Quantum efficiencies scission

Experimental determination of the quantum efficiency of photosensitive polymers of the sort that are used in one-component positive resist systems is a more complex experimental undertaking. Here the quantum efficiency is defined as the number of main chain scissions that occurs per photon absorbed. Guillet and coworkers at the University of Toronto have... 

As we mentioned before, G(s) is a structure dependent constant that describes the number of scissions per unit absorbed dose and in that sense can be compared to a photochemical quantum efficiency. The G(s) of a radiation-sensitive polymer is a figure of merit that can be used in comparing one material with another. There is a very high correlation between G(s) values to gamma radiation (the radiation most commonly used for determining G(s)) and high sensitivity for lithographic materials used in either electron beam, ion beam or x-ray exposure. 

The radiation chemical yields are expressed in terms of G-values. G(scission), G(s), equals the number of main chain scissions produced per 100 eV of energy absorbed and G (cross-linking), G(x), the number of crosslinks formed per 100 eV absorbed. The G-value is a structure dependent constant similar to quantum efficiency in photochemistry. 

Fig. 4, Wavelength sensitivity of the estimated quantum efficiency of chain scission in polyfmethyl methaciylate) exposed to near-monochromatic radiation. Based on data in [16]...

This catalysis is normally a disadvantage but can be used to accelerate degradation. Further, hydroperoxides are very sensitive to photolysis at solar wavelengths. The extinction coefficients are very low, but the quantum efficiency for scission is close to 1. Hydroperoxide photolysis can be sensitized by pigments. 

This leads to a chain scission without producing radicals. The quantum efficiency is much higher than for the Type I reaction (typically aroimd 0.02) but is still low in polymers because of the constraints on formation of the six-membered transition state. It has been shown that ketone groups in polymers are pho-tolyzed almost exclusively by the Norrish II mechanism, whereas aldehydes photolyze almost completely by the Norrish I mechanism (114). Since aldehydes do not normally survive in oxidative conditions, being rapidly oxidized to acid, the Norrish II mechanism is more important. 

This gives a chain scission without producing radicals. The quantum efficiency is much higher than for the Type I reaction (typically around 0.02) but is still low in polymers because of the constraints on formation of the six-membered transition state. 

Hydroperoxides have very low extinction coefficients in the solar UV range, much lower than carbonyl groups. However, the photolysis of a hydroperoxide leads to scission of the 0-0 bond with a quantum efficiency of at least unity and sometimes greater. Thus hydroperoxide photolysis is an efficient initiator of photo-oxidation. 

Both reactions are responsible for the chain scission of the polymer chain. At ordinary temperatures the Type II predominates and is the major cause of chain breaking. The reaction is unaffected by the mobility of the polymer molecules but does depend on the freedom of internal motion of the chain. The Type I reaction, which involves free radical intermediates, is strongly temperature dependent. At 25 °C it accounts for only about 10% of the total reaction, but at 120 °C it has quantum efficiency about the same as Type II. At this temperature the total quantum yield for photoreaction is about 0.05, the remainding energy (95% of the total energy absorbed) being dissipated as heat. 

The / -scission of the tertiary radical IX so produced provides another, potentially efficient, method of causing main-chain scission in the polymeric solid phase. Similar high quantum yields for the Norrish type I process were... 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

A number of workers have looked at the effect of photooxidation and photodynamic sensitizers on DNA. Rose Bengal photosensitizes strand breaks in double-stranded, supercoiled, pBR322 DNA the effect follows first-order kinetics with respect to light fluence and dye concentration. The reaction is substantially more efficient in the absence of oxygen, but the quantum yield of strand breaks in air is only 10 8. The results are consistent with the initiation of chain scission by Rose Bengal triplet, with some additional mechanism coming into play in the presence of oxygen. 

Isopropyl chloride and (erf-butyl chloride were studied to determine the influence, on the primary process, of increasing alkyl substitution on the a-carbon atom. The data listed in Table V show that the efficiency of the isotopically specific primary step gradually decreases as the alkyl substitution on the a-carbon atom increases (assuming that the primary quantum yield for C-Cl bond scission remains at unity). 

---