Shengrui Yu

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.

Photodissociation dynamics of acetylene via the (c)over-tilde (1)pi(u) electronic state

Photodissociation of acetylene has been studied using the H-atom Rydberg tagging time-of-flight technique at two excitation wavelengths (148.35 and 151.82 nm) in the vacuum ultraviolet region. Product translational energy distributions have been obtained from the H-atom time-of-flight spectra. Experimental results indicate that the C(2)H product is mainly populated in the A state. Clear trans-bend nu(2) and C-C stretch nu(3) vibrational progressions of the C(2)H(A) product in the product internal energy distribution were observed. The anisotropy parameter obtained from experiment is clearly translational energy dependent for both photolysis wavelengths. The anisotropy parameters at the two photolysis wavelengths were also found to be significantly different from each other, suggesting different dissociation dynamics for the two photolysis wavelengths.

Vacuum Ultraviolet Photodissociation Dynamics of Isocyanic Acid: The Hydrogen Elimination Channel

Photodissociation dynamics of the H-atom channel from HNCO photolysis between 124 and 137 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. Two dissociation channels, H + NCO (X(2)Π) and H + NCO(A(2)Σ(+)), have been observed. The former channel involves two different dissociation pathways; one is a slow predissociation pathway through internal conversion from the excited state to the S0 state, and the other is a fast predissociation pathway through internal conversion from the excited state to the S1 state. The latter channel dominates by a prompt dissociation via coupling to the S2 state. As the photon energy increases, dissociation on the ground state S0 becomes dominant. Vibrational structures are observed in both the NCO(X) and NCO(A) channels, which can be assigned to the bending mode excitation with some stretching vibrational excitation.

Photodissociation dynamics of C4H2 at 164.41 nm: Competitive dissociation pathways

Photodissociation dynamics of C4H2 at 164.41 nm through the Rydberg state R((1)Σu) have been studied using the high-resolution H atom Rydberg tagging technique. Experimental evidences show that two different predissociation pathways exist to form the ground C4H (X(2)Σ(+)) and electronically excited C4H (A(2)Π) products: the former has statistical and isotropic translational energy distribution through internal conversion (IC) to the ground state, while the latter has non-statistical and anisotropic translational energy distribution through IC to the excited repulsive state. The vibrational progressions of C4H (A(2)Π) products have also been observed and assigned.

State-to-state differential cross-sections for the reactive scattering of H*(n) with o-D2

Full quantum-state resolved differential cross-sections of the H*(n) + o-D2 → HD + D*(n′) reaction have been measured for the first time using the Rydberg H-atom time-of-flight method. Experimental results show that the angular distributions of HD product rotational states show a strong preference for forward scattering. This result is considerably different to that predicted by full quantum mechanical calculations on the corresponding ion–molecule reaction, suggesting that the ionic core and Rydberg electron coupling cannot be neglected in the Rydberg H-atom reactive scattering with D2 and, therefore, that the Fermi independent-collider model is not valid in describing the dynamics of Rydberg atom reactions with molecules.