DC Slice Imaging Research Articles

Dissociative Ionization and Coulomb Explosion of CHBrCl2 in Intense Near-Infrared Femtosecond Laser Fields

We experimentally demonstrate the dissociative photoionization of CHBrCl2 molecules in a femtosecond laser field by time-of-flight mass spectrum and dc-slice imaging technology. The results suggest that the low kinetic energy components are from the dissociative ionization process of single-charged molecular ions. The angular distribution of fragment Cl+ ions can be attributed to the features of dissociative state and molecular configuration, and that of Br+ ions results from the electronic wave-packet evolution and combination of the multi-dissociation processes. The high kinetic energy components are from the Coulomb explosion of multi-charged molecular ions, and the error of the C-Br distance involved in the Coulomb explosion can be explained by the movement of the effective charge center of the polyatomic molecule.

BrCl+ elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields.

By using a dc-slice imaging technique, photodissociation of 1,2-C2H4BrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl+ in an 80 fs laser field. The occurrence of fragment ion BrCl+ in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl+ in this experiment. The relative quantum yield of the BrCl+ channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl+ and its partner ion C2H4+, we concluded that BrCl+ came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-C2H4BrCl2+. With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl+ detachment from 1,2-C2H4BrCl2+ has been examined in detail. According to the ab initio calculations, two transition state structures tended to correlate with the reactant 1,2-C2H4BrCl2+ and the products BrCl+ + C2H4+. In this entire dissociation process, the C–Br and C–Cl bond lengths were observed to elongate asymmetrically, that is, the C–Br chemical bond broke firstly, and subsequently a new Br–Cl chemical bond started to emerge while the C–Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl+ detachment.

Dissociative Ionization and Coulomb Explosion of Molecular Bromocyclopropane in an Intense Femtosecond Laser Field.

The dissociative ionization and Coulomb explosion of molecular bromocyclopropane (BCP) has been experimentally investigated by time-of-flight mass spectrum and dc-slice imaging technology. The sliced 2D images, kinetic energy releases and angular distributions of the fragment ions are obtained under the intense femtosecond laser fields (8.0 × 1013–2.0 × 1014 W/cm2). The results indicated that the low kinetic energy release (KER) components come from dissociative ionization of BCP+, while the high KER components come from Coulomb explosion of BCP2+. The chemical reaction path of BCP+ has been calculated by ab initio calculation, furthermore, the C-Br bond cleavage involved Coulomb explosion channels have been revealed, and the corresponding dehydrogenation mechanism has been confirmed.

Dehydrogenation involved Coulomb explosion of molecular C2H4FBr in an intense laser field

The dissociative double ionization (DDI) of molecular 1-fluo-2-bromoethane (FBE) in an intense laser field has been investigated by dc-slice imaging technology. The DDI channels involved with dehydrogenation are revealed and it’s believed both the charge distribution and the bound character of real potential energy surfaces of parent ions play important roles in the dissociation process. The relationship between the potential energy surfaces of the precursor species and the photofragment ejection angles are also discussed and analyzed. Furthermore, the competition between the DDI channels has been studied and the CC bond cleavages dominate the DDI process at relative higher laser intensity.

Mixed transitions in the UV photodissociation of propargyl chloride revealed by slice imaging and multireference ab initio calculations.

Resonance-enhanced multiphoton ionization (REMPI) and DC slice imaging were used to detect photoproducts Cl (2P3/2), spin-orbit excited Cl* (2P1/2), and C3H3 in the photodissociation of propargyl chloride at 212 and 236 nm. Cl and Cl* translational energy distributions peak at high recoil energy, suggesting impulsive energy release in the C-Cl coordinate. Near 236 nm, photofragment angular distributions show rapidly changing anisotropy across the main peak for both Cl and Cl*, indicating excitations arising from in-plane or out-of-plane π systems. At 212 nm, the distribution is broader and isotropic for both Cl products. Ionization of C3H3 at 212 nm via 1 + 1 REMPI provides additional insight into these processes. A portion of the C3H3+ distribution is momentum-matched to the Cl, but in addition, there is a component assigned to cyclic C3H3+ formed from dissociative ionization of propargyl chloride. Multireference ab initio calculations show that excitations to three triplet states contribute at the long wavelength region, while predissociation from the first excited singlet state plays a role at the shorter wavelength.

