Pickup and reactions of molecules on clusters relevant for atmospheric and interstellar processes

In this perspective, we review experiments with molecules picked up on large clusters in molecular beams with the focus on the processes in atmospheric and interstellar chemistry. First, we concentrate on the pickup itself, and we discuss the pickup cross sections. We measure the uptake of different atmospheric molecules on mixed nitric acid-water clusters and determine the accommodation coefficients relevant for aerosol formation in the Earth's atmosphere. Then the coagulation of the adsorbed molecules on the clusters is investigated. In the second part of this perspective, we review examples of different processes triggered by UV-photons or electrons in the clusters with embedded molecules. We start with the photodissociation of hydrogen halides and Freon CF₂Cl₂ on ice nanoparticles in connection with the polar stratospheric ozone depletion. Next, we mention reactions following the excitation and ionization of the molecules adsorbed on clusters. The first ionization-triggered reaction observed between two different molecules picked up on the cluster was the proton transfer between methanol and formic acid deposited on large argon clusters. Finally, negative ion reactions after slow electron attachment are illustrated by two examples: mixed nitric acid-water clusters, and hydrogen peroxide deposited on large Ar_N and (H₂O)_N clusters. The selected examples are discussed from the perspective of the atmospheric and interstellar chemistry, and several future directions are proposed.

Matrix Isolation Spectroscopy and Nuclear Spin Conversion of Propyne Suspended in Solid Parahydrogen

Parahydrogen (pH₂) quantum solids are excellent matrix isolation hosts for studying the rovibrational dynamics and nuclear spin conversion (NSC) kinetics of molecules containing indistinguishable nuclei with nonzero spin. The relatively slow NSC kinetics of propyne (CH₃CCH) isolated in solid pH₂ is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm^-1 region. Detailed analyses of a variety of parallel (ΔK = 0) and perpendicular (ΔK=±1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but K rotation of the methyl group around the C₃ symmetry axis still persists. However, this single-axis K rotation is significantly hindered for propyne trapped in solid pH₂ such that the energies of the K rotational states do not obey simple energy-level expressions. The NSC kinetics of propyne follows first-order reversible kinetics with a 287(7) min effective time constant at 1.7 K. Intensity-intensity correlation plots are used to determine the relative line strengths of individual ortho- and para-propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective A″ constant of propyne.