Christopher G. Bailey

Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network

We report autodetachment spectra of the mass-selected, anionic water clusters, (H2O)n−, n=2, 3, 5–9, 11 in the OH stretching region (3000–4000 cm−1), and interpret the spectra with the aid of ab initio calculations. For n⩾5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm−1, which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the n=5–11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear “chainlike” (H2O)n− species. We test this assignment by recording the spectral pattern of the cooled (argon solvated) HDO⋅(D2O)5− isotopomer over the entire OH stretching frequency range.

Vibrational predissociation spectroscopy of the (H2O)6−⋅Arn, n⩾6, clusters

Solvation of (H2O)6− with several argon atoms suppresses the strong direct photodetachment background in the bare hexamer anion, allowing vibrational predissociation spectroscopy to be carried out in a background-free regime. In addition to the previously reported autodetaching resonances [C. G. Bailey, J. Kim, and M. A. Johnson, J. Phys. Chem. 100, 16782 (1996)] in the single donor hydrogen bonding region (∼3300 cm−1), the predissociation spectra reveal many weak bands scattered throughout the mid infrared (3200–3750 cm−1). Most of these new bands are evident in the bare hexamer spectrum after signal averaging, indicating that they are isolated using predissociation but not induced by solvation. The most intense bands display much stronger redshifts (>30 cm−1 by n=15) than the matrix shifts typically found for the neutral water clusters, indicating that these bands are unique to the negative ion.

Vibrational predissociation spectra of I−·(H2O): isotopic labels and weakly bound complexes with Ar and N2

Abstract Vibrational predissociation spectra in the region of the OH stretch are reported for I−·(H2O), I−·HDO and the weakly bound complexes I−·H2O·Ar and I−·H2O·N2. The I−·HDO system consists of two isomers in Cs geometry, each displaying a similar frequency to one of the two bands observed for I−·(H2O), indicating that the motions of the two hydrogens are weakly coupled. Rotational structure is partially resolved in the 3700 cm−1 band and assigned to Q-heads of a B-type transition. Two members of the previously observed quartet [Johnson et al., Chem. Phys. Lett. 260 (1996) 551] in I−·(H2O) around 3400 cm−1 (corresponding to the hydrogen-bonded OH) persist upon complexation with either Ar or N2.

Vibronic effects in the photon energy‐dependent photoelectron spectra of the CH3CN− dipole‐bound anion

Photoelectron spectra are reported for the ‘‘dipole‐bound’’ CH3CN− negative ion at three photodetachment energies (1.165, 2.331, and 3.496 eV), where the anion is prepared by photodissociation of the I−⋅CH3CN ion–molecule complex. While all three spectra are dominated by a single feature centered near zero electron binding energy, as expected for a dipole‐bound anion, vibrational structure is also observed and found to depend strongly on the photodetachment energy. This observation indicates that the vibrational excitation is not exclusively due to distortion between the ion and neutral, but also involves non‐‘‘Franck–Condon’’ effects. The origin of the energy dependence is traced to excitation of the πCN* shape resonance corresponding to the valence or ‘‘chemical’’ anion. The vibrational envelope of the nonresonant spectrum is surprisingly similar to the infrared spectrum of neutral acetonitrile, suggesting that even this excitation may not result from intramolecular distortions. We develop a simple mode...

Dipole‐bound excited states of the I−⋅CH3CN and I−⋅(CH3CN)2 ion–molecule complexes: Evidence for asymmetric solvation

Dipole‐bound excited states are reported for the I−⋅CH3CN and I−⋅(CH3CN)2 cluster ions, located just below their vertical electron detachment energies (determined using negative ion photoelectron spectroscopy). The absorption cross sections for excitation to these states are observed to increase with increasing dipole moments of the solvent molecules in the I−⋅M series (M=methyl iodide, acetone, acetonitrile). Photoexcitation at the peak of the transition to the dipole‐bound state results exclusively in the dipole‐bound fragment ion, M−. The photoelectron spectrum of the CH3CN− fragment was also recorded by sequential two‐photon absorption in the I−⋅CH3CN parent, indicating that the excess electron is indeed weakly bound (≤10 meV) with very little intramolecular distortion evident upon electron detachment. The I−⋅(CH3CN)2 cluster displays two absorption bands, one below each of the two features in the photoelectron spectrum. The most intense band correlates with the weaker, lower binding energy photoelect...

