Sample exchange techniques

Flow Cells in Transient Absorption Spectroscopy. Transient absorption spectroscopy of

molecules in solution requires the continuous replacement of the pumped sample volume, mainly in order to avoid photodegradation and accumulation of photoproducts. Usually a custom-made flow cell was used which is described in detail in Ref. [34]. In brief, the design of the cell is optimized to minimize the optical path length through dispersing material, namely the windows and the solution. The pump and probe pulses have to pass two 200 µm thick fused silica windows and the solution layer of about 120 µm. The small amount of dis- persive material provides that the chirp introduced to the pulses is small. The thin solution layer furthermore ensures that the group velocity mismatch (GVM) between pump and probe pulse [35] remains on a small level. Both – the chirp of the pulses and the GVM – should be as small as possible for a high temporal resolution. The short optical path length through dispersive material additionally provides that non-linear interactions between pump and probe pulse in the material such as two-photon absorption, stimulated Raman amplification and cross-phase modulation [29] are kept on a small level.

Figure 2.7: a) Influence of the flow cell windows on the measured early signal. Observed signal of Ph₂CHPPh₃+BF₄¯ in CH₂Cl₂ at 329 nm after UV excitation (black) compared to the signal measured in an empty flow cell (blue). b) Comparison of the transient signals at 329 nm after UV excitation of Ph₂CHCl in CH₃CN around time-zero in the flow cell (black) and the liquid jet (pur- ple). The signal of neat CH₃CN in the liquid jet is shown in blue.

These interactions lead to signal contributions within 100 fs to 150 fs around time-zero masking the earliest dynamics of photo-initiated processes (see Figure 2.7). Particularly dis-

turbing are negative absorption changes since they are hardly distinguishable from ultrafast signal rises (τ≈ 100 fs) originating from the molecule. Hereby, the major part of the negative signal contributions seems to stem from the flow cell windows (see Figure 2.7a). However, the achievable time resolution with flow cells of ~70 fs over the entire detection window is suitable to study the major part of the dynamics of the molecules studied in this work.

Figure 2.8: Scheme of the liquid jet setup and the concentration control. The pump-probe meas- urement is carried out with the solution in the liquid jet. The precursor concentration increase by solvent evaporation is quantified by an independent UV/Vis spectrophotometer and is compen- sated by a feedback loop which controls the addition of neat solvent. Picture: The liquid jet used in the transient experiments. The solution is colored in blue for the sake of visibility.

Liquid Jet Setup. In order to study ultrafast dynamics in the sub-100 fs range, in which the

ultrafast photo-induced bond cleavage of benzhydryl chloride is predicted to take place (see Section 3.2) [14], the time resolution of the setup has to be improved and the spurious signal contributions of the flow cell windows have to be removed. Therefore, a wire-guided liquid jet was constructed as a second system which allows for the generation of even thinner solu- tion layers as with the flow cell without any additional dispersive material to be passed by the pulses. It consists of two stainless steel wires (150 µm diameter) in a distance of 2 to 3 mm. Between these wires a thin solution film flows downwards which is sustained by sur- face tension (see Figure 2.8) [45-48]. The wires lead directly into a reservoir providing minimal back reflections from the end of the layer. The thickness of the sample layer was usually 50 µm to 60 µm but it can be easily varied between 20 µm to 200 µm [49]. The use of a jet in transient absorption measurements leads to a tremendous decrease of the coherent signal contributions compared with a flow cell (see Figure 2.7b). Especially the negative

signals are eliminated to a large extent. A further advantage of the jet compared to the flow cell is that precipitation of the sample on the cell windows can be avoided.

Closed-loop control of the Sample Concentration by Online Broadband Measurements. In

contrast to the flow cell the liquid jet is not a closed system – due to the open surface of the jet and the reservoir, the solvent evaporates constantly (e.g., ethanol evaporates in the liquid jet setup with a rate of ~ 2.1 ml/h). Without any compensation this would lead to a strong increase in the precursor concentration making any time-dependent measurement impossible. Therefore, a closed-loop control was developed which keeps the optical density and thereby the concentration of the solution in the jet on a constant level [50]. The optical density is measured in a reference flow cell by a home-made, mobile UV/Vis absorption spectropho- tometer consisting of a fiber-coupled light source, a fiber coupled spectrometer and four off- axis parabolic mirrors (see Figure 2.8). The light from the source, ranging spectrally from 210 nm to above 1000 nm, is collimated after the fiber output and focused into the sample down to a diameter of ~ 400 µm allowing for even smallest sample volumes to be measured. After recollimation of the beam and coupling into the second fiber, the light is guided to the small spectrometer where the spectrum is recorded. The use of off-axis parabolic mirrors hereby allows for an efficient imaging of the beam free of astigmatism. For moderate optical densities < 1.5 the UV/Vis absorption spectra measured within single seconds are compara- ble in quality to spectra recorded within minutes with commercially available spectropho- tometers (see Figure 2.9a).

Figure 2.9: a) Absorption spectrum of a holmium oxide glass filter measured with a commercial Perkin-Elmer Lambda 750 (black line) and the home-made absorption spectrophotometer (red line). b) Stabilized optical density of indole in ethanol in the liquid jet setup at 270 nm by com- pensation of evaporation losses through the control loop.

At the beginning of each measurement, a transmission spectrum of the neat solvent is re- corded. A second transmission spectrum of the sample then allows for the determination of the initial optical density. The OD at the excitation wavelength serves as set-point for the

control loop. During a transient absorption measurement in the jet the optical density in the reference flow cell is steadily compared with the value of the set-point. A computer con- trolled syringe pump then adds neat solvent to the sample with a rate determined by a PID control. This allows to keep the concentration of the sample extremely stable for several hours (see Figure 2.9b). For a measurement of indole in ethanol in the liquid jet setup, the maximum deviation from the initial optical density during three hours was smaller then 0.7% compared to the mean value. The standard deviation was ~ 0.2% of the mean value.

The on-line measurement of absorption spectra during transient absorption measurements additionally allows for the recognition of a possible change or a (photo-)degradation of the sample: the absorption band of the molecule can undergo changes or even new absorption bands of generated (photo-)products can arise. Both effects can be detected by absorption spectroscopy which allows for a reliable identification of changes of the sample.