The Role of Excited Oxygen Molecules in the Formation of the Secondary Ozone Layer at 87 to 97 km

The secondary ozone layer is located at elevations of 87 to 97 km in the upper mesosphere – lower thermosphere. It overlaps with the ionospheric D-layer. Daytime intensive UV radiation is dissociating O2 molecules to O atoms and photoexcitating O2 molecules up to 11.07eV level. Ozone photolysis between the wavelengths of 118.7–121.6 nm produces three oxygen atoms from one ozone molecule. Collision reactions of O2(BΣu) and O2(XΣg, υ≥26) with O2(XΣg, υ=0) produce additional oxygen atoms. The number of oxygen atoms is maintained at such a high level that a small but significant ozone concentration survives. UV radiation weakens radically during the night. The number of O atoms shows no diurnal variation in the MLT. This leads to a ten-fold increase of ozone concentration over the course of the night. Dissociative recombination of O2 (entered via diffusion from above) and reactions of O (P) atoms with excited O2 molecules generate O(S) atoms. The quenching of O(S)→O(D) emits the green nightglow. The reactions of O(D) with ozone and O2 absorption of UV nightglow produce O2(cΣu, A’∆u and AΣu). When these molecules relax, they emit the O2 UV nightglows. The relaxations of O2(a∆g) and O2(bΣg) emit infrared nightglows.


Introduction
Before the 1970s, the mesospheric-lower thermospheric (MLT) layer was located too far from the ground for the purposes of obtaining good observation results. On the other hand, for rocket research it was located too near, as they were targeting objects that were further away. Thomas and Bowman [1] examined the diurnal variations of constituents in an oxygen-hydrogen atmosphere. They compared the results of available experimental data, and suggested that their laboratory tests demonstrated atmospheric oxygen chemistry in the Schumann-Runge band region as well.
Hays and Roble [2] found that stellar ultraviolet light near 2500 Å is attenuated in the Earth's upper atmosphere in the 60-to 100-km region due to strong absorption in the Hartley continuum of ozone. They found that the night-time ozone number density has a bulge in its vertical profile with a peak of 1 to 2 x 10 8 cm -3 at approximately 83km and a minimum near 75km.
Miller and Ryder [3] also measured the concentration of ozone in the mesosphere and lower thermosphere at sunset using the occultation technique. They found a minimum in the O 3 concentration near 80km and a secondary O 3 maximum around 90km.
Nowadays, there is a special Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency's Envisat satellite. This instrument measures ozone using the stellar occultation method [5]. The number density of ozone has a strong maximum region around 90km (see Figure 1). This is known as the secondary ozone maximum [5,6]. The night-time mixing ratio of ozone in this maximum zone is comparable to that found in the stratospheric maximum (around 8 to 9 ppm). Daytime mixing ratios are substantially smaller but are significantly higher than those observed elsewhere above or (immediately) below [5,7].
Rogers et al. [8] measured the ozone in a layer of 80 to 104 km by the microwave line of 11.072454 GHz. The Gaussian distribution of the ozone mixing ratio was centred at 92km with the full width at maximum of 10km. The location of the secondary ozone maximum can be set between altitudes of 87 to 97 km. An important detail is that this ozone layer is a part of the ionospheric D-region as well [9].

Aims of the Study
The aim of this meta-study is to provide a basic understanding of the formation and dynamics of energetically excited oxygen molecules, and of their importance in the generation of the secondary ozone layer. The study is based on the available literature of excited O 2 molecules and excited O atoms in the secondary ozone layer. Data from the literature regarding atmospheric nightglows on Earth and Venus are used as crucial evidence.

Formulas Used in Calculations
The formation enthalpies of reactions are calculated by formula (1). If difference ∆H 0 <0, reaction is proceeding spontaneously, it is an exergonic reaction.
The formations of enthalpies (kJ/mol) are: for ozone 142.

