Electronic Relaxation Research Articles

Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and space

Measurements of the electronic circular dichroism (CD) are highly sensitive to the absolute configuration and conformation of chiral molecules and supramolecular assemblies and have therefore found widespread application in the chemical and biological sciences. Here, we demonstrate an approach to simultaneously follow changes in the CD and absorption response of photoexcited systems over the ultraviolet−visible spectral range with 100 fs time resolution. We apply the concept to chiral polyfluorene copolymer thin films and track their electronic relaxation in detail. The transient CD signal stems from the supramolecular response of the system and provides information regarding the recovery of the electronic ground state. This allows for a quantification of singlet−singlet annihilation and charge-pair formation processes. Spatial mapping of chiral domains on femtosecond time scales with a resolution of 50 μm and diffraction-limited steady-state imaging of the circular dichroism and the circularly polarised luminescence (CPL) of the films is demonstrated.

Optical Control of Multistage Phase Transition via Phonon Coupling in MoTe_{2}.

The temporal characters of laser-driven phase transition from 2H to 1T^{'} has been investigated in the prototype MoTe_{2} monolayer. This process is found to be induced by fundamental electron-phonon interactions, with an unexpected phonon excitation and coupling pathway closely related to the nonequilibrium relaxation of photoexcited electrons. The order-to-order phase transformation is dissected into three substages, involving energy and momentum scattering processes from optical (A_{1}^{'} and E^{'}) to acoustic phonon modes [LA(M)] in subpicosecond timescale. An intermediate metallic state along the nonadiabatic transition pathway is also identified. These results have profound implications on nonequilibrium phase engineering strategies.

3D laser Raman micro-spectroscopy of Er3+ and Yb3+ co-activated nano-composite phosphosilicate for industrial photonics applications

Phosphosilicate thin film and monolith co-doped with Er3+-Yb3+ were prepared by self-modified Sol-Gel. Spin coating was used to deposit the prepared samples. The calculated crystallite sizes were found to be 26 and 28 nm for (S22P0.5E0.5YM) sintered at 500 and 1000 °C, respectively. A thickness of 1.6 μm was obtained for (S22P0.5E0.5YM). Laser-based Raman microspectroscopy presents the homogenous distribution of the Er3+&Yb3+ ions. The Er3+&Yb3+ absorption coefficients were measured. The radiative properties for the metastable level 4I13/2 were determined using The Judd–Ofelt model. The electron relaxation time (τr) slightly increased with the Yb3+ ions concentration up to 19.41 ms. Photoluminescence increased with increasing the Yb3+ ions. Pumping the 4F transition 4I13/2 → 4I15/2 of Erbium ions with 650 nm laser emits at around 1500 nm. This study introduces promising results for the prepared samples to support the planar optical waveguide amplifier system that could be used in communication and laser applications. However its cost-effectiveness, on-site measurability is still an interesting matter challenge.

Electronic relaxation and dissociation dynamics in formaldehyde: pump wavelength dependence.

The effect of the incident UV pump wavelength on the subsequent excited state dynamics, electronic relaxation, and ultimate dissociation of formaldehyde is studied using first principles simulation and Coulomb explosion imaging (CEI) experiments. Transitions in a vibronic progression in the à ← X̃ absorption band are systematically prepared using a tunable UV source which generates pulses centered at 304, 314, 329, and 337 nm. We find, both via ab initio simulation and experimental results, that the rate of excited state decay and subsequent dissociation displays a prominent dependence on which vibronic transition in the absorption band is prepared by the pump. Our simulations predict that nonadiabatic transition rates and dissociation yields will increase by a factor of >100 as the pump wavelength is decreased from 337 to 304 nm. The experimental results and theoretical simulations are in broad agreement and both indicate that the dissociation rate plateaus rapidly after ≈2 ps following an ultrafast sub-ps rise.

