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Excimer energy diagram
Excimer energy diagram

An excimer (originally short for excited dimer) is a short-lived polyatomic molecule formed from two species that do not form a stable molecule in the ground state. In this case, formation of molecules is possible only if such atom is in an electronic excited state.[1] Heteronuclear molecules and molecules that have more than two species are also called exciplex molecules (originally short for excited complex). Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is very short, on the order of nanoseconds.

Formation and decay

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Molecular orbitals

Under the molecular orbital formalism, a typical ground-state molecule has electrons in the lowest possible energy levels. According to the Pauli principle, at most two electrons can occupy a given orbital, and if an orbital contains two electrons they must be in opposite spin states. The highest occupied molecular orbital is called the HOMO and the lowest unoccupied molecular orbital is called the LUMO; the energy gap between these two states is known as the HOMO–LUMO gap. If the molecule absorbs light whose energy is equal to this gap, an electron in the HOMO may be excited to the LUMO. This is called the molecule's excited state.

Excimers are only formed when one of the dimer components is in the excited state. When the excimer returns to the ground state, its components dissociate and often repel each other. The wavelength of an excimer's emission is longer (smaller energy) than that of the excited monomer's emission. An excimer can thus be measured by fluorescent emissions.

Because excimer formation is dependent on a bimolecular interaction, it is promoted by high monomer density. Low-density conditions produce excited monomers that decay to the ground state before they interact with an unexcited monomer to form an excimer.

Usage note

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The term excimer (excited state dimer) is, strictly speaking, limited to cases in which a true dimer is formed; that is, both components of the dimer are the same molecule or atom. The term exciplex refers to the heterodimeric case; however, common usage expands excimer to cover this situation.

Examples and use

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Excimer lasers
Laser Reagents Emission peak
XeCl Xe + Cl2 308 nm
KrF Kr + NF3 248 nm
ArF Ar + F2 193 nm

Heterodimeric diatomic complexes involving a noble gas and a halide, such as xenon chloride, are common in the construction of excimer lasers, which are excimers' most common application. These lasers take advantage of the fact that excimer components have attractive interactions in the excited state and repulsive interactions in the ground state. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light (excimer lamps).[2]

The molecule pyrene is another canonical example of an excimer that has found applications in biophysics to evaluate the distance between biomolecules.[3]

In organic chemistry, many reactions occur through an exciplex, for example, those of simple arene compounds with alkenes.[4] The reactions of benzene and their products depicted are a [2+2]cycloaddition to the ortho product (A),[5] a [2+3]cycloaddition to the meta product (B)[6] and the [2+4]cycloaddition to the para product (C)[7] with simple alkenes such as the isomers of 2-butene. In these reactions, it is the arene that is excited.

Arene photocycloadditions
Arene photocycloadditions

As a general rule, the regioselectivity is in favor of the ortho adduct at the expense of the meta adduct when the amount of charge transfer taking place in the exciplex increases.

Generation techniques

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For a noble gas dimer or noble gas halide it takes a noble gas atom in an excited electronic state to form an excimer molecule. Sufficiently high energy (approximately 10 eV) is required to obtain a noble gas atom in the lowest excited electronic state, which provides the formation of an excimer molecule. The most convenient way to excite gases is by an electric discharge. That is why such excimer molecules are generated in a plasma (see excimer molecule formation) or through high energy electron beams.

Fluorescence quenching

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Exciplexes provide one of the three dynamic mechanisms by which fluorescence is quenched. A regular exciplex has some charge-transfer (CT) character, and in the extreme case there are distinct radical ions with unpaired electrons. If the unpaired electrons can spin-pair to form a covalent bond, then the covalent bonding interaction can lower the energy of the charge transfer state. Strong CT stabilisation has been shown to lead to a conical intersection of this exciplex state with the ground state in a balance of steric effects, electrostatic interactions, stacking interactions, and relative conformations that can determine the formation and accessibility of bonded exciplexes.[8]

