Dc Slice Ion Imaging Research Articles

Multi-ionization of the Cl2 molecule in the near-infrared femtosecond laser field.

The multi-electron ionization and subsequent dissociation of the Cl2 molecule in a near-infrared femtosecond laser field was investigated via the dc-sliced ion imaging technique. The single charged molecular ions, Cl2+, dissociate from two excited states, 2Πu and 2Σg+, with the electrons ionized from the HOMO−1 and HOMO−2 orbital, respectively. For the multi-charged molecular ions, Cl2n+ (n = 2–8), our results showed that the stretch of the inter-nuclear distance benefitted the ionization of the electrons to produce highly-charged molecular ions. In addition, compared with the traditional charge resonance enhanced ionization (CREI) model, the critical distance (Rc) for the Cl2 molecule in our experiment was a short range that depended on the charge state rather than a single point.

Channel-resolved multiorbital double ionization of molecular Cl2 in an intense femtosecond laser field

We measure the sequential double ionization and the subsequent Coulomb explosion of molecular ${\mathrm{Cl}}_{2}$ in an intense femtosecond laser field by using the dc-sliced ion imaging technique. The results indicate that not only the highest occupied molecular orbital (HOMO) but also the next two lower-lying molecular orbitals are involved in three distinguished reaction pathways. Pathways ${(1,\phantom{\rule{0.28em}{0ex}}1)}^{2}$ and ${(1,\phantom{\rule{0.28em}{0ex}}1)}^{3}$ are both dissociated from ${\mathrm{Cl}}_{2}{\phantom{\rule{0.16em}{0ex}}}^{2+}\ensuremath{\rightarrow}{\mathrm{Cl}}^{+}+{\mathrm{Cl}}^{+}$, but with the two electrons removed from HOMO and HOMO-2 in reverse sequence, and populated on the ionic excited states $^{3}\mathrm{\ensuremath{\Pi}}_{g}$ and $^{1}\mathrm{\ensuremath{\Pi}}_{g}$, respectively. The kinetic energy release differences observed in these two channels are ascribed to the nuclear motion during the ionization process. For pathway ${(1,\phantom{\rule{0.28em}{0ex}}1)}^{1}$ (also dissociated from ${\mathrm{Cl}}_{2}{\phantom{\rule{0.16em}{0ex}}}^{2+}\ensuremath{\rightarrow}{\mathrm{Cl}}^{+}+{\mathrm{Cl}}^{+}$) populated on the ionic excited states $^{1}\mathrm{\ensuremath{\Pi}}_{g}$, the isotropic angular distribution of the fragment ions is attributed to the combination of the electron density distribution of HOMO-1, the vibrational electronic wave-packet evolution, and the field excitation. Our results propose a feasible method to manipulate the electronic dynamics which takes place in the attosecond time domain via accurately tuned laser parameters of a femtosecond laser field.

Dissociative ionization of CH2Br2 in 800 and 400 nm femtosecond laser fields

We experimentally demonstrate the dissociative ionization of CH2Br2 molecules irritated by 800 and 400nm femtosecond laser fields using time-of-flight mass spectra and dc-sliced ion imaging technology. Our results show that in both laser fields, CH2Br2 can ionize to CH2Br2+, then CH2Br2+ break one CBr bond to produce fragments CH2Br(+) and Br(+). The charge assignment is determined by the ionization energy of the fragments. Additionally, in 400nm laser fields, the CH2Br2+ can overpass a transition state to form an intermediate CH2BrBr+, then the intermediate break CBr bond to produce Br2 or Br2+ via elimination channel.

Coulomb explosion of CS2 molecule under an intense femtosecond laser field**Project supported by the National Natural Science Foundation of China (Grant Nos. 51132004 and 11474096), and the Science and Technology Commission of Shanghai Municipality, China (Grant No. 14JC1401500). We acknowledge the support of the NYU-ECNU Institute of Physics at NYU Shanghai, China.

