Picosecond and nanosecond laser flash photolysis study of some halogenated benzene derivatives

Abstract

Picosecond laser photolyses of benzyl chloride, benzyl bromide and benzyl iodide vapour have been examined by monitoring the time profiles of fluorescence and absorption of benzyl radicals formed in the 266 nm photolysis. The decays of fluorescence for these molecules were well represented by two exponential functions. The short-lifetime component has a value of ca. 1 ns, and the longer one 5–9 ns. The time-resolved absorption of benzyl radicals obtained in the photolysis of benzyl chloride (0.5 Torr) showed that the rise time of benzyl radical absorption, 6 ns, could be attributed to the decomposition rate of benzyl chloride molecules under collision-free conditions, while the rise time for benzyl radicals formed in the photolyses of benzyl bromide and benzyl iodide was ca. 1 ns. Thus the photodecomposition rate of benzyl chloride is in good correlation to the value of the slow component and those of benzyl bromide and benzyl iodide to values of short components.

The formation and collisional deactivation of the excited HFB molecules was studied directly by monitoring the ultraviolet absorption spectra of vibrationally highly excited hexafluorobenzene (HFB) molecules produced by excitation with an ArF(193 nm) laser. The absorption spectrum observed at time t= 0 is attributed to S^*₀(hot hexafluorobenzene with an internal energy of 639 kJ mol^–1). This transient absorption spectrum can be simulated as a part of the spectrum of the S₃(¹E_1u)â†� S₀ transition at 3050 K. The decay time profile of the internal energy was calculated from the observed absorption decay profile of the hot molecule using the relation between the absorbance by hot molecules and the internal energy. Thus the average energy 〈ΔE〉 transferred per collision was estimated by two different models; energy-independent and energy-dependent function for the decay of the internal energy. The values of 〈ΔE〉 obtained (table 2) indicate that the energy-dependent model may give reasonable values for 〈ΔE〉, but that the energy-independent model is less ambiguous than the energy-dependent model.