Crossed beam scattering experiments with optimized energy resolution

Crossed molecular beam scattering experiments in which the energy of the collision is varied can reveal valuable insight into the collision dynamics. The energy resolution that can be obtained depends mainly on the velocity and angular spreads of the molecular beams; often, these are too broad to resolve narrow features in the cross sections like scattering resonances. The collision energy resolution can be greatly improved by making appropriate choices for the beam velocities and the beam intersection angle. This method works particularly well for situations in which one of the beams has a narrow velocity spread, and we here discuss the implications of this method for crossed beam scattering experiments with Stark-decelerated beams.     

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.     

The excitation function for Li + HF → LiF + H at collision energies below 80 meV

We have measured the dependence of the relative integral cross section of the reaction Li + HF → LiF + H on the collision energy (excitation function) using crossed molecular beams. By varying the intersection angle of the beams from 37° to 90° we covered the energy range 25 meV ≤ E(tr) ≤ 131 meV. We observe a monotonous rise of the excitation function with decreasing energy over the entire energy range indicating that a possible translational energy threshold to the reaction is significantly smaller than 25 meV. The steep rise is quantitatively recovered by a Langevin-type excitation function based on a vanishing threshold and a mean interaction potential energy ∝R(-2.5) where R is the distance between the reactants. To date all threshold energies deduced from ab initio potentials and zero-point vibrational energies are at variance with our results, however, our findings support recent quantum scattering calculations that predict significant product formation at collision energies far below these theoretical thresholds.     

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.     

Experimental investigation of the molecular ions of the xylenes

A mass spectrometric method of distinguishing between molecular ions of the three isomeric xylenes (dimethylbenzenes) was sought, in light of recent findigs that photoexcited ions could be distinguished via measurements of kinetic energy release accompanying expulsion of a methyl radical. Provided the molecular ions are formed with low internal energies, reproducible differences were found between relative intensities of collision induced reactions of higher critical energies, than for methyl expulsion. These differences exist both for collision energies in the kilovolt range (double focusing mass spectrometers) and in the range of a few tens of volts (triple quadrupole instrument). Though statistically significant, these differences were small. The mechanism of isomerization and fragmentation was investigated via isotopic labelling studies and measurements of kinetic energy release. Most of the present findings can be rationalized in terms of the most recent version of established mechanisms for reactions of ionized methylbenzenes.     

Generation of intense secondary pulsed molecular beams of high kinetic energy controllable by a powerful IR laser

A method for generation of intense secondary pulsed molecular beams and beams of radicals of high kinetic energy controllable by a powerful IR laser is described. A pressure shock (shock wave) is used as a source of secondary beams. The pressure shock is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beams were studied. Their intensities and the degree of gas cooling in them were shown to be comparable with the corresponding characteristics of the unperturbed primary beam. The acceleration of molecules in the secondary beam is achieved due to vibrational excitation of them by high-power IR laser pulse in the pressure shock and subsequent vibrational to translational (V–T) relaxation, which occurs when a gas expands through the orifice into a vacuum. Intense [10²⁰ molecules/(sr s)] beams of SF₆ and CF₃I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases. The SF₆ molecular beams with kinetic energies approximately from 2.5 to 2.7 eV with carrier gases H₂, He and CH₄ (SF₆/carriergas=1/10) were obtained. The possibility of generation of intense beams of cold radicals by this method is demonstrated. The intense beams of cold and accelerated CF₃ radicals were generated when the CF₃I molecules in the shock were dissociated by high-power CO₂ laser radiation. The spectral and energetic characteristics of acceleration of SF₆ and CF₃I molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules.     

