H. W. Sarkas

Photoelectron spectra of the alkali metal cluster anions: Na−n=2−5, K−n=2−7, Rb−n=2−3, and Cs−n=2−3

Obtention des spectres de photoelectrons a 2,540 eV sur faisceaux supersoniques. Deduction des energies de detachement vertical et des affinites electroniques adiabatiques, dont on analyse les variations avec n

Using cluster studies to approach the electronic structure of bulk water: Reassessing the vacuum level, conduction band edge, and band gap of water

Aqueous cluster studies have lead to a reassessment of the electronic properties of bulk water, such as band gap, conduction band edge, and vacuum level. Using results from experimental hydrated electron cluster studies, the location of the conduction band edge relative to the vacuum level (often called the V0 value) in water has been determined to be −0.12 eV⩽V0⩽0.0 eV, which is an order of magnitude smaller than most experimental values in the literature. With V0=−0.12 eV and making use of the calculated solvation energy of OH in water, the band gap of water is determined to be 6.9 eV. Again, this is smaller than many literature estimates. In the course of this work, it is shown that due to water’s ability to reorganize about charge (1) photoemission thresholds of water or anionic defects in water do not determine the vacuum level, and (2) there is almost no probability of accessing the bottom of the conduction band of water with a vertical/optical process from water’s valence band. The results are pres...

Negative ion photoelectron spectroscopy of solvated electron cluster anions, (H2O)n− and (NH3)n−

The photodetachment spectra of (H2O)n=2−69/− and (NH3)n=41−1100/− have been recorded, and vertical detachment energies (VDEs) were obtained from the spectra. For both systems, the cluster anion VDEs increase smoothly with increasing sizes and most species plot linearly withn−1/3, extrapolating to a VDE (n=∞) value which is very close to the photoelectric threshold energy for the corresponding condensed phase solvated electron system. The linear extrapolation of this data to the analogous condensed phase property suggests that these cluster anions are gas phase counterparts to solvated electrons, i.e. they are embryonic forms of hydrated and ammoniated electrons which mature with increasing cluster size toward condensed phase solvated electrons.

Ammonia cluster anions and their relationship to ammoniated (solvated) electrons: The photoelectron spectra of (NH3)n=41–1100−

We report the negative ion photoelectron spectra of (NH3)n=41–1100−, recorded using 2.540 eV photons. The largest cluster anion in this series has a diameter of approximately 4.3 nm. The vertical detachment energies (VDEs) of these cluster anions increase smoothly from 0.55 eV for n=41 to 1.05 eV for n=1100. The VDEs throughout this size range are linear with n−1/3 and extrapolate to a VDE (n=∞) value, which is very close to the measured photoelectric threshold energy of condensed phase ammoniated electrons. The linear extrapolation of this data to an analogous condensed phase property implies that these cluster anions are gas-phase counterparts to ammoniated electrons, i.e., they are embryonic forms of ammoniated electrons which will mature with increasing cluster size to become condensed phase-solvated electrons. The VDE data further implies that these embryonic ammoniated electrons were generated in solid ammonia environments, consistent with the source conditions under which they were produced.

Lithium cluster anions: Photoelectron spectroscopy and ab initio calculations

Structural and energetic properties of small, deceptively simple anionic clusters of lithium, Li(n)(-), n = 3-7, were determined using a combination of anion photoelectron spectroscopy and ab initio calculations. The most stable isomers of each of these anions, the ones most likely to contribute to the photoelectron spectra, were found using the gradient embedded genetic algorithm program. Subsequently, state-of-the-art ab initio techniques, including time-dependent density functional theory, coupled cluster, and multireference configurational interactions methods, were employed to interpret the experimental spectra.

