Dissociative field ionization of H2 and HD

Abstract

Distributions in kinetic energy of the atomic ions resulting from field ionization of H2 and HD are reported. For fields of 1.4−2.6 V/Å, the distributions of H+ from HD are characterized by a series of peaks which cannot be ascribed to field induced resonance states. This structure can be understood by examining the motion of vibrationally excited molecular ions in the high electrostatic field necessary for field ionization during the period immediately following field ionization. The stability of a molecular ion in such fields is a strong function of the angle between the molecular ion axis and the field. Thus, not all molecular ions produced with sufficient vibrational energy to dissociate, if properly oriented, do in fact dissociate in a time comparable to the first one−half vibrational period. Such ions dissociate after a time of the order of (3/2), (5/2), or (7/2), ..., vibrational periods, depending on the time required for the field to induce rotation of the molecule ion to the unstable orientation. These dissociations, periodic in time, result in periodicities, i.e., peaks, in the kinetic energy distribution of H+ from HD. Because the energy differences between these peaks vary as the square of the electrostatic field, they can be used as a sensitive method of field calibration. The effect of these molecular processes on the kinetic energy distributions of the other atomic ions resulting from the dissociation of H2 and HD are discussed.