Free Carrier Emergence and Onset of Electron–Phonon Coupling in Methylammonium Lead Halide Perovskite Films

Sub-10 fs resolution pump–probe experiments on methylammonium lead halide perovskite films are described. Initial response to photoexcitation is assigned to localized hot excitons which dissociate to free carriers. This is attested to by band integrals of the pump–probe spectra where photoinduced bleaching rises abruptly 20 fs after photoexcitation. Later stages of spectral evolution are consistent with hot carrier cooling, during which state filling induced bleaching of interband and exciton transitions curiously more than doubles. Electron coupling to optical phonons is observed as periodic spectral modulations in the pump–probe data of both films. Fourier analysis identifies active phonons at ∼100 and 300 wavenumbers pertaining to the lead-halide framework and organic cation motions, respectively. Coupling strengths estimated from the depth of these modulations are in the weak coupling limit, in agreement with values extracted from temperature dependent emission line shape analysis. These findings support free carriers in these materials existing as large polarons. Accordingly, these modes are probably not dictating the moderate carrier mobility in this material

Reflectivity Effects on Pump–Probe Spectra of Lead Halide Perovskites: Comparing Thin Films <i>versus</i> Nanocrystals

Due to the sizable refractive index of lead halide perovskites, reflectivity off their interface with air exceeds 15%. This has prompted a number of investigations into the prominence of photoreflective contributions to pump–probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI₃) perovskite. By analyzing how complex refractive index changes impact the two experiments, results demonstrate that changes in absorption and not reflection dominate transient transmission measurements in thin films of these materials. None of the characteristic spectral signatures reported in such experiments are exclusively due to or even strongly affected by changes in sample reflectivity. This finding is upheld by another experiment where a methyl ammonium lead iodide (MAPbI₃) perovskite film was formed on high-index flint glass and probed after pump irradiation from either face of the sample. We conclude that interpretations of ultrafast pump–probe experiments on thin perovskite films in terms of photoinduced changes in absorption alone are qualitatively sound, requiring relatively minor adjustments to factor in photoreflective effects

Probing Ultrafast Photochemistry of Retinal Proteins in the Near-IR: Bacteriorhodopsin and Anabaena Sensory Rhodopsin vs Retinal Protonated Schiff Base in Solution

Photochemistry of bacteriorhodopsin (bR), anabaena sensory rhodopsin (ASR), and all-trans retinal protonated Schiff base (RPSB) in ethanol is followed with femtosecond pump–hyperspectral near-IR (NIR) probe spectroscopy. This is the first systematic probing of retinal protein photochemistry in this spectral range. Stimulated emission of the proteins is demonstrated to extend deep into the NIR, and to decay on the same characteristic time scales previously determined by visible probing. No signs of a transient NIR absorption band above λ_pr > 1.3 μm, which was recently reported and is verified here for the RPSB in solution, is observed in either protein. This discrepancy demonstrates that the protein surroundings change photochemical traits of the chromophore significantly, inducing changes either in the energies or couplings of photochemically relevant electronic excited states. In addition, low-frequency and heavily damped spectral modulations are observed in the NIR signals of all three systems up to 1.4 μm. By background subtraction and Fourier analysis they are shown to resemble wave packet signatures in the visible, stemming from multiple vibrational modes and by analogy are assigned to torsional wave packets in the excited state of the retinal chromophore. Differences in the vibrational frequencies between the three samples and the said discrepancy in transient spectra are discussed in terms of opsin effects on the RPSB electronic structure

Membrane Independence of Ultrafast Photochemistry in Pharaonis Halorhodopsin: Testing the Role of Bacterioruberin

Ultrafast photochemistry of pharaonis halorhodopsin (p-HR) in the intact membrane of Natronomonas pharaonis has been studied by photoselective femtosecond pump–hyperspectral probe spectroscopy with high time resolution. Two variants of this sample were studied, one with wild-type retinal prosthetic groups and another after shifting the retinal absorption deep into the blue range by reducing the Schiff base linkage, and the results were compared to a previous study on detergent-solubilized p-HR. This comparison shows that retinal photoisomerization dynamics is identical in the membrane and in the solubilized sample. Selective photoexcitation of bacterioruberin, which is associated with the protein in the native membrane, in wild-type and reduced samples, demonstrates conclusively that unlike the carotenoids associated with some bacterial retinal proteins the carrotenoid in p-HR does not act as a light-harvesting antenna

pH Dependence of Anabaena Sensory Rhodopsin: Retinal Isomer Composition, Rate of Dark Adaptation, and Photochemistry

Microbial rhodopsins are photoactive proteins, and their binding site can accommodate either all-trans or 13-cis retinal chromophore. The pH dependence of isomeric composition, dark-adaptation rate, and primary events of Anabaena sensory rhodopsin (ASR), a microbial rhodopsin discovered a decade ago, are presented. The main findings are: (a) Two p<i>K</i>_a values of 6.5 and 4.0 assigned to two different protein residues are observed using spectroscopic titration experiments for both ground-state retinal isomers: all-trans, 15-anti (AT) and 13-cis, 15-syn (13C). The protonation states of these protein residues affect the absorption spectrum of the pigment and most probably the isomerization process of the retinal chromophore. An additional p<i>K</i>_a value of 8.5 is observed only for 13C-ASR. (b) The isomeric composition of ASR is determined over a wide pH range and found to be almost pH-independent in the dark (>96% AT isomer) but highly pH-dependent in the light-adapted form. (c) The kinetics of dark adaptation is recorded over a wide pH range, showing that the thermal isomerization from 13C to AT retinal occurs much faster at high pH rather than under acidic conditions. (d) Primary photochemical events of ASR at pH 5 are recorded using VIS hyperspectral pump–probe spectroscopy with <100 fs resolution and compared with the previously recorded results at pH 7.5. For AT-ASR, these are shown to be almost pH-independent. However, photochemistry of 13C-ASR is pH-dependent and slowed down in acidic environments