As an exception to the conventional radical ion pair model, this mode of covalent bond formation is of interest to photochemistry research, as well as the many biological fields using fluorescence spectroscopy techniques. Evidence for the bonded exciplex intermediate has been given in studies of steric and Coulombic effects on the quenching rate constants and from extensive density functional theory computations that show a curve crossing between the ground state and the low-energy bonded exciplex state.[9]

See also

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References

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Excimer

View on Grokipedia
from Grokipedia
An excimer, short for "excited dimer," is a short-lived polyatomic formed from two —typically identical atoms or molecules—that do not form a stable bond in their but associate weakly when one is in an electronically . This complex is characterized by a bound excited electronic state and a repulsive or dissociative , leading to rapid dissociation upon relaxation and emission of characteristic or visible light with broad, structureless spectra. Excimers were first identified in the through studies of aromatic hydrocarbons in solution, where shifts revealed the formation of these transient . Their unique photophysical properties, including high quantum yields for emission and short lifetimes on the order of nanoseconds, make them valuable in various fields. In , excimers facilitate energy transfer and processes, influencing reaction pathways in polymers and biological systems. Recent advances leverage excimer formation for designing probes that detect biomolecular conformations with high sensitivity, aiding applications in and . The most prominent application of excimers is in excimer lasers, which utilize gas mixtures like argon fluoride (ArF) or krypton fluoride (KrF) to generate pulsed radiation at wavelengths such as 193 nm or 248 nm. These lasers are essential in manufacturing for deep ultraviolet lithography, enabling the production of microchips with features below 10 nm, and in medicine for procedures like , where precise tissue occurs without thermal damage. Additionally, excimer-based technologies extend to organic light-emitting diodes (OLEDs), where controlled excimer emission enhances in displays and .

Fundamentals

Definition

An excimer, short for "excited dimer," is a short-lived molecular formed by the association of two identical atoms or molecules that exists as a stable only in an electronically , while dissociating in the . This term was coined in to describe such transient dimers observed in studies of aromatic hydrocarbons. In the ground state, the dimer interaction is typically weak and characterized by van der Waals forces, rendering it unstable and prone to rapid dissociation, whereas excitation leads to a stronger interaction, often involving charge-transfer character, that stabilizes the complex for a brief period. The general notation for a homonuclear excimer is (AA)*, where A represents the identical atomic or molecular units. This distinguishes excimers from exciplexes, which are the heteronuclear analogs formed from dissimilar , denoted as (AB)* where A ≠ B. Excimers can also extend to multimers beyond simple dimers, but the core concept remains the excitation-dependent bonding.

Historical Background

The concept of the excimer emerged from spectroscopic studies of aromatic hydrocarbons in the late and early , as researchers sought to explain delayed and energy transfer phenomena in solution. In 1960, B. Stevens and E. Hutton observed structureless emission bands in solutions, attributing them to the formation of a short-lived excited dimer stable only in its electronically ; they coined the term "excimer" (from "excited dimer") to describe this and its role in reversible photoassociation processes. This marked the initial experimental recognition of excimers, distinguishing them from ground-state dimers through their dissociative ground potential and bound excited potential. Theoretical foundations for excimers were laid in the 1960s by Robert S. Mulliken, whose provided insights into their electronic structure. Mulliken predicted that excimer binding in the often involves charge-transfer configurations, where partial between the dimer components stabilizes the upper state while the remains repulsive; this model explained the broad, red-shifted emission spectra observed experimentally. His seminal contributions, including analyses of potential energy curves for such systems, influenced subsequent interpretations of excimer and dynamics. Key milestones in excimer research occurred in the with the realization of excimer lasers, leveraging the in rare gas halide systems. In 1975, the first rare gas halide excimer lasers were demonstrated, including XeBr at 282 nm by S. K. Searles and G. A. Hart, followed by reports from J. J. Ewing and C. A. Brau on KrF at 248 nm and XeF at 354 nm using electron-beam excitation to achieve high-efficiency output. These developments transformed excimers from spectroscopic curiosities into practical light sources. The broader impact of photochemical research on excited states was underscored by the 1986 , awarded to , , and John C. Polanyi for elucidating the dynamics of elementary chemical processes.

