W. C. Lineberger

Binding energies in atomic negative ions

This article updates a ten‐year‐old review of this subject [J. Chem. Phys. Ref. Data 4, 539 (1975)]. A survey of the electron affinity determinations for the elements up to Z=85 is presented, and based upon these data, a set of recommended electron affinities is established. Recent calculations of atomic electron affinities and the major semiempirical methods are discussed and compared with experiment. The experimental methods which yield electron binding energy data are described and intercompared. Fine structure splittings of these ions and excited state term energies are given.

Photoelectron spectroscopy of metal cluster anions: Cu−n, Ag−n, and Au−n

Negative ion photoelectron spectra of Cu−n, Ag−n(n=1–10), and Au−n (n=1–5) are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 6–9 meV. The metal cluster anions are prepared in a flowing afterglow ion source with a cold cathode dc discharge. In the spectra of Cu−2, Ag−2, and Au−2, the M2 X 1Σ+g←M−2 X 2Σ+u transitions are vibrationally resolved. We analyze these spectra to yield the adiabatic electron affinities, vibrational frequencies, bond length changes, and dissociation energies. The a 3Σ+u triplet states of Cu2 and Ag2 are also observed. Using experimental and theoretical data, we assign the major features in the Cu−3 and Ag−3 spectra to the transition from the linear ground state of the anion (M−31Σ+g) to an excited linear state of the neutral (M3 2Σ+u). The Au−3 spectrum is attributed to a two‐photon process, photodissociation followed by photodetachment of the Au− or Au−2 fragment. For larger clusters, we measure the threshold and vertical detachment energies...

A study of the singlet and triplet states of vinylidene by photoelectron spectroscopy of H2C=C−, D2C=C−, and HDC=C−. Vinylidene–acetylene isomerization

The X 1A1, a 3B2, and b 3A2 states of vinylidene are observed in the ultraviolet (351.1–364.0 nm) photoelectron spectra of X 2B2 H2CC−, X 2B2 D2CC−, and X 2A’ HDCC−. The X 1A1 state exhibits vibrational structure well above the barrier for isomerization to acetylene. A strict lower bound to the lifetime of the singlet state against rearrangement is τ>0.027 ps, with an estimate of τ≊0.04–0.2 ps based on a simulation of the line shapes including rotational broadening. A vibrational analysis of the singlet and lower triplet state bands provides vibrational frequencies and estimates of the changes of molecular geometries between the anion and the neutral species. A qualitative potential energy surface for the CH2 rock mode, which closely corresponds to the reaction coordinate for isomerization, is extracted from the experimental data. The adiabatic electron affinity is EA(X 1A1 H2CC)=0.490±0.006 eV and the triplet term energies are T0(a 3B2 H2CC)=2.065±0.006 eV and T0(b 3A2 H2CC)=2.754±0.020 eV. Exp...

Photoelectron spectroscopy of mass‐selected metal cluster anions. I. Cu−n, n=1–10

Negative ion photoelectron spectra of Cu−n (n=1–10) are reported for the 0–2.4 eV region at an instrumental resolution of 10 meV. The cluster anions were prepared in a flowing afterglow ion source incorporating a cold cathode dc discharge. This very simple source provides a convenient, general method to prepare continuous beams of near‐thermal metal cluster ions at intensities (up to 10−11 A) sufficient for spectroscopic or chemical studies. Photoelectron spectra of the copper cluster anions yield measurements for vertical electron binding energies and adiabatic electron affinities as a function of cluster size. The overall trend observed is well described by the classical spherical drop electrostatic model. In addition, quantum effects are apparent in the higher electron affinities generally observed for clusters containing odd numbers of atoms. Excited electronic states in the photoelectron spectra show that the transition energy in the neutral molecule decreases rapidly with cluster size. Vibrational s...

Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of C2H− and CD2−

Photoelectron spectra are reported for the CH2(X 3B1)+e−←CH−2 (X 2B1) and CH2(a 1A1)+e−←CH−2 (X 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to <1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 c...

