Crossed-beam Conditions Research Articles

Reaction dynamics of Al + O₂ → AlO + O studied by a crossed-beam velocity map imaging technique: vib-rotational state selected angular-kinetic energy distribution.

Oxidation reaction of a gas-phase aluminum atom by a molecular oxygen was studied by a crossed-beam condition at 12.4 kJ/mol of collision energy. A (1+1) resonance-enhanced multiphoton ionization (REMPI) via the D(2)Σ(+)-X(2)Σ(+) transition of AlO was applied to ionize the product. The REMPI spectrum was analyzed to determine rotational state distributions for v = 0-2 of AlO. For several vib-rotational states of AlO, state selected angular and kinetic energy distributions were determined by a time-sliced ion imaging technique for the first time. Kinetic energy distributions were well represented by that taken into account initial energy spreads of collision energy and the population of the spin-orbit levels of the counter product O((3)P(J)) determined previously. All angular distributions showed forward and backward peaks, and the forward peaks were more pronounced than the backward one for the states of low internal energy. The backward peak intensity became comparable to the forward one for the states of high internal energy. These results and the rotational state distributions suggested that the reaction proceeds via an intermediate which has a lifetime comparable to or shorter than its rotational period.

Vector correlation between the alignment of reactant N2 (A 3Σu+) and the alignment of product NO (A 2Σ+) rotation in the energy transfer reaction of aligned N2 (A 3Σu+) + NO (X 2Π) → NO (A 2Σ+) + N2 (X 1Σg+)

The vector correlation between the alignment of reactant N2 (A (3)Σu(+)) and the alignment of product NO (A (2)Σ(+)) rotation has been studied in the energy transfer reaction of aligned N2 (A (3)Σu(+)) + NO (X (2)Π) → NO (A (2)Σ(+)) + N2 (X (1)Σg(+)) under the crossed beam condition at a collision energy of ~0.07 eV. NO (A (2)Σ(+)) emission in the two linear polarization directions (i.e., parallel and perpendicular with respect to the relative velocity vector v(R)) has been measured as a function of the alignment of N2 (A (3)Σu(+)) along its molecular axis in the collision frame. The degree of polarization of NO (A (2)Σ(+)) emission is found to depend on the alignment angle (θ(v(R))) of N2 (A (3)Σu(+)) in the collision frame. The shape of the steric opacity function at the two polarization conditions turns out to be extremely different from each other: The steric opacity function at the parallel polarization condition is more favorable for the oblique configuration of N2 (A (3)Σu(+)) at an alignment angle of θ(v(R)) ~ 45° as compared with that at the perpendicular polarization condition. The alignment of N2 (A (3)Σu(+)) is found to give a significant effect on the alignment of NO (A (2)Σ(+)) rotation in the collision frame: The N2 (A (3)Σu(+)) configuration at an oblique alignment angle θ(v(R)) ~ 45° leads to a parallel alignment of NO (A (2)Σ(+)) rotation (J-vector) with respect to v(R), while the axial and sideways configurations of N2 (A (3)Σu(+)) lead to a perpendicular alignment of NO (A (2)Σ(+)) rotation with respect to vR. These stereocorrelated alignments of the product rotation have a good correlation with the stereocorrelated reactivity observed in the multi-dimensional steric opacity function [H. Ohoyama and S. Maruyama, J. Chem. Phys. 137, 064311 (2012)].

Crossed-beam and reduced dimensionality studies of the state-to-state integral cross sections of the Cl + HCD3(v) → HCl(v′) + CD3 reaction

The state-to-state integral cross sections of the Cl+HCD3(v)→HCl(v′)+CD3 reaction have been studied experimentally and theoretically. Reactions involving both Cl(2P3/2) and Cl(2P1/2) were analysed. In the experiment, a time-sliced velocity imaging detection method under crossed-beam condition was used. For theoretical study, a multiple potential energy surface model is employed. A combined reduced dimensionality quantum dynamics and quantum chemistry method is used with explicit treatment of spin–orbit coupling. Good agreement between experiment and theory is obtained, especially for the energy threshold dependence of the cross sections.