Number Densities of Oxygen Atom in the MLT
According to Lin et al. [12] at an altitude of about 101 km the model-dependent peak of atomic oxygen density is 3.6 (±1.9) × 10 11 cm −3 (or in terms of the number of atomic oxygen 4.5 x 10 11 ). In a later study by Hedin et al. [13] the maximum concentration of atomic oxygen (4.7 x 10 11 atoms in cubic cm) was achieved at an altitude of 95 km.
At an altitude of 105km, the number density of O atoms is 2.8 x 10 11 , at an altitude of 120km it is 8 x 10 10 and at an altitude of 150km it is 8 x 10 9 [14]. The number of O atoms reaches their atmospheric maximum in the MLT. At an altitude of 120km the number density of O 2 molecules is 6 x 10 10 [14], which is less than the number density of O atoms. At an altitude of 91km the number of atomic oxygen is 1.4 x 10 11 during night and day [12].

Absorption of UV, Visible and IR Radiation by Oxygen Species
In the thermosphere, solar UV radiation is absorbed via photoionization by hydrogen, oxygen and nitrogen atoms and molecules and helium atoms. It leads to almost complete absorption of UV radiation shorter than 102nm in the thermosphere above the MLT layer.
In order to be photodissociated or photoexcitated by a 76 The Role of Excited Oxygen Molecules in the Formation of the Secondary Ozone Layer at 87 to 97 km non-ionizing photon, a molecule must absorb on the wavelength of the photon. These absorptions define the absorptive optical thickness of the atmosphere towards the UV radiation. Oxygen atoms absorb UV radiation at wavelengths between 10nm and 91nm [15]. An oxygen atom has the ground state O( 3 P), which is a triplet state. Higher energy forms, singlet oxygen atoms, are formed if the electrons pair up, like in the first excited state O( 1 D) and second excited state O( 1 S). Radiative lifetime of O( 1 S) is only 0.84s and that of O( 1 D) is 114s [16].
Oxygen molecule absorbs UV radiation at wavelengths from 58 to 260 nm. The absorption cut-off of UV photons by O 2 molecules is 260nm in 1 atm. air pressure and 250nm in zero pressure [17].
For dissociation, a photon must have so much energy that it exceeds the dissociation energy of the molecule. Due to the properties of the electron configuration, the excited O 2 molecules have individual dissociation ranges within which they are more or less metastable. If the UV photon raises an electron to such a high level in which it is in the dissociation range of a certain excited state, the O 2 molecule is photoexcitated rather than dissociated.
The spectrum of O 2 from 102 to 125 nm is very complex (see Figure 2) [18]. Lyman β line at 102.57nm is known to penetrate down to about 86 km level [19]. The ionization potential of oxygen molecules is 12.07eV, which is equal to the wavelength of 102.78nm. O 2 molecules absorb weakly at the Lyman β line [20] so the ionization of O 2 is possible in the entire MLT layer as well.  . Survey transmittance spectrum of the visible and near IR bands of O2 [24] Environment and Ecology Research 6(1): 74-85, 2018 77 UV at the Lyman α line (121.567nm) can penetrate down to 70km level [19]. Even though its O 2 absorption cross-section is small, about 1 x 10 -24 cm 2 [21], it is excitating O 2 molecules. Its attenuating about ten-fold between 100 to 80 km is, of course, mainly due to ozone molecules [19]. Between the Lyβ and Lyα lines, there are eight bands (centred at 107, 109. 110, 113, 114.5, 116 and 117 nm) with equally small O 2 UV absorption cross-sections than Lyα (see Figure 2). Especially through these windows extreme UV radiation (EUV) of 102.6 to 124 nm is able to enter into the MLT zone. UV photons in this range can photoexcitate highly excited O 2 molecules having energies between 10.6 to 11.07 eV (see Table 1).
Photodissociation of O 2 molecules in the Schumann-Runge continuum is due to the UV radiation of 125-175 nm [17]. This continuum is the primary source of oxygen atoms in the MLT layer and above it. UV photons at this range contain energy of 7.1 to 9.9 eV/mol (see Table 1). Products are either O( 3 P) atom + metastable O( 1 D) atom or O( 3 P) atom + metastable O( 1 S) atom [17].
The short wavelength ends of the Schumann-Runge continuum (125-132.5 nm) UV photons photoexcitate O 2 molecules with energies between 9.35 and 9.92 eV.
The UV photons with λ<134.1nm ionize NO molecules, which fact is largely responsible for the existence of the ionospheric D-region, and it's overlapping with the MLT layer [23].
Smith and Newnham [25] measured the near-infrared (NIR) absorption cross-sections and integrated absorption intensities in gas phase of oxygen and nitrogen mixtures. Monomer and binary cross-sections of the O 2 (a 1 Δ g )←O 2 (X 3 Σ g ) − (0,0) and (1,0) bands and underlying continuum absorptions of oxygen were centred at 1.06 and 1.27 μm.
Ozone is the most important UV absorber of all oxygen species. Watanabe et al. [18] have measured the ozone UV absorption spectrum over the spectral range of 100-220 nm, and Tu and Nee [26] over the range of 105-200 nm. Ozone absorbs strongly UV radiation entering the MLT layer. Ozone absorbs visible and NIR light as well.
Recently, Gorshlev et al. [27] have measured cross-sections of ozone with great accuracy over the spectral range of 213-1100 nm. Even though UV absorption by ozone is a much studied topic, there remains disagreement regarding the absorption minimum between the Huggins and Chappuis bands (350-425 nm), and especially near 380 nm due to low absorption coefficients and the difficulties of preparing pure ozone for study [27].

