An intense pulsed positron source has been developed using a buffer gas trap to accumulate large numbers of positrons and create a dense plasma, which may then be bunched and spatially focused. Areal densities of more than 3 ϫ 10 10 e + cm −2 have been achieved in a subnanosecond pulse producing an instantaneous positron current of more than 10 mA. We describe various aspects of the device including a detection technique specifically developed for use with intense positron pulses. Two applications are also described as well as future experiments such as the formation of positronium molecules and the positronium Bose-Einstein condensate.
We have created a high-density gas of interacting positronium (Ps) atoms by irradiating a thin film of nanoporous silica with intense positron bursts and measured the Ps lifetime using a new single-shot technique. When the positrons were compressed to 3:3 10 10 cm ÿ2 , the apparent intensity of the orthopositronium lifetime component was found to decrease by 33%. We believe this is due to a combination of spin exchange quenching and Ps 2 molecule formation associated with colliding pairs of oppositely polarized triplet positronium atoms. Our data imply an effective cross section for this process of 2:9 10 ÿ14 cm ÿ2 .
Recent developments in positron trapping technology have made possible experimentation with dense interacting positronium gases. Along with these capabilities comes a need for suitable measurement techniques, and accordingly we have developed a method to measure positronium lifetimes from a single intense burst of positrons. Our method is based on recording the anode signal from a photomultiplier with a fast oscilloscope following a short-time positron burst which allows us to measure transitory effects as well as high density positronium interactions.
Recent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric aerosol formation. Such findings have stimulated theoretical studies with the aim of understanding the interaction of organic acids with common aerosol nucleation precursors like bisulfate (HSO4(-)). We report a combined negative ion photoelectron spectroscopic and theoretical investigation of molecular clusters formed by HSO4(-) with succinic acid (SUA, HO2C(CH2)2CO2H), HSO4(-)(SUA)n (n = 0-2), along with HSO4(-)(H2O)n and HSO4(-)(H2SO4)n. It is found that one SUA molecule can stabilize HSO4(-) by ca. 39 kcal/mol, three times the corresponding value that one water molecule is capable of (ca. 13 kcal/mol). Molecular dynamics simulations and quantum chemical calculations reveal the most plausible structures of these clusters and attribute the stability of these clusters to the formation of strong hydrogen bonds. This work provides direct experimental evidence showing significant thermodynamic advantage by involving organic acid molecules to promote formation and growth in bisulfate clusters and aerosols.
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