Molecular Structure and Properties

Electronic Configuration

Excimers exhibit a distinctive electronic configuration where the is characterized by weak binding or effective repulsion, primarily due to the that causes significant overlap repulsion between closed-shell atoms at short internuclear distances, countered only by shallow van der Waals attractions at larger separations (binding energy approximately 0.001 eV for systems like Xe₂). In contrast, the forms a stable bond through configurations involving charge-transfer or Rydberg character, resulting in attractive curves where the excited-state potential V*(R) lies below the ground-state potential V(R), with typical dissociation energies ranging from 0.1 to 1 eV. This arises from the excited-state wavefunction's ability to mitigate Pauli repulsion while incorporating electrostatic and dispersion attractions. For rare gas excimers, such as those formed by or , the orbital description highlights the role of electron promotion in the . The features a closed-shell configuration with paired electrons in (σ_g) and antibonding (σ_u) molecular orbitals derived from the ns valence shells, leading to net repulsion at equilibrium distances. Upon excitation, one is promoted from a valence orbital to a higher-lying antibonding Rydberg orbital (e.g., σ_u Rydberg), creating an open-shell structure that reduces orbital overlap repulsion and enables , often modeled as a hole-particle pair where the "hole" in the valence shell interacts attractively with the Rydberg . The of the is commonly approximated by the to capture its anharmonic bonding characteristics: V∗(R)=De(1−e−a(R−Re))2V^*(R) = D_e \left(1 - e^{-a(R - R_e)}\right)^2 where DeD_e represents the well depth (0.1–1 eV), ReR_e the equilibrium internuclear distance (typically 3–4 for rare gas excimers), and aa the parameter controlling the potential's width; the , by comparison, has a negligible well depth D0≈0D_0 \approx 0. This formulation effectively describes the bound nature of the excimer, facilitating broad emission continua upon relaxation to the repulsive . Symmetry plays a crucial role in distinguishing homonuclear excimers from heteronuclear exciplexes. In homonuclear cases, like Ar₂ or Kr₂, the presence of an inversion center leads to gerade (g, even parity) and ungerade (u, odd parity) classifications for molecular orbitals and states (e.g., ground state ¹Σ_g⁺, lowest excited state ¹Σ_u⁺), enforcing parity selection rules for electronic transitions. Heteronuclear exciplexes, such as XeCl, lack this inversion symmetry (belonging to C_{∞v} point group), resulting in states without g/u labels and altered potential curves due to unequal atomic contributions to bonding.

Spectroscopic Characteristics

Excimers exhibit broad, structureless emission bands in the ultraviolet-visible range, primarily arising from the governing transitions between a bound and a repulsive , which favors vertical transitions to a continuum of dissociative levels. This lack of vibrational structure results from the rapid dissociation following emission, preventing resolved progressions and producing continua rather than discrete lines. For rare gas excimers, these emission bands typically span wavelengths from 100 to 400 nm, with specific examples including the second continuum of Xe₂* peaking at approximately 170 nm. Similarly, Kr₂* emits around 150 nm and Ar₂* near 130 nm, reflecting the progression of atomic energy levels in heavier rare gases. The emission lifetimes of these excimers, typically on the order of 10–100 ns, correlate with the Stokes shift through the underlying dissociation dynamics; larger shifts indicate greater separation between potential minima, influencing the radiative decay rate via reduced oscillator strength overlap. For Xe₂*, the radiative lifetime of the lowest excited ungerade state is about 60 ns, modulated by non-radiative pathways tied to the dissociative character. Absorption spectra of rare gas excimers show weak features in the due to the repulsive nature of the potential, which lacks bound levels for significant , while the bound enables strong absorption to higher electronic configurations. This asymmetry contributes to enhanced in the , where effects amplify vibrational signatures through increased changes. The temperature dependence of excimer emission bands manifests as narrowing at low temperatures, as reduced thermal population of vibrational levels in the minimizes bandwidth broadening from hot-band contributions. In supersonic expansions or matrix isolation, this effect yields sharper continua, such as for Ar₂* at cryogenic conditions, highlighting the role of vibrational cooling in .

