Time-resolved spectroscopy | EPFL Graph Search

In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10−16 seconds. All time-resolved spectra are suitable to be analyzed using the two-dimensional correlation method for a correlation map between the peaks. Transient-absorption spectroscopy Flash photolysis Ultrafast laser spectroscopy Transient-absorption spectroscopy (TAS), also known as flash photolysis, is an extension of absorption spectroscopy. Ultrafast transient absorption spectroscopy, an example of non-linear spectroscopy, measures changes in the absorbance/transmittance in the s

Ultrafast dynamics of photoinduced charge separation in cyanine- and polymer-based organic photovoltaic systems

Jelissa Risse

Organic photovoltaics (OPV) have the potential to provide low-cost solar-to-electricity converting devices. Improving such devices requires a deeper understanding of the ultrafast photoinduced processes occurring after light absorption. We have performed femtosecond (fs) transient absorption spectroscopy, as well as time-resolved electroabsorption (Stark effect) on model OPV systems to follow electron and hole transfer dynamics, as well as charge carrier motion in the active layer to the electrodes. Solid-state cyanine borate (Cy3-B) films undergo intra-ion pair reductive quenching on the picosecond (ps) timescale. We have found that when intermixing Cy3-B with a fullerene acceptor (PCBM), photoexcitation leads to the appearance of oxidized Cy3 in less than 70 fs. We correlated the rate and yield of Cy3 oxidation to the PCBM loading. We then investigated a cyanine Cy3-P in a bilayer geometry with fullerene C60. In this case, ultrafast electron transfer in less than 100 fs is followed by slower dissociation of interfacial charge transfer states in the presence of an electric field. A photoinduced Stark effect observed in neat C60 enabled to identify an initially delocalized excited state, which localizes in 360 fs. Lastly, we analyzed how microstructural changes in polymer pBTTT:PCBM blends impact free charge carrier formation and motion. We identified that the intermixed phase efficiently produces charge carriers within 100 fs, and that pure fullerene and pBTTT domains attract the charge carriers separating them further apart. This is ascribed to an energy cascade toward the neat domains. This thesis reveals ultrafast charge transfer at donor-acceptor interfaces in such OPV systems by means of fs transient absorption spectroscopy. Results emphasize that a second step is needed for charge separation in order to successfully compete with charge pair recombination in these systems. An electric field applied can act as a driving force to dissociate interfacial charge transfer states. Pure donor or acceptor domains attract holes or electrons, respectively, and reduce geminate charge recombination in blends. These second steps in charge separation were derived from photoinduced electroabsorption dynamics in transient absorption measurements, as well as time-resolved electroabsorption based on the Stark effect.

EPFL2015

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis

Florian Le Formal, Min Ling, Yimeng Ma

It has been more than 40 years since Fujishima and Honda demonstrated water splitting using TiO2, yet there is still no clear mechanism by which surface holes on TiO2 oxidize water. In this paper, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent and transient absorption spectroscopy, we measure both the kinetics of electron extraction (t(50%) approximate to 200 mu s, 1.5V(RHE)) and the kinetics of hole oxidation of water (t(50%) approximate to 100 ms, 1.5V(RHE)) as a function of applied potential, demonstrating the water oxidation by TiO2 holes is the kinetic bottleneck in this water-splitting system. Photoinduced absorption spectroscopy measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2 holes under conditions of photoelectrochemical water oxidation. Under these conditions, we find that the surface density of these holes increases nonlinearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is third order with respect to surface hole density, with a rate constant k(WO) = 22 +/- 2 nm(4).s(-1). Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions, the rate law was second order with respect to surface hole density, indicative of a change in reaction mechanism. Although a change in reaction order was observed, the rate of reaction did not change significantly over the wide pH range examined (with TOFs per surface hole in the region of 20-25 s(-1) at similar to 1 sun irradiance). This showed that the rate-limiting step does not involve OH- nucleophilic attack and demonstrated the versatility of TiO2 as an active water oxidation photocatalyst over a wide range of pH.

Amer Chemical Soc2017

Ultrafast Photoelectron Spectroscopy of Liquid Samples

José Javier Pancracio Ojeda Andara

This thesis focuses on the commissioning and characterization of HARMONIUM, an ultrafast tunable EUV source based on high-harmonic generation for performing photoelectron spectroscopy of liquids. In ultrafast photoelectron spectroscopy a short pump pulse induces an excited state in a molecular system while a time-delayed short pulse interrogates the electronic distribution of the transient species. This method is an important tool in photochemistry as it combines features like element specificity and surface sensitivity with the absence of forbidden transitions and the ability to retrieve absolute binding energies. Ultrafast photoelectron spectroscopy of liquid media became possible by combining efficient turbomolecular pumping with a liquid micro jet in vacuum from which collisionless evaporation takes place. In HARMONIUM, it is possible to choose between high energy (up to 0.16 eV) or high temporal (up to 40 fs) resolution modes by selecting one of the 4 different gratings in a pulse-preserving single-grating EUV monochromator. High photon flux ranging from

2.3\cdot10^{11}

photons/s at 36 eV to

1.5\cdot10^{8}

photons/s at 102 eV has been demonstrated. Tunability of the laser repetition rate allows distributing the laser power among the maximum number of pulses per second for a given EUV photon energy, thereby minimizing EUV-induced space charges without compromising the acquisition time of photoelectron spectra. An ellipsoidal mirror in a 4:1 configuration yields an EUV focal spot size similar to the

\sim

\mu

m diameter of the liquid jet. Apart from its implementation on photoelectron spectroscopy of liquid samples, the versatility of this beamline makes it suitable to study different states of matter. To this end, HARMONIUM will be coupled to a molecular beam and an angle-resolved photoemission spectroscopy (ARPES) end stations. A photoelectron spectrum of a liquid sample is composed of electrons emitted from the liquid surface and those emitted from the surrounding evaporated molecules. In this thesis, a systematic approach for separating the gas contribution from the liquid+gas photoelectron spectrum is presented. This procedure opens the possibility to acquire photoelectron spectra of dilute aqueous media under conditions of moderate electrokinetic charging. In this regime, preliminary photoelectron measurements of protonated water in the absence of counterions suggest that changes in the water orbitals take place as a function of the hydrogen bond coordination. The laser assisted photoelectric effect in liquids is fully described for the first time. Convolution of its retrieved response function with the unpumped photoelectron spectrum reproduces the pumped photoelectron spectrum. This contribution is important as usually the laser assisted photoelectric effect masks photoinduced dynamics transients in time-resolved photoelectron spectra around the region of zero time delay. For the first time ultrafast photoelectron spectroscopy is used to temporally resolve photo induced dynamics in an aqueous metal complex. Impulsive photoreduction of the iron center followed by ultrafast oxidation in

\sim

550 fs was shown to take place upon excitation of the ligand-to-metal charge-transfer transition of aqueous ferricyanide solution.

EPFL2016