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Mechanistic Insight into the Carbon Dots: Protonation induced Photoluminescence
摘要： Although several theories have been proposed, the mechanism of complex photoluminescence in carbon dots (CNDs) is a central quest till date. This report presents pH dependent steady state and time resolved spectroscopy study which identifies a possible origin of the complex photoluminescence in CNDs. The multiple emissive species created by the excited state protonation-deprotonation reaction at certain pH gives rise to inhomogeneous broadening and consequently excitation dependent multicolour emission. The origin of the excited state dynamics is attributed to the significant change of the proton dissociation between ground and excited state. We present a new model on protonation dynamics and show how it affects the emissive states in CNDs.
关键词： time resolved fluorescence,origin of photoluminescence,pH dependence,Carbon dots,Excited state protonation
更新于2025-09-10 09:29:36
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Assessment of Gate Width Size on Lifetime-Based F?rster Resonance Energy Transfer Parameter Estimation
摘要： F?rster Resonance Energy Transfer (FRET) enables the observation of interactions at the nanoscale level through the use of fluorescence optical imaging techniques. In FRET, fluorescence lifetime imaging can be used to quantify the fluorescence lifetime changes of the donor molecule, which are associated with proximity between acceptor and donor molecules. Among the FRET parameters derived from fluorescence lifetime imaging, the percentage of donor that interacts with the acceptor (in proximity) can be estimated via model-based fitting. However, estimation of the lifetime parameters can be affected by the acquisition parameters such as the temporal characteristics of the imaging system. Herein, we investigate the effect of various gate widths on the accuracy of estimation of FRET parameters with focus on the near-infrared spectral window. Experiments were performed in silico, in vitro, and in vivo with gate width sizes ranging from 300 ps to 1000 ps in intervals of 100 ps. For all cases, the FRET parameters were retrieved accurately and the imaging acquisition time was decreased three-fold. These results indicate that increasing the gate width up to 1000 ps still allows for accurate quantification of FRET interactions even in the case of short lifetimes such as those encountered with near-infrared FRET pairs.
关键词： fluorescence lifetime,F?rster Resonance Energy Transfer (FRET),gated ICCD,near infrared (NIR) dyes,time-resolved imaging,gate width,in vivo imaging
更新于2025-09-10 09:29:36
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Time-resolved universal temperature measurements using NaYF <sub/>4</sub> :Er <sup>3+</sup> ,Yb <sup>3+</sup> upconverting nanoparticles in an electrospray jet
摘要： Hexagonal upconverting nanoparticles (UCNPs) of NaYF4:Er3+,Yb3+ (ca. 300 nm) have been widely used to measure the temperature at the nanoscale using luminescence ratio thermometry. However, several factors limit their applications. For example, changes in the peak shape, mainly is the S-band emission, hinders their ability to be used as a universal temperature sensor. Herein, we introduce a universal calibration protocol for NaYF4:Er3+,Yb3+ upconverting nanoparticles that is robust to environmental changes and gives a precise temperature measurement. We used this new procedure to calculate the temperature profile inside a Taylor cone generated with an electrospray jet. Inside the Taylor cone the fluid velocity increases toward the tip of the cone. A constant acquisition length leads to a decrease in excitation and acquisition time. This decrease in excitation time causes a peak shape change that corrupts the temperature measurement if the entire peak shape is integrated in the calibration. Our universal calibration circumvents this problem and can be used for time-resolved applications. The temperature at the end of the Taylor cone increases due to the creation of a whispering gallery mode cavity with 980 nm excitation. We use time-resolved energy balance equations to support our optical temperature measurements inside the Taylor cone. We believe that the findings of this paper provide a foundation for time-resolved temperature measurements using NaYF4:Er3+,Yb3+ upconverting nanoparticles and can be used to understand temperature-dependent reactions such as protein unfolding inside microjet/microdroplets and microfluidic systems.
关键词： upconverting nanoparticles,microjet,nanothermometry,electrospray,temperature measurement,time-resolved measurement
更新于2025-09-10 09:29:36
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Coulomb explosion imaging of CH <sub/>3</sub> I and CH <sub/>2</sub> ClI photodissociation dynamics
摘要： The photodissociation dynamics of CH3I and CH2ClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide m/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH2ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion.
关键词： pixel imaging mass spectrometry,time-resolved,CH3I,velocity map imaging,photodissociation dynamics,Coulomb explosion imaging,CH2ClI
更新于2025-09-10 09:29:36
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Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator
摘要： Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nano-oscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a non-linear 'bullet' mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal.
关键词： nano-contact spin Hall nano-oscillator,Time-resolved scanning Kerr microscopy,bullet mode,injection locking,ferromagnetic resonance
更新于2025-09-10 09:29:36
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Time-resolved double-resonance spectroscopy: Lifetime measurement of the 6?1Σg+(7,31) electronic state of molecular sodium
摘要： We report on the lifetime measurement of the 6 1Σ+g (7, 31) state of Na2 molecules, produced in a heat-pipe oven, using a time-resolved spectroscopic technique. The 6 1Σ+g (7, 31) level was populated by two-step two-color double resonance excitation via the intermediate A 1Σ+u (8, 30) state. The excitation scheme was done using two synchronized pulsed dye lasers pumped by a Nd:YAG laser operating at the second harmonics. The fluorescence emitted upon decay to the final state was measured using a time-correlated photon counting technique, as a function of argon pressure. From this, the radiative lifetime was extracted by extrapolating the plot to collision-free zero pressure. We also report the calculated radiative lifetimes of the Na2 6 1Σ+g ro-vibrational levels in the range of v = 0–200 with J = 1 and J = 31 using the LEVEL program for bound-bound and the BCONT program for bound-free transitions. Our calculations reveal the importance of the bound-free transitions on the lifetime calculations and a large difference of about a factor of three between the J = 1 and J = 31 for the v = 40 and v = 100, respectively, due to the wavefunction alternating between having predominantly inner and outer well amplitude.
