Subsequent Light Pulse Research Articles

Simulation of coaxial time-of-flight measurements using SiPM as detector

Modern LiDAR sensors find increasing use in safety–critical applications. Therefore, highly accurate modeling of the system’s behavior under demanding environmental conditions is necessary. In this paper, we present a modular structure to accurately simulate the amplified raw detector signal of a direct time-of-flight LiDAR system for coaxial transmitter–receiver optics using a silicon photomultiplier (SiPM) as a detector. The SiPM causes strong non-linearities of the response due to its response to subsequent light pulses. To verify the model’s predictions, single-point measurements for targets of different reflectivity at defined distances were performed. Statistical analysis shows an R-squared value greater than 0.990 for simulated and measured signal amplitude levels. Noise modeling shows good accordance with the performed measurements for different target irradiance levels. The presented results have a guiding significance in the modeling of the complex signal processing chain of LiDAR systems based on SiPM detectors, as it enables the prediction of key parameters of the system early in the development process. Hence, unnecessary costs by design flaws can be mitigated.

A simple routine for quantitative analysis of light and dark kinetics of photochemical and non-photochemical quenching of chlorophyll fluorescence in intact leaves.

Paper describes principles and application of a novel routine that enables the quantitative analysis of the photochemical O–J phase of the variable fluorescence F v associated with the reversible photo-reduction of the secondary electron acceptor QA of photosystem II (PSII) in algae and intact leaves. The kinetic parameters that determine the variable fluorescence F PP(t) associated with the release of photochemical quenching are estimated from 10 µs time-resolved light-on and light-off responses of F v induced by two subsequent light pulses of 0.25 (default) and 1000 ms duration, respectively. Application of these pulses allows estimations of (i) the actual value of the rate constants k L and k AB of the light excitation (photoreduction of QA) and of the dark re-oxidation of photoreduced QA ({text{Q}}_{text{A}}^{ - }), respectively, (ii) the actual maximal normalized variable fluorescence [nF v] associated with 100 % photoreduction of QA of open RCs, and (iii) the actual size β of RCs in which the re-oxidation of {text{Q}}_{text{A}}^{ - } is largely suppressed (QB-nonreducing RC with k AB ~ 0). The rate constants of the dark reversion of Fv associated with the release of photo-electrochemical quenching F PE and photo-electric stimulation F CET in the successive J–I and I–P parts of the thermal phase are in the range of (100 ms)−1 and (1 s)−1, respectively. The kinetics of fluorescence changes during and after the I–P phase are given special attention in relation to the hypothesis on the involvement of a Δµ H+-dependent effect during this phase and thereafter. Paper closes with author’s personal view on the demands that should be fulfilled for chlorophyll fluorescence methods being a correct and unchallenged signature of photosynthesis in algae and plants.

Influence of magnetocrystalline anisotropy on the magnetization dynamics of magneticmicrostructures

The study of magnetodynamics using stroboscopic time-resolved x-ray photoemissionelectron microscopy (TR-XPEEM) involves an intrinsic timescale provided by the pulsestructure of the synchrotron radiation. In the usual multi-bunch operation mode, the timespan between two subsequent light pulses is too short to allow a relaxation of thesystem into the ground state before the next pump–probe cycle starts. Usinga deflection gating mechanism described in this paper we are able to pick thephotoemission signal resulting from selected light pulses. Thus, PEEM measurements canbe carried out in a flexible timing scheme with longer delays between two lightpulses. Using this technique, the magnetodynamics of both Permalloy and ironstructures have been investigated. The differences in the dynamic response on a shortmagnetic field pulse are discussed with respect to the magnetocrystalline anisotropy.

GABA interacts with photic signaling in the suprachiasmatic nucleus to regulate circadian phase shifts

Circadian rhythms of physiology and behavior in mammals are driven by a circadian pacemaker located in the suprachiasmatic nucleus of the hypothalamus. The majority of neurons in the suprachiasmatic nucleus are GABAergic, and activation of GABA receptors in the suprachiasmatic nucleus can induce phase shifts of the circadian pacemaker both in vivo and in vitro. GABA also modulates the phase shifts induced by light in vivo, and photic information is thought to be conveyed to the suprachiasmatic nucleus by glutamate. In the present study, we examined the interactions between GABA receptor agonists, glutamate agonists, and light in hamsters in vivo. The GABA A receptor agonist muscimol and the GABA B receptor agonist baclofen were microinjected into the suprachiasmatic nucleus at circadian time 13.5 (early subjective night), followed immediately by a microinjection of N-methyl- D-aspartate (NMDA). Both muscimol and baclofen significantly reduced the phase shifting effects of NMDA. Further, coadministration of tetrodotoxin with baclofen did not alter the inhibition of NMDA by baclofen, suggesting a postsynaptic mechanism for the inhibition of NMDA-induced phase shifts by baclofen. Finally, the phase shifting effects of microinjection of muscimol into the suprachiasmatic nucleus during the subjective day were blocked by a subsequent light pulse. These data suggest that GABA regulates the phase of the circadian clock through both pre- and postsynaptic mechanisms.

