Mechanisms and dynamics of the metastable decay in Ar2+

Delayed fragmentation of weakly bound Kr2

We report the experimental observation of the delayed fragmentation of the weakly bound dimer Kr2+ produced through the single ionization of Kr2 by a femtosecond laser field. The observed time delay between ionization and fragmentation, which reflects the survival time of the resulting Kr2+, is measured on the microsecond timescale. A detailed analysis of the kinetic energy releases of the ejected fragments and photoelectrons suggests that this delayed fragmentation arises from the radiative decay of the long-lived Kr2+, transitioning from the bound state II(1/2u) to the repulsive state I(1/2g).

On the size and structure of helium snowballs formed around charged atoms and clusters of noble gases

Helium nanodroplets doped with argon, krypton, or xenon are ionized by electrons and analyzed in a mass spectrometer. HenNgx(+) ions containing up to seven noble gas (Ng) atoms and dozens of helium atoms are identified; the high resolution of the mass spectrometer combined with advanced data analysis make it possible to unscramble contributions from isotopologues that have the same nominal mass but different numbers of helium or Ng atoms, such as the magic He20(84)Kr2(+) and the isobaric, nonmagic He41(84)Kr(+). Anomalies in these ion abundances reveal particularly stable ions; several intriguing patterns emerge. Perhaps most astounding are the results for HenAr(+), which show evidence for three distinct, solid-like solvation shells containing 12, 20, and 12 helium atoms. This observation runs counter to the common notion that only the first solvation shell is solid-like but agrees with calculations by Galli et al. for HenNa(+) [J. Phys. Chem. A 2011, 115, 7300] that reveal three shells of icosahedral symmetry. HenArx(+) (2 ≤ x ≤ 7) ions appear to be especially stable if they contain a total of n + x = 19 atoms. A sequence of anomalies in the abundance distribution of HenKrx(+) suggests that rings of six helium atoms are inserted into the solvation shell each time a krypton atom is added to the ionic core, from Kr(+) to Kr3(+). Previously reported strong anomalies at He12Kr2(+) and He12Kr3(+) [Kim , J. H.; et al. J. Chem. Phys. 2006, 124, 214301] are attributed to a contamination. Only minor local anomalies appear in the distributions of HenXex(+) (x ≤ 3). The distributions of HenKr(+) and HenXe(+) show strikingly similar, broad features that are absent from the distribution of HenAr(+); differences are tentatively ascribed to the very different fragmentation dynamics of these ions.

Ne2+[II (1/2)u]: radiative decay and electronic predissociation

Metastable fragmentation of neon dimer ions has been investigated by measuring and analyzing high resolution kinetic energy release distributions. Data were obtained by a modified MIKE (mass-analyzed ion kinetic energy) scan technique. Due to the high energy resolution it was possible to distinguish two different reaction mechanisms in the micros time regime which produce Ne+ ions with different kinetic energy distributions. Theoretical studies based on ab initio calculations of potential energy curves allowed the assignment of the reactions to specific electronic transitions in the excited Ne2+ ion. The unusual bimodal kinetic energy release distribution arises from competition between radiative and non radiative decay of the long-lived Ne2+ II(1/2)u state.

Experimental evidence for the existence of an electronically excited state of the proposed dihydrogen radical cation He-H-H-He+

Survival of the weakest: The existence of a new class of centrosymmetric radical cations in which H(2) bridges two identical main group elements was recently proposed in this journal by Uggerud and co-workers. By growing complexes inside helium nanodroplets at subkelvin temperatures, we obtained experimental evidence for the existence of the most weakly bound member of this class, He-H-H-He(+) (see picture), although in a metastable, electronically excited state.In a recent report, Uggerud and co-workers (A. Krapp et al., Chem. Eur. J. 2008, 14, 4028) proposed the existence of a new class of radical cations in which a dihydrogen bridges two identical main group elements. Upon electron impact ionization of helium nanodroplets doped with one or more H(2) molecules we observe various He(x)H(y) (+) cluster ions, including He(2)H(2) (+), which would belong to the proposed class of radical cations. Mass-analyzed kinetic energy scans reveal that the ion is metastable; it dissociates in the field-free region of the mass spectrometer. One reaction is into HeH(2) (+) + He with a low kinetic energy release of 15+/-4 meV. Surprisingly, another unimolecular reaction is observed, into HeH(+) + HeH (or He + H). The probability of this reaction is an order of magnitude higher, and the average kinetic energy release is four times larger. These findings suggest the presence of a metastable electronically excited state; they are consistent with the proposed linear, centrosymmetric ion structure of He-H-H-He(+).

On the R-dependence of the spin-orbit coupling constant: Potential energy functions of Xe(2) (+) by high-resolution photoelectron spectroscopy and ab initio quantum chemistry

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe(2) has been measured between 97 350 and 108 200 cm(-1), following resonant two-photon excitation via selected vibrational levels of the C 0(u) (+) Rydberg state of Xe(2). Transitions to three of the six low-lying electronic states of Xe(2) (+) could be observed. Whereas extensive vibrational progressions were observed for the transitions to the I(32g) and I(32u) states, only the lowest vibrational levels of the II(12u) state could be detected. Assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts and from the modeling of the potential energy curves. Adiabatic ionization energies, dissociation energies, and vibrational constants are reported for the I(32g) and the I(32u) states. Multireference configurational interaction and complete active space self-consistent field calculations have been performed to investigate the dependence of the spin-orbit coupling constant on the internuclear distance. The energies of vibrational levels, measured presently and in a previous investigation (Rupper et al., J. Chem. Phys. 121, 8279 (2004)), were used to determine the potential energy functions of the six low-lying electronic states of Xe(2) (+) using a global model that includes the long-range interaction and treats, for the first time, the spin-orbit interaction as dependent on the internuclear separation.

Dissociative charge transfer and collision induced dissociation of Ar2+ and Ar3+ clusters in collisions with argon atoms at keV energies

The dynamics of dissociative charge transfer and collision induced dissociation of Ar(2) (+) and Ar(3) (+) clusters colliding with Ar atoms at 4.8 keV has been investigated using a novel multifragment detection scheme that maps the postcollision vectors of all particles simultaneously. Estimation of internal energies and measurement of pre- and postcollision vectors enables a full description of reaction dynamics. The prominence of electronic excitation in defining the dynamics of these collision systems is demonstrated. The dissociation dynamics of Ar(3) (+) clusters is distinctly different from that of Ar(2) (+). This is attributed to a combination of lower internal energies and predominantly triangular T-shape structure of the Ar(3) (+) ion.

Lifetime and yield of metastable Ar2+ ions

Ar2(+) ions produced in a cooled supersonic expansion by electron-impact ionization are accelerated at 2.5 keV and kept during few milliseconds inside a linear electrostatic trap. The lifetime of the metastable Ar2(+) ion is determined from the measurement of the rate of the argon atoms escaping the trap. The lifetime and the relative metastable populations are measured as a function of the pressure and temperature in the supersonic expansion, i.e., of the mean cluster size. Possible mechanisms responsible for the metastable formation are discussed.

Nonthermal power law decay of metal dimer anions

The metastable decay of dimer anions of Cu and Ag has been measured in a storage ring. The decay is found to proceed nonexponentially and is well described by a power law with an exponent of -1. This signals the presence of a continuum of decay constants in the ensemble. This quasicontinuum can be provided by the quantum mechanical tunneling decay of high angular momentum states populated in the source. Numerical calculations for dimers of a variety of elements suggest that this decay behavior can be expected for a wide range of species.