Kaijun Yuan

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.

Photochemistry of the Water Molecule: Adiabatic versus Nonadiabatic Dynamics

Water and light are two common constituents of both the earths 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.

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.

Coordination Number of Li+ in Nonaqueous Electrolyte Solutions Determined by Molecular Rotational Measurements

The coordination number of Li(+) in acetonitrile solutions was determined by directly measuring the rotational times of solvent molecules bound and unbound to it. The CN stretch of the Li(+) bound and unbound acetonitrile molecules in the same solution has distinct vibrational frequencies (2276 cm(-1) vs 2254 cm(-1)). The frequency difference allows the rotation of each type of acetonitrile molecule to be determined by monitoring the anisotropy decay of each CN stretch vibrational excitation signal. Regardless of the nature of anions and concentrations, the Li(+) coordination number was found to be 4-6 in the LiBF4 (0.2-2 M) and LiPF6 (1-2 M) acetonitrile solutions. However, the dissociation constants of the salt are dependent on the nature of anions. In 1 M LiBF4 solution, 53% of the salt was found to dissociate into Li(+), which is bound by 4-6 solvent molecules. In 1 M LiPF6 solution, 72% of the salt dissociates. 2D IR experiments show that the binding between Li(+) and acetonitrile is very strong. The lifetime of the complex is much longer than 19 ps.

Probing Ion/Molecule Interactions in Aqueous Solutions with Vibrational Energy Transfer

Interactions between model molecules representing building blocks of proteins and the thiocyanate anion, a strong protein denaturant agent, were investigated in aqueous solutions with intermolecular vibrational energy exchange methods. It was found that thiocyanate anions are able to bind to the charged ammonium groups of amino acids in aqueous solutions. The interactions between thiocyanate anions and the amide groups were also observed. The binding affinity between the thiocyanate anion and the charged amino acid residues is about 20 times larger than that between water molecules and the amino acids and about 5-10 times larger than that between the thiocyanate anion and the neutral backbone amide groups. The series of experiments also demonstrates that the chemical nature, rather than the macroscopic dielectric constant, of the ions and molecules plays a critical role in ion/molecule interactions in aqueous solutions.

Nonstatistical spin dynamics in photodissociation of H2O at 157nm

Photodissociation of H2O via the A band at 157nm has been reinvestigated using the high resolution H atom Rydberg tagging technique. The spin-orbit population distributions were found to be highly nonstatistical for the OH(v=0,1,2) product channels, while nearly statistical for the OH(v=4) channel. These results suggest that the dissociation dynamics of H2O at 157nm is remarkably different for different vibrationally excited OH channels. The result presented here is not entirely consistent with the direct dynamical picture of the A band dissociation of H2O.

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.

Competition between direct and indirect dissociation pathways in ultraviolet photodissociation of HNCO.

Photodissociation dynamics of HNCO at photolysis wavelengths between 200 and 240 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. At low photon energy excitation, the product translational energy distribution is nearly statistical and the angular distribution is isotropic, which is consistent with an indirect dissociation mechanism, i.e., internal conversion from S1 to S0 surface and dissociation on S0 surface. As the photon energy increases, a direct dissociation pathway on S1 surface opens up. The product translational energy distribution appears to be quite nonstatistical and the product angular distribution is anisotropic. The fraction of direct dissociation pathway is determined to be 36 ± 5% at 202.67 nm photolysis. Vibrational structures are observed in both direct and indirect dissociation pathways, which can be assigned to the NCO bending mode excitation with some stretching excitation.

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.