2008

Making More-Complex Molecules Using Superthermal Atom/Molecule Collisions

Atoms adsorbed on cold surfaces react with energetic impinging atoms.

A method of making more-complex molecules from simpler ones has emerged as a by-product of an experimental study in outer-space atom/ surface collision physics. The subject of the study was the formation of CO2 molecules as a result of impingement of O atoms at controlled kinetic energies upon cold surfaces onto which CO molecules had been adsorbed. In this study, the O/CO system served as a laboratory model, not only for the formation of CO2 but also for the formation of other compounds through impingement of rapidly moving atoms upon molecules adsorbed on such cold interstellar surfaces as those of dust grains or comets. By contributing to the formation of increasingly complex molecules, including organic ones, this study and related other studies may eventually contribute to understanding of the origins of life.

Image
These Plots of the CO2+ Reading of the mass spectrometer versus desorption temperature have been interpreted as signifying that CO2 had been adsorbed on the surface in the reaction O + CO →CO2. The plot labeled “Laser Off” is that of a background reading that must be subtracted from the total-reading “Laser On” reading to obtain the net CO2+-ion count from the superthermal reaction.
In the study, CO was adsorbed onto a cryo-cooled surface, then the surface was exposed to a beam of ground-state O atoms at a kinetic energy of 10 eV per atom. After an exposure time of 135 minutes, the surface was retracted from the O-atom beam into the field of view of a quadrupole mass spectrometer. The reaction products were desorbed by heating the cold surface according to a defined temperature-vs.-time schedule (temperature- programmed desorption). Desorbed molecules were ionized, then detected in the mass spectrometer. The temperature dependence of the CO2 peak in the mass-spectrometer readout (see figure) indicated that large quantities of CO2 were desorbed; this observation was taken to be evidence for the reaction O + CO(s) → CO2(s).

Generalizing the method used in this study, it may be possible, for example, to make simple and more-complex amines, even amino acids by reacting ice mixtures of CH4 and NH3 with superthermal O and H beams. In general, by choice of atomic projectiles, kinetic energies, atomic quantum states, surfaces, and exposure times, it may be possible to create new molecular species and stabilize them on the solid surfaces on which they were created.

This work was done by Brian Shortt, Ara Chutjian, and Otto Orient of Caltech for NASA’s Jet Propulsion Laboratory.
NPO-41300

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