Photodissociation of propargyl chloride at 193 nm

The photodissociation of propargyl chloride (C3H3Cl) has been studied at 193 nm. Ion imaging experiments with state-selective detection of the Cl atoms and single-photon ionization of the C3H3 radicals were performed, along with measurements of the Cl + C3H3 and HCl + C3H2 recoil kinetic energy distributions, using a scattering apparatus with electron bombardment ionization detection to resolve the competing Cl and HCl elimination channels. The experiments allow the determination of the Cl (2P3/2) and Cl (2P1/2) (hereafter Cl) branching fractions associated with the C-Cl bond fission, which are determined to be 0.5 +/- 0.1 for both channels. Although prior translational spectroscopy studies by others had concluded that the low velocity signal at the Cl+ mass was due to daughter fragments of the HCl elimination products, the present work shows that Cl atoms are produced with a bimodal recoil kinetic energy distribution. The major C-Cl bond fission channel, with a narrow recoil kinetic energy distribution peaking near 40 kcal/mol, produces both Cl and Cl, whereas the minor (5%) channel, partitioning much less energy to relative kinetic energy, produces only ground spin-orbit state Cl atoms. The maximum internal energy of the radicals produced in the low-recoil-kinetic-energy channel is consistent with this channel producing electronically excited propargyl radicals. Finally, in contrast to previous studies, the present work determines the HCl recoil kinetic energy distribution and identifies the possible contribution to this spectrum from propargyl radicals cracking to C3+ ions in the mass spectrometer.     

Dynamics of Cl (2Pj) atom formation in the photodissociation of fumaryl chloride (ClCO - CH = CH - COCl) at 235 nm: a resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) study

The photodissociation dynamics of fumaryl chloride (ClCO-CH═CH-COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl ((2)P(3/2)) and Cl* ((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, β(i). The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(E(T)), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.     

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 from 235 to 291 nm using velocity map ion imaging. We find that Cl(2P(3/2))+O(1D(2)) is the dominant channel above the O(1D(2)) threshold with minor contributions from the Cl(2P(J))+O(3P(J)) and Cl(2P(1/2))+O(1D(2)) channels. We have measured the photofragment angular distributions for each dissociation channel and find that the A 2pi state reached via a parallel transition carries most of the oscillator strength above the O(1D(2)) threshold. The formation of O(3P(J)) fragments with positive anisotropy is evidence of curve crossing from the A 2pi 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(1D(2)) threshold to be 38,050+/-20 cm(-1) by measuring O(1D(2)) quantum yield in the wavelength range of 260-270 nm. We also report on the predissociation dynamics of ClO below the O(1D(2)) threshold. We find that the branching ratio of Cl(2P(3/2))/Cl(2P(1/2)) is 1.5+/-0.1 at both 266 and 291 nm. The rotational depolarization of the anisotropy parameters of the Cl(2P(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.     

Ultraviolet photodissociation of iodine monochloride (ICl) at 235, 250, and 265 nm

ICl photolysis in the ultraviolet region of the spectrum (235-265 nm) is studied using the Slice Imaging technique. The Cl?((2)P(1/2))/Cl((2)P(3/2)) and the I?((2)P(1/2))/I((2)P(3/2)) branching ratio between the I((2)P(3/2)) + Cl((2)P(3/2))∕Cl?((2)P(1/2)) and I?((2)P(1/2)) + Cl((2)P(3∕/2))∕Cl?((2)P(1/2)) channels is extracted from the respective iodine and chlorine photofragment images. We find that ground state chlorine atoms (Cl((2)P(3/2))) are formed nearly exclusively with excited state iodine atoms (I?((2)P(1/2))), while excited spin-orbit chlorine atoms (Cl?((2)P(1/2))) are concurrently produced only with ground state iodine atoms (I((2)P(3/2))). We conclude that photolysis of ICl in this UV region is a relatively "clean" source of spin-orbit excited chlorine atoms that can be used in crossed molecular beam experiments.     

