Photodissociation of water in the à band revisited with new potential energy surfaces

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 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.

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.

Striking Isotopologue-Dependent Photodissociation Dynamics of Water Molecules: The Signature of an Accidental Resonance

Investigations of the photofragmentation patterns of both light and heavy water at the state-to-state level are a prerequisite for any thorough understanding of chemical processing and isotope heterogeneity in the interstellar medium. Here we reveal dynamical features of the dissociation of water molecules following excitation to the C̃(010) state using a tunable vacuum ultraviolet source in combination with the high-resolution H(D)-atom Rydberg tagging time-of-flight technique. The action spectra for forming H(D) atoms and the OH(OD) product state distributions resulting from excitation to the C̃(010) states of H₂O and D₂O both show striking differences, which are attributable to the effects of an isotopologue-specific accidental resonance. Such accidental-resonance-induced state mixing may contribute to the D/H isotope heterogeneity in the solar system. The present study provides an excellent example of competitive state-to-state nonadiabatic decay pathways involving at least five electronic states.

Product fine-structure resolved photodissociation dynamics: the A band of H2O

The photodissociation dynamics of H2O in its first absorption band is investigated on an accurate potential energy surface based on a large number of high-level ab initio points. Several ro-vibrational states of the parent molecule are considered. Different from most previous theoretical studies, the spin-orbit and Λ-doublet populations of the open-shell OH fragment are reported from full-dimensional wave packet calculations. The populations of the two spin-orbit manifolds are in most cases close to the statistical limit, but the Λ-doublet is dominated by the A(") component, thanks largely to the fast in-plane dissociation of H2O(Ã(1)A('')). Comparisons with experimental data and a Franck-Condon model are generally very good, although some discrepancies exist.

State-resolved spectroscopy of high vibrational levels of water up to the dissociative continuum

We summarize here our experimental studies of the high rovibrational energy levels of water. The use of double-resonance vibrational overtone excitation followed by energy-selective photofragmentation and laser-induced fluorescence detection of OH fragments allowed us to measure previously inaccessible rovibrational energies above the seventh OH-stretch overtone. Extension of the experimental approach to triple-resonance excitation provides access to rovibrational levels via transitions with significant transition dipole moments (mainly OH-stretch overtones) up to the dissociation threshold of the O-H bond. A collisionally assisted excitation scheme enables us to probe vibrations that are not readily accessible via pure laser excitation. Observation of the continuous absorption onset yields a precise value for the O-H bond dissociation threshold, 41 145.94 ± 0.15 cm(-1). Finally, we detect long-lived resonances as sharp peaks in spectra above the dissociation threshold.

State-to-state photodissociation dynamics of triatomic molecules: H2O in the B band

State-to-state photodissociation dynamics of H(2)O in its B band has been investigated quantum mechanically on a new set of non-adiabatically coupled potential energy surfaces for the lowest two (1)A' states of H(2)O, which are developed at the internally contracted multi-reference configuration interaction level with the aug-cc-pVQZ basis set. Quantum dynamical calculations carried out using the Chebyshev propagator yield absorption spectra, product state distributions, branching ratios, and differential cross sections, which are in reasonably good agreement with the latest experimental results. Particular focus is placed here on the dependence of various dynamical observables on the photon energy. Detailed analyses of the dynamics have assigned the diffuse structure in absorption spectrum to short-time recurring dynamics near the HOH conical intersection. The non-adiabatic dissociation to the ground state OH product via the HOH conical intersection is facile, direct, fast, and produces rotationally hot OH(X̃) products. On the other hand, the adiabatic channel on the excited state leading to the OH(Ã) product is dominated by long-lived resonances, which depend sensitively on the potential energy surfaces.

