Cyclohexane pulse radiolysis

Hummel, A. and Luthjens, L. H., Ionization in the track of a high energy electron in liquid cyclohexane Pulse radiolysis of solutions of biphenyl in cyclohexane, /. Chem. Phys., 59, 654, 1973. 

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

Additional experiments were done in mixtures of alcohol alkane [16,17]. The spectra and kinetics were measured in mixtures of 1-propanol n-hexane. Some experiments were done in cyclohexane, where the behavior was qualitatively similar however, the exact concentration where spectra and kinetics changed depended on the alkane [16]. Additional experiments observed the shift of the final spectrum of the solvated electron in supercritical ethane-methanol mixtures. These experiments were done using standard pulse radiolysis techniques and thus we were unable to observe the kinetics [19]. 

Early pulse radiolysis studies of alkanes at room temperature showed that the solvated electron absorption begins around 1 pm and increases with increasing wavelength to 1.6 pm for -hexane, cyclohexane, and 2-methylbutane [77]. More complete spectra for three liquid alkanes are shown in Fig. 4. The spectrum for methylcyclohexane at 295 K extends to 4 pm and shows a peak at 3.25 pm [78]. At the maximum, the extinction coefScient is 2.8 x 10 cm The spectrum for 3-methyloctane at 127 K, shown in Fig. 4, peaks around 2 pm. The peak for methylcyclohexane is also at 2 pm at lower temperature. Recently, the absorption spectra of solvated electrons in 2-methylpentane, 3-methylpentane, cA-decalin, and methylcyclohexane glasses have been measured accurately at 77 K [80]. For these alkanes, the maxima occur at 1.8 pm, where the extinction coefScient is 2.7 x 10 cm. ... 

To the best of our knowledge, pulse radiolysis transient absorption studies of neat CS2 have not been reported. CS2 anion in 0.1 M cyclohexane and 0.1 M THF solutions appears as a single 275-nm peak [35] there is no charge-resonance band that can be attributed to the dimer anion, at early (< 10 nsec) or later times. 

The absorption spectrum observed in the pulse radiolysis of solid films of polystyrene is shown in Figure 5. The absorption spectrum around 540 nm is also very similar to the absorption spectrum of polystyrene excimer observed in irradiated polystyrene solutions in cyclohexane as reported previously (2,3). The absorption with the maximum at 410 nm was reported previously and was assigned to anionic species (13,14). The longer life absorptions, attributed to triplet excited polystyrene repeat units and nonidentifiable free radicals, were observed in a wave length region < 400 nm. The absorption spectrum of CMS films obtained in pulse radiolysis showed a peak around 320 nm and a very broad absorption around 500 nm as shown in Figure 6. 

CMS and Polystyrene Solutions in Cyclohexane. Both monomer and excimer fluorescences were observed in the pulse radiolysis of polystyrene solution in cyclohexane. The decay curves of monomer and excimer fluorescences at 287 and 360 nm are shown in Figures 7(a) and (b), respectively. Energy migration on the polymer chain has been discussed elsewhere (15). The dependences of the decay of monomer fluorescence and the rise of excimer fluorescence on the... 

Figure 7. The decay curves obtained from pulse radiolysis of polystyrene solution in cyclohexane (a) monomer and (b) excimer fluorescence monitored at 287 nm and 360 nm, respectively.

The absorption band around 520 nm is very similar to that of polystyrene excimer (2,3,5). The decay follows first order kinetics with a lifetime of 20 ns. The decay rate agrees with that of the excimer fluorescence and excimer absorption. The longer life absorptions, attributed to the triplet states and free radicals (2,5), were observed at wave lengths <400 nm, although the anionic species of polystyrene with the absorption maximum at 410 nm as seen in solid films (cf. Figure 5) was not observed. Figure 9 shows the absorption spectrum observed in the pulse radiolysis of CMS solution in cyclohexane. 

Figure 10 shows the absorption observed in the pulse radiolysis of benzyl chloride solutions in cyclohexane using the same radiolysis system. The... 

Figure 8. Transient absorption spectrum obtained by pulse radiolysis of 200 mM polystyrene solution in cyclohexane.