Excitation Dynamics

Formation Mechanisms

Excimers in rare gases primarily form through collision processes involving an excited atom and two ground-state atoms, such as A + A + A* → A₂* + A, where A represents a rare gas atom like or . This pathway dominates in the gas phase due to the need for a third body to carry away excess energy, stabilizing the weakly bound excimer state. In some systems, Penning contributes indirectly by generating metastable states or ions that lead to excimer formation via subsequent associations, particularly in mixtures of rare gases. Photoexcitation routes also play a key role, including direct absorption by monomers to produce excited states that then associate, or pooling collisions between two excited monomers, such as A* + A* → A₂* + A. These processes are efficient in optically pumped systems, where selective excitation of atomic levels enhances excimer yields without requiring high densities of ground-state atoms. The kinetics of excimer formation are described by rate equations of the form d[A₂*]/dt = k_f [A]² [A*], where k_f is the three-body rate constant. For excimers, typical values range from (0.3 ± 0.1) × 10⁻³² cm⁶ s⁻¹ for the ¹u state to (4 ± 4) × 10⁻³² cm⁶ s⁻¹ for the ⁰⁺_u state, reflecting the state's electronic configuration and . In the gas phase, excimer yield increases with pressure due to the quadratic dependence on atomic density in the three-body rate, favoring higher concentrations for efficient formation. Temperature exerts a negative influence, as lower temperatures enhance collision efficiency and reduce dissociation, thereby boosting excimer stability and yield in rare gas vapors. In solution-phase environments, particularly organic media, exciplex formation—a heteroatomic analog of excimers—relies on solvent-mediated diffusion of excited and ground-state species to enable association. Solvent polarity modulates this process by stabilizing charge-transfer character in the exciplex, with polar solvents promoting formation through enhanced electron donor-acceptor interactions, while nonpolar media favor neutral exciplexes. Fluidity of the solvent is essential, as it allows the necessary molecular encounters absent in rigid phases.

Decay Pathways

Excimers deactivate through a variety of decay pathways following their formation, encompassing both radiative and non-radiative mechanisms that return the system to the . Radiative decay proceeds via of a as the excimer transitions from the bound to the repulsive , producing characteristic continuum . The efficiency of this process is quantified by the radiative φ = k_r / (k_r + k_nr), where k_r is the radiative rate constant and k_nr aggregates all non-radiative rates; for rare gas excimers like Ar₂*, φ is typically low due to competing channels. Non-radiative decay pathways dominate in many excimers, particularly van der Waals types, and include predissociation, where vibrational motion in the overcomes a potential barrier to dissociate into excited atoms; , involving radiationless transition to a lower electronic state via vibronic ; and collision-induced , where interactions with ground-state atoms or other transfer without emission. These processes are pressure-dependent, with higher densities enhancing quenching rates. For Ar₂*, predissociation is prevalent, contributing to short effective lifetimes even as the intrinsic radiative lifetime approaches 3 μs. The total decay lifetime τ of an excimer is given by τ = 1 / (k_r + k_d + k_q [Q]), where k_d denotes the dissociation (predissociation) rate constant and k_q [Q] the pseudo-first-order rate, with [Q] the concentration of quenchers such as ground-state atoms. This formulation highlights how environmental factors modulate deactivation, with τ for Ar₂* observed around 100–3000 ns depending on conditions, reflecting a balance between intrinsic and collisional rates. Branching ratios between pathways vary by excimer and conditions but often favor non-radiative dissociation over emission; for Ar₂*, approximately 90% of decays proceed via dissociation compared to emission, underscoring the inefficiency of radiative output in systems and the role of fast predissociation from higher vibrational levels populated during formation. Energy dissipation in excimer decay ultimately involves heat release upon separation to the , as the atoms repel and is partitioned as translational motion, contributing to thermalization in the surrounding medium without output in non-radiative cases.

