Merged-beams for slow molecular collision experiments

Molecular collisions can be studied at very low relative kinetic energies, in the milliKelvin range, by merging codirectional beams with much higher translational energies, extending even to the kiloKelvin range, provided that the beam speeds can be closely matched. This technique provides far more intensity and wider chemical scope than methods that require slowing both collision partners. Previously, at far higher energies, merged beams have been widely used with ions and/or neutrals formed by charge transfer. Here, we assess for neutral, thermal molecular beams the range and resolution of collision energy that now appears attainable, determined chiefly by velocity spreads within the merged beams. Our treatment deals both with velocity distributions familiar for molecular beams formed by effusion or supersonic expansion, and an unorthodox variant produced by a rotating supersonic source capable of scanning the lab beam velocity over a wide range.     

Crossed ablated beams: CAB

We have examined the feasibility of performing crossed molecular beam chemistry with pulsed beams ablated from surfaces. Crossed ablated beams (CAB) could be formed by laser photodissociation, photoejection or thermal desorption involving either adsorbate molecules or substrate. We note that: (i) reagents can be formed in high flux pulses with temporal widths as short as the laser duration, (ii) reaction product signal levels are several orders of magnitude higher than for conventional crossed molecular beam experiments, (iii) the short duration of the beam crossing allows the product time-of-flight spectra to display excellent translational energy resolution, (iv) the method lends itself to the generation of free radical beams, permitting radical-radical reactions to be studied under single collision conditions, (v) reactions with cross-sections as low as 0.01 Ų should be readily observable. Other attributes of CAB include aligned reagents or beams of complexes.     

State-to-state differential and relative integral cross sections for rotationally inelastic scattering of H2O by hydrogen

State-to-state differential cross sections (DCSs) for rotationally inelastic scattering of H(2)O by H(2) have been measured at 71.2 meV (574 cm(-1)) and 44.8 meV (361 cm(-1)) collision energy using crossed molecular beams combined with velocity map imaging. A molecular beam containing variable compositions of the (J = 0, 1, 2) rotational states of hydrogen collides with a molecular beam of argon seeded with water vapor that is cooled by supersonic expansion to its lowest para or ortho rotational levels (J(KaKc) = 0(00) and 1(01), respectively). Angular speed distributions of fully specified rotationally excited final states are obtained using velocity map imaging. Relative integral cross sections are obtained by integrating the DCSs taken with the same experimental conditions. Experimental state-specific DCSs are compared with predictions from fully quantum scattering calculations on the most complete H(2)O-H(2) potential energy surface. Comparison of relative total cross sections and state-specific DCSs show excellent agreement with theory in almost all details.     

F+H2→HF+H反应的全量子态分辨的散射动力学研究

A crossed molecular beams, state-to-state scattering study was carried out on the F+H2→HF+H reaction at the collision energy of 5.02 kJ/mol, using the highly sensitive H atom Rydberg tagging time-of-flight method. All the peaks in the TOF spectra can be clearly assigned to the ro-vibrational structures of the HF product. The forward scattering of the HF product at v′=3 has been observed. The small forward scattering of the HF product at v′=2 has also been detected. Detailed theoretical analysis is required in order to fully understand the dynamical origin of these forward scattering products at this high collision energy.     

Inelastic scattering from glyoxal: collision kinematics rather than the interaction potential dominates rotational channel selection

Relative cross sections have been obtained for the rotationally and rovibrationally inelastic scattering of S1 trans-glyoxal (CHO-CHO) in its zero point level with K' = 0 from the target gases H2, D2, and He. Emphasis is placed on using crossed molecular beam conditions that provide several choices of collision kinematics (center-of-mass collision energy, relative velocity, center-of-mass collision momentum) for each collision pair. The cross sections define the state-to-state competition among numerous rotational channels involving destination states with DeltaK' ranging from 1 to >15 for collisions with each target gas and under every kinematic condition. They also resolve a similar rotational competition among rovibrational channels where the torsion nu7' is collisionally excited. The cross section sets also allow the relative overall magnitudes of the two types of scattering to be compared. The primary motivation of these experiments concerns the rotationally inelastic scattering. Earlier studies with rare gases and fixed kinematics demonstrated that the distribution of rotational cross sections is remarkably similar from one collision pair to another. The new data show that the competition among rotational channels actually has a small but distinct dependence on kinematic conditions. Data analysis shows that the dependence is a systematic function of the available collision momentum and entirely unrelated to the identity of the target gases, including the heavier rare gases used in earlier studies. The competition among the rotational energy transfer channels and its kinematic heritage is discussed in the context of a classical hard ellipse model of linear momentum to angular momentum conversion much used with room temperature systems. When adapted to our beam conditions, the resulting account of the rotational scattering is accurate and provides insight into the collisional details.     