Translational energy dependence of the Cl + CH4(v b = 0, 1) reactions: a joint crossed-beam and quasiclassical trajectory study

The Cl + CH4(v b = 0, 1) reactions were studied over a wide range of collision energies, from threshold up to 20 kcal mol−1, using joint experimental and theoretical methods. Experiments were performed under crossed-beam conditions using a time-sliced velocity imaging detection method. Both the pair-correlated integral and differential cross sections were measured. Theoretically, quasiclassical trajectory calculations were performed on a highly accurate ab initio potential energy surface. The computed results show very good agreement with experimental findings. The underlying reaction mechanisms are as below: the formation of HCl(v′ = 0) products is mainly governed by direct scatterings, whereas the HCl(v′ = 1) channel is mediated by a time-delayed mechanism, likely invoking reactive resonances. In addition, the spin-orbit excited Cl*(2 P 1/2) reactivity was experimentally characterized. The results compare favourably with a recently reported reduced dimensionality quantum dynamics calculation, and with the previous reports on the isotopically analogous reactions. Possible involvement of resonances in this spin-orbit nonadiabatic process is suggested and awaits further investigations.

Alignment Effect of N2(A3Σu+) in the Energy Transfer Reaction of Aligned N2(A3Σu+) + NO(X2Π) → NO(A2Σ+) + N2(X1Σg+)

Steric effect in the energy transfer reaction of N(2)(A(3)Σ(u)(+)) + NO(X(2)Π) → NO(A(2)Σ(+)) + N(2)(X(1)Σ(g)(+)) has been studied under crossed beam conditions at a collision energy of ~0.07 eV by using an aligned N(2)(A(3)Σ(u)(+)) beam prepared by a magnetic hexapole. The emission intensity of NO(A(2)Σ(+)) has been measured as a function of the magnetic orientation field direction (i.e., alignment of N(2)(A(3)Σ(u)(+))) in the collision frame. A significant alignment effect on the energy transfer probability is observed. The shape of the steric opacity function turns out to be most reactive at the oblique configuration of N(2)(A(3)Σ(u)(+)) with an orientation angle of γ(v(R)) ~ 45° with respect to the relative velocity vector (v(R)), which has a good correlation with the spatial distribution of the 2pπ(g)* molecular orbital of N(2)(A(3)Σ(u)(+)). We propose the electron exchange mechanism in which the energy transfer probability is dominantly controlled by the orbital overlap between N(2)(2pπ(g)*) and NO(6σ).

Experimental and theoretical study of the chemi-ionization in thermal collisions of Ne Rydberg atoms

The state dependence of associative ionization of the $ns$ and $nd$ Rydberg series of Ne atoms with $5<n<18$ at thermal collision energies is investigated experimentally using a combination of synchrotron radiation and mass spectrometry in a single-beam experiment. The $n$ dependence of the collisional and associative ionization of the Ne Rydberg atoms have been calculated using the dipole resonant mechanism, in particular the associative ionization rates for $5<{n}_{\mathrm{eff}}<20$ have been calculated for cell, single beam, and crossed beam conditions. Refinements of the theory implemented previously have been necessary in order to apply the theory to the collisions of rare gas atoms. The results of the theoretical model for the $n$ dependency of the associative ionization rates of the Ne Rydberg atoms are compared to the experimental data.

Reaction dynamics of Mo + O2 → MoO + O studied by a crossed-beam velocity map imaging technique

The oxidation reaction dynamics of gas-phase molybdenum atoms by oxygen molecules was studied under a crossed-beam condition. The product MoO was detected by a time-of-flight mass spectrometer combined with laser multi-photon ionization. An acceleration lens system designed for the ion-velocity mapping condition, a two-dimensional (2D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products at three collision energies: 10.0, 17.8, and 50.0 kJ/mol. The angular distributions showed forward and backward peaks, whose relative intensities changed by the collision energy. While two peaks had similar intensities at low collision energies, the forward peak became dominant at the highest collision energy, 50 kJ/mol. The product kinetic energy distributions showed a good correlation with the initial collision energies, i.e., almost the same energy as the collision energy appeared as the product kinetic energy. These results suggested that the reaction proceeds via an intermediate complex, and the lifetime of the complex becomes shorter than its rotational period at high collision energy.

Time-sliced ion-velocity imaging study of the reaction Y + O2→ YO + O

The oxidation reaction dynamics of the gas-phase yttrium atoms by oxygen molecules was studied under crossed-beam conditions. The product YO was detected using a time-of-flight mass spectrometer combined with laser single-photon ionization. An acceleration lens system designed for the ion-velocity mapping conditions, a two-dimensional (2-D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products. Two ionization wavelengths were used for the internal (vibrational and/or electronic) energy selective detection of YO. The single photon of the shorter wavelength (202.0 nm) can ionize all states of YO(X (2)Σ, A' (2)Δ, and A (2)Π), while electronically excited YO(A' and A) are dominantly ionized at a longer wavelength (285.0 nm). Time-sliced images were converted to the velocity and angular distributions in the center-of-mass frame. The general features of the velocity distributions of YO, determined at two wavelengths, were well represented by those expected from the statistical energy disposal model. The forward-backward symmetry was also observed for two images. These results suggest that the reaction proceeds via long-lived intermediates, and that this mechanism is consistent with previous chemiluminescence/LIF studies.