Generation Methods

Experimental Techniques

One primary method for generating excimers in laboratory settings is electrical discharge pumping, which involves applying high-voltage pulses to ionize gas mixtures containing rare gases and . This technique creates a plasma where electrons and ions collide with precursor atoms, leading to excimer formation via three-body recombination or processes. For instance, in mixtures of and (Kr/F₂), discharge pumping efficiently produces KrF excimers, achieving output efficiencies of approximately 2% at . The method is particularly suited for scalable systems but requires careful control of discharge uniformity to prevent arcing and ensure homogeneous excitation. Electron beam excitation provides an alternative approach, employing high-energy electron beams (typically 10-20 keV) to directly deposit energy into high-pressure rare gas targets, promoting uniform ionization and excimer formation. A thin ceramic foil often separates the electron gun from the gas cell to maintain pressure differences while allowing beam penetration. This technique has been used to study excimer production in dense argon or xenon, where the rapid energy input favors the formation of bound excited dimers over dissociated products. It excels in high-density environments but demands robust vacuum systems to handle beam-generated byproducts. Photolysis represents a photochemical route to excimer generation, utilizing laser irradiation of halogen-containing precursors in rare gas matrices to produce reactive halogen atoms or ions that bind with the host gas. For example, 193 nm ArF excimer laser photolysis of alkyl halides like CCl₄ in gas dissociates the precursor via single-photon absorption, yielding Cl atoms that react with Xe to form XeCl excimers through mechanisms. This method allows precise control over excitation wavelength and is valuable for studying transient excimers at low temperatures, though it typically yields lower densities compared to discharge or beam techniques. Detection of these short-lived species relies on time-resolved spectroscopy, which captures the characteristic broadband emission from excimer decay, often using laser-induced fluorescence to resolve lifetimes on the nanosecond scale. For rare gas excimers like Ar₂*, time-resolved fluorescence following pulsed excitation reveals formation kinetics and pressure-dependent lifetimes, typically in the 10-100 ns range. Complementary mass spectrometry detects ionic excimers or fragmentation products, enabling identification of transient species via time-of-flight analysis after ionization. Key challenges in these experiments include ensuring gas purity to maximize excimer yields, as trace impurities such as oxygen act as efficient quenchers through collisional energy transfer, reducing emission intensities by up to 50% at parts-per-million levels. Rigorous degassing and purification protocols, often involving cryogenic traps, are essential to mitigate such quenching effects.

Theoretical Approaches

Theoretical approaches to excimers primarily involve computational methods to predict their formation, stability, and decay, focusing on potential energy surfaces (PES) and dynamical simulations for weakly bound systems like rare gas dimers. Ab initio quantum chemistry techniques, such as complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI), are widely used to compute accurate PES for excimer states, particularly for rare gas dimers where electronic correlation and multiconfigurational character are crucial. For instance, MRCI calculations with large basis sets have been applied to the argon dimer (Ar₂), yielding PES that reproduce the binding energies and emission continua of the ¹Σ_u⁺ and ³Σ_u⁺ excimer states, with dissociation energies around 0.15–0.20 eV for the excited states. These methods build on the electronic configuration of excimers, where promotion of an electron to a Rydberg or antibonding orbital leads to repulsive ground states and bound excited states, but detailed configurations are addressed elsewhere. Semiclassical models, including classical trajectory simulations on ab initio or model PES, simulate collision-induced excimer formation by tracking atomic trajectories under the influence of . Such approaches have been employed for Ar₂ excimers in conditions, using diatomics-in-molecules PES for the Ar₃ system to model collisions leading to vibrational relaxation and stabilization of the ³Σ_u⁺ state, revealing formation cross-sections on the order of 10⁻¹⁶ cm² at thermal energies. Rate constant calculations for excimer dissociation often rely on Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which assumes statistical energy redistribution in the quasibound excimer before dissociation. In rare gas clusters, RRKM predicts dissociation rates for vibrationally excited excimers, with lifetimes scaling inversely with excess energy above the barrier, typically yielding rates of 10⁸–10¹⁰ s⁻¹ for Ar₂* at room temperature. Software packages like Gaussian and MOLPRO facilitate these computations, enabling geometry optimization and PES scanning for excimer structures. MOLPRO, with its efficient MRCI implementation, has been used to optimize the equilibrium distance of Ar₂ excimers at approximately 3.5 Å for the ³Σ_u⁺ state, while Gaussian supports CASSCF optimizations for initial wavefunction guesses. Despite advances, theoretical treatments face limitations in accurately capturing van der Waals interactions in excimer ground and low-lying states, where basis set superposition errors and incomplete correlation recovery can overestimate binding by 10–20%.