A study of the low-lying electronic states of Fe2 and Co2 by negative ion photoelectron spectroscopy

The anions Fe−2 and Co−2 were prepared and cooled to 300 K in a flowing afterglow ion source, and the low‐lying electronic states of the neutral dimers were probed by negative ion photoelectron spectroscopy. Previous ab initio studies of Fe2 and Co2 have predicted single 4s–4s bonds, and extremely high densities of low‐lying states due to the small energy cost in transferring electrons among nonbonding 3d orbitals. In contrast to the complex photoelectron spectra implied by these calculations, the observed spectra are remarkably simple. It is argued that this spectral simplicity implies a greater role for the 3d electrons in the iron and cobalt dimer bonds. These data also provide values for the electron affinities of the neutral dimers (0.902±0.008 eV Fe2, 1.110±0.008 eV Co2), the bond elongation on electron attachment (0.08±0.02 A Fe2, Co2), and the vibrational frequencies of the anions (250±20 cm−1 Fe−2, 240±15 cm−1 Co−2). Related studies of the atomic anions yield improved values for the electron affi...

Photoabsorption and photofragmentation studies of Ar+n cluster ions

We have measured the photoabsorption spectra of mass selected Ar+n clusters, n=3–40, from 355–1064 nm. The smaller clusters, n<15, display a visible photoabsorption spectrum similar to Ar+3, i.e., a broad, intense band peaking near 520 nm. From n=15–20 this photoabsorption band shifts smoothly to a longer wavelength, peaking near 600 nm for Ar+20. This band does not change appreciably as n increases from 20 to 40. These results clearly demonstrate that the Ar+n clusters have photophysical properties quite different from those of Ar+2. We have also studied the photoabsorption and subsequent photofragmentation of Ar+n cluster ions, n=3–60, at selected visible wavelengths. The ionic photofragment distributions both indicate that photofragmentation proceeds through the loss of individual Ar atoms and place an upper bound of 90 meV on the cluster ion binding energy in the large cluster limit.

OH− and OD− threshold photodetachment

Threshold photodetachment cross sections for OH− and OD− at photon energies close to their electron binding energies have been obtained using a crossed tunable laser‐negative ion beam apparatus at a resolution of 25 μeV. The data reveal many clear features associated with photodetachment of single rotational levels of the negative ion, producing single rotational levels of the neutral and a near zero energy electron. From the frequencies of the observed thresholds, the spectroscopic constants of OH− and OD− are found. The rotational constants of the vibrational ground state are B0 = 18.7409(45) cm−1 and D0 = 2.052(45)×10−3 cm−1 for OH− and B0 = 9.9852(48) cm−1 and D0 = 0.553(33)×10−3 cm−1 for OD−. The electron affinities of OH and OD are measured to be 14 741.03(17) cm−1 and 14 723.92(30) cm−1, respectively. The observed bound–free transitions satisfy selection rules only slightly different from the selection rules observed in bound–bound spectroscopy. The relative intensities of the transitions show the ...

Transition-State Spectroscopy of Cyclooctatetraene

The 351-nanometer photoelectron spectrum of the planar cyclooctatetraene radical anion (COT·−) shows transitions to two electronic states of cyclooctatetraene (COT). These states correspond to the D4h 1A1g state, which is the transition state for COT ring inversion, and the D8h 3A2u state. The electron binding energy of the 1A1g transition state is 1.099 ± 0.010 electron volts, which is lower by 12.1 ± 0.3 kilocalories per mole than that of the 3A2u state. The photoelectron spectrum shows that the singlet lies well below the triplet in D8h COT and confirms ab initio predictions that the molecule violates Hunds rule. Vibrational structure is observed for both features and is readily assigned by use of a simple potential energy surface.

Laser photodetachment electron spectrometry of methoxide, deuteromethoxide, and thiomethoxide: Electron affinities and vibrational structure of CH30, CD30, and CH3S

Photodetachment of the three anions CH3O−, CD3O−, and CH3S− by a fixed‐frequency argon ion laser at 488 nm and subsequent energy analysis of the photoelectrons yields the photoelectron spectra of these species. From the spectra, electron affinities were determined: E.A.(CH3O) = (1.570±0.022) eV, E.A.(CD3O) = (1.552±0.022) eV, E.A.(CH3S) = (1.882±0.024) eV. From the vibrational structure appearing in the spectra, and the shifts observed upon deuteration, the predominant motion excited in the neutral upon photodetachment of CH3O− and CD3O− is found to be the symmetric hydrogen umbrella bend at 1325±30 and 1010±30 cm−1, respectively. In CH3S both the symmetric hydrogen bend and the carbon–sulfur bond stretch (680±40 cm−1) are excited. From the observed hot bands, some vibrational frequencies of the negative ions are also derived.