Photodissociation Dynamics Of H2O Research Articles

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.

Photodissociation dynamics of H2O at 111.5 nm by a vacuum ultraviolet free electron laser.

Photodissociation dynamics of H2O via the F̃ state at 111.5 nm were investigated using the high resolution H-atom Rydberg "tagging" time-of-flight (TOF) technique, in combination with the tunable vacuum ultraviolet free electron laser at the Dalian Coherent Light Source. The product translational energy distributions and angular distributions in both parallel and perpendicular directions were derived from the recorded TOF spectra. Based on these distributions, the quantum state distributions and angular anisotropy parameters of OH (X) and OH (A) products have been determined. For the OH (A) + H channel, highly rotationally excited OH (A) products have been observed. These products are ascribed to a fast direct dissociation on the B̃1A1 state surface after multi-step internal conversions from the initial excited F̃ state to the B̃ state. While for the OH (X) + H channel, very highly rotationally excited OH (X) products with moderate vibrational excitation are revealed and attributed to the dissociation via a nonadiabatic pathway through the well-known two conical intersections between the B̃-state and the X̃-state surfaces.

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.

Photodissociation dynamics of H2O: Effect of unstable resonances on the $\tilde B\,^{1}{\rm \!A}_{1}$B̃1A1 electronic state

We report a tunable vacuum ultraviolet photodissociation study of H(2)O from different unstable resonances in the B̃(1)A(1) electronic state, using the H-atom Rydberg tagging technique. The quantum state resolved OH product translational energy distributions and angular distributions have been measured. Experimental results illustrate, for the first time, that excitation to the different unstable resonances has very different effect on the OH(X) and OH(A) product channels. The OH(X) product rotational distributions vary only slightly, while the OH(A) product rotational distributions and state-resolved angular distributions change dramatically as the photolysis energy increases. Effect of parent rotational excitation on the OH(A) product has also been observed. Through careful simulations to the experimental spectra, OH(A)∕OH(X) branching ratios have been determined at five photolysis wavelengths. The general agreement between theory and experiment in the branching ratios is good. The branching ratios for the OH(A) product from different parent rotational levels are close to the nuclear spin-statistics value, which is also consistent with the extremely low rotational temperature of the H(2)O beam in the current experiment.

Semiclassical Molecular Dynamics Simulations of the Excited State Photodissociation Dynamics of H2O in the AB1 Band

The photodissociation dynamics of H2O in the A1B1 band is investigated by implementing a recently developed time-sliced semiclassical initial value representation method (Burant, J. C.; Batista, V. S. J. Chem. Phys. 2002, 116, 2748). The capabilities of the computational method are explored as applied to calculations of partial and total photodissociation cross sections associated with highly excited vibrational states of H2O. The direct comparison of our semiclassical results with full quantum mechanical calculations shows that the semiclassical approach is able to describe the H2O spectroscopy and the overall photodissociation dynamics in excellent agreement with full quantum mechanical results.

Photodissociation of H2O at 121.6 nm: A state-to-state dynamical picture

Photodissociation dynamics of H2O at 121.6 nm have been studied using the H atom Rydberg “tagging” time-of-flight technique and by quasiclassical trajectory (QCT) calculations. Product kinetic energy distributions and angular distributions have been measured. From these distributions, rovibronic distributions of the OH radical product as well as the state resolved angular anisotropy parameters were determined. The dissociation energy D00(H–OH) is determined to be 41151±5 cm−1. Two clear alternations in the OH(X,v=0) rotational distribution have been observed, with each alternation corresponding to an oscillation in the anisotropy distribution. These oscillations had been attributed to the dynamical interference between the two conical intersection pathways. Further theoretical modeling in this work strongly supports this argument. Very highly vibrationally excited OH(X) products (up to v=9) have also been observed. These are ascribed to interconversion of H–O–H bending (H–H vibration) and O–H vibration in O–H–H geometries. The effect of parent rotational excitation on the OH(A) product state distribution and anisotropy distribution was observed for the first time. Experimental results also show clear evidence for the triple dissociation channel, O(3P)+2H. Accurate branching ratios of different product channels have been determined. Results of detailed QCT calculations agree well with the experimental results in this work.

