The primary stages of the charge carrier photogeneration in C60 films studied by the 100-fs laser pulse pump-probe method

The primary stages of photoinduced processes are studied in thin C₆₀ films by the femtosecond laser pump-probe method. The films were excited by 100-fs laser pulses with photon energies above (wavelengths 345 and 367 nm) and below (645 nm) the mobility threshold, the fraction of excited molecules being no more than several percent. Upon probing in the spectral range from 400 to 1100 nm, several regions with substantially different decay kinetics were observed in the difference spectrum, which is caused by the simultaneous presence of several relaxing components. The appearance of the 465-and 500-nm bleaching bands in the difference spectrum upon excitation by photons with energies both above and below the mobility threshold, which are typical for electroabsorption spectra, suggests that charge carriers are produced in both these cases. The observed dependence of relaxation on the oxygen amount in the sample volume suggests that during excitation both charged (electrons and holes) and neutral (excited molecules) components are produced. The fraction of charged components is greater upon excitation into the fundamental band. The appearance of the 500-nm absorption band delayed by 10^?13–10^?14 s, the delay being increased in the presence of oxygen, was attributed to the formation of excited anions due to the capture of electrons by C₆₀ molecules. It is concluded that upon excitation of the films by photons with the energy below the mobility threshold, charge carriers are produced due to two-photon absorption rather than due to singlet-singlet annihilation. When the films are excited by photons above the mobility threshold, the primary charge carriers are produced by direct optical excitation.     

A plug‐and‐measure diagnostic tool for flux,resolution and beam size of a soft X‐ray beamline

A compact multi‐functional diagnostic tool has been installed for commissioning beamlines at the Taiwan Light Source. For a photon beam, the instrument can measure the photon flux, energy resolution and beam size, consecutively with a photodiode or gold mesh, a windowless gas cell and a movable fluorescence screen viewport. Two gratings with ruling densities of 350 and 1000 lines mm^?1 and dispersing photons of energies from 80 to 1200 eV were calibrated with a photon flux of 10¹¹ photon s^?1 at slit openings of 50 µm × 50 µm; a maximum resolving power of greater than 10000 was certified with an absorption spectra of gaseous samples. Pressure differences over four orders of magnitude were achieved between the ion chamber and the flux measurement chamber with a single capillary differential pumping stage. A sequence of beam profiles was measured by moving continuously in the vicinity of the nominal focal positions. This tool is useful in commissioning or trouble‐shooting at most beamlines in a synchrotron facility.     

Spectroscopic investigation of the ionization kinetics in XeCl laser discharges by Xe* density measurements

Time and spatially resolved absorption measurements of Xe^* particle number densities performed at an X-ray preionized self-sustained XeCl^*-laser discharge were used to investigate the reaction kinetics of such discharges and to test the validity of the commonly-made assumption that they are fit well by spatially homogeneous numerical models. It turned out that the Xe^* density distribution over the discharge cross section remains homogeneous for less than 100 ns only. Comparing the experimental results of the homogeneous discharge phase with zero-dimensional model calculations, we conclude that induced emission modeled using two excited xenon levels must be taken into account for the quantitative interpretation of the absorption measurements. For the interpretation of the Xe 6s-6p absorption measurements even a multilevel model will be necessary. Further we conclude from the relative courses and the absolute values of the Xe^* particle number densities that realistic HCl kinetics should contain three vibrationally excited levels and stepwise excitation processes as proposed by Dem'yanov et al. [8].     

Single-molecule detection using continuous wave excitation of two-photon fluorescence

Two-photon fluorescence (TPF) is one of the most important discoveries for biological imaging. Although a cw laser is known to excite TPF, its application in TPF imaging has been very limited due to the perceived low efficiency of excitation. Here we directly excited fluorophores with an IR cw laser used for optical trapping and achieved single-molecule fluorescence sensitivity: discrete stepwise photobleaching of enhanced green fluorescent proteins was observed. The single-molecule fluorescence intensity analysis and on-time distribution strongly indicate that a cw laser can generate TPF detectable at the single-molecule level, and thus opens the door to single-molecule TPF imaging using cw lasers.     

Computational model for simultaneous Q switching and mode-locking in a solid state laser with a saturable absorber

A numerical model of rate equations for a four-level solid state laser with Cr⁴⁺:YAG saturable absorber including excited state absorption ESA is presented. The cavity is divided into a large number of disks and the model is solved for each disk and its local corresponding photon flux. The flux array is shifted for each recurrence simulating the movement of photons inside the cavity during the round trip. This simulator can describe the mode-locking phenomenon and can be used to simulate simultaneous mode locking and Q switching with a saturable absorber.     

