DC Slice Imaging Research Articles

Crossed-beam dc slice imaging of chlorine atom reactions with pentane isomers

We present the first in a series of detailed investigations of the reaction of Cl atoms with hydrocarbons in crossed beams. This study focuses on reaction with the pentane isomers n-pentane, isopentane, and neopentane. These crossed-beam ion imaging experiments rely on "universal" 157 nm probe of the pentyl radical product, with dc sliced detection providing the scattering distributions directly. The results for the different target reactants were studied at two collision energies: approximately 5 and approximately 9 kcal/mol. The scattering results are quite similar for all reactants, suggesting that the nature of the abstraction site has surprisingly little influence on the dynamics. The angular distributions are broad with a backscattered peak at low collision energy and a sharp forward peak at high collision energy. The translational energy distributions are strongly coupled to the angular distributions and show, for the forward scattered products, a sharp peak at 80% of the collision energy for all reactants at both collision energies. The sideways-scattered products show the lowest fraction of energy in translation, while the backscattered translational energy distribution peaks at the lowest energy but extends to the highest, showing evidence of coupling of the internal degrees of freedom and the reaction exoergicity, into translation. The results are discussed in view of the extensive literature on related systems.

Dynamics of CN+alkane reactions by crossed-beam dc slice imaging

The hydrogen atom abstraction reactions of CN (X (2)Sigma(+)) with alkanes have been studied using the crossed molecular beam technique with dc slice ion imaging at collision energies of 7.5 and 10.8 kcalmol. The product alkyl radical images were obtained via single photon ionization at 157 nm for the reactions of CN (X (2)Sigma(+)) with n-butane, n-pentane, n-hexane, and cyclohexane. From analysis of the images, we obtained the center-of-mass frame product angular distributions and translational energy distributions directly. The results indicate that the products are largely backscattered and that most of the available energy ( approximately 80%-85%) goes to the internal energy of the products. The reaction dynamics is discussed in light of recent kinetics data, theoretical calculations, and results for related halogen and oxygen atom reactions.

Roaming Dynamics in Acetone Dissociation

DC slice imaging has been employed to study the photodissociation dynamics of acetone at 230 nm, with detection of the CO photoproduct via the B (v' = 0) (1)Sigma(+) <-- X (v'' = 0) (1)Sigma(+) transition. A bimodal translational energy distribution observed in the CO fragments points to two distinct dissociation pathways in the 230 nm photolysis of acetone. One pathway results in substantial translational energy release (E(ave) approximately 0.3 eV) along with rather high rotational excitation (up to J'' = 50) of CO, and is attributed to the thoroughly investigated stepwise mechanism of bond cleavage in acetone. The other dissociation pathway leads to rotationally cold CO (J'' = 0-20) with very little energy partitioned into translation (E(ave) approximately 0.04 eV) and in this way it is dynamically similar to the recently reported roaming mechanism found in formaldehyde and acetaldehyde dissociation. We ascribe the second dissociation pathway to an analogous roaming dissociation mechanism taking place on the ground electronic state following internal conversion. For acetone, this would imply highly vibrationally excited ethane as a coproduct of rotationally cold CO, with the ethane formed above the threshold for secondary decomposition. We estimate that about 15% of the total CO fragments are produced through the roaming pathway. Rotational populations were obtained using a new Doppler-free method that simply relies on externally masking the phosphor screen under velocity map conditions in such a way that only the products with no velocity component along the laser propagation direction are detected.

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.

Energy dependence of the roaming atom pathway in formaldehyde decomposition

Recently, a new mechanism of formaldehyde decomposition leading to molecular products CO and H(2) has been discovered, termed the "roaming atom" mechanism. Formaldehyde decomposition from the ground state via the roaming atom mechanism leads to rotationally cold CO and vibrationally hot H(2), whereas formaldehyde decomposition through the conventional molecular channel leads to rotationally hot CO and vibrationally cold H(2). This discovery has shown that it is possible to have multiple pathways for a reaction leading to the same products with dramatically different product state distributions. Detailed investigations of the dynamics of these two pathways have been reported recently. This paper focuses on an investigation of the energy dependence of the roaming atom mechanism up to 1500 cm(-1) above the threshold of the radical channel, H(2)CO-->H+HCO. The influence of excitation energy on the roaming atom and molecular elimination pathways is reported, and the branching fraction between the roaming atom channel and molecular channel is obtained using high-resolution dc slice imaging and photofragment excitation spectroscopy. From the branching fractions and the reaction rates of the radical channel, the overall competition between all three dissociation channels is estimated. These results are compared with recent quasiclassical trajectory calculations on a global H(2)CO potential energy surface.

