Experiments on positronium negative ions

This paper is a review of new developments in the experimental field of the efficient emission of the positronium negative ion, a bound state of one positron and two electrons, from alkali-metal coated surfaces. Following the prediction of its existence by Wheeler in 1946, this ion has been one of the most attractive subjects for theorists to investigate in the field of positron physics, resulting in a large number of studies. Though the first observation of the ions, formed using a carbon thin film, was made in 1981 by Mills, there have been few experimental works since, limited by the low-production efficiency achievable for nearly 30 years. In 2008, a dramatic effect on the ion emission from tungsten surfaces was discovered upon coating it with Cs. This has opened the door to a new era of experimental investigations of this ion. This paper reports the investigations performed so far. Advances discussed include efficient formation of the ion, its photodetachment and the production of an energy-tunable positronium beam based on the technique of the photodetachment