Randall S. Urdahl

An experimental determination of the heat of formation of C2 and the CH bond dissociation energy in C2H

Abstract The rotational cutoff levels of C 2 (B′ 1 Σ + g , ν″ = 1,2), produced in the multiphoton dissociation of jet-cooled acetylene at 193 nm, have been measured using laser-induced fluorescence. This data is used to calculate the values Δ H f,0 (C 2 ) = 194.8 ± 0.5 kcal/mol and D 0 (CCH) = 112.0 ± 0.8 kcal/mol, in excellent agreement with the latest theoretical calculations.

Implications of C2H photochemistry on the modeling of C2 distributions in comets

Laboratory studies of the secondary photolysis of the C2H radical are summarized and used to explain some discrepancies between models of C2 emission in comets. These studies show that several states of the C2 radicals produced in the photolysis of C2H2 at 193 nm have bimodal rotational distributions when plotted as a Boltzmann diagram. They also establish that the C2 radicals are formed with varying degrees of vibrational excitation, so that if they are formed in a similar manner in comets, the C2 radicals must start out with this initial vibrational excitation.

Laser-induced flourescence studies of C3 formation and isomerization in the 193 nm photolysis of allene and propyne

Abstract The quantum state distributions of the C3 radical from the UV multiphoton dissociation of allene and propyne in a pulsed molecular beam have been measured. The observed distributions for both molecules are identical and are much colder than a phase space model would predict, implying that direct dissociation is occurring in one step and isomerization in another. It is concluded that both allene and propyne undergo internal conversion from the S1 state to the vibrationally excited S0 ground state of allene followed by molecular elimination of H2. The observed C3 is then produced by molecular elimination of H2 from the secondary photolysis of C3H2.

Detection of C2(B’ 1Σ+g) in the multiphoton dissociation of acetylene at 193 nm

The first observation of C2(B’ 1Σ+g) in the 193 nm photodissociation of jet‐cooled acetylene is reported. Nascent C2(B’ 1Σ+g, v‘=0,1,2) is detected using laser‐induced fluorescence via the D 1Σ+u state.

Non-adiabatic interactions in excitedC 2 H molecules and their relationship toC 2 formation in comets

A unified picture of the photodissociation of theC2H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC2 are formed during the photodissociation of theC2H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC2 radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC2 fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC2H radical. The photochemically active states are the 22∑+, 22II, 32II, and 32∑+, with photodissociation rate constants of 1.0×10−6, 4.0×10−6, 0.7×10−6, and 1.3×10−6s−1, respectively. These rate constants lead to a total photochemical lifetime of 1.4×105 s.