The photolytic cage effect is studied over a wide density range of several bath gases in the laser flash photolysis of iodine at 694.3 nm. Particular attention is paid to measurements in the ‘‘simple’’ solvents Xe and Kr, which are compared with molecular dynamics calculations. Measurements in polyatomic gases and liquids are also reported, as well as the variation of the cage effect with temperature and applied magnetic field. While measuring the cage effect under such a wide variety of conditions, we simultaneously obtain the second order rate constants for iodine atom combination, which help to establish the detailed mechanism of this elementary chemical reaction.

lnstitut de chimie minerale et Reaction of dithiophosphinate salts with hydrated ThCI, in the ratio 4 : 1 in ethanolic solution yields the complexes [Th(S,PR,),] where R = Me, Et, Pr', Ph. CsH11, OEt, or OPr'. The air-stable complexes have been characterised by their elemental analyses, i.r., and n.m.r. spectra, and the crystal structures have been determined for R = Me and C6H11. Crystals of [Th(S,PMe,),] are tetragonal, space group P4,2,2, with a = 10.521 (1) and c = 11.936(1) A (R = 0.030). Crystals of [Th{S,P(C,H,,),),] are monoclinic, space group C2/c, with a = 42.521 (5), b = 11.754(2), c = 23.945(2) A, and p = 100.285(8)" (R = 0.048). The thorium atoms are surrounded by eight sulphur atoms (av. Th-S = 2.910 A) in a dodecahedra1 arrangement (the methyl compound having crystallographic 222 symmetry). In both cases the M(L-L), mmmm isomer is formed.

The molecule 1,1,2-trifiuroethane (TFE) was used in experiments to determine the population distribution of excited molecule3 produced by infrared multiphoton absorption induced by high power TEA C0 2 1asers operating at 1079.85 cm-1 [9.6I"m R (22) line]. Optoacoustic measurements of absorbed laser power proVided a measure of the mean energy of the population distribution, wl11je very low pressure photolysis measurements of the collision-free decomposition yield gave information about the high,energy tan of the distribution. The experimental results were accurately simulated using a Master Equa.tion model that incorporated Quack's statisticaldynamical theory of infrared mUltiphoton absorption (cases B and C), RRKM unimolecuJar reactions (three channels), and collisional energy transfer. The computer simulations included known TFE mo1ec'lJlar properties and oIlly four Ildjustable pl~ra.meters, which were very highly constrained in order to fit the {lxperiInt"Iltal data.. From the simuJatio,lS, 'We conclude that the opticaJ coupling matrix elements are dnunaHcaHy reduced in ma.g:llitude for energies above the reaction thresholds. This effect is symptomatic of the vibrational anllannonicity due to the presence of the reaction channels, even in molecuk>s that have not yet reacted, resulting in vibrational frequency shifts ofthe absorption lines out of resonance with the laser line. This effect is expected to be present and ohservable in oth!!r highly vibrationally excited molecules.

Infrared emission spectra were obtained for 1, 1, 2-trifluorethane (TFE) excited by infrared multiphoton absorption (1079.85 cm−1). The emission features show that the HF reaction product is formed in vibrational states up to about v=3. Furthermore, emission attributed to F–C≡C–H was observed near 3320 cm−1, indicating that the difluoroethylene primary products of TFE decomposition undergo secondary photolysis; since the difluoroethylene products at room temperature do not absorb laser light, they must be formed vibrationally excited. The emission from the C–H stretch modes of TFE was readily identified near 2980 cm−1 and the emission intensity was obtained as a function of laser fluence. These data are in excellent agreement with predictions based on the theoretical expression for fluorescence intensity and the reconstructed populations determined by the Master Equation calculations described in the preceding paper. These results provide additional support for the accuracy of the reconstructed population distributions and for the theory relating infrared fluorescence intensity to total vibrational energy in polyatomic molecules.

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