Hahkjoon Kim | Duksung Women's University (original) (raw)

Papers by Hahkjoon Kim

Research paper thumbnail of Lipid Nanotube Formation from Streptavidin−Membrane Binding

Langmuir, 2008

A novel transformation of giant lipid vesicles to produce nanotubular structures was observed upo... more A novel transformation of giant lipid vesicles to produce nanotubular structures was observed upon the binding of streptavidin to biotinylated membranes. Unlike membrane budding and tubulation processes caused by proteins involved with endocytosis and vesicle fusion, streptavidin is known to crystallize at near the isoelectric point (pI 5 to 6) into planar sheets against biotinylated films. We have found, however, that at neutral pH membranes of low bending rigidity (<10kT), such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), spontaneously produce tubular structures with widths ranging from micrometers to below the diffraction limit (<250 nm) and lengths spanning up to hundreds of micrometers. The nanotubes were typically held taut between surface-bound vesicles suggesting high membrane tension, yet the lipid nanotubes exhibited a fluidic nature that enabled the transport of entrained vesicles. Confocal microscopy confirmed the uniform coating of streptavidin over the vesicles and nanotubes. Giant vesicles composed of lipid membranes of higher bending energy exhibited only aggregation in the presence of streptavidin. Routes toward the development of these highly curved membrane structures are discussed in terms of general proteinmembrane interactions.

Research paper thumbnail of Joint Statistical Analysis of Multichannel Time Series from Single Quantum Dot−(Cy5) n Constructs

Journal of Physical Chemistry B, 2008

One of the major challenges in single-molecule studies is how to extract reliable information fro... more One of the major challenges in single-molecule studies is how to extract reliable information from the inevitably noisy data. Here, we demonstrate the unique capabilities of multichannel joint statistical analysis of multispectral time series using Föster resonance energy transfer (FRET) in single quantum dot (QD)-organic dye hybrids as a model system. The multispectral photon-by-photon registration allows model-free determination of intensity change points of the donor and acceptor channels independently. The subsequent joint analysis of these change points gives high-confidence assignments of acceptor photobleaching events despite the interference from background noise and from intermittent blinking of the QD donors and acceptors themselves. Finally, the excited-state lifetimes of donors and acceptors are calculated using the joint maximum likelihood estimation (MLE) method on the donor and acceptor decay profiles, guided by a four-state kinetics model.

Research paper thumbnail of Multiparameter Fluorescence Spectroscopy of Single Quantum Dot−Dye FRET Hybrids

Journal of The American Chemical Society, 2007

Förster resonance energy transfer (FRET) between donor and acceptor chromophores provides a sensi... more Förster resonance energy transfer (FRET) between donor and acceptor chromophores provides a sensitive mechanism for probing interactions between the chromophores and with their local environment. As donor chromophores, luminescent semiconductor quantum dots (QD) offer significant advantages over fluorescent dyes including superior photostability, broad absorption, and narrow, size-tunable emission. As a result, there is rapidly growing interest in QD-organic dye hybrids for FRET-based sensing. 1-4 At the single-particle level, these hybrids have potential for measuring the local environment of nanostructures and studying macromolecule conformational fluctuations. For most purposes multiple acceptors are bound to a single QD hub that serves as the sole energy donor. A major hurdle to using QD-dye hybrids in single-particle applications is the QD's complicated, intermittent emission, or blinking, 5-8 whose kinetic mechanism remains unknown. 9 The single-particle FRET signal in these QD-multiple-acceptor systems must be distinguished from acceptor dye photobleaching events, flickering dye emission caused by the QD donor blinking, and direct excitation of the acceptors. Here we demonstrate that the combined use of multiparameter fluorescence spectroscopy 10,11 and modelfree statistical analysis 12 can address these issues. These methods should also be applicable to analyze the complex excited-state dynamics in other multichromophore systems, where various types of multiparameter single-molecule spectroscopy have previously been employed. Our multiparameter single-molecule microscope measures the wavelength, emission delay relative to excitation (excited-state lifetime), and chronological time (intensity) for each detected photon. By measuring changes of multiple spectral parameters synchronized through correlated emission intensity jumps, high confidence event assignments can be obtained even in low signal-to-background environments. The model-free statistical analysis quantitatively determines the time of emission intensity changes, taking into account the magnitude of the change and the number of photons used in the evaluation. We have performed multiparameter FRET measurements for bulk samples of various QD605-Cy5 hybrids made with biotinylated QDs and Cy5 labeled streptavidin. panels a and b show results with, on average, about 5 and 1 dyes per QD. The FRET efficiency, indicated by the ratio of acceptor to donor emission in the spectra, reflects the number of acceptors. The time-resolved excited-state kinetics provides additional information about the energy transfer process (cf. ,d). For example, with many acceptors present (cf. , energy transfer to the acceptors reduces the donor excited-state lifetime to ∼3.7 ns from its acceptor-free value of ∼10.5 ns. On the other hand, with only one acceptor (cf. ), the decay of the acceptor emission is prolonged from ∼1.4 ns intrinsic lifetime to ∼7.4 ns because energy transfer can occur throughout the long donor lifetime. This result verifies that the acceptor emission is due to FRET and not direct excitation. The apparent slow rise of the acceptor emission transient (cf. insets of ,d) results from competition between the intrinsic deexcitation of the acceptor and energy transfer from the donor. A coupled four-state kinetic model involving the excited and ground states of donor and acceptor explains this process. This model uses a single exponential, exp(-t/τ D ), to describe the donor emission decay, where τ D is the donor decay with acceptors. The acceptor emission is described by exp(-t/τ D )exp(-t/τ A ), where τ A is the intrinsic de-excitation time constant of the acceptor without the donor. The bulk-level characterization, however, contains convoluted contributions from the static distribution of number of acceptor dyes around a QD donor, as well as those from the dynamic QD emission blinking and bleaching events. These issues are resolved in single-particle studies as discussed below.