Interfacial energy exchange and reaction dynamics in collisions of gases on model organic surfaces

Molecular beam scattering experiments and molecular dynamics simulations have been combined to develop an atomic-level understanding of energy transfer, accommodation, and reactions during collisions between gases and model organic surfaces. The work highlighted in this progress report has been motivated by the scientific importance of understanding fundamental interfacial chemical reactions and the relevance of reactions on organic surfaces to many areas of environmental chemistry. The experimental investigations have been accomplished by molecular beam scattering from ω-functionalized self-assembled monolayers (SAMs) on gold. Molecular beams provide a source of reactant molecules with precisely characterized collision energy and flux; SAMs afford control over the order, structure, and chemical nature of the surface. The details of molecular motion that affect energy exchange and scattering have been elucidated through classical-trajectory simulations of the experimental data using potential energy surfaces derived from ab initio calculations. Our investigations began by employing rare-gas scattering to explore how alkanethiol chain length and packing density, terminal group relative mass, orientation, and chemical functionality influence energy transfer and accommodation at organic surfaces. Subsequent studies of small molecule scattering dynamics provided insight into the influence of internal energy, molecular orientation, and gas–surface attractive forces in interfacial energy exchange. Building on the understanding of scattering dynamics in non-reactive systems, our work has recently explored the reaction probabilities and mechanisms for O₃ and atomic fluorine in collisions with a variety of functionalized SAM surfaces. Together, this body of work has helped construct a more comprehensive understanding of reaction dynamics at organic surfaces.     

Average cluster size determination in supersonic beams from angular distribution measurements after scattering by a buffer gas

A method for the determination of average cluster size in supersonic beams is presented. Based on angular distribution broadening of the beams caused by passing through a buffer gas, this method is well suited for in situ determination of the mean cluster size when the apparatus contains a movable detector with sufficient spatial resolution. The shape and width of the beam profile after scattering by a buffer gas are evaluated theoretically as functions of buffer gas pressure and atom-cluster collision cross-section. Experimental results are presented for an argon beam, yielding average cluster sizes between 300 and 7000 atoms depending on the stagnation pressure. Simple criteria to assess the applicability of the method to a given experimental situation are discussed. The average cluster sizes determined in this work agree quite satisfactorily with previously published values for similar beam generation conditions.     

Crossed beams study of the reaction 1CH2+C2H2-->C3H3+H

The reaction of electronically excited singlet methylene (1CH2) with acetylene (C2H2) was studied using the method of crossed molecular beams at a mean collision energy of 3.0 kcal/mol. The angular and velocity distributions of the propargyl radical (C3H3) products were measured using single photon ionization (9.6 eV) at the advanced light source. The measured distributions indicate that the mechanism involves formation of a long-lived C3H4 complex followed by simple C-H bond fission producing C3H3+H. This work, which is the first crossed beams study of a reaction involving an electronically excited polyatomic molecule, demonstrates the feasibility of crossed molecular beam studies of reactions involving 1CH2.     

Scattering experiments with hydrogen atoms. I. Differential collision cross sections for H + Ar,D + Ar and H + CH4

Differential collision cross section measurements for the scattering of hydrogen and deuterium atoms from argon and methane have been carried out with a crossed beams scattering apparatus which uses an oscillating supersonic beam as scattering target and a cryogenic bolometer as beam detector. Diffraction oscillations have been clearly resolved. The data are analyzed with a best fit computing procedure in terms of simple intermolecular energy functions. Well depth parameters for both Ar and CH₄ are 60% larger as compared with those predicted by the geometric mean combination rule while the experimental minimum of the well positions are 10% smaller as given by the arithmetic mean combining rule.     