Dissociative photoionization of 1,2-dichloroethane in intense near-infrared femtosecond laser field

We experimentally demonstrate the dissociative photoionization of 1,2-C2H4Cl2 molecules in femtosecond laser field by time-of-flight mass spectrum and dc-slice imaging technology. Our results show the low kinetic energy components are from the dissociative ionization process of singly charged molecular ions, and the positive charge assignment are greatly influenced by the appearance energy of the fragment ions. The high kinetic energy components result from Coulomb explosion of multi-charged molecular ions, and the different angular distribution of these fragments along CC and CCl bond dissociation can be explained by the potential energy surfaces of the molecular ions.

H Abstraction Channels in the Crossed-Beam Reaction of F + 1-Propanol, 1-Butene and 1-Hexene by DC Slice Imaging.

We report a crossed molecular beam study of the reaction dynamics of fluorine atom with 1-propanol, 1-butene, and 1-hexene. The product alkoxy and alkenyl radicals were detected via dc slice imaging by 157 nm single photon ionization at collision energies around 10 kcal mol-1. The analyzed data is interpreted with the aid of theoretical investigation of the relevant potential energy surfaces. The translational energy distribution and center-of-mass angular distribution of F + 1-propanol is quite similar to our previous results for F + n-butane, albeit with an increased fraction of the available energy in translation. In F atom reaction with alkenes, we also detected the HF formation channel. The low translational energy release and presence of significant backward scattering suggests the importance of an addition/elimination mechanism. Our selective single photon ionization probe allows us to examine the dynamics in minor channels in these systems. Although the probe is not sensitive to reaction at vinylic H sites, theoretical calculations consistently suggest a lower barrier from the addition complex to HF elimination involving vinylic H atoms.

Convenient (1 + 1) probe of S(1D2) and application to photodissociation of carbonyl sulfide at 216.9 nm

We introduce a 1+1 probe of S(1D) at 216.9nm through the (4s 3So) state, and demonstrate it in one-color experiments to examine the photodissociation of OCS using DC slice imaging. As seen in previous work, the S(1D) image shows a bimodal distribution corresponding to two dissociation pathways of the parent molecule. The slice images are rotationally resolved across the entire product velocity profile. Separate two-color experiments allow us to determine that the probe is insensitive to orbital polarization in the sulfur atom under the conditions studied.

Imaging NO elimination in the infrared multiphoton dissociation of nitroalkanes and alkyl nitrites

We present a DC slice imaging study of the decomposition of C2, C3 and C4 nitroalkanes and alkyl nitrites, focusing on the NO elimination channel, possibly a minor pathway. Infrared multiphoton dissociation (IRMPD) is used to induce dissociation on the ground electronic state under collisionless conditions. The channels that produced NO as a product were studied and compared among the target molecules to gain a better understanding of the isomerization of the nitroalkanes prior to dissociation. Trends in the total translational energy and NO rotational temperatures obtained from the images are discussed.

Crossed-beam DC slice imaging of fluorine atom reactions with linear alkanes.

We report the reaction dynamics of F atom with selected alkanes studied by crossed beam scattering with DC slice ion imaging. The target alkanes are propane, n-butane, and n-pentane. The product alkyl radicals are probed by 157 nm single photon ionization following reaction at a collision energy of ∼10 kcal mol(-1). The analyzed data are compared with the corresponding theoretical studies. Reduced translational energy distributions for each system show similar trends with little of the reaction exoergicity appearing in translation. However, the pentane reaction shows a somewhat smaller fraction of available energy in translation than the other two, suggesting greater energy channeled into pentyl internal degrees of freedom. The center-of-mass angular distributions all show backscattering as well as sharp forward scattering that decreases in relative intensity with the size of the molecule. Possible reasons for these trends are discussed.

Observation of hydrogen migration in cyclohexane under an intense femtosecond laser field.