Quantum Chemistry Notations
The spectroscopic state of atoms and molecules is conveniently summarized by the use of spectroscopic term symbols. The Greek capital letters Σ, Π or Δ are used as the principal symbol for the state of the molecule. It tells the resultant angular momentum of all the electrons with respect to the internuclear axis Λ, which may have values 0, 1 and 2. When |Λ| is 0, the letter is Σ; when |Λ| is 1, the letter is Π and when |Λ| is 2, the letter is Δ. In atoms, the notation is analogous to the roman letters S, P and D. The multiplicity (singlet = 1, triplet = 3) is denoted by a superscript number to the left side of the letter [28,29].
A closed molecular group, whether σ or π, must give a 1 Σ term, as the resultant angular momentum and the multiplicity must both be zero. A superscript + oron the right side indicates whether the total wave function is symmetric or antisymmetric with respect to the reflection in a plane containing the principal axis. A subscript g or u on the right side is given corresponding to the gerade or ungerade molecular symmetry. The product of any number of g functions or an even number of u functions results in a g function [28].
The lowest state of the molecule (either Σ, Π or Δ) is labelled X. The excited states are then labelled alphabetically, starting with A, B, and so on to indicate the first excited state, second excited state, and so on. Uppercase letters are used for states with the same multiplicity as the ground state, while lowercase letters are used for states with a different multiplicity. Occasionally, new states are found which lie in between previously assigned states. These states are labelled A′ or B′. Thus, for O 2 , the lowest state is X 3 Σ g − , followed, in terms of increasing energy, by a 1 ∆ g , b 1 Σ g

Quantum Chemical Rules: Obeyed and Disobeyed by O 2 Molecules and Atoms
Generally, according to Wigner's spin selection rules [31], chemical reactions between triplet and singlet reactants are forbidden. So the transitions Σ→Δ, Δ→Σ, Σ + →Σand Σ -→Σ + are forbidden. In practice, all other elements more or less obey these rules, except excited O 2 molecules.
Transitions (and vice versa) Due to excitation, O 2 molecules have extra energy, with the aid of which they are able to go against the quantum chemical rules. Of course, transitions/reactions may be slower and yields may subsequently be smaller than in the allowed reactions. Nature has overcome this issue by the set-up of two ozone layers in the atmosphere. The outcome is that no harmful UV radiation is able to enter the troposphere through both of them. Only that part of UV radiation that helps our body to produce D-vitamin, and which is so vital for human life, is able to enter [33]. O 2 is called a life supporting and saving molecule for a reason.
In the 1980s chemical models predicted a lower density for ozone in the MLT layer than that which was observed [34]. One reason for this may have been that the quantum chemical rules of forbidden transitions/reactions for oxygen species were taken too literally in the chemical models of those times.