Research paper thumbnail of Photolysis dynamics of diastereoselectivity in the photocyclization of o-ethoxybenzophenone and o-2,2,2-trifluoroethoxybenzophenone

Journal of Photochemistry and Photobiology A-chemistry, 2011

The photochemical reactions of o-ethoxybenzophenone (R-1) and o-2,2,2-trifluoroethoxybenzophenone... more The photochemical reactions of o-ethoxybenzophenone (R-1) and o-2,2,2-trifluoroethoxybenzophenone (R-2) in cyclohexane solutions of 0.7 M pyridine have been investigated using time-resolved laser flash photolysis to understand diastereoselectivity in the photocyclization of the two compounds. Whereas the formation times of biradical intermediates (7 ns) are the same for the two compounds, the lifetime of the biradical intermediate of R-2 (200 ns) is eight times larger than that of R-1 (25 ns), explaining why diastereoselectivity is much lower in the photocyclization of R-2.

Research paper thumbnail of Photochemistry of 1-( o-tolyl)-1-benzoylcyclopropane and 2-( o-tolyl)-2-benzoyloxirane: Determination of ring opening rates of oxiranylcarbinyl radicals

Tetrahedron Letters, 1999

Photolvsis of the title ('ottll?Ollllds results itl e.pJicient hydrogen tl'ttlt.xJer re(tt'tion t... more Photolvsis of the title ('ottll?Ollllds results itl e.pJicient hydrogen tl'ttlt.xJer re(tt'tion to prod.ce 1.5-I~iradical intermediate, which does aot.form in the photolysis

Research paper thumbnail of Photolysis Dynamics of Cyclopropyl and Oxiranyl Aryl Ketones: Drastic Ring-Activation Effect of Oxygen

Journal of Physical Chemistry A, 2001

Research paper thumbnail of Photodissociation of the BrO radical using velocity map ion imaging: Excited state dynamics and accurate D0 0 "BrO… evaluation

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-28... more We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-281.5 nm͒ and below ͑355 nm͒ the A 2 ⌸ 3/2 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O͑ 1 D 2 ͒ threshold the contribution of the direct excitation to states other than the A 2 ⌸ 3/2 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O͑ 1 D 2 ͒ threshold provides an accurate and direct determination of the A 2 ⌸ 3/2 state dissociation threshold of 35418± 35 cm −1 , leading to a ground state bond energy of D 0 0 ͑BrO͒ = 55.9± 0.1 kcal/ mol.

Research paper thumbnail of Anisotropy of photofragment recoil as a function of dissociation lifetime, excitation frequency, rotational level, and rotational constant

Quantum mechanical calculations of photofragment angular distributions have been performed as a f... more Quantum mechanical calculations of photofragment angular distributions have been performed as a function of the frequency of excitation, the lifetime of the dissociative state, the rotational level, and the rotational constant. In the limit of high J values and white, incoherent excitation, the general results are found to agree exactly with both those of Mukamel and Jortner ͓J. Chem. Phys. 61, 5348 ͑1974͔͒ and those of Jonah ͓J. Chem. Phys. 55, 1915 ͑1971͔͒. Example calculations describe how the anisotropy is dependent on the degree of broadening, the rotational constant, the initial rotational level, and the frequency of excitation. Applications are also made to interpret experimental results on the photodissociation of ClO via the 11-0, 10-0, and 6-0 bands of the A 2 ⌸ 3/2 -X 2 ⌸ 3/2 transition and on the photodissociation of O 2 via the 0-0 band of the E 3 ⌺ u − -X 3 ⌺ g − transition.

Research paper thumbnail of Anisotropy of photofragment recoil as a function of dissociation lifetime, excitation frequency, rotational level, and rotational constant

Chemical Physics, 2006

Quantum mechanical calculations of photofragment angular distributions have been performed as a f... more Quantum mechanical calculations of photofragment angular distributions have been performed as a function of the frequency of excitation, the lifetime of the dissociative state, the rotational level, and the rotational constant. In the limit of high J values and white, incoherent excitation, the general results are found to agree exactly with both those of Mukamel and Jortner [J. Chem. Phys. 61, 5348 (1974)] and those of Jonah [J. Chem. Phys. 55, 1915 (1971)]. Example calculations describe how the anisotropy is dependent on the degree of broadening, the rotational constant, the initial rotational level, and the frequency of excitation. Applications are also made to interpret experimental results on the photodissociation of ClO via the 11-0, 10-0, and 6-0 bands of the A 2Π3/2-X 2Π3/2 transition and on the photodissociation of O2 via the 0-0 band of the E 3Σu--X 3Σg- transition.

Research paper thumbnail of Photodissociation of the BrO radical using velocity map ion imaging: Excited state dynamics and accurate D00(BrO) evaluation

Chemical Physics, 2006

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278-28... more We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278-281.5 nm) and below (355 nm) the A 2Π3/2 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O(1D2) threshold the contribution of the direct excitation to states other than the A 2Π3/2 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O(1D2) threshold provides an accurate and direct determination of the A 2Π3/2 state dissociation threshold of 35418+/-35 cm-1, leading to a ground state bond energy of D00(BrO)=55.9+/-0.1 kcal/mol.

Research paper thumbnail of Vibrational state-dependent predissociation dynamics of ClO (A 2?3/2): Insight from correlated fine structure branching ratios

Physical Chemistry Chemical Physics, 2006

We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velo... more We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velocity-map ion-imaging. Experimental final correlated state branching ratios, i.e. Cl( 2 P J=3/2,1/2 ) þ O( 3 P J=2,1,0 ) channels, have been measured for v 0 = 6-11. We find that the branching ratios are highly variable and depend strongly on v 0 , providing a window into the v 0 -dependent predissociation mechanism. A comparison of the experimental results with the recent model of Lane et al. (I. C. Lane, W. H. Howie and A. J. Orr-Ewing, Phys. Chem. Chem. Phys. , 1999, 1, 3087) in both the diabatic and adiabatic limits suggests that the dynamics are closer to the diabatic limit. The overall Cl( 2 P J ) branching ratios are in good agreement with the diabatic model results. There are significant differences, however, between theory and experiment at the correlated state level, demonstrating the sensitivity of correlated measurements to the role of the exit channel coupling in the predissociation dynamics. The results highlight the need for more sophisticated quantum dynamical calculations to describe the correlated fine structure branching ratios in this system.