Photochemistry of the water molecule: adiabatic versus nonadiabatic dynamics

Water and light are two common constituents of both the earth's atmosphere and interstellar space. Consequently, water photodissociation is a central component of the chemistry of these environments. Electronically excited molecules can dissociate adiabatically (on a single potential energy surface, or PES) or nonadiabatically (with transfer between PESs), and water serves as a prototype for understanding these two processes in unimolecular dissociation. In recent years, extensive experimental and theoretical studies have been focused on water photolysis, particularly on the primary product of the dissociation, the OH radical. The use of the high-resolution H-atom Rydberg tagging technique, in combination with various vacuum ultraviolet (VUV) sources, has spurred significant advances in water photochemistry. As the excitation energy increases, different excited electronic states of water can be reached, and the mutual interactions between these states increase significantly. In this Account, we present the most recent developments in water photodissociation that have been derived from the study of the four lowest electronic excited states. The Ã(1)B(1) state photodissociation of H(2)O has been studied at 157.6 nm and was found to be a fast and direct dissociation process on a single repulsive surface, with only vibrational excitation of the OH(X(2)Π) product. In contrast, the dissociation of the B̃(1)A(1) state was found to proceed via two main routes: one adiabatic pathway leading to OH(A(2)Σ(+)) + H, and one nonadiabatic pathway to OH(X(2)Π) + H through conical intersections between the B̃ state and the ground state X̃(1)A(1). An interesting quantum interference between two conical intersection pathways has also been observed. In addition, photodissociation of H(2)O between 128 and 133 nm has been studied with tunable VUV radiation. Experimental results illustrate that excitation to the different unstable resonances of the state has very different effects on the OH(X(2)Π) and OH(A(2)Σ(+)) product channels. The C̃(1)B(1) state of H(2)O is a predissociative Rydberg state with fully resolved rotational structures. A striking variation in the OH product state distribution and its stereodynamics has been observed for different rotational states. There are two kinds of nonadiabatic dissociation routes on the C̃ state. The first involves Renner-Teller (electronic Coriolis) coupling to the B̃ state, leading to rotationally hot and vibrationally cold OH products. The second goes through a newly discovered homogeneous nonadiabatic coupling to the à state, leading to rotationally cold and vibrationally hot OH products. But the D̃(1)A(1) state shows no rotational structure and leads to a fast, homogeneous, purely electronic predissociation to the B̃ state. These studies demonstrate the truly fascinating nature of water photochemistry, which is extremely variable because of the different electronic states and their interactions. The results also provide a rather complete picture of water photochemistry and should be helpful in the modeling of interstellar chemistry, with its abundant VUV radiation.

Two-photon photodissociation dynamics of H2O via the D electronic state

Photodissociation dynamics of H(2)O via the D state by two-photon absorption have been investigated using the H-atom Rydberg tagging time-of-flight technique. The action spectrum of the D<--X transition band has been measured. The predissociation lifetime of the D state is determined to be about 13.5 fs. The quantum state-resolved OH product translational energy distributions and angular distributions have also been measured. By carefully simulating these distributions, quantum state distributions of the OH product as well as the state-resolved angular anisotropy parameters were determined. The most important pathway of the H(2)O dissociation via the D state leads to the highly rotationally excited OH(X,v=0) products. Vibrationally excited OH(X) products (up to v=10) and electronically excited OH(A,v=0,1,2) have also been observed. The OH(A)/OH(X) branching ratios are determined to be 17.9% at 244.540 nm (2omega(1)=81,761.4 cm(-1)) and 19.9% at 244.392 nm (2omega(2)=81,811 cm(-1)), which are considerably smaller than the value predicted by the theory. These discrepancies are attributed to the nonadiabatic coupling effect between the B and D surfaces at the bent geometry.

Desorption of hydroxyl radicals in the vacuum ultraviolet photolysis of amorphous solid water at 90 K

We have studied the desorption dynamics of OH radicals from the 157 nm photodissociation of amorphous solid water (ASW) as well as H(2)O(2) deposited on an ASW surface at 90 K. The translational and internal energy distributions of OH were measured using resonance-enhanced multiphoton ionization methods. These distributions are compared to reported molecular dynamics calculations for the condensed phase photodissociation of water ice and also reported results for the gas phase photodissociation of H(2)O at 157 nm. We have confirmed that OH radicals are produced from two different mechanisms: one from primary photolysis of surface H(2)O of ASW, and the other being secondary photolysis of H(2)O(2) photoproducts on the ASW surface after prolonged irradiation at 157 nm.