Determination of the relative photodetachment cross sections of the two isomers of (H2O)6− using saturated photodetachment

Abstract We report relative photodetachment cross sections for the two isomers of (H 2 O) 6 − at 1.165 eV by monitoring the photoelectron spectrum as a function of laser fluence. The cross sections of the two isomers differ by only about a factor of three, much closer to each other than suggested by recent calculations by Kim et al.

Autodetachment from vibrational levels of the O−2 A 2Πu resonance across its dissociation limit by photoexcitation from O−2 X 2Πg

We report the observation of resonance structure in the photodetachment spectrum of O−2 in the 4 eV range, which results from the excitation of autodetaching vibrational levels of the O−2 A–X transition near the dissociation limit. The evolution of the resonances with increasing vibration is simply explained using continuity of the inner part of the vibrational wave functions across the dissociation threshold. This affords the possibility of investigating the DA process at the half‐collision, in a kind of ‘‘correspondence limit’’ where the outer turning point slowly recedes and the vibrational wave function incrementally adopts the character of the dissociation continuum. Photoexcitation near one of the resonances results in the population of significantly higher vibrational levels in the O2 a1Δg state (which are cleanly resolved) than the typical ‘‘Franck–Condon’’ pattern observed for nonresonant photodetachment. Finally, hot‐band structure is also observed in the detachment spectrum, allowing us to extr...

Observation of the dipole‐bound excited state of the I−⋅acetone ion‐molecule complex

Photofragmentation action spectra of the I−⋅acetone ion‐molecule complex reveal the existence of an excited state, located just below the electron detachment threshold, which is thought to correspond to a dipole‐bound state derived primarily from the electric dipole moment of acetone. The excited state relaxes by fragmentation into the acetone anion, a dipole‐bound ground state anion. This type of excited state should be a general property of ion‐molecule complexes, X−⋅M, where M has a significant dipole moment and X does not, and is a microscopic precursor of the charge transfer to solvent bands observed for anions in dipolar solvents.

On the vibrational fine structure in the near‐threshold photofragmentation spectrum of the I−⋅CH3I complex: Spectroscopic observation of nonadiabatic effects in electron‐molecule scattering

Photofragmentation of the I−⋅CH3I ion‐molecule complex is observed to accompany photoexcitation in the vicinity of its electron detachment thresholds. The I− photofragment action spectrum displays a vibrational progression in the ν3 (largely C–I stretching) mode of neutral CH3I, the same mode which is excited upon photodetachment of the complex. The extent of this vibrational activity in the I−⋅CH3I photoelectron spectrum is found to strongly depend on the photodetachment energy, becoming very pronounced as the photon energy approaches the detachment threshold. This indicates that the vibrational features in the photoelectron spectrum arise from non‐Franck–Condon effects. These observations of selective excitation of ν3 in both the photoelectron and photofragmentation spectra are correlated to nonadiabatic effects arising from the repulsive state of the CH3I− anion, which is thought to evolve into a resonance near the equilibrium separation of neutral CH3I. The I−⋅CH3I photochemistry is discussed in the c...

The charge transfer excited state of the I−⋅CH3I SN2 reaction intermediate: Photoinduced intracluster dissociative attachment

We describe the reaction dynamics which occur upon excitation of the I−⋅CH3I ion–molecule complex to the first optically allowed excited electronic state. Photoelectron spectroscopy of I−⋅CH3I confirms the identification of the species as essentially charge localized, where the observed vibrational fine structure is found to be consistent with small distortions of the CH3I neutral upon complexation to form a stable intermediate in the SN2 identity reaction. A narrow photofragmentation band lies just below the vertical electron detachment energy and is assigned to the I−⋅CH3I→I...[CH3I]− charge transfer excited state. The recoil energy imparted to the I− fragment is only about 10% of the available energy, indicating that most of the energy is lost to the methyl group as expected for an impulsive dissociation. The I− photoproduct is preferentially ejected along the electric vector of the laser with an anisotropy parameter β of +0.5±0.2. This requires that photoabsorption occurs to a repulsive state which di...