Introduction

Oxygen is the third most abundant element in the Universe, but its molecular form (O₂) is very rare. Besides on the Earth, molecular oxygen has only been detected in two interstellar clouds^1,2, in the moons of Jupiter³ and Saturn⁴, and on Mars⁵. Geologically based arguments suggested that Earth’s original atmosphere had no oxygen and was composed mostly of H₂O, CO₂, and N₂, with only small amounts of CO and H₂⁶. Therefore, how oxygen is produced in the primitive atmosphere is a fundamentally important issue in the evolution of the early primitive atmosphere. Before the emergence of the oxygen-rich atmosphere due to the “great oxidation event”^7,8, about 2.33 billion years ago, which allowed the Earth to evolve into a living planet, a small amount of oxygen was already present, and this was previously attributed to an abiotic formation mechanism involving photodissociation of CO₂ by vacuum ultraviolet (VUV) light, followed by three-body recombination processes⁹:

In contrast, photodissociation of H₂O, one of the dominant oxygen carriers¹², has long been assumed to proceed mainly to produce hydroxyl (OH) and hydrogen (H) atom primary products, and contribute limitedly to the O₂ production⁹. Recently, abundant molecular O₂ in the coma of comet 67P/Churyumov–Gerasimenko, which is dominated by H₂O, CO, and CO₂, has been detected¹³. Interestingly, a strong correlation between O₂ and H₂O has been identified, indicating the O₂ formation is linked to H₂O in the comet. One plausible explanation for the strong O₂–H₂O correlation would be that the O₂ is produced by radiolysis or photolysis of water, or single collisions of energetic H₂O⁺ with surfaces¹⁴. However, the existing photochemical reaction mechanisms underestimate the O₂ abundance¹³. Thus, the detailed O₂ production mechanism in the coma of comets is still unclear.

The photodissociation of water has been the subject of many experimental studies, which have revealed fascinating dynamics arising from strongly coupled electronic states with strikingly different potential energy surfaces (PESs)^15,16. Excitation to the first excited singlet ($$ \tilde{A}$$¹B₁) state of H₂O at wavelengths λ ~160 nm results in direct O−H bond fission yielding an H atom plus a ground state hydroxyl radical, OH(X²Π), with little internal excitation^17–19. The absorption cross-section to the second excited singlet ($$ \tilde{B}$$¹A₁) state is maximal at λ ~128 nm. Excitation at these wavelengths results in a (minor) direct dissociation channel to electronically excited OH(A²Σ⁺) + H products. The major dissociation process yields ground state OH(X) + H products following non-adiabatic transitions at conical intersections between the $$ \tilde{B}$$ and $$ \tilde{X}$$ state PESs at linear H–O–H and H–H–O geometries^20–23.

Additional fragmentation pathways, named three-body dissociation (TBD), become accessible energetically at shorter photolysis wavelengths, e.g.:

Herein we report the studies of the H₂O photochemistry in the VUV region using the DCLS and the H-atom Rydberg tagging time-of-flight (HRTOF) technique. These experiments allow quantitative determination of the relative importance of the binary dissociation and the TBD processes following photoexcitation of H₂O in the 90–110 nm region. The present results show conclusively that the H₂O TBD process is an important pathway to form oxygen in the interstellar space.

Results and discussion

The H₂O sample was generated in a supersonic beam, with a rotational temperature estimated to be about 10 K. The H₂O molecules were photoexcited to different Rydberg states²⁷ (see Supplementary Fig. 2 and Supplementary Note 2). The dissociated H atom fragments were then detected using the HRTOF technique (see Supplementary Fig. 1 and Supplementary Note 1). TOF spectra of the H atoms resulting from H₂O photodissociation at λ = 107.4 nm have been recorded, with the detection axis aligned parallel and perpendicular to the polarization vector of the VUV-FEL radiation. Knowing both the distance traveled by the H atom from the photodissociation area to the detector and its mass, the TOF spectra can be converted into the distributions of total kinetic energy release (TKER), which is derived using the following equation, $$ E_{\mathrm{KE}}=\frac{1}{2}{m}_{\mathrm{H}}\left(1+\frac{m_{\mathrm{H}}}{m_{\mathrm{OH}}}\right){\left(\frac{d}{t}\right)}^2$$, where d is the flying path length of H atom (d ≈ 28 cm) from the photodissociation region to the detector, t is the measured time of flight²⁸. Using the TKER distributions obtained in the parallel and perpendicular directions, we can construct a 3-dimensional (3D) flux diagram of the H atom fragments. Figure 1 shows the 3D product flux diagrams in two regions of the kinetic energy with rich structures. The tall feature at low kinetic energy (Fig. 1A) shows a large product angular anisotropy, whereas the product anisotropy in the higher kinetic energy region (Fig. 1B) is rather small.