Importance of O 2 Vibrational Energies
Molecular vibrations are caused by thermal infrared wavelengths and molecular rotations are caused by microwave and far-IR wavelengths. The vibrational energy levels E(υ) within one electronic state (E el ) are schematically described by the Morse potential energy curve [35]. Vibrational energies are typically much smaller than electronical energies, and rotational energies E(J) are much smaller than vibrational energies.
Energy of O 2 (a 1 ∆ g , υ = 0) is 94.3 kJ/mol and that of O 2 (a 1 ∆ g , υ = 1) is 112.3 kJ/mol. So vibrational energy is as much as 19% from the E el of the O 2 (a 1 ∆ g , υ = 1) molecule. The energy of O 2 (a 1 ∆ g , υ = 10) is 19695cm -1 or 235.7kJ/mol [37]. It exceeds 2.5 times the E el of the υ = 0 molecule. So in case of O 2 molecules, it is necessary to deal with vibrational energies as well. For O 2 molecules, E (J) is much smaller than E (υ). Rotational energies are important in advanced research work regarding, for example, the identification of the emissions from differently excited oxygen molecules [38].
Copeland et al. [22] photoexcitated O 2 (X 3 Σ g -) with a laser beam between the wavelengths 242 to 252 nm to O 2 (A 3 Σ u + ) at vibrational levels of υ = 6 to the highest level of υ = 11. This proves that an important formation mechanism of the excited states of O 2 (c 1 Σ u -, A' 1 ∆ u and A 3 Σ u + ) is photoexcitation of O 2 (X 3 Σ g -) between UV cut-off and dissociation wavelengths of the O 2 molecule.
The highest vibrational level of O 2 (A 3 Σ u + ) is only 100 cm -1 (or 1.2 kJ/mol) from the O 2 (X 3 Σ g -, υ = 0) dissociation limit. However, the yields of O atoms in the dissociation of υ = 11 to was similar to that of υ = 10. This shows that within its dissociation range (see Table 1), the O 2 (A 3 Σ u + ) molecule is rather stable. It is likely that if the total energy available in a reaction of O 2 (X 3 Σ g -) fits within the dissociation limits of the O 2 (A 3 Σ u + ), it is likely to form.
The O 2 (B 3 Σ u -) molecule contains energy of 6.12 eV, which is higher than 5.12 eV, the dissociation energy of an O 2 (X 3 Σ g -, υ = 0). Rotational and vibrational energy levels E (υ, J) above the dissociation energy can still be stable if they are below the maximum of the potential barrier. A quantum mechanical tunnelling effect is used to provide a rationale for predissociation [29].
Predissociation was first described in 1924 for sulphur molecule S 2 . Typical to the phenomenon is that no rotational spectra can be resolved, even in cases when absorption maxima and minima have been observed [43].
In the emission spectrum of the oxygen molecule, no bands in the Schumann-Runge system, O 2 (B 3 Σ u -→ O 2 (X 3 Σ g -, having υ > 2 have ever been observed. The absence of these emission bands as well as evidence of broadened rotation lines in the υ > 2 absorption bands suggests that in these vibration levels of O 2 (B 3 Σ u -) state undergo predissociation [44].
The qualitative description of the phenomenon of predissociation is considered adequate, but it is insufficient for quantitative calculations of lifetimes and dissociation probabilities [43]. That is why the radiative lifetime of O 2 (B 3 Σ u -) remains enigmatic. Transitions in predissociation were supposed to be radiationless [44]. The wavelength range of the transition O 2 (B 3 Σ u -→ X 3 Σ g -) is 211-566 nm. [45]. This would apply to molecules of O 2 (B 3 Σ u -, υ = 0, 1 and 2). However, there is no explanation for why these states relax by radiating, but when υ increases > 2, the relaxation would be radiationless.
Predissociation does not affect the energy content of O 2 (B 3 Σ u − ). Within its 7700 cm -1 (92.1 kJ/mol) dissociation energy, there is room for 19 different vibronic states of O 2 (B 3 Σ u -) molecules [40]. So in the supposed radiationless radiation, the O 2 (B 3 Σ u -, υ > 2) molecules lose no energy. When O 2 (X 3 Σ g − , υ = 0) absorbs a UV photon in the Schumann-Runge bands, its internuclear distance (R) increases from the value 1.2075358 Å to the value 1.6042799 Å for O 2 (B 3 Σ u -) [39]. The double bond between the oxygen atoms in the O 2 (B 3 Σ u − ) molecule probably becomes weaker. Considering the great E el and E (υ), the O 2 (B 3 Σ u -, υ > 2) molecules would become more exposed towards all possible collision reactions, supposing its radiative lifetime is sufficiently long.
Sick et al. [46] have actually demonstrated that O 2 (B 3 Σ u − ) can undergo collisional reactions. It may be that due to the predissociation phenomenon (lack of rotation) the molecule internally strengthens against radiative relaxation. This would allow the O 2 (B 3 Σ u -, υ > 2) molecules to be destroyed in collisions. In this case, no emission spectra would be generated.
An increase of internuclear distance, predissociation via tunneling effect and the fact that transition O 2 (B 3 Σ u -)←O 2 (X 3 Σ g -) is optically allowed [16] are obviously important mechanisms to aid the oxygen molecule in absorbing UV radiation at the Schumann-Runge bands without dissociation into two O( 3 P) atoms + excessive heat. The molecules are formed when one electron is moved from (2σ g ) 2 MO to (3σ u ) 2 MO. The number of electrons on the last four MOs becomes 2σ g =1, (1π u ) 4 =4, (1π g ) 2 =2 and 3σ u =1. In 1 Π u and 3 Π u , one electron is moved from 1π u MO to 3σ u MO [39].
The excited oxygen molecules in this category belong to the energy range of 9.35 to 11.07 eV (see Table 1). They are formed by direct photoexcitation: 1) UV photons of λ < 131 nm produce O 2 (β 3 Σ u + ) and O 2 (a 1 Σ u + ) and 2) UV photons of λ<112 produce 3 Σ u + , 1 ∆ u and 1 Π u . England et al. [47] have made a comprehensive vibronic assignment of the O 2 ( 3 Π u ) states in the region 104 to 120 nm. They report measurements of 17 3 Π u ←X 3 Σ g bands in the 85,800-93,000 cm −1 (10.64-11.53 eV) region of the photoabsorption spectra of 16

