Tunable VUV photochemistry using vacuum ultraviolet free electron laser combined with H-atom Rydberg tagging time-of-flight spectroscopy

Photodissociation dynamics of SO2 via the G̃1B1 state: The O(1D2) and O(1S0) product channels

Produced by both nature and human activities, sulfur dioxide (SO2) is an important species in the earth's atmosphere. SO2 has also been found in the atmospheres of other planets and satellites in the solar system. The photoabsorption cross sections and photodissociation of SO2 have been studied for several decades. In this paper, we reported the experimental results for photodissociation dynamics of SO2 via the G̃1B1 state. By analyzing the images from the time-sliced velocity map ion imaging method, the vibrational state population distributions and anisotropy parameters were obtained for the O(1D2) + SO(X3Σ-, a1Δ, b1Σ+) and O(1S0) + SO(X3Σ-) channels, and the branching ratios for the channels O(1D2) + SO(X3Σ-), O(1D2) + SO(a1Δ), and O(1D2) + SO(b1Σ+) were determined to be ∼0.3, ∼0.6, and ∼0.1, respectively. The SO products were dominant in electronically and rovibrationally excited states, which may have yet unrecognized roles in the upper planetary atmosphere.

Exploring the vacuum ultraviolet photochemistry of astrochemically important triatomic molecules

The recently constructed vacuum ultraviolet (VUV) free electron laser (FEL) at the Dalian Coherent Light Source (DCLS) is yielding a wealth of new and exquisitely detailed information about the photofragmentation dynamics of many small gas-phase molecules. This Review focuses particular attention on five triatomic molecules-H2O, H2S, CO2, OCS and CS2. Each shows excitation wavelength-dependent dissociation dynamics, yielding photofragments that populate a range of electronic and (in the case of diatomic fragments) vibrational and rotational quantum states, which can be characterized by different translational spectroscopy methods. The photodissociation of an isolated molecule from a well-defined initial quantum state provides a lens through which one can investigate how and why chemical reactions occur, and provides numerous opportunities for fruitful, synergistic collaborations with high-level ab initio quantum chemists. The chosen molecules, their photofragments and the subsequent chemical reaction networks to which they can contribute are all crucial in planetary atmospheres and in interstellar and circumstellar environments. The aims of this Review are 3-fold: to highlight new photochemical insights enabled by the VUV-FEL at the DCLS, notably the recently recognized central atom elimination process that is shown to contribute in all of these triatomic molecules; to highlight some of the potential implications of this rich photochemistry to our understanding of interstellar chemistry and molecular evolution within the universe; and to highlight other and future research directions in areas related to chemical reaction dynamics and astrochemistry that will be enabled by increased access to VUV-FEL sources.

The vibronic state dependent predissociation of H2S: determination of all fragmentation processes

Photochemistry plays a significant role in shaping the chemical reaction network in the solar nebula and interstellar clouds. However, even in a simple triatomic molecule photodissociation, determination of all fragmentation processes is yet to be achieved. In this work, we present a comprehensive study of the photochemistry of H₂S, derived from cutting-edge translational spectroscopy measurements of the H, S(¹D) and S(¹S) atom products formed by photolysis at wavelengths across the range 155-120 nm. The results provide detailed insights into the energy disposal in the SH(X), SH(A) and H₂ co-fragments, and the atomisation routes leading to two H atoms along with S(³P) and S(¹D) atoms. Theoretical calculations allow the dynamics of all fragmentation processes, especially the bimodal internal energy distributions in the diatomic products, to be rationalised in terms of non-adiabatic transitions between potential energy surfaces of both ¹A' and ¹A'' symmetry. The comprehensive picture of the wavelength-dependent (or vibronic state-dependent) photofragmentation behaviour of H₂S will serve as a text-book example illustrating the importance of non-Born-Oppenheimer effects in molecular photochemistry, and the findings should be incorporated in future astrochemical modelling.

Photodissociation of H2S: A New Pathway for the Production of Vibrationally Excited Molecular Hydrogen in the Interstellar Medium

Hydrogen sulfide (H₂S) is the most abundant S-bearing molecule in the solar nebula. Although its photochemistry has been studied for decades, the H₂ fragment channel is still not well-understood. Herein, we describe the photodissociation dynamics of H₂S + hv → S(¹S) + H₂(X¹Σ_g⁺) with the excitation wavelength of 122 nm ≤ λ ≤ 136 nm. The results reveal that the H₂(X) fragments formed are significantly vibrationally excited, with the quantum yields of ∼87% of H₂(X) fragments populated in vibrational levels v″ = 3, 4, 5, and 6. Theoretical analysis suggest that these H₂ products are formed on the H₂S 4¹A' state surface following a nonadiabatic transition via an avoided crossing from the 3¹A' to 4¹A' state. The estimated quantum yield of the S(¹S) + H₂ channel is ∼0.05, implying this channel should be incorporated into the appropriate interstellar chemistry models.