Shock wave model for sputtering biomolecules using massive cluster impacts

A shock wave model is proposed to explain certain features of recently reported spectra obtained by massive duster impact (MCI) mass spectrometry. It is suggested that clusters that impact glycerol matrices with energies/nucleon in the range 0.01 eV/u < E/N < 1.0 eV/u provide an extremely soft method for sputtering intact biomolecules, Compared to the high energy/nucleon characteristic of atomic or molecular ion primary beams (typically < 50 eV/u), massive cluster primary beams possess much lower energies/nucleon, which are insufficient to cause appreciable ionization and radiation damage of matrix material. Moreover, fragmentation products of parent molecular ions are effectively lower. With these benefits, MCI spectra show lower chemical noise background and enhanced signalto-noise ratios. Rankine-Hugoniot analysis of the shock conditions is used to arrive at an estimate of the heat retained in the collision-affected matrix volume after bombardment by a characteristic cluster. For a cluster collision resulting in a 26.8 GPa shock pressure, by analogy with water data, rapid heating of the shocked volume to 1000 °C or more is plausible. In a beam consisting of clusters distributed in size and charge, an estimate is made for the range of cluster sizes over which hyrodynamic shock wave theory applies.     

Reactions of O+ with CnH2n+2, n=2-4: a guided-ion beam study

We have measured absolute reaction cross sections for the interaction of O(+) with ethane, propane, and n-butane at collision energies in the range from near thermal to approximately 20 eV, using the guided-ion beam (GIB) technique. We have also measured product recoil velocity distributions using the GIB time-of-flight (TOF) technique for several product ions at a series of collision energies. The total cross sections for each alkane are in excess of 100 A(2) at energies below approximately 2 eV, and in each case several ionic products arise. The large cross sections suggest reactions that are dominated by large impact parameter collisions, as is consistent with a scenario in which the many products derive from a near-resonant, dissociative charge-transfer process that leads to several fragmentation pathways. The recoil velocities, which indicate product ions with largely thermal velocity distributions, support this picture. Several product ions, most notably the C(2)H(3) (+) fragment for each of the alkanes, exhibit enhanced reaction efficiency as collision energy increases, which can be largely attributed to endothermic channels within the dissociative charge-transfer mechanism.     

Quantum-state-resolved CO2 scattering dynamics at the gas-liquid interface: dependence on incident angle

Energy transfer dynamics at the gas-liquid interface have been probed with a supersonic molecular beam of CO2 and a clean perfluorinated-liquid surface in vacuum. High-resolution infrared spectroscopy measures both the rovibrational state populations and the translational distributions for the scattered CO2 flux. The present study investigates collision dynamics as a function of incident angle (thetainc = 0 degrees, 30 degrees, 45 degrees, and 60 degrees), where column-integrated quantum state populations are detected along the specular-scattering direction (i.e., thetascat approximately thetainc). Internal state rovibrational and Doppler translational distributions in the scattered CO2 yield clear evidence for nonstatistical behavior, providing quantum-state-resolved support for microscopic branching of the gas-liquid collision dynamics into multiple channels. Specifically, the data are remarkably well described by a two-temperature model, which can be associated with both a trapping desorption (TD) component emerging at the surface temperature (Trot approximately TS) and an impulsive scattering (IS) component appearing at hyperthermal energies (Trot > TS). The branching ratio between the TD and IS channels is found to depend strongly on thetainc, with the IS component growing dramatically with increasingly steeper angle of incidence.     

Quantum state-resolved study of the four-atom reaction OH- (X1sigma+) + D2 (X1sigma(g)+, v = 0) --> HOD (X1A', v') + D- (1S)

The D+ transfer reaction between OH- (X1sigma+) and D2 was studied with crossed molecular beam experiments and quantum chemical calculations at collision energies of 89 and 68 kJ/mol. The D- product ions were observed and measured for the first time in the crossed beam experiments. The center-of-mass (c.m.) flux distributions of the D- product ions exhibit significant asymmetry, and their maxima are close to the velocity and direction of the precursor D2 beam. The data are consistent with a direct mechanism that occurs on a time scale significantly less than a rotational period of the transient complex formed by approaching reactants. The D+ transfer results primarily in the excitation of the H-O-D bending vibrational mode of the molecular product. The experimental observation is in agreement with theoretical results showing that, during the D+ transfer, the H-O-D bond angle changes significantly.     