Research paper thumbnail of Design and characterization of late-mixing flash pyrolytic reactor molecular-beam source

We report on the design and characterization of an intense pulsed beam source for the generation ... more We report on the design and characterization of an intense pulsed beam source for the generation of expansion-cooled radical species. The source combines both dual pulsed valve late mixing for coexpansion of reactive gases with a pyrolytic end nozzle as a flash kinetic reactor. As a demonstration of the source we present evidence of BrO and ClO radical production and subsequent photodissociation studies of these species using velocity map ion imaging. The radicals are characterized by a rotational temperature of 150± 40 K although substantially lower temperatures should be possible.

Research paper thumbnail of The ultraviolet photodissociation of jet-cooled ClO and BrO radicals

Chemical Physics, 2002

The photodissociation dynamics of jet-cooled ClO and BrO radicals have been studied using resonan... more The photodissociation dynamics of jet-cooled ClO and BrO radicals have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Four asymptotic electronic channels are energetically allowed for ClO photodissociation at 235 nm, although the dominant products observed at 235 nm are Cl( 2 P 3/2 ) and O( 1 D 2 ). Polarization-dependent time-of-flight profiles indicate that this channel has an anisotropy parameter of ␤ϭ1.2Ϯ0.2. Evidence for O( 3 P J ) products was detected in coincidence with Cl( 2 P 1/2 ) fragments, and we estimate that this channel constitutes Ͻ3% at this wavelength. In the photodissociation of BrO at 355 nm we observe two asymptotic electronic state channels, Br( 2 P 3/2 )ϩO( 3 P 2 ) and Br( 2 P 1/2 ) ϩO( 3 P 2 ) in a 0.75:0.25 ratio. Both channels have an anisotropy parameter of ␤ϭ1.5Ϯ0.1. Based on the measured asymptotic velocities of both channels, we have directly determined the Br-O bond dissociation energy to be 55.8Ϯ1.0 kcal/mol, providing a heat of formation for the BrO radical at 298 K of 29.7Ϯ1.0 kcal/mol in good agreement with previous spectroscopic determinations.

Research paper thumbnail of Photodissociation of the BrO radical using velocity map ion imaging: Excited state dynamics and accurate D00(BrO) evaluation

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-28... more We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-281.5 nm͒ and below ͑355 nm͒ the A 2 ⌸ 3/2 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O͑ 1 D 2 ͒ threshold the contribution of the direct excitation to states other than the A 2 ⌸ 3/2 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O͑ 1 D 2 ͒ threshold provides an accurate and direct determination of the A 2 ⌸ 3/2 state dissociation threshold of 35418± 35 cm −1 , leading to a ground state bond energy of D 0 0 ͑BrO͒ = 55.9± 0.1 kcal/ mol.

Research paper thumbnail of The UV photodissociation dynamics of ClO radical using velocity map ion imaging

Chemical Physics, 2005

We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical fro... more We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical from 235 to 291 nm using velocity map ion imaging. We find that Cl(2P3/2)+O(1D2) is the dominant channel above the O(1D2) threshold with minor contributions from the Cl(2PJ)+O(3PJ) and Cl(2P1/2)+O(1D2) channels. We have measured the photofragment angular distributions for each dissociation channel and find that the A 2Π state reached via a parallel transition carries most of the oscillator strength above the O(1D2) threshold. The formation of O(3PJ) fragments with positive anisotropy is evidence of curve crossing from the A 2Π state to one of several dissociative states. The curve crossing probability increases with wavelength in good agreement with previous theoretical calculations. We have directly determined the O(1D2) threshold to be 38 050+/-20 cm-1 by measuring O(1D2) quantum yield in the wavelength range of 260-270 nm. We also report on the predissociation dynamics of ClO below the O(1D2) threshold. We find that the branching ratio of Cl(2P3/2)/Cl(2P1/2) is 1.5+/-0.1 at both 266 and 291 nm. The rotational depolarization of the anisotropy parameters of the Cl(2P3/2) fragments provides predissociation lifetimes of 1.5+/-0.2 ps for the 9-0 band and 1.0+/-0.4 ps for the 8-0 band, in reasonable agreement with previous spectroscopic and theoretical studies.

Research paper thumbnail of Photodissociation dynamics of Cl 2O at 235 nm using velocity map ion imaging

Journal of Photochemistry and Photobiology A-chemistry, 2011

We have investigated the photodissociation dynamics of expansion-cooled Cl 2 O at 235 nm using ph... more We have investigated the photodissociation dynamics of expansion-cooled Cl 2 O at 235 nm using photofragment velocity map ion imaging coupled with resonance-enhanced multiphoton ionization (REMPI) to measure the speed and angular distributions of the Cl( 2 P 3/2 ) and Cl( 2 P 1/2 ) products. We observe speed-dependent spatial anisotropy for both Cl fragments from the imaging data and attribute this to a fraction of nascent ClO, which undergoes secondary photodissociation. We have employed a Monte-Carlo forward-convolution scheme to analyze the data and obtain the speed distribution and anisotropy parameters for the ClO + Cl( 2 P 3/2,1/2 ) channels. We find that the speed distribution of the Cl( 2 P 3/2 ) + ClO channel is bimodal with an anisotropy parameter of 1.2 ± 0.2, whereas the speed distribution of the Cl( 2 P 1/2 ) + ClO channel consists of a single component with an anisotropy parameter of 1.0 ± 0.2. We have determined branching ratios for Cl( 2 P 3/2 ) + ClO and Cl( 2 P 1/2 ) + ClO channels to be 0.97 ± 0.02:0.03 ± 0.02. Based on the translational energy distribution of Cl( 2 P 3/2 ) + ClO channel, 28% of the parent molecule undergoes three-body dissociation.

