Stephan E. Barlow

Laboratory measurement of water nucleation using a laminar flow tube reactor

A Laminar Flow Tube Reactor (LFTR) was used to study the nucleation of water vapor. Computational analysis was carried out to define the operating conditions of the LFTR suitable for water nucleation measurements. An interface between the LFTR and a mass spectrometer was developed to analyze the chemical content of the freshly nucleated water particles. Contaminants were detected in the initial configuration of the LFTR. As a result, improvements were made to the LFTR to achieve ultrahigh purity conditions in the system. The nucleation rate of water vapor as a function of supersaturation was measured over the temperature range 210–250 K. The first measurement of the nucleation rate of water at a temperature of 210 K was obtained. Reasonable agreement with the classical theory predictions is observed for temperatures in the range 230–250 K. However, below 220 K, classical theory begins to overestimate nucleation rates compared to experimental data and the disagreement grows with decreasing temperature. The...

The laminar flow tube reactor as a quantitative tool for nucleation studies: Experimental results and theoretical analysis of homogeneous nucleation of dibutylphthalate

A laminar flow tube reactor was designed and constructed to provide an accurate, quantitative measurement of a nucleation rate as a function of supersaturation and temperature. Measurements of nucleation of a supersaturated vapor of dibutylphthalate have been made for the temperature range from −30.3 to +19.1 °C. A thorough analysis of the possible sources of experimental uncertainties (such as defining the correct value of the initial vapor concentration, temperature boundary conditions on the reactor walls, accuracy of the calculations of the thermodynamic parameters of the nucleation zone, and particle concentration measurement) is given. Both isothermal and the isobaric nucleation rates were measured. The experimental data obtained were compared with the measurements of other experimental groups and with theoretical predictions made on the basis of the self-consistency correction nucleation theory. Theoretical analysis, based on the first and the second nucleation theorems, is also presented. The crit...

‐charge‐induced acceleration of ions emitted by laser‐irradiated surfaces

Pulsed‐laser‐irradiated surfaces sometimes emit positive ions at energies several volts higher than one would expect, even at modest (<0.1 J/cm2) fluences. A mechanism that can account for this phenomenon is discussed. Intense surface photoemission of electrons during the laser pulse leads to the formation of a space‐charge layer near the surface. If the laser fluence were constant, the ions would accelerate and subsequently decelerate as they pass through this steady potential well. As the laser pulse ends, however, some ions may undergo extended acceleration as this space‐charge layer moves away from the surface. The maximum possible ion acceleration is analytically calculated and the acceleration for a range of realistic experimental parameters is numerically predicted.

Determination of Analytic Potentials from Finite Element Computations

Abstract We present here a general approach for determining analytic electrical potentials from the results of finite difference calculations. Such an approach allows us to explore effects of realistic electrode structures that are not directly amenable to analytic treatment. We then use this technique to find the effects of various modifications of hyperbolic electrode structures of some practical interest by way of example.

Dynamical acceleration effects in laser‐induced particle emission from surfaces

Pulsed laser irradiated surfaces sometimes emit electrons or ions at energies several volts higher than what one would expect, even at modest (<0.1 J/cm2) fluences. We discuss a mechanism that could account for this. When the photon energy is above the work function, a 0.1 J/cm2 laser fluence can easily yield thousands of amps of electron emission. The electron’s self repulsion (space‐charge) creates a negative potential high enough to return most of the emitted electrons back to the surface after a micron‐scale excursion. The establishment and evolution of this space‐charge layer involves plasma‐like dynamics that can accelerate a few electrons (or ions if present) to several volts. Computer simulations and earlier experimental work have shown how the highest energy photoemitted electrons can be accelerated to 5.4 times their initial energy. We have continued this study to demonstrate the completely different way that ions are accelerated. The electron space‐charge potential at the peak of a typical lase...

The mass multiplet in Penning trap ion cyclotron resonance mass spectroscopy

A model for the behavior of closely spaced masses—the “mass multiplet”—in a Penning trap mass spectrometer is developed. The model shows how these species separate from each other (or not) following resonant excitation, and gives quantitative criteria for mass separation. More surprisingly, the model shows the species tend to drift to different mean cyclotron radii. This latter effect undoubtedly plays a role in the problem of abundance determinations in Penning trap mass spectrometers.

Analytic potentials for realistic electrodes

We show how LaPlace’s equation can be accurately solved when the boundary conditions are not amenable to direct analytic treatment. This problem arises for nearly all real electrodes. Our approach systematically combines numerical relaxation techniques with analytic expansions to produce a provably unique solution.

Laminar Flow Tube Reactor interface with quadrupole mass spectrometer

Nucleation of supersaturated water vapor was studied using the Laminar Flow Tube Reactor (LFTR) technique. In order to check the presence of contaminants in the freshly nucleated water particles, the LFTR has been connected to a mass spectrometer. Trace amounts contaminants arising from the substances used as a circulating liquid to maintain temperatures of the LFTR have been detected. The results of the mass spectroscopic analysis are in full agreement with the observed values of nucleation rate.

Experimental nucleation studies with a laminar flow tube reactor

A Laminar Flow Tube Reactor (LFTR) has been used as a quantitative tool for nucleation measurements. It has been specially designed and constructed in order to minimize the potential experimental inaccuracies. Careful attention has been paid to the temperature conditions inside the LFTR. Other sources of experimental uncertainties, such as initial vapor concentration conditions, calculations of the parameters of the nucleation zone, and particle concentration measurements have been thoroughly analyzed as well. The nucleation rate dependence as a function of supersaturation and temperature has been measured for the dibutyl phthalate, ethylene glycol, and glycerol. The experimental results have been compared with the other experimental data and with the theoretical predictions.