This paper investigates whether photoemission can be nearly completely suppressed in a non-atomic system even when the photon energy exceeds the electron-detachment threshold. The proposed mechanism is an extreme Cooper minimum, in which positive and negative contributions to the transition dipole matrix element cancel. Complete suppression is difficult when multiple outgoing electron partial waves contribute, because their amplitudes generally do not vanish at the same energy. To address this challenge, the authors designed highly symmetric metal clusters, or superatoms, whose outermost S-like orbital emits predominantly through a single p-wave channel. The primary experimental system was Ag₁₂W⁻, consisting of a tungsten atom enclosed within an icosahedral cage of 12 silver atoms. Density functional theory identified this structure as the most stable, with no competing structural isomers within approximately 0.8 eV. Its predicted electronic configuration includes one electron in a superatomic 2S orbital. The researchers produced mass-selected cluster anions by magnetron sputtering and recorded photoelectron images at 55 photon energies between 1.46 and 4.00 eV. They independently measured photodetachment action spectra from 1.46 to 5.51 eV by monitoring laser-induced depletion of trapped anions. Calculations using the ezDyson program aided the interpretation of the spectra and electron angular distributions. Ag₁₂W⁻ exhibited a vertical detachment energy of 1.46 eV, consistent with the value calculated at the PBEPBE/LANL2DZ level. Its photoelectron spectrum contained a single narrow peak with a width of 39 meV, indicating little structural relaxation following electron removal. The angular distributions closely resembled those of the atomic silver anion, supporting the S-like character of the emitting orbital. Crucially, both experimental methods showed that photodetachment became nearly undetectable at photon energies of approximately 2.0–2.5 eV. The minimum occurred near 2.3 eV, corresponding to a photoelectron kinetic energy of approximately 0.85 eV. Emission recovered at higher energies, with a small spectral peak near 3.2 eV and a broad, intense band near 4.3 eV. The simulations reproduced the suppression by describing the bound electron with a Kohn–Sham orbital and the emitted electron with a Coulomb wave using an adjustable effective charge of +0.37e. At a kinetic energy of 0.85 eV, the calculated positive and negative radial contributions to the transition dipole integral canceled. Away from the minimum, the measured angular anisotropy parameter reached the atomic limit of 2 near a photon energy of 2.7 eV and throughout the 3.3–4.0 eV range. Near the minimum, it approached zero as the dominant p-wave contribution vanished and weak higher-angular-momentum contributions became relatively important. An isoelectronic cluster, Ag₁₂Mo⁻, exhibited analogous suppression near a photon energy of 2.1 eV, corresponding to an electron kinetic energy of approximately 0.87 eV. The authors conclude that nearly spherical superatoms offer a tunable platform for controlling photoemission through cluster composition and size, potentially enabling studies of emission time delays and nondipole effects. Several qualifications remain, however. The experimental yield approaches zero but is not established as mathematically zero, and icosahedral symmetry permits weak residual partial waves. The theoretical treatment is phenomenological, employs an adjusted continuum potential, and omits resonant autodetachment processes that may explain discrepancies in the angular distributions. Absolute action-spectrum cross sections also have estimated systematic uncertainties of 30–50%, primarily because of uncertainty in ion–laser overlap. Overall, the results strongly support a Cooper-minimum interpretation, although broader applicability beyond the two demonstrated clusters remains a prediction.
- Scientists studied tiny balls of atoms called clusters, including one with a tungsten atom inside a cage of 12 silver atoms.
- Light can knock tiny bits called electrons out of these clusters, but some light energies made almost no electrons come out.
- This happened because two parts of the electron’s escape process canceled each other, like opposite pushes.
- Scientists saw this effect in two kinds of clusters, and electrons came out again when the light had more energy.
- These clusters might help scientists control how electrons escape, but more tests are needed to learn how well this works.
Definitions
- atom: A tiny building block that makes up things around us.
- cluster: A small group of atoms joined together.
- tungsten: A kind of metal.
- silver: A kind of metal often used to make jewelry.
- electron: A tiny part of an atom that has a negative electric charge.
- energy: Something light carries that can help an electron escape.
- canceled: Balanced each other out, leaving little or no effect.
When More Light Produces Almost No Electrons
Give an electron enough energy, and it should escape. That simple picture underlies photoemission, the process in which light ejects electrons from matter. But having enough energy does not guarantee a successful exit. A new study shows how a carefully designed metal cluster can nearly stop emitting electrons at particular photon energies—even though those photons exceed the energy needed for electron detachment.
In “Zero photoemission from icosahedral superatoms,” Yuta Suzuki, Mamoru Tanaka, Yoshi-ichi Suzuki, Akira Terasaki, and Takuya Horio investigate this striking suppression. Their results reveal how quantum-mechanical cancellation can make a tiny, highly symmetric cluster nearly unresponsive to light within a specific energy window.
The Quantum Cancellation Behind the Disappearing Signal
Photoemission depends on more than the energy balance between an incoming photon and a bound electron. It also depends on a transition dipole matrix element, a mathematical quantity describing how effectively light connects the electron’s initial and outgoing states.
