Erwin Thiel

Singlet molecular oxygen photosensitized by Rhodamine dyes: correlation with photophysical properties of the sensitizers

Abstract By measuring its IR phosphorescence the formation of singlet molecular oxygen 1 O 2 photosensitized by rhodamine dyes is directly proved. The 1 O 2 formation rate is compared with that expected from the low probability (≈1%) of intersystem crossing of the photosensitizers. The quantum yield for triplet population and the triplet lifetime of the investigated dyes is measured by using a laser-scanning-microscopy technique. The influence of quenching agents (nitrobenzene and COT) is discussed. It results that the formation of 1 O 2 can be prevented effectively by quenching of the S 1 or T state of the photosensitizer. The influence of the molecular ground-state oxygen 3 O 2 concentration [ 3 O 2 ] is investigated. The presence of the paramagnetic 3 O 2 leads to an increased S 1 →T intersystem crossing rate of the photosensitizers and therefore to a reinforced formation of singlet molecular oxygen. It is found for rhodamine 6G as well as for rose bengal that in air-saturated acetonitrile nearly the half of the excited dye triplets are quenched by molecular oxygen. The 1 O 2 concentration can be significantly reduced by decreasing the 3 O 2 concentration below its air saturated level.

Intersystem crossing rate constants of rhodamine dyes: influence of the amino-group substitution

Abstract A time-resolved laser-scanning-microscopy technique is described. Using this technique we measured intersystem crossing rate constants of structurally related rhodamine dyes. Although rhodamine dyes are distinguished by a low probability of intersystem crossing (

New method to investigate weakly populated transient states

Abstract A new method to study transients in the microsecond region was developed. It utilizes spatial separation of excitation and probing by employing a fast-flowing jet stream. For excitation a cw laser is used. The time resolution is determined by the flow velocity and the focal diameter of the laser beam. Since excitation is not by way of short pulses, noise is low. Absorbance changes −4 could be detected even in the presence of intense fluorescence. The triplet parameters of rhodamine 6G and related laser dyes have been determined in ethylene glycol.

Photoreactions of rhodamine dyes in basic solvents

Abstract Using a highly sensitive spectrometer we measured the absorption of transient states of different dyes in liquid solution. Analysing the temporal and spectral dependency of the absorption we determined lifetimes as well as absorption spectra of transient states. The kinetic parameters are specifically influenced by interaction with certain bases. On investigating the acid–base equilibrium of the triplet state of different rhodamine dyes we found the deprotonated triplet state to be an efficient source of a long-lived transient species. We suggest a reaction scheme which explains the dependence of the observed triplet lifetime on the concentration of amine. The possible consequences for the photostability of the dye are discussed.

Continuous wave dye laser pumped by a HeNe laser

Abstract Continuous wave operation of a dye laser pumped at 633 nm by a HeNe laser is reported. A jet of a cooled solution of Oxazine 1 in ethanol was used as the laser medium in an asymmetric spherical cavity. With mirrors of high reflectivity a threshold as low as 10 mW could be achieved.

Incoherently pumped continuous wave dye laser

Abstract Continuous wave operation of a dye laser, pumped by an incoherent light source, is reported. A jet of a water-based solution of Rhodamine 6G is used as the laser medium in a spherical cavity with high reflectivity mirrors. Two high pressure arcs generated by electrical discharge between tungsten electrodes serve as pump source. They produce a power density of 0.5–10 kW/cm 2 in the jet causing the dye to lase at 615 nm.

Photophysical and photochemical properties of electronically excited fluorescent dyes: a new type of time-resolvedlaser-scanning spectroscopy

The capacity of a new type of time-resolved laser-scanning spectroscopy is discussed. Due to the developed modulation technique we obtain a high sensitivity in the absorption measurement as well as a very efficient suppression of the background absorption. The new technique has been used to investigate photophysical and photochemical properties of fluorescent dyes. All presented results are discussed in view of potential applications like dye laser and fluorescence labelling.

Precise fluorescence measurement for determination of photophysical properties of dyes

Abstract The authors present a double frequency modulation technique to determine the fluorescence power with an accuracy of at least 10 −5 . Quantitative analysis of the modulation transfer from the excitation to the fluorescence power is used to derive the triplet state lifetime τ DS and the rate constant for intersystem crossing k ED of fluorescent dyes. For that purpose the correlation between the population of transient states and the decrease of fluorescence power is measured. The experimental results of rhodamine 6G are presented. The influence of molecular oxygen is also examined. The photophysical parameters can be determined with high accuracy even if absorption of excited states, photoproducts or other molecules is present. This sensitivity is sufficient to determine the presence of a few hundred molecules in excited states.

Continuous-wave dye laser pumped by a high-pressure argon arc

Continuous-wave operation of a Rhodamine 6G dye laser, incoherently pumped by a high-pressure argon arc, has been achieved. A special electrode design reduces melting of the electrode tips, and thus the arc provides the necessary brightness for periods of the order of hours.

Laser-raster spectrometer for time-resolved recording of transient absorption

For measurements of transient absorption a laser-raster technique has been found to be surprisingly sensitive. It utilizes spatial separation of excitation and probing by employing a fast-flowing jet stream. The time resolution is determined by the flow velocity and the focal diameters of the continuous excitation and probing laser beams. Changes in transmission can be detected in the spectral region from 350 to 1000 nm. A dual-frequency modulation technique is used to achieve a sensitivity of 10(-4). The time resolution is 0.5 mus. The performance of this spectrometer is demonstrated by measurement of the absorption of the transient states of Rhodamine 6G dye in ethylene glycol.