Efficient Femtosecond Energy Transfer from Carotenoid to Retinal in <i>Gloeobacter</i> Rhodopsin–Salinixanthin Complex

The retinal proton pump xanthorhodopsin (XR) was recently found to function with an attached carotenoid light harvesting antenna, salinixanthin (SX). It is intriguing to discover if this departure from single chromophore architecture is singular or if it has been adopted by other microbial rhodopsins. In search of other cases, retinal protein encoding genes in numerous bacteria have been identified containing sequences corresponding to carotenoid binding sites like that in XR. <i>Gloeobacter</i> rhodopsin (GR), exhibiting particularly close homology to XR, has been shown to attach SX, and fluorescence measurements suggest SX can function as a light harvesting (LH) antenna in GR as well. In this study, we test this suggestion in real time using ultrafast transient absorption. Results show that energy transfer indeed occurs from S₂ of SX to retinal in the GR–SX composite with an efficiency of ∼40%, even higher than that in XR. This validates the earlier fluorescence study, and supports the notion that many microbial retinal proteins use carotenoid antennae to harvest light

Three-Pulse Femtosecond Spectroscopy of PbSe Nanocrystals: 1S Bleach Nonlinearity and Sub-Band-Edge Excited-State Absorption Assignment

Above band-edge photoexcitation of PbSe nanocrystals induces strong below band gap absorption as well as a multiphased buildup of bleaching in the 1S_e1S_h transition. The amplitudes and kinetics of these features deviate from expectations based on biexciton shifts and state filling, which are the mechanisms usually evoked to explain them. To clarify these discrepancies, the same transitions are investigated here by double-pump–probe spectroscopy. Re-exciting in the below band gap induced absorption characteristic of hot excitons is shown to produce additional excitons with high probability. In addition, pump–probe experiments on a sample saturated with single relaxed excitons prove that the resulting 1S_e1S_h bleach is not linear with the number of excitons per nanocrystal. This finding holds for two samples differing significantly in size, demonstrating its generality. Analysis of the results suggests that below band edge induced absorption in hot exciton states is due to excited-state absorption and not to shifted absorption of cold carriers and that 1S_e1S_h bleach signals are not an accurate counter of sample excitons when their distribution includes multiexciton states

Impurity Sub-Band in Heavily Cu-Doped InAs Nanocrystal Quantum Dots Detected by Ultrafast Transient Absorption

The effect of Cu impurities on the absorption cross section, the rate of hot exction thermalization, and on exciton recombination processes in InAs quantum dots was studied by femtosecond transient absorption. Our findings reveal dynamic spectral effects of an emergent impurity sub-band near the bottom of the conduction band. Previously hypothesized to explain static photophysical properties of this system, its presence is shown to shorten hot carrier relaxation. Partial redistribution of interband oscillator strength to sub-band levels reduces the band edge bleach per exciton progressively with the degree of doping, even though the total linear absorption cross section at the band edge remains unchanged. In contrast, no doping effects were detected on absorption cross sections high in the conduction band, as expected due to the relatively high density of sates of the undoped QDs

Rewriting the Story of Excimer Formation in Liquid Benzene

Formation of benzene excimer following UV excitation of the neat liquid is monitored with femtosecond spectroscopy. A prompt rise component in excimer transient absorption, which contradicts the classical scenario of gradual reorientation and pairing of the excited monomers, is observed. Three-pulse experiments in which the population of evolving excimers is depleted by a secondary dump pulse demonstrate that the excimer absorption band is polarized along the interfragment axis. The experiments furthermore prove that the subsequent 4-fold increase in excimer absorption over ∼50 ps is primarily due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers forming excimers in a delayed fashion due to unfavorable initial geometry. Results are analyzed in light of recent studies of local structure in the liquid benzene combined with advanced electronic structure calculations. The prompt absorption rise is ascribed to excited states delocalized over nearby benzene molecules, which are sufficiently close and nearly parallel in the pure liquid. Such low-symmetry structures, which differ considerably from the optimized structures of isolated benzene dimer and solid benzene, are sufficiently abundant in liquid benzene. Electronic structure calculations confirm the orientation of transition dipoles of the excimers along the interparticle axis and demonstrate how slow refinement of the intermolecular geometry leads to a significant increase in the excimer absorption strength

Novel Spectral Decay Dynamics of Hot Excitons in PbSe Nanocrystals: A Tunable Femtosecond Pump–Hyperspectral Probe Study

Ultrafast exciton cooling in highly monodisperse PbSe nanocrystals is followed with tunable pump–hyperspectral near-IR probe spectroscopy. Unexpected kinetic and spectral correlations between induced bleach and absorption features are revealed, which are incompatible with standard models for excited nanocrystal absorption. Interband optical excitation immediately generates a sharp bleach feature near the 1S_h1S_e transition which is unchanged during exciton thermalization, while pumping well above the band edge induces an intense absorption at frequencies just below the band edge which decays concurrently with a buildup of renewed absorbance at the 1P_h1P_e peak during exciton cooling. Transient spectra of hot single and double excitons are nearly indistinguishable, arguing against the controversial involvement of Auger cooling in the rapid dissipation of excess energy in excited PbSe QDs. Finally, quantitative signal analysis shows no signs of multiexciton generation up to photon energies four times the sample band gap