Deciphering the nature of the reactive resonance in F + CHD3: correlated differential cross-sections of the two isotopic channels

Following up a previous publication (J. Chem. Phys. 121, 813 (2004)) on the observation of a reactive resonance in the integral cross-section of the title reaction, this report elucidates the resonance fingerprints in the pair-correlated differential cross section (CDCS). By using a time-sliced, velocity-mapped (ion) imaging technique, we measured the DCS in a product pair-correlated manner under the crossed-beam conditions. Here, the focus is on the major vibration states of the two isotopic product channels: the CD3(v = 0) + HF(v) and CHD2(v = 0) + DF(v). A distinct ridge structure near reaction threshold that followed by oscillatory forward-backward peaking features is clearly revealed in the E c (collision energy)-evolution of the angular distribution of the paired products. The observed patterns are attributed to the characteristic imprints of a resonant reaction pathway. More intriguing is the observation that these resonance signatures occur at about the same collision energies in both HF + CD3 and DF + CHD2 isotope channels. To account for the experimental findings, the nature of the resonance states is delineated, which invokes two geometrical isomers of the F-H-CD3 and F-D-CHD2 configurations in the transition state region. The facile isomerization process of the resonant complexes causes the resonance wave function being delocalised over the two isomeric configurations, thus yielding similar resonance imprints in both isotopic product channels.

Imaging the pair-correlated dynamics and isotope effects of the Cl+CH2D2 reaction

By using a time-sliced (ion) velocity-imaging technique, the title reaction was investigated to interrogate the pair-correlated dynamics of HCl (DCl) in concomitance with the CHD(2) or CHD(2) ground-state product under the crossed-beam conditions. Product pair-correlated excitation functions, vibrational branching ratios, and angular distributions were obtained over a wide range of collision energies, from 2 to 22 kcal/mol. Two distinct reaction mechanisms were uncovered. Both the dominant ground-state reaction of Cl + CH(2)D(2)(v = 0) --> HCl(v = 0)/DCl(v = 0) + CHD(2)(0(0))/CH(2)D(0(0)) and the reaction forming the same product pairs from bend-excited CH(2)D(2) reactants proceed through direct mechanisms, which shift from a rebound dynamics near thresholds to a peripheral dynamics at higher E(c). Reactivity of forming the HCl(v = 1)/DCl(v = 1,2) + CHD(2)(0(0))/CH(2)D(0(0)) product pairs from ground-state reactants is small, yet shows dynamics pattern characteristics of a resonant reaction pathway. Significant spin-orbit reactivity of Cl(*)((2)P(1/2)) was discovered, and its mechanism appears to be also mediated by the resonant reaction pathway. Comparing the dynamical attributes of the H- and D-atom transfer channels, remarkable isotope effects are found-in line with the previous findings in the Cl + CH(4)/CD(4) and Cl + CHD(3) reactions.

Development of a stable source of atomic oxygen with a pulsed high-voltage discharge and its application to crossed-beam reactions

To investigate the reactions of oxygen atoms with ethene and silane in a crossed-beam condition, we developed a stable, highly intense, and short-pulsed source of atomic oxygen with a transient high-voltage discharge. Mixtures of O(2) and He served as discharge media. Utilizing a crossed molecular-beam apparatus and direct vacuum-ultraviolet ionization, we measured the temporal profiles of oxygen atoms and the time-of-flight spectra of reaction products. With O(2) 3% seeded in He as a discharge medium, oxygen atoms might have a full width as small as 13.5 micros at half maximum at a location 193 mm downstream from the discharge region. Most population of oxygen atoms is in the ground state (3)P but some in the first excited state (1)D, depending on the concentration of precursor O(2). This discharge device analogously generates carbon, nitrogen, and fluorine atoms from precursors CO, N(2), and F(2), respectively.

Disentangling mode-specific reaction dynamics from overlapped images

The hydrogen abstraction reaction between atomic chlorine and C-H stretch-excited CHD(3) was studied under crossed-beam conditions. Prior to collisions, an infrared (IR) laser was used to pump up a fraction of CHD(3) to nu(1) = 1. A time-sliced velocity imaging technique was exploited to image the recoil velocity distribution of the state-selected product CD(3)(nu = 0). For energetic reasons, the IR-on image shows severely overlapped features arising from both the excited and the un-pumped ground-state reagents. A novel threshold method was then developed to directly determine the fraction of IR-excited CHD(3) reagents, which in turn enables us to disentangle the state-selected dynamics from the overlapped images. The results reveal significant differences from previous experimental reports.