Ionic O 2 + Molecules
The first of the O 2 + ions (with the lowest energy of 12.07 eV), is X 2 Π g which has lost one 1π g electron. The O 2 (a 4 Π ui ) and O 2 (A 2 Π u , molecules have lost one 1π u electron. The O 2 (b 4 Σ g -), O 2 (C 2 ∆ g ) and O 2 (B 2 Σ u -) molecules have lost one 3σ g electron. The O 2 (c 4 Σ u -) molecule has lost one 2σ u electron. [39]. The energy range of ionic O 2 molecules is between 12.07-24.56 eV, which is equal to the wavelength range 102.7 nm-50.5 nm.
The O 2 + ion has 11 valence electrons, of which eight are bonding electrons and three antibonding electrons. Thus, its bond order is ½(8-3) = 2½. The experimental dissociation energy and bond length of the O 2 (X 2 Π g ) ion are 643 kJ/mol and 1.123 Å, respectively [39]. In terms of dissociation energy, losing one electron makes the O 2 (X 2 Π g ) molecule "stronger" than O 2 (X³Σ g -). However, the ionic O 2 molecules have a "weak" side -absorption of an electron changes their bond order from 2.5 to 2.0. So a single electron with its 32.8 kJ/mol energy [48] can dissociate ionic O 2 (X 2 Π g ) molecule in the dissociative recombination reaction (DR) [49]: (4) The lifetime of O 2 + ions in the night depends mostly on the available electron concentration. O 2 + ion concentrations peak at 110 km [23]. It is quite possible that some O 2 (X 2 Π g ) molecules are produced in the MLT layer and that at 110 km, more O 2 (X 2 Π g ) and other O 2 + ions enter into the MLT layer via diffusion. During the night the reaction (4) may have importance, as a continuous additional source of O( 1 S) atoms.  (7) In this channel the result would be production of two unpaired oxygen species. This would be important for maintaining both the daytime and night-time ozone and oxygen atom concentration.

