Stephen L. Howard

Laser-based flame species profile measurements: A comparison with flame model predictions☆

Abstract Two laser-based diagnostic methods, resonance-enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF), were used for relative concentration measurements of seven species, O, H, OH, CH, CO, HCO, and CH 3 , in low pressure stoichiometric methane oxygen , ethylene oxygen , and ethane oxygen premixed hydrocarbon flames. For the well-understood methane oxygen flame, measured relative density profiles for these seven species were in good agreement with flame modeling calculations. This agreement justifies the use of the stoichiometric methane oxygen flame as a standard of comparison by which relative density profiles, measured for other hydrocarbon flames, may be placed on an absolute density scale. This approach was applied to measurements for the ethylene oxygen and ethane oxygen flames. Peak HCO densities for these flames appear to be overstimated by model calculations based on current kinetic models of C 1 and C 2 chemistry.

A hybrid tandem supersonic beam mass spectrometer for the study of collision-induced dissociation of ions in the energy range <1 to 3000 eV

Abstract A hybrid tandem mass spectrometer has been constructed to study the dynamics of collision-induced dissociation processes in the energy range of less than 1 eV to several keV. A mass- and energy-analyzed high-energy ion beam is decelerated to low energies by a series of cylindrical and rectangular tube lenses. The decelerated ion beam collides with a supersonic neutral beam at right angles. Energy and mass analysis of the fragment ions is performed by a novel hemispherical energy analyzer followed by a quadrupole mass filter. The detector system can be rotated about the collision center to provide angular analysis of scattered fragment ions. Ion beams of moderate intensity have been obtained in the entire energy range, it is especially significant that good intensity is obtained in the lower threshold energy range of 0.2–5 eV. The performance of the instrument has been evaluated by an extensive series of ion transmission, focus, and energy measurements and by comparison of CID results with known mstastable and collision-induced dissociation processes.

Collision-induced dissociation reaction dynamics of the acetone molecular ion

Abstract The collision-induced dissociation (CID) reaction CH3COCH+3 + Ar → CH3CO+ + CH3 + Ar has been studied with a tandem hybrid mass spectrometer (a high-resolution mass spectrometer coupled with a supersonic neutral beam, which uses post-collision energy and mass analysis) at ion laboratory energies in the range 4.5–300 eV. Kinetic energy and angular distributions of the fragment CH3CO+ ions were measured and scattering contour diagrams for this process were constructed. The results show that the dissociation proceeds via low-impact parameter collisions with extensive momentum transfer. At 4.5 eV, the product is primarily backward scattered with some intensity at the center-of-mass (completely inelastic collision). At higher energies the product ion is progressively more forward scattered but over the energy range investigated the maximum intensity never shifts to 0°. Collectively, these results suggest that impulsive mechanisms dominate CID reactions at all energy ranges for this ion and that the dominant mechanism is kinematically different at low and high energy.

Evidence for a long-lived excited state of CH4+ from a beam scattering study of the collision-induced dissociation of CH4+ to CH2+ at low energy

Abstract A crossed-beam study of the collision-induced dissociation of CH 4 + by Ar was carried out at a center-of-mass (c.m.) collision energy of 5.5 eV. The scattering shows three patterns for the formation of CH 2 + , (1) large-angle scattering at preferred impact parameters with little internal excitation of the products, (2) scattering near the c.m. with nearly all collision energy transferred into products internal energy and (3) superelastic scattering, i.e. conversion of internal energy to translational energy, implying the reaction is initiated by a long-lived excited state of CH 4 + generated by electron impact ionization of methane. No previous evidence exists, to our knowledge, that excited states of CH 4 + thus generated may have microsecond lifetimes.