Research paper thumbnail of Vibrational state dependent predissociation dynamics of CIO(A2PI3/2): Insight from correlated fine structure branching ratios

Physical Chemistry Chemical Physics, 2006

We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velo... more We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velocity-map ion-imaging. Experimental final correlated state branching ratios, i.e. Cl( 2 P J=3/2,1/2 ) þ O( 3 P J=2,1,0 ) channels, have been measured for v 0 = 6-11. We find that the branching ratios are highly variable and depend strongly on v 0 , providing a window into the v 0 -dependent predissociation mechanism. A comparison of the experimental results with the recent model of Lane et al. (I. C. Lane, W. H. Howie and A. J. Orr-Ewing, Phys. Chem. Chem. Phys. , 1999, 1, 3087) in both the diabatic and adiabatic limits suggests that the dynamics are closer to the diabatic limit. The overall Cl( 2 P J ) branching ratios are in good agreement with the diabatic model results. There are significant differences, however, between theory and experiment at the correlated state level, demonstrating the sensitivity of correlated measurements to the role of the exit channel coupling in the predissociation dynamics. The results highlight the need for more sophisticated quantum dynamical calculations to describe the correlated fine structure branching ratios in this system.

Research paper thumbnail of The OH-Initiated Oxidation of 1,3Butadiene in the Presence of O 2 and NO: A Photolytic Route To Study Isomeric Selective Reactivity

Journal of Physical Chemistry A, 2005

We report the study of the isomeric selective OH-initiated oxidation of 1,3-butadiene in the pres... more We report the study of the isomeric selective OH-initiated oxidation of 1,3-butadiene in the presence of O 2 and NO using the LP/LIF technique. The photolysis of monodeuterated 1-iodo-3-buten-2-ol provides only one of the possible OD-butadiene adducts, the minor addition channel product, simplifying the oxidation mechanism. We find, based on analysis of OD time-dependent traces that prompt rearrangement of initial -hydroxyalkyl radicals to R-hydroxyalkyl radicals occurs in agreement with RRKM/ME theoretical predictions. We report a rate constant of (3.3(1.0) × 10 -11 cm 3 molecules -1 s -1 for deuterium abstraction from the R-hydroxyalkyl radical at 298 (2 K. Our approach demonstrates the feasibility of isomeric selective kinetic studies of the OH-initiated oxidation of unsaturated hydrocarbons.

Research paper thumbnail of The UV photodissociation dynamics of ClO radical using velocity map ion imaging

We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical fro... more We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical from 235 to 291 nm using velocity map ion imaging. We find that Cl͑ 2 P 3/2 ͒ +O͑ 1 D 2 ͒ is the dominant channel above the O͑ 1 D 2 ͒ threshold with minor contributions from the Cl͑ 2 P J ͒ +O͑ 3 P J ͒ and Cl͑ 2 P 1/2 ͒ +O͑ 1 D 2 ͒ channels. We have measured the photofragment angular distributions for each dissociation channel and find that the A 2 ⌸ state reached via a parallel transition carries most of the oscillator strength above the O͑ 1 D 2 ͒ threshold. The formation of O͑ 3 P J ͒ fragments with positive anisotropy is evidence of curve crossing from the A 2 ⌸ state to one of several dissociative states. The curve crossing probability increases with wavelength in good agreement with previous theoretical calculations. We have directly determined the O͑ 1 D 2 ͒ threshold to be 38 050± 20 cm −1 by measuring O͑ 1 D 2 ͒ quantum yield in the wavelength range of 260-270 nm. We also report on the predissociation dynamics of ClO below the O͑ 1 D 2 ͒ threshold. We find that the branching ratio of Cl͑ 2 P 3/2 ͒ /Cl͑ 2 P 1/2 ͒ is 1.5± 0.1 at both 266 and 291 nm. The rotational depolarization of the anisotropy parameters of the Cl͑ 2 P 3/2 ͒ fragments provides predissociation lifetimes of 1.5± 0.2 ps for the 9-0 band and 1.0± 0.4 ps for the 8-0 band, in reasonable agreement with previous spectroscopic and theoretical studies.

Research paper thumbnail of Investigation of the Atmospheric Oxidation Pathways of Bromoform and Dibromomethane: Initiation via UV Photolysis and Hydrogen Abstraction

Journal of Physical Chemistry A, 2004

A computational study of the oxidation of CH 2 Br 2 and CHBr 3 initiated via both UV photolysis a... more A computational study of the oxidation of CH 2 Br 2 and CHBr 3 initiated via both UV photolysis and abstraction of an H atom by OH/Cl radicals has been performed. We have calculated the energetics associated with the addition of O 2 to the substituted bromomethyl radicals and the subsequent addition of NO to the peroxy radicals to form energized peroxy nitrite molecules. The peroxy nitrite molecules are predicted to dissociate rapidly to form alkoxy radicals (CH 2 BrO and CHBr 2 O) and NO 2 , and the kinetics of these reactions have been determined using Rice-Ramsperger-Kassel-Marcus/master equation calculations. We additionally find that the reaction of the peroxy radicals with HO 2 may directly lead to significant production of alkoxy radicals, a pathway that is unimportant in nonbrominated analogues. We predict that the alkoxy radicals will dissociate rapidly via C-Br bond cleavage. The atmospheric implications of these results will be discussed.