Kinetics of the reactions of Cl*(2P(1/2)) and Cl(2P(3/2)) atoms with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH at 295 K

The title reactions were studied using laser flash photolysis/laser-induced-fluorescence (FP-LIF) techniques. The two spin-orbit states, Cl*(2P(1/2)) and Cl(2P(3/2)), were detected using LIF at 135.2 and 134.7 nm, respectively. Measured reaction rate constants were as follows (units of cm3 molecule(-1) s(-1)): k(Cl(2P(3/2))+CH3OH) = (5.35 +/- 0.24) x 10(-11), k(Cl(2P(3/2))+C2H5OH) = (9.50 +/- 0.85) x 10(-11), k(Cl(2P(3/2))+n-C3H7OH) = (1.71 +/- 0.11) x 10(-10), and k(Cl(2P(3/2))+i-C3H7OH) = (9.11 +/- 0.60) x 10(-11). Measured rate constants for total removal of Cl*(2P(1/2)) in collisions with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH were (1.95 +/- 0.13) x 10(-10), (2.48 +/- 0.18) x 10(-10), (3.13 +/- 0.18) x 10(-10), and (2.84 +/- 0.16) x 10(-10), respectively; quoted errors are two-standard deviations. Although spin-orbit excited Cl*(2P(1/2)) atoms have 2.52 kcal/mol more energy than Cl(2P(3/2)), the rates of chemical reaction of Cl*(2P(1/2)) with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH are only 60-90% of the corresponding Cl(2P(3/2)) atom reactions. Under ambient conditions spin-orbit excited Cl* atoms are responsible for 0.5%, 0.5%, 0.4%, and 0.7% of the observed reactivity of thermalized Cl atoms toward CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH, respectively.     

Quantum yields for Cl(2P(j)) atom formation from the photolysis of chlorofluorocarbons and chlorinated hydrocarbons at 193.3 nm

Cl(2P(3/2)) and Cl*(2P(1/2)) atoms produced from the photodissociation of chlorofluorocarbons (CFCs) and chlorinated hydrocarbons at 193.3 nm have been detected quantitatively by a technique of vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy at 135.2 and 134.7 nm for j = 1/2 and 3/2, respectively. The quantum yields for total Cl-atom formation in the 193.3 nm photolysis at 295 +/- 2 K have been determined to be 1.03 +/- 0.09, 1.01 +/- 0.08, 1.03 +/- 0.08, 1.03 +/- 0.10, 1.41 +/- 0.14, 1.02 +/- 0.08, and 0.98 +/- 0.08 for CF2Cl2, CFCl3, CH2Cl2, CHCl3, CCl4, CHFCl2, and CCl3CF3, respectively. Those results suggest that the single C-Cl bond rupture always occurs in the photolysis of these molecules except for CCl4. Formation of two Cl atoms partly takes place in the photodissociation of CCl4. The quantum yields for total Cl-atom formation in the 193.3 nm photolysis of CHBr2Cl and CHBrClCF3 are 0.27 +/- 0.02 and 0.28 +/- 0.02, respectively, which suggests that the C-Br bond rupture is a main channel in the photodissociation processes. The branching ratios between the spin-orbit states, Cl*(2P(1/2)) and Cl(2P(3/2)), have also been determined for the photodissociation of the chlorinated compounds at 193.3 nm. The UV photodissociation processes giving rise to formation of Cl(2P(j)) atoms from the chlorinated compounds studied here have been discussed.     

A 4D wave packet study of the CH3I photodissociation in the A-band. Comparison with femtosecond velocity map imaging experiments

The time-resolved photodissociation dynamics of CH(3)I in the A-band has been studied theoretically using a wave packet model including four degrees of freedom, namely the C-I dissociation coordinate, the I-CH(3) bending mode, the CH(3) umbrella mode, and the C-H symmetric stretch mode. Clocking times and final product state distributions of the different dissociation (nonadiabatic) channels yielding spin-orbit ground and excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch ν(1) and umbrella ν(2) modes) CH(3) fragments have been obtained and compared with the results of femtosecond velocity map imaging experiments. The wave packet calculations are able to reproduce with very good agreement the experimental reaction times for the CH(3)(ν(1), ν(2))+I*((2)P(1/2)) dissociation channels with ν(1) = 0 and ν(2) = 0,1,2, and also for the channel CH(3)(ν(1) = 0, ν(2) = 0)+I((2)P(3/2)). However, the model fails to predict the experimental clocking times for the CH(3)(ν(1), ν(2))+I((2)P(3/2)) channels with (ν(1), ν(2)) = (0, 1), (0, 2), and (1, 0), that is, when the CH(3) fragment produced along with spin-orbit ground state I atoms is vibrationally excited. These results are similar to those previously obtained with a three-dimensional wave packet model, whose validity is discussed in the light of the results of the four-dimensional treatment. Possible explanations for the disagreements found between theory and experiment are also discussed.     

