More Like this

1. Direct mapping of curve-crossing dynamics in IBr by attosecond transient absorption spectroscopy

   https://doi.org/10.1126/science.aax0076  

   Kobayashi, Yuki ; Chang, Kristina F. ; Zeng, Tao ; Neumark, Daniel M. ; Leone, Stephen R. ( July 2019 , Science)

   The electronic character of photoexcited molecules can abruptly change at avoided crossings and conical intersections. Here, we report direct mapping of the coupled interplay between electrons and nuclei in a prototype molecule, iodine monobromide (IBr), by using attosecond transient absorption spectroscopy. A few-femtosecond visible pulse resonantly excites the B ( Π 3 0 + ) , Y(0 + ), and Z(0 + ) states of IBr, and the photodissociation dynamics are tracked with an attosecond extreme-ultraviolet pulse that simultaneously probes the I-4 d and Br-3 d core-level absorption edges. Direct comparison with quantum mechanical simulations unambiguously identifies the absorption features associated with adiabatic and diabatic channels at the B/Y avoided crossing and concurrent two-photon dissociation processes that involve the Y/Z avoided crossing. The results show clear evidence for rapid switching of valence-electronic character at the avoided crossing. 

   more » « less
2. Hot-carrier dynamics in InAs/AlAsSb multiple-quantum wells

   https://doi.org/10.1038/s41598-021-89815-y  

   Piyathilaka, Herath P. ; Sooriyagoda, Rishmali ; Esmaielpour, Hamidreza ; Whiteside, Vincent R. ; Mishima, Tetsuya D. ; Santos, Michael B. ; Sellers, Ian R. ; Bristow, Alan D. ( May 2021 , Scientific Reports)

   A type-II InAs/AlAs$$_{0.16}$$${}_{0.16}$Sb$$_{0.84}$$${}_{0.84}$multiple-quantum well sample is investigated for the photoexcited carrier dynamics as a function of excitation photon energy and lattice temperature. Time-resolved measurements are performed using a near-infrared pump pulse, with photon energies near to and above the band gap, probed with a terahertz probe pulse. The transient terahertz absorption is characterized by a multi-rise, multi-decay function that captures long-lived decay times and a metastable state for an excess-photon energy of$$>100$$$>100$meV. For sufficient excess-photon energy, excitation of the metastable state is followed by a transition to the long-lived states. Excitation dependence of the long-lived states map onto a nearly-direct band gap ($$E{_g}$$$E{}_g$) density of states with an Urbach tail below$$E{_g}$$$E{}_g$. As temperature increases, the long-lived decay times increase$$$<E{}_g$, due to the increased phonon interaction of the unintentional defect states, and by phonon stabilization of the hot carriers$$>E{_g}$$$>E{}_g$. Additionally, Auger (and/or trap-assisted Auger) scattering above the onset of the plateau may also contribute to longer hot-carrier lifetimes. Meanwhile, the initial decay component shows strong dependence on excitation energy and temperature, reflecting the complicated initial transfer of energy between valence-band and defect states, indicating methods to further prolong hot carriers for technological applications.

    

   more » « less
3. Validation of Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM) With Long‐Term GOES MAGED Measurements of keV Electron Fluxes at Geostationary Orbit

   https://doi.org/10.1029/2018SW002028  

   Ganushkina, N. Yu ; Sillanpää, I. ; Welling, D. ; Haiducek, J. ; Liemohn, M. ; Dubyagin, S. ; Rodriguez, J. V. ( May 2019 , Space Weather)

   Surface charging by keV (kiloelectron Volt) electrons can pose a serious risk for satellites. There is a need for physical models with the correct and validated dynamical behavior. The 18.5‐month (2013–2015) output from the continuous operation online in real time as a nowcast of the Inner Magnetosphere Particle Transport and Acceleration Model (IMPTAM) is compared to the GOES 13 MAGnetospheric Electron Detector (MAGED) data for 40, 75, and 150 keV energies. The observed and modeled electron fluxes were organized by Magnetic Local Time (MLT) and IMPTAM driving parameters; the observed Interplanetary Magnetic Field (IMF)B_Z,B_Y, and |B|; the solar wind speedV_SW; the dynamic pressureP_SW; andKpandSYM‐Hindices. The peaks for modeled fluxes are shifted toward midnight, but the ratio between the observed and modeled fluxes at around 06 MLT is close to 1. All the statistical patterns exhibit very similar features with the largest differences of about 1 order of magnitude at 18–24 MLT. Based on binary event analysis, 20–78% of threshold crossings are reproduced, but Heidke skill scores are low. The modeled fluxes are off by a factor of 2 in terms of the median symmetric accuracy. The direction of the error varies with energy: overprediction by 50% for 40 keV, overprediction by 2 for 75 keV, and underprediction by 18% for 150 keV. The revealed discrepancies are due to the boundary conditions developed for ions but used for electrons, absence of substorm effects, representations of electric and magnetic fields which can result in not enough adiabatic acceleration, and simple models for electron lifetimes.

