Simon W. North

Evidence for stepwise dissociation dynamics in acetone at 248 and 193 nm

The technique of molecular beam photofragment translational spectroscopy has been used to study the dissociation of acetone following S1←S0 (248 nm) and S2←S0 (193 nm) excitation. Excitation at 248 nm resulted in the production of CH3 and CH3CO with 14.2±1.0 kcal/mole on average of the available energy appearing as translation of the photofragments. Comparison of the measured 〈ET〉 with values reported at 266 nm suggest that the energy partitioning is dominated by the exit barrier caused by an avoided crossing on the potential energy surface. A substantial fraction (30±4%) of the nascent acetyl radicals from the primary dissociation contain sufficient energy to undergo spontaneous secondary decomposition. From the onset of the truncation of the CH3CO P(ET) a threshold of 17.8±3.0 kcal/mole for the dissociation of the acetyl radical has been determined in agreement with recent results on the photodissociation of acetyl chloride. The translational energy release in the dissociation of CH3CO closely matches t...

The Multiplexed Chemical Kinetic Photoionization Mass Spectrometer: A New Approach To Isomer-resolved Chemical Kinetics

We have developed a multiplexed time- and photon-energy-resolved photoionization mass spectrometer for the study of the kinetics and isomeric product branching of gas phase, neutral chemical reactions. The instrument utilizes a side-sampled flow tube reactor, continuously tunable synchrotron radiation for photoionization, a multimass double-focusing mass spectrometer with 100% duty cycle, and a time- and position-sensitive detector for single ion counting. This approach enables multiplexed, universal detection of molecules with high sensitivity and selectivity. In addition to measurement of rate coefficients as a function of temperature and pressure, different structural isomers can be distinguished based on their photoionization efficiency curves, providing a more detailed probe of reaction mechanisms. The multiplexed three-dimensional data structure (intensity as a function of molecular mass, reaction time, and photoionization energy) provides insights that might not be available in serial acquisition, as well as additional constraints on data interpretation.

Primary and secondary processes in the 193 nm photodissociation of vinyl chloride

We have investigated the photodissociation of vinyl chloride (H2CCHCl) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used vacuum ultraviolet synchrotron radiation for product photoionization. We have observed five primary dissociation channels following an initial π*←π excitation. The majority of Cl atoms originate from an excited-state dissociation. The remaining dissociation channels are consistent with competition on the ground electronic state following internal conversion from the optically prepared state. These channels include atomic and molecular hydrogen elimination, HCl elimination, and a translationally slow Cl elimination channel. We have also identified and characterized two secondary decomposition channels: (1) the elimination of Cl from chlorovinyl radicals following the primary atomic hydrogen elimination channel, and (2) hydrogen atom elimination from vinyl radica...

The ultraviolet photodissociation dynamics of pyrrole

Abstract Photofragment translational spectroscopy was used to study the photodissociation of pyrrole at 193 and 248 nm under collision-free conditions. Five primary dissociation channels were observed at 193 nm. Two channels resulted from cleavage of the NH bond to yield H + pyrrolyl radical with one channel following internal conversion (IC) to the ground state (≈21%) and the other originating from electronically excited pyrrole (≈30%). Two dissociation channels involved elimination of HCN following IC. One channel producing HCN + vinylmethylene (≈25%) following ring opening and hydrogen migration and the other proceeding via a bridged 3H-pyrrole intermediate to form HCN+cyclopropene (≈24%). The last channel at 193 nm involved IC to the ground state followed by ring opening and NC bond cleavage to form NH+CHCCHCH 2 ( D 0 (NH) =88±2 kcal/mol, D 0 (CH) = 112.5±1 kcal/mol and Δ H r (pyrrolyl radical) = 62±2 kcal/mol were determined.

No Straight Path: Roaming in Both Ground- and Excited-State Photolytic Channels of NO3 → NO + O2

Exclusive Roaming How do polyatomic molecules fall apart? The basic model, supported by centuries of chemical theory and experiment, invokes a series of internal rearrangements that lead to a fleeting high-energy transition-state geometry from which lower-energy products emerge. Over the past decade, several molecules have been shown to manifest a competing dissociation mechanism such that alongside trajectories that pass through the transition state, there are energetically accessible pathways that roam around it. Grubb et al. (p. 1075; see the Perspective by Jordan and Kable) showed that the light-induced reaction of NO3 to form NO and O2 proceeded exclusively by roaming. Although there are distinct pathways to the products in two different electronic states, neither one passes through a conventional transition state. A chemical reaction proceeds exclusively by mechanisms that do not pass through a conventional transition state. Roaming mechanisms have recently been observed in several chemical reactions alongside trajectories that pass through a traditional transition state. Here, we demonstrate that the visible light–induced reaction NO3 → NO + O2 proceeds exclusively by roaming. High-level ab initio calculations predict specific NO Λ doublet propensities (orientations of the unpaired electron with respect to the molecular rotation plane) for this mechanism, which we discern experimentally by ion imaging. The data provide direct evidence for roaming pathways in two different electronic states, corresponding to both previously documented photolysis channels that produce NO + O2. More broadly, the results raise intriguing questions about the overall prevalence of this unusual reaction mechanism.