ARIA—A VUV Beamline for EuPRAXIA@SPARC_LAB

EuPRAXIA@SPARC_LAB is a new Free Electron Laser (FEL) facility that is currently under construction at the Laboratori Nazionali di Frascati of the INFN. The electron beam driving the FEL will be delivered by an X-band normal conducting LINAC followed by a plasma wakefield acceleration stage. It will be characterized by a small footprint and will deliver ultra-bright photon pulses for experiments in the water window to the user community. In addition to the soft-X-rays beamline already planned in the project, we propose the installation of a second photon beamline with seeded FEL pulses in the range between 50 and 180 nm. Here, we will present the FEL generation scheme, the layout of the dedicated beamline and the potential applications of the FEL radiation source in this low energy range.

Rotational and nuclear-spin level dependent photodissociation dynamics of H2S

The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces. But even in a small molecule like hydrogen sulphide (H₂S), the influence of non-adiabatic transitions is not yet well understood. Here we report high resolution translational spectroscopy measurements of the H and S(¹D) photoproducts formed following excitation of H₂S to selected quantum levels of a Rydberg state with ¹B₁ electronic symmetry at wavelengths λ ~ 139.1 nm, revealing rich photofragmentation dynamics. Analysis reveals formation of SH(X), SH(A), S(³P) and H₂ co-fragments, and in the diatomic products, inverted internal state population distributions. These nuclear dynamics are rationalised in terms of vibronic and rotational dependent predissociations, with relative probabilities depending on the parent quantum level. The study suggests likely formation routes for the S atoms attributed to solar photolysis of H₂S in the coma of comets like C/1995 O1 and C/2014 Q2.

The photodissociation dynamics of small molecules in the vacuum ultraviolet range can have key implications for astrochemical modelling, but revealing such dynamical details is a challenging task. Here the authors, combining high resolution experimental techniques, provide a detailed description of the fragmentation dynamics of selected rotational levels of a predissociated Rydberg state of H₂S.

Photodissociation Dynamics of H2O via the Ẽ' (1B2) Electronic State

Photodissociation dynamics of H₂O via the Ẽ'¹B₂ state were studied using the high-resolution H atom photofragment translational spectroscopy method, in combination with the tunable vacuum ultraviolet free electron laser (VUV FEL). The measured translational energy spectra allow us to determine the respective quantum state population distributions for the nascent OH(X²Π) and OH(A²Σ⁺) photofragments. Analyses of the quantum state population distributions show both the ground and electronically excited OH fragments to be formed with moderate vibrational excitation but with highly rotational excitation. Unlike the dissociation via the lower-lying electronic states, where OH(X) is the major fragment, the OH(A) products are predominant via the Ẽ' state. These products are mainly ascribed to a fast dissociation on the B̃¹A₁ state surface after nonadiabatic transitions from the initial excited Ẽ' state to the B̃ state. Meanwhile, another dissociation pathway from the Ẽ' state to the ¹B₂ 3pb₂ state, followed by coupling to the ¹A₂ 3pb₂ state, is also observed, which yields the OH(X) + H and O(³P) + 2H products.

The role of the three body photodissociation channel of water in the evolution of dioxygen in astrophysical applications

A recent experiment at the Dalian Coherent Light Source (DCLS) has provided measurements of the partial cross sections for the photodissociation of water vapor over an unprecedented range of wavelengths in the vacuum ultraviolet (VUV) region. It was found that the three body dissociation channel, H + H + O(3P/1D), becomes prominent at wavelengths shorter than the Lyman α-line at 121.6 nm. The present work explores the kinetic consequences of this discovery for several astrophysically motivated examples. The irradiation of a dilute low-temperature gas by unscreened solar radiation, similar to early stage photochemical processing in a comet coma, shows significant increase in the production of O2-molecules at shorter times, <1 day, that might physically correspond to the photochemical reaction zone of the coma. Several examples of planetary atmospheres show increased O-atom production at high altitudes but relatively little modification of the equilibrium O2 concentrations predicted by conventional models.

