Supersonic velocimetry in a shock tube using laser enhanced ionisation and planar laser induced fluorescence

 A novel flow-tagging technique is presented which was employed to measure gas velocities in the free stream of a shock tube. This method is based on the laser spectroscopic techniques of Laser-Enhanced Ionisation (LEI) and Laser-Induced Fluorescence (LIF). The flow in the shock tube is seeded with small amounts of sodium, and LEI is used to produce a substantial depletion of neutral sodium atom concentration in a well-defined region of the flow, by using two wavelength-resonance excitation and subsequent collisional ionisation. At a specific time delay, single-laser-pulse planar LIF is utilised to produce a two-dimensional (2-D) inverse image of the depleted tagged region downstream of the flow. By measuring the displacement of the tagged region, free stream velocities in a shock tube were determined. Large variations in the concentration of sodium seeded into the flow were observed and even in the presence of these large variations accurate free-stream velocity measurements were obtained. The experimentally determined value for velocity compares very well with the predicted velocity. Received: 25 March 1996/Revised version: 8 July 1996     

Phenole

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H + HOONO2 → products: Temperature-dependent rate constant

The absolute reaction rate constant of the title reaction was measured in a stirred-flow reactor under H-atom-rich conditions at seven temperatures from 226 to 315 K. Carbon monoxide was added to convert any OH radicals produced back to H atom by way of the reaction OH + CO → H + CO₂. The reaction rate constants were essentially constant between 248 and 315 K: (k ± 2σ) = (2.46 ± 0.35) × 10^?14 cm³/s. At temperatures lower than 248 K, the measured rate constant became larger at lower temperatures, possibly due to heterogeneous effects. An hypothesis is advanced that may explain the surprisingly slow rate constant that is virtually independent of temperature, but more experiments are required to determine the dynamical reaction pathway.     

Infrared multiphoton chemistry: Comparison of theory and experiment,solution of the master equation

An energy-grained master equation has been used to model the multiphoton pumping process in SF₆. It was found that an energy-dependent absorption cross section that was approximately inversely dependent on the level of excitation was needed to reproductive experimental data on the fraction decomposed per pulse.     

The hearing aid feedback path: mathematical simulations and experimental verification

Acoustic feedback in hearing aids has received little attention in the literature. Feedback occurs when stability conditions of the open-loop transfer function of an in situ hearing aid are violated. Solving the feedback problem will first require knowledge of the open-loop transfer function. Included in the open-loop transfer function is the acoustical path by which sound emanating from the earmold vent returns to the microphone (i.e., the feedback path). Reported herein are two different mathematical procedures for simulating transfer functions of the feedback path of an eyeglass-type hearing aid. In one procedure the vent exit was modeled as a point source of sound located on a flat plane, while it was treated as a point source on a sphere in the other. Results of laboratory experiments indicate that the mathematical models accurately predict those acoustic phenomena for which they were intended: point sources on plane and spherical baffles. Results of manikin experiments showed both models to be less accurate for simulating the feedback path around the human head. The maximum difference between experiment and theory was 6 dB at one frequency. Surprisingly, the flat-baffle model produced better agreement with experimental results than did the sphere model.     

Infrared multiphoton dissociation yields via a versatile new technique: intensity,fluence, and wavelength dependence for CF3I

Intensity, fluence, and wavelength dependence of the collisionless infrared multiphoton dissociation yield for CF₃I have been determined via “titration” of radical photoproducts and molecular-beam-sampling-mass spectrometry. The importance of nonlinear effects in the pumping of this molecule is confirmed.