Time-resolved spectrometer with a single photon counting technique and its application

Abstract

A time-resolved spectrometer was made with a singlephoton counting technique. This instrument is suitable for measuring fluorescence lifetime and transient behavior of fluorescence. And the time-resolved emission spectrum of a spark discharge is also recorded. The light source and fluorescence were dispersed with a small monochromator. In a single photon counting technique, the time of the emission of individual fluorescence photon is measured electronically with Ortec 437 A time to amplitude converter(TAC), the reference time zero being the initial rise of current pulse related in time to the pulsed light. The time-distribution of single photons is accumlated with NAIG D161 and D171 multichannel pulse height analyzer. RCA 8850 photomultiplier was used for detecting single photons. The time-resolution obtained experimentally was about 0.1 nsec. The pulsed light source which is simple to be constructed and its use was developed. This lamp filled with air or hydrogen at 1 atm was opereated in a relaxiation mode at a usual repetition rate of 10 kHz. The decay time and the width of half-maximum as measured with the above apparatus were 0.5 nsec and 1.5 nsec. In the lamp operated in air, most of the emission is between 300 nm and 400 nm, which is primarily from the C³π-B³π transition of nitrogen.
In this paper, the typical air-filled flash lamp decay and the time-resolved spectrum of the hydrogen flash lamp are shown. The use of the apparatus is illustrated by salicylic acid. At pH4, the fluorescence lifetime was 4.9 nsec, which correspond to that of singly ionized species, whereas in 6 N KOH solution the lifetime of doubly ionized salicylic acid was 3.9 nsec. In organic solvents such as methanol, the decay curve of fluorescence was composed of two exponents. The shorter lifetime was 0.6 nsec and the longer was 4.4 nsec. The shorter lifetime seems to correspond to that of the zwitterion formed due to intramolecular proton transfer in the excited state. The existence of the longer exponent shows that a slight amount of salicylic acid exists as singly ionized species in the ground state.