlying dynamics and energy dependence of the roaming pathway.

Distribution of Internal States of CO from O (1D) + CO Determined with Time-Resolved Fourier Transform Spectroscopy

Following collisions of O (1D) with CO, rotationally resolved emission spectra of CO (1 < or = v < or = 6) in the spectral region 1800-2350 cm(-1) were detected with a step-scan Fourier transform spectrometer. O (1D) was produced by photolysis of O3 with light from a KrF excimer laser at 248 nm. Upon irradiation of a flowing mixture of O3 (0.016 Torr) and CO (0.058 Torr), emission of CO (v < or = 6) increases with time, reaches a maximum approximately 10 micros. At the earliest applicable period (2-3 micros), the rotational distribution of CO is not Boltzmann; it may be approximately described with a bimodal distribution corresponding to temperatures approximately 8000 and approximately 500 K, with the proportion of these two components varying with the vibrational level. A short extrapolation from data in the period 2-6 micros leads to a nascent rotational temperature of approximately 10170 +/- 600 K for v = 1 and approximately 1400 +/- 40 K for v = 6, with an average rotational energy of 33 +/- 6 kJ mol(-1). Absorption by CO (v = 0) in the system interfered with population of low J levels of CO (v = 1). The observed vibrational distribution of (v = 2):(v = 3):(v = 4):(v = 5):(v = 6) = 1.00:0.64:0.51:0.32:0.16 corresponds to a vibrational temperature of 6850 +/- 750 K. An average vibrational energy of 40 +/- 4 kJ mol(-1) is derived based on the observed population of CO (2 < or = v < or = 6) and estimates of the population of CO (v = 0, 1, and 7) by extrapolation. The observed rotational distributions of CO (1 < or = v < or = 3) are consistent with results of previous experiments and trajectory calculations; data for CO (4 < or = v < or = 6) are new.

Photolysis of oxalyl chloride (ClCO)2 at 193 nm: emission of CO(v

Upon photolysis of oxalyl chloride at 193 nm, time-resolved and rotationally resolved emission of CO(v<or=6, J<or=60) in the spectral region 1850-2350 cm(-1) was detected with a step-scan Fourier-transform spectrometer under nearly collisionless conditions. Boltzmann-type rotational distributions of CO correspond to temperatures 3520+/-110 (v=1) to 2300+/-610 K (v=6), with an average rotational energy of 23+/-2 kJ mol(-1). The average vibrational energy of CO is estimated to be 26+/-4 kJ mol(-1) according to observed vibrational populations of v=1-6 and that of v=0 predicted with a surprisal analysis. Combining the average internal energy of CO determined in this work and average translational energies of photofragments Cl and CO determined previously by Hemmi and Suits, we propose a four-body dissociation mechanism producing one pair of translationally rapid and internally excited CO and one pair of translationally rapid Cl, each with similar energies, to account for the energy balance. Formation of translationally slow ClCO, Cl, and CO reported previously by Hemmi and Suits might be rationalized with a second channel involving emission of electronically excited intermediates. We observed no emission of ClCO near 1880 cm(-1), indicating that surviving ClCO has little vibrational excitation in the C-O stretching mode.

Photolysis of Oxalyl Chloride (ClCO)2 at 248 nm: Emission of CO(v‘ ⩽ 3, J‘ ⩽ 51) Detected with Time-Resolved Fourier Transform Spectroscopy

Upon photolysis of oxalyl chloride at 248 nm, rotationally resolved emission of CO (v ≤ 3) in the spectral region 1900−2300 cm-1 was detected with a step-scan time-resolved Fourier transform spectrometer under nearly collisionless conditions. Boltzmann-like rotational distributions of CO (v = 1 and 2) correspond to temperatures ∼2290 ± 100 and 1910 ± 130 K, respectively, with average rotational energy of 16 ± 2 kJ mol-1; several lines of CO (v = 3) were also observed, but their weak intensity precludes quantitative analysis. The average vibrational energy of CO is estimated to be 10 ± 3 kJ mol-1 according to observed vibrational populations of v = 1 and 2 and that of v = 0 and 3 predicted with surprisal analysis. Combining the average internal energy of CO determined in this work and average translational energies of photofragments Cl, CO, and ClCO determined previously by Suits and co-workers, we derived a revised energy balance. We observed no emission of ClCO near 1880 cm-1, indicating that ClCO (v1 = 1) decomposes rapidly and that surviving ClCO has little vibrational excitation in the CO stretching mode. Most CO produced from decomposition of ClCO is in its vibrational ground state with small rotational excitation, undetectable with our technique.

