Experiment of spectroscopy in excited state

A method to form excitation state for measuring optical absorption is described. The method introduced is to modulate number of the electrons in excited state by chopping the excitation photons. Light from an Ar laser or a Xe lamp is chopped and projected on the sample in a cryostat. The sample used was an F-colored alkali halide. The life of the excited state of F-center is about 1 μsec. The infrared light passing vertically to the exciting photons and absorbed at the F-center was detected with a lock-in amplifier. For the probe light source, a globar lamp was used. The detectors used were Ge: Cu and Hg Cd Te. Detection capability was 109--11. The cryostat used was that alkali halide or KRS-5 was adhered with electron wax. Moisture in the air must be avoided carefully. When the frequency of the chopper is larger than the inverse number of the life of excited state, detection of signal is impossible. Therefore, the chopper frequency must be variable. The maximum frequency obtained with a mechanical chopper is 10 kHz, but a supersonic modulater can produce higher frequency. An example obtained by the present apparatus is shown in a figure. (Kato, T.)