Observation of fine‐structure transitions in argon charge transfer at low energies using a crossed‐beam technique

The crossed‐beam method was used to investigate the charge–transfer reaction of Ar+(2P3/2) with Ar(1S0) as a function of collision energy over the range of 2–100 eV (center of mass). Both the exactly resonant channel and the endoergic fine‐structure transition with ΔJ=1 were detected and the cross section ratio σ3/2→1/2/(σ3/2→1/2+σ3/2→3/2) was found first to increase with increasing collision energy to a maximum value of about 0.5 at 30 eV and then decrease at higher energy. The relative abundance of the fine‐structure transition channel at low energy is much higher than that predicted by the accepted general theory for these processes. It is suggested that a curve‐crossing mechanism not previously considered and the perturbation of trajectories at low energies resulting from the strongly bound Ar+2 intermediate may be responsible for the unexpected observations.

Quenching of Ar+(2P12) at high pressures in an electron-impact ion source

Abstract Investigations of the non-reactive scattering of Ar+ by N2 using a crossed-beam apparatus reveal that the two spin-orbit states of Ar+ are back-scattered into different regions of velocity space (center-of-mass reference frame); this fact permits the ratio of these two states in the primary ion beam to be monitored as a function of ion source conditions. Adjustments of source pressure and ionizing current density are the key parameters which control the extent of quenching of the Ar+ (2P 1 2 ) initially formed by electron impact on Ar. Superelastic scattering of electrons is suggested as a probable mechanism and a rate coefficient, k ≥ 8 × 10−11 cm3 s−1, is estimated for the reaction Ar+2P 1 2 ) + e− → Ar+(2P 3 2 ) + e− + 0.18 eV

Analysis of the energy window for the quantum-state and angular scattering specificity in Ar+ charge transfer with N2

Abstract Previous kinematic analysis of the charge-transfer reaction of Ar+ with N2 has shown an energy window near 1 eV collision energy where quantum-specific and angular-specific scattering is observed. Reactant energy and angular distributions were used to determine the center-of-mass energy distribution from the nominal center-of-mass energies previously reported. The resultant distributions show that the maximum width of the energy “window” is 0.31 eV with a median collision energy of 0.91 eV rather than a width of 0.9 eV with a median energy near 1 eV as previously reported.

Observation of fine-structure transitions in rare gas charge transfer at surprisingly low energies using a crossed-molecular beam technique

Abstract The crossed-beam method was used to investigate the charge-transfer reaction of Kr+(2P 3 2 ) with Kr(1S0), using a nearly pure Kr+(2P 3 2 ) ion beam. The resonant charge-transfer reaction is well described by a standard theoretical rectilinear-trajectory model. The endoergic fine-structure transition with ΔJ = 1 was also detected. This product is scattered at a definite angle, suggesting a short-range interaction which selects a particular impact parameter. Both channels are in general accord with accepted theories for ion/atom charge exchange, but the energy range at which the endothermic channel is observed is much lower than predicted.

A high transmission hemispherical energy analyzer for ion spectrometry

A hemispherical energy analyzer was constructed by using a novel approach to control the fringing electrostatic field. It provides several properties useful in ion spectrometers: namely, rather simple fabrication and compact size, high transmission efficiency at moderate resolution, and the capability to adjust resolution by changing the intersphere potentials. A computer program was developed to evaluate ion trajectories through the hemispherical analyzer. Data obtained from the trajectories were used to predict the characteristics of the analyzer. Experiments performed to determine the kinetic energy dependence of the absolute transmission and the resolution functions are in accord with theoretical calculations.

Charge-transfer reaction of helium ions with nitrogen

Abstract The charge-transfer reaction of helium ions with nitrogen molecules was investigated in order to gain insight into the role of vibrational-energy transfer in reaction dynamics. This paper presents kinematic data at moderate energies of the charge-transfer reaction of He + with N 2 (X 1 Σ + g , v = 0). The preferred reaction channels were formation of electronically excited N + 2 (A 2 Π + u ) via a direct mechanism and from highly scattered ground-state N + 2 (X 2 Σ + g , v = 0−4) with contribution from N + 2 (B 2 Σ + u ) produced via impulsive mechanisms.