Two photon photoemission of deposited silver clusters

We use time resolved two photon photoemission to study the stability of size selected silver clusters deposited onto highly oriented pyrolytic graphite (HOPG) substrates. Size-selected Agn+ clusters (n=2-9) are deposited at low coverage onto HOPG surfaces at liquid nitrogen temperatures. After deposition, the samples are irradiated by a series of ultrashort laser pulse pairs. Photoelectrons created by two photon photoemission are collected in a magnetic bottle type time-of-flight photoelectron spectrometer. Their kinetic energy distribution is recorded as a function of the delay time between subsequent light pulses. With the exception of Ag3 the size dependence of the photoelectron spectra reveals a pronounced odd/even effect, which is well known for gas phase silver clusters. This indicates that the deposited clusters retain their size and identity on the sample. The lifetime of the photoexcitation rises with cluster size. This is attributed to an increasing electronic density of states for larger clusters.

Blocking the phase-shifting effect of neuropeptide Y with light.

Previous studies have indicated that the neuropeptide Y input from the intergeniculate leaflet of the lateral geniculate nucleus to the suprachiasmatic nucleus is the final part of a non-photic phase shifting pathway to pacemakers in hamsters, or that neuropeptide Y is necessary for other pathways to be effective. Experiments in which two stimuli are presented during the same circadian cycle have shown that phase shifts in response to at least two non-photic stimuli are attenuated by a subsequent light pulse during the subjective day. This study was conducted to investigate the neural site of the blocking effect of light on non-photic stimuli. Experiment 1 showed that phase shifts in response to induced wheel-running during the subjective day are greatly attenuated by a subsequent light pulse. Experiment 2 showed that phase shifts in response to injections of neuropeptide Y in the middle of the subjective day were also greatly reduced by a subsequent light pulse. These results provide some insight about the site of the blocking action of light on non-photic phase shifts. Because there is evidence indicating that neuropeptide Y may mediate phase shifts in response to induced activity, and because light was able to block phase shifts produced by neuropeptide Y, we conclude that, in blocking activity-induced shifts, light must act downstream from the release of neuropeptide Y into the suprachiasmatic nucleus.

AN ANALYSIS OF PHOTO‐INDUCED FRUITING IN Coprinus congregatus*

Abstract— Experiments are described to determine the effect of short irradiations on the initiation and differentiation of sporophore primordia in Coprinus congregatus and on the morphogenesis in darkness of such photo‐induced primordia. Photoinduction is characterized either by development in darkness of etiolated primordia, or by a subthreshold state where no visible primordia are recognizable but where interaction with a subsequent light pulse can be shown. The photo‐induced primordia abort after about 4.5 to 7.5 days in darkness. The photoinduction itself is memorized for a longer period (8.5 to 9.5 days) in darkness.

Anticholinergic agents do not block light-induced circadian phase shifts

Depletion of brain acetylcholine (ACh) stores by central infusions of hemicholinium-3 (HC-3) did not prevent the circadian phase-shifting effects of subsequent light pulses in rats. Animals treated with atropine likewise had normal responses to light pulses. Neither delay nor advance shifts were prevented. These data do not support the view that ACh is the primary mediator of photoentrainment in the rat. The previously reported effects of carbachol injections may be non-specific.

Laser photodetachment spectra of hexafluorobenzene anion in nonpolar liquids

Aromatic anions absorb in the visible region of the spectrum and energetic considerations suggest that in solution photodetachment may be an important process at these wavelengths. This study reports electron photodetachment cross sections for C/sub 6/F/sub 6//sup -/ for wavelengths between 415 and 700 nm. The anion is generated by electron attachment to solute C/sub 6/F/sub 6/ during an x-ray pulse. Detachment is observed as a change in conductivity induced by a subsequent light pulse from a tunable dye laser. The threshold values are reported for tetramethylsilane, n-butane, cyclopentane, n-pentane, 2,2,4-trimethylpentane, 2,2 dimethylbutane, and neopentane. The thresholds are consistent with an electron affinity of 1.09 +- 0.04 eV for the C/sub 6/F/sub 6/ molecule. Comparison of the data to the absorption spectrum of C/sub 6/F/sub 6//sup -/ shows that for photon energies above threshold the major process is photodetachment. The relationship of photodetachment data to other properties of the excess electrons in these solvents are discussed.