Research paper thumbnail of Lipid Nanotube Formation from Streptavidin−Membrane Binding

Langmuir, 2008

A novel transformation of giant lipid vesicles to produce nanotubular structures was observed upo... more A novel transformation of giant lipid vesicles to produce nanotubular structures was observed upon the binding of streptavidin to biotinylated membranes. Unlike membrane budding and tubulation processes caused by proteins involved with endocytosis and vesicle fusion, streptavidin is known to crystallize at near the isoelectric point (pI 5 to 6) into planar sheets against biotinylated films. We have found, however, that at neutral pH membranes of low bending rigidity (<10kT), such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), spontaneously produce tubular structures with widths ranging from micrometers to below the diffraction limit (<250 nm) and lengths spanning up to hundreds of micrometers. The nanotubes were typically held taut between surface-bound vesicles suggesting high membrane tension, yet the lipid nanotubes exhibited a fluidic nature that enabled the transport of entrained vesicles. Confocal microscopy confirmed the uniform coating of streptavidin over the vesicles and nanotubes. Giant vesicles composed of lipid membranes of higher bending energy exhibited only aggregation in the presence of streptavidin. Routes toward the development of these highly curved membrane structures are discussed in terms of general proteinmembrane interactions.

Research paper thumbnail of Joint Statistical Analysis of Multichannel Time Series from Single Quantum Dot−(Cy5) n Constructs

Journal of Physical Chemistry B, 2008

One of the major challenges in single-molecule studies is how to extract reliable information fro... more One of the major challenges in single-molecule studies is how to extract reliable information from the inevitably noisy data. Here, we demonstrate the unique capabilities of multichannel joint statistical analysis of multispectral time series using Föster resonance energy transfer (FRET) in single quantum dot (QD)-organic dye hybrids as a model system. The multispectral photon-by-photon registration allows model-free determination of intensity change points of the donor and acceptor channels independently. The subsequent joint analysis of these change points gives high-confidence assignments of acceptor photobleaching events despite the interference from background noise and from intermittent blinking of the QD donors and acceptors themselves. Finally, the excited-state lifetimes of donors and acceptors are calculated using the joint maximum likelihood estimation (MLE) method on the donor and acceptor decay profiles, guided by a four-state kinetics model.

Research paper thumbnail of Multiparameter Fluorescence Spectroscopy of Single Quantum Dot−Dye FRET Hybrids

Journal of The American Chemical Society, 2007

Förster resonance energy transfer (FRET) between donor and acceptor chromophores provides a sensi... more Förster resonance energy transfer (FRET) between donor and acceptor chromophores provides a sensitive mechanism for probing interactions between the chromophores and with their local environment. As donor chromophores, luminescent semiconductor quantum dots (QD) offer significant advantages over fluorescent dyes including superior photostability, broad absorption, and narrow, size-tunable emission. As a result, there is rapidly growing interest in QD-organic dye hybrids for FRET-based sensing. 1-4 At the single-particle level, these hybrids have potential for measuring the local environment of nanostructures and studying macromolecule conformational fluctuations. For most purposes multiple acceptors are bound to a single QD hub that serves as the sole energy donor. A major hurdle to using QD-dye hybrids in single-particle applications is the QD's complicated, intermittent emission, or blinking, 5-8 whose kinetic mechanism remains unknown. 9 The single-particle FRET signal in these QD-multiple-acceptor systems must be distinguished from acceptor dye photobleaching events, flickering dye emission caused by the QD donor blinking, and direct excitation of the acceptors. Here we demonstrate that the combined use of multiparameter fluorescence spectroscopy 10,11 and modelfree statistical analysis 12 can address these issues. These methods should also be applicable to analyze the complex excited-state dynamics in other multichromophore systems, where various types of multiparameter single-molecule spectroscopy have previously been employed. Our multiparameter single-molecule microscope measures the wavelength, emission delay relative to excitation (excited-state lifetime), and chronological time (intensity) for each detected photon. By measuring changes of multiple spectral parameters synchronized through correlated emission intensity jumps, high confidence event assignments can be obtained even in low signal-to-background environments. The model-free statistical analysis quantitatively determines the time of emission intensity changes, taking into account the magnitude of the change and the number of photons used in the evaluation. We have performed multiparameter FRET measurements for bulk samples of various QD605-Cy5 hybrids made with biotinylated QDs and Cy5 labeled streptavidin. panels a and b show results with, on average, about 5 and 1 dyes per QD. The FRET efficiency, indicated by the ratio of acceptor to donor emission in the spectra, reflects the number of acceptors. The time-resolved excited-state kinetics provides additional information about the energy transfer process (cf. ,d). For example, with many acceptors present (cf. , energy transfer to the acceptors reduces the donor excited-state lifetime to ∼3.7 ns from its acceptor-free value of ∼10.5 ns. On the other hand, with only one acceptor (cf. ), the decay of the acceptor emission is prolonged from ∼1.4 ns intrinsic lifetime to ∼7.4 ns because energy transfer can occur throughout the long donor lifetime. This result verifies that the acceptor emission is due to FRET and not direct excitation. The apparent slow rise of the acceptor emission transient (cf. insets of ,d) results from competition between the intrinsic deexcitation of the acceptor and energy transfer from the donor. A coupled four-state kinetic model involving the excited and ground states of donor and acceptor explains this process. This model uses a single exponential, exp(-t/τ D ), to describe the donor emission decay, where τ D is the donor decay with acceptors. The acceptor emission is described by exp(-t/τ D )exp(-t/τ A ), where τ A is the intrinsic de-excitation time constant of the acceptor without the donor. The bulk-level characterization, however, contains convoluted contributions from the static distribution of number of acceptor dyes around a QD donor, as well as those from the dynamic QD emission blinking and bleaching events. These issues are resolved in single-particle studies as discussed below.

