Joe Ho

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...

Electronic and vibrational structure of transition metal trimers: Photoelectron spectra of Ni−3, Pd−3, and Pt−3

The transition metal trimer anions Ni−3, Pd−3, and Pt−3 are prepared in a flowing afterglow ion source with a cold cathode dc discharge. The low‐lying electronic states of the neutral trimers are probed by 488 nm negative ion photoelectron spectroscopy at an electron kinetic energy resolution of 5–12 meV. Each trimer exhibits multiple low‐lying electronic states. Vibrational progressions are observed in tripalladium and triplatinum. The adiabatic electron affinities are found to be EA(Ni3)=1.41±0.05 eV, EA(Pd3)≲1.5±0.1 eV, and EA (Pt3)=1.87±0.02 eV.

Photoelectron spectroscopy of nickel group dimers: Ni−2, Pd−2, and Pt−2

Negative ion photoelectron spectra of Ni−2, Pd−2, and Pt−2 are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 8–10 meV. The metal cluster anions are prepared in a flowing afterglow ion source. Each dimer exhibits multiple low‐lying electronic states and a vibrationally resolved ground state transition. Franck–Condon analyses yield the anion and neutral vibrational frequencies and the bond length changes between anion and neutral. The electron affinities are determined to be EA(Ni2)=0.926±0.010 eV, EA(Pd2)=1.685±0.008 eV, and EA(Pt2)=1.898±0.008 eV. The electronic configurations of the ground states are tentatively assigned. Comparison of the nickel group dimers to the coinage metal dimers sheds light on the d orbital contribution to the metal bonding in the nickel group dimers.

A study of the electronic structures of Pd2- and Pd2 by photoelectron spectroscopy

The ultraviolet negative ion photoelectron spectrum of Pd−2 is presented for electron binding energies up to 3.35 eV. The anion is prepared by sputtering in a flowing afterglow ion source. Multiple low‐lying electronic states of Pd2, all unidentified previously, are observed with resolved vibrational structure. The spectrum shows two strong electronic bands, each with similar vibrational progressions. Franck–Condon analyses are carried out on the two transitions and molecular constants are extracted for the anion and the two neutral electronic states. With the help of simple molecular orbital arguments and ab initio calculations, these two electronic bands are assigned as the triplet ground state (3Σ+u) and a singlet excited state (1Σ+u). The adiabatic electron affinity is E.A.(Pd2)=1.685±0.008 eV and the singlet excitation energy T0(1Σ+u) is 0.497±0.008 eV (4008±65 cm−1 ). The bonding in the palladium dimers is discussed and we find that the anion bond strength is 1.123±0.013 eV stronger than that of the...

Photoelectron spectroscopy of negatively charged bismuth clusters: Bi−2, Bi−3, and Bi−4

We have recorded the 351 nm photoelectron spectra of Bi−2, Bi−3, and Bi−4. The spectrum of Bi−2 shows transitions to at least seven electronic states of Bi2 neutral, four of which are observed with vibrational resolution. Term energies, bond lengths, and vibrational frequencies are obtained for the anion ground state and for the first three excited states of Bi2. These results are compared to previous spectroscopic measurements and to the ab initio calculations presented in the accompanying paper. The photoelectron spectrum of Bi−3 reveals some of the electronic structure of Bi3 and the results are discussed in comparison to recent theoretical work. Adiabatic electron affinities are obtained for Bi2 [1.271(8) eV] and for Bi3 [1.60(3) eV]. The electron affinity of Bi4 is estimated from the onset of photodetachment to be 1.05(10) eV.

Photoelectron spectroscopy of group IV heavy metal dimers: Sn−2, Pb−2, and SnPb−

Negative ion photoelectron spectra of Sn−2, SnPb−, and Pb−2 are presented for electron binding energies up to 3.35 eV. Each spectrum exhibits multiple electronic bands, most of which contain resolved vibrational structure. Franck‐Condon analyses yield spectroscopic parameters (re, ωe, and Te) for the anion ground states and the neutral excited states. Adiabatic electron affinities are determined to be: EA(Sn2)=1.962±0.010 eV, EA(Pb2)=1.366±0.010 eV, and EA(SnPb)=1.569±0.008 eV. The anion dissociation energies D0(Sn−2) and D0(Pb−2) are derived from the electron affinities and the neutral dissociation energies. For SnPb−, the dissociation energy difference D0(SnPb−)−D0(SnPb) is precisely measured. Based on the present data, previous experiments and ab initio calculations, we assign most of the observed bands to the corresponding neutral low‐lying electronic states.

Photoelectron spectroscopy of small antimony cluster anions: Sb−, Sb2−, Sb3−, and Sb4−

We report the 351 nm photoelectron spectra of Sb−, Sb2−, Sb3−, and Sb4−. The electron affinity of atomic Sb is measured to be 1.046(5) eV. The Sb2− photoelectron spectrum displays rich vibrational and electronic structure. Low‐lying electronically excited states are observed for both the anion and the neutral. Several features in both the 351 and 364 nm photoelectron spectra of Sb2− cannot be explained as Franck–Condon processes, indicating that we are accessing autodetaching resonances of the negative ion at these wavelengths. The adiabatic electron affinity of Sb2 is determined to be 1.282(8) eV. For the photoelectron spectra of Sb3− and Sb4−, the observed electronic structure is explained in terms of recently reported ab initio calculations. The adiabatic electron affinity of Sb3 is estimated to be 1.85(3) eV, and an upper bound on the electron affinity of Sb4 is reported, EA(Sb4)≤1.00(10) eV. The vertical detachment energies of Sb3− and Sb4− to the neutral ground states are determined to be 1.90(2) an...