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Without the need to construct complicated potential energy surfaces, a multicenter impulsive model is developed to characterize the dynamical feature of conventional transition state (TS) and roaming pathways in the photodissociation of formaldehyde, H2CO → CO + H2. The photofragment energy distributions (PED) resulting from the roaming mechanism are found to closely correlate to a particular configuration that lies close to the edge of the plateau-like intrinsic reaction coordinate, whereas such a PED is associated with the configuration at the saddle point when the conventional TS pathway is followed. The evaluated PED results are consistent with those by experimental findings and quasi-classical trajectory calculations. Following impulsive analysis, the roaming pathway can be viewed as a consequence of energy transfer events between several vibrational modes. For H2CO, the available energy initially accumulated at the C-H bond is transferred to other transitional mode(s) via stretching-bending coupling, and finally to the HH stretching.

Time-resolved Fourier transform infrared emission spectroscopy is employed in the photolysis of propionaldehyde (CH3CH2CHO) at 248 nm to characterize the role of the roaming pathway. High-resolution spectra of CO are analyzed to yield a single Boltzmann rotational distribution for each vibrational level (ν = 1-4) with small rotational and large vibrational energy disposals. A roaming saddle point is found containing two far separated moieties of HCO and CH3CH2 with a weak interaction between them. Quasiclassical trajectory calculations on this configuration yield the CO energy flow behavior, consistent with the findings. The rate constant along the roaming pathway is evaluated to be larger by >1-2 orders of magnitude than those along tight transition state or three-body dissociation pathways. This work implies that the roaming mechanism plays an increasingly important role in aliphatic aldehydes as the molecular size becomes larger.

The behavior of Λ-doublet resolved rotational energy transfer (RET) by Ar collisions within the SH(X(2)Π, v''=0) state is characterized. The matrix elements of terms in the interaction potential responsible for interference effects are calculated to explain the propensity rules for collision-induced transitions within and between spin-orbit manifolds. In this manner, the physical mechanisms responsible for the F(1)-F(1), F(2)-F(2), and F(1)-F(2) transitions may be reasonably identified. As collision energy increases, the propensity for collisional population of the final e or f level is replaced by the e/f-conserving propensity. Such a change in propensity rule can be predicted in terms of energy sudden approximation at high J limit for the pure Hund's case scheme., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The rotational energy transfer (RET) by Ar collisions within the SH X (2)Π (v'' = 0, J'' = 0.5-10.5) state is characterized. The integral cross sections as a function of collision energy for each rotational transition are calculated using a quantum scattering method in which the constructed potential energy functions are based on a ground state potential energy surface (PES) reported previously. On the other hand, a laser-induced excitation fluorescence technique is employed to monitor the relaxation of the rotational population as a function of photolysis-probe delay time following the photodissociation of H(2)S at 248 nm. The rotational population evolution is comparable to its theoretical counterpart based on calculated Λ-resolved RET rate constants. The propensity in Λ-resolved RET transitions is found to approximately resemble the case of OH(X (2)Π, v'' = 0) + Ar. The Λ-averaged RET collisions are also analyzed and result in several propensity rules in the transitions. Most propensity rules are similar to those observed in the collisions of SH(A (2)Σ(+)) by Ar. However, the behavior of the conserving ratio, defined as rate constants for spin-orbit conserving transition divided by those for spin-orbit changing transition, shows distinct difference from those described by Hund's case (b)., (© The Owner Societies 2011)

Br(2) molecular elimination is probed in the photodissociation of 1,1- and 1,2-C(2)H(4)Br(2) isomeric forms at 248 nm by using cavity ring-down absorption spectroscopy. Their photodissociation processes differ markedly from each other. The quantum yield of the Br(2) fragment in 1,2-dibromoethane is 0.36+/-0.18, in contrast to a value of 0.05+/-0.03 in 1,1-dibromoethane. The vibrational population ratios of Br(2)(v=1)/Br(2)(v=0) are 0.8+/-0.1 and 0.5+/-0.2 for 1,2- and 1,1-dibromoethanes, respectively. The Br(2) yield densities are found to increase by a factor of 35% and 190% for 1,2- and 1,1-dibromoethanes within the same temperature increment. In the ab initio potential energy calculations, the transition state (TS) along the adiabatic ground state surface may correlate to the Br(2) products. The TS energy for 1,2-dibromoethane is well below the excitation energy at 483 kJ/mol, whereas that for 1,1-dibromoethane is slightly above. Such a small TS energy barrier impedes the photodissociation of the ground state 1,1-dibromoethane such that the production yield of Br(2) may become relatively low, but rise rapidly with the temperature. The TS structure shows a larger bond distance of Br-Br in 1,2-dibromoethane than that in 1,1-dibromoethane. That explains why the former isomer may result in hotter vibrational population of the Br(2) fragments.

An experimental and theoretical investigation of rotational energy transfers (RET) of CH involving the B (2)Sigma(-) (v=0, 0