Analysis of209Bi and238U photofission cross section in the quasi-deuteron region of photonuclear absorption

An analysis of the photofission reactions in the quasi-deuteron energy range of photonuclear absorption (30–140 MeV) has been performed for²⁰⁹Bi and²³⁸U nuclei. Experimental cross section data available in the literature have been compared with calculated values obtained from a model in which the incoming photon is assumed to be absorbed by a neutron-proton pair (Levinger's quasi-deuteron photoabsorption), followed by a mechanism of evaporation-fission competition for the excited residual nuclei. The model has been shown to reproduce the main experimental features of²⁰⁹Bi and²³⁸U photofission cross section, although unexplained differences still remain in the case of²³⁸U-fission by 30– 50 MeV incident photons.     

Laser excited time-resolved low-temperature luminescence of nucleic acid bases single crystals

The first results on low temperature (80 K) one photon fluorescence of single crystals of nucleic acids bases excited by picosecond light pulses are reported. The excitation to the first absorption band of the crystals was realized by tuning the fundamental frequency of the passive modelocked Nd:YAG laser into UV spectral region. Our spectroscopic measurements confirm previous assumptions about the excimer character of the fluorescence of the crystals. Kinetic parameters of complex fluorescence decays show a strong dependence on the exciting wavelength.     

Time-resolved UV absorption of polyimide

The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm² excitation and recovers with a fast time constant 34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS ₁ and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S ₁) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.     

Sub- and super-Poissonian photon statistics of single-molecule fluorescence blinking

An analysis of intermittent fluorescence is presented for a single molecule driven by a continuous-wave laser field. The interruptions of fluorescence are caused by transition of the molecule to a triplet state. A method previously developed to calculate photon distribution for continuous-wave fluorescence is applied to analyze photon statistics of fluorescence blinking. The probability w _N (T) of counting N photons over a time interval T is derived for intermittent fluorescence. The photons counted over relatively short intervals are found to have a sub-Poissonian (narrower than Poisson) distribution. The photon distribution over intervals longer than the mean off time has a complicated form with two maxima; i.e., a super-Poissonian (wider than Poisson) distribution is obtained.     

Ultrafast optical responses of three-level systems in β-carotene: Resonance to a high-lying n 1 A g − excited state

Ultrafast optical phenomena in all-trans-β-carotene have been investigated by femtosecond absorption and fluorescence spectroscopy. Following a resonant pump pulse, both fluorescence and absorbance changes have a decay time of 150 fs. The signals are assigned to the lowest optically allowed singlet excited state, 1¹ B _u ⁺ . Transmittance changes induced by nonresonant pump pulses depend on the pump photon energy. They are interpreted in terms of the ac Stark effect in three-level systems and two-photon absorption of the pump and probe pulses generating a high-lying n ¹ A _g ^? state.     

On the mechanism of rhodamine 6G fluorescence quenching

The fluorescence of the widely used TPF dye rhodamine 6G is quenched by photons of the ruby as well as the Nd-glass laser. Taking into consideration the polarisation of the ground- and excited-state absorptions of rhodamine 6G a simple model of quenching results. The main process is an S₁ absorption with subsequent leave of the ordinary rhodamine 6G singlet system. For Nd-laser photons the cross section of the concerned transition following from our quenching experiments is σS₁→S₂ = 1.2×10^-16 cm².     

Short-pulse, high-brightness X-ray production with the PLEIADES Thomson-scattering source

PLEIADES is a compact, tunable, high-brightness, ultra-short-pulse, Thomson-scattering X-ray source. Picosecond pulses of hard X-rays (10–200 keV) are created by colliding an ultra-relativistic (20–100 MeV), picosecond-duration electron beam with a high-intensity, sub-picosecond, 800-nm laser pulse. Initial operation of this source has produced 78-keV X-rays with 10⁶ photons per pulse using a 57-MeV, 0.3-nC, 50-m rms width electron beam and a 180-mJ, 15-m rms width laser pulse. The angular distribution, energy, and energy spectrum of the source are found to agree well with theory and simulations. Source optimization is expected to increase X-ray output to between 10⁷ and 10⁸ photons per pulse with a peak brightness approaching 10²⁰ photons/s/0.1% bandwidth/mm²/mrad². PACS 41.50.+h; 07.85.Fv; 41.75.Ht; 42.62.-b     

GZK photons as ultra-high-energy cosmic rays

We calculate the flux of “GZK photons,” namely, the flux of ultra-high-energy cosmic rays (UHECRs) consisting of photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so-called GZK effect. We show that for primary nucleons, the GZK-photon fraction of the total UHECR flux is between 10⁻⁴ and 10^–2 above 10¹⁹ eV and up to the order of 0.1 above 10²⁰ eV. The GZK-photon flux depends on the assumed UHECR spectrum, the slope of the nucleon flux at the source, and the distribution of sources and intervening backgrounds. Detection of this photon flux would open the way for UHECR gamma-ray astronomy. Detection of a larger photon flux would imply the emission of photons at the source or new physics. We compare the photon fractions expected for GZK photons and the minimal fractions predicted by top-down models. We find that the photon fraction above 10¹⁹ eV is a crucial test for top-down models. The text was submitted by the authors in English.     