The photodissociation dynamics of tetrachloroethylene

We present a direct current slice imaging study of tetrachloroethylene (C(2)Cl(4)) photodissociation, probing the resulting ground state Cl ((2)P(3/2)) and spin-orbit excited state Cl* ((2)P(1/2)) products. We report photofragment images, total translational energy distributions and the product branching ratio of Cl*/Cl following dissociation at 235 and 202 nm, obtained using a two-color reduced-Doppler dissociation/probe. Near 235 nm, the Cl translational energy distribution shows a peak at the limit of the available energy, indicating a direct dissociation through a σ*(C-Cl) ← π (C=C) transition, which is superimposed on a broader underlying distribution. The ground state Cl image and associated translational energy distribution at 202 nm is broad and peaked at lower energy, suggesting either internal conversion to the ground state or a lower excited state prior to dissociation. The Cl* images are similarly broad at both wavelengths. The branching ratio is presented as a function of recoil energy, but after integration shows a near-statistical average of Cl:Cl* as 70:30 at both wavelengths. All the images are largely isotropic, with anisotropy parameters (β) of 0.05 ± 0.03.     

Photodissociation of molecular bromine in solid H2 and D2: spectroscopy of the atomic bromine spin-orbit transition

We report 355 nm photodissociation studies of molecular bromine (Br2) trapped in solid parahydrogen (pH2) and orthodeuterium (oD2). The product Br atoms are observed via the spin-orbit transition ((2)P(1/2)<-- (2)P(3/2)) of atomic bromine. The quantum yield (Phi) for Br atom photoproduction is measured to be 0.29(3) in pH2 and 0.24(2) in oD2, demonstrating that both quantum solids have minimal cage effects for Br2 photodissociation. The effective Br spin-orbit splitting increases when the Br atom is solvated in solid pH2 (+1.1%) and oD2 (+1.5%); these increases are interpreted as evidence that the solvation energy of the Br ground fine structure state ((2)P(3/2)) is significantly greater than the excited state ((2)P(1/2)). Molecular bromine induced H2 infrared absorptions are detected in the Q1(0) and S1(0) regions near 4150 and 4486 cm(-1), respectively, which allow the relative Br2 concentration to be monitored as a function of 355 nm photolysis.     

Photodissociation dynamics of allyl bromide at 234, 265, and 267 nm

The photodissociation dynamics of allyl bromide was investigated at 234, 265, and 267 nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br* (2P1/2) and Br (2P3/2) atoms. The Br fragments show a bimodal translational energy distribution, while the Br* fragments reveal one translational energy distribution. The vertical excited energies and the mixed electronic character of excited states were calculated at ab initio configuration interaction method. It is presumed that the high kinetic energy bromine atoms are attributed to the predissociation from 1(pipi*) or 1(pisigma*) state to the repulsive 1(nsigma*) state, and to the direct dissociation from 3(nsigma*) and 3(pisigma*) states, while the low kinetic energy bromine atoms stem from internal conversion from the lowest 3(pipi*) state to 3(pisigma*) state.     

Iron monoxide photodissociation

The photodissociation of (56)FeO was studied by means of the velocity map imaging technique. A molecular beam of iron atoms and iron monoxide molecules was created using an electrical discharge with an iron electrode in a supersonic expansion of molecular oxygen. The ground state iron atom Fe((5)D(4)) and FeO concentrations in the molecular beam have been estimated. The dissociation energy of the FeO X (5)Delta ground electronic state was found to be D(0) (0)(FeO)=4.18+/-0.01 eV. The effective absorption cross section of FeO at 252.39 nm (vac), leading to the Fe((5)D(4))+O((3)P) dissociation channel, is approximately 1.2 x 10(-18) cm(2). A (1+1) resonantly enhanced multiphoton ionization spectrum of (56)FeO in the region 39 550-39 580 cm(-1) with rotational structure has been observed, but not assigned. Angular distributions of Fe((5)D(4)) and Fe((5)D(3)) products for the channel FeO-->Fe((5)D(4,3))+O((3)P) have been measured at several points in the 210-260 nm laser light wavelength region. The anisotropy parameter varies strongly with wavelength for both channels.     