Research paper thumbnail of Photolysis dynamics of diastereoselectivity in the photocyclization of o-ethoxybenzophenone and o-2,2,2-trifluoroethoxybenzophenone

Journal of Photochemistry and Photobiology A-chemistry, 2011

The photochemical reactions of o-ethoxybenzophenone (R-1) and o-2,2,2-trifluoroethoxybenzophenone... more The photochemical reactions of o-ethoxybenzophenone (R-1) and o-2,2,2-trifluoroethoxybenzophenone (R-2) in cyclohexane solutions of 0.7 M pyridine have been investigated using time-resolved laser flash photolysis to understand diastereoselectivity in the photocyclization of the two compounds. Whereas the formation times of biradical intermediates (7 ns) are the same for the two compounds, the lifetime of the biradical intermediate of R-2 (200 ns) is eight times larger than that of R-1 (25 ns), explaining why diastereoselectivity is much lower in the photocyclization of R-2.

Research paper thumbnail of Photochemistry of 1-( o-tolyl)-1-benzoylcyclopropane and 2-( o-tolyl)-2-benzoyloxirane: Determination of ring opening rates of oxiranylcarbinyl radicals

Tetrahedron Letters, 1999

Photolvsis of the title ('ottll?Ollllds results itl e.pJicient hydrogen tl'ttlt.xJer re(tt'tion t... more Photolvsis of the title ('ottll?Ollllds results itl e.pJicient hydrogen tl'ttlt.xJer re(tt'tion to prod.ce 1.5-I~iradical intermediate, which does aot.form in the photolysis

Research paper thumbnail of Photolysis Dynamics of Cyclopropyl and Oxiranyl Aryl Ketones: Drastic Ring-Activation Effect of Oxygen

Journal of Physical Chemistry A, 2001

Research paper thumbnail of Photodissociation of the BrO radical using velocity map ion imaging: Excited state dynamics and accurate D0 0 "BrO… evaluation

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-28... more We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-281.5 nm͒ and below ͑355 nm͒ the A 2 ⌸ 3/2 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O͑ 1 D 2 ͒ threshold the contribution of the direct excitation to states other than the A 2 ⌸ 3/2 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O͑ 1 D 2 ͒ threshold provides an accurate and direct determination of the A 2 ⌸ 3/2 state dissociation threshold of 35418± 35 cm −1 , leading to a ground state bond energy of D 0 0 ͑BrO͒ = 55.9± 0.1 kcal/ mol.

Research paper thumbnail of Anisotropy of photofragment recoil as a function of dissociation lifetime, excitation frequency, rotational level, and rotational constant

Quantum mechanical calculations of photofragment angular distributions have been performed as a f... more Quantum mechanical calculations of photofragment angular distributions have been performed as a function of the frequency of excitation, the lifetime of the dissociative state, the rotational level, and the rotational constant. In the limit of high J values and white, incoherent excitation, the general results are found to agree exactly with both those of Mukamel and Jortner ͓J. Chem. Phys. 61, 5348 ͑1974͔͒ and those of Jonah ͓J. Chem. Phys. 55, 1915 ͑1971͔͒. Example calculations describe how the anisotropy is dependent on the degree of broadening, the rotational constant, the initial rotational level, and the frequency of excitation. Applications are also made to interpret experimental results on the photodissociation of ClO via the 11-0, 10-0, and 6-0 bands of the A 2 ⌸ 3/2 -X 2 ⌸ 3/2 transition and on the photodissociation of O 2 via the 0-0 band of the E 3 ⌺ u − -X 3 ⌺ g − transition.

Research paper thumbnail of Anisotropy of photofragment recoil as a function of dissociation lifetime, excitation frequency, rotational level, and rotational constant

Chemical Physics, 2006

Quantum mechanical calculations of photofragment angular distributions have been performed as a f... more Quantum mechanical calculations of photofragment angular distributions have been performed as a function of the frequency of excitation, the lifetime of the dissociative state, the rotational level, and the rotational constant. In the limit of high J values and white, incoherent excitation, the general results are found to agree exactly with both those of Mukamel and Jortner [J. Chem. Phys. 61, 5348 (1974)] and those of Jonah [J. Chem. Phys. 55, 1915 (1971)]. Example calculations describe how the anisotropy is dependent on the degree of broadening, the rotational constant, the initial rotational level, and the frequency of excitation. Applications are also made to interpret experimental results on the photodissociation of ClO via the 11-0, 10-0, and 6-0 bands of the A 2Π3/2-X 2Π3/2 transition and on the photodissociation of O2 via the 0-0 band of the E 3Σu--X 3Σg- transition.

Research paper thumbnail of Photodissociation of the BrO radical using velocity map ion imaging: Excited state dynamics and accurate D00(BrO) evaluation

Chemical Physics, 2006

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278-28... more We have studied the photodissociation dynamics of expansion-cooled BrO radical both above (278-281.5 nm) and below (355 nm) the A 2Π3/2 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O(1D2) threshold the contribution of the direct excitation to states other than the A 2Π3/2 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O(1D2) threshold provides an accurate and direct determination of the A 2Π3/2 state dissociation threshold of 35418+/-35 cm-1, leading to a ground state bond energy of D00(BrO)=55.9+/-0.1 kcal/mol.

Research paper thumbnail of Vibrational state-dependent predissociation dynamics of ClO (A 2?3/2): Insight from correlated fine structure branching ratios

Physical Chemistry Chemical Physics, 2006

We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velo... more We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velocity-map ion-imaging. Experimental final correlated state branching ratios, i.e. Cl( 2 P J=3/2,1/2 ) þ O( 3 P J=2,1,0 ) channels, have been measured for v 0 = 6-11. We find that the branching ratios are highly variable and depend strongly on v 0 , providing a window into the v 0 -dependent predissociation mechanism. A comparison of the experimental results with the recent model of Lane et al. (I. C. Lane, W. H. Howie and A. J. Orr-Ewing, Phys. Chem. Chem. Phys. , 1999, 1, 3087) in both the diabatic and adiabatic limits suggests that the dynamics are closer to the diabatic limit. The overall Cl( 2 P J ) branching ratios are in good agreement with the diabatic model results. There are significant differences, however, between theory and experiment at the correlated state level, demonstrating the sensitivity of correlated measurements to the role of the exit channel coupling in the predissociation dynamics. The results highlight the need for more sophisticated quantum dynamical calculations to describe the correlated fine structure branching ratios in this system.