The effect of electron–phonon and electron-impurity scattering on the electronic transport properties of silicon/germanium superlattices

Lattice strain environment of superlattices causes shift of electronic bands and electron relaxation times modulated electronic transport properties.

Shallow and deep trap states of solvated electrons in methanol and their formation, electronic excitation, and relaxation dynamics.

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (emet−) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions. The simulation identified four distinct trapping states with different OH coordination numbers. The simulation results also revealed that electronic transitions of emet− are primarily due to charge transfer between electron trapping sites (cavities) formed by OH and methyl groups, and that these transitions differ from hydrogenic electronic transitions involving aqueous solvated electrons (eaq−). Such charge transfer also explains the alkyl-chain-length dependence of the photoabsorption peak wavelength and the excited-state lifetime of solvated electrons in primary alcohols.

Ultrafast carrier kinetics at surface and interface of Sb<sub>2</sub>Se<sub>3</sub> film by transient reflectance

Antimony selenide (Sb<sub>2</sub>Se<sub>3</sub>) is a promising low-cost and environmentally-friendly semiconductor photovoltaic material. The power conversion efficiency of Sb<sub>2</sub>Se<sub>3</sub> solar cells has been improved to <inline-formula><tex-math id="Z-20220322113243-1">\begin{document}$ \sim $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211714_Z-20220322113243-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211714_Z-20220322113243-1.png"/></alternatives></inline-formula>10% in the past few years. The carrier recombination transfer dynamics is significant factor that affects the efficiency of Sb<sub>2</sub>Se<sub>3</sub> solar cells. In this work, carrier recombination on the Sb<sub>2</sub>Se<sub>3</sub> surface and carrier transfer dynamics at the CdS/Sb<sub>2</sub>Se<sub>3</sub> heterojunction interface are systematically investigated by surface transient reflectance. According to the evolution of relative reflectance change <inline-formula><tex-math id="M2">\begin{document}${{\Delta }{R}}/{{R}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211714_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20211714_M2.png"/></alternatives></inline-formula>, the carrier thermalization and band gap renormalization time of Sb<sub>2</sub>Se<sub>3</sub> are determined to be in a range from 0.2 to 0.5 ps, and carrier cooling time is estimated to be about 3－4 ps. Our results also demonstrate that both free electron and shallow-trapped electron transfer occur at the Sb<sub>2</sub>Se<sub>3</sub>/CdS interface after photo excitation. Our results present a method of explaining the transient reflectance of Sb<sub>2</sub>Se<sub>3</sub> and enhancing the understanding of carrier kinetics at Sb<sub>2</sub>Se<sub>3</sub> surface and Sb<sub>2</sub>Se<sub>3</sub>/CdS interface.

Hot carrier relaxation in CsPbBr3 nanocrystals: electron-hole asymmetry and shape effects.

Optimization of the optoelectronic performance of lead halide perovskite (LHP) nanocrystals calls for understanding and manipulation of their hot carrier relaxation processes. In this work, the hot carrier relaxation in a nanocube (NC) and a nanoplate (NPL) of CsPbBr3 is studied using non-adiabatic molecular dynamics based on first-principles calculations. Strong electron-hole asymmetry in the relaxation processes is observed. Regardless of the nanocrystal shape, the hot hole cooling rate is much faster than that of hot electrons. Moreover, while the hot-hole relaxation is insensitive to the excitation energy, faster relaxation of hot electrons is observed with a lower excitation energy. The origin of the asymmetry is associated with the orbital characters and density of states at the band edges. The hot-hole relaxation is strongly affected by the shape of the nanocrystal. It is faster in the NPL than in the NC. This is attributed to the larger atomic displacements in the NPL due to its higher surface/volume ratio. These results provide theoretical insights for fundamental understanding of excited-state dynamics in LHPs and may help the development of hot-carrier optoelectronic devices.

Photostability of 2,6-diaminopurine and its 2'-deoxyriboside investigated by femtosecond transient absorption spectroscopy.

