Abstract
Heterogeneous chemistry of atmospheric compounds on large clusters and nanoparticles is investigated in relevance to aerosol chemistry in the Earth's atmosphere, and to astrochemistry on interstellar ice-grains. In our experiment, individual molecules are adsorbed on nanoparticles as they fly in a molecular beam through a pickup cell, and the molecules coagulate to clusters on the nanoparticles.
Subsequent excitation by UV photons and positive or negative ionization by electrons trigger intracluster reactions, and their products are probed by a mass spectrometry. In the present experiment, we pickup molecular oxygen O2 on large argon clusters, and ionize the clusters positively by 70 eV electron or negatively via a low-energy electron attachment at energies below 10 eV.
We observe series of mixed Arx.O2y clusters in both negative and positive mass spectra. In addition, the electron attachment produces a rich spectrum of negatively charged ions including molecular oxygen (O2)n- clusters, mixed cluster series of Ar.O2n-, Arn.O2-, Arn.O3-.
The most striking observation was Arn- cluster series, which could not be generated directly after electron attachment to pure argon clusters. It seems that presence of oxygen can stabilize the negative charge on pure Ar nanoparticles from which the oxygen is then evaporated.
This interesting phenomenon will be discussed. Pickup and reactions of other atmospherically relevant molecules, e.g. hydrogen peroxide, on Ar and other, e.g. water ice, nanoparticles will be presented too.