Multiphoton dissociative ionization of tert-pentyl bromide near 265 nm

We report on the photodissociation dynamics of tert-pentyl bromide near 265 nm investigated by time-sliced velocity map imaging. The speed and angular distributions have been analyzed for both the ground-state Br((2)P(3∕2)) atom (denoted Br) and the spin-orbit excited-state Br((2)P(1∕2)) atom (denoted Br*). The speed distributions of Br and Br* atoms are all found to consist of three Gaussian components, which correlate to three independent dissociation pathways on the excited potential energy surfaces: (1) the high translational energy (E(T)) component from the prompt dissociation along the C-Br stretching mode, (2) the middle E(T) component from the repulsive mode along the C-Br stretching coupled with some bending motions, and (3) the low E(T) component from the repulsive mode along the C-Br stretching coupled with more bending motions. More interestingly, we have also observed the tert-C(5)H(11)(+) ions in 263-267 nm. The near-zero kinetic energy distributions extracted from the three tert-C(5)H(11)(+) images near 265 nm show the typical characteristics that are attributable to multiphoton dissociative ionization, suggesting the existence of a neutral superexcited state of the parent tert-pentyl bromide molecule. The contribution of bromine atoms formed in this dissociative ionization channel adds in the total relative distribution of low E(T) component in the Br*(Br) formation channel, which reasonably explains the abnormal distributions observed in between the middle and low E(T) components in the Br*(Br) formation channel.     

Vibrational structure, spin-orbit splitting, and bond dissociation energy of Cl2+(X2 Pi g) studied by zero kinetic energy photoelectron spectroscopy and ion-pair formation imaging method

The isotopomer-resolved vibrational and spin-orbit energy structures of Cl(2) (+)(X (2)Pi(g)) have been studied by one-photon zero kinetic energy photoelectron spectroscopy. The spin-orbit energy splitting for the ground vibrational state is determined as 717.7+/-1.5 cm(-1), which greatly improves on the accuracy of the previously reported data. This value is found to be in good agreement with the ab initio quantum chemical calculation taking account of the inner shell electron correlation. The first adiabatic ionization energy (IE) of Cl(2) is determined as 92 645.9+/-1.0 cm(-1). Using the ion-pair formation imaging method to discriminate signals of Cl(+)((1)D(2)) from those of Cl(+)((3)P(j)), the threshold for ion-pair (E(tipp)) production, Cl(+)((1)D(2))+Cl(-)((1)S(0))<--Cl(2)(X (1)Sigma(g) (+)), is determined as 107 096(-2) (+8) cm(-1). By using the determined IE and E(tipp) for Cl(2) and also the reported IE and electronic affinity for chlorine atom, the bond dissociation energies of Cl(2)(X (1)Sigma(g) (+)) and Cl(2) (+)(X (2)Pi(g)) have been determined as 19 990(-2) (+8) and 31 935.1(-2) (+8), respectively.     

Measurement of Br photofragment orientation and alignment from HBr photodissociation: production of highly spin-polarized hydrogen atoms

The orientation and alignment of the (2)P(3/2) and (2)P(1/2) Br photofragments from the photodissociation of HBr is measured at 193 nm in terms of a(q) ((k))(p) parameters, using slice imaging. The A (1)Pi state is excited almost exclusively, and the measured a(q) ((k))(p) parameters and the spin-orbit branching ratio show that the dissociation proceeds predominantly via nonadiabatic transitions to the a (3)Pi and 1 (3)Sigma(+) states. Conservation of angular momentum shows that the electrons of the nascent H atom cofragments (recoiling parallel to the photolysis polarization) are highly spin polarized: about 100% for the Br((2)P(1/2)) channel, and 86% for the Br((2)P(3/2)) channel. A similar analysis is demonstrated for the photodissociation of HCl.     

Chloroacetone photodissociation at 193 nm and the subsequent dynamics of the CH3C(O)CH2 radical--an intermediate formed in the OH + allene reaction en route to CH3 + ketene