关键词： radiative lifetime,lifetime measurement,Na2 molecules,bound-bound transitions,time-resolved spectroscopic technique,bound-free transitions,double resonance excitation
更新于2025-09-09 09:28:46
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Time-resolved second harmonic generation with single-shot phase sensitivity
摘要： A time-resolved, phase-sensitive second harmonic generation (SHG) method to probe the excited state dynamics of interfacial species is presented. It is based on an interference measurement between the SHG from a sample and a local oscillator generated at a reference surface in which an entire interference pattern is recorded in a single shot by using a spatially varying phase unit comprised of a pair of wedges that sandwich the reference sample. In combination with 30 kHz modulation of the experiment, shot-to-shot pump-probe measurements are presented. The technique is characterised by measuring the time-resolved change in the amplitude and phase of the interference pattern due to the excited state dynamics of the dye malachite green at the air/water interface. The key attributes of the technique are its excellent phase stability and sensitivity, and relatively short data acquisition times.
关键词： air/water interface,excited state dynamics,time-resolved,second harmonic generation,malachite green,phase-sensitive,interference measurement
更新于2025-09-09 09:28:46
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Theory of time-resolved Raman scattering in correlated systems: Ultrafast engineering of spin dynamics and detection of thermalization
摘要： Ultrafast characterization and control of many-body interactions and elementary excitations are critical to understanding and manipulating emergent phenomena in strongly correlated systems. In particular, spin interaction plays an important role in unconventional superconductivity, but efficient tools for probing spin dynamics, especially out of equilibrium, are still lacking. To address this question, we develop a theory for nonresonant time-resolved Raman scattering, which can be a generic and powerful tool for nonequilibrium studies. We also use exact diagonalization to simulate the pump-probe dynamics of correlated electrons in the square-lattice single-band Hubbard model. Different ultrafast processes are shown to exist in the time-resolved Raman spectra and dominate under different pump conditions. For high-frequency and off-resonance pumps, we show that the Floquet theory works well in capturing the softening of bimagnon excitation. By comparing the Stokes and anti-Stokes spectra, we also show that effective heating dominates at small pump fluences, while a coherent many-body effect starts to take over at larger pump amplitudes and frequencies on resonance to the Mott gap. Time-resolved Raman scattering thereby provides the platform to explore different ultrafast processes and design material properties out of equilibrium.
关键词： thermalization,Hubbard model,spin dynamics,correlated systems,time-resolved Raman scattering,Floquet theory
更新于2025-09-09 09:28:46
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Time-Resolved Spectroscopic and Density Functional Theory Investigation of the Photogeneration of a Bifunctional Quinone Methide in Neutral and Basic Aqueous Solutions
摘要： Binol quinone methides (BQMs) can be generated from 1,1′-(2,2′-dihydroxy-1,1′-binaphthyl-6,6′-diyl)bis(N,N,N-trimethylmethanamiuium) bromide (BQMP-b) in a 1:1 MeCN:H2O mixed solution via a ground state intramolecular proton transfer (GSIPT), as mentioned in our previously reported studies. Here, the photoreaction of BQMP-b in neutral and basic aqueous solution (pH = 7, 10, 12) was investigated to explore the possible mechanisms and the key intermediates produced in the process of the photoreaction and to examine whether they are different from those in a neutral mild-mixed MeCN:H2O solution. The studies were conducted using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) in conjunction with results from density functional theory (DFT) computations. The results showed that BQMP-b was deprotonated initially and produced BQMs species more effectively through an E1bc elimination reaction in a strong basic aqueous condition (pH = 12), which differed from the reaction pathway that took place in the solution with pH = 7 or 10. A related single naphthol ring molecule 1-(6-hydroxynaphthalen-2-yl)-N,N,N-trimethylmethanaminium bromide (QMP-b) that did not contain a second naphthol ring was also investigated. The related reaction mechanisms are elucidated in this work, and it is briefly discussed how the mechanisms vary as a function of aqueous solution pH conditions.
关键词： E1bc elimination reaction,time-resolved resonance Raman,DFT calculation,bifunctional quinone methides
更新于2025-09-09 09:28:46
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Time-resolved structure analysis of piezoelectric crystals by X-ray diffraction under alternating electric field
摘要： Rare-earth substitution effects on atomic motions in resonantly vibrating piezoelectric oscillators of langasite-type crystals, namely, La3Ga5SiO14 (LGS) and Nd3Ga5SiO14 (NGS), are revealed by time-resolved X-ray crystal structure analysis under alternating electric fields. Deformations of Ga–O–Ga and Ga–O–Ga/Si bond angles accompanying deformations of RE–O (RE: La or Nd) bond lengths found in LGS are suppressed in NGS. Alternatively, rigid GaO6 octahedra are deformed in NGS. The decreases in RE–O bond lengths and Ga–O–Ga and Ga–O–Ga/Si bond angles caused by the substitution of La by Nd would make the bond lengths and angles more difficult to deform under electric fields; hence, the piezoelectric constants of NGS are smaller than those of LGS.
关键词： rare-earth substitution,piezoelectric crystals,alternating electric field,X-ray diffraction,time-resolved structure analysis
更新于2025-09-09 09:28:46