First ultraviolet absorption band of methane: Anab initiostudy

Quantum mechanical calculations of the cross sections for photodissociation of CH4 and CD4 in the 1t2-->3s band are presented. The potential energy surfaces for the three states correlating with the 1 1T2 state at tetrahedral geometries are calculated. The elements of the (3x3) matrix representing the electronic Hamiltonian in the diabatic basis are expanded in powers of nuclear coordinates, up to the second order. The expansion coefficients are based on accurate multireference configuration interaction calculations. The electronically nonadiabatic dynamics is treated with the multiconfiguration time-dependent Hartree approach. All nine internal degrees of methane are included in the quantum dynamics simulations. The calculated cross section agrees well with experiment. Semiclassical calculations using the reflection principle suggest that the peaks in the spectrum correspond to the three adiabatic electronic states correlating with the 1 1T2 state at Td geometries. However, the non-Born-Oppenheimer terms in the Hamiltonian have a strong effect on the positions of the peaks in the absorption spectrum. The results of semiclassical calculations, which neglect these terms, are therefore quite different from the accurate quantum results and experiment.

Enceladus' Water Vapor Plume

The Cassini spacecraft flew close to Saturn's small moon Enceladus three times in 2005. Cassini's UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturn's E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.

Stretching of hydrogen-bonded OH in the lowest singlet excited electronic state of water dimer

The lowest singlet excited electronic state of water monomer in the gas phase is strictly dissociative along a OH stretch coordinate but changes its nature when the stretched OH moiety is hydrogen bonded to a neighboring water molecule. This work extends previous exploration of the water dimer excited singlet potential-energy surface, using computational methods that are reliable even at geometries well removed from the ground-state equilibrium. First, the hydrogen-bonded OH moiety is stretched far enough to establish the existence of a barrier that is sufficient to support a quasibound vibrational state of the OH oscillator near the Franck-Condon region. Second, the constraint of an icelike structure is relaxed, and it is found that a substantial fraction of liquidlike structures also supports a quasibound vibrational state. These potential-energy explorations on stretching of the hydrogen-bonded OH moiety in a water dimer are discussed as a model for understanding the initial dynamics upon excitation into the lowest excited singlet state of condensed water. The possibility is raised that the excited-state lifetime may be long enough to allow for exciton migration, which would provide a mechanism for energy transport in condensed water phases.

Hydroxyl tagging velocimetry method optimization: signal intensity and spectroscopy

The previously demonstrated nonintrusive time-of-flight molecular velocity tagging method, hydroxyl tagging velocimetry (HTV), has shown the capability of operating both at room temperature and in flames. Well-characterized jets of either air (nonreacting cases) or hydrogen-air diffusion flames (reacting cases) are employed. A 7 x 7 OH line grid is generated first through the single-photon photodissociation of H2O by a approximately 193 nm pulsed narrowband ArF excimer laser and is subsequently revealed by a read laser sheet through fluorescence caused by A2sigma+(v' = 3) <-- X2pi(i)(v'' = 0), A2sigma+(v' = 1) <-- X2pi(i)(v'' = 0), or A2sigma+(v' = 0) < or = X2pi(i)(v'' = 0) pumping at approximately 248, approximately 282, or approximately 308 nm, respectively. A detailed discussion of the spectroscopy and relative signal intensity of these various read techniques is presented, and the implications for optimal HTV performance are discussed.

Generalized nuclear Fukui functions in the framework of spin-polarized density-functional theory

An extension of Cohen's nuclear Fukui function is presented in the spin-polarized framework of density-functional theory (SP-DFT). The resulting new nuclear Fukui function indices PhiNalpha and PhiSalpha are intended to be the natural descriptors for the responses of the nuclei to changes involving charge transfer at constant multiplicity and also the spin polarization at constant number of electrons. These generalized quantities allow us to gain new insights within a perturbative scheme based on DFT. Calculations of the electronic and nuclear SP-DFT quantities are presented within a Kohn-Sham framework of chemical reactivity for a sample of molecules, including H2O, H2CO, and some simple nitrenes (NX) and phosphinidenes (PX), with X=H, Li, F, Cl, OH, SH, NH2, and PH2. Results have been interpreted in terms of chemical bonding in the context of Berlin's theorem, which provides a separation of the molecular space into binding and antibinding regions.

