Vibronic Structure of the 3s and 3p Rydberg States of the Allyl Radical

Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

Resistively heated silicon carbide microreactors are widely applied as continuous sources to selectively prepare elusive and reactive intermediates with astrochemical, catalytic, or combustion relevance to measure their photoelectron spectrum. These reactors also provide deep mechanistic insights into uni- and bimolecular chemistry. However, the sampling conditions and effects have not been fully characterized. We use cation velocity map imaging to measure the velocity distribution of the molecular beam signal and to quantify the scattered, rethermalized background sample. Although translational cooling is efficient in the adiabatic expansion from the reactor, the breakdown diagrams of methane and chlorobenzene confirm that the molecular beam component exhibits a rovibrational temperature comparable with that of the reactor. Thus, rovibrational cooling is practically absent in the expansion from the microreactor. The high rovibrational temperature also affects the threshold photoelectron spectrum of both benzene and the allyl radical in the molecular beam, but to different degrees. While the extreme broadening of the benzene TPES suggests a complex ionization mechanism, the allyl TPES is in fact consistent with an internal temperature close to that of the reactor. The background, room-temperature spectra of both are superbly reproduced by Franck-Condon simulations at 300 K. On the one hand, this leads us to suggest that room-temperature reference spectra should be used in species identification. On the other hand, analysis of the allyl iodide pyrolysis data shows that iodine atoms often recombine to form molecular iodine on the chamber surfaces. Such sampling effects may distort the chemical composition of the scattered background with respect to the molecular beam signal emanating directly from the reactor. This must be considered in quantitative analyses and kinetic modeling.

Velocity map imaging study of the photodissociation dynamics of the allyl radical

The photodissociation of the allyl radical (CH₂[double bond, length as m-dash]CH-CH₂˙) following excitation between 216 and 243 nm has been investigated employing velocity map imaging in conjunction with resonance enhanced multiphoton ionization to detect the hydrogen atom and CH₃(ν = 0) produced. The translational energy distributions for the two fragments are reported and analyzed along with the corresponding fragment ion angular distributions. The results are discussed in terms of the different reactions pathways characterizing the hydrogen atom elimination and the minor methyl formation. On one hand, the angular analysis provides evidence of an additional mechanism, not reported before, leading to prompt dissociation and fast hydrogen atoms. On the other hand, the methyl elimination channel has been characterized as a function of the excitation energy and the contribution of three reaction pathways: single 1,3-hydrogen shift, double 1,2-hydrogen shift and through the formation of vinylidene have been discussed. Contrary to previous predictions, the vinylidene channel, which plays a significant role at lower energies, seems to vanish following excitation on the E[combining tilde]²B₁(3p_x) excited state at λ≤ 230 nm.

Femtosecond dynamics of the 2-methylallyl radical: A computational and experimental study

We investigate the photodynamics of the 2-methylallyl radical by femtosecond time-resolved photoelectron imaging. The experiments are accompanied by field-induced surface hopping dynamics calculations and the simulation of time-resolved photoelectron intensities and anisotropies, giving insight into the photochemistry and nonradiative relaxation of the radical. 2-methylallyl is excited at 236 nm, 238 nm, and 240.6 nm into a 3p Rydberg state, and the subsequent dynamics is probed by multiphoton ionization using photons of 800 nm. The photoelectron image exhibits a prominent band with considerable anisotropy, which is compatible with the result of theory. The simulations show that the initially excited 3p state is rapidly depopulated to a 3s Rydberg state, from which photoelectrons of high anisotropy are produced. The 3s state then decays within several 100 fs to the D₁ (nπ) state, followed by the deactivation of the D₁ to the electronic ground state on the ps time scale.