Present and future positron-molecular scattering experiments

Progress to date (June, 1999) of the Oak Ridge positron scattering project is reviewed. Results from our positron time-of-flight mass spectrometer include ionization and fragmentation of some halomethanes, positronium formation below the threshold for double ionization of Ne and the heavier noble gases, and the production of HeH⁺ by positron impact on mixtures of He and H₂. New directions for future experimental work are discussed. These entail the construction and testing of a new spectrometer which measures both the masses and recoil energies of charged products, including secondary electrons. We hope to use an intense positron beam, such as the one at Lawrence Livermore National Laboratory or a similar beam, as the ionizing agent for the new spectrometer, which contains a pulsed supersonic molecular beam. With such an instrument, a range of new phenomena can be studied.     

Influence of reagent collision energy on the rotational distribution of BaO in the reaction of SO2 with Ba

The rotational distribution of the product BaO of the crossed molecular beam reaction of Ba with SO₂ was studied with interest in its dependence on collision energy. The distribution was probed by laser-induced fluorescence at collision energies ranging from 1.2 to 7.2 kcal/mol. The rotational excitation was found to increase very slowly with collision energy. The observed distribution was compared with calculations based on the phase space theory and the transition state theory. As a result, the phase space theory reproduced the observed distribution only at low collision energy, while the transition state theory reproduced it satisfactorily over a wide range of collision energy. This feature was interpreted in terms of the angular momentum restriction involved in the reaction. The present result was consistent with the angular distribution and recoil velocity spectrum study.     

Aerodynamical acceleration and rotational-vibrational temperatures in seeded supersonic molecular beams

The time-of-flight (TOF) technique was used to study the aerodynamical acceleration in seeded supersonic molecular beams of heavy molecules and light seeding gas. We also studied the correlation between the degree of aerodynamic acceleration achieved, and rotational-vibrational temperatures as measured using the laser-induced fluorescence (LIF) technique. The velocity slip (difference) between helium and hydrogen carrier gases and iodine and aniline heavy molecules was determined in free-jet expansion by TOF measurements and compared with rotational temperatures measured by LIF. The helium translational temperature was found to be abnormally high and dependent on teh heavy-molecule concentration, even at concentration as low as 400 ppm. In the case of iodine it was found that the rotational degrees of freedom were equilibrated with the helium or hydrogen seeding gas translational and slip temperatures, although this temperature was more than an order of magnitude higher than theoretical predictions obtained for the pure-gas expansion. In aniline, the rotational temperature is found to be higher than the gas-dynamic temperature and rotational relaxation is incomplete. The heavy-molecule kinetic energy increases linearly with the light-gas pressure up to = 50% of its maximum available kinetic energy. The possibility of making accurate heavy-molecule kinetic-energy measurements using the LIF technique is discussed. It is claimed that the existence of velocity slip can largely effect theoretical calculations concerning vibrational and rotational relaxation in seeded supersonic beams.     

Modification of the velocity distribution of H(2) molecules in a supersonic beam by intense pulsed optical gradients

We report the acceleration and deceleration of H(2) molecules in a supersonic molecular beam by means of its interaction with an intense optical gradient from a nanosecond far-off-resonant optical pulse. The strong optical gradients are formed in the interference pattern of two intense optical pulses at 532 nm. The velocity distribution of the molecular beam, before and after the applied optical pulse, is measured by a velocity-mapped ion imaging technique. Changes in velocity up to 202 m s(-1)+/- 61 m s(-1) are observed in a molecular beam initially travelling at a mean speed of 563 m s(-1). We report the dependence of this change in velocity with the strength of the optical gradient applied.     

Mean Gas Cluster Size Determination from Cluster Beam Cross-Section

This paper describes the simple experimental method of size determination of gas clusters in molecular beams formed from supersonic jets. Mean cluster size N is calculated from broadening of the transverse profile of beam intensity at a fixed distance behind the skimmer. The described method allows determining the mean sizes of the clusters of any pure gases. It does not require the building of some special models, or determination of empirical constants. Due to the high intensity of the supersonic beams, the measurements do not require any complex highly sensitive equipment. The effectiveness of the present method is validated by measurements in a cluster beams of test gases (easily condensable CO₂, Ar, and weakly condensable N₂) and the beam of C₂H₄ (ethylene), formed from a supersonic jet behind conical nozzles. The certainty of measured characteristics is confirmed by the results of numerical simulations. By using the described method the mean cluster sizes from 50 to 2000 molecules per cluster were determined. The correctness of the obtained cluster sizes of CO₂ and Ar is proved by comparison with results of other authors, obtained by other experimental methods, and estimations according to the empirical correlations using condensation scaling parameter Г^*.     