Fig. 1

The kinetic energy release spectra from H₂O photodissociation.

A The 3D product contour diagram from the photodissociation of H₂O at 107.4 nm for the total kinetic energy release (TKER) between 0 and 18,000 cm⁻¹; B the 3D product contour diagram for the TKER between 650 and 18,000 cm⁻¹. C The product TKER distributions at λ = 107.4 nm with the detection axis parallel and perpendicular to the polarization vector of the VUV-FEL radiation. The energetic limits of the two TBD channels (E_KEmax ~580 and ~16,448 cm⁻¹) are marked, and the inset displays the same spectra on an expanded vertical scale. D The product TKER distribution at λ = 107.4 nm with the detection axis at 54.7° (magic angle) to the polarization direction, along with the simulated TKER distributions for the O(¹D) + 2H and O(³P) + 2H TBD channels. Source data are provided as a Source Data file.

For detailed analysis and feature assignment, the TKER distributions in parallel and perpendicular directions at λ = 107.4 nm are plotted in Fig. 1C, while the product TKER distribution from photodissociation of H₂O at the magic angle (with detection angle of 54.7° relative to the polarization direction) is shown in Fig. 1D. These distributions show both sharp and broad features. Using the energy conservation relationship appropriate for a binary photodissociation process:

Given the above analysis, the product TKER distribution in Fig. 1D can be divided into three components: O(¹D) + 2H, O(³P) + 2H, and OH + H. The first two components are broad features, while the third comprises sharp structures attributable to ro-vibrational levels of the OH(A) and OH(X) products. From Fig. 1D, the O(¹D) + 2H channel obviously has the highest intensity in the kinetic energy <600 cm⁻¹, yet it has never been reported previously in H₂O photodissociation. The O(³P) + 2H channel clearly makes the major contribution in the higher kinetic energy region. This indicates that the TBD is likely the dominant process following VUV photoexcitation of H₂O at 107.4 nm.

Branching ratios for the O(¹D) + 2H, O(³P) + 2H, and OH + H fragmentation channels have been estimated by simulating the TKER distribution (Fig. 1D) using the three components shown in Fig. 2. Integrating the areas under the respective distributions returns relative H atom yields for the three channels. Recognizing that two H atoms are formed in each TBD process, the relative H atom yields are used to determine the branching ratio, e.g. 67% at 107.4 nm for the TBD channels (Table 1).

Fig. 2

The simulated kinetic energy release spectra from H₂O photodissociation.

TKER distributions determined from simulating the (A) O(¹D) + 2H, (B) O(³P) + 2H, and (C) OH + H product yields from photodissociation of H₂O at λ = 107.4 nm. The sharp features in the H + OH product yield have been assigned to population of ro-vibrational levels of the X and A states of the OH radical. Source data are provided as a Source Data file.

Table 1

The branching ratios for the binary and TBD channels following the H₂O photodissociation at different VUV wavelengths (nm).

Dissociation channel	Photolysis wavelength (nm)
	109.0	107.4	106.7	105.7	101.3	98.1	96.2	94.5	92.0
TBD (O(1D/3P) + 2H)	0.35	0.67	0.76	0.62	0.77	0.72	0.79	0.86	0.86
Binary dissociation: (H + OH)	0.65	0.33	0.24	0.38	0.23	0.28	0.21	0.14	0.14

Photodissociation of H₂O has also been investigated at eight more VUV wavelengths between 92 and 109 nm, and a similar data analysis procedure is applied at these photolysis wavelengths (Supplementary Fig. 3 and Supplementary Note 3). The branching ratios determined for the binary and TBD channels at each wavelength are listed in Table 1. At 109.0 nm, only one TBD channel (O(³P) + 2H) exists because the O(¹D) + 2H channel is energetically not accessible. The results mean that oxygen atoms (¹D and ³P), not OH radicals, are the major oxygen-containing products from H₂O photolysis at λ < 107.4 nm, in striking contrast to the dominant binary photofragmentation (i.e. H + OH) behavior displayed by H₂O photochemistry at longer VUV wavelengths¹⁵.