We experimentally demonstrate the Coulomb explosion process of CS2 molecule under a near-infrared (800 nm) intense femtosecond laser field by a DC-sliced ion imaging technique. We obtain the DC-sliced images of these fragment ions S+, S2+, CS+, and CS2+ by breaking one C–S bond, and assign their Coulomb explosion channels by considering their kinetic energy release and angular distribution. We also numerically simulate the dissociation dynamics of parent ions by a Coulomb potential approximation, and obtain the time evolution of Coulomb energy and kinetic energy release, which indicates that the dissociation time of parent ions decreases with the increase of the charge number k. These experimental and theoretical results can serve as a useful benchmark for those researchers who work in the related area.

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.

Reaction dynamics of Cl + butanol isomers by crossed-beam sliced ion imaging

Butanol is now prominent among the prototype renewable biofuels. We have studied oxidation of a variety of butanol isomers under single collision conditions using chlorine atom as the oxidizing agent to gain detailed insight into the energetics and dynamics of these reactions. The interaction of chlorine atom radicals with butanol isomers: n-butanol, iso-butanol, sec-butanol, and tert-butanol have been studied by crossed-beam dc slice ion imaging techniques. The hydroxybutyl radicals generated from the H-abstraction processes were probed by single photon ionization using an F2 excimer laser. After background subtraction and density-to-flux correction of the raw images, translational energy distribution and product angular distributions were generated. At low collision energy, the hydroxyalkyl products are backscattered with respect to the alcohol beam and the scattering shifts to the forward direction as the collision energy is increased. The translational energy distributions are reminiscent to that of Cl + pentane reactions we studied earlier, i.e. a sharp forward peak -80% of the collision energy appears at the high collision energy. Isomer-specific details of the reactions will be discussed.

Dynamics of H and D abstraction in the reaction of Cl atom with butane-1,1,1,4,4,4-d6

We report the primary (D-atom) and secondary (H-atom) abstraction dynamics of chlorine atom reaction with butane-1,1,1,4,4,4-d(6). The H- and D-atom abstraction channels were studied over a range of collision energies: 10.4 kcal mol(-1) and 12.9 kcal mol(-1); 5.2 kcal mol(-1) to 12.8 kcal mol(-1), respectively, using crossed molecular beam dc slice ion imaging techniques. Single photon ionization at 157 nm was used to probe the butyl radical products resulting from the H- and D-atom abstraction reactions. These two channels manifest distinct dynamics principally in the translational energy distributions, while the angular distributions are remarkably similar. The reduced translational energy distribution for the primary abstraction showed marked variation with collision energy in the backward direction, while the secondary abstraction showed this variation in the forward direction.

Imaging the dynamics of chlorine atom reactions with alkenes

We report a study of chlorine atom reactions with a series of target monounsaturated alkene molecules: 1-pentene, 1-hexene, 2-hexene, and cyclohexene. These reactions were studied using crossed-beam dc slice ion imaging at collision energies of 4 and 7 kcal/mol. Images of the reactively scattered alkenyl radical products were obtained via single photon ionization at 157 nm. The angular distributions at low collision energy are largely isotropic, suggesting the formation of a complex that has a lifetime comparable to or longer than its rotational period, followed by HCl elimination. At high collision energy, the distributions show a sharp forward peak superimposed on the isotropic component accounting for approximately 13% of the product flux. The translational energy distributions peak near zero for the backscattered product, in sharp contrast to the results for alkanes. In the forward direction, the translational energy distributions change dramatically with collision energy. At the high collision energy, a sharp forward peak at approximately 80% of the collision energy appears, quite reminiscent of results of our recent study of Cl+pentane reactions. The scattering distributions for all target molecules are similar, suggesting similarity of the reaction dynamics among these molecules. Ab initio calculations of the energetics and ionization energies for the various product channels were performed at the CBS-QB3 level to aid in interpreting the results.

PHOTODISSOCIATION DYNAMICS OF METHYLAMINE CATION AND ITS RELEVANCE TO TITAN'S IONOSPHERE

Photodissociation of CH3NH2 + has been studied using the DC sliced ion imaging technique and ab initio calculations in order to understand the formation of HCNH+, an important molecule in Titan's ionosphere. Our experimental and theoretical observations show that hydrogen loss from CH3NH2 + has two channels: one giving rise to the triplet species CH3NH+, while the other product is CH2NH2 +. The latter then decomposes further to form HCNH+. H2 loss from CH3NH2 + has only one channel, yielding CH2NH+. This species further loses H to form HCNH+. The branching ratio of the H, H2, and H+H2 loss channels is found to be 4.2:1:2.5. This is ascribed to the fact that, at these energies, the H loss has one stable triplet product channel, while most of the H2 loss product further decomposes to HCNH+.

