Fragmentation dynamics of metastable hydrogen ion clusters H5+, H7+, and H9+: experiment and theory

Abstract

Data are presented on the metastable H2 loss from H5+, D5+, H7 , and Hg+ ions formed in a high-pressure, temperature-variable ion source with coaxial electron beam, ion exit geometry. The metastable reactions occur in the second field free region of a reverse geometry mass spectrometer and require parent ion lifetimes of ~10-6 s. Substantial structure is observed in the kinetic energy distributions, derived from the metastaWe peak shapes, for Hs+, D5+, and H7+. Statistical theory modeling predicts a smooth P(Et) with no structure and a lifetime in the 10~9 s range for H5+. A theoretical model is developed for H5+ that combines ab initio potential surfaces with a high-order WKB tunneling formalism. This model nicely reproduces the observed structure and predicts metastable reactions with lifetimes of ~10"6 s. The structure is due to a superposition of tunneling from various bound variational levels of the H3+-H2 stretch for different angular momentum states of the H5+ molecule. A Boltzmann distribution of J0 and v states can only account for the lowest energy portion of P(Et). The experimental distribution is used to extract a J0 distribution required by the model to fit the data. The extracted distribution has substantial intensity at the highest values of J0 that support bound variations (J0 = 29 for H5+). Several possible mechanisms are suggested that could generate these high values of J0 under our experimental conditions. Statistical phase space theory accurately fits the P(Et) for metastable H2 loss from H9+ indicating tunneling is a minor process, while H2 loss from H7+ appears to occur both by tunneling and statistical processes.

On the formation mechanisms of hydrogen ionic clusters

“…The experimental work on the hydrogen clusters has been concentrated on enthalpy variation measurements (∆H) [16,24,25,26,31,33] and collisional induced dissociation (CID) studies [34,35,36,37]. The ∆H measurements for the H + n clusters and for the most general X + (H 2 ) k clusters have been an important source of information about the cluster energetic properties and have guided the theoretical studies.…”

“…Recently, more attention has been dedicated to the H + 5 cluster. Its potential energy surface has been studied at high levels of calculation [13,14] and its presence in some interstellar environments in concentrations up two times higher than that of the H + 3 molecule has been investigated [15].Besides the H + 3 molecular ion, a large variety of atomic and molecular cations has been considered as a core for H 2 clustering: the first-column Li + , Na + and K + ions [16][17][18][19][20][21][22] Table I summarizes some properties of these X + (H 2 ) k hydrogen clusters.The experimental work on the hydrogen clusters has been concentrated on enthalpy variation measurements (∆H) [16,24,25,26,31,33] and collisional induced dissociation (CID) studies [34,35,36,37]. The ∆H measurements for the H + n clusters and for the most general X + (H 2 ) k clusters have been an important source of information about the cluster energetic properties and have guided the theoretical studies.…”

Multiple H+3 fragment production in single collision of fast H+ n clusters with He atoms

“…Besides, we note that the proportion of D 2 in the H 2 gas of the source is less than 150 ppm. Moreover, it has been shown that deuterium appears mainly in even mass clusters [ 12]. Thus, the HD + pollution is negligeable (~10-5).…”

“…Thus, the single-collision conditions in the gas jet are also fulfilled for the double and triple production processes in the target thickness range x=<l.15 1014atoms/cm 2. This target thickness range corresponds to the dissociation of at the most 30% of the H + clusters in the gas jet [12] (a aiss= (28.8_+2.5) 10-16cm2). In Fig.…”

A mass and time-of-flight spectroscopy study of the formation of clusters in free-jet expansions of normal D2

“…Since both the even and odd clusters have similar binding energies 65 the lack of even-numbered cluster ions in most of the previous studies has been attributed to kinetic factors 66 related to the small nonreactive probability of the H 2 + ion core. 67 Recently, however, Tachikawa has speculated that H 4 + and H 6 + ions might exist at least in the solid if the excessive energy is rapidly and efficiently removed from the complexes.…”