Applications

Excimer Lasers

Excimer lasers were first demonstrated in 1970 by Nikolai Basov, V.A. Danilychev, and Yu.M. Popov at the P.N. Lebedev Physical Institute in , using a xenon dimer (Xe₂) excimer emitting at 172 nm via electron beam pumping. This breakthrough established the foundation for (UV) laser technology based on excimer formation in gas mixtures. These lasers operate through pulsed excitation of a gas mixture typically comprising a (such as , , or ) and a (such as or ), using either high-voltage electrical discharge or electron beam pumping to create a plasma where excimers form transiently. For instance, the fluoride (ArF) excimer lases at 193 nm when the mixture is ionized, producing as the excimer dissociates after release, minimizing . The process relies on short, high-energy pulses to sustain the discharge or beam, enabling efficient in the UV spectrum. Common excimer laser types include krypton fluoride (KrF) at 248 nm, widely used in for its balance of power and ; xenon chloride (XeCl) at 308 nm, applied in medical procedures like ; and ArF at 193 nm for advanced high-resolution patterning in fabrication. Output characteristics feature high peak powers on the order of 100 MW, pulse durations of 10-20 ns, and wall-plug efficiencies ranging from 1% to 5%, allowing for pulse energies of 10 mJ to 1 J at repetition rates up to 1 kHz. The short wavelengths of excimer lasers provide high and resolution, making them essential for precision applications requiring sub-micron features. However, they necessitate regular gas replenishment due to the corrosive nature of the components, which degrade over time and limit operational lifetime to billions of pulses without maintenance. Recent developments as of 2025 include the application of excimer lasers in (ICF). Companies like Xcimer Energy have demonstrated prototype electron-beam-pumped KrF excimer lasers capable of producing record-long pulses for fusion experiments, aiming for commercial fusion power plants by the mid-2030s.

Photochemical and Material Processing

Excimers play a pivotal role in photochemical reactions by facilitating dimerization es, as exemplified by the photodimerization of , where the excimer serves as a key intermediate in fluid solutions. In this reaction, excited anthracene molecules form a short-lived excimer with a lifetime of approximately 1-1.5 nanoseconds, leading to the formation of dianthracene upon decay, a process that alters the molecule's photophysical in confined environments like aqueous nanocavities. Similarly, excimer photolysis contributes to formation; for instance, 193 nm ArF excimer irradiation of oxygen molecules dissociates O₂ into oxygen atoms, which recombine to produce O₃ through a three-body . In material processing, excimer lamps enable precise surface modifications by breaking molecular bonds without thermal damage. Xenon excimer lamps emitting at 172 nm are particularly effective for and , as their vacuum-ultraviolet photons directly cleave C-H, C-O, and other organic bonds on substrates like polymers and , removing contaminants or patterning surfaces at ambient conditions. This wavelength also supports sterilization by generating that inactivate microorganisms on surfaces, offering a mercury-free alternative for and industrial applications. For sterilization efficacy, exposure times of several minutes achieve high microbial reduction rates on materials like PDMS. Excimer irradiation facilitates polymer processing through photo-crosslinking and controlled degradation, essential for fabrication. In poly(methylmethacrylate) (PMMA), 248 nm KrF excimer exposure induces cross-linking at fluences below 100 mJ/cm², raising the temperature from 106°C by forming intermolecular bonds that compete with chain scission at higher doses. For poly(ethylene terephthalate) (PET), 222 nm KrCl excimer lamps in bifunctional atmospheres like octadiene promote surface cross-linking, increasing modulus and resistance while reducing crystallinity in the near-surface layer. These modifications enhance adhesion and enable patterning for without altering bulk properties. Environmental applications leverage excimer-generated radicals for via (AOPs). KrCl excimer lamps at 222 nm photolyze or to produce hydroxyl radicals (•OH), which degrade recalcitrant organics like humic with reduction rates up to 65%—significantly higher than ozonation alone. In , these lamps yield 3.7 to 13.1 times more •OH than low-pressure UV systems, improving contaminant mineralization through peroxyl and other radical pathways. Quantum yields for key reactions, such as O(¹D) formation in photolysis, reach up to 0.9, underscoring the efficiency of excimer-driven AOPs in practical purification setups.