Non-adiabatic bending dissociation of OCS: the effect of bending excitation on the transition probability

The UV photodissociation dynamics of OCS starting from the bending excited (0 1 0) state was compared to that from the ground vibrational state (0 0 0). The rotational distribution of CO(X 1Σ + ) was measured by (2+1) resonance enhanced multiphoton ionization, in which dissociations from the two different initial states were discriminated from the translational energies of the CO fragments. Quantitative analysis revealed that the non-adiabatic transition probability in dissociation starting from (0 1 0) was 0.21 that is similar to but slightly smaller than the value, 0.34, from (0 0 0).

Rotationally resolved photoionization: Influence of the 4σ→kσ shape resonance on CO+(B 2Σ+) rotational distributions

We present experimental and theoretical results on rotational distributions of CO+(B 2Σ+) photoions. Rotational distributions were determined for both the v+=0 and v+=1 vibrational levels following photoionization of cold (T0≈9 K) neutral CO target molecules. Data were generated using dispersed ionic fluorescence over a wide range of photoelectron kinetic energies, 0⩽Ek⩽120 eV, which allows one to interrogate the ionization dynamics. This wide spectral coverage permits illustrative comparisons with theory, and calculated spectra are presented to interpret the data. In particular, the comparison between theory and experiment serves to identify the strong continuum resonant enhancement at hνexc≈35 eV in the l=3 partial wave of the 4σ→kσ ionization channel, as this feature has profound effects on the ion rotational distributions over a wide range of energy. Second, there are differences between the rotational substructure for the v+=0 and v+=1 vibrational levels. All of the experimentally observed features and trends are reproduced by theory, and the consequences of these comparisons are discussed.

Glyoxal photodissociation. II. An ab initio direct classical trajectory study of C2H2O2→CO+H2CO

Photodissociation of glyoxal via the H2CO+CO channel has been investigated by ab initio classical trajectory calculations using Becke’s three-parameter hybrid functional method with split valence and polarized basis set [B3LYP/6-311G(d,p)]. To model the experimental conditions, trajectories were started from a microcanonical ensemble at the transition state with 8.5 kcal/mol excess energy distributed among the vibrational modes and the transition vector. The CO product was produced with a broad rotational distribution but with almost no vibration excitation. When combined with the results from the H2+2CO channel, the calculated vibrational and rotational distributions of CO are in excellent agreement with the experimental observations. The rotational distribution of H2CO was very broad ranging up to J=85. The H2CO product has significant vibrational excitation in the out-of-plane bending, CH2 rocking, CH2 scissoring, and CO stretching modes. For both the H2+2CO and the CO+H2CO channels, the majority of available energy was partitioned into translations.

State Correlations in the Unimolecular Dissociation of Ketene

Doppler-broadened absorption spectra of selected rotational levels of singlet CH2 fragments from the 308-nm photolysis of ketene have been observed by frequency-modulated transient absorption spectroscopy in a slit jet expansion. Nascent Doppler profiles of CH2 transitions have been analyzed to yield the internal energy distributions of coincident CO fragments at a total energy 2333 cm-1 above the singlet dissociation threshold. The vibrational branching ratio of CO (υ = 1):(υ = 0) is determined in coincidence with selected rotational levels of CH2. The coincident rotational distributions of CO (υ = 0) are also estimated and found to differ from simple statistical predictions, revealing a relative deficiency of those pairs in which both fragments have a low internal energy.