Manipulation of slow molecular beams by static external fields

Deflection by magnetic or electric field gradients has long been used to analyze or to alter the translational trajectories of neutral gas-phase atoms or molecules. Recent work has developed sources of slow, cold molecular beams that offer means to enhance markedly the attainable deflections, which are inversely proportional to the translational kinetic energy. The sensitivity and resolution can thus be much increased, typically by factors of 10(2)-10(4). We illustrate ways to exploit this enhanced deflection capability, particularly when balancing electric and magnetic deflections. Chemical scope can be greatly extended by utilizing feeble but ubiquitous interactions, especially the induced electric dipole due to the molecular polarizability and magnetic moments resulting from molecular rotation or nuclear spins. We also examine the effect of non-Maxwellian velocity distributions produced by supersonic expansions or by quantum statistics (pertinent for ultracold beams). Generic plots are provided, employing dimensionless variables, to facilitate the design and interpretation of experiments with deflections amplified by low kinetic energy.     

High Resolution Crossed Molecular Beams Study of the H+HD→H2+D Reaction

The H+H₂ reaction is the simplest chemical reaction system and has long been the prototype model in the study of reaction dynamics. Here we report a high resolution experimental investigation of the state-to-state reaction dynamics in the H+HD→H₂+D reaction by using the crossed molecular beams method and velocity map ion imaging technique at the collision energy of 1.17 eV. D atom products in this reaction were probed by the near threshold 1+1'(vacuum ultraviolet+ultraviolet) laser ionization scheme. The ion image with both high angular and energy resolution were acquired. State-to-state differential cross sections was accurately derived. Fast forward scattering oscillations, relating with interference effects in the scattering process, were clearly observed for H₂ products at H₂(v'=0,j'=1) and H₂(v'=0,j'=3) rovibrational levels. This study further demonstrates the importance of measuring high-resolution differential cross sections in the study of state-to-state reaction dynamics in the gas phase.     

A bright, slow cryogenic molecular beam source for free radicals

We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam source capable of producing bright beams of free radicals and refractory species. Details of the beam properties (brightness, forward velocity distribution, transverse velocity spread, rotational and vibrational temperatures) are measured under varying conditions for the molecular species SrF. Under typical conditions we produce a beam of brightness 1.2 × 10(11) molecules/sr/pulse in the X(2)Σ(+)(v = 0, N(rot) = 0) state, with 140(m/s) forward velocity and a rotational temperature of ≈ 1 K. This source compares favorably to other methods for producing beams of free radicals and refractory species for many types of experiments. We provide details of construction that may be helpful for others attempting to use this method.     

Scattering resonances in slow NH3-He collisions

We theoretically study slow collisions of NH(3) molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10(-4) cm(-1) to 130 cm(-1), using fully converged quantum close-coupling calculations. To describe the interaction between the NH(3) molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH(3) beam.     

Reaction dynamics of OH+(3Sigma-)+C2H2 studied with crossed beams and density functional theory calculations

The reactions between OH+(3Sigma-) and C2H2 have been studied using crossed ion and molecular beams and density functional theory calculations. Both charge transfer and proton transfer channels are observed. Products formed by carbon-carbon bond cleavage analogous to those formed in the isoelectronic O(3P)+C2H2 reaction, e.g., 3CH2 + HCO+, are not observed. The center of mass flux distributions of both product ions at three different energies are highly asymmetric, with maxima close to the velocity and direction of the precursor acetylene beam, characteristic of direct reactions. The internal energy distributions of the charge transfer products are independent of collision energy and are peaked at the reaction exothermicity, inconsistent with either the existence of favorable Franck-Condon factors or energy resonance. In proton transfer, almost the entire reaction exothermicity is transformed into product internal excitation, consistent with mixed energy release in which the proton is transferred with both the breaking and forming bonds extended. Most of the incremental translational energy in the two higher-energy experiments appears in product translational energy, providing an example of induced repulsive energy release.     

Quasi-classical trajectory-gaussian binning study of the OH + D2 → HOD(v1',v2',v3') + D angle-velocity and vibrational distributions at a collision energy of 0.28 eV

The angle-velocity and product vibrational state distributions for the OH + D(2) reaction at a collision energy of 0.28 eV have been calculated using the quasi-classical trajectory-gaussian binning (QCT-GB) method and the Wu-Schatz-Lendvay-Fang-Harding (WSLFH) analytical potential energy surface. Comparison with high resolution molecular beam experiments shows that, differing from what happens when using the standard QCT method (i.e., histogram binning), very good results are obtained for both distributions. Hence, the strong differences previously observed between QCT and experimental results mainly come from an inadequate pseudoquantization of HOD rather than from other quantum effects. This is probably the first time that such a high level of agreement between theory and high resolution experimental data has been found in polyatomic reaction dynamics.     

