Previous experiments on the TEA CO₂ laser multiphoton dissociation of tetramethyl-dioxetan (TMD) have been extended to low fluence levels (<20 J cm^–2) and low pressure (ca. 10^–3 Torr*). The reaction, followed by observing the accompanying luminescence, proceeds at a rate much slower than the laser pulse width. Thus direct determination of the unimolecular rate constant of this low-threshold-energy reaction (ca. 25 kcal mol^–1) is possible. The extent of activation can be estimated by comparing to rates measured on the same system using overtone excitation by Crim et al. [J. Chem. Phys., 1981, 75, 1752]. Analysis of the results using R.R.K.M. theory is constrained by this comparison, and allows in principle the derivation of vibrational population distribution of the reacting molecules. A fourfold increase in the laser fluence leads to an almost 10³ yield increase. The average energy of the reacting molecules at the low-fluence end used is between 3000 and 4000 cm^–1 above the dissociation threshold. Better defined values can probably be obtained by either calibrating the overtone-excitation results, or by isolating the emission of a single species and using rate equations.

The emission is shown to be due to at least two species: a fast-decaying component and the thermalized triplet. The former is strongly quenched by collisions, and its relative contribution to the total yield is increased upon increasing the fluence. Its emission spectrum is very broad and apparently structureless. The thermalized triplet can be observed separately by using a red cut-off filter. It is formed by a unimolecular dissociation and not (up to pressures of ca. 40 mTorr) by collisional processes. Extrapolation to low-fluence, high-pressure conditions show that the main product is the triplet, in agreement with thermal, liquid-solution results.