Impulsive Rise Research Articles

Early Prediction of Monkeypox Virus Outbreak Using Machine Learning

At the onset of an infectious disease, such as the monkeypox virus (MPXV), surveillance data is crucial in keeping track of the outbreak’s progression. The surveillance data for MPXV received considerable attention after multiple European countries recorded cases. Historical data obtained from May 9, 2022, to August 10, 2022, were used to model the cumulative case trajectories of MPXV in five countries. Our study employed autoregressive integrated moving averages (ARIMA), neural network autoregression (NNETAR), exponential smoothing (ETS), and seasonal naïve regression (SNAÏVE) for training and evaluation. The paper makes the following contributions: (1) enhanced model stability with the Box-Cox transformation as a preprocessing step, (2) experimentation with both linear and non-linear models, and (3) simulation of the top five countries during the impulsive rise in cases of MPXV. The results were evaluated using three metrics: root mean square error (RMSE), mean square error (MAE), and mean absolute percentage error (MAPE). The ARIMA (0,1,3) (1,0,0)[7] model yielded the lowest percentage error of 5.16 in the holdout set for MAPE in France observations. The ETS (A, A, A) model, the lowest percentage error in the holdout set for MAE was 7.35 in Germany. Regarding the NNETAR (1,1,2) [7] model, the lowest percentage error in the holdout observations for RMSE was 8.33 in Spain, 2.75 in the United Kingdom (UK), and 8.05 in the United States of America (USA) in that order. Based on these findings, we can conclude that while the transformation proved crucial for model performance, it was not necessary for all experiments, as ARIMA remained dominant in France and the ETS model in Germany. At the same time, NNETAR model outperformed in cumulative case counts in Spain, the UK, and the USA. Our experimentation allows for early identification and contributes to a better understanding of forecasting MPXV cases using combinations of both linear and nonlinear models.

Preflare Processes, Flux Rope Activation, Large-scale Eruption, and Associated X-class Flare from the Active Region NOAA 11875

Abstract We present a multiwavelength analysis of the eruption of a hot coronal channel associated with an X1.0 flare (SOL2013-10-28T02:03) from the active region NOAA 11875 by combining observations from Atmospheric Imaging Assembly/Solar Dynamics Observatory (SDO), Helioseismic and Magnetic Imager/SDO, Reuven Ramaty High Energy Solar Spectroscopic Imager, and Hiraiso Radio Spectrograph. EUV images at high coronal temperatures indicated the presence of a hot channel at the core of the active region from the early preflare phase evidencing the preexistence of a quasi-stable magnetic flux rope. The hot channel underwent an activation phase after a localized and prolonged preflare event occurring adjacent to one of its footpoints. Subsequently, the flux rope continued to rise slowly for ≈16 minutes during which soft X-ray flux gradually built-up characterizing a distinct precursor phase. The flux rope transitioned from the state of slow rise to the eruptive motion with the onset of the impulsive phase of the X1.0 flare. The eruptive expansion of the hot channel is accompanied by a series of type III radio bursts in association with the impulsive rise of strong hard X-ray nonthermal emissions that included explicit hard X-ray sources of energies up to ≈50 keV from the coronal loops and ≈100 keV from their footpoint locations. Our study contains evidence that preflare activity occurring within the spatial extent of a stable flux rope can destabilize it toward eruption. Moreover, sudden transition of the flux rope from the state of slow rise to fast acceleration precisely bifurcated the precursor and the impulsive phases of the flare, which points toward a feedback relationship between early coronal mass ejection dynamics and the strength of the large-scale magnetic reconnection.

Study on Acoustic Detection of the Working State of Pulse Detonation Engine

AbstractThe experiment system of pulse detonation engine is set up to investigate the acoustic signature of detonation acoustic. The research on detonation acoustic characteristic of pulse detonation engine in three kinds of working states, deflagration stage, detonation stage and external detonation stage, is carried out. Results from the test show that the maximal PImpact, PJet1, τ+ and τInterval are obtained in external detonation stage. In deflagration stage, the concentrate of PDE acoustic signal energy is obvious. The energy of impulsive rise of impact noise and jet noise is an important part of PDE acoustic in deflagration stage. In detonation stage, a concentration of energy distribution is appeared in low frequency, and the energy of medium-high frequency is decreased. In external detonation stage, energy distribution is more concentrated in low frequency. The value of η11/η5 can be used to detect the working state of pulse detonation engine. When η11/η5 is less than 5, pulse detonation engine is in deflagration stage, however, when η11/η5 is large than 5, pulse detonation engine is in detonation stage or external detonation stage.

