A nanosecond near-infrared step-scan Fourier transform absorption spectrometer: Monitoring singlet oxygen, organic molecule triplet states, and associated thermal effects upon pulsed-laser irradiation of a photosensitizer

A step-scan Fourier transform (FT) near infrared (IR) spectrometer has been used to record nanosecond time-resolved absorption spectra of singlet molecular oxygen (a(1)Delta(g)-->b(1)Sigma(g)(+)) created by pulsed-laser irradiation of a photosensitizer dissolved in a solvent. In addition to oxygen's a-->b transition at similar to5200 cm(-1), other laser-induced changes in absorbance can be monitored over the spectral range similar to4000-11 000 cm(-1). These include transients derived from the singlet oxygen sensitizer, and examples are provided with triplet state absorption spectra of several molecules in benzene as well as water. In this report, issues pertinent to the use of a step-scan FT spectrometer to monitor weak laser-induced near-IR signals are discussed. Specifically, key modifications to a commercially available instrument are described, and the relevant sources of noise from electronic components are outlined. Transient laser-induced thermal effects such as thermal lenses and temperature-dependent shifts in solvent absorption bands can also interfere with the detection of a given signal, and these complications are likewise described. The significance of these issues is apparent when monitoring oxygen's a-->b transition in D(2)O, a scenario in which the present detection limits of this technique are embodied. Much of the information in this report transcends the specific problem of singlet oxygen detection, however, and addresses issues general to the use of a step-scan FT spectrometer for time-resolved IR measurements. (C) 2002 American Institute of Physics.