We experimentally demonstrate hydrogen migration in photoionization and photofragmentation processes of cyclohexane (C6H12) under a near-infrared (800 nm) intense femtosecond laser field by the dc-slice imaging technique, and the observation of fragment ions CH3(+), C2H5(+), and C3H7(+) can be regarded as direct evidence that chemical bond rearrangement processes associated with hydrogen migration occur in the dissociative ionization process of cyclohexane. We measure the sliced images of fragment ions CH3(+), C2H5(+), and C3H7(+) and calculate their corresponding kinetic energy release (KER) and angular distributions, and it is confirmed that high-KER components of these fragment ions result from two-body Coulomb explosion of the doubly charged parent ion C6H12(2+) whereas low-KER components result from dissociative ionization of the singly charged parent ion C6H12(+). Moreover, we measure the maximally attainable relative yields of fragment ions CH3(+), C2H5(+), and C3H7(+), which approach 4.5%, 4.0%, and 3.0%, respectively.

Visible/Infrared Dissociation of NO3: Roaming in the Dark or Roaming on the Ground?

We present a DC slice imaging study of roaming dynamics in the photodissociation of the nitrate radical, NO3, contrasting pure visible excitation with a combination of visible and CO2 laser excitation at 10.6 μm. Images of specific rotational levels of NO are seen to reflect dissociation on the ground and first excited electronic states, as reported in previous work. The branching is obtained for specific rotational levels by comparison to quasiclassical trajectory calculations of the dynamics on these two surfaces. The results for the visible dissociation are found to be very similar to the combination of visible and infrared, raising questions about the nature of the coupling of these surfaces, the extent to which roaming takes place on both, and how the final product branching is determined.

Dynamics of chlorine atom reactions with hydrocarbons: insights from imaging the radical product in crossed beams.

We present a comprehensive overview of our ongoing studies applying dc slice imaging in crossed molecular beams to probe the dynamics of chlorine atom reactions with polyatomic hydrocarbons. Our approach consists in measuring the full velocity-flux contour maps of the radical products using vacuum ultraviolet "soft" photoionization at 157 nm. Our overall goal is to extend the range of chemical dynamics investigations from simple triatomic or tetraatomic molecules to systematic investigations of a sequence of isomers or a homologous series of reactants of intermediate size. These experimental investigations are augmented by high-level ab initio calculations which, taken together, reveal trends in product energy and angular momentum partitioning and offer deep insight into the reaction mechanisms as a function of structure, bonding patterns, and kinematics. We explore these issues in alkanes, for which only direct reactive encounters are found, and in unsaturated hydrocarbons, for which an addition-elimination mechanism competes with direct abstraction. The results for alkene addition-elimination in particular suggest a new view of these reactions: The only pathway to HCl elimination is accessed by means of roaming excursions of the Cl atom from the strongly bound adduct.

Concerted elimination of Br2+ resulting from the Coulomb explosion of 1,2-dibromoethane in an intense femtosecond laser field

Concerted elimination pathway leading to Br2+ and C2H4+ from 1,2-dibromoethane molecule has been investigated in 800nm femtosecond laser field by dc-slice imaging technology. The kinetic energy release and angular distributions of Br2+ and C2H4+ demonstrate that Br2+ results from the two-body Coulomb explosion of the doubly charged parent ion. Ab initio calculations show that the doubly charged precursor overcomes a small energy barrier (0.18eV) and then dissociates into Br2+ and C2H4+ through a synchronous concerted elimination mechanism. Moreover, the relative yield of the Br2+ channel is obtained, and it remains about 3.4% when the laser intensity exceeds 1.0×1014W/cm2.

Dynamics of Cl + propane, butanes revisited: a crossed beam slice imaging study

We report velocity-flux contour maps for H-D abstraction in selected Cl + alkane reactions measured by means of crossed beam scattering combined with universal DC slice imaging. The studied hydrocarbons are propane and its two selectively deuterated isotopologues, namely 1,1,1,3,3,3-propane-d6 and 2,2-propane-d2, n-butane and isobutane (2-methyl-propane), with detection of the hydrocarbon radical product by 157 nm single photon ionization. Data are obtained at collision energies of 12-13 kcal mol(-1) using a high-density atomic chlorine radical source combining Cl2 photolysis with ablation. All presented scattering distributions involving secondary and tertiary abstractions show distinct differences. Their comparisons allow for revisiting the dynamical picture of these reactions in terms of the nature of the abstraction sites, radical product energy disposal, and H vs. D abstraction. Results are discussed in the light of previous work and ab initio thermochemical calculations, along with proposals to future directions for investigation.

