Direct electron attachment to fast hydrogen in 10⁻⁹ contrast 10¹⁸ W cm⁻² intense laser solid target interaction

Abstract

The interaction of an ultra-short (<30 fs), high-contrast(<10 −9 ), highintensity (>10 18 Wcm −2 ) laser pulse with a solid target is not generally known to produce and accelerate negative ions. The transient accelerating electrostatic-fields are so strong that they ionize any atom or negative ion at the target surface. In spite of what may appear to be unfavourable conditions, here it is reported that H − ions extending up to 80 keV are measured from such an interaction. The H − ion flux is about 0.1% that of the H + ions at 20 keV. These measurements employ a recently developed temporally-gated Thomson parabola ion spectrometry diagnostic which significantly improves signal-to-noise ratios. Electrons that co-propagate with the fast protons cause a two-step charge-reduction reaction. The gas phase three-body attachment of electrons to fast neutral hydrogen atoms accounts for the measured H − yield. It is intriguing that such a fundamental gas-phase reaction, involving the attachment of an electron to a hydrogen atom, has not been observed in laboratory experiments previously. Laser-produced plasma offers an alternative environment to the conventional charged particle beam experiments, in which such atomic physics processes can be investigated.

Creating negative ion beams from neutral gases using a negative hydrogen ion source

“…This neutralized hydrogen beam would then pass through the beamline and collect a free electron downstream of the accelerator, negatively ionizing it, before reaching the spectrometer so that it is detected at the Faraday cup. The attachment of a free electron to a neutral hydrogen atom is not typically found to be a likely reaction, but Tata et al suggest that it is a possible mechanism [11]. The neutral hydrogen beam would pass through the gas cell without acceleration at the energy 𝑉 𝐵 and collect a free electron in between the end of the accelerator unit and the mass spectrometer, a distance of approximately 80 cm.…”