Ultraviolet radiation (UVR) from the sun is essential for the prebiotic syntheses of nucleotides, but it can also induce photolesions such as the cyclobutane pyrimidine dimers (CPDs) to RNA or DNA oligonucleotide in prebiotic Earth. 2,6-Diaminopurine (26DAP) has been proposed to repair CPDs in high yield under prebiotic conditions and be a key component in enhancing the photostability of higher-order prebiotic DNA structures. However, its electronic relaxation pathways have not been studied, which is necessary to know whether 26DAP could have survived the intense UV fluxes of the prebiotic Earth. We investigate the electronic relaxation mechanism of both 26DAP and its 2'-deoxyribonucleoside (26DAP-d) in aqueous solution using steady-state and femtosecond transient absorption measurements that are complemented with electronic-structure calculations. The results demonstrate that both purine derivatives are significantly photostable to UVR. It is shown that upon excitation at 287 nm, the lowest energy 1ππ* state is initially populated. The population then branches following two relaxation coordinates in the 1ππ* potential energy surface, which are identified as the C2- and C6-relaxation coordinates. The population following the C6-coordinate internally converts to the ground state nonradiatively through a nearly barrierless conical intersection within 0.7 ps in 26DAP or within 1.1 ps in 26DAP-d. The population that follows the C2-relaxation coordinate decays back to the ground state by a combination of nonradiative internal conversion via a conical intersection and fluorescence emission from the 1ππ* minimum in 43 ps and 1.8 ns for the N9 and N7 tautomers of 26DAP, respectively, or in 70 ps for 26DAP-d. Fluorescence quantum yields of 0.037 and 0.008 are determined for 26DAP and 26DAP-d, respectively. Collectively, it is demonstrated that most of the excited state population in 26DAP and 26DAP-d decays back to the ground state via both nonradiative and radiative relaxation pathways. This result lends support to the idea that 26DAP could have accumulated in large enough quantities during the prebiotic era to participate in the formation of prebiotic RNA or DNA oligomers and act as a key component in the protection of the prebiotic genetic alphabet.

Exciton-vibrational dynamics induces efficient self-trapping in a substituted nanoring.

Cycloparaphenylenes, being the smallest segments of carbon nanotubes, have emerged as prototypes of the simplest carbon nanohoops. Their unique structure-dynamics-optical properties relationships have motivated a wide variety of synthesis of new related nanohoop species. Studies of how chemical changes, introduced in these new materials, lead to systems with new structural, dynamics and optical properties, expand their functionalities for optoelectronics applications. Herein, we study the effect that conjugation extension of a cycloparaphenylene through the introduction of a satellite tetraphenyl substitution has on its structural and dynamical properties. Our non-adiabatic excited state molecular dynamics simulations suggest that this substitution accelerates the electronic relaxation from the high-energy band to the lowest excited state. This is partially due to efficient conjugation achieved between specific phenyl units as introduced by the tetraphenyl substitution. We observe a particular exciton redistribution during relaxation, in which the tetraphenyl substitution plays a significant role. As a result, an efficient inter-band energy transfer takes place. Besides, the observed phonon-exciton interplay induces a significant exciton self-trapping. Our results encourage and guide the future studies of new phenyl substitutions in carbon nanorings with desired optoelectronic properties.

Ultrafast electronic dynamics and vibrational dynamics of SWCNT under alkylation and annealing

Using ultrashort visible pulse laser, we have performed transient absorption spectroscopy to study the electronic relaxation and vibrational dynamics for three SWCNT samples of non-substituted SWCNT, alkylated SWCNT, and annealed alkylated SWCNT. Estimated lifetimes of ∼100 fs and ∼1 ps can be assigned to intraband and interband relaxation processes, respectively. Compared with the non-substituted sample, the intraband transition rate was found to be 7% smaller for the alkylated sample and nearly equal to the annealed alkylated sample within 2%. It implies that the intraband relaxation rate is not affected by alkylation itself but inhibited by excess chemical modification. Conversely, the interband relaxation rate shows no difference between the three samples. Short-time Fourier transform analysis of the measured traces of pump–probe delay dependent difference absorbance visualizes the change of molecular vibration frequency reflecting the energy transfer from (6,5)-SWCNT to (6,4)-SWCNT and the molecular structure change during the interband transition.

