Ronald R. Herm

Molecular Beam Kinetics: Reactions of Alkali Atoms with NO2 and CH3NO2

Magnetic and electric deflection analysis of the scattering of Cs + NO2 shows that the principal product is a paramagnetic, polar molecule. Magnetic analysis of the K + NO2 system indicates that the scattered signal is paramagnetic; a similar study of Na + NO2 shows a small yield of diamagnetic product. For the analogous reactions with CH3NO2, the product is diamagnetic and has a pseudo‐first‐order Stark effect. From these data and thermochemical arguments the principal alkali‐containing products are identified as: for Cs + NO2, a 2Σ electronic state of CsO; for Na + NO2, probably a 2Π state of NaO; for M + CH3NO2, almost certainly MNO2 in a singlet state. The NO2 results indicate that the ground state of the MO molecule changes from 2Π for LiO (the only species which had been previously observed) to 2Σ for CsO. The usual differential surface ionization detection fails for Cs + NO2 and consequently only a very rough estimate of the scattering is obtained; this indicates that the total reaction cross secti...

Photodissociation of NaBr, Nal, and KI vapors and collisional quenching of Na* (3 2P), K* (4 2P), and K* (5 2P) by foreign gases

Fluorescence from excited alkali atoms (A*) may be produced by photodissociation of alkali halide (AX) vapor. Fluorescence efficiencies have been determined as a function of the photodissociation wavelength, λ0, for Na* (3 2P) from NaBr and for K* (4 2P) and K* (5 2P) from KI. Employing the Stern‐Volmer relation, cross sections, Qq, for the collisional quenching of the A* electronic excitation may be determined from the attenuation of the A* fluorescence that is observed upon introduction of a foreign gas. Because A* may be produced with different average speeds by varying λ0, this method permits the determination of the dependence of Qq on relative collision speed, g. Employing this method, Qq was determined to decrease monotonically with increasing g for Na* (3 2P)+Br2 (AX = NaBr) and K* (4 2P)+C2H4, CF3Cl, and SO2 (AX = KI). Moreover, values of Qq were determined at a particular g value for K* (4 2P)+I2 and K* (5 2P)+I2, HCl, and DCl. Alternatively, premixing the quenching gas in a large (∼ 100‐fold) e...

Photodissociation of NaI Vapor and the Energy Dependence of the Quenching of Na* (3p2P) by Foreign Gases

Fluorescence of the Na* (3p 2P) D lines is observed upon photodissociation of NaI vapor by 1900–2500 A radiation obtained through a monochromator from a high current, low pressure H2 arc continuum source; the properties of this source are briefly described. The D lines fluorescence efficiency exhibits a threshold at ∼ 2500 A, a relative maximum at ∼ 2225 A, and a relative minimum at ∼ 2050 A. An expression is derived for the distribution in laboratory speeds of an atom produced by photodissociation of a diatomic molecule at thermal equilibrium. This expression is then employed to calculate the distributions in speeds of the photodissociatively generated Na* and these computed Na* speed distributions are employed to analyze the observed attenuations of the D‐lines fluorescence upon addition of foreign gases. In this manner, the dependences on relative collision velocity g of the cross sections Qq for collisional quenching of Na* by CO2, C2H4, CH3CN, CF3Cl, C6H6, SO2, and I2 have been determined. Over the r...

Crossed Beam Collision Mechanics: Reactions of Ca, Sr, and Ba with HI and Limits on D0° for CaI, SrI, and BaI

Angular distributions of MI (M=Ca, Sr, or Ba) products scattered from crossed beams of HI and M are reported and compared with derived expressions for the angular distributions of the velocities of the center‐of‐mass. These comparisons indicate that the reaction threshold relative kinetic energies, E*, are 5, 4, and 2.5 kcal/mole for the Ca, Sr, and Ba+HI reactions, respectively. Energy conservation and these measured E* values establish rigorous lower bounds for D0°(MI) of 64 (CaI), 65 (SrI), and 66 (BaI) kcal/mole.

Crossed beams chemistry: Reactions of Ba, Sr, Ca, and Mg with Cl2 and Br2

Reactions of Ba, Sr, Ca, and Mg with Cl2 and Br2 have been studied in a crossed beam apparatus employing an electron bombardment ionizer‐mass filter detector. Product center‐of‐mass (c.m.) recoil energy and angle distributions have been fit to the measured monohalide product (MX) laboratory (LAB) angular distributions by averaging the c.m.→LAB transformations over the measured (nonthermal) beam speed distributions. These experiments give no indications of a dihalide product (MX2) and indicate that the formation of MX2 (by a two‐body radiative association) cannot account for more than a small fraction (<∼5%) of the reactive collisions. All eight reactions favor forward product scattering (i.e., MX scattered in the direction defined by the incident M) with relatively low recoil energies (∼10%–20% of the reaction exoergicity). For reaction with either halogen molecule, the fraction of the product scattered into the forward c.m. hemisphere increases in the sequence: Mg, Ca, Sr, Ba. Similarities and difference...

