Tunable vacuum ultraviolet radiation generated by four-wave mixing will be used to probe collisional energy transfer and photodissociation. Collisional relaxation of the S1D velocity distribution by rare gases has been measured to learn to what extent this simple process can be described by a hard-sphere, elastic interaction. E yielding V transfer was studied from S1D to CO and N2, both by examining the Doppler profile of the relaxed atoms and by direct measurement of the COv,J distribution. Finally, the photodissociation of O3 and N2O have been investigated by monitoring the Doppler profiles of the resulting O1D lines. This integrated program of molecular dynamics studies using vacuum ultraviolet radiation has enhanced our knowledge both of the chemical physics of these basic processes and of the interaction of high energy photons with small molecules found in the upper atmosphere. Vacuum Ultraviolet Light, Molecular Dynamics, Ozone, Nitrous Oxide, CO.