Specific chemical bond relaxation unraveled by analysis of shake-up satellites in the oxygen single site double core hole spectrum of CO2

Important electronic relaxation and reorganization effects induced by single site double core hole formation and associated shake-up excitation are investigated in CO2 by means of an highly accurate computation method using non orthogonal basis set. Connection between the two main satellite bands and the two (non equivalent) carbon oxygen bonds are highlighted.

Theoretical and experimental investigation of femtosecond laser processing fused silica

By tracking the spatiotemporal distribution of the free electron density/temperature and laser intensity, the ablation threshold, depth and crater shape of fused silica for femtosecond laser processing are investigated theoretically and experimentally. The electron dynamics as well as the transient optical and thermophysical properties of laser irradiated fused silica is quantitatively determined. The numerical model is validated by comparing the calculated threshold fluence, depth and crater shape of ablation with the experimental data at the wavelength of 800 nm. The electron relaxation time at different laser fluences and pulse durations throughout the photoionization and impact ionization processes are probed. In the present paper, it is found that (1) the electron relaxation time significantly affects the material optical properties and femtosecond laser energy absorption. The optical properties change dramatically, and the fused silica becomes opaque during laser irradiation. Moreover, the transition from electron-phonon collision to electron-ion collision accompanies by the laser ablation of fused silica in the femtosecond laser irradiation process. (2) The experimentally observed saturation of the ablation depth at high laser fluence is elucidated by the proposed model, which is due to the significantly changed optical reflectivity and absorption coefficient. Both theoretical simulations and experimental observations found that laser fluence has a strong influence on the shape of the ablation crater. The ablation volume increases sharply with increasing laser fluence for femtosecond laser irradiation compared to that for picosecond laser irradiation. (3) With the increment of laser fluence, a saturation of the ablation depth removal efficiency and ablation efficiency occurs, followed by slight decrements. The ablation depth removal efficiency peaks at laser fluence close to 1.4 times of the ablation threshold. Whereas, the accuracy is slightly low due to the higher sensitivity of the ablation characteristics (ablation crater depth and ablation volume) to the shorter pulse laser. For the laser fluence higher than 3.5 times of the ablation threshold, good repeatability over a very wide fluence range enables accurate processing results, because a more consistent flat-bottom ablation profile tends to appear. However, the heat-affected zone leads to a decrement of the processing quality compared to that of laser close to the ablation threshold.

Probing the electronic relaxation pathways and photostability of the synthetic nucleobase Z via laser interfaced mass spectrometry.

The photostability of synthetic (unnatural) nucleobases is important in establishing the integrity of new genetic alphabets, and critical for developing healthy semisynthetic organisms. Here, we report the first study to explore the photostability and electronic decay pathways of the synthetic nucleobase, Z (6-amino-5-nitro-2(1H)-pyridone), combining UV laser photodissociation and collisional dissociation measurements to characterise the decay pathways across the region from 3.1-4.9 eV. Photoexcitation across this region produced the m/z 138 ion as the dominant photofragment, mirroring the dominant fragment produced upon higher-energy collisional excitation. Analysis of the ion-yield production curve profile for the m/z 138 ion indicates that it is produced following ultrafast excited state decay with boil off of the OH functional group of Z from the hot electronic ground state. Electronic structure calculations provide physical insight into why this is the dominant fragmentation pathway, since a node in the electron density along the C-OH bond is found for all tautomers of Z. While the dominant decay pathway for Z is consistent with ultrafast excited state decay, we also identify several minor dissociative photochemistry decay pathways, associated with intrinsic photoinstability. The results presented here can be used to guide the development of more photostable synthetic nucleobases.