We use a combination of crossed laser-molecular beam experiments and velocity map imaging experiments to investigate the primary photofission channels of chloroacetone at 193 nm; we also probe the dissociation dynamics of the nascent CH(3)C(O)CH(2) radicals formed from C-Cl bond fission. In addition to the C-Cl bond fission primary photodissociation channel, the data evidence another photodissociation channel of the precursor, C-C bond fission to produce CH(3)CO and CH(2)Cl. The CH(3)C(O)CH(2) radical formed from C-Cl bond fission is one of the intermediates in the OH + allene reaction en route to CH(3) + ketene. The 193 nm photodissociation laser allows us to produce these CH(3)C(O)CH(2) radicals with enough internal energy to span the dissociation barrier leading to the CH(3) + ketene asymptote. Therefore, some of the vibrationally excited CH(3)C(O)CH(2) radicals undergo subsequent dissociation to CH(3) + ketene products; we are able to measure the velocities of these products using both the imaging and scattering apparatuses. The results rule out the presence of a significant contribution from a C-C bond photofission channel that produces CH(3) and COCH(2)Cl fragments. The CH(3)C(O)CH(2) radicals are formed with a considerable amount of energy partitioned into rotation; we use an impulsive model to explicitly characterize the internal energy distribution. The data are better fit by using the C-Cl bond fission transition state on the S(1) surface of chloroacetone as the geometry at which the impulsive force acts, not the Franck-Condon geometry. Our data suggest that, even under atmospheric conditions, the reaction of OH with allene could produce a small branching to CH(3) + ketene products, rather than solely producing inelastically stabilized adducts. This additional channel offers a different pathway for the OH-initiated oxidation of such unsaturated volatile organic compounds, those containing a C=C=C moiety, than is currently included in atmospheric models.     

Probing the barrier for CH2CHCO --> CH2CH + CO by the velocity map imaging method

This work determines the dissociation barrier height for CH2CHCO --> CH2CH + CO using two-dimensional product velocity map imaging. The CH2CHCO radical is prepared under collision-free conditions from C-Cl bond fission in the photodissociation of acryloyl chloride at 235 nm. The nascent CH2CHCO radicals that do not dissociate to CH2CH + CO, about 73% of all the radicals produced, are detected using 157-nm photoionization. The Cl(2P(3/2)) and Cl(2P(1/2)) atomic fragments, momentum matched to both the stable and unstable radicals, are detected state selectively by resonance-enhanced multiphoton ionization at 235 nm. By comparing the total translational energy release distribution P(E(T)) derived from the measured recoil velocities of the Cl atoms with that derived from the momentum-matched radical cophotofragments which do not dissociate, the energy threshold at which the CH2CHCO radicals begin to dissociate is determined. Based on this energy threshold and conservation of energy, and using calculated C-Cl bond energies for the precursor to produce CH2CHC*O or C*H2CHCO, respectively, we have determined the forward dissociation barriers for the radical to dissociate to vinyl + CO. The experimentally determined barrier for CH2CHC*O --> CH2CH + CO is 21+/-2 kcal mol(-1), and the computed energy difference between the CH2CHC*O and the C*H2CHCO forms of the radical gives the corresponding barrier for C*H2CHCO --> CH2CH + CO to be 23+/-2 kcal mol(-1). This experimental determination is compared with predictions from electronic structure methods, including coupled-cluster, density-functional, and composite Gaussian-3-based methods. The comparison shows that density-functional theory predicts too low an energy for the C*H2CHCO radical, and thus too high a barrier energy, whereas both the Gaussian-3 and the coupled-cluster methods yield predictions in good agreement with experiment. The experiment also shows that acryloyl chloride can be used as a photolytic precursor at 235 nm of thermodynamically stable CH2CHC*O radicals, most with an internal energy distribution ranging from approximately 3 to approximately 21 kcal mol(-1). We discuss the results with respect to the prior work on the O(3P) + propargyl reaction and the analogous O(3P) + allyl system.     

Neutral photodissociation of superexcited states of molecular iodine

The formation of high-n Rydberg atoms from the neutral dissociation of superexcited states of I(2) formed by resonant two-photon excitation of molecular iodine using an ArF laser has been investigated. The high-n Rydberg atoms I* are formed by predissociation of the optically excited molecular Rydberg states I*(2)[R(B (2)Sigma(g) (+))] converging on the I(2) (+)(B (2)Sigma(g) (+)) state of the ion. Measurement of the kinetic energy release of the Rydberg I* fragments allowed the identification of the asymptotic channels as I*[R((3)P(J))]+I((2)P(32)), where the I*[R((3)P(J))] are Rydberg atoms converging on the I(+)((3)P(J)) states of the ion with J=2, 1, and 0. In the case of the I*[R((3)P(2))] fragments, the average Rydberg lifetime is observed to be 325+/-25 micros. Based on experiments on the variation of the Rydberg atom signal with the field ionizing strength, the distribution of Rydberg levels peaks at about 25-50 cm(-1) below the ionization limit.     

