Michael A. Everest

Discrete Sums for the Rapid Determination of Exponential Decay Constants

Several computational methods are presented for the rapid extraction of decay time constants from discrete exponential data. Two methods are found to be comparably fast and highly accurate. They are corrected successive integration and a method involving the Fourier transform (FT) of the data and the application of an expression that does not assume continuous data. FT methods in the literature are found to introduce significant systematic error owing to the assumption that data are continuous. Corrected successive integration methods in the literature are correct, but we offer a more direct way of applying them which we call linear regression of the sum. We recommend the use of the latter over FT-based methods, as the FT methods are more affected by noise in the original data.

State-selected studies of the reaction of NH3+ (ν1, ν2) with D2

Abstract The title reaction has been studied in a guided ion beam instrument at hyperthermal energies, resulting in ion products NH 3 D + and NH 2 D + . Ammonia ions were prepared with differing amounts of excitation in the in-plane symmetric stretching ( ν 1 ) and the out-of-plane symmetric bending mode ( ν 2 ) using 2+1 resonance-enhanced multiphoton ionization. Comparison of the reactivity of two nearly isoenergetic states ( ν 1 =0, ν 2 =5; internal energy, E int =0.60 eV and ν 1 =1, ν 2 =2; E int =0.63 eV) with differing concerted atom motions indicates that this reaction is not mode selective.

Reaction of state-selected ammonia ions with methane

We have investigated the reaction of ammonia ions with methane molecules (CD4) over the collision energy range of 0.5–10.0 eV and for ammonia ion vibrational states ranging from ν2=1–10. Under these conditions, the two main product channels are NH3D+ and CD3+. The cross section for formation of NH3D+ is enhanced with increasing internal energy at collision energies below 6.0 eV, and independent of internal energy at higher collision energies. The enhancement is greater for forward-scattered products indicating that ammonia-ion vibrational energy enhances reactivity at large impact parameters. The mechanism for formation of CD3+ involves collision-induced dissociation of CD4 (or NH3+) which leads to the formation of a short-lived [NH3CD3]+ ([NH2CD4]+) complex, which then decays to products. This reaction is found not to be vibrationally mode selective, which is consistent with the hypothesis that mode selectivity in reactions of ammonia ions is driven by the Franck–Condon overlap whenever charge transfer i...

Evanescent-Wave Cavity Ring-Down Ellipsometry.

We introduce the new technique of evanescent-wave cavity ring-down ellipsometry (EW-CRDE), used for the measurement of ellipsometric angles of samples at a solid-gas or solid-liquid interface, and achieve phase-shift measurements with precision of ∼0.01°. We demonstrate the technique by measuring the time-dependent refractive index of methanol-water mixtures and thin films at the liquid/fused-silica interface, showing that the monitoring of monolayers on microsecond time scales using EW-CRDE should be achievable.

Development of cavity ring-down ellipsometry with spectral and submicrosecond time resolution

Cavity-enhanced ellipsometry, using nanosecond pulsed lasers and without moving parts, is demonstrated to have submicrosecond time resolution. The ellipsometric phase angles are measured from the Fourier transform of the cavity ring-down experimental signals, with a sensitivity 0.01 degrees. The technique is applied to highly reflective surfaces, including total internal reflection, where the samples are placed within the evanescent wave. The technique can be generalized to broadband sources, such as from supercontinuum generation, allowing spectral resolution of thin films and monolayer samples.

Vibrational and collisional energy effects in the reaction of ammonia ions with methylamine

We have investigated the reactions of vibrationally state-selected ammonia ions with d3-methylamine over the center-of-mass collisional energy range of 0.5 to 10.0 eV and for ammonia ion vibrational states ranging from ν2=1–9. Under these conditions, five major products appear: NH4+, NH3D+, CD2NH2+, CD3NH2+, and CD3NH3+. The cross section for each product is a decreasing function of collision energy and also a decreasing function of energy in the ν2 mode of the ammonia ion, except for CD2NH2+ that shows about a twofold enhancement with increasing internal energy, most notably at low-collision energies. Examination of the velocity scattering profiles shows that the mechanism for formation of each major product does not involve complex formation in this energy range. Branching ratios for each product are measured, and a comparison is presented for CD2NH2+ and CD3NH2+ arising from reactions with ammonia ions prepared in two nearly isoenergetic states. One state has no quanta in the symmetric stretch and five...

(2+1) laser-induced fluorescence of spin-polarized hydrogen atoms.

We report the measurement of the spin polarization of hydrogen (SPH) atoms by (2+1) laser-induced fluorescence, produced via the photodissociation of thermal HBr molecules with circularly polarized 193 nm light. This scheme, which involves two-photon laser excitation at 205 nm and fluorescence at 656 nm, offers an experimentally simpler polarization-detection method than the previously reported vacuum ultraviolet detection scheme, allowing the detection of SPH atoms to be performed more straightforwardly, from the photodissociation of a wide range of molecules and from a variety of collision experiments.