Field-induced ion-pair formation in molecular hydrogen

Coincidence ion pair production (cipp) spectroscopy of diiodine

Coincidence ion pair production (I⁺ + I^-) (cipp) spectra of I₂ were recorded in a double imaging coincidence experiment in the one-photon excitation region of 71 600-74 000 cm^-1. The I⁺ + I^- coincidence signal shows vibrational band head structure corresponding to iodine molecule Rydberg states crossing over to ion-pair (I⁺I^-) potential curves above the dissociation limit. The band origin (ν⁰), vibrational wavenumber (ω_e) and anharmonicity constants (ω_ex_e) were determined for the identified Rydberg states. The analysis revealed a number of previously unidentified states and a reassignment of others following a discrepancy in previous assignments. Since the ion pair production threshold is well established, the electric field-dependent spectral intensities were used to derive the cutoff energy in the transitions to the rotational levels of the 7pσ(1/2) (v' = 3) state.

An Evaluation of Gas Phase Enthalpies of Formation for Hydrogen-Oxygen (HxOy) Species

We have compiled gas phase enthalpies of formation for nine hydrogen-oxygen species (HxOy) and selected recommended values for H, O, OH, H2O, HO2, H2O2, O3, HO3, and H2O3. The compilation consists of values derived from experimental measurements, quantum chemical calculations, and prior evaluations. This work updates the recommended values in the NIST-JANAF (1985) and Gurvich et al. (1989) thermochemical tables for seven species. For two species, HO3 and H2O3 (important in atmospheric chemistry) and not found in prior thermochemical evaluations, we also provide supplementary data consisting of molecular geometries, vibrational frequencies, and torsional potentials which can be used to compute thermochemical functions. For all species, we also provide supplementary data consisting of zero point energies, vibrational frequencies, and ion reaction energetics.

Lifetimes of heavy-Rydberg ion-pair states formed through Rydberg electron transfer

The lifetimes of K(+)..Cl(-), K(+)..CN(-), and K(+)..SF(6)(-) heavy-Rydberg ion-pair states produced through Rydberg electron transfer reactions are measured directly as a function of binding energy using electric field induced detachment and the ion-pair decay channels discussed. The data are interpreted using a Monte Carlo collision code that models the detailed kinematics of electron transfer reactions. The lifetimes of K(+)..Cl(-) ion-pair states are observed to be very long, >100 micros, and independent of binding energy. The lifetimes of strongly bound (>30 meV) K(+)..CN(-) ion pairs are found to be similarly long but begin to decrease markedly as the binding energy is reduced below this value. This behavior is attributed to conversion of rotational energy in the CN(-) ion into translational energy of the ion pair. No long-lived K(+)..SF(6)(-) ion pairs are observed, their lifetimes decreasing with increasing binding energy. This behavior suggests that ion-pair loss is associated with mutual neutralization as a result of charge transfer.

Spectroscopic observation and characterization of H(+)H(-) heavy Rydberg states

A series of discrete resonances was observed in the spectrum of H2, which can be unambiguously assigned to bound quantum states in the 1/R Coulombic potential of the H+H- ion-pair system. Two-step laser excitation was performed, using tunable extreme ultraviolet radiation at lambda = 94-96 nm in the first step, and tunable ultraviolet radiation in the range lambda = 310-350 nm in the second step. The resonances, detected via H+ and H2+ ions produced in the decay process, follow a sequence of principal quantum numbers (n = 140-230) associated with a Rydberg formula in which the Rydberg constant is mass scaled. The series converges upon the ionic H+H- dissociation threshold. This limit can be calculated without further assumptions from known ionization and dissociation energies in the hydrogen system and the electronegativity of the hydrogen atom. A possible excitation mechanism is discussed in terms of a complex resonance. Detailed measurements are performed to unravel and quantify the decay of the heavy Rydberg states into molecular H2+ ions, as well as into atomic fragments, both H(n = 2) and H(n = 3). Lifetimes are found to scale as n3.

Ion-pair dissociation of N2O in the 16.25-16.41 eV: dynamics and electronic structure

The ion-pair dissociation dynamics of N(2)O -->(XUV) N(2)(+)(X (2)Sigma(g)(+), v) + O(-)((2)P(j)) at 16.248, 16.271, 16.389, and 16.411 eV have been studied using the velocity map imaging method and tunable XUV laser. The electronic structures of the ion-pair states have been studied by employing the ab initio quantum chemical calculation. The translational energy distributions and the angular distributions of the photofragments have been measured. The results show that about 40% of available energies are transformed into the translational energies, and the first excited vibrational states are populated most strongly for all four excitation energies. The anisotropy parameters beta are approximately 1. The ab initio calculations at the level of CASSCF6-311++g(3df) show that the equilibrium geometries of the ion-pair states are nonlinear with bond lengths R(N-N) = 1.10 A, R(N-O) = 2.15 A, and bond angle N-N-O = 103 degrees, respectively. The ion-pair states are formed by electron migration from the bonding sigma orbital of N[triple bond]N to the antibonding sigma orbital localized primarily on the O atom. Combining the experimental and theoretical results, it is concluded that the ion-pair dissociation occurs via predissociation of Rydberg states with (1)Sigma(+) symmetry, which converges to the ion-core N(2)O(+)(A (2)Sigma(+)).

