Multiphoton dissociation of ethyl chloride at 3.3 μm: Excitation mechanism and rate equations analysis

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Abstract
Tunable 3.3 μm laser pulses were used to excite the CH stretching modes of ethyl chloride. Energy deposition vs fluence was measured optoacoustically. For 3.3 μm excitation, absorbed energy increases almost linearly with fluence, while for 10 μm excitation there is substantial saturation. Dissociation yields were measured as a function of fluence and of wavelength by gas chromatographic determination of C2H4 from C2H5Cl+nhν→C2H4+HCl. Much higher yields were observed for 3.3 μm excitation than for CO2 laser 10 μm excitation. Sharp resonances in the 3.3 μm dissociation yield spectrum match peaks in the fundamental and overtone absorption spectra. Overtone spectra show that for many rotational states the vCH=2 level (6000 cm−1) is in the quasicontinuum and that vCH=3 is always in the quasicontinuum. The resonant nature of the excitation allows the rate equations description for transitions in the quasicontinuum and continuum to be extended to the discrete levels. Absorption cross sections are estimated from ordinary IR spectra. A set of cross sections which is constant or slowly decreasing with increasing vibrational excitation gives good fits to both absorption and dissociation yield data. The much smaller dissociation yields and the saturated absorption at 10 μm reflect bottlenecking in the discrete levels. As ethyl chloride pressure is increased from 0.2 to 2 Torr the fraction dissociated decreases by nearly a factor of 2. Only about 10% further decrease occurs from 2 to 6 Torr. The rate equations model gives a similar curve when the strong collision assumption is made for vibrational relaxation and RRKM dissociation rates are used.