H elimination and metastable lifetimes in the UV photoexcitation of diacetylene

We present an experimental investigation of the UV photochemistry of diacetylene under collisionless conditions. The H loss channel is studied using DC slice ion imaging with two-color reduced-Doppler detection at 243 nm and 212 nm. The photochemistry is further studied deep in the vacuum UV, that is, at Lyman-alpha (121.6 nm). Translational energy distributions for the H + C(4)H product arising from dissociation of C(4)H(2) after excitation at 243, 212, and 121.6 nm show an isotropic angular distribution and characteristic translational energy profile suggesting statistical dissociation from the ground state or possibly from a low-lying triplet state. From these distributions, a two-photon dissociation process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a one-photon dissociation process prevails. The results are interpreted with the aid of ab initio calculations on the reaction pathways and statistical calculations of the dissociation rates and product branching. In a second series of experiments, nanosecond time-resolved phototionization measurements yield a direct determination of the lifetime of metastable triplet diacetylene under collisionless conditions, as well as its dependence on excitation energy. The observed submicrosecond lifetimes suggest that reactions of metastable diacetylene are likely to be less important in Titan's atmosphere than previously believed.

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

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

State-selected imaging of HCCO radical photodissociation dynamics

We present a dc sliced ion imaging study of HCCO radical photodissociation to CH and CO at 230 nm. The measurements were made using a two-color reduced Doppler probe strategy. The CO rotational distribution was consistent with a Boltzmann distribution at 3500 K. Using the dc slice ion imaging approach, we obtained CO images for various rotational levels of CO (v=0). The results are largely consistent with earlier work, albeit with a significant 0.9 eV peak seen previously in the translational energy distributions absent in our state-selected imaging study.

Photodissociation of Spatially Aligned Acetaldehyde Cations

Photofragment translational energy and angular distributions are reported for the photodissociation of acetaldehyde cations in the wavelength range 354-363 nm obtained using the DC slice ion imaging technique. Vibrationally selected parent ions were produced by 2+1 resonance-enhanced multiphoton ionization (REMPI) via the 3s<--n Rydberg transition, with photodissociation resulting from absorption of a fourth additional photon. Three product channels were observed: HCO+, CH3CO+, and CH4+. The angular distributions reveal that all product channels have a predominantly parallel recoil anisotropy although the lower beta2 parameter of CH3CO+ indicates the concomitant presence of a perpendicular component. Furthermore, the distinct angular distribution of the CH3CO+ fragments shows a large value of the higher order Legendre polynomial term, providing evidence that acetaldehyde cations are spatially aligned during the ionization process.

Correlated vH2 and jCO product states from formaldehyde photodissociation: Dynamics of molecular elimination

A detailed study of the photoinduced molecular elimination pathway of formaldehyde on the ground state surface was carried out using high-resolution dc slice ion imaging. Detailed correlated H(2) rovibrational and CO rotational product quantum state distributions were measured by imaging spectroscopically selected CO velocity distributions following photodissociation at energies from approximately 1800 to approximately 4100 cm(-1) above the barrier to molecular elimination. Excitation to the 2(1)4(1), 2(1)4(3), 2(2)4(1), 2(2)4(3), and 2(3)4(1) bands of H(2)CO are reported here. The dependence of the product rovibrational distributions on excitation energy are discussed in light of a dynamical model which has been formulated to describe the strong product state correlations observed.

DC slice ion imaging of the ultraviolet photodissociation of BrCN

The photodissociation of jet-cooled BrCN molecules has been investigated by DC slice ion imaging of the Br atom product at wavelengths of 193 and 234 nm. The images reveal that both the ground state ( 2P 3/2) and spin–orbit excited ( 2P 1/2) bromine atoms are produced at both photolysis energies studied. The translational energy measurements show that at 193 nm the CN fragments are vibrationally cold, while the vibrational excitation is much higher in dissociation at 234 nm. Furthermore, the CN vibrational excitation in the spin–orbit excited Br product channel is higher at both wavelengths studied.

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.