ION PAIR DISSOCIATION: Spectroscopy and Dynamics

Ion pair dissociation processes may be studied using coherent vacuum ultraviolet laser sources in a manner entirely analogous to photoelectron spectroscopy, albeit with the anion playing the role of a heavy electron. If the excitation energy is above the dissociation energy and the kinetic energy of the fragment is measured using ion imaging, this approach is termed ion pair imaging spectroscopy (IPIS) and is related to conventional photoelectron spectroscopy. If the excitation energy is just below the dissociation energy and pulsed-field dissociation is employed, this approach is analogous to mass analyzed threshold ionization (MATI) spectroscopy and is termed threshold ion pair production spectroscopy (TIPPS). These approaches provide a novel means of investigating ion thermochemistry and spectroscopy and superexcited state decay dynamics at high resolution.

Velocity map imaging study of the O2 ion-pair production at 17.499 eV: simultaneous parallel and perpendicular transitions

The fine structure resolved photofragment O(-)((2)P(j)) image from the O(2) ion-pair production at 17.499 eV has been recorded. The branching ratio for producing the low energy spin-orbit O(-)((2)P(3/2)) component to the high energy spin-orbit O(-)((2)P(1/2)) component is 1:0.78 and the optical transitions for them correspond to perpendicular and parallel transitions, respectively. The anisotropy parameters, 1.64 for channel producing O(-)((2)P(1/2)) and -0.35 for O(-)((2)P(3/2)), suggest that the dissociation proceeds via the states with symmetry (3)Sigma(u)(-) and (3)Pi(u), respectively. Although the main mechanisms for the O(2) ion-pair production are the predissociation via the intermediate Rydberg states, the direct dissociation mechanism for the channel producing O(-)((2)P(1/2)) may also be involved.

A combined zero electronic kinetic energy spectroscopy and ion-pair dissociation imaging study of the F2+(XΠg2) structure

Rotationally resolved pulsed field ionization and zero electronic kinetic energy photoelectron spectra for the transition F(2) (+)(X (2)Pi(g))<--F(2)(X (1)Sigma(g) (+)) have been recorded using the extreme ultraviolet coherence radiation. The vibrational energy spacings, rotational constants, and spin orbit coupling constants for the first three vibrational states of F(2) (+)(X (2)Pi(g)) have been determined accurately. The first adiabatic ionization potential (IP) of F(2) is determined as IP(F(2))=126 585.7+/-0.5 cm(-1). To determine the threshold E(tipp) for ion-pair production of F(2), the images of F(-)((1)S(0)) in the velocity mapping conditions have also been recorded at the photon energy of 126 751 cm(-1). Taking the Stark effect into account, the E(tipp) is determined as E(tipp)(F(2))=126 045+/-8 cm(-1) (15.628+/-0.001 eV). By combing the IP(F(2)) and the E(tipp)(F(2)) determined in this work and together with the reported ionization potential and electronic affinity of the F atom, the bond dissociation energies of F(2) and F(2) (+) are determined as D(0)(F(2))=1.606+/-0.001 eV and D(0)(F(2) (+))=3.334+/-0.001 eV, respectively.

Vacuum ultraviolet spectroscopy and chemistry by photoionization and photoelectron methods

The recent developments of vacuum ultraviolet (VUV) laser and third generation synchrotron radiation sources, together with the introduction of pulsed field ionization (PFI) schemes for photoion-photoelectron detection, have had a profound impact on the field of VUV spectroscopy and chemistry. Owing to the mediation of near-resonant autoionizing states, rovibronic states of ions with negligible Franck-Condon factors for direct photoionization can be examined by VUV-PFI measurements with rotational resolutions. The VUV-PFI spectra thus obtained have provided definitive ionization energies (IEs) for many small molecules. The recent synchrotron-based PFI-photoelectron-photoion coincidence experiments have demonstrated that dissociative photoionization thresholds for a range of molecules can be determined to the same precision as in PFI-photoelectron measurements. Combining appropriate dissociation thresholds and IEs measured in PFI studies, thermochemical data for many neutrals and cations can be determined with unprecedented precision. The further development of two-color excitation-ionization schemes promises to expand the scope of spectroscopic and chemical applications using the photoionization-photoelectron method.

Thermochemical aspects of proton transfer in the gas phase

The beginning of the twentieth century saw the development of new theories of acidity and basicity, which are currently well accepted. The thermochemistry of proton transfer in the absence of solvent attracted much interest during this period, because of the fundamental importance of the process. Nevertheless, before the 1950s, few data were available, either from lattice energy evaluations or from calculations using the emerging molecular orbital theory. Advances in mass spectrometry during the last 40 years allowed studies of numerous systems with better accuracy. Thousands of accurate gas-phase acidities or basicities are now available, for simple atomic and molecular systems and for large biomolecules. The intrinsic effect of structure on the Brønsted basic or acidic properties of molecules and the influence of solvents have been unravelled. In this tutorial, the basics of the thermodynamic principles involved are given, and the mass spectrometric techniques are briefly reviewed. Advances in the design and measurements of gas-phase superacids and superbases are described. Recent studies concerning biomolecules are also evoked.