Electronically excited O 2 (B³Σ u -)
The Schumann-Runge bands have been observed in hydrogen, carbon monoxide and ammonia flames. This suggests that there may be a reaction common to all flames such as [52]:

Ozone Photolysis and Reactions Thereof
The ozone molecule is a kind of primus motor in many of the reactions in the MLT layer. Its readiness to react is enhanced by the participation of excited ozone molecules [53]. Wayne [54] has calculated limiting wavelengths for the formation of energetically different combinations of O + O 2 in ozone photolysis (see Table 2).
When ozone is photolyzed by photons of visible light, λ< 611nm, are produced O 2 (a 1 ∆ g ) + O( 3 P). With UV photons λ < 310 nm, the products are O 2 (a 1   Tu and Nee [26] suggested that when a UV photon at the wavelength λ = 121.6 nm dissociates ozone, the result is direct formation of three oxygen atoms from one ozone molecule: The ∆H 0 of the starting products (ozone and energy of UV photon) of the reaction (14)  . So between the wavelengths of 118.6 to 121.6 nm there is a window in which three oxygen atoms are formed from one ozone molecule.

Formation of O( 1 S) Atom
In the photolysis of ozone UV photons at wavelengths 230 < λ < 234 nm produce O( 1 S) and O 2 (X 3 Σ g -) (see Table  2). This reaction is feasible at night as well.

Nightglows as Evidences of the Night-time Dynamics of O and O 2 Species in the MLT Layer
Nightglows are the emission of UV, visible or IR light by excited atoms and molecules when they are relaxing to the lower excited states or ground states in the upper atmosphere. Nightglows in the MLT layer last throughout the night with only a little falling off [59]. Understanding the nightglow dynamics is vital to understanding the dynamics of oxygen species during the night in the secondary ozone zone.

82
The Role of Excited Oxygen Molecules in the Formation of the Secondary Ozone Layer at 87 to 97 km 3.10.1. IR and Visible Nightglows These nightglows are due to the relaxing of excited molecules O 2 (a 1 ∆ g and b 1 Σ g + ). The transitions O 2 (a 1 ∆ g , υ=0, 1) → O 2 (X 3 Σ g -) are called the IR atmospheric system. IR photons are emitted at wavelengths at 1060 nm) and 1269 nm. The transition O 2 (b 1 Σ g + , υ = 0 and 1) → O 2 (X 3 Σ g -) is called the Atmospheric system. The emitting wavelengths are 761.9 nm and 864.5 nm [60].

