Single-Quantum External Photoelectron Effect (SQEPE)

• SQEPE is a one-photon photoemission process where a single light quantum excites and ejects an electron from a bound state into vacuum, with the electron described by a density matrix.
• The approach uses one-photon and three-step photoemission models to capture threshold energetics, transport, escape probabilities, and matrix-element effects in diverse materials.
• Experiments reveal that SQEPE highlights vectorial field effects and nonlocal conductivity in metals, while also enabling quantum state tomography for atomic ionization processes.

Single-Quantum External Photoelectron Effect (SQEPE) denotes the one-photon external photoemission process in which absorption of a single light quantum produces a single electron that escapes from a bound state into vacuum. In the atomic limit, this may be realized as ionization by a single extreme-ultraviolet photon, with the emitted electron treated as a quantum subsystem described by a density matrix rather than only by a momentum distribution. In metals, semiconductors, and organic semiconductors, SQEPE is commonly formulated within one-photon photoemission and three-step photoemission models, where threshold energetics, transport, escape, and matrix-element effects determine the observed quantum efficiency or yield. Across these settings, the “external” qualifier emphasizes emission into vacuum, while “single-quantum” distinguishes the strictly linear, first-order regime from multiphoton or biphotonic channels (Laurell et al., 2023, Pedersoli et al., 2012, Li et al., 2011, Nakazawa et al., 1 Oct 2025)

1. Definition, scope, and threshold conditions

In metallic photocathodes, SQEPE is explicitly identified with single-photon photoemission from a solid surface into vacuum. For an electron initially at energy $E_i$ referenced to vacuum, the single-photon condition is

$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$

with maximum emitted-electron kinetic energy

$E_{\text{kin,max}} = h\nu - \Phi.$

For copper irradiated at $h\nu = 6.28\ \text{eV}$, the reported work functions are $\Phi_{\text{poly}}=4.65\ \text{eV}$ and $\Phi_{(111)}=4.94\ \text{eV}$, giving $E_{\text{kin,max}}\approx 1.63\ \text{eV}$ for polycrystalline Cu and $E_{\text{kin,max}}\approx 1.34\ \text{eV}$ for Cu(111) (Pedersoli et al., 2012).

In positive electron affinity semiconductor photocathodes, the same one-photon external-emission logic is expressed through the band gap $E_g$, electron affinity $E_a$, and Schottky reduction $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$0. The essential threshold is

$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$1

or more generally

$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$2

This is the condition for a single absorbed photon to raise an electron from an occupied state to the vacuum continuum. The three-step picture—photon absorption and excitation, transport to the surface, and escape over the barrier—provides the standard microscopic interpretation of SQEPE in such materials (Li et al., 2011).

In atomic ionization, the threshold language is replaced by the continuum transition induced by a single XUV photon. The cited work studies helium and argon photoionized by an ultrashort XUV pulse of photon energy $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$3, generated via high-order harmonic generation, under conditions such that single-photon ionization dominates. The emitted electron is not treated as a classical particle following a single trajectory, but as a continuum wave packet that may be entangled with the residual ion (Laurell et al., 2023).

Organic-semiconductor studies use SQEPE to denote ordinary one-photon photoemission from occupied states into vacuum, explicitly distinguishing it from biphotonic electron emission (BEE). In that literature, SQEPE encompasses one-photon emission from the HOMO, from occupied in-gap states, and from the singly occupied molecular orbital (SOMO) of anions (Nakazawa et al., 1 Oct 2025).

2. State descriptions and observables

A central distinction across SQEPE research is between classical yield-based descriptions and full quantum-state descriptions. In the atomic implementation, the photoelectron is described by a density operator $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$4 in the continuum energy basis,

$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$5

The diagonal elements $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$6 are the populations measured in conventional photoelectron spectroscopy, while the off-diagonal elements encode coherences between different continuum energies. The state purity is

$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$7

with $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$8 for a pure state and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$9 for a mixed state. This formulation makes SQEPE a problem in open quantum systems whenever the residual ion is not observed (Laurell et al., 2023).

For solids, the most common observable is the quantum efficiency $E_{\text{kin,max}} = h\nu - \Phi.$0, the number of emitted electrons per incident photon. In metals, a purely scalar picture $E_{\text{kin,max}} = h\nu - \Phi.$1 is inadequate when matrix elements depend strongly on field orientation. The copper measurements therefore use a phenomenological decomposition of the absorbed field into parallel and perpendicular components at the surface,

$E_{\text{kin,max}} = h\nu - \Phi.$2

or, for pure $E_{\text{kin,max}} = h\nu - \Phi.$3-polarization,

$E_{\text{kin,max}} = h\nu - \Phi.$4

Here $E_{\text{kin,max}} = h\nu - \Phi.$5 quantifies how much more efficient the normal field component $E_{\text{kin,max}} = h\nu - \Phi.$6 is than the parallel component $E_{\text{kin,max}} = h\nu - \Phi.$7 in generating photoelectrons (Pedersoli et al., 2012).