    

   more » « less
4. Recent development of carbon quantum dots regarding their optical properties, photoluminescence mechanism, and core structure

   https://doi.org/10.1039/c8nr10059d  

   Mintz, Keenan J. ; Zhou, Yiqun ; Leblanc, Roger M. ( March 2019 , Nanoscale)

   Carbon quantum dots (CDs) are a relatively new class of carbon nanomaterials which have been studied very much in the last fifteen years to improve their already favorable properties. The optical properties of CDs have drawn particular interest as they display the unusual trait of excitation-dependent emission, as well as high fluorescence quantum yields (QY), long photoluminescence (PL) decay lifetimes, and photostability. These qualities naturally lead researchers to apply CDs in the field of imaging (particularly bio-imaging) and sensing. Since the amount of publications regarding CDs has been growing nearly exponentially in the last ten years, many improvements have been made in the optical properties of CDs such as QY and PL lifetime. However, a great deal of confusion remains regarding the PL mechanism of CDs as well as their structural properties. Therefore, presented in this review is a summary and discussion of the QYs and PL lifetimes reported in recent years. The effect of method as well as precursor has been evaluated and discussed appropriately. The current theories regarding the PL mechanism of CDs are discussed, with special attention to the concept of surface state-controlled PL. With this knowledge, the improvement of preparation and applications of CDs related to their optical properties will be easily accomplished. Further improvements can be made to CDs through the understanding of their structural and optical properties. 

   more » « less
5. Comparative study of rare-earth doped low-phonon fluoride and chloride crystals for mid-IR laser potential

   E. Brown, Z. D. ( April 2021 , Proceedings SPIE 11723, Laser Technology for Defense and Security XVII)

   A comparative study was performed on the mid-infrared emission properties of trivalent erbium (Er3+) and holmium (Ho3+) doped fluorides (BaF2, NaYF4) and ternary chloride-based crystals (CsCdCl3, KPb2Cl5,). All crystals were grown by vertical Bridgman technique. Following optical excitation at 800 nm, all Er3+ doped fluorides and chlorides exhibited mid-infrared emissions at ~4500 nm at room temperature. The mid-infrared emission at 4500 nm, originating from the 4I9/2 -> 4I11/2 transition, showed long emission lifetime values of ~11.6 ms and ~3.2 ms for Er3+ doped CsCdCl3 and KPb2Cl5 crystals, respectively. In comparison, Er3+ doped and BaF2 and NaYF4 demonstrated rather short lifetimes in the microsecond range of ~47 us and ~205 us, respectively. Temperature dependent decay time measurements were performed for the 4I9/2 excited state for Er3+ doped BaF2, NaYF4, and CsCdCl3 crystals. We noticed that the emission lifetimes of Er3+:CsCdCl3 were nearly independent of the temperature, whereas significant emission quenching of 4I9/2 level was observed for both Er3+ doped fluoride crystals. The temperature dependence of the multiphonon relaxation rate for 4.5 um mid-IR emissions was determined for the studied Er3+ doped fluorides using the well-known energy-gap law. Using ~890 nm excitation, all studied Ho3+ doped fluorides and chlorides exhibited mid-infrared emissions at ~3900 nm originating from the 5I5 -> 5I6 transition. The longest emission lifetime of the 5I5 level was determined to be ~14.55 ms from the Ho3+:CsCdCl3 crystal. The room temperature stimulated emission cross-sections for the Er3+ 4I9/2 -> 4I11/2 and Ho3+ 5I5 -> 5I6 transitions were determined using the Füchtbauer-Landenburg equation. Among the studied crystals, Er3+ doped chlorides are more than two orders of magnitude better in terms of emission lifetimes and sigma-tau product than the fluoride crystals. 

   more » « less