Different regions of the electron’s wave function can contribute positively or negatively to this quantity. At certain energies, those contributions cancel, producing a dip in emission known as a Cooper minimum. If the cancellation is sufficiently complete, the electron signal can become extraordinarily small.
Achieving this in a system larger than an atom is difficult. An escaping electron can leave through several quantum channels, called partial waves. Suppressing one channel does not necessarily suppress the others, because their minima generally occur at different energies.
Why Build a Superatom?
The researchers addressed that problem using superatoms: clusters whose collective electronic states resemble atomic orbitals. Their main target, Ag₁₂W⁻, contains a tungsten atom surrounded by an icosahedral cage of 12 silver atoms. Calculations identified this arrangement as the most stable structure, with no competing structural isomers within approximately 0.8 electronvolts.
Its predicted electronic configuration places one electron in a superatomic 2S orbital. Because this orbital is S-like and the cluster is nearly spherical, detachment proceeds predominantly through a single p-wave channel. That makes the cluster a promising setting for an unusually deep Cooper minimum.
Two Experiments Find the Same Missing Emission
The team produced cluster anions using magnetron sputtering and selected them by mass. They then examined photodetachment using two independent approaches:
- Photoelectron imaging at 55 photon energies between 1.46 and 4.00 electronvolts, measuring the emitted electrons and their angular distributions.
- Photodetachment action spectroscopy between 1.46 and 5.51 electronvolts, tracking how laser exposure depleted trapped anions.
Ag₁₂W⁻ had a vertical detachment energy of 1.46 electronvolts, consistent with the density functional theory prediction. Its photoelectron spectrum showed one narrow peak, only 39 millielectronvolts wide, indicating little structural relaxation after electron removal. The angular distributions also closely resembled those of the atomic silver anion, supporting the S-like character of the emitting orbital.
The central result appeared in both measurements: photodetachment became nearly undetectable between approximately 2.0 and 2.5 electronvolts. The deepest suppression occurred near 2.3 electronvolts, corresponding to an outgoing electron kinetic energy of about 0.85 electronvolts.
Crucially, emission returned at higher energies. The action spectrum showed a small peak near 3.2 electronvolts and a broad, intense band near 4.3 electronvolts. The cluster was therefore not simply unable to lose its electron; its emission probability depended strongly on the photon energy.
What the Calculations Reveal
Simulations using the ezDyson program supported the cancellation picture. The researchers described the bound electron with a Kohn–Sham orbital and the outgoing electron with a Coulomb wave, using an adjustable effective charge of +0.37e. At a kinetic energy of 0.85 electronvolts, the calculated positive and negative radial contributions to the transition dipole integral canceled.
The electron emission directions supplied another clue. Away from the minimum, the measured angular anisotropy parameter reached the atomic limit of 2 near a photon energy of 2.7 electronvolts and across 3.3–4.0 electronvolts. Near the minimum, it approached zero. This behavior is consistent with the dominant p-wave contribution disappearing, allowing weak higher-angular-momentum channels to become relatively important.
A second, isoelectronic cluster, Ag₁₂Mo⁻, showed analogous suppression near a photon energy of 2.1 electronvolts. Its corresponding electron kinetic energy, approximately 0.87 electronvolts, was remarkably close to that of the tungsten-containing cluster.
Nearly Zero, With Important Qualifications
The evidence strongly supports an extreme Cooper minimum, but “zero” requires care. The measured yield approached zero; it was not established as mathematically zero. Icosahedral symmetry still permits weak residual partial waves. The theoretical model also uses an adjusted continuum potential and omits resonant autodetachment processes, which may explain some angular-distribution discrepancies. Absolute action-spectrum cross sections carry estimated systematic uncertainties of 30–50%, mainly from uncertainty in ion–laser overlap.
Even with those limitations, the study establishes nearly spherical superatoms as a compelling platform for controlling photoemission. Changing cluster composition and size could offer new ways to tune emission and explore time delays or nondipole effects. That broader versatility remains a prediction beyond the two demonstrated clusters—but the central lesson is already clear: enough photon energy does not always mean an electron gets out.
Photoemission is expected to occur when the photon energy exceeds the binding energy of an electron, with the photoemission yield generally remaining non-zero above this threshold. Yet, it can be suppressed at specific photon energies, a phenomenon known as the Cooper minimum. Although complete suppression represents an extreme limit of this phenomenon, such zero photoemission has previously been reported only for atoms with an outermost s-type orbital and has never been observed in non-atomic systems. Here, we demonstrate the deliberate realization of zero photoemission from Ag12W-, an artificial icosahedral superatom, by observing a vanishing photoemission cross section 0.85 eV above the detachment threshold and reproducing this behavior in theoretical simulations. These simulations predict that zero photoemission can be common in nearly spherical nanoclusters, a prediction we experimentally confirm using Ag12Mo-, an isoelectronic counterpart of Ag12W-. Because superatoms offer systematic control over both size and elemental composition using the periodic table, our findings establish a route toward active control of photoemission in finite-size quantum systems and stimulate further investigations of underexplored physical phenomena such as nondipole effects.
