Abstract

In condensed media, chemical reactions can be initiated with a great deal of geometric specificity, since the forces holding the molecules in place ensure that the reactants are aligned and oriented relative to one another. Unfortunately, the surrounding medium is in constant interaction with the reactive site and no signature characteristic of the eigenstates of isolated product species can be obtained. On the other hand, binary encounters in the gas phase are especially well suited for detecting nascent excitations. However, with isotropic samples the initial conditions are unbiased so that all angles and impact parameters are present. To exploit the virtues of each of these environments, we use weakly bound van der Waals-type complexes as precursors for studying elementary processes. A prototypical example is CO₂ HBr, where the potential is minimum with the nuclei along a straight line. These complexes are prepared at 2K by supersonic expansion, and photodissociation of the HBr constituent propels the H-atom toward the CO₂ with initial alignment, etc., determined mainly by zero-point fluctuations of the complex. Nascent products (e.g., OH, CO) can be detected by standard spectroscopic methods before collisions become troublesome. The technique can be used to peruse the system’s ability to avoid low energy channels (e.g., with an SCOHX precursor, can OH + CS be favored over SH + CO?), as well as provide stringent tests for different kinds of calculation.

© 1986 Optical Society of America