Quasiclassical trajectories on a finite element density functional potential energy surface: The C++H2O reaction revisited

A new method for the representation of potential energy surfaces (PESs) based on the p version of the finite element method is presented and applied to the PES of the [COH2]+ system in order to study the C++H2O-->[COH]++H reaction through the quasiclassical trajectory method. Benchmark ab initio computations have been performed on the most relevant stationary points of the PES through a procedure that incorporates basis set extrapolations, the contribution of the core correlation energy, and scalar relativistic corrections. The electronic structure method employed to compute the many points needed to construct the PES is a hybrid density functional approach of the B3LYP type with geometry-dependent parameters, which improves dramatically the performance with respect of the B3LYP method. The trajectory computations shed light on the behavior of the COH2+ complex formed in the collision. At a fixed relative translational energy of 0.62 eV, which corresponds to the crossed beam experiments [D. M. Sonnenfroh et al., J. Chem. Phys. 83, 3985 (1985)], the complex dissociates significantly into the reactants (37%). However, the behavior for a thermal sampling at T=300 K is significantly different because only 9% of the trajectories where capture occurs lead to dissociation into the reactants. The latter kind of behavior is coherent with the view that simple ion-molecule reactions proceed quite often at the capture rate provided it is corrected by the fraction of the electronic states which, being nearly degenerate for the reactants, become attractive at short distances. For both T=300 K and crossed beam conditions, the trajectory computations indicate that COH2+ is the critical intermediate, in agreement with a recent work [Y. Ishikawa et al., Chem. Phys. Lett. 370, 490 (2003)] and in contrast with the interpretation of the crossed beam experiments. Besides, virtually all trajectories generate COH++H (>99%), but a significant proportion of the isoformyl cation is formed with enough vibrational energy as to surmount the COH+-HCO+ isomerization barrier, about 37% at T=300 K.

State-correlation matrix of the product pair from F + CD4→ DF(ν′) + CD3(0 v20 0)

The title reaction was investigated under crossed-beam conditions at three different collision energies, E(c) = 8.4, 2.76 and 1.46 kcal mol(-1). The combination of using a (2 + 1) resonance-enhanced multiphoton ionization for tagging state-specific CD(3) products and exploiting a time-sliced velocity imaging for ion detection allows us to reveal the coincident information of the two product pairs in a state-correlated manner. The pair-correlated results are reported for the two product vibrators -- (v(2) = 0, v'), (v(2) = 1, v'), (v(2) = 2, v') and (v(2) = 3, v')-and the dynamics attributes we examined include product state distributions, energy disposals and angular distributions. Together with our earlier communications, a rather complete picture of the correlated dynamics of the title reaction emerges. One of the major findings, the anti-correlated excitations of the two product vibrators at all four energies of this study, can qualitatively be understood by kinematics arguments.

Detection of O(PJ3) atoms formed by reaction, Al+O2→ AlO+O under crossed-beam condition

The vacuum ultraviolet laser-induced fluorescence technique was employed to detect the oxygen atoms formed by the reaction, Al+O(2)--> AlO+O. The measurements were carried out under the crossed-beam condition at 12.2 kJmol of collision energy. The relative populations of three spin-orbit states of O((3)P(J)) were determined to be 3.8, 1.0, and 0.2 for J=2, 1, and 0, respectively. They show nonstatistical populations, i.e., more population in O((3)P(2)) and less population in O((3)P(0)) than the statistical expectation. These populations were almost identical for two Al beam conditions where the relative concentrations of two spin-orbit states of Al, (2)P(1/2), and (2)P(3/2), are different. These results suggest that the reaction of Al with O(2) proceeds via an intermediate complex where the memory of the initial spin-orbit state is lost. Deviation from the statistical population of O((3)P(J)) implies the occurrence of the interaction among potential surfaces in the exit channel.

Observation of a reactive resonance in the integral cross section of a six-atom reaction: F+CHD3.

The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF+CD3 product channel is surmized, which echoes the reactive resonances found previously for the F+HD-->HF+D reaction and more recently for the F+CH4 reaction.