Effect of molecular rotation on the atomic alignment dependence in the oriented Ar (3P2) + CF3H reaction

Atomic alignment effect for the CF3* formation in the oriented Ar (3P2, MJ = 2) + CF3H reaction has been investigated at different two CF3H beam conditions: effusive and supersonic beams. The chemiluminescence intensity of CF3* was measured as a function of the magnetic orientation field direction in the collision frame. A significant contribution of rank 4 moment was recognized. The cross-section for each magnetic M'(J) substate in the collision frame, sigma|M'(J)|, was determined to be sigma(|M'(J)|=0):sigma(|M'(J)|=1):sigma(|M'(J)|=2) = 1.00:0.84 +/- 0.02:0.88 +/- 0.02 for the effusive CF3H beam condition. The atomic alignment effect was found to significantly depend on the CF3H beam condition. For the supersonic beam condition, sigma(|M'(J)|=0&1) was changed to be smaller than sigma(|M'(J)|=2).     

Fast,very fast,and ultra-fast gas chromatography-mass spectrometry of thermally labile steroids,carbamates, and drugs in supersonic molecular beams

Gas chromatography-mass spectrometry (GC-MS) analyses of thermally labile compounds have been studied by using a short column fast gas chromatograph, coupled with fly-through electron ionization in supersonic molecular beams. Thirty-two compounds, which include steroids, carbamate pesticides, antibiotic drugs, and other pharmaceutical compounds, have been analyzed and the details of their GC-MS analysis are provided. The ability to analyze thermally labile compounds is discussed in relation to the speed of analysis. A new term, “speed enhancement factor” (SEF), is defined as the product of column length reduction and the carrier gas linear velocity increase, as compared with normal GC-MS conditions. Fast, very fast, and ultra-fast GC-MS are defined with a SEF in the ranges of 5–30, 30–400, and 400–4000, respectively. Trade-offs in the degree of dissociation, speed, gas chromatograph resolution, and sensitivity were studied and examined with thermally labile molecules. The experimental factors that affect the dissociation are described with emphasis on its reduction. We claim that the use of supersonic molecular beams for sampling and ionization provides the ultimate capability in the GC-MS of thermally labile compounds. The obtained 70-eV electron ionization mass spectra are shown, and an enhanced relative abundance of the molecular ion is demonstrated together with library search capability of these mass spectra, which is better than that reported with particle beam liquid chromatography-mass spectrometry. The performance of fast GC-MS in supersonic molecular beams is compared with other methods of fast GC-MS and with particle beam liquid chromatography-mass spectrometry.     

Dynamics of ionization of H2 by Ne*(3P) investigated by electron spectroscopy

The Penning ionization reaction Ne*(2p(5)3s 3P)+H2-->[NeH2]+ +e- has been studied in crossed supersonic molecular beams with electron-energy analysis at four collision energies E = 1.83, 2.50, 3.16, and 3.89 kcal/mol. The electron kinetic-energy spectra, which directly reflect the ionizing transition region, show resolved peaks assignable to v' = 0-4 of H2+. The vibrational populations deviate systematically from Franck-Condon behavior, suggesting that the discrete-continuum coupling increases with H2 bond stretching. Each peak displays both increasing breadth and increasing blueshift with increasing E, and the blueshift also increases with increasing v'. The first two properties are consistent with a predominantly repulsive excited-state potential-energy surface, while the last is speculated to be a reflection of the rHH dependence of the ionic surface. Quantum scattering calculations based on ab initio potential surfaces for the excited and ionic states in spherical and infinite-order-sudden rigid rotor approximations are in semiquantitative agreement with the measurements. Discrepancies suggest changes in the imaginary, absorptive part of the excited surface, which probably can be best effected by multiproperty fitting calculations.