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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.
