Three-Body Photodissociation of Water Molecule: An Important Prebiotic Oxygen Source


 The provenance of oxygen on the Earth and other Solar planetary bodies is a fundamental issue. It has been widely accepted that the only prebiotic pathway to produce oxygen in the Earth’s primitive atmosphere was via vacuum ultraviolet (VUV) photodissociation of CO2 and subsequent two O atom recombination. Here, we provide experimental evidence of three-body dissociation (TBD) of H2O to produce O atoms in both 1D and 3P states upon vacuum ultraviolet (VUV) excitation using the newly developed tunable VUV free electron laser. Experimental results show that the TBD is the dominant pathway in the VUV H2O photochemistry at wavelengths between 90 and 107.4 nm. The relative abundance of water in the interstellar space with its exposure to intense VUV radiation suggests that the TBD of H2O and subsequent O atoms recombination should be an important prebiotic O2-production, which may need to be incorporated into interstellar photochemical models.


Introduction
Oxygen is the third most abundant element in the Universe, but its molecular form (O 2 ) is very rare.
Besides on the Earth, molecular oxygen has only been detected in two interstellar clouds [1] , [2] , in the moons of Jupiter [3] and Saturn [4], and on Mars [5]. Geologically based arguments suggested that Earth's original atmosphere had no oxygen and was composed mostly of H 2 O, CO 2 , and N 2 , with only small amounts of CO and H 2 [6]. 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" (GOE) [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 CO 2 by vacuum ultraviolet (VUV) light, followed by three-body recombination processes [9], where M is a third body to carry off the excess energy in the reaction process. Direct O 2 production pathways via VUV photodissociation of CO 2 [10] and dissociative electron attachment to CO 2 [11] have recently been identi ed. These ndings provide new insights into the sources of O 2 in Earth's early atmosphere.
In contrast, photodissociation of H 2 O, one of the dominant oxygen carriers [12], has long been assumed to proceed mainly to produce hydroxyl (OH) and hydrogen (H) atom primary products, and contribute limited  [14]. However, the existing photochemical reaction mechanisms seem hard to predict the measured O 2 abundance 13 . Thus, the detailed O 2 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 rst excited singlet ( [21], [22], [23] . Additional fragmentation pathways, named three-body dissociation (TBD), become accessible energetically at shorter photolysis wavelengths, e.g. where the threshold energies (E th ) for these fragmentation channels are given in parentheses [24].

Results And Discussion
The H 2 O sample is generated in a supersonic beam, with a rotational temperature estimated to be about 10 K. The H 2 O molecules were photoexcited to different Rydberg states [28] (see Fig. S1 and S2 in the Supplementary Materials (SM))). The dissociated H-atom fragments were then detected using the HRTOF technique (see Methods Section). TOF spectra of the H atoms resulting from H 2 O photodissociation at l = 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 travelled 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) [29]. Using the TKER distributions obtained in the parallel and perpendicular directions, we can construct a 3-dimensional (3D) ux diagram of the H-atom fragments. Fig. 1 shows the 3D product ux 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.
For detailed analysis and feature assignment, the TKER distributions in parallel and perpendicular directions at l = 107.4 nm are plotted in Fig. 1C, while the product TKER distribution from photodissociation of H 2 O 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, where, E int (H 2 O) and E int (OH) are the internal energies of H 2 O and OH, respectively, E KE is the product total kinetic energy and D 0 (H-OH) is the dissociation energy of H 2 O [30]. We can assign all of the sharp structures to speci c ro-vibrational levels of the OH product in the X and A states formed via the binary dissociation channel, H+OH (X or A, v, N). In addition to these sharp structures, the TKER spectra show two broad features: one with E KE £ 600 cm -1 that has a large angular anisotropy, and an underlying feature that spans the range of 600 £ E KE £ 16000 cm -1 which displays a much smaller angular anisotropy. These broad features are obviously not from the binary dissociation channel, H + OH. Based on energy conservation, the maximum kinetic energy for the H-atom product from the TBD channels (3) and (4) at 107.4 nm are 16448 cm -1 and 580 cm -1 , respectively. These limits of the two TBD channels match well with the upper limits of the two broad features in the distributions (Fig. 1). Thus, the intense broad feature at E KE < 600 cm -1 is assigned to the O ( 1 D) + 2H channel, while the broad underlying signal extending to E KE ~ 16000 cm -1 is attributed to the O ( 3 P) + 2H products.
Given the above analysis, the product TKER distribution in Fig. 1D can be divided into three components: Branching ratios for the O ( 1 D) + 2H, O ( 3 P) + 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).
Photodissociation of H 2 O has also been investigated at eight more VUV wavelengths between 92 nm and 109 nm, and a similar data analysis procedure is applied at these photolysis wavelengths (see Fig. S3 in the SM). The branching ratios determined for the binary and TBD channels at each wavelength are listed in 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 (see Fig. S5 and S6). 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 Fig. S7).
The conclusion that the TBD is the dominant decay process following excitation of H 2 O 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 H 2 O by using: J H2O = Φ λ Γ σ λ dλ, where Φ λ is the solar photon ux, Γ is the fragment quantum yield, and σ λ is the photodissociation cross section [31]. Fig. 4 collects together the wavelength dependences of the solar photon ux in the early period [32], the total photoabsorption cross sections of the parent H 2 O molecule in the VUV region (90-200 nm) [33] and the production yields of O atoms at studied photolysis wavelengths. Convoluting the solar photon ux, the photoabsorption cross sections, and the production yields implies that ~21% of the photoexcitation events of H 2 O will result in O atoms. Considering the water abundance in widely interstellar circumstances, like in interstellar clouds 2,3 and in the comets 67P 13, [34], oxygen production from water photolysis should be an important process. The following recombination of oxygen atoms will produce molecular oxygen.
In addition, it is well known that 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 10,35 . 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) [35], 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 meteorites 6 ) populated at the top of the atmosphere could absorb the VUV photons and dissociate. Given [H 2 O] is 10 times abundant than [CO 2 ] in the atmosphere during this early, chaotic period of Earth 6 (See Fig. S4 in the SM), the O-production rate from H 2 O VUV photochemistry could be 3 times larger than that of CO 2  atmosphere [37] , [38] . Thus, the production of O ( 1 D) 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.