Molecular beams with a tunable velocity

The merging of molecular beam methods with those of accelerator physics has yielded new tools to manipulate the motion of molecules. Over the last few years, decelerators, lenses, bunchers, traps, and storage rings for neutral molecules have been demonstrated. Molecular beams with a tunable velocity and with a tunable width of the velocity distribution can now be produced, and are expected to become a valuable tool in a variety of physical chemistry and chemical physics experiments. Here we present a compact molecular beam machine, capable of producing 3D spatially focused packets of state-selected accelerated or decelerated molecules.     

Use of central-field potentials for describing H2(D2) elastic scattering by other molecules

Differential elastic scattering cross sections have been measured for the systems H₂ + O₂, SF₆, NH₃, CO and CH₄ and for D₂ + O₂, SF₆, and NH₃ using crossed molecular beams. These experiments represent a wide variation in the size, anisotropy and initial relative collision energy E of the scattering partners, and of the corresponding de Broglie wavelengths. In all cases, rapid quantum oscillations have been resolved. From these differential cross sections, central-field potentials have been obtained which were independent of the energy and the isotopic composition of the hydrogen molecule used, as required for such potentials to be physically meaningful. Therefore, anisotropy effects do not seem important in describing the differential elastic scattering of these H₂(D₂) systems.     

Exploring the accuracy level of new potential energy surfaces for the F + HD reactions: from exact quantum rate constants to the state-to-state reaction dynamics

Exact quantum reactive scattering calculations in the collision energy range 1-250 meV have been carried out for both the isotopic product channels of the title system. The dynamical studies compares an ab initio potential energy surface (PES) recently appeared in the literature (J. Chem. Phys., 2008, 129, 011103) with other phenomenological PESs. Vibrational branching ratios, cross sections and rate constants are presented and compared with molecular beam scattering experiments as well as with chemical kinetics data. In particular, the agreement of the vibrational branching ratios with experimental measurements is improved with respect to previous studies on other PESs, mainly because of the presence of a broad peak in the HF(v' = 3) integral cross section completely absent in the previous simulations. This feature, observed by molecular beam experiments, is the fingerprint of a new reaction mechanism operative in the dynamics described by the new PES. A conjecture for its origin, able to explain many of its characteristic aspects, is analyzed and discussed.     

Intermolecular interactions of H2S with rare gases from molecular beam scattering in the glory regime and from ab initio calculations

Integral cross sections for collisions of rotationally hot H2S molecules with rare gas atoms (Ne, Ar, and Kr) have been measured, in the collision energy range of 10-60 kJ mol(-1), using a molecular beam apparatus operating under high resolution both in angle and in velocity. A well resolved glory pattern has been measured which permitted the accurate characterization of the intermolecular potentials both at long range (in the attractive region) and at intermediate distances (in the well region). Considering the conditions used in the experiments, the obtained potentials must be considered very close to the spherical averages of the full intermolecular potential energy surfaces. Extensive ab initio calculations have also been carried out in parallel in order to characterize energy minima in the potential energy surfaces and energy barriers associated to the motion of the rare gas atoms around H2S. An assessment of the relative role of the various interaction components has been also attempted: the combined analysis of experimental and theoretical results suggests that H2S-rare gas aggregates are mainly bound by nearly isotropic noncovalent interactions of the van der Waals type.     

Crossed beams and theoretical studies of the dynamics of hyperthermal collisions between Ar and ethane

Crossed molecular beams experiments and classical trajectory calculations have been used to study the dynamics of Ar+ethane collisions at hyperthermal collision energies. Experimental time-of-flight and angular distributions of ethane molecules that scatter into the backward hemisphere (with respect to their original direction in the center-of-mass frame) have been collected. Translational energy distributions, derived from the time-of-flight distributions, reveal that a substantial fraction of the collisions transfer abnormally large amounts of energy to internal excitation of ethane. The flux of the scattered ethane molecules increased only slightly from directly backward scattering to sideways scattering. Theoretical calculations show angular and translational energy distributions which are in reasonable agreement with the experimental results. These calculations have been used to examine the microscopic mechanism for large energy transfer collisions ("supercollisions"). Collinear ("head-on") or perpendicular ("side-on") approaches of Ar to the C-C axis of ethane do not promote energy transfer as much as bent approaches, and collisions in which the H atom is "sandwiched" in a bent Ar...H-C configuration lead to the largest energy transfer. The sensitivity of collisional energy transfer to the intramolecular potential energy of ethane has also been examined.