To determine the nature of excitons in organic solids and search for one photon forbidden optical transitions a perturbation technique is required. A large (of order 10^6 sinusoidal electric field) is applied to the sample at 10 K and the perturbed optical absorption spectrum measured using lock-in detection. Careful fitting of the measured spectrum to first and second order differentials of the linear absorption spectrum reveals the nature of the exciton, i.e. Frenckel or Charge transfer and new features that arise from transfer of oscillator strength 9to the forbidden states).
Bentham dual light source, Xe and W (300nm to 2.5μm)
Femto Si and GaAs fast high band width detectors
Trek 10:10 high voltage amplifier
Perkin Elmer 7512 digital lock-in amplifier
Leybold 10K displex cryostat and Lakesure 352 controller