Using slow measurement systems to measure fast excited-state
kinetics with nonlinear rate-competitive optical bleaching
Stephen E. Bialkowski and Agnès Chartier
Department of Chemistry and Biochemistry
Utah State University
Logan, UT 84322-0300
USA
Oral: Area 6; Nonlinear phenomena and inverse problems
Several apparatuses and methods for determining energy- and irradiance-dependent photothermal refraction signals, including photothermal deflection and lens, have been developed for both gas and liquid phase samples. This presentation will address practical aspects and considerations for experiments designed to obtain optical and excited state kinetic information from nonlinear irradiance-dependent signals. The discussions will be illustrated by example data.
The fact that the photothermal refraction signal amplitude is proportional to excitation irradiance has prompted researchers to use more powerful lasers in order to decrease detection limits. Although higher irradiance enhances the signal, it may also result in nonlinear effects. Absorption-based nonlinear effects are due mostly to dynamic excited state population changes producing optical saturation, bleaching, and multiple- photon absorption. Most nonlinear effects become more problematic using short-pulsed excitation lasers where instantaneous irradiances are high and the excitation time scales become short compared to complete ground state recovery. Nonlinear effects result in analytical calibration problems that are difficult to solve for all but the most controlled laser sources.
We have observed nonlinear bleaching effects in both pulsed and continuous laser excited experiments. These are observed using even modest (mW) optical powers or (m J) pulsed laser energies. Although this may be thought to limit the utility of photothermal spectroscopy to the analysis of all but the most favorable analytes, the nonlinear absorption also opens up new areas of application. In particular, the irradiance or integrated irradiance-dependent photothermal signals may be analyzed yielding a variety of excited state kinetic and optical absorption cross section data. Due to the enhanced sensitivity of photothermal refraction spectroscopy over transmission experiments, the data are easily obtained using low concentration samples.
Irradiance-dependent photothermal refraction signal data obtained for organic dye substances are used to illustrate the method. Using continuous excitation, the data can be used to determine excited triplet state absorption cross sections and lifetimes of the excited metastable triplet states. An additional parameter is found when using pulsed laser excitation sources. In particular, we have been able to deduce rate constants for T2 to T1 relaxation after excited state excitation. These sub-nanosecond lifetimes can be deduced by competitive excitation. The data is rather easily obtained and does not require complicated excitation-probe experimentation typically employed for excited state studies. The data is more directly related to excited state absorption effects thought to be useful for the design of optical limiters.