S ( D 2 1 ) atomic orbital polarization in the photodissociation of OCS at 193nm: Construction of the complete density matrix

The absolute velocity-dependent alignment and orientation for S(1D2) atoms from the photodissociation of OCS at 193 nm were measured using the dc slice imaging method. Three main peaks ascribed to specific groups of high rotational levels of CO in the vibrational ground state were found, with rotationally resolved rings in a fourth slow region ascribed to weak signals associated with excited vibrational states of CO. The observed speed-dependent beta and polarization parameters support the interpretation that there are two main dissociation processes: a simultaneous two-surface (A' and A") excitation and the initial single-surface (A') excitation followed by the nonadiabatic crossing to ground state. At 193 nm photodissociation, the nonadiabatic dissociation process is strongly enhanced relative to longer wavelengths. The angle- and speed-dependent S(1D2) density matrix can be constructed including the higher order (K = 3,4) contributions for the circularly polarized dissociation light. This was explicitly done for selected energies and angles. It was found in one case that the density matrix is sensitively affected by the rank 4 terms, suggesting that the higher order contributions should not be overlooked for an accurate picture of the dissociation dynamics in this system.

Rotationally resolved reactive scattering: Imaging detailed Cl+C2H6 reaction dynamics

The hydrogen atom abstraction reaction of Cl (2P3/2) with ethane has been studied using the crossed molecular beam technique with dc slice imaging at collision energies from 3.2 to 10.4 kcal/mol. The products HCl (v,J) (v = 0, J = 0-5) were state-selectively detected using 2+1 resonance enhanced multiphoton ionization. The images were used to obtain the center-of-mass frame product angular distributions and translational energy release distributions. Two general features were found in all probed HCl quantum states at 6.7 kcal/mol collision energy, and these features have distinct translational energy release and angular distributions, as described for HCl (v = 0, J = 2) in a recent preliminary report [Li et al., J. Chem. Phys. 124, 011102 (2006)]. The results for HCl (v = 0, J = 2) at four collision energies were also compared to investigate the energy-dependent dynamics. We discuss the reaction in terms of a variety of models of polyatomic reaction dynamics. The dynamics of this well studied system are more complicated than can be accounted for by a single mechanism, and the results call for further theoretical and experimental investigations.

The roaming atom pathway in formaldehyde decomposition

We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H(2) and CO following excitation to the 2(1)4(1) and 2(1)4(3) transitions in S(1). The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, j(CO)=5-45). These high-resolution measurements reveal the correlated internal state distribution in the H(2) cofragments. The results show that rotationally hot CO (j(CO) approximately 45) is produced in conjunction with vibrationally "cold" H(2) fragments (v=0-5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H(2)CO-->H+HCO) we also find formation of rotationally cold CO (j(CO)=5-28) correlated to highly vibrationally excited H(2) (v=6-8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this "roaming" mechanism are the focus of this paper. The correlations between the vibrational states of H(2) and rotational states of CO formed following excitation on the 2(1)4(3) transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel.

Multiphoton processes of CO at 230 nm

High resolution kinetic energy release spectra were obtained for C(+) and O(+) from CO multiphoton ionization followed by dissociation of CO(+). The excitation was through the CO (B (1)Sigma(+)) state via resonant two-photon excitation around 230 nm. A total of 5 and 6 photons are found to contribute to the production of carbon and oxygen cations. DC slice and Megapixel ion imaging techniques were used to acquire high quality images. Major features in both O(+) and C(+) spectra are assigned to the dissociation of some specific vibrational levels of CO(+)(X (2)Sigma(+)). The angular distributions of C(+) and O(+) are very distinct and those of various features of C(+) are also different. A dramatic change of the angular distribution of C(+) from dissociation of CO(+)(X (2)Sigma(+), nu(+) = 1) is attributed to an accidental one-photon resonance between CO(+)(X (2)Sigma(+), nu(+) = 1) and CO(+)(B (2)Sigma(+), nu(+) = 0) and explained well by a theoretical model. Both kinetic energy release and angular distributions were used to reveal the underlying dynamics.