Direct Observation of the C + S2 Channel in CS2 Photodissociation

Carbon disulfide (CS₂) is a typical triatomic molecule. Its photodissociation process has generally been assumed to proceed to CS and S primary products via single bond fission. However, recent theoretical calculations suggested that an exit channel to produce C + S₂ should also be energetically accessible. Here, we report the direct experimental evidence for the C + S₂ channel in CS₂ photodissociation by using the velocity map ion imaging technique with two-photon UV and one-photon vacuum UV (VUV) excitations. The detection of the C (³P) products illustrates that the ground state and the electronically excited states of S₂ coproducts are formed within highly excited vibrational states. The very weak anisotropic distributions indicate relatively slow dissociation processes. The possible dissociation mechanism involves molecular isomerization of CS₂ to linear-CSS from the excited ¹B₂ (2¹Σ⁺) state via vibronic coupling with the ¹Π state followed by an avoided crossing with the ground state surface. Our results imply that the S₂ molecules observed in comets might be primarily formed in CS₂ photodissociation.

Electronically Excited OH Super-rotors from Water Photodissociation by Using Vacuum Ultraviolet Free-Electron Laser Pulses

The fragmentation dynamics of water in a superexcited state play an important role in the ionosphere of the planets and in the photodissociation region (PDR) of the planetary nebula. In this Letter, we experimentally study the fragmentation dynamics of H₂O with the energy above its ionization potential initiated by vacuum ultraviolet free-electron laser pulses. The experimental results indicate that the binary fragmentation channels H + OH and the triple channels O + 2H both present at 96.4 nm photolysis. Electronically excited OH super-rotors (v = 0, N ≥ 36, or v = 1, N ≥ 34), with the internal energy just above the OH (A) dissociation energy, are observed for the first time, which are only supported by the large centrifugal barriers. An absolute cross section of these super-rotors is estimated to be 0.7(±0.3) × 10^-18 cm². The tunnelling rates of these extremely rotationally excited states are also analyzed. This work shows a spectacular example of energy transfer from a photon to fragment rotation through photodissociation.

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light. Ethane is an important constituent in the atmospheres of the gas giants in our solar system. The present work describes translational spectroscopy studies of the VUV photochemistry of ethane using tuneable radiation in the wavelength range 112 ≤ λ ≤ 126 nm from a free electron laser and event-triggered, fast-framing, multi-mass imaging detection methods. Contributions from at least five primary photofragmentation pathways yielding CH2, CH3 and/or H atom products are demonstrated and interpreted in terms of unimolecular decay following rapid non-adiabatic coupling to the ground state potential energy surface. These data serve to highlight parallels with methane photochemistry and limitations in contemporary models of the photoinduced stratospheric chemistry of the gas giants. The work identifies additional photochemical reactions that require incorporation into next generation extraterrestrial atmospheric chemistry models which should help rationalise hitherto unexplained aspects of the atmospheric ethane/acetylene ratios revealed by the Cassini-Huygens fly-by of Jupiter.

Ultraviolet photolysis of H2S and its implications for SH radical production in the interstellar medium

Hydrogen sulfide radicals in the ground state, SH(X), and hydrogen disulfide molecules, H₂S, are both detected in the interstellar medium, but the returned SH(X)/H₂S abundance ratios imply a depletion of the former relative to that predicted by current models (which assume that photon absorption by H₂S at energies below the ionization limit results in H + SH photoproducts). Here we report that translational spectroscopy measurements of the H atoms and S(¹D) atoms formed by photolysis of jet-cooled H₂S molecules at many wavelengths in the range 122 ≤ λ ≤155 nm offer a rationale for this apparent depletion; the quantum yield for forming SH(X) products, Γ, decreases from unity (at the longest excitation wavelengths) to zero at short wavelengths. Convoluting the wavelength dependences of Γ, the H₂S parent absorption and the interstellar radiation field implies that only ~26% of photoexcitation events result in SH(X) products. The findings suggest a need to revise the relevant astrochemical models.

Sulfur is abundant in the Universe, but the observed abundance ratio of SH to H₂S doesn’t agree with astrochemical models. The authors measure product state-resolved translational energy spectra of photoproducts in a jet-cooled H₂S beam as a function of wavelength, showing that SH yield is lower than assumed in the models.

