Robert E. Imhof

Optothermal transient emission radiometry

Reports the development of a new technique-optothermal transient emission radiometry (OTTER)-for studying optical absorption, thermal diffusivities and associated properties in opaque materials. In essence, the technique makes use of the information content of the thermal emission transient observed by means of a wideband infrared detector, following pulsed optical excitation of the material. Some preliminary observations, including a study of several commercial pigments are presented.

Pulse fluorometry using simultaneous acquisition of fluorescence and excitation

We report a new method of measuring fluorescence lifetimes which uses the single‐photon technique and two detection channels with matched impulse response for simultaneous acquisition of fluorescence and excitation (SAFE). This differential arrangement is shown to correct automatically for variations in the optical pulse profile during the measurement, thus eliminating a common source of error. It can be used to improve precision and sensitivity with any pulsed light source such as a flashlamp, laser, or synchrotron. A routing system separates photomultiplier coincidences from the dual detection channels into different memory segments of a multichannel analyzer (MCA) using a single time‐to‐amplitude converter (TAC). Comprehensive data are presented on tuning the single‐photon response of the Philips range of XP2020Q photomultipliers. Results obtained using a coaxial flashlamp to excite a dilute solution of PPO in ethanol give a lifetime of 1.63±0.02 ns in good agreement with that obtained using convention...

A multiplexed single-photon instrument for routine measurement of time-resolved fluorescence anisotropy

The authors report a new type of instrument, the Edinburgh Instruments Model 2997 differential fluorometer, for the measurement of the decay of fluorescence anisotropy. The instrument incorporates three multiplexed detection channels for simultaneous measurement of two polarised fluorescence decays and the excitation pulse, using time-correlated single-photon counting. This arrangement corrects automatically for temporal and intensity fluctuations of the spark source. A study of the probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in a white oil reveals that the instrument can measure rotational correlation times with subnanosecond time resolution in a third of the time of conventional methods.

Kinetic interpretation of fluorescence decays

ABSTRACT We review the interpretation of reconvolution analysis of single photon fluorescence decay data with regard to the time resolution achievable and sources of error encountered. Examples are presented for complex fluorescence decays in a wide range of applications including polymers, membranes, scintillators and solar collectors.

Fluorescence quenching of perylene by Co2+ ions via energy transfer in viscous and non-viscous media

Abstract Non-radiative energy transfer from singlet excited perylene to Co2+ ions has been studied in methanol, ethylene glycol and glycerol by time-correlated single-photon counting and steady state quenching experiments. In highly viscous glycerol the decay curves are represented by the expression given by Forster for long-range energy transfer. The experimental critical distance R0 = 13.2 A is in good agreement with the theoretical value. In methanol and ethylene glycol the decay is modified owing to translational motion and can be described by the function exp(— at — bt 1 2 with reasonable values for the reaction sphere and diffusion coefficients.

Multiplexed array fluorometry

The authors demonstrate the simultaneous measurement of fluorescence decays at different emission wavelengths using single-photon timing array detection.

PPO excimers in lipid bilayers studied using single-photon timing array detection

Abstract We report fluorescence decay studies of 2,5-diphenyloxazole (PPO) excimers in small unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) using single-photon timing array detection at multi-wavelengths. Evidence is presented for complex kinetics caused by the presence of isolated PPO molecules and ground state aggregates which give rise to more than one excimer emission. The results are a clear demonstration of probe aggregation being a dominant mechanism for excimer formation in lipid bilayers.

Evidence for donor-donor energy transfer in lipid bilayers : perylene fluorescence quenching by CO2+ ions

Abstract Electronic dipole—dipole energy transfer between perylene molecules and from perylene to cobalt(II) ions has been observed in lipid bilayers below the gel to liquid-crystalline phase transition. It is shown that due to clustering donor—donor energy transfer has to be taken into account at the low acceptor concentrations when analysing the fluorescence decay data in terms of Forster kinetics. This correction then gives an interaction radius for energy transfer from perylene to Co 2+ of ≈ 13.4 A, which is comparable to the theoretical value and also to that found in glycerol.

New opto-thermal radiometry technique using wavelength-selective detection

The authors report the development of a new opto-thermal radiometry technique, using wavelength-selective detection. The measurement principle is similar to that of opto-thermal transient emission radiometry (OTTER), but includes a grating monochromator as part of the signal detection system, to enable opto-thermal transients at selected infrared wavelengths to be studied. The technique offers the ability to separate the effects associated with the absorption of exciting light from those of infrared emission. Changes of opto-thermal decay time with emission wavelength can therefore be related to the infrared absorption spectrum of the sample. First observations on TiO2 excited by UV laser light are presented.

Near‐infrared spark source excitation for fluorescence lifetime measurements

We have extended the range of excitation wavelengths from spark sources used in single photon timing fluorometry into the near infrared by means of the all‐metal coaxial flashlamp filled with an argon‐hydrogen gas mixture. At 750 nm this mixture gives ∼15 times the intensity available from pure hydrogen for a comparable pulse duration. Measurements are demonstrated by using the laser dye IR‐140 in acetone, for which a fluorescence lifetime of 1.20 ns is recorded.