An Electronic Method of Measuring Molecular Lifetimes

Abstract

An apparatus is described for following in detail changes in light intensity that occur in times of the order of one microsecond. It has been applied to measure the decay of fluorescence and to measure the decay of light from a nitrogen discharge. The light to be measured falls on an electron multiplier tube which is connected to a cathode‐ray oscilloscope, on which the decay curves and a time scale are observed. In measuring fluorescence, a ``square'' pulse of exciting light is produced by a Kerr cell whose potential difference is controlled by the sweep circuit of the oscilloscope. For the fluorescence of diacetyl vapor a mean lifetime of 1.40×10⁻³ sec. was obtained, for diacetyl dissolved in water and in CS₂ 1.0×10⁻⁶ sec. and 1.9×10⁻⁶ sec., respectively. With the nitrogen discharge tube in place of the Kerr cell, photographs were taken at several pressures of the decay of the light of red and blue band systems, and of the discharge tube current which decreases more rapidly than the light. The blue system (C³Π→B³Π) decays exponentially with a mean lifetime (extrapolated to zero pressure) of 7.7×10⁻⁶ sec. The red system (B³Π→A³Σ) decay can be resolved into two exponentials with mean lifetimes of 29×10⁻⁶ sec. and about 1.5×10⁻⁶ sec., respectively.