UV-vis transient absorption belongs to a family of pump-probe techniques. The general principle behind this approach consists of exciting a sample with an ultrashort but relatively narrowband light pulse (pump) and probing the photoexcited volume with another ultrashort, but broadband, pulse (Figure 2.3). Since light travels 1 mm in space during
~3.3 ps, a delay between two pulses can be controlled with the help of an optical delay line to mechanically vary a pulse’s pathlength. The alignment and precision of the delay lines is a crucial bottleneck for this type of experiment, therefore only relatively short pump-probe time delays (up to few nanoseconds) can be achieved reliably in this way.
Figure 2.3 also shows an exemplary transient absorption spectrum and its constituent spectral components. A TA spectrum is a difference spectrum calculated according to
Δ𝐴 = −𝑙𝑜𝑔 𝐼‚"S 𝐼‚"S•Žd•
𝐼@T$•Žd•
𝐼@T$
(2.2)
where the intensities (𝐼) of the signal (sig) and reference (ref) probe beams with and without preceding pump pulse are recorded. Hence, it effectively quantifies the probe absorbance changes caused by the pump pulse that arrived beforehand. If there is a new absorption band appearing in the spectra as a result of the preceding excitation pulse, there will be a positive signal in the TA spectrum corresponding to the excited-state absorption (ESA). If the pump pulse causes a photoreaction in the sample, the product of the reaction might absorb some light as well, and its signature will also appear as a positive signal similarly to the ESA contribution (Figure 2.3).
Figure 2.3 | Principle of the ultrafast pump-probe experiment (top). An exemplary TA spectrum is shown in the bottom. Various contributions making up the spectrum are color-coded and their origin is explained in the energy diagram on the right side. In this case a hypothetical photochemical reaction takes place and positive absorptions of both excited educts as well as newly formed ground-state photoproducts are seen in the spectrum.
Molecules promoted in the excited state might be stimulated by incoming photons of the probe beam to return to their ground state. This
process causes the appearance of the negative signal of the stimulated emission which is proportional to the spontaneous emission (fluorescence) spectrum of the compound
SE = 𝐹 𝜈 𝜈6/𝑑𝜈 (2.3)
where 𝐹 𝜈 represents the fluorescence spectrum of the molecule on a wavenumber scale. Additionally, since some molecules are promoted to the excited state, there will be less absorption (and hence negative contribution to the TA spectrum) in the region where stationary ground-state absorption is situated (ground-state bleach – GSB). Thus, the transient absorption signal in the UV-vis spectral region is typically a complex blend of various, usually strongly overlapping, contributions. Their disentanglement is rarely straightforward and various approaches are utilized to separate different absorbing species.
The optical scheme of our electronic UV-vis transient absorption setup is shown in Figure 2.4. A Ti:Sapphire regenerative amplifier (Spitfire, Spectra-Physics) seeded by a Ti:Sapphire mode-locked oscillator (Tsunami, Spectra-Physics) produces 800 nm pulses (although the output could be tuned within 785-815 nm range) with a repetition rate of 1 kHz.
The output is split in two parts that are used to produce pump and probe pulses. The pump part is chopped at half the repetition rate of the regen (500 Hz) and sent through a delay line to a 700 µm thick nonlinear crystal (BBO type I, Castech) producing the second harmonic at 400 nm. This beam is sent through a zero-order half-wave plate to control its polarization, which was always set at the magic angle (MA = 54.7°) with respect to the probe. Afterward, it is focused shortly before the sample, so that the excitation spot size in the cell is approximately 300 µm in diameter and is blocked after the sample.
The second part of the initial laser output serves as the probe. It is focused on a constantly moving 3 mm thick CaF2 plate producing chirped white light supercontinuum due to the self-phase modulation process. The spectrum of the continuum spans a wide range of wavelengths from
<320 nm to the NIR. However, due to the lens materials and coatings of the mirrors in the imaging part, the high-frequency spectral cut-off is at
~360 nm, while on the other edge the spectrum is limited at ~720 nm due to the use of the filter cutting the remaining 800 nm contribution. The probe beam goes through a wire-grid polarizer fixing its polarization vertical in the laboratory frame and is split into two parts by a beamsplitter. The first part serving as the reference beam is focused directly on the entrance slit of a 163 mm spectrograph (Andor Technology) equipped with a 512×58 pixel back-thinned CCD camera (Hamamatsu, assembled by Entwicklungsbüro Stresing, Berlin). The second part is focused tightly onto the sample and serves as the signal probe beam.