Photodissociation of the water dimer: three-dimensional quantum dynamics studies on diabatic potential-energy surfaces

The results are presented of three-dimensional model studies of the photodissociation of the water dimer following excitation in the first absorption band. Diabatic potential-energy surfaces are used to investigate the photodissociation following excitation of the hydrogen bond donor molecule and of the hydrogen bond acceptor molecule. In both cases, the degrees of freedom considered are the two OH-stretch modes of the molecule being excited, and the dimer stretch vibration. The diabatic potentials are based on adiabatic potential surfaces computed with the multireference configuration-interaction method, and the dynamics of dissociation was studied using the time-dependent wave-packet method. The dynamics calculations yield a donor spectrum extending over roughly the same range of frequencies as the spectrum of the water monomer computed at the same level of theory. The acceptor spectrum has the same width as the monomer spectrum, but is shifted to the blue by 0.4-0.5 eV. The dimer spectrum obtained by averaging the donor and the acceptor spectrum is broader than the monomer spectrum, with the center of the dimer first absorption band shifted to the blue by about 0.2 eV relative to the monomer band. Our reduced dimensionality calculations do not find the red tail predicted for the dimer first absorption band by Harvey et al. [J. Chem. Phys. 109, 8747 (1998)]. This conclusion also holds if preexcitation of the dimer stretch vibration with one or two quanta is considered.

Excited electronic states of small water clusters

The lowest electronic states that are initially formed upon excitation of small water clusters having a central water molecule with one stretched OH bond are studied with electronic structure methods. It is found that in water dimer, trimer, and pentamer the lowest excited singlet and triplet states are each nondissociative for stretching of an OH bond that is hydrogen bonded in an icelike configuration to a neighboring water molecule. This is in marked contrast to the behavior of an isolated gas phase water monomer, where it is well known that the lowest excited state is strongly dissociative upon OH stretching. The conclusions of this study may serve as a basis to interpret recent experimental evidence that suggests a significant lifetime for excited water in irradiated thin ice films, and may also have important implications for the behavior of excitation of liquid water.

Excited state electronic structures and dynamics of NOCl: A new potential function set, absorption spectrum, and photodissociation mechanism

A set of analytical potential energy surfaces (PESs) for six singlet excited states of NOCl are constructed based on multireference configuration interaction calculations. The total absorption cross section at the energy range of 2-7 eV is calculated by quantum dynamics calculations with the present PESs and transition dipole moments. The calculated absorption spectrum agrees well with the experiment. It is also found that the A band with the absorption maximum at 6.3 eV is attributed to the transition to the 4 1A' state, though the excitations to the 3 1A' and 3 1A" states contribute to the spectrum at the energy range between 4 and 5 eV. The spin-forbidden transitions are concluded to be negligibly weak. The mechanism of photodissociation reaction at the energy region corresponding to the A band is examined. The nonadiabatic transition rates from the 4 1A' state to lower singlet and triplet states are estimated by Fermi's golden rule, and the transitions to the 1 1A' and 3 1A' states induced by vibronic coupling are found to be the predominant dissociation pathways. The experimentally observed energy dependence of the recoil anisotropy of the fragments is discussed based on the calculated nonadiabatic transition rates.

State-resolved dynamics of photofragmentation

We discuss experiments on the dynamics of photodissociation that employ methods to select the energy, sometimes quantum states, of the reactant and to determine the quantum states and energy, sometimes also the orientation and alignment, of products. A summary of new advances of experimental methods is followed by applications to photodissociation of various types. Representative examples of simple bond fission, molecular elimination, and three-body dissociation with determined electronic states-sometimes the orientation of their angular momentum-of product atoms or distributions of electronic and internal states of product molecules illustrate the detailed information and insight that one can derive from such experiments. Photodissociation of van der Waals complexes, ions, species adsorbed on surfaces, and species in solution is excluded from this review.