Branching of negative streamers in free flight

We recently have shown that a negative streamer in a sufficiently high homogeneous field can branch spontaneously due to a Laplacian instability, rather than approach a stationary mode of propagation with fixed radius. In our previous simulations, the streamer started from a wide initial ionization seed on the cathode. We here demonstrate in improved simulations that a streamer emerging from a single electron branches in the same way. In fact, though the evolving streamer is much more narrow, it branches after an even shorter propagation distance.Comment: 4 pages, 3 figure

Stability of negative ionization fronts: regularization by electric screening?

We recently have proposed that a reduced interfacial model for streamer propagation is able to explain spontaneous branching. Such models require regularization. In the present paper we investigate how transversal Fourier modes of a planar ionization front are regularized by the electric screening length. For a fixed value of the electric field ahead of the front we calculate the dispersion relation numerically. These results guide the derivation of analytical asymptotes for arbitrary fields: for small wave-vector k, the growth rate s(k) grows linearly with k, for large k, it saturates at some positive plateau value. We give a physical interpretation of these results.Comment: 11 pages, 2 figure

Some perspectives in electron momentum spectroscopy experiments

The basic challenges of electron momentum spectroscopy (EMS) as it has guided our research efforts at Carleton University are outlined. These challenges are : (a) Are the spectroscopic factors obtained in EMS equivalent to the spectroscopic factors observed in photoelectron spectroscopy ? (b) Can experimental momentum profiles be related to the chemical reactivity of complex molecular systems ? (c) Are the basic assumptions of (e,2e) reaction theory valid for complex molecular systems ? Insights based on our efforts to address these questions are presented

Surface Plasmon Resonance-Laser Desorption/ lonization-Time-of-Flight Mass Spectrometry

The laser desorption/ionization (LDI) process is investigated under surface plasmon resonance (SPR) conditions using time-of-flight mass spectrometry (TOFMS). We demonstrate that LDI-TOFMS at the SPR angle requires a lower minimum laser fluence for the production of silver monomer and cluster cations from ablation of a thin silver film substrate. In the LDI of gramicidin S deposited on a thin silver film substrate, the largest intensity for the molecular cation peak occurs when the laser light is incident on the substrate at a specific SPR angle. These results fully confirm SPR enhancement of the LDI process. The capability to perform SPR-LDI on a larger molecular weight analyte (1141 amu for gramicidin S) represents a new milestone beyond the previous achievement with rhodamine B (479 amu). A better understanding of the SPR mechanism is gained with respect to the substrate metals (silver vs aluminum), desorption (microscopic vs mesoscopic), and ionization (chemical vs multiphoton). These findings may be useful in the future design of SPR-LDI techniques for better TOFMS analysis of higher mass biomolecules

Far-infrared absorption measurements on thin polymer films

Interference fringes in the far infrared spectra of thin films are the result of the coherent adding up of waves and can influence the spectral interpretation by concealing smaller features. A number of methods have been developed in order to eliminate such oscillating features from the transmittance spectrum. The paper critically evaluates the total transmissionreflection technique and applies the method to quantify the spectra of different polymer film

Far infrared studies on Nafion and perfluoroimide acid PFIA and their alkali salts

The terahertz far infrared spectra lt;300 cm of perfluorinated sulfonic acid Nafion NR211 polymer and perfluoroimide acid PFIA polymer and their alkali M salts have been analyzed and the results arepresented. Pronounced features in the spectra of these ionomers that correlate systematically with thecorresponding cation mass are reported and from their spectral position the force constants are derived.The average vibrational force constants for Nafion M and PFIA M are found to be 54 7 and 39 4 N m,respectively. Such terahertz far infrared signatures probe the detailed structure of the Nafion M andPFIA M ionic clusters and, in turn, provide benchmarks for elucidating the ionomer water channels or water molecules located in the ionomer water interface upon hydration. Qualitative trends in thevibrational energies of Nafion and PFIA can be explained by consideration of electronic and or structural ionic domain size effect