The Laser Two-photon Photolysis of Liquid Carbon Tetrachloride†

Abstract

The two-photon photolysis of liquid CCl4 with 25 ps pulses of 266 nm light has been studied and compared with similar studies with high energy radiation. Both neutral and ionic species are produced from excited states and ionization. The emphasis of the study is on the ionic processes, while some data related to excited states and free radicals are presented. In both radiolysis and photolysis, a solvent separated charged pair, CCl3+ // Cl-, exhibiting a lambda(max) at 475 nm, is observed that exhibits a total growth over 38 to 100 ps. Solutes with ionization potentials less than that of CCl4 (11.47 eV) reduce the yield of the 475 nm species producing radical cations of the solute. The efficiency of this process is about 10-fold larger in radiolysis compared with photolysis. Analysis of the data suggest that the lower energy of two-photon photolysis produces a charge pair CCl4+ // CCl4-, which decays in about 3 ps to CCl4+ // Cl-. This species then decays to CCl3+ // Cl-. The lifetime of the growth of the 475 nm is measured as 46 ps. These studies clearly show areas where radiolysis and photolysis can be quite similar and also areas where the vast difference in excitation energy introduces stark differences in the observed radiation and photoinduced chemistry.

Photoisomerization and Photoinduced Reactions in Liquid CCl4 and CHCl3

“…Zhang and Thomas photoexcited CCl4 at 266 nm and monitored the rise of absorption at a single wavelength of 532 nm: a single-exponential fit to their data gave a time constant of 46 ps. 22 A similar analysis of the visible band growth from our measurements shown in Fig. 3, which benefit from superior time resolution and signal-tonoise ratio, gives an exponential rise time constant of 37 ± 2 ps.…”

“…that two-photon photolysis of CCl4 at 266 nm gave the same broad visible absorption band as radiolysis, and retained the assignment to the CCl3 + ||Cl -ion pair. 22 In these experiments, it was recognized that the two-photon absorption at 266 nm corresponds to a total energy of 9.34 eV, which is significantly lower than the gas phase ionization energy (IE) of CCl4 (11.47 eV 40 ). This IE might be reduced in the liquid, with some reports arguing a reduction of as much as 1.5 eV.…”

“…here by CCl3 + ||Cl -, [20][21][22] or a charge-transfer complex CCl3 + Cl -, 23 although the mechanisms of formation are suggested to differ for the two excitation methods. On the basis of evidence from the observed kinetics of growth of the spectral band, and with the support of electronic structure calculations, we propose an assignment to an isomeric form of CCl4, denoted by isoCCl3-Cl.…”

Higher Excited States in Multiphoton Photochemical Reactions “Hint” Toward Rapid Chemistry

“…The prominent feature is ionization of the liquid, both in radiolysis and two-photon excitation. A detailed study 4 shows the same general principles by both methods of excitation, but the kinetics of the observed ions differ in the two methods of excitation. For example, twophoton excitation of CCl 4 produces the solvent-separated ion pair, CCl 3 + ∥Cl − .…”

“…For example, two-photon excitation of CCl 4 produces the solvent-separated ion pair, CCl 3 + ∥Cl – . This species is formed following reactions and The yield of CCl 3 + ∥Cl – is reduced by addition of molecules of lower ionization potential, for example, C 6 H 12 .…”