Effects of electric fields and collisions on highly excited rubidium atoms

The effects of static and pulsed electric fields on the multiphoton ionization (MPI) of rubidium atoms at both low (atomic beam) and high (heat pipe) densities are studied using tunable OPO lasers. Two-photon excitation of np states is induced by the external electric field at both low and high densities. In addition, np signal is also seen at very low electric fields in the heat pipe, providing evidence for collision mixing as well as field mixing. At low Rb densities strong resonance features are observed in the energy region between the zero field limit (IP) and the field ionization limit. In addition, collisional detachment and charge transfer between excited ns and nd Rb Rydberg states and nozzle-jet cooled polar molecules (acetonitrile and acetone) are studied under crossed-beam conditions. The formation of dipole bound anions for acetone is only seen under nozzle jet expansion conditions and the maximum in the Rydberg electron transfer (RET) rate versusn depends upon the expansion gas (\(n_{\rm max}\) increases in the order H2, He, Ne, Ar, Xe). For acetone (low dipole moment and large \(n_{\rm max}\)), collisional detachment dominates the charge transfer, whereas for acetonitrile (high dipole moment and low \(n_{\rm max}\)), charge transfer is seen to dominate the creation of Rb+.

Velocity map imaging of ion-molecule reaction products: Co+(3F4)+isobutane

The velocity map imaging technique is applied to mass-selected CoC3H6++CH4 and CoC4H8++H2 elimination products from the Co+(3F4)+isobutane reaction studied under crossed-beam conditions at 0.21 eV collision energy. For both reactions we obtain the joint scattering probability distribution P(E,Θ), where E and Θ are the product translational energy and scattering angle. The fraction of available energy deposited into product translation is 0.4 for H2, compared with 0.1 for CH4. For the CH4 product, the angular distribution is forward-backwards symmetric and sharply peaked at Θ=0 and 180°. P(E,Θ) is not separable into the product of an energy and an angular function; rather, the angular distribution peaks more sharply at higher translational energy. Evidently, incipient CoC3H6++CH4 products equilibrate in the Co+(C3H6)(CH4) exit-channel well, from which they decay statistically. The product translational energy distribution P(E) is consistent with orbiting-transition state phase-space theory with no exit-channel barrier. In addition, the energy-integrated angular distribution T(Θ) is consistent with the predictions of the early statistical complex decay model of Miller and Herschbach for fragmentation from a transition state that is a prolate top. In sharp contrast, P(E) for the CoC4H8++H2 products exhibits a substantial hot, nonstatistical tail towards high energy. Perhaps the H2 channel has a late potential energy barrier some 0.5 eV above products, but we view this explanation as highly unlikely. Instead, we suggest that the potential energy from an earlier multicenter transition state is funneled efficiently, and highly nonstatistically, into product translation. This surprising conclusion may apply to H2 products for the entire family of reactions of the late-3D series transition metal cations Fe+, Co+, and Ni+ with alkanes.

Nonstatistical Translational Energy Distribution of H2 Elimination Products from Co+(3F4) + Propane

The velocity map imaging technique is applied to mass-selected CoC2H4+ + CH4 and CoC3H6+ + H2 elimination products from the Co+(3F4) + propane reaction studied under crossed-beam conditions at 0.19 eV collision energy. For both products, we obtain the joint scattering probability distribution P(E,ϑ), where E and ϑ are the product translational energy and scattering angle. Both angular distributions are forward-backward symmetric and only modestly anisotropic. Both product translational energy distributions P(E) are in fairly good agreement with earlier results of Bowers and co-workers, indicating that the earlier distributions were dominated by reaction of ground-state Co+. The fraction of the total available energy deposited into product translation is 0.13 for CH4 and 0.44 for H2. For CH4 products, P(E) is cold and would be readily fit by orbiting-transition-state phase-space theory (OTS−PST) without an exit-channel barrier. Evidently, incipient CoC2H4+ + CH4 products equilibrate in the Co+(C2H4)(CH4) e...

Ion-pair formation in near-thermal energy collisions of sodium clusters with electron acceptors

Sodium clusters (Nax, x⩽100) were collided with A=TCNQ, C84, C60, Br2, or SF6 at near-thermal energies under crossed beam conditions. Efficient single collisional ionization was observed for all A. Resulting cations and anions were studied by mass spectroscopy. Resonant electron transfer followed by charge separation to yield parent cluster cation (Nax+*) and anion (A−*) pairs is an important channel—given sufficient center of mass kinetic energy. It is the only charging process observed for the fullerenes and TCNQ. For Br2 and SF6, electron transfer can give rise to either charge separation or bond rearrangement/chemi-ionization. We discuss factors influencing the energetics, inelasticity, and probability of electron transfer/ion-pair formation in these systems.