Practice question

Most dispersive or Fourier-transform infrared spectrometers record percent transmittance, percentage of incident light passing through sample, plotted against wavenumber. Baseline corresponds to 100 percent transmission; when molecular vibration absorbs radiation, transmitted intensity falls, creating downward trough that appears as inverted peak relative to absorption scale. Depth correlates with concentration and transition strength. This trough is not emission line from excited state luminescence, reflection point from attenuated total reflectance crystal, or random electronic noise artifact. Interpreting troughs as absorption bands allows assignment of bond types and assessment of sample purity.