FIELD DESORPTION OF H-3 AND FIELD DISSOCIATION OF H-3+

Ab-initio calculations using a non-local spin-density approximation ha ve been done for linear and triangular H-3+ ions in an external homoge nous electric field. From these calculations it is predicted that line ar H-3+ is not stable above 2 V/angstrom if its molecular axis is para llel to the field vector, whereas triangular H-3+ resists field dissoc iation up to at least 3.1 V/angstrom. Linear H-3 is formed at kink sit es on the surface of the field emitter. Laser-stimulated field desorpt ion of that H-3 could lead to linear H-3+. In spite of the rotation of the H-3+ ion, a majority should field-dissociate in fields greater th an 2.4 V/angstrom. However, if the linear H-3 is bending during laser- stimulated field desorption the more stable triangular H-3+ will be fo rmed upon field ionization. The H-3+ field dissociation for fields bet ween 2.4 and 3.1 V/angstrom was experimentally investigated using lase r pulse correlated ion pair spectroscopy in combination with a pulsed- laser atom probe. During these measurements a total of 605 H-3+ ions a rrived at the time-of-flight detector, but only one event occurred whi ch could be attributed to H-3+ field dissociation. However, H-2+, form ed by field ionization of the H-3+ field dissociation product H-2, cou ld have been field-dissociated also. Therefore the H-2+ field-dissocia tion probability has been calculated for the case where the H-2+ molec ular axis is parallel to the field vector. Taking this maximum dissoci ation probability of H-2+ into account, it followed from processing of the measured yields that the H-3+ field-dissociation probability is s maller than that of field-desorbed H-2+ for fields up to 3.1 V/angstro m. Hence, it is inferred that linear H-3 bends during laser-stimulated field desorption, resulting in a more stable triangular H-3+ after fi eld ionization.