Fig. 4.

FTIR difference spectra recorded at 4 cm^− 1 resolution for BR (red) at 230 K, AR3 (green) at 230 K and AR3 (blue) at 240 K in the 3,800–3,500 cm^− 1 region. Each spectrum represents the average of at least 18 difference spectra, with each spectrum being the average of 1,000 individual scans. Spacing of Y-axis (difference absorbance) markers is 2.5 mOD for BR, and 0.15 mOD for AR3 at 230 and 240K. Note that in the AR3 difference spectrum recorded at 240 K corresponds to a mixture of the M and N intermediates. For lighting sequence used to obtain difference spectrum, see Section 3