Reaction Scheme of CMS Resists. The transient absorption spectrum shown in Figure 6 and observed for irradiated CMS films is mainly composed of two components as based on pulse radiolysis data of solid films of CMS and polystyrene, and CMS and polystyrene solutions in cyclohexane, chloroform, and carbon tetrachloride. An absorption with a maxima at 320 nm and 500 nm as due to the charge transfer radical-complex of the phenyl ring of CMS and chlorine atom (see Figure 14) and an absorption with maxima at 312 and 324 nm is due to benzyl type radicals (see Figure 11). 

Hummel and Luthjens [398] formed electron—cation pairs in cyclohexane by pulse radiolysis. With biphenyl added to the solvent, biphenyl cations and anions were formed rapidly on radiolysis as deduced from the optical spectra of the solutions. The optical absorption of these species decreased approximately as t 1/2 during the 500 ns or so after an 11ns pulse of electrons. The much lower mobility of the molecular biphenyl anion (or cation) than the solvated electron, es, (solvent or cation) increases the timescale over which ion recombination occurs. Reaction of the solvated electron with biphenyl (present in a large excess over the ions) produces a biphenyl anion near to the site of the solvated electron localisation. The biphenyl anion can recombine with the solvent cation or a biphenyl cation. From the relative rates of ion-pair reactions (electron-cation, electron—biphenyl cation, cation—biphenyl anion etc.), Hummel and Luthjens deduced that the cation (or hole) in cyclohexane was more mobile than the solvated electron (cf. Sect. 2.2 [352, 353]). 

Chemical evidence for the existence of electrons in irradiated cyclohexane was obtained from pulse radiolysis studies of solutions containing aromatic solutes27. Because of the lifetime of the pulse these experiments only allowed the determination of ions still surviving after 10 6 sec. With benzophenone and anthracene as scavengers transient absorption peaks at 700 nm and 730 nm respectively, were obtained. These were consistent with the known spectra of the benzophenone and anthracene radical ions and are most simply accounted for by assuming direct electron capture by these solutes. Positively charged ion radicals may also be produced since these are likely to have similar spectra. Ion yields can be calculated since the absorption coefficients are known, but these yields necessarily represent the sum of the positive and negative ion yields. Some results are shown in Fig. 3. 

Du, Y., Wu, J., Li, F., Liu, A. 1999. A pulse radiolysis study on energy transfer mechanism in trioctylphosphineoxide-cyclohexane and tributylphosphate-cyclohexane systems by using benzophenone as a probe. Radiat. Phys. Chem. 54 455 161. 

The pulse radiolysis studies of liquid alkanes have relevance to the radiolysis of polyethylene and related polymers. In liquid alkanes at ambient temperature, the reaction intermediates such as alkane radical-cations, olefin radical-cations, olefine dimer-cations, excited states, and alkyl radicals have been observed after the electron-pulse irradiation [90-93]. According to the nanosecond and subnanosecond studies by Tagawa et al., the observed species were alkane radical cations, excited states, and alkyl radicals in n-dodecane excited states and cyclohexyl radical were observed in cyclohexane, and only radicals in neopentane [91, 93]. Olefin radical-cations were also detected in cyclohexane containing carbon tetrachloride [92],... 

Electron Beam Resist Reactions of CMS. The lifetime of the excimer fluorescence of CMS observed in pulse radiolysis of CMS solutions in cyclohexane and tetrahydrofuran (THF) is almost independent of chloromethylation ratio from 0% to 24%. The intensity of the excimer fluorescence decreases with increasing degree chloromethylation indicating that the precursor of the excimer is scavenged by the chloromethylated part of CMS. In this case, an electron (quasi-free electron in cyclohexane and solvated electron in tetrahydrofuran, which are the precursors of the excimer), is scavenged by the chloromethyl group. The excited singlet state... 

The absorption due to the substituted benzyl type polymer radical, Pi is observed in pulse radiolysis of CMS solutions in cyclohexane and THF. An electron in cyclohexane or THF reacts with CMS resulting in formation of Pi and Cl by dissociative electron capture (reaction (10)). 

Tagawa S, Katsumura Y, Tabata Y. (1979) The ultra-fast process of picosecond time-resolved energy transfer in liquid cyclohexane by picosecond single-pulse radiolysis. Chem Phys Lett 64 258-260. 

Trifimac AD, Sauer MC, Jonah CF. (1985) Pulse-radiolysis of cyclohexane The identity of the high-mobiKty positive ion. Chem Phys Lett 113 316-319. 

---