Coulomb explosion and dissociative ionization of 1,2-dibromoethane under an intense femtosecond laser field

Coulomb explosion and dissociative ionization of 1,2-dibromoethane are experimentally investigated in a near-infrared (800 nm) femtosecond laser field by dc-slice imaging technology.

Crossed-Beam Slice Imaging of Cl Reaction Dynamics with Butene Isomers

We present a crossed-beam imaging study of the reaction of chlorine atoms with several butene isomers. A high-intensity pulsed ablation Cl source is used with DC slice imaging and single-photon ionization detection at 157 nm to record the velocity-flux contour maps for these reactions. The target unsaturated hydrocarbons are 1-butene, trans-2-butene, cis-2-butene, and isobutene (2-methylpropene). Data are obtained at collision energies of ~13.0 kcal·mol(-1). Distinct differences in the scattering distributions and in particular the coupling of angular and translational energy release provide insight into the dynamics of this little-studied class of reactions. We find that these distributions reflect the energetics for competition between addition/elimination and direct abstraction in line with ab initio thermochemical data. A possible role for Cl atom roaming mediating the addition/elimination pathway is suggested.

Dissociative double ionization of 1-bromo-2-chloroethane irradiated by an intense femtosecond laser field

The dissociative double ionization and multi-photon ionization of 1-bromo-2-chloroethane (BCE) irradiated by the 800 nm femtosecond laser field have been investigated by dc-slice imaging technology. The charged parent ion ratio [BCE(2+)]/[BCE(+)] was measured, and the corresponding ionization process including non-sequential double ionization and sequential double ionization was analyzed. The sliced images of different photo-dissociated ions were detected, and the corresponding kinetic energy release (KER) distributions were calculated and extracted. Furthermore, the dissociative double ionization channels, attributed to the cleavage of the C-C, C-Br, and C-Cl bonds by the Coulombic repulsive forces, were discussed, and the revised equilibrium distance R(e)*, the energy ratio E(exp)/E(coul), and the value a=√(R(e)*)/(E(exp)/E(coul)) were calculated.

Conformationally selective photodissociation dynamics of propanal cation

We have previously reported experimental evidence for conformationally selective dissociation of propanal cation that was interpreted, on the basis of ab initio multiple spawning calculations, as arising from distinct dynamics in the excited state manifold of the cation. Two conical intersections (CIs) are accessible from Franck-Condon points on the dark state; however, different conformers prefer different CIs and quench to different regions on the ground state. In this paper, we extend our initial report to include experimental results for the partially deuterated propanal cation as well as detailed characterization of the ground state potential energy surface and statistical calculations of the ground state dissociation dynamics. The DC slice imaging experiments show a bimodal velocity distribution for H elimination with the observed branching ratio of the two channels different for the cis and gauche conformers. H(D)-elimination experiments from deuterated propanal cation support the dissociation mechanism proposed in the earlier report. We further investigate reaction rates on the ground state using Rice-Ramsperger-Kassel-Marcus theory. We find that the experimental results are consistent with a mechanistic picture where the ground state dissociation is statistical, and conformer specificity of the dissociation products arises because of the different populations in distinct ground state isomers after photoexcitation due to ultrafast quenching to the ground state.

Crossed-Beam Imaging of the H Abstraction Channel in the Reaction of CN with 1-Pentene

The reaction of the cyano radical with 1-pentene has been studied in crossed molecular beams experiments at a range of collision energies from 5 to 9 kcal/mol. In these studies, the pentenyl radical product is probed using single-photon ionization at 157 nm, and the product flux contour map is obtained directly using DC slice imaging. The results clearly demonstrate the presence of the hydrogen abstraction channel in this reaction, which has largely been neglected in studies of analogous systems. The reaction likely gives rise to the resonantly stabilized C5H9 radical. The yield of this channel is estimated by comparison to the reaction of the cyano radical with n-pentane. The results have implications for hydrocarbon growth and nitrile incorporation in formation of haze particles on Saturn’s moon, Titan.