Hans-Jürgen Deyerl

Photodissociation dynamics of the propargyl radical

The photochemistry and photodissociation dynamics of the propargyl radical, C3H3, upon UV excitation is investigated by time- and frequency-resolved detection of hydrogen atoms. From a statistical analysis of the data, we conclude that formation of cyclopropenylidene, c-C3H2, is the dominant reaction channel. Around 22% of the excess energy is released into the translational degrees of freedom. By varying the excitation energy between 265 and 240 nm, microcanonical rates for the loss of a hydrogen atom can be obtained as a function of excess energy. The experimental rates, on the order of several 106 s−1, are in good agreement with Rice–Ramsperger–Kassel–Marcus (RRKM) calculations, provided a scaling factor is used for the vibrational frequencies, to account for the effects of anharmonicity. The interpretation is confirmed in experiments using monodeuterated propargyl radicals, H2CCCD, indicating a mechanism that proceeds via an initial [1,2] H-shift, followed by cyclization.

Photodissociation dynamics of the allyl radical

The photochemistry and photodissociation dynamics of the allyl radical upon ultraviolet (UV) excitation is investigated in a molecular beam by using time- and frequency-resolved photoionization of hydrogen atoms with Lyman-α-radiation. The UV states of allyl decay by internal conversion to the ground state, forming vibrationally hot radicals that lose hydrogen atoms on a nanosecond time scale. Two channels are identified, formation of allene directly from allyl, and isomerization from allyl to 2-propenyl, with a subsequent hydrogen loss, resulting in both allene and propyne formation. The branching ratio is between 2:1 and 3:1, with direct formation of allene being the dominant reaction channel. This channel is associated with site-selective loss of hydrogen from the central carbon atom, as observed in experiments on isotopically labeled radicals. Ab initio calculations of the reaction pathways and Rice–Ramsperger–Kassel–Marcus (RRKM) calculations of the rates are in agreement with the mechanism and branching ratios. From the measured Doppler profiles a translational energy release of 14±1 kcal/mol is calculated. The calculated value of 66 kcal/mol for the barrier to the 1,2 hydrogen shift from allyl radical to 2-propenyl is confirmed by the experimental data.

Kinetics and dynamics in the photodissociation of the allyl radical

The direct observation of the products, kinetics and translational energy release from the photodissociation of the allyl radical, C3H5, upon excitation in the near-uv is reported. A statistical analysis of the data shows that they are in agreement with allene formation being the dominant H-loss reaction channel.

Transition state dynamics of the OH+H2O hydrogen exchange reaction studied by dissociative photodetachment of H3O2−

Dynamics in the transition state region of the bimolecular OH + H2O-->H2O + OH hydrogen exchange reaction have been studied by photoelectron-photofragment coincidence spectroscopy of the H3O2- negative ion and its deuterated analog D3O2-. The data reveal vibrationally resolved product translational energy distributions. The total translational energy distribution shows a vibrational progression indicating excitation of the antisymmetric stretch of the water product. Electronic structure calculations at the QCISD level of theory support this analysis. Examination of the translational energy release between the neutral products reveals a dependence on the product vibrational state. These data should provide a critical test of ab initio potential energy surfaces and dynamics calculations.

Photoelectron–photofragment coincidence study of OHF−: transition state dynamics of the reaction OH + F → O + HF

A photoelectron-photofragment coincidence (PPC) study of the dissociative photodetachment of OHF at a photon energy of 4.80 eV is presented. The correlated electron kinetic energy (eKE) and translational energy release (E(T)) into the O + HF + e- products yield information on the potential energy surface close to the transition state of the neutral reaction OH + F --> O + HF. The correlation spectrum shows two different features in the energetically allowed O + HF product channel: (a) diagonal ridges, resulting from direct dissociative photodetachment (DPD) and (b) areas with higher E(T) in the neutral fragments from nonadiabatic dissociation. The total translational energy spectrum (E(TOT) = eKE + ET) reveals a vibrationally resolved product state distribution. These results are discussed in the context of recent theoretical studies of the dissociative photodetachment of OHF-.