Nightglows Due to the Oxygen Green Line and Red
Line Emissions The green nightglow of the O( 1 S→ 1 D) transition is at a wavelength of 557.7 nm and is located at two elevations. The first peaks at an altitude between 300 and 160 km in the thermosphere [61] and the second one in the MLT layer at an altitude of 94 km [62].
The transition of O( 1 D)→O( 3 P) produces the red nightglow at 630 nm and at 636.4 nm at an altitude of 300 to 160 km. Its intensity is 1000 R at the close of the day but diminishes to 50 R by midnight [61]. The green line nightglow at the 170 km diminishes soon after sunset from 1000 R to 300 R and by the midnight much more [63]. (1 rayleigh equals 10 8 photons per cm 2 column per second [61]).
In the altitude range of 300 to 160 km the most important sources of O( 1 S) are O 2 + ions are generated during the daytime, and are then exhausted by midnight by electrons and N atoms. The formation of O 2 + ions by the EUV from geocorona, the formation of O( 1 S) by the three-body reaction of O+O+O atoms and the contribution from the galactic background [63] continues to provide so much O( 1 S) that the falling-off stops at the 50 R level.
In the MLT layer O( 1 S) is steadily formed throughout the night, so the intensity of the green line nightglow emission stays at 337 R [64]. Figure 4 presents the entire UV nightglow radiation of Earth [65]. It is generated (1)   Nightglows have been found on Venus as well [67,68]. The UV nightglows due to O 2 (c 1 Σ − u →X 3 Σ g -) and O 2 (A' 3 ∆ u →a 1 ∆ g ) emissions have a greater intensity in rayleighs on Venus than they do on Earth [68]. The ozone layer on Venus at 100 kilometres above the planet's surface is considerably less dense compared to the MLT ozone layer at night [69].

Ultraviolet Nightglows
So a likely conclusion is that on Earth during the night, the MLT ozone layer absorbs part of the UV photons emitted during nightglow. This absorbed UV radiation may have an important auxiliary role in the night-time dynamics of excited oxygen molecules and ozone.

Nightglow Dynamics: Economic Use of Available Energy
Energy reserved in the MLT layer during the daytime in the excited O 2 molecules and their replenishment by diffusion from above may not be enough to allow the nightglows to run throughout the night with steady intensity. Additional energy is provided by EUV from geocorona (producing highly excited O 2 molecules), UV from the nightglows of NO δ (producing O 2 (B 3 Σ u -) and NO γ bands (producing O 2 (A 3 Σ u + ), and UV generated by the nightglows of O 2 (A 3 Σ u + , c 1 Σ u and A' 3 ∆ u ) molecules (causing photolysis of ozone).
Visible photons from the oxygen green line produce O 2 (a 1 ∆ g ) via the photolysis of ozone Quenching of O( 1 S) provides O( 1 D) which reactions with ozone produce O 2 (A 3 Σ u + , c 1 Σ u and A' 3 ∆ u ) molecules. In the MLT ozone layer the use of energy is highly efficient. Energy-rich compounds are upgraded from "waste" energy, and with no cost to the "middle man of ozone", as these reactions provide also O atoms, which produce ozone.

Conclusions
During the daytime in the MLT layer the EUV photons (102.8 to 124 nm) are photoexcitating O 2 molecules up to 11.07 eV level. O 2 (B 3 Σ u -, υ = 0-19) molecules are formed via photoexcitation in the Schumann-Runge bands.
Dissociation of O 2 molecules in the Schumann-Runge and Herzberg continuums produces O atoms. In the photolysis of ozone there is a window between the wavelengths of 118.7 to 121.6 nm in which three oxygen atoms are formed from one ozone molecule. Vibrationally excited O 2 (X³Σ g -, υ ≥ 26) molecules are formed via ozone photolysis., Both O 2 (B 3 Σ u -) and (O 2 (X³Σ g -, υ ≥ 26) are able to react with O 2 (X³Σ g -, υ = 0) to produce O and O 3 . The O atom concentration reaches its atmospheric maximum in the MLT layer.
Due to the abundance of O atoms, a small but significant ozone concentration (as compared to the regions above or immediately below) remains during the daytime, even though ozone is then rapidly destroyed by photolysis.
During the night, solar UV radiation is radically reduced. Ozone, being the "weak" oxygen species, gains the most.
The number of O atoms shows no diurnal variation in the MLT layer. This leads to a ten-fold increase of ozone concentration over the course of the night.
The MLT ozone zone coincides with the ionospheric D-ring, so there is a small concentration of electrons at night as well.
At an altitude of 110 km, O 2 + ions enter the MLT layer via diffusion. Highly energetic neutral oxygen molecules (9.35 to 11.07 eV) generated during the daytime in the MLT layer are also replenished by diffusion from above.