Lina Cheng

Nonadiabatic dissociation dynamics in H2O: Competition between rotationally and nonrotationally mediated pathways

The photochemistry of H2O in the VUV region is important in interstellar chemistry. Whereas previous studies of the photodissociation used excitation via unbound states, we have used a tunable VUV photolysis source to excite individual levels of the rotationally structured C̃ state near 124 nm. The ensuing OH product state distributions were recorded by using the H-atom Rydberg tagging technique. Experimental results indicate a dramatic variation in the OH product state distributions and its stereodynamics for different resonant states. Photodissociation of H2O(C̃) in rotational states with k′a = 0 occurs exclusively through a newly discovered homogeneous coupling to the à state, leading to OH products that are vibrationally hot (up to v = 13), but rotationally cold. In contrast, for H2O in rotationally excited states with k′a > 0, an additional pathway opens through Coriolis-type coupling to the B̃ state surface. This yields extremely rotationally hot and vibrationally cold ground state OH(X) and electronically excited OH(A) products, through 2 different mechanisms. In the case of excitation via the 110 ← 000 transition the H atoms for these 2 product channels are ejected in completely different directions. Quantum dynamical models for the C̃-state photodissociation clearly support this remarkable dynamical picture, providing a uniquely detailed illustration of nonadiabatic dynamics involving at least 4 electronic surfaces.

Tunable VUV photochemistry using Rydberg H-atom time-of-flight spectroscopy

In this article, we report an experimental method for studying tunable vacuum ultraviolet (VUV) photochemistry using the H-atom Rydberg tagging technique. In this method, two VUV laser light beams were generated using nonlinear four-wave mixing scheme in a single Kr gas cell: one VUV beam is fixed at the 121.6 nm wavelength to probe the H-atom product through the Lyman alpha transition, the other beam can be tunable for photodissociating molecules in the wavelength range lambda(VUV)=121-190 nm. Preliminary results on the H(2)O photodissociation in the B state are reported here. These results suggest that the experimental method is a powerful tool for investigating photodissociation dynamics in the VUV region for molecules involving H-atom processes.

Photodissociation dynamics of H2O: Effect of unstable resonances on the 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.

Two-photon photodissociation dynamics of H2O via the D̃ 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.

Product rotational Franck-Condon oscillations in HOD (Jka,kc) dissociation

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the dissociation of jet-cooled HOD molecules following excitation to individual rovibrational levels of its (1B1) Rydberg state near 124 nm. Spectra have been recorded for both the D + OH and H + OD dissociation channels. The branching ratios between OH/OD(X , high v, low N), OH/OD(X , low v, high N), and OH/OD(A ) channels vary between different parent rotational levels, and between the OH and OD products. The variation between the OH and OD channels can be attributed to an isotopic mass effect on the rotational axes which influences non-adiabatic coupling strengths. In addition, there is a population alternation with product rotational quantum number N for the OH/OD(X, high v, low N) product, most striking for the OH case, the sense of which is correlated with the value of the parent pseudo-quantum number . It is shown that this is a consequence of recoil forces which lead to a closing of the HOD angle to near 90° accompanying non-adiabatic transfer from the state to the (1B1) state. The oscillation in population then derives from the symmetry properties around this angle of the product rotational wavefunctions. This suggests that the symmetry-induced population oscillation can only occur for a narrowly defined angular dissociation path. Model calculations also explain the different structure of the spin-orbit doubled low N band heads for each v(OH/OD) which also alternates for even and odd . The necessary conditions to observe these phenomena are discussed.

Vibronically induced decay paths from the C̃1B1-state of water and its isotopomers.

The photochemistry of the water molecule has revealed a wealth of quantum phenomena, which arise from the involvement of several coupled electronic states with very different potential energy surfaces. Most recently, dissociation from single rotational levels of its C̃(1)B1 state near 124 nm has been attributed to a vibronically coupled decay via the lower Ã-state surface, despite a large vertical energy gap of 2.8 eV. Similar conclusions have been reached for subsequent experimental data for D2O. The present paper presents further experimental data for HOD and for both the H+OD(X) and D+OH(X) products. Unlike the cases for H2O and D2O, the vibrational populations for hydroxyl products do not follow a smooth distribution with v(OH∕OD). In particular, for OH there is a clear alternation in population for all the strong peaks, with odd v favoured over even v. These experimental data are analysed using new MRCI+Q calculations, which have been used to generate potential surfaces and associated non-adiabatic matrix elements for transition from the adiabatic C̃-state to lower unbound potential surfaces; and hence, to guide dynamical calculations using time-dependent wavepackets. It is concluded that although there is a minor contribution from the C̃→ à decay route, the major route follows C̃→ (1)A2 → Ã. This is mediated through two regions of near degeneracy of the elusive (1)A2 surface with C̃ for short bonds ca. 0.8 Å; and between (1)A2 and à with long bonds ≥2 Å, thereby bridging the vertical energy gap. The striking population alternation for the D+OH(X) products is attributed to dynamic symmetry breaking on the (1)A2 surface.

Rotational State Specific Dissociation Dynamics of HOD -> H plus OD via Two-Photon Excitation to the (C)over-tilde Electronic State

The dissociation dynamics of HOD via two-photon excitation to the C̃ state have been investigated using the H-atom Rydberg tagging time-of-flight (TOF) technique. The H-atom action spectrum for the C̃ ← X̃ transition shows resolved rotational structure. Product translational energy distributions and angular distributions have also been recorded for the H + OD channel for three excited levels each with k(a)′ = 2. From these distributions, quantum state distributions and angular anisotropy parameters (β2 and β4) for the OD product were determined. These results are consistent with the nonadiabatic predissociation picture illustrated in the one-photon dissociation process for H2O. The heterogeneous dissociation pathway via Coriolis coupling is the dominant dissociation process in the present study. A high proportion of the total available energy is deposited into the rotational energy of the OD product. The anisotropic recoil distributions reveal the distinctive contributions from the alignment of the excited states and the dissociation process. Comparisons are also made between the results for HOD and H2O via the equivalent rotational transitions. The OH bond energy, D(o)(H−OD), of the HOD molecule is also determined to be 41283.0 ± 5 cm(-1).

Photodissociation of HOD via the C̃1B1 State: OD/OH Branching Ratio and OD Bond Dissociation Energy

Photodissociation of jet-cooled HOD via the C state around 124 nm has been studied using the H(D)-atom Rydberg tagging time-of-flight technique. Rotational state resolved action spectrum and the product translational energy distribution spectra have been recorded for both D+OH and H+OD dissociation channels. Product channel OH/OD branching ratios for the individual C-X rotational transition have been determined. A comparison is also given with the B-X and A-X transitions. In addition, the dissociation energy of the OD bond in HOD has been determined accurately to be 41751.3±5 cm−1.