Figure 2.4 | Optical scheme of our UV-vis transient absorption setup. Image adapted from Dr. Romain Letrun.
After passing through the sample it is focused onto the entrance slit of an identical spectrograph-CCD camera combination. Since every second pump beam is blocked by the chopper, 𝐼‚"S and 𝐼‚"S•Žd• (as well as 𝐼@T$ and 𝐼@T$•Žd•) correspond to the ith and (i-1)th consecutive laser pulses, whereas 𝐼‚"S and 𝐼@T$ (as well as 𝐼‚"S•Žd• and 𝐼@T$•Žd•) come from the same ith pulse.
Referencing and a single shot detection with statistical outlier filtering
implemented using in-house code allows for significantly improved signal-to-noise ratio and a fast optimized data readout make this TA setup very time-efficient.
The instrument response function has a full width at half-maximum (FWHM) of <170 fs as obtained from measurements of the optical Kerr effect (OKE) in neat solvents. The absorbance of the sample is usually kept below 0.3 in a quartz cuvette with 1.0 mm pathlength. For the results reported in this thesis between 250 to 1000 spectra were recorded at each time delay and then averaged. About 500 time delays were recorded to obtain a single time scan. This procedure was repeated 4-6 times and the resulting scans were averaged to produce final data, which were preprocessed and analyzed. Samples were bubbled with nitrogen gas during the experiment to constantly refresh the excitation volume and avoid sample decomposition. The stability of the samples was verified by measuring their steady-state absorption spectra before and after time-resolved experiments and, if some decomposition was detected (which was normally not the case), the pump power was reduced and the experiment repeated on a freshly prepared sample until no decomposition was observed in the steady-state spectra. Under these conditions measurement of one system (one compound in one solvent) took less than 20 minutes. Given enough volume present in the cuvette even highly volatile solvents, such as diethyl ether, could be used without substantial evaporation during the experiment.
Transient absorption data require some preprocessing prior to any data interpretation or even visual inspection is carried out. First of all, background subtraction is performed by subtracting the average of a few spectra acquired well before time zero (typically at -5 to -3 ps). These spectra contain contributions from spontaneous emission and pump scatter in the sample. Such a subtraction suppresses these contributions assuming that they stay constant throughout the measurement. Precautions are taken to minimize the scattering light, but usually the data from 5-10 pixels around the pump wavelength are withdrawn from the analysis.
Secondly, wavelength calibration of the spectrograph has to be performed. A holmium oxide glass filter is used as a reference for
pixel-to-wavelength conversion due to its many narrow absorption bands throughout the whole visible range.
Thirdly, a correction for the chirp of the probe beam is mandatory.
The white light supercontinuum is characterized by a strong positive group-delay dispersion (GDD) due to the manner in which it is generated and the optical elements traversed on its way to the sample. Therefore, the pump-probe temporal overlap, known as time zero, exhibits a pronounced wavelength dependence across the experimental spectral window. The OKE of neat solvents was recorded for each solvent in order to eliminate this experimental artifact. The electronic part of this signal has a well-defined Gaussian shape and a constant sign over the entire spectral region.
The maximum of the Gaussian is determined for each wavelength and is fitted with a polynomial128
tg 𝜆 = 𝑎 +10’
𝜆/ 𝑏 +10”
𝜆f 𝑐 (2.4)
allowing accurate determination of time zero for each spectral data point.
The TA data are then corrected by interpolation in the time domain. Such a correction is absolutely crucial in order to perform a global kinetic analysis over many wavelengths – the technique that we have frequently resorted to throughout this work. Additionally, the OKE measurement provides an estimation of the IRF, which is also wavelength-dependent alike time zero.
After such a preprocessing, the data are ready to be inspected and analyzed. Usually, the first 150 fs are discarded due to the presence of coherent artifacts within the IRF. This is detrimental only if ultrafast sub-IRF processes take place since their lifetime and amplitude are severely overestimated. However, in the vast majority of cases reported in this dissertation, this was not the case.