Conclusions
In the existing interstellar photochemical model, reactions (1) and (2) are the major pathways to produce prebiotic O 2 . In this work, we propose an alternative prebiotic O 2 pathway: atomic oxygen production from the TBD of water, followed by oxygen recombination reactions. Recent International Ultraviolet Explorer (IUE) satellite observation of pre-main-sequence stars suggested that the nascent sun has emitted more than 10 times VUV radiation than it does today [39]. This implies that oxygen formation by VUV photoinduced TBD of H 2 O 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 model. Furthermore, the TBD of H 2 O may well be important for oxygen evolution in the atmospheres of all water-rich terrestrial planets [40].

Methods
The experiments employ a newly constructed apparatus for molecular photochemistry, which is centered on the vacuum ultraviolet free electron laser (VUV-FEL) beam line at the Dalian Coherent Light Source (DCLS) 27 . Brie y, the VUV-FEL facility runs in the high gain harmonic generation (HGHG) mode, in which the seed laser is injected to interact with the electron beam in the modulator (Fig. S1). The seeding pulse, in the wavelength range (λ seed ) 240-360 nm, can be generated from a picosecond Ti:sapphire laser pulse.
The electron beam is generated from a photocathode RF gun, and accelerated to the beam energy of 300 MeV by 7 S-band accelerator structures, with a bunch charge of 500 pC. The micro-bunched beam is then sent through the radiator, which is tuned to the 2nd/3rd/4th harmonic of the seed wavelength, and coherent FEL radiation with wavelength λ seed /2, λ seed /3 or λ seed /4 is emitted. Optimization of the linear accelerator yields a high quality electron beam with emittance of ~1.5 mm·mrad, energy spread of ~1‰, and pulse duration of ~1.5 ps. In this work, the VUV-FEL operates at 10 Hz, and the maximum pulse energy is >100 μJ/pulse. The output wavelength is continuously tunable in the range 50-150 nm and the typical spectral bandwidth of the VUV-FEL output is 30~50 cm -1 .
The high-n H atom Rydberg tagging time-of-ight (HRTOF) technique used in this work was pioneered by Welge and coworkers [41]. The key point of this technique is the 1+1′ (VUV+UV) excitation of the H atom. The rst step involves VUV laser excitation of the H atom from its n=1 ground state to the n=2 state by absorbing one l = 121.57 nm photon. In the second step, the H (n=2) atom is excited with a UV (l ~365 nm) photon to a high-n (n=30-80) Rydberg state. Charged species formed in the interaction region are extracted from the TOF axis by a small electric eld (~20V/cm) placed across this region. Rydberg tagged neutral H atoms y a known distance (d »280 mm) from the interaction region to a rotatable microchannel plate (MCP) Z-stack detector located close behind a grounded ne metal grid. After passing through the grid, the Rydberg atoms are immediately eld-ionized by the electric eld (~2000V/cm) applied between the grid and the front plate of the Z-stack MCP detector. The signal detected by the MCP is ampli ed by a fast pre-ampli er and counted by a multichannel scaler.

Competing interests
The authors declare no competing interests. Table   Table 1 The branching ratios for the binary and TBD channels following H 2 O photodissociation at different VUV wavelengths (nm). The maximum uncertainty on the branching ratios is ±10%.