Harmonic Forces Linear Model for Reactions of Cs Atoms with Alkyl Iodides

The Cs plus alkyl iodide reaction dynamics are investigated by means of a classical, linear, four‐particle model, Cs–I–CH2–R, where R is H, CH3, C2H5, or C3H7, with linear harmonic restoring forces between adjacent particles. The full reaction exothermicity is initially partitioned between potential energy of Cs–I extension and I–CH2 compression, and the trajectories of the four particles are followed until the I–CH2 distance reaches a critical extension, at which point the reaction is assumed to be complete. The model is examined as a function of two parameters, the I–CH2 force constant, k2, and the initial repulsive energy in I–CH2 compression, P. By varying k2, the model spans the spectrum of possible direct interaction reaction dynamics from the limit of the adiabatic, very slow CsI–CH2R separation (low k2 limit) to the limit of impulsive release of the I–CH2 compression (high k2 limit). In contrast to previous calculations, we obtain a good qualitative fit to the experimentally reported product recoi...

The A+Bx condensation reaction: Crossed nozzle beams of Br2 and (Cl2)x or (NH3)x clusters

Nozzle beams of Br2 and Cl2 or NH3 have been crossed in a molecular beam scattering apparatus; the Cl2 or NH3 beam contained (Cl2)x or (NH3)x clusters distributed such that the intensity of a given cluster, Fx, decreased with increasing x for 1⩽x⩽∼50. Mass, angular, and time‐of‐flight spectra of the scattered neutral species all establish that the A+Bx→ABx* bimolecular condensation reaction is being observed. However, working from the data, it is not possible to distinguish between detection of a long‐lived ABx* metastable complex or of a decomposition product formed with low recoil velocity. Product angular distributions are confined to a small region of laboratory scattering angle Θ and peak at small but positive Θ (Θ=0° and 90° defined by cluster and Br2 beam directions, respectively). It is pointed out that this sharp peaking at small Θ is due to a number of experimental factors, including a Jacobian factor varying as sin−2Θ, and should be a universal characteristic of such condensation reactions in c...

Product magnetic defection slotted disk velocity analysis molecular beam kinetics: Li+CH3NO2, CCl4, and CH3I

Recoil velocity spectra of LiNO2, LiCl, and LiI reactively scattered from crossed thermal beams of Li and CH3NO2, CCl4, or CH3I have been measured by positioning a combination of an inhomogeneous electromagnet (to deflect aside nonreactively scattered Li atoms) and a slotted disk velocity analyzer between the beam collision zone and surface ionization detector. Transformation of these data into the center‐of‐mass (CM) coordinate system provides contour maps of the differential reaction cross sections. The CM product angular distributions from the three reactions are quite different: Li+CH3I favors backward scattering (i.e., a reversal in Li atom direction during the reaction) although the CM distribution is poorly resolved due to unfavorable kinematics; Li+CCl4 favors sideways scattering; and Li+CH3NO2 favors forward scattering. The product recoil energy distributions are quite similar for all three reactions and indicate potential energy surfaces with substantial repulsion between the products. These results are discussed in relation to previous work on related reactions and interpreted in terms of the electronic structure of ion‐pair intermediates.

Product magnetic deflection‐slotted disk velocity analysis molecular beams kinetics: LiO(X 2Π) and LiO(A 2Σ) from Li+NO2

Product magnetic deflection‐slotted disk velocity analysis establishes both LiO(X 2Π) and LiO(A 2Σ) as products of the Li+NO2 bimolecular reaction. Velocity distributions of LiO(X 2Π) reactively scattered from crossed thermal beams of Li and NO2 are reported. Transformation of these data into the center‐of‐mass coordinate system provides a contour map of the differential reactive cross section which indicates that (a) the reaction mechanism is direct, i.e., the lifetime of the LiNO2 intermediate is less than its rotational period; (b) the LiO(X 2Π) product is preferentially scattered forward, i.e., in the direction defined by the Li velocity; (c) an appreciable fraction of the reaction exothermicity (∼45%) appears as product recoil energy; and (d) distributions in product recoil angle and energy are weakly coupled with forward scattering favoring higher recoil energy.

ELASTIC DIFFERENTIAL CROSS SECTIONS AND INTERMOLECULAR POTENTIALS FOR Ar + CH4 AND Ar + NH3

Differential elastic cross sections are reported for CH4 + Ar (E = μg2/2 = 8.43 kJ/mole) and NH3 + Ar (E = 8.31 kJ/mole) in the region of the rainbow angles. Quantum interference undulations are apparently observed as well for CH4 + Ar and, possibly, NH3 + Ar. The measurements are fit to spherically symmetric intermolecular potentials yielding well depths and equilibrium intermolecular separations of 1.32 kJ/mole and 3.82 A for CH4 + Ar and 1.32 kJ/mole and 3.92 A for NH3 + Ar.