An investigation of nonadiabatic interactions in Cl(2Pj) + D2 via crossed-molecular-beam scattering

We have determined limits on the cross section for both electronically nonadiabatic excitation and quenching in the Cl((2)P(j)) + D(2) system. Our experiment incorporates crossed-molecular-beam scattering with state-selective Cl((2)P(12,32)) detection and velocity-mapped ion imaging. By colliding atomic chlorine with D(2), we address the propensity for collisions that result in a change of the spin-orbit level of atomic chlorine either through electronically nonadiabatic spin-orbit excitation Cl((2)P(32)) + D(2)-->Cl(*)((2)P(12)) + D(2) or through electronically nonadiabatic spin-orbit quenching Cl(*)((2)P(12)) + D(2)-->Cl((2)P(32)) + D(2). In the first part of this report, we estimate an upper limit for the electronically nonadiabatic spin-orbit excitation cross section at a collision energy of 5.3 kcal/mol, which lies above the energy of the reaction barrier (4.9 kcal/mol). Our analysis and simulation of the experimental data determine an upper limit for the excitation cross section as sigma(NA)< or =0.012 A(2). In the second part of this paper we investigate the propensity for electronically nonadiabatic spin-orbit quenching of Cl(*) following a collision with D(2) or He. We perform these experiments at collision energies above and below the energy of the reaction barrier. By comparing the amount of scattered Cl(*) in our images to the amount of Cl(*) lost from the atomic beam we obtain the maximum cross section for electronically nonadiabatic quenching as sigma(NA)< or =15(-15) (+44) A(2) for a collision energy of 7.6 kcal/mol. Our experiments show the probability for electronically nonadiabatic quenching in Cl(*) + D(2) to be indistinguishable to that for the kinematically identical system of Cl(*) + He.     

Photodissociation dynamics of CH(2)Br(2) near 234 and 267 nm

The photodissociation dynamics of CH(2)Br(2) was investigated near 234 and 267 nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton (REMPI) ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br ((2)P(3/2)) and Br* ((2)P(1/2)) atoms. The obtained translational energy distributions of Br and Br* are found consist of two components which should be come from the radical channel and secondary dissociation process, respectively. It is suggested that the symmetry reduction from C(2v) to C(s) during photodissociation invokes a non-adiabatic coupling between the 2B(1) and A(1) states. Consequently, the higher internal energy distribution of Br channel than Br* formation channel and the broader translational energy distribution of the former are presumed correlate with a variety of vibrational excitation disposal at the crossing point resulting from the larger non-adiabatic crossing from 2B(1) to A(1) state than the reverse crossing. Moreover, the measured anisotropy parameter beta indicate that fragments recoil along the Br-Br direction mostly in the photodissociation.     

Photodissociation of cyclobutyl bromide at 234 nm studied using velocity map imaging

This study investigates the 234 nm photodissociation dynamics of cyclobutyl bromide using a two-dimensional photofragment velocity imaging technique. The spin-orbit ground- and excited-state Br(2P) atoms are state-selectively detected via [2+1] resonance enhanced multiphoton ionization (REMPI), whereas the cyclobutyl radicals are ionized using 157 nm laser light. The Br(2P(3/2)) and the Br(2P(1/2)) atoms and their c-C4H7 radical cofragments evidence a single-peaked, Gaussian-shaped translational energy distribution ranging from approximately 14 to approximately 39 kcal/mol and angular distributions with significant parallel character. The Br(2P(1/2))/ Br(2P(3/2)) spin-orbit branching ratio is determined to be 0.11 +/- 0.07 by momentum match between the Br(2P) photofragments and the recoiling c-C4H7 fragments, assuming a uniform photoionization probability of the c-C4H7 radicals with an internal energy range of 10-35 kcal/mol. The REMPI line strength ratio for the detection of Br(2P(3/2)) and Br(2P(1/2)) atoms at 233.681 and 234.021 nm, respectively, is therefore derived to be 0.10 +/- 0.07. The measured recoil kinetic energies of the c-C4H7 radicals, and the resulting distribution of internal energies, indicates some of the radicals are formed with total internal energies above the barrier to isomerization and subsequent dissociation, but our analysis indicates they may be stable due to the substantial fraction of the internal energy which is partitioned to rotational energy of the radicals.