Excited states under pressure

Microwave or radio frequencies above 1 MHz that are appHed to a gas under low pressure produce high energy electrons, which can interact with organic substrates in the vapor and soHd state to produce a wide variety of reactive intermediate species cations, anions, excited states, radicals, and ion radicals. These intermediates can combine or react with other substrates to form cross-linked polymer surfaces and cross-linked coatings or films (22,23,29). 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

Because of the properties of the metal carbonyls, the experimental work required new techniques, such as the use of carbon monoxide under pressure in the laboratory, and the design of special apparatus. The results obtained always kept us in a state of excitement, thereby stimulating us to make further studies. Starting from modest beginnings it has been shown that the metal carbonyls in no way represent an isolated area of chemistry. Herein major results of our wrork have been summarized with no pretension to making a complete review. 

In an actual experiment, it is frequently not possible to work under conditions where there are no relaxation effects. The usual reason for this is that the intensity of the fluorescence becomes too weak to observe as the concentration of excited molecules is reduced. The lowest pressures which can be used are defined by a number of parameters the strength of the transition, the power of the laser and the detection efficiency of the system are among the most important. It therefore follows that, in interpreting the results of lifetime measurements, one must consider carefully the possible effects of rotational and vibrational redistribution in the excited state. In a regular unperturbed state where there is little or no change in radiative lifetime with changes in rotational and vibrational level, the effects of relaxation are not observable so long as the fluorescence is still detected with the same efficiency. However, if the excited state is perturbed, for example by predissociation, then the effects of redistribution must be carefully studied. 

Visible chemiluminescence has been observed from Ba + N20 under single collision conditions [338, 342], although the nature of the excited state of BaO is uncertain. Similarities between the observed spectra from Ba + NzO and Ba + 03 were noted [338]. Higher pressure studies show emission characteristic of BaO (A S), but it is believed that this... 

It is clear that if we are to use photolysis to study the reaction of radicals with oxygen it would be preferable to use conditions under which decomposition of the parent molecule occurs primarily from the singlet excited state. However, there is good evidence that methyl isopropyl ketone129 and 2-pentanone141 can be deactivated from the singlet excited state by sufficient pressures of biacetyl. 

Under the conditions of their experiment, a zero-pressure lifetime of 0.3 qs was measured for the vibrationally relaxed triplet state at all excitation wavelengths. Since pressures were not low enough to ensure isolated molecule conditions, it is not known from these experiments whether the strong energy dependence of the triplet lifetime observed in other carbonyls will occur in propynal. 

The association of radicals of intermediate complexity may be expected to show pressure dependence under proper conditions. Their lifetime, subject to correction for large entropy changes in the energized state and the contribution of excited states, may be predicted from Eq. (XI.3.4). The pressure range for showing third-order behavior is then predictable from Table XI.2, subject to further correction for the efficiency of deactivation. 

If one assumes that the densities of the electronically excited states, from which the observed emissions originate, are directly proportional to those of the ground state, the emission intensity profiles reflect approximately the species concentrations, especially at constant pressure. Therefore, the above results may indicate that, under the same operating conditions, almost the same quantity of CH radicals exist in CH4 and CH3OH cascade arc plasmas. From the OES spectra assignment, CH radicals are the only reactive species that could be attributed to the growth of plasma polymers in both CH4 and CH3OH cascade arc plasmas. 

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