Theoretical and experimental research of femtosecond laser processing fused silica

The ablation threshold, depth and crater shape of fused silica for femtosecond laser processing are investigated theoretically and experimentally. Based on tracking the spatiotemporal distribution of the free electron density, free electron temperature, and laser intensity, the electron dynamics as well as the transient optical and thermophysical properties of femtosecond laser irradiated fused silica are quantitatively determined. The numerical model is validated by comparing the calculated threshold fluence, depth and crater shape of ablation with the experimentally measured ones at a wavelength of 800 nm. The free electron relaxation time at different laser fluences and pulse durations throughout the photoionization process and impact ionization process are probed. In the present work, the findings are as follows. 1) The electron relaxation time significantly affects the material optical properties and femtosecond laser energy absorption. The optical properties change dramatically. The fused silica becomes opaque for the case of laser irradiation with fluence higher than the ablation threshold. Moreover, the transition from electron-phonon collision to electron-ion collision accompanies with the femtosecond laser ablation of fused silica. 2) By using the proposed model, the experimentally observed saturation of ablation depth at high laser fluence is elucidated by the significant change of optical reflectivity and absorption coefficient. Both the results of theoretical simulation and experimental observation indicate that laser fluence has a strong influence on the shape of the ablation crater. The ablation volume increases sharply with the increase of laser fluence for femtosecond laser irradiation, compared with that for picosecond laser irradiation. 3) With the increase of femtosecond laser fluence, the ablation depth removal efficiency and ablation efficiency are both saturated, followed by slight decrements. The peak of ablation depth removal efficiency peak occurs at the femtosecond laser fluence close to 1.4 times of the ablation threshold. While the accuracy is slightly low due to the higher sensitivity of the ablation characteristics (ablation crater depth and ablation volume) to the shorter femtosecond laser pulse. For the femtosecond laser fluence higher than 3.5 times of the ablation threshold, good repeatability over a very wide fluence range can achieve accurate processing results, because a more consistent flat-bottom ablation profile tends to appear. However, the heat-affected zone leads the processing quality to degrade, compared with the scenario of femtosecond laser fluence close to the ablation threshold.

Ageing of PEEK/Carbon Fibre composite under electronic irradiations: Influence on mechanical behaviour and charge transport

Polymer used in satellite manufacturing are exposed to surface charging phenomenon due to electronic irradiations. This phenomenon can induce electrostatic discharges (ESD) which can lead to failures. In order to limit this phenomenon, we propose to develop PEEK composite reinforced with Short Carbon Fibres (SCF). We studied the behaviour of these composites in pristine state and after electronic irradiations compared to PEEK. In pristine state, fibres induce a faster surface electron relaxation. This behaviour shows that SCF can reduce ESD risks in space which is a first validation of the concept. After irradiations, PEEK ageing mechanisms are not modified by the presence of fibres. However, SCF stabilise both the evolution of mechanical behaviour and surface potential relaxation compared to irradiated PEEK. This stabilisation comes from two contributions: fibres stabilise the ageing of the matrix and hides the influence of this ageing on composite mechanical and electrical behaviours.

Increased Photostability of the Integral mRNA Vaccine Component N1 -Methylpseudouridine Compared to Uridine.