In positive electron affinity semiconductors, the three-step model is written as an energy integral,

$E_{\text{kin,max}} = h\nu - \Phi.$8

where $E_{\text{kin,max}} = h\nu - \Phi.$9 is the excitation probability, $h\nu = 6.28\ \text{eV}$0 the transport probability, and $h\nu = 6.28\ \text{eV}$1 the escape probability. The escape step is constrained by the normal kinetic-energy condition

$h\nu = 6.28\ \text{eV}$2

which leads to a maximum escape angle

$h\nu = 6.28\ \text{eV}$3

This formalism retains the single-photon character of SQEPE while explicitly incorporating band structure, optical constants, transport, and field-lowered barriers (Li et al., 2011).

In organic semiconductors, the angle-integrated SQEPE spectrum is expressed as

$h\nu = 6.28\ \text{eV}$4

where $h\nu = 6.28\ \text{eV}$5 is the density of occupied initial states, $h\nu = 6.28\ \text{eV}$6 the final-state DOS, $h\nu = 6.28\ \text{eV}$7 the transport probability, and $h\nu = 6.28\ \text{eV}$8 the surface-transmission probability. In constant final state yield spectroscopy (CFS-YS), fixing $h\nu = 6.28\ \text{eV}$9 yields

$\Phi_{\text{poly}}=4.65\ \text{eV}$0

so that $\Phi_{\text{poly}}=4.65\ \text{eV}$1 directly maps the occupied DOS if the $\Phi_{\text{poly}}=4.65\ \text{eV}$2-dependence of the matrix element is weak (Nakazawa et al., 1 Oct 2025).

3. Measurement architectures

The most complete quantum-state measurement of SQEPE reported in the cited literature is the KRAKEN protocol for photoelectron quantum state tomography. An ultrashort XUV pump creates a continuum superposition. A delayed bichromatic IR probe with frequencies $\Phi_{\text{poly}}=4.65\ \text{eV}$3 and $\Phi_{\text{poly}}=4.65\ \text{eV}$4 then drives a second photon absorption from intermediate continuum energies $\Phi_{\text{poly}}=4.65\ \text{eV}$5 and $\Phi_{\text{poly}}=4.65\ \text{eV}$6 into a common final energy $\Phi_{\text{poly}}=4.65\ \text{eV}$7, satisfying

$\Phi_{\text{poly}}=4.65\ \text{eV}$8

These are two indistinguishable quantum paths to the same final state. Scanning the XUV–IR delay $\Phi_{\text{poly}}=4.65\ \text{eV}$9 produces oscillations at the beat frequency $\Phi_{(111)}=4.94\ \text{eV}$0,

$\Phi_{(111)}=4.94\ \text{eV}$1

with

$\Phi_{(111)}=4.94\ \text{eV}$2

By scanning $\Phi_{(111)}=4.94\ \text{eV}$3 and $\Phi_{(111)}=4.94\ \text{eV}$4, the experiment accesses multiple sub-diagonals of $\Phi_{(111)}=4.94\ \text{eV}$5. Bayesian estimation with Hamiltonian Monte Carlo then reconstructs a positive, unit-trace density matrix folded with the independently measured spectrometer response function. The method is described as informationally complete within the experimental bandwidth and resolution (Laurell et al., 2023).

Metallic SQEPE experiments typically measure total photocurrent and time-of-flight spectra while varying incidence angle and polarization. In copper, ultraviolet pulses of $\Phi_{(111)}=4.94\ \text{eV}$6, $\Phi_{(111)}=4.94\ \text{eV}$7, and peak intensity $\Phi_{(111)}=4.94\ \text{eV}$8 irradiate Cu polycrystal and Cu(111) under ultra-high vacuum. The key observable is the angle-dependent quantum efficiency $\Phi_{(111)}=4.94\ \text{eV}$9, often normalized to $E_{\text{kin,max}}\approx 1.63\ \text{eV}$0, for pure $E_{\text{kin,max}}\approx 1.63\ \text{eV}$1- and $E_{\text{kin,max}}\approx 1.63\ \text{eV}$2-polarization. The energy distribution is monitored continuously to verify that emission remains in the linear, single-photon regime and to exclude space-charge distortions (Pedersoli et al., 2012).