Novel molecular elimination mechanism in formaldehyde photodissociation: the roaming H atom pathway

We present a state-correlated experimental investigation of formaldehyde (H2CO) dissociation to H2 and CO following excitation to a series of vibrational bands in the first electronically excited state, S1. The CO was detected by resonance-enhanced multiphoton ionization at various rotational states of CO (J = 5–45) and the CO velocity distributions were measured using state-resolved DC Slice Imaging. These high-resolution measurements reveal the internal state distribution of the correlated H2 cofragments. The results show that the rotationally hot CO (JCO = 40) is produced in conjunction with vibrationally cold H2 fragments (ν = 0–3), consistent with dissociation through the celebrated skewed transition state. After excitation of formaldehyde at energies near and above the threshold for dissociation to radical products (H2CO → H+HCO), a second molecular elimination channel appears which is characterized by rotationally cold CO (J ̃ 5–15) correlated with highly vibrationally excited H2 (ν = 5–7). These products are formed through a novel roaming H-atom mechanism that involves intramolecular H abstraction and avoids the region of the transition state to molecular elimination entirely. The current measurements give insight into the energy dependence of the branching of these different reaction mechanisms.

Megapixel ion imaging with standard video

We present an ion imaging approach employing a real-time ion counting method with standard video. This method employs a center-of-mass calculation of each ion spot (more than 3×3pixels spread) prior to integration. The results of this algorithm are subpixel precision position data of the corresponding ion spots. These addresses are then converted to the final image with user selected resolution, which can be up to ten times higher than the standard video camera resolution (640×480). This method removes the limiting factor imposed by the resolution of standard video cameras and does so at very low cost. The technique is used in conjunction with dc slice imaging, replacing the local maximum searching algorithm developed by Houston and co-workers [B. Y. Chang, R. C. Hoetzlein, J. A. Mueller, J. D. Geiser, and P. L. Houston, Rev. Sci. Instrum. 69, 1665 (1998)]. The performance is demonstrated using HBr and DBr photodissociation at 193nm with 3+1 resonance enhanced multiphoton ionization detection of hydrogen and deuterium atom products. The measured velocity resolution for DBr dissociation is 0.50% (δv∕v), mainly limited in this case by the bandwidth of the photolysis laser. Issues affecting slice imaging resolution and performance are also discussed.

O(1D2) orbital orientation in the ultraviolet photodissociation of ozone

We present the absolute velocity-dependent orbital orientation for O(1D2) atoms produced from the photodissociation of ozone in the 248-285 nm region obtained using the DC slice imaging method. The results are analyzed in terms of laboratory frame anisotropy parameters describing distinct excitation and dissociation mechanisms possessing characteristic angular distributions. The results show negligible orbital orientation produced in dissociation by circularly polarized light, but strong recoil speed-dependent orientation following photolysis by linearly polarized light at all wavelengths studied. The origin of this polarization is ascribed to nonadiabatic transitions at avoided crossings and at long range.

DC Slice Imaging of CH3Cl Photolysis at 193.3 nm

The DC slice imaging method is used to record energy- and angle-resolved distributions for both ground-state Cl (2P3/2) and spin−orbit excited Cl* (2P1/2) photofragments produced from the A-band photolysis of CH3Cl at 193.3 nm. The dissociation is found to be highly impulsive, with ∼90% of the available energy being released into translation. The angular distributions are predominantly perpendicular in the case of both the ground state and spin−orbit excited chlorine atom fragments, as characterized by recoil-energy averaged values of β = −0.46 and β* = −0.74 respectively, although we find considerable variation in both channels as a function of CH3 internal energy. In contradiction with previous measurements, we conclude that the transition to the 1Q1 state dominates much of the A-band profile, and this has significant implications for the description of the dissociation dynamics. Our findings are rationalized in terms of similar measurements performed with other methyl halides and their H atom substitut...

Orbital polarization from DC slice imaging: S(1D2) alignment in the photodissociation of ethylene sulfide

Abstract DC slice imaging [Rev. Sci. Instrum. 74 (2003) 2530], a recently developed high-resolution `slicing' approach that directly provides the full 3D product distribution in imaging experiments, has been adapted to yield the absolute speed-dependent angular momentum alignment anisotropy parameters in a photodissociation experiment. We present the theoretical machinery for interpretation of a small basis set of sliced experimental images recorded under different laser polarization geometries and then demonstrate its application in the study of electronic orbital angular momentum alignment of excited state S( 1 D 2 ) atoms following 193.3 nm photodissociation of ethylene sulfide. We find the slicing approach to be a highly sensitive and widely applicable technique for the study of orbital polarization, even in instances where the magnitude of such effects is small. The approach outlined here represents a direct route to the determination of recoil-angle dependent orbital alignment for a continuous range of photofragment kinetic energy release.