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Abstract

Ab-initio calculations using a non-local spin-density approximation have been done for linear and triangular H⁺₃ ions in an external homogenous electric field. From these calculations it is predicted that linear H⁺₃ is not stable above 2 V/Å if its molecular axis is parallel to the field vector, whereas triangular H⁺₃ resists field dissociation up to at least 3.1 V/Å.

Linear H₃ is formed at kink sites on the surface of the field emitter. Laser-stimulated field desorption of that H₃ could lead to linear H⁺₃. In spite of the rotation of the H⁺₃ ion, a majority should field-dissociate in fields greater than 2.4 V/Å. However, if the linear H₃ is bending during laser-stimulated field desorption the more stable triangular H⁺₃ will be formed upon field ionization.

The H⁺₃ field dissociation for fields between 2.4 and 3.1 V/Å was experimentally investigated using laser pulse correlated ion pair spectroscopy in combination with a pulsed-laser atom probe. During these measurements a total of 605 H⁺₃ ions arrived at the time-of-flight detector, but only one event occurred which could be attributed to H⁺₃ field dissociation. However, H⁺₂, formed by field ionization of the H⁺₃ field dissociation product H₂, could have been field-dissociated also. Therefore the H⁺₂ field-dissociation probability has been calculated for the case where the H⁺₂ molecular axis is parallel to the field vector. Taking this maximum dissociation probability of H⁺₂ into account, it followed from processing of the measured yields that the H⁺₃ field-dissociation probability is smaller than that of field-desorbed H⁺₂ for fields up to 3.1 V/Å. Hence, it is inferred that linear H₃ bends during laser-stimulated field desorption, resulting in a more stable triangular H⁺₃ after field ionization.