Semiconductor implementations are usually model-driven rather than tomographic. For K$E_{\text{kin,max}}\approx 1.63\ \text{eV}$3CsSb at $E_{\text{kin,max}}\approx 1.63\ \text{eV}$4, numerical integration of the three-step QE formula uses measured $E_{\text{kin,max}}\approx 1.63\ \text{eV}$5, $E_{\text{kin,max}}\approx 1.63\ \text{eV}$6, the penetration depth $E_{\text{kin,max}}\approx 1.63\ \text{eV}$7, transport parameters, phonon-scattering inputs, and the applied field. The reported theoretical value is $E_{\text{kin,max}}\approx 1.63\ \text{eV}$8, compared with experimental values of $E_{\text{kin,max}}\approx 1.63\ \text{eV}$9 at Brookhaven and Cornell and up to $E_{\text{kin,max}}\approx 1.34\ \text{eV}$0 at LBNL (Li et al., 2011).

In organic semiconductors, the decisive spectroscopic architecture is the combined use of $E_{\text{kin,max}}\approx 1.34\ \text{eV}$1-dependent high-sensitivity ultraviolet photoelectron spectroscopy, photoelectron yield spectroscopy (PYS), and CFS-YS. This combination allows discrimination between slope-1 onsets

$E_{\text{kin,max}}\approx 1.34\ \text{eV}$2

which signify SQEPE from HOMO, in-gap states, or anion SOMO, and horizontal onsets in $E_{\text{kin,max}}\approx 1.34\ \text{eV}$3 versus $E_{\text{kin,max}}\approx 1.34\ \text{eV}$4, which signify BEE (Nakazawa et al., 1 Oct 2025).

4. Material-specific manifestations

Helium and argon illustrate two distinct quantum realizations of SQEPE. In helium, single-photon ionization leaves the residual ion in a unique ionic ground state, so tracing out the ion yields an almost pure photoelectron wave packet. The reconstructed density matrix $E_{\text{kin,max}}\approx 1.34\ \text{eV}$5 is nearly circular in the $E_{\text{kin,max}}\approx 1.34\ \text{eV}$6 plane after correcting for spectrometer resolution, and the measured purity is $E_{\text{kin,max}}\approx 1.34\ \text{eV}$7, consistent with the theoretical value $E_{\text{kin,max}}\approx 1.34\ \text{eV}$8. In argon, by contrast, spin-orbit interaction in the residual ion produces the $E_{\text{kin,max}}\approx 1.34\ \text{eV}$9 and $E_g$0 ionic states separated by $E_g$1. The electron is then entangled with the ion, and the reduced electron state becomes mixed: $E_g$2 Experimentally, $E_g$3 is elongated along the diagonal, $E_g$4 agrees with $E_g$5, and the concurrence inferred from the reduced density matrix is $E_g$6, compared with $E_g$7 (Laurell et al., 2023).

Copper shows a different aspect of SQEPE: strong vectorial sensitivity to the optical field at the interface. For polycrystalline Cu, the maximum quantum efficiency is $E_g$8 at $E_g$9 in $E_a$0-polarization, about a factor of 4 larger than at normal incidence. The corresponding pseudo-Brewster angle predicted by Fresnel absorption is $E_a$1, so the experimental maximum lies $E_a$2 beyond the absorption maximum. Fits yield $E_a$3 for polycrystalline Cu and $E_a$4 for Cu(111), showing that $E_a$5 is 9–13 times more effective than $E_a$6 in generating photoelectrons. The proposed microscopic explanation is the nonlocal conductivity tensor near the metal–vacuum interface, where rapid spatial variation of the field—especially the normal component—adds a nonlocal term to the photoemission matrix element (Pedersoli et al., 2012).

In positive electron affinity semiconductors such as K$E_a$7CsSb, SQEPE is governed by the combined action of threshold energetics, optical absorption, transport, and escape. The parameter set used in the cited calculation includes $E_a$8, $E_a$9, $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$00, and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$01 at $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$02. Since $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$03, the photon energy is moderately above the threshold $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$04, and a one-photon QE of 4.69% is obtained. The reported dependencies show that larger $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$05 and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$06 lower QE by raising the threshold and reducing the escape cone, while longer relaxation times increase QE by improving transport to the surface (Li et al., 2011).

Organic semiconductors add an additional layer of complexity because SQEPE can originate from in-gap states and anion SOMO states, while BEE may coexist. In Alq$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$07, the HOMO ionization energy is $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$08 and the effective work function is $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$09. A low-$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$10 slope-1 onset yields $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$11, identifying SQEPE from anion SOMO. A strong, $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$12-independent onset at $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$13 is assigned to BEE via singlet–anion fusion, consistent with a calculated value of $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$14 using $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$15 and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$16. High-$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$17 CFS-YS at $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$18 then suppresses BEE and reveals an exponential in-gap DOS with $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$19, together with a Gaussian SOMO peak at $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$20 and onset at $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$21–$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$22, over six orders of magnitude (Nakazawa et al., 1 Oct 2025).