Research paper thumbnail of Design and characterization of late-mixing flash pyrolytic reactor molecular-beam source

We report on the design and characterization of an intense pulsed beam source for the generation ... more We report on the design and characterization of an intense pulsed beam source for the generation of expansion-cooled radical species. The source combines both dual pulsed valve late mixing for coexpansion of reactive gases with a pyrolytic end nozzle as a flash kinetic reactor. As a demonstration of the source we present evidence of BrO and ClO radical production and subsequent photodissociation studies of these species using velocity map ion imaging. The radicals are characterized by a rotational temperature of 150± 40 K although substantially lower temperatures should be possible.

Research paper thumbnail of The ultraviolet photodissociation of jet-cooled ClO and BrO radicals

Chemical Physics, 2002

The photodissociation dynamics of jet-cooled ClO and BrO radicals have been studied using resonan... more The photodissociation dynamics of jet-cooled ClO and BrO radicals have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Four asymptotic electronic channels are energetically allowed for ClO photodissociation at 235 nm, although the dominant products observed at 235 nm are Cl( 2 P 3/2 ) and O( 1 D 2 ). Polarization-dependent time-of-flight profiles indicate that this channel has an anisotropy parameter of ␤ϭ1.2Ϯ0.2. Evidence for O( 3 P J ) products was detected in coincidence with Cl( 2 P 1/2 ) fragments, and we estimate that this channel constitutes Ͻ3% at this wavelength. In the photodissociation of BrO at 355 nm we observe two asymptotic electronic state channels, Br( 2 P 3/2 )ϩO( 3 P 2 ) and Br( 2 P 1/2 ) ϩO( 3 P 2 ) in a 0.75:0.25 ratio. Both channels have an anisotropy parameter of ␤ϭ1.5Ϯ0.1. Based on the measured asymptotic velocities of both channels, we have directly determined the Br-O bond dissociation energy to be 55.8Ϯ1.0 kcal/mol, providing a heat of formation for the BrO radical at 298 K of 29.7Ϯ1.0 kcal/mol in good agreement with previous spectroscopic determinations.

Research paper thumbnail of Photodissociation of the BrO radical using velocity map ion imaging: Excited state dynamics and accurate D00(BrO) evaluation

We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-28... more We have studied the photodissociation dynamics of expansion-cooled BrO radical both above ͑278-281.5 nm͒ and below ͑355 nm͒ the A 2 ⌸ 3/2 state threshold using velocity map ion imaging. A recently developed late-mixing flash pyrolytic reactor source was utilized to generate an intense BrO radical molecular beam. The relative electronic product branching ratios at 355 nm and from 278 to 281.5 nm were determined. We have investigated the excited state dynamics based on both the product branching and the photofragment angular distributions. We find that above the O͑ 1 D 2 ͒ threshold the contribution of the direct excitation to states other than the A 2 ⌸ 3/2 state and the role of curve crossing is considerably larger in BrO compared to that observed for ClO, in agreement with recent theoretical studies. The measurement of low velocity photofragments resulting from photodissociation just above the O͑ 1 D 2 ͒ threshold provides an accurate and direct determination of the A 2 ⌸ 3/2 state dissociation threshold of 35418± 35 cm −1 , leading to a ground state bond energy of D 0 0 ͑BrO͒ = 55.9± 0.1 kcal/ mol.

Research paper thumbnail of The UV photodissociation dynamics of ClO radical using velocity map ion imaging

Chemical Physics, 2005

We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical fro... more We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical from 235 to 291 nm using velocity map ion imaging. We find that Cl(2P3/2)+O(1D2) is the dominant channel above the O(1D2) threshold with minor contributions from the Cl(2PJ)+O(3PJ) and Cl(2P1/2)+O(1D2) channels. We have measured the photofragment angular distributions for each dissociation channel and find that the A 2Π state reached via a parallel transition carries most of the oscillator strength above the O(1D2) threshold. The formation of O(3PJ) fragments with positive anisotropy is evidence of curve crossing from the A 2Π state to one of several dissociative states. The curve crossing probability increases with wavelength in good agreement with previous theoretical calculations. We have directly determined the O(1D2) threshold to be 38 050+/-20 cm-1 by measuring O(1D2) quantum yield in the wavelength range of 260-270 nm. We also report on the predissociation dynamics of ClO below the O(1D2) threshold. We find that the branching ratio of Cl(2P3/2)/Cl(2P1/2) is 1.5+/-0.1 at both 266 and 291 nm. The rotational depolarization of the anisotropy parameters of the Cl(2P3/2) fragments provides predissociation lifetimes of 1.5+/-0.2 ps for the 9-0 band and 1.0+/-0.4 ps for the 8-0 band, in reasonable agreement with previous spectroscopic and theoretical studies.

Research paper thumbnail of Photodissociation dynamics of Cl 2O at 235 nm using velocity map ion imaging

Journal of Photochemistry and Photobiology A-chemistry, 2011

We have investigated the photodissociation dynamics of expansion-cooled Cl 2 O at 235 nm using ph... more We have investigated the photodissociation dynamics of expansion-cooled Cl 2 O at 235 nm using photofragment velocity map ion imaging coupled with resonance-enhanced multiphoton ionization (REMPI) to measure the speed and angular distributions of the Cl( 2 P 3/2 ) and Cl( 2 P 1/2 ) products. We observe speed-dependent spatial anisotropy for both Cl fragments from the imaging data and attribute this to a fraction of nascent ClO, which undergoes secondary photodissociation. We have employed a Monte-Carlo forward-convolution scheme to analyze the data and obtain the speed distribution and anisotropy parameters for the ClO + Cl( 2 P 3/2,1/2 ) channels. We find that the speed distribution of the Cl( 2 P 3/2 ) + ClO channel is bimodal with an anisotropy parameter of 1.2 ± 0.2, whereas the speed distribution of the Cl( 2 P 1/2 ) + ClO channel consists of a single component with an anisotropy parameter of 1.0 ± 0.2. We have determined branching ratios for Cl( 2 P 3/2 ) + ClO and Cl( 2 P 1/2 ) + ClO channels to be 0.97 ± 0.02:0.03 ± 0.02. Based on the translational energy distribution of Cl( 2 P 3/2 ) + ClO channel, 28% of the parent molecule undergoes three-body dissociation.