Photodissociation dynamics of CH2BrCl studied using resonance enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry

The photodissociation dynamics of CH2BrCl have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Polarization dependent time-of-flight profiles were collected for a range of wavelengths from 248 to 268 nm, corresponding to the red wing of the absorption spectrum. Forward convolution fits to the data have provided translational energy distributions and anisotropy parameters over the entire wavelength range for both Br(2P3/2) and Br*(2P1/2). The average translational energies for the Br and Br* channels are 20 and 23 kcal/mol, respectively. The measured anisotropy parameters indicate that both channels arise preferentially from a parallel transition and that the relative contribution of this transition increases with decreasing wavelength. Nonadiabatic transitions appear to play a smaller role in CH2BrCl dissociation than in its monohalogenated analogues, specifically CH3Br. We suggest that this difference is the result of the intrinsic Cs symmetry and lower...

Two-component molecular tagging velocimetry utilizing NO fluorescence lifetime and NO 2 photodissociation techniques in an underexpanded jet flowfield

We report the application of molecular tagging velocimetry (MTV) toward two-component velocimetry as demonstrated in an underexpanded free jet flowfield. Two variants of the MTV technique are presented: 1) electronic excitation of seeded nitric oxide (NO) with gated fluorescence imaging (fluorescence lifetime) and 2) photodissociation of seeded NO2 followed by NO fluorescence imaging (NO2 photodissociation). The seeded NO fluorescence lifetime technique is advantageous in low-quenching, high-velocity flowfields, while the photodissociation technique is useful in high-quenching environments, and either high- or low-velocity flowfields due to long lifetime of the NO photoproduct. Both techniques are viable for single-shot measurements, with determined root mean squared results for streamwise and radial velocities of ~5%. This study represents the first known application of MTV utilizing either the fluorescence lifetime or the photodissociation technique toward two-component velocity mapping in a gaseous flowfield. Methods for increasing the spatial resolution to be comparable to particle-based tracking techniques are discussed.

Determination of the barrier height to CH3CO dissociation

Abstract The photodissociation of acetyl chloride at 248 nm has been investigated by the technique of photofragment translational spectroscopy. A comparison of the transitional energy distributions required to fit the chlorine atom and the acetyl radical indicates that a significant fraction (≈ 35%) of the CH 3 CO fragments undergo secondary decomposition to CH 3 and CO. From analysis of the center-of-mass translational energy distributions a value of 17 ± 1 kcal/mol for the barrier height to acetyl radical dissociation has been determined.

Molecular Tagging Using Vibrationally Excited Nitric Oxide in an Underexpanded Jet Flowfield

We report a laser diagnostic technique which relies on planar laser-induced fluorescence of vibrationally excited nitric oxide (NO v=1 ) molecules produced from the 355 nm photodissociation of seeded NO 2 for molecular tagging velocimetry applications. The technique was applied toward an axisymmetric highly underexpanded jet flowfield to yield single-component (streamwise) velocity maps. Detection of the photodissociated NO v=1 molecules would be valuable in flow environments where molecular tagging velocimetry would be highly desirable, but where there are also significant background concentrations of NO. The technique would also be valuable in high-quenching and/or low-velocity flow conditions due to the long-lived nature of the photodissociated NO molecules. Single-shot streamwise velocity uncertainties were about 5% and could be lowered by increasing signal to noise. In addition, the vibrational relaxation of NO was explored in support of a U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative project and it was found that the vibrational decay of NO was heavily dependent on collisional vibrational relaxation with oxygen atom formed through NO 2 photodissociation.

Kinetic studies of OH-initiated reactions of isoprene

We report laboratory kinetic studies of isoprene reactions initiated by the hydroxyl radical OH, using a turbulent flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. The rate constants for the reaction of isoprene with OH have been measured in the pressure range of 70 to 120 torr at 298±2 K and are found to be independent of pressure with an averaged value of (10.1±0.8)×10−11 cm3 molecule−1 s−1. The error limit given is within 1 standard deviation; a systematic error is estimated to be ±15%. We also describe direct observation of the OH-isoprene adduct based on ion-molecule reactions by using the CIMS method. The formation of the OH-isoprene adduct was used to extract the rate constant between OH and isoprene; within the uncertainty of the experiments the results were consistent with those obtained from the observed disappearance of OH. By monitoring the formation of the OH-isoprene adduct in the presence of oxygen molecules, an overall rate constant between OH-isoprene adduct and O2 has been first determined, with an averaged value of (2.8±0.7)×10−15 cm3 molecule−1 s−1 at 76 torr and an estimated systematic error of ±50%. Atmospheric implications of the present results to the photochemical oxidation of isoprene are discussed.