QUASI-PERIODIC FLUCTUATIONS AND CHROMOSPHERIC EVAPORATION IN A SOLAR FLARE RIBBON OBSERVED BY HINODE/EIS, IRIS, AND RHESSI

ABSTRACT The Hinode/Extreme-ultraviolet Imaging Spectrometer (EIS) obtained rapid cadence (11.2 s) EUV stare spectra of an M7.3 flare ribbon in AR 12036 on 2014 April 18. Quasi-periodic (P ≈ 75.6 ± 9.2 s) intensity fluctuations occurred in emission lines of O iv, Mg vi, Mg vii, Si vii, Fe xiv, and Fe xvi during the flare's impulsive rise, and ended when the maximum intensity in Fe xxiii was reached. The profiles of the O iv–Fe xvi lines reveal that they were all redshifted during most of the interval of quasi-periodic intensity fluctuations, while the Fe xxiii profile revealed multiple components including one or two highly blueshifted ones. This indicates that the flare underwent explosive chromospheric evaporation during its impulsive rise. Fluctuations in the relative Doppler velocities were seen, but their amplitudes were too subtle to extract significant quasi-periodicities. RHESSI detected 25–100 keV hard-X-ray sources in the ribbon near the EIS slit's pointing position during the peaks in the EIS intensity fluctuations. The observations are consistent with a series of energy injections into the chromosphere by nonthermal particle beams. Electron densities derived with Fe xiv (4.6 × 1010 cm−3) and Mg vii (7.8 × 109 cm−3) average line intensity ratios during the interval of quasi-periodic intensity fluctuations, combined with the radiative loss function of an optically thin plasma, yield radiative cooling times of 32 s at 2.0 × 106 K, and 46 s at 6.3 × 105 K (about half the quasi-period); assuming Fe xiv's density for Fe xxiii yields a radiative cooling time of 103 s (13 times the quasi-period) at 1.4 × 107 K.

Direct observation of the energy release site in a solar flare by SDO/AIA, Hinode/EIS, and RHESSI

We present direct evidence for the detection of the main energy release site in a non-eruptive solar flare, SOL2013-11-09T06:38UT. This GOES C2.7 event was characterised by two flaring ribbons and a compact, bright coronal source located between them, which is the focus of our study. We use imaging from SDO/AIA, and imaging spectroscopy from RHESSI to characterise the thermal and non-thermal emission from the coronal source, and EUV spectroscopy from the Hinode/EIS, which scanned the coronal source during the impulsive peak, to analyse Doppler shifts in Fe XII and Fe XXIV emission lines, and determine the source density. The coronal source exhibited an impulsive emission lightcurve in all AIA filters during the impulsive phase. RHESSI hard X-ray images indicate both thermal and non-thermal emission at the coronal source, and its plasma temperature derived from RHESSI imaging spectroscopy shows an impulsive rise, reaching a maximum at 12-13 MK about 10 seconds prior to the hard X-ray peak. High redshifts associated with this bright source indicate downflows of 40-250 km/s at a broad range of temperatures, interpreted as loop shrinkage and/or outflows along the magnetic field. Outflows from the coronal source towards each ribbon are also observed by AIA images at 171, 193, 211, 304 and 1600 A. The electron density of the source obtained from a Fe XIV line pair is $10^{11.50}$ which is collisionally thick to electrons with energy up to 45-65 keV, responsible for the source's non-thermal X-ray emission. We conclude that the bright coronal source is the location of the main release of magnetic energy in this flare, with a geometry consistent with component reconnection between crossing, current-carrying loops. We argue that the energy that can be released via reconnection, based on observational estimates, can plausibly account for the non-thermal energetics of the flare.

Stellar ejecta from falling comet-like bodies: young stars

AbstractHigh-resolution spectral observations of young stars with dense protoplanetary discs like Beta Pictoris led to the discovery of variable emission lines of metal atoms, Na, Fe etc., that indicate the presence of fluxes of comet-like evaporating bodies falling onto the stars, FEBs. Assuming the presence of stellar atmospheres similar to the solar one, we show that passages of the FEBs through the stellar chromosphere and photosphere with velocities around 600 km/s will be accompanied by aerodynamic crushing of the nuclei, transverse expansion of the crushed matter, “explosion” of the flattened nuclei in a relatively very thin sub-photosphere layer due to sharp deceleration, and impulse production of a hot plasma. The impulsive rise of the layer's temperature and density lead to the generation of a strong “blast” shock wave and shock wave-induced ejection/eruption of hot plasma into space above the chromosphere. Observations of such impact-induced high-temperature phenomena are of interest for the physics/prognosis of stellar/solar flares as well as physics of comets.