Vibrationally mediated photolysis dynamics of H2O in the vOH=3 manifold: Far off resonance photodissociation cross sections and OH product state distributions

Vibrationally mediated photodissociation dynamics of water on the first excited electronic state surface (Ã) has been studied with slit jet-cooled H2O prepared in the complete polyad of vOH=3 overtone stretch levels (|03+〉, |12+〉, |12−〉, and |03−〉). (Notationally, |n1n2±〉 refers to symmetric/antisymmetric combinations of local mode OH stretch excitation, roughly corresponding to n1 and n2 quanta in the spectator and photolyzed OH bond, respectively.) At 248 nm photolysis wavelength the Condon point for bond cleavage occurs in the classically forbidden region, primarily sampling highly asymmetric H+OH exit valley geometries of the potential energy surface. Rotational, vibrational, spin orbit, and lambda doublet distributions resulting from this “far off resonance” photodissociation process are probed via laser induced fluorescence, exploiting the high efficiency laser excitation and light collection properties of the slit jet expansion geometry. Only vibrationally unexcited OH products are observed for both |12±〉 and |03±〉 initial excitation of H2O, despite different levels of vibration in the spectator OH bond. This is in contrast with “near-resonance” vibrationally mediated photolysis studies by Crim and co-workers in the |04−〉 and |13−〉 manifold, but entirely consistent with theoretical predictions from a simple two-dimensional quantum model. Photolysis out of the rotational ground H2O state (i.e., JKaKc=000) yields OH product state distributions that demonstrate remarkable insensitivity to the initial choice of H2O vibrational stretch state, in good agreement with rotational Franck–Condon models. However, this simple trend is not followed uniformly for rotationally excited H2O precursors, which indicates that these Franck–Condon models are insufficient and suggests that exit channel interactions do play a significant role in photodissociation dynamics of H2O at the fully state-to-state level.

Photodissociation dynamics of H2O at 121.6 nm: Effect of parent rotational excitation on reaction pathways

Photodissociation dynamics of H2O at 121.6 nm through the B̃ 1A1′ state have been studied using the high-resolution H atom Rydberg tagging technique. Experimental evidences show two different dissociation pathways to form the ground OH (X,v=0) products: dissociation through the B̃–X̃ conical intersection, and dissociation through B̃–Ã Coriolis coupling. By preparing the H2O molecules at higher rotational temperatures, dissociation through the B̃–Ã Coriolis coupling pathway can be enhanced.

New trends in the state-to-state photodissociation dynamics of H2O(Ã)

The water molecule, rotationally state selected in the third and fourth OH stretching overtone (‖04〉−, ‖05〉−) and stretch–bend combination (‖04−2〉) levels, has been photodissociated via the à state at λ≂282 nm. The photofragment rotational state distributions, determined by OH(A–X) laser induced fluorescence (LIF), differ from those reported previously by Andresen and co-workers, in which water was initially prepared in the ‖01〉− level and photodissociated at 193 nm, and from those by Crim and co-workers, in which H2O was photodissociated via the ‖04〉− level at wavelengths shorter than 282 nm. These differences become more pronounced with increasing angular momentum in the parent water molecule and with an increasing number of quanta in the intermediate OH stretching overtone state. The Franck–Condon theory of Balint-Kurti, previously employed successfully to account for the product state distributions arising from the 193 nm photodissociation of H2O‖01〉−, qualitatively reproduces the trends observed in the present study if it is assumed that dissociation occurs preferentially from extended RH-OH configurations of the ‖04〉− and ‖05〉− overtones. The product OH state distributions are thus shown to be sensitive indicators of the bending and rotational motions of H2O(X̃) in the wide amplitude stretching region of the ground state surface.

Spectroscopy and photodissociation dynamics of H2O: Time-dependent view

We present a study of H2O photodissociation dynamics in the first excited state. The absorption and emission spectra are calculated quantum mechanically using a time-dependent wave packet propagation technique. The excited state potential energy surface is obtained from ab initio data generated by Palma et al., while the ground state is a model by Reimers and Watts cast in a simple analytical form and constructed by fitting to infrared data. The variation of the transition dipole moment is taken into account by using an analytical functional form which is adjusted to fit the low resolution experimental emission spectrum. The calculated absorption spectrum is very similar to both the experimental one and results obtained with other methods. The high resolution emission spectrum is calculated for the first time and shows excellent agreement with the recently measured experimental spectrum by Hudson and co-workers. Dynamics on both the ground and excited states are examined.