Fluorescence Quenching

Fluorescence quenching of excimers involves the deactivation of the excited dimer state by external agents, preventing radiative decay and thereby suppressing emission. This process is particularly relevant for rare gas excimers, where quenchers interact with the weakly bound excited state, leading to non-radiative relaxation or energy redistribution. The primary quenching mechanisms include collisional quenching, as seen with molecular oxygen (O₂), where physical collisions transfer energy or induce dissociation without chemical change; energy transfer, in which the excimer's excitation is passed to another species; and electron transfer, involving charge exchange that destabilizes the excimer. For instance, collisional quenching by O₂ proceeds via two-body interactions with rate constants typically on the order of 10^{-10} cm³ molecule⁻¹ s⁻¹, as measured for the triplet Ne₂ excimer under near-atmospheric conditions. The extent of quenching is quantitatively described by the Stern-Volmer relation: I0I=1+Kqτ[Q],\frac{I_0}{I} = 1 + K_q \tau [Q], where I0I_0 and II are the fluorescence intensities without and with quencher, respectively, KqK_q is the bimolecular quenching rate constant, τ\tau is the unquenched excimer lifetime, and [Q][Q] is the quencher concentration. For rare gas systems, Kq≈10−10K_q \approx 10^{-10} cm³ molecule⁻¹ s⁻¹, reflecting near-gas-kinetic collision efficiencies for effective quenchers like certain rare gases acting on excited states. Quenching significantly shortens excimer lifetimes and lowers fluorescence yields, which can diminish laser output efficiency in excimer-based devices or slow photochemical reaction rates by reducing available excited species. Specific quenchers such as promote exciplex formation through reactive quenching channels, for example, where a rare gas excimer interacts with Cl₂ to yield a rare gas exciplex alongside dissociation products, with rate constants around 4 × 10^{-10} cm³ molecule⁻¹ s⁻¹. In , O₂ quenching plays a key role in limiting excimer persistence in trace rare gas environments, influencing VUV photolysis rates and radical formation in the upper atmosphere. These effects are experimentally assessed using Stern-Volmer plots of intensity or decay rates versus quencher concentration.

Terminological Notes

The term "excimer" specifically refers to an electronically excited dimer formed from two identical molecular entities that do not form a stable bond in the , dissociating upon de-excitation. In contrast, an "exciplex" denotes a similar excited complex but involving two different molecular species (heteronuclear), maintaining the requirement of a non-bonding . This distinction underscores the homodimeric nature of excimers versus the heterodimeric character of exciplexes, a convention established to clarify spectroscopic and photochemical behaviors in literature. Historically, the term "excimer" was introduced by Stevens and Hutton in to describe collisionally formed excited dimers that dissociate in the , distinguishing them from excited states of stable dimers. Early usage in the often encompassed any transient excited complex, including those without strict dissociative ground states, but subsequent refinements restricted it to exhibiting bound excited states and repulsive ground-state potentials, aligning with observed structureless emission spectra. A common terminological error involves applying "excimer" to stable ground-state dimers, such as the cyclobutane pyrimidine dimers formed in DNA upon ultraviolet irradiation, which persist as covalent lesions rather than dissociating after de-excitation. These biological photoproducts, unlike true excimers, require enzymatic repair mechanisms like nucleotide excision repair due to their stability. The International Union of Pure and Applied Chemistry (IUPAC) formalized this in its 2007 Compendium of Chemical Terminology (Gold Book), defining an excimer as "an electronically excited dimer, 'non-bonding' in the ground state," emphasizing its transient, dissociative character to prevent misuse for persistent species. In interdisciplinary contexts, the term retains core meaning but varies in emphasis: photochemistry literature focuses on organic excimers for fluorescence studies, while plasma physics applies it to rare-gas excimers in discharge media for laser applications, where formation occurs via electron-impact excitation rather than collisional encounters.

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