Photoelectron spectroscopy of lithium hydride anion

We present negative ion photoelectron spectra of the smallest stable molecular negative ion, the lithium hydride anion. Photoelectron spectra, recorded using 2.540 eV photons, are reported for the LiH(D) [X 1Σ+]+e−←LiH(D)−[X 2Σ+] transitions of 7LiH− and 7LiD−. Adiabatic electron affinities of 0.342±0.012 eV and 0.337±0.012 eV were determined for 7LiH and 7LiD, respectively. The experimentally determined electron affinities led to anion dissociation energy (D0) values of 2.017±0.021 eV for 7LiH− and 2.034±0.021 eV for 7LiD− relative to their Li[2S1/2]+H−(D−)[1S0] asymptotes. Franck–Condon analyses yielded the following molecular parameters for the ground state of 7LiH−: Be=6.43±0.18 cm−1, re=1.724±0.025 A, and ωe=920±80 cm−1; and the following parameters for the ground state of 7LiD−: Be=3.62±0.06 cm−1, re=1.724±0.015 A, and ωe=650±45 cm−1. In addition, we have observed the alkali hydride anions: 7LiH−2, 7LiD−2, Li2D−, NaD−, NaD−2, NaD−3, and NaD−4. No photodetachment signal was observed for the lithium d...

Photoelectron spectroscopy of alkali metal tetramer anions: The anomalous spectrum of Li−4

We present the photoelectron spectrum of Li−4. This spectrum displays a spectral pattern that is strikingly different from that of the other alkali tetramer anions. Using the photoelectron spectrum of Li−4 along with our previously measured photoelectron spectra of Na−4, K−4, and Rb−4 plus other existing evidence, we find that Li−4 does not have a linear geometry, as do the tetramer anions of sodium, potassium, and rubidium. This observation indicates that for both anions and neutrals, lithium clusters appear to take on higher dimensional structures at smaller sizes than do sodium and probably other alkali clusters. By examining the clues found in its photoelectron spectrum, we then speculate as to what the structure of Li−4 may be and also summarize the present state of theoretical progress on this problem.

PHOTOELECTRON SPECTROSCOPY OF COLOR CENTERS IN NEGATIVELY CHARGED CESIUM IODIDE NANOCRYSTALS

We present the photoelectron spectra of negatively charged cesium iodide nanocrystals recorded using 2.540 eV photons. The species examined were produced using an inert gas condensation cluster ion source, and they ranged in size from (CsI)−n=13 to nanocrystal anions comprised of 330 atoms. Nanocrystals showing two distinct types of photoemission behavior were observed. For (CsI)−n=13 and (CsI)−n=36−165, a plot of cluster anion photodetachment threshold energies vs n−1/3 gives a straight line extrapolating (at n−1/3=0, i.e., n=∞) to 2.2 eV, the photoelectric threshold energy for F centers in bulk cesium iodide. The linear extrapolation of the cluster anion data to the corresponding bulk property implies that the electron localization in these gas‐phase nanocrystals is qualitatively similar to that of F centers in extended alkali halide crystals. These negatively charged cesium iodide nanocrystals are thus shown to support embryonic forms of F centers, which mature with increasing cluster size toward conde...

The smoke ion source: a device for the generation of cluster ions via inert gas condensation

We report the development of an ion source for generating intense, continuous beams of both positive and negative cluster ions. This device is the result of the marriage of the inert gas condensation method with techniques for injecting electrons directly into expanding jets. In the preliminary studies described here, we have observed cluster ion size distributions ranging fromn=1−400 for Pbn+ and Pbn−, and fromn=12−5700 for Lin−.

Characterization of theX2 ∑ u + state of7Li 2 − via negative ion photoelectron spectroscopy

The negative ion photoelectron spectrum of7Li2− is reported at 488 nm (2.540 eV). Three electronic bands are observed in this spectrum and are assigned to the following photodetachment transitions:7Li2,X1 ∑g++e− ←7Li2−,X2 ∑u+;7Li2,a3 ∑u++e− ←7Li2−,X2 ∑u+; and7Li2,A1 ∑u++e− ←7Li2−,X2 ∑u+. The electron affinity of7Li2 is determined to be 0.437±0.009 eV, leading to an anion dissociation energy,D0, of 0.865±0.022 eV for the ground state of7Li2−. A Franck-Condon analysis of the7Li2,X1 ∑g++e− ←7Li2−,X2 ∑u+ band yields the following spectroscopic constants for the ground state of7Li2−:Be=0.502±0.005 cm−1,re=3.094±0.015 Å, and ωe=232±35 cm−1.