N1 -Methylation of pseudouridine (m1 ψ) replaces uridine (Urd) in several therapeutics, including the Moderna and BioNTech-Pfizer COVID-19 vaccines. Importantly, however, it is currently unknown if exposure to electromagnetic radiation can affect the chemical integrity and intrinsic stability of m1 ψ. In this study, the photochemistry of m1 ψ is compared to that of uridine by using photoirradiation at 267 nm, steady-state spectroscopy, and quantum-chemical calculations. Furthermore, femtosecond transient absorption measurements are collected to delineate the electronic relaxation mechanisms for both nucleosides under physiologically relevant conditions. It is shown that m1 ψ exhibits a 12-fold longer 1 ππ* decay lifetime than uridine and a 5-fold higher fluorescence yield. Notably, however, the experimental results also demonstrate that most of the excited state population in both molecules decays back to the ground state in an ultrafast time scale and that m1 ψ is 6.7-fold more photostable than Urd following irradiation at 267 nm.

Influence of La3+ ion doping on electrical conduction and thermal stability of NiO nanostructures

Ni1-xLaxO (x = 0.00, 0.01, and 0.03) nanostructures have been synthesized by the conventional sol-gel method, and further the electrical conduction and thermal stability have been investigated. It is observed that ε′ decreases with increasing frequency and does not exhibit any significant change with temperature T < 150 °C. It attains a high value ε′ ∼103-104 (75 kHz and 300 °C) for NiO sample. The high value of ε′ in pure and LNO samples might be explained via electron relaxation–mode-coupling model, associated with oxygen vacancies (OVs) which become dominant at high temperature. The high-temperature dielectric relaxation behavior was also observed in all the samples. The calculated activation energies (Ucond and Uac) from DC and AC conductivities, revealing that the relaxation behavior associated with the migration of OVs as confirmed by the EPR spectroscopy. On the other hand, a highly doped (x = 0.03) LNO sample shows high thermal stability than pure NiO, which needs a much higher degradation temperature, i.e., above 500 °C. The thermal activation energy was also determined in the temperature range of 80–150 °C.

Ultrafast relaxation of photoinjected nonthermal electrons in the Γ valley of GaAs studied by time- and angle-resolved photoemission spectroscopy

We study ultrafast relaxation of nonthermal electrons confined within the \ensuremath{\Gamma} valley of GaAs, based on the momentum-resolved transient electron distribution functions determined using time- and angle-resolved photoemission spectroscopy at 293 K. To elucidate the fundamental processes that lead to the electronic quasithermalization at the nonthermal regime of relaxation, the dynamics in both $p$- and $n$-type samples of GaAs are investigated. In $p$-type GaAs, photoinjected electrons form the electron ensembles quasiequilibrated only in the momentum space within 100 fs of excitation by the Coulomb interaction among photoinjected electrons. However, the ensembles maintain strong nonthermal populations in the energy space. They are quasithermalized only after certain time delays of a few hundred femtoseconds, which depend on the excess energy of photoinjected electrons. These features in $p$-type samples are not sensitively dependent on the excitation density in the range from $1.5\ifmmode\times\else\texttimes\fi{}{10}^{16}$ to $5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$. On the other hand, in $n$-type samples, which include the cold electrons populated near the conduction-band minimum with the density of $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$, photoinjected electrons are quasithermalized within 170 fs of excitation. The main interaction for the fast quasithermalization is attributed to the inelastic scattering among the cold electrons and photoinjected electrons by a dynamics screened Coulomb interaction like the electronic thermalization in metals.

Magnetic and dielectric properties of BaFe1/2Sn1/2O3-δ ceramics

Ceramic samples of barium ferrostannate BaFe1/2Sn1/2O3-δ have been prepared by the sol-gel and hydrothermal synthesis methods. To reduce the number of oxygen vacancies, annealing in oxygen has been performed. The heat capacity, magnetization, Mössbauer and dielectric spectroscopy data point to the appearance of antiferromagnetic ordering below ∼55 K followed by further spin glass magnetic ordering below 15 K. The giant dielectric response seems to be due to the relaxation of electrons trapped by oxygen vacancies. Comparison of the Fe K-edge X-ray absorption spectra of BaFe1/2Sn1/2O3-δ confirmed substantial difference from those of the similar stoichiometric BaFe1/2Nb1/2O3 perovskite.