5. Misconceptions, interpretive pitfalls, and limitations

A recurrent misconception is that SQEPE is exhausted by threshold equations or by a kinetic-energy spectrum. The atomic tomography results show otherwise: standard photoelectron spectroscopy measures only $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$23, whereas the full SQEPE state requires reconstruction of both populations and coherences. The helium–argon comparison further shows that reduced purity in the photoelectron state need not be an experimental artifact. In argon it is intrinsic and arises from tracing out the spin-orbit-split ionic core; the mixedness is therefore a manifestation of ion–electron entanglement rather than noise (Laurell et al., 2023).

A second misconception is that metallic QE is simply proportional to absorbed optical power. The copper data contradict that scalar picture. The strong enhancement in $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$24-polarization, the shift of the QE maximum beyond the pseudo-Brewster angle, and the fitted values $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$25 and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$26 require an explicit distinction between $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$27 and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$28. The same study also argues against explaining the effect primarily by roughness or crystal symmetry: AFM gives $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$29 for polycrystalline Cu and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$30 for Cu(111), yet the vectorial enhancement is comparable in both, and the polycrystal should average out crystallographic symmetry effects (Pedersoli et al., 2012).

A third interpretive pitfall concerns derivative photoelectron yield spectroscopy in organics. If PYS were only the integral of a one-photon DOS-weighted emission probability, its derivative or second derivative could approximate the DOS under additional assumptions on matrix elements and transmission. The cited work shows that this can fail because low-energy photons generate excitons and anions, and BEE can dominate the low-$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$31 signal. In Alq$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$32, PYS shows yield below $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$33, non-monotonic peaks at 4.9, 3.7, and 3.2 eV, and derivative spectra that can even become negative; these features do not represent a physical DOS. CFS-YS at sufficiently high $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$34 is therefore proposed as the reliable route for DOS determination (Nakazawa et al., 1 Oct 2025).

The three-step semiconductor model also carries explicit approximations. The cited K$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$35CsSb study notes uncertainties from non-monochromatic drive light, stoichiometry variation, uncertainty in the absorption coefficient, omission of additional scattering channels, approximation of the Fermi–Dirac function by a step function, uncertainty in the Fermi level, and disagreement in the reported values of $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$36 and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$37. This suggests that even in nominally simple one-photon SQEPE, accurate QE prediction depends sensitively on material-specific inputs (Li et al., 2011).

6. Significance and emerging directions

SQEPE has become a unifying framework linking the photoelectric effect, surface photoemission, and quantum-state reconstruction. In the atomic implementation, full tomography bridges photoelectron spectroscopy and quantum information by making purity, entanglement, and reduced density matrices directly measurable. This suggests a description of one-photon ionization in which the photoelectron is a quantum subsystem, and the ionization process acts as a preparation-and-readout channel for electronic quantum states (Laurell et al., 2023).

In metallic photocathodes, SQEPE refines the first step of the three-step model by replacing scalar absorption with a weighted functional of $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$38 and $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$39. The immediate practical consequence is that optimal operation is not obtained by maximizing Fresnel absorption alone. For Cu at $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$40, the reported optimum is $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$41-polarization at $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$42, not at the pseudo-Brewster angle, yielding $$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$43 QE. A plausible implication is that photocathode design in the single-photon regime should be framed in terms of interface electrodynamics and nonlocal response rather than reflectivity alone (Pedersoli et al., 2012).

In positive electron affinity semiconductors, SQEPE remains the standard basis for predictive QE modeling. The K$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$44CsSb calculation indicates that a purely single-photon, three-step treatment can reach quantitative agreement with experiment by and large when band gap, electron affinity, optical constants, and transport parameters are reasonably specified. This supports continued use of three-step integral models for photocathodes in accelerator applications, while also indicating that threshold and transport parameters remain the dominant sources of uncertainty (Li et al., 2011).

In organic semiconductors, SQEPE has become inseparable from methodological questions about what photoelectron yields actually measure. The key advance is not only the identification of one-photon emission from in-gap and SOMO states, but also the separation of these channels from BEE. High-$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$45 CFS-YS reveals DOS over six orders of magnitude in Alq$$E_f = E_i + h\nu \ge 0,\qquad E_i \le -\Phi,\qquad h\nu \ge \Phi,$$46, while the BEE process itself is implicated as both a carrier-generation pathway and a degradation pathway in organic optoelectronic devices. This establishes SQEPE as both a spectroscopic probe of occupied states and a reference process against which higher-order channels must be isolated if low-energy photoemission data are to be interpreted correctly (Nakazawa et al., 1 Oct 2025)