State to state He–CO rotationally inelastic scattering

Relative integral cross sections for rotational excitation of CO in collisions with He were measured at energies of 72 and 89 meV. The cross sections are sensitive to anisotropy in the repulsive wall of the He–CO interaction. The experiments were done in crossed molecular beams with resonance enhanced multiphoton ionization detection. The observed cross sections display interference structure at low Δj, despite the average over the initial CO rotational distribution. At higher Δj, the cross sections decrease smoothly. The results are compared with cross sections calculated from two high quality potential energy surfaces for the He–CO interaction. The ab initio SAPT surface of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)] agrees with the data better than the XC(fit) surface of Le Roy et al. [Farad. Disc. 97, 81 (1994)].

Nascent rovibrational distributions of CO(d 3Δi,e 3Σ−,a′ 3Σ+) produced in the dissociative recombination of CO2+ with electrons

The d 3Δi–a 3Πr, e 3Σ−–a 3Πr, and a′ 3Σ+–a 3Πr transitions of CO resulting from the dissociative recombination of CO2+(X̃ 2Πg:0,0,0) with electrons have been observed from the He afterglow reaction of CO2. The formation rate constants of CO(d), CO(e), and CO(a′) were estimated to be 1.6×10−7, 3.3×10−9, and 2.4×10−7 cm3 s−1, respectively. The vibrational and rotational distributions of CO(d:v′=0–6,e:v′=2,3,a′=3–11) were determined. Most of available excess energies (91%∼98%) were deposited into the vibrational energy of CO(d,e,a′) and the relative translational energies of the products, indicating that CO(d,e,a′) were produced by direct curve crossings between the entrance e−/CO2+(X̃ 2Πg:0,0,0) potential and repulsive CO(d,e,a′)+O(3P) potentials with linear geometries. The vibrational distributions of CO(d) and CO(a′) slightly shifted to lower states than those in photodissociation at a similar excitation energy. A simple statistical model was unable to explain the observed vibrational distributions obtained by dissociative recombination.

Ionization process of collision of Ne(3P0,2) with CO under molecular beam condition

The ionization of CO with metastable Ne(3P0,2) in a molecular beam was studied by measuring the emission spectra of CO+ (A2II1-X2Σ+). The nascent vibrational and rotational distributions of CO+ (A) were calculated by spectral simulation and the results are discussed.

Correlated distributions in the photodissociation of HNCO to NH(X 3Σ−, a 1Δ) + CO(X 1Σ+) near the barrier on S 1

The photodissociation of HNCO is examined at 230.1 nm, near the threshold to NH(a 1 Δ) + CO(X 1 Σ +). CO products are generated from NH(X 3 Σ −) + CO(X 1 Σ +) and NH(a 1 Δ) + CO(X 1 Σ +). Although the two contributions overlap spectroscopically, they are resolved in velocity space when using photofragment ion imaging. For both channels the CO rotational distribution is cold and non-statistical. A high resolution variant of imaging is employed to obtain NH(a 1Δ) distributions correlating with specific J CO levels. Combining these measurements with NH(a 1Δ) photofragment yield spectra and rotational distributions, we establish that decomposition to NH(a 1Δ) + CO(X 1 Σ) evolves over a small barrier on S 1, estimated at 400–600 cm −1.

State-specific neutral time-of-flight of CO from ketene photodissociation at 351 nm: The internal energy distribution of CH2(X̃ 3B1)

Metastable time-of-flight (TOF) spectroscopy was used to measure the translational energy distribution of specific rotational states of CO formed from ketene photodissociation (CH2CO→CH2+CO) at 351 nm. This distribution could be directly related to the internal energy distribution of the other fragment (X̃ 3B1 CH2) formed in the reaction, thereby giving a correlated distribution of the internal states of the fragments. This technique overcomes the spectral complexity associated with detection the X̃ 3B1 state CH2. Previous measurements of the CO rotational distribution were simulated theoretically using the impulsive model and zero-point vibrational energy considerations. These models predicted that the rotational distributions of CO and CH2 should be uncorrelated, that ∼10% of the CH2 should be vibrationally excited with one quantum in the bending mode, and that the rotational energy distribution of CH2 should peak near zero. Measurements presented in this paper show a slight anticorrelation of CO and CH2 rotations, no vibrational excitation of CH2 and Gaussian-like rotational energy distributions of CH2 that peak at ∼1 kcal/mole and have a full width at half-maximum of ∼0.8 kcal/mol. Qualitative explanations for this behavior are presented.