State-to-state photodissociation dynamics of CO2 around 108 nm: the O(1S) atom channel

State-to-state photodissociation of carbon dioxide (CO2) via the 3p1Πu Rydberg state was investigated by the time-sliced velocity map ion imaging technique (TSVMI) using a tunable vacuum ultraviolet free electron laser (VUV FEL) source. Raw images of the O(1S) products resulting from the O(1S) + CO(X1Σ+) channel were acquired at the photolysis wavelengths between 107.37 and 108.84 nm. From the vibrational resolved O(1S) images, the product total kinetic energy releases and the vibrational state distributions of the CO(X1Σ+) co-products were obtained, respectively. It is found that vibrationally excited CO co-products populate at as high as v = 6 or 7 while peaking at v = 1 and v = 4, and most of the individual vibrational peaks present a bimodal rotational structure. Furthermore, the angular distributions at all studied photolysis wavelengths have also been determined. The associated vibrational-state specific anisotropy parameters (β) exhibit a photolysis wavelength-dependent feature, in which the β-values observed at 108.01 nm and 108.27 nm are more positive than those at 107.37 nm and 107.52 nm, while the β-values have almost isotropic behaviour at 108.84 nm. These experimental results indicate that the initially prepared CO2 molecules around 108 nm should decay to the 41A' state via non-adiabatic coupling, and dissociate in the 41A' state to produce O(1S) + CO(X1Σ+) products with different dissociation time scales.

Photodissociation dynamics of H2O and D2O via the D[combining tilde](1A1) electronic state

Photodissociation dynamics of H₂O and D₂O via the D[combining tilde] state by one-photon excitation have been investigated using the H/D atom Rydberg tagging time-of-flight technique. The TOF spectra of the H/D-atom product in both parallel and perpendicular polarizations have been measured. Product translational energy distributions and angular distributions have been derived from TOF spectra. By simulating these distributions, quantum state distributions of the OH/OD product as well as the state-resolved angular anisotropy parameters were determined. The most important pathway of H₂O/D₂O dissociation via the D[combining tilde] state leads to highly rotationally excited OH/OD(X, v = 0) products, while vibrationally excited OH/OD products with v≥ 1 comprise only one third of the total OH/OD(X) population. The branching ratios of OH(A)/OH(X) and OD(A)/OD(X) have also been determined, 1.0/3.0 for H₂O at 122.12 nm and 1.0/2.2 for D₂O at 121.95 nm, which are reasonably consistent with the values predicted by the previous theory.

Generating Third Harmonic Vacuum Ultraviolet Light with a TiO2 Metasurface

Dielectric metasurfaces have recently been shown to provide an excellent platform for the harmonic generation of light due to their low optical absorption and to the strong electromagnetic field enhancement that can be designed into their constituent meta-atoms. Here, we demonstrate vacuum ultraviolet (VUV) third harmonic generation from a specially designed dielectric metasurface consisting of a titanium dioxide (TiO₂) nanostructure array. The metasurface was designed to enhance the generation of VUV light at a wavelength of 185 nm by tailoring its geometric design parameters to achieve an optical resonance at the fundamental laser wavelength of 555 nm. The metasurface exhibits an enhancement factor of nominally 180 compared to an unpatterned TiO₂ thin film of the same thickness, evidence of strong field enhancement at the fundamental wavelength. Mode analysis reveals that the origin of the enhancement is an anapole resonance near the pump wavelength. This work demonstrates an effective strategy for the compact generation of VUV light that could enable expanded access to this useful region of the electromagnetic spectrum.

Hydroxyl super rotors from vacuum ultraviolet photodissociation of water

Hydroxyl radicals (OH) play a central role in the interstellar medium. Here, we observe highly rotationally excited OH radicals with energies above the bond dissociation energy, termed OH "super rotors", from the vacuum ultraviolet photodissociation of water. The most highly excited OH(X) super rotors identified at 115.2 nm photolysis have an internal energy of 4.86 eV. A striking enhancement in the yield of vibrationally-excited OH super rotors is detected when exciting the bending vibration of the water molecule. Theoretical analysis shows that bending excitation enhances the probability of non-adiabatic coupling between the [Formula: see text] and [Formula: see text] states of water at collinear O-H-H geometries following fast internal conversion from the initially excited [Formula: see text] state. The present study illustrates a route to produce extremely rotationally excited OH(X) radicals from vacuum ultraviolet water photolysis, which may be related to the production of the highly rotationally excited OH(X) radicals observed in the interstellar medium.

Probing solvation and reactivity in ionized polycyclic aromatic hydrocarbon–water clusters with photoionization mass spectrometry and electronic structure calculations

Synchrotron based mass spectrometry coupled with theoretical calculations provides insight into polycyclic aromatic hydrocarbon water interactions.