Wachewsky et al. (J. Phys. Chem. 100, 11559 (1996)) examined the UV absorption spectrum of CH₃I, a species of interest in connection with stratospheric ozone chemistry. They found the integrated absorption coefficient to be dependent on temperature and pressure to an extent inconsistent with internal structural changes in isolated CH₃I molecules; they explained the changes as due to dimerization of a substantial fraction

of the CH₃I, a process which would naturally be pressure and temperature dependent.

(a) Compute the integrated absorption coefficient over a triangular lineshape in the range 31 250 to 34 483 cm⁻¹ and a maximal molar absorption coefficient of 150 dm³mol⁻¹ cm⁻¹ at 31 250 cm⁻¹.

(b) Suppose 1.0 per cent of the CH₃I units in a sample at 2.4 Torr and 373K exists as dimers. Compute the absorbance expected at 31 250 cm⁻¹ in a sample cell of length 12.0 cm.

(c) Suppose 18 per cent of the CH₃I units in a sample at 100 Torr and 373K exists as dimers. Compute the absorbance expected at 31 250 cm⁻¹ in a sample cell of length 12.0 cm; compute the molar absorption coefficient which would be inferred from this absorbance if dimerization was not considered.

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E/(dm³ mol-¹ cm-¹) 10 8 CO 2 0 22 30 Wavenumber, >/(10³ cm-¹) 26 34