Simplicity Beneath Complexity: Counting Molecular Electrons Reveals Transients and Kinetics of Photodissociation Reactions

Time-resolved pump-probe gas-phase X-ray scattering signals, extrapolated to zero momentum transfer, provide a measure of the number of electrons in a system, an effect that arises from the coherent addition of elastic scattering from the electrons. This allows to identify reactive transients and determine the chemical reaction kinetics without the need for extensive scattering simulations or complicated inversion of scattering data. We examine the photodissociation reaction of trimethylamine and identify two reaction paths upon excitation to the 3p state at 200 nm: a fast dissociation path out of the 3p state to the dimethyl amine radical (16.6±1.2 %) and a slower dissociation via internal conversion to the 3s state (83.4±1.2 %). The time constants for the two reactions are 640±130 fs and 74±6 ps, respectively. Additionally, it is found that the transient dimethyl amine radical has a N-C bond length of 1.45±0.02 Å and a C-N-C bond angle of 118°±4°.

Carbon dioxide photolysis from 150 to 210 nm: singlet and triplet channel dynamics, UV-spectrum, and isotope effects

We present a first principles study of the carbon dioxide (CO2) photodissociation process in the 150- to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + singlet channel threshold at ~167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ~0.5 nm without scaling the intensity. The observed structure in the 150- to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the and potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. The cross-sections of individual CO2 isotopologues ((12)C(16)O2, (12)C(17)O(16)O, (12)C(18)O(16)O, (13)C(16)O2, and (13)C(18)O(16)O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on mass-independent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of (17)O containing CO2 more efficient.

Observation of NH X(3)Σ(-) as a Primary Product of Methylamine Photodissociation: Evidence of Roaming-Mediated Intersystem Crossing?

3+1 Resonance-enhanced multiphoton ionization and photofragment excitation spectroscopy have been used to identify NH X(3)Σ(-) as a primary product of methylamine photodissociation after state-specific excitation to the S1 state. On the basis of standard thermochemical data, NH X(3)Σ(-) can be formed only in conjunction with closed-shell CH4 coproducts, indicating that dissociation must occur on the T1 surface. It is proposed that the mechanism for the formation of triplet NH and CH4 involves intramolecular abstraction between frustrated radical products and is an example of roaming-mediated intersystem crossing.