Survey of Ar-tagged predissociation and vibrationally mediated photodetachment spectroscopies of the vinylidene anion, C2H2-.

We report predissociation spectra of Ar-tagged C(2)H(2)(-) and C(2)D(2)(-) anions, and explore vibrationally mediated photodetachment from various vibrational levels of the bare C(2)H(2)(-) ion using velocity-map imaging. Intense photodetachment resonances are observed in the C-H stretching region that are strongly correlated with vibrational hot bands in the anion photoelectron spectra, indicating that one-color, resonant two-photon photodetachment (R2PD) is complicated by excitation of vibrationally excited states with autodetaching upper levels embedded in the continuum. Isolation of the R2PD spectrum was achieved using a two-color, IR-IR scheme in which vibrational excitation and photodetachment were carried out in two separate laser interaction regions.

Spectroscopic study of the ion-radical H-bond in H4O2+.

The primary event in the ionization of water involves rapid proton transfer, leading to charge localization on H(3)O(+) and the creation of a hydroxyl radical. We trap the nascent [H(3)O(+).(*)OH] exit channel intermediate in the bimolecular reaction by Ar-mediated ionization of the neutral water dimer and characterize the nature of this ion-radical complex using vibrational predissociation spectroscopy of the Ar-tagged species. The resulting bands involving the displacement of the bridging proton are broad and appear as a strong triplet centered around 2000 cm(-1). The observed band pattern is analyzed with theoretical calculations to identify the origin of the anhamonic effects evident in the spectrum. In the course of this work, expressions were derived for treating the coupling terms within a sinc-DVR. Although this level of treatment did not reveal the assignment of the triplet structure, its characteristic approximately 100 cm(-1) spacing suggests activity involving the frustrated rotation of the hydroxyl radical upon excitation of the bridging-proton vibration parallel to the heavy atom axis. The behavior of this system is considered in the context of that reported previously for the related H(5)O(2)(+), H(3)O(2)(-), and F(-).H(2)O complexes.

Intermolecular proton binding in the presence of a large electric dipole: Ar-tagged vibrational predissociation spectroscopy of the CH3CN x H+ x OH2 and CH3CN x D+ x OD2 complexes.

We report Ar-predissociation vibrational spectra of the binary proton-bound hydrates of acetonitrile (AN), AN x H(+) x OH(2) and AN x D(+) x OD(2), in the 600-3800 cm(-1) energy range. This complex was specifically chosen to explore the nature of the intermolecular proton bond when there is a large difference between the electric dipole moments of the two tethered molecules. Sharp, isotope-dependent bands in the vicinity of 1000 cm(-1) are traced to AN x H(+) x OH(2) vibrations involving the parallel displacement of the shared proton along the heavy atom axis, nu(sp)(parallel). These transitions lie much lower in energy than anticipated by a recently reported empirical trend which found the nu(sp)(parallel) fundamentals to be strongly correlated with the difference in proton affinities (DeltaPA) between the two tethered molecules (Roscioli et al., Science, 2007, 316, 249). The different behavior of the AN x H(+) x OH(2) complex is discussed in the context of the recent theoretical prediction (Fridgen, J. Phys. Chem A., 2006, 110, 6122) that a large disparity in dipole moments would lead to such a deviation from the reported (DeltaPA) trend.

Isolating the spectra of cluster ion isomers using Ar-"tag" -mediated IR-IR double resonance within the vibrational manifolds: Application to NO2- *H2O.

We demonstrate a method for isolating the vibrational predissociation spectra of different structural isomers of mass-selected cluster ions based on a population-labeling double resonance scheme. This involves a variation on the "ion dip" approach and is carried out with three stages of mass selection in order to separate the fragment ion signals arising from a fixed-frequency population-monitoring laser and those generated by a scanned laser that removes population of species resonant in the course of the scan. We demonstrate the method on the Ar-tagged NO(2) (-)H(2)O cluster, where we identify the spectral patterns arising from two isomers. One of these structures features accommodation of the water molecule in a double H-bond arrangement, while in the other, H(2)O attaches in a single ionic H-bond motif where the nominally free OH group is oriented toward the N atom of NO(2) (-). Transitions derived from both the NO(2) (-) and H(2)O constituents are observed for both isomers, allowing us to gauge the distortions suffered by both the ion and solvent molecules in the different hydration arrangements.

Probing isomer interconversion in anionic water clusters using an Ar-mediated pump-probe approach: Combining vibrational predissociation and velocity-map photoelectron imaging spectroscopies.

We present the first results from an experiment designed to explore barriers for interconversion between isomers of cluster anions using an Ar-cluster mediated pump-probe technique. In this approach, anions are generated with many Ar atoms attached, and one of the isomers present is selectively excited by tuning an infrared laser to one of the isomer's characteristic vibrational resonances. The excited cluster is then cooled by evaporation of Ar atoms, and the isomer distribution in the lighter daughter ions is measured after secondary mass selection by recording their photoelectron spectra using velocity-map imaging. We apply the method to the water hexamer anion, (H(2)O)(6) (-), which is known to occur in two isomeric forms with different electron-binding energies. We find that conversion of the high-binding (type I) form to the low-binding (type II) isomer is not efficiently driven in (H(2)O)(6) (-) with excitation energies in the 0.4 eV range even though it is possible to create both isomers in abundance in the ion source. This observation is discussed in the context of the competition between isomerization and electron autodetachment, which depends on the relative positions of the neutral and ionic potential surfaces along the isomerization pathway. Application of the method to the more complex heptamer ion, however, does reveal that interconversion is available among the highest binding isomer classes (I and I(')).