Research paper thumbnail of Vibrational state dependent predissociation dynamics of CIO(A2PI3/2): Insight from correlated fine structure branching ratios

Physical Chemistry Chemical Physics, 2006

We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velo... more We have studied the v 0 -dependent predissociation dynamics of the ClO A 2 P 3/2 state using velocity-map ion-imaging. Experimental final correlated state branching ratios, i.e. Cl( 2 P J=3/2,1/2 ) þ O( 3 P J=2,1,0 ) channels, have been measured for v 0 = 6-11. We find that the branching ratios are highly variable and depend strongly on v 0 , providing a window into the v 0 -dependent predissociation mechanism. A comparison of the experimental results with the recent model of Lane et al. (I. C. Lane, W. H. Howie and A. J. Orr-Ewing, Phys. Chem. Chem. Phys. , 1999, 1, 3087) in both the diabatic and adiabatic limits suggests that the dynamics are closer to the diabatic limit. The overall Cl( 2 P J ) branching ratios are in good agreement with the diabatic model results. There are significant differences, however, between theory and experiment at the correlated state level, demonstrating the sensitivity of correlated measurements to the role of the exit channel coupling in the predissociation dynamics. The results highlight the need for more sophisticated quantum dynamical calculations to describe the correlated fine structure branching ratios in this system.

Research paper thumbnail of The OH-Initiated Oxidation of 1,3Butadiene in the Presence of O 2 and NO: A Photolytic Route To Study Isomeric Selective Reactivity

Journal of Physical Chemistry A, 2005

We report the study of the isomeric selective OH-initiated oxidation of 1,3-butadiene in the pres... more We report the study of the isomeric selective OH-initiated oxidation of 1,3-butadiene in the presence of O 2 and NO using the LP/LIF technique. The photolysis of monodeuterated 1-iodo-3-buten-2-ol provides only one of the possible OD-butadiene adducts, the minor addition channel product, simplifying the oxidation mechanism. We find, based on analysis of OD time-dependent traces that prompt rearrangement of initial -hydroxyalkyl radicals to R-hydroxyalkyl radicals occurs in agreement with RRKM/ME theoretical predictions. We report a rate constant of (3.3(1.0) × 10 -11 cm 3 molecules -1 s -1 for deuterium abstraction from the R-hydroxyalkyl radical at 298 (2 K. Our approach demonstrates the feasibility of isomeric selective kinetic studies of the OH-initiated oxidation of unsaturated hydrocarbons.

Research paper thumbnail of The UV photodissociation dynamics of ClO radical using velocity map ion imaging

We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical fro... more We have studied the wavelength-dependent photodissociation dynamics of jet-cooled ClO radical from 235 to 291 nm using velocity map ion imaging. We find that Cl͑ 2 P 3/2 ͒ +O͑ 1 D 2 ͒ is the dominant channel above the O͑ 1 D 2 ͒ threshold with minor contributions from the Cl͑ 2 P J ͒ +O͑ 3 P J ͒ and Cl͑ 2 P 1/2 ͒ +O͑ 1 D 2 ͒ channels. We have measured the photofragment angular distributions for each dissociation channel and find that the A 2 ⌸ state reached via a parallel transition carries most of the oscillator strength above the O͑ 1 D 2 ͒ threshold. The formation of O͑ 3 P J ͒ fragments with positive anisotropy is evidence of curve crossing from the A 2 ⌸ state to one of several dissociative states. The curve crossing probability increases with wavelength in good agreement with previous theoretical calculations. We have directly determined the O͑ 1 D 2 ͒ threshold to be 38 050± 20 cm −1 by measuring O͑ 1 D 2 ͒ quantum yield in the wavelength range of 260-270 nm. We also report on the predissociation dynamics of ClO below the O͑ 1 D 2 ͒ threshold. We find that the branching ratio of Cl͑ 2 P 3/2 ͒ /Cl͑ 2 P 1/2 ͒ is 1.5± 0.1 at both 266 and 291 nm. The rotational depolarization of the anisotropy parameters of the Cl͑ 2 P 3/2 ͒ fragments provides predissociation lifetimes of 1.5± 0.2 ps for the 9-0 band and 1.0± 0.4 ps for the 8-0 band, in reasonable agreement with previous spectroscopic and theoretical studies.

Research paper thumbnail of Investigation of the Atmospheric Oxidation Pathways of Bromoform and Dibromomethane: Initiation via UV Photolysis and Hydrogen Abstraction

Journal of Physical Chemistry A, 2004

A computational study of the oxidation of CH 2 Br 2 and CHBr 3 initiated via both UV photolysis a... more A computational study of the oxidation of CH 2 Br 2 and CHBr 3 initiated via both UV photolysis and abstraction of an H atom by OH/Cl radicals has been performed. We have calculated the energetics associated with the addition of O 2 to the substituted bromomethyl radicals and the subsequent addition of NO to the peroxy radicals to form energized peroxy nitrite molecules. The peroxy nitrite molecules are predicted to dissociate rapidly to form alkoxy radicals (CH 2 BrO and CHBr 2 O) and NO 2 , and the kinetics of these reactions have been determined using Rice-Ramsperger-Kassel-Marcus/master equation calculations. We additionally find that the reaction of the peroxy radicals with HO 2 may directly lead to significant production of alkoxy radicals, a pathway that is unimportant in nonbrominated analogues. We predict that the alkoxy radicals will dissociate rapidly via C-Br bond cleavage. The atmospheric implications of these results will be discussed.