The dissociation dynamics of the two TBD channels are also quite interesting. Since the two H atoms in the water molecule are equivalent, if they dissociate simultaneously it should yield a narrow H atom kinetic energy distribution, peaking at an E_EK value close to half of the available energy (Supplementary Figs. 5 and 6 and Supplementary Note 5). However, the observed distributions are much broader than the narrow distributions for a simultaneous concerted process, implying that both TBD processes are due to mostly a sequential dissociation mechanism. Possible dissociation routes for the two channels are illustrated in Fig. 3 (and more details in Supplementary Fig. 7 and Supplementary Note 6). The water molecule undergoes efficient non-adiabatic coupling from the initial excited nd Rydberg states to the $$ \tilde{D}$$ state. Path 1 illustrates the possible direct dissociation route from the $$ \tilde{D}$$ state to form O(¹D) + 2H products. The overall anisotropy parameter of this channel is about 0.8, according with a fast and direct dissociation process. Paths 2 and 3 illustrate plausible routes for the more complicated O(³P) + 2H dissociation paths: from the $$ \tilde{D}$$ state to the $$ \tilde{B}$$ state and then to the ground state via the two CIs between the $$ \tilde{B}$$ and $$ \tilde{X}$$ state PESs, following the initial internal conversion from the excited nd Rydberg states to the $$ \tilde{D}$$ state. The averaged anisotropy parameter of this channel is considerably smaller (~0.2). This also accords with the dissociation undergoing several internal conversions, and with the more scrambling that leads to small product anisotropy.

Fig. 3

Illustration of the TBD mechanisms of H₂O upon VUV excitation.

Photoexcitation populates the nd Rydberg state (nd RS) which undergo efficient non-adiabatic coupling to the $$ \tilde{D}$$ state. The light blue arrow displays the photoexcitation process and the gray oval represents the crossing region between the nd RS and the $$ \tilde{D}$$ state. Path 1 illustrates the possible direct dissociation route from the $$ \tilde{D}$$ state to form O(¹D) + 2H products (the yellow curve). Paths 2 and 3 illustrate plausible routes for the more complicated O(³P) + 2H dissociation paths: from the $$ \tilde{D}$$ state to the $$ \tilde{B}$$ state and then to the ground state via the two conical intersections (CI #1 or CI #2), following the initial internal conversion from the excited nd RS to the $$ \tilde{D}$$ state (the red curves). This picture is consistent with the observed angular anisotropy for the two channels: the O(¹D) + 2H channel is a more direct dissociation process with larger angular anisotropy, while the O(³P) + 2H channel is a more complicated dissociation process with smaller angular anisotropy. The PES contour color from blue to red represents the potential energy from 0 to 12 eV.

The conclusion that the TBD is the dominant decay process following excitation of H₂O at these VUV wavelengths could have profound implications for our understanding of the source of oxygen production. For quantitative assessment, we have calculated the fragment-dependent photodissociation rate of H₂O by using: J_H2O = ʃ Φ_λΓσ_λdλ, where Φ_λ is the solar photon flux, Γ is the fragment quantum yield, and σ_λ is the photodissociation cross-section³⁰. Figure 4 collects together the wavelength dependences of the solar photon flux in the early period³¹, the total photoabsorption cross-sections of the parent H₂O molecule in the VUV region (90–200 nm)³² and the production yields of O atoms at studied photolysis wavelengths. Convoluting the solar photon flux, the photoabsorption cross-sections, and the production yields implies that ~21% of the photoexcitation events of H₂O will result in O atoms. Considering the water abundance in widely interstellar circumstances, like in interstellar clouds^2,3 and in the moons of planets in the Solar System (e.g., the comets 67P^13,33), oxygen production from the water photolysis should be an important process. The following recombination of oxygen atoms will produce molecular oxygen.