Photocaloric spectroscopy of the excited state absorption and of the fluorescence quantum efficiency in fluorescent material

A compensation photocalorimetric technique is described to determine the excited state absorption cross section σ₂ of fluorescent materials at the pump wavelength and also their fluorescence quantum efficiency Q_F. Excitation with a continuous, moderate power laser suffices. σ₂ is inferred from the pump intensity dependence of the heat conversion efficiency which is analyzed for materials without fluorescence upconversion. The σ₂ obtained is most useful as a calibration point for pump-and-probe excited state absorption measurement, for example, for an analysis of the optical pumping efficiency of laser materials. The technique is successfully tested with the well-characterized ruby (Al₂O₃:Cr³⁺) up to high excited state population densities.     

Photoemission of protons from 4He in the ⊿-resonance region

High resolution proton energy spectra from the ⁴He(γ,p) reaction have been measured with tagged photons in the range E_γ = 130–525 MeV using the large Mainz NaI(Tl) spectrometer at \([Theta _p^{lab} = 37.1^circ ]\) . Three separate reaction channels were identified, viz. ⁴He(γ,p)t two-body breakup, ⁴He photodisintegration via two-nucleon photon absorption processes and the quasifree pion production channel. Differential cross sections are presented for each of these channels as a function of photon energy. The sum of the two-nucleon photon absorption and the quasifree pion production differential cross sections, in the CM system, resembles that of the corresponding free-nucleon differential cross section when Fermi motion is taken into account.     

Synthesis and Spectroscopic Properties of Novel Fluorescent Compounds Containing Bis-pyrazole Ring

A series of pyrazoline derivatives containing pyrazole group were synthesized and characterized by means of ¹H NMR, FT-IR, MS and elemental analysis, and their UV–vis absorption behavior and fluorescent properties were also measured. Moreover, the influence of metal ions on spectral properties of synthetic products was examined with compound A ₅ as an example. It has been found that all synthetic pyrazoline derivatives show two remarkable absorption peaks at about 260 and 360 nm and the maximum emission peak at 445–467 nm. Especially, the joined of Co²⁺ can quench the fluorescence of compound A ₅ obviously.     

Ultra-short efficient laser-driven hard X-ray source operated at a kHz repetition rate

Ultra-short bursts of hard X-ray radiation are generated by interaction of high-contrast femtosecond laser pulses with a jet of liquid Ga. The X-ray emission shows a strong dependence on the angle of incidence and polarization of the laser beam, consistent with the processes of resonant absorption and vacuum heating. As much as 60% of the total X-ray emission consists of [_Ga]K radiation (9.22–9.25 keV) with a photon flux of 6×10⁹ photons/(ssrad). Using this novel X-ray source, static diffraction patterns from a GaAs(111) crystal were recorded with an acquisition time of 2 s. PACS 52.38.Ph; 52.59.Px; 52.50.Jm     

Photon time interval distributions of cathodoluminescence light from a YVO4 − Eu3+ phosphor

Measured photon time interval distributions and spectral noise densities justify the assumption of a Poisson distribution for the number of excitations per bombarding electron. A lifetime of 0.50 ± 0.02 ms was found for the ⁵D₀ excited Eu³⁺ state.     

Collective dissociative desorption of water molecules from glass produced by IFP-2000 flash tubes

When the radiation from an IFP-2000 lamp interacts with the surface crazed layer of glass, there is a weak spectral absorption not only in the vacuum ultraviolet but also in the range 420 > > 290 nm. The adsorbed H₂O molecules are desorbed mainly by the dissociation, for the concentration of molecular hydrogen in the products is 80–84%. At intensities E600 W/cm² and E4600 W/ cm², in accordance with the working conditions, the desorption becomes collective. The rate of integral photon absorption then attains 10¹⁹ photons/cm²·sec. The transition to collective desorption in the first case (at relatively low intensity) is due to the production of a plasma in the material as a result of a sliding discharge struck at the surface of the glass.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 48–51, April, 1981.     

Tachyonic γ-ray spectra of active galactic nuclei: Evidence for intrinsic spectral curvature

Tachyonic spectral densities of ultra-relativistic electron populations are fitted to the γ-ray spectra of two TeV blazars, the BL Lacertae objects 1ES 0229+200 and 1ES 0347-121. The spectral maps are compared to Galactic TeV sources, the γ-ray binary LS 5039 and the supernova remnant W28. In contrast to TeV photons, the extragalactic tachyon flux is not attenuated by interaction with the cosmic background light; there is no absorption of tachyonic γ-rays via pair creation, as tachyons do not interact with infrared background photons. The curvature of the observed γ-ray spectra is intrinsic, caused by the Boltzmann factor of the electron densities, and reproduced by a tachyonic cascade fit. In particular, the curvature in the spectral map of the Galactic microquasar is more pronounced than of the two extragalactic γ-ray sources. Estimates of the thermodynamic parameters of the thermal or, in the case of supernova remnant W28, shock-heated nonthermal electron plasma generating the tachyon flux are obtained from the spectral fits.