Fig. 4

The wavelength dependences of O atom quantum yields.

Plot showing (A) the wavelength dependences of the reconstructed VUV solar flux (90–200 nm, the black lines) at ~10 My (10 My = 1 × 10⁷ years, reconstructed from ref. ³¹. The VUV solar flux at modern period or the interstellar radiation field (ISRF)⁴¹ also can be used, which may modify the yield of O-production a little, but the final conclusion holds), the total absorption (σ_tot, the solid blue curve)³² and photoionization (σ_ion, the dotted blue curve) cross-sections⁴² of H₂O, and (B) the quantum yield for forming O-atom photoproducts (O(³P/¹D) + 2H), Γ, determined in the present work (the red dot). It is noted that the predissociation rate of H₂O is sufficiently fast that the fluorescence quantum yield must be negligible, so the total photodissociation cross-section will be almost the same as the photoabsorption cross-section. The polynomial function through the latter data is used to derive the reported overall O product quantum yield (the black curve in B). The quantum yields at λ = 111.5, 115.2, 121.57 nm are obtained from refs. ^{15, 24, 26}. The error bars represent the standard deviation (1σ) of three times measurements. Source data are provided as a Source Data file.

In addition, it is well known that the water photolysis has nothing to do with oxygen production in the Earth’s atmosphere under equilibrium conditions due to VUV photon screening by the thick atmosphere^10,34. However, in the earliest period of Earth, i.e., the period approaching to clement conditions on the earliest Earth followed by the current Earth–Moon system formed, the surface of Earth remained quite hot (>1000 K)³⁴, all of the water on the Earth was vaporized to the atmosphere and part of water clouds (emitted from volcanos or delivered by carbonaceous chondrite meteorites⁶) populated at the top of the atmosphere could absorb the VUV photons and dissociate. Given [H₂O] is ten times abundant than [CO₂] in the atmosphere during this early, chaotic period of Earth⁶ (see Supplementary Fig. 4 and Supplementary Note 4), the O production rate from H₂O VUV photochemistry could be three times larger than that of CO₂ in the same VUV wavelength region, via TBD processes: N_H2O(O)/N_CO2(O) = (J_H2O(O) × [H₂O])/(J_CO2(O) × [CO₂]) = ~3, where J_H2O(O) = ~5.2 × 10⁻⁵ s⁻¹ (Fig. 4), J_CO2(O) = ~1.8 × 10⁻⁴ s⁻¹ (Supplementary Fig. 8 and Supplementary Note 7), [H₂O] and [CO₂] are the densities of H₂O and CO₂, respectively. Since the molecular oxygen generation process should be the same in the three-body recombination process (Eq. (2)), this analysis implies that the H₂O photochemistry might be an important prebiotic source of O₂ in Earth’s early atmosphere.

From the experimental results, it seems that more than one-third of O atoms produced from H₂O TBD process populate in the metastable ¹D state. The generation of O(¹D) atoms from photodissociation of H₂O in a significant amount is also very interesting because the metastable O(¹D) atom is highly reactive³⁵. It can react with almost all the gases emitted into the atmosphere. For instance, the reaction of O(¹D) with methane could be a significant source of formaldehyde in the earth’s primitive atmosphere^36,37. Thus, the production of O(¹D) atoms from the exposure of water to VUV radiation, and the subsequent reactions of these atoms, could have been important drivers in the evolution of the earliest atmosphere.

In the existing interstellar photochemical model, reactions (1) and (2) are the major pathways to produce prebiotic O₂. In this work, we propose an alternative prebiotic O₂ pathway: atomic oxygen production from the TBD of water, followed by oxygen recombination reactions. Recent International Ultraviolet Explorer satellite observation of pre-main-sequence stars suggested that the nascent Sun has emitted more than ten times VUV radiation than it does today³⁸. This implies that oxygen formation by VUV photoinduced TBD of H₂O is likely an important process in the coma of comets, in the interstellar clouds and even in Earth’s primitive atmosphere, and thus needs to be incorporated into interstellar photochemical models. Furthermore, the TBD of H₂O may well be important for the oxygen evolution in the atmospheres of all water-rich terrestrial planets³⁹.

Methods