Toward understanding the early stages of an impulsively accelerated coronal mass ejection

The expanding magnetic flux in coronal mass ejections (CMEs) often forms a cavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of an impulsively accelerated CME on 25 March 2008, which displays a well-defined extreme ultraviolet (EUV) and white-light cavity of nearly circular shape already at low heights ~ 0.2 Rs. The center height h(t) and radial expansion r(t) of the cavity are obtained in the whole height range of the main acceleration. We interpret them as the axis height and as a quantity proportional to the minor radius of a flux rope, respectively. The three-dimensional expansion of the CME exhibits two phases in the course of its main upward acceleration. From the first h and r data points, taken shortly after the onset of the main acceleration, the erupting flux shows an overexpansion compared to its rise, as expressed by the decrease of the aspect ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident with the impulsive rise of the acceleration and is followed by a phase of very gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~ 1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by flux conservation around a rising flux rope of decreasing axial current and by the addition of flux to a growing, or even newly forming,flux rope by magnetic reconnection. Further analysis will be required to decide which of these contributions is dominant. The data also suggest that the horizontal component of the impulsive cavity expansion (parallel to the solar surface) triggers the associated EUV wave, which subsequently detaches from the CME volume.

Numerical oscillatory on the simulation of thermoacoustic waves

AbstractThe SIMPLE algorithm for compressible flows is introduced to predict the thermoacoustic wave in a one‐dimensional closed region. The thermoacoustic waves are generated by an impulsive rise of the temperature on the left wall, while the other walls are kept at the initial temperature. Four different schemes are employed to deal with the convection‐diffusion terms, i.e., CD, FUD, QUICK, and MUSCL. The calculations and analysis show that numerical oscillation occurs under all four schemes, affected by the intensity of the waves, type of schemes, and other factors. The results are beneficial for the further investigation of the thermoacoustic waves and high efficiency scheme development. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 265– 275, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20162

Chromospheric Evaporation in a Remote Solar Flare-like Transient Observed at High Time Resolution with SOHO 's CDS and RHESSI

We present EUV light curves and Doppler velocity measurements for a small, remote flarelike transient observed at high time resolution (9.8 s) with SOHO's CDS during a GOES M1.6 solar flare. The EUV observations include a brief precursor and an impulsive peak followed by a more gradual rise and decline of emission. Hard X-ray light curves obtained with RHESSI reveal a small burst just before the EUV impulsive rise, and another burst at the time of the more gradual EUV peak. RHESSI images show no emission at the location of the EUV transient due to limitations in dynamic range. During the impulsive phase we measure simultaneous, cospatial downward velocities ~30 km s-1 in the chromospheric line of He I at 584.3 A and the transition region line of O V at 629.7 A, and upward velocities ~20 km s-1 in the coronal line of Si XII at 520.7 A. Fe XIX emission at 592.2 A emerged during the impulsive phase and revealed upward velocities approaching 150 km s-1. These observations demonstrate that flarelike explosive chromospheric evaporation occurred at a location remote from the primary region of particle acceleration, apparently driven by electron beams from the primary acceleration region.

Evidence for the Formation of a Large-Scale Current Sheet in a Solar Flare

We present X-ray evidence for the formation of a large-scale current sheet in a flare observed by the Ramaty High-Energy Solar Spectroscopic Imager on 2002 April 15. The flare occurred on the northwest limb, showing a cusp-shaped flare loop in the rise phase. When the impulsive rise in hard X-rays (>25 keV) began, the cusp part of the coronal source separated from the underlying flare loop and remained stationary for about 2 minutes. During this time, the underlying flare loops shrank at ~9 km s-1. The temperature of the underlying loops increased toward higher altitudes, while the temperature of the coronal source increased toward lower altitudes. These results indicate that a current sheet formed between the top of the flare loops and the coronal source during the early impulsive phase. After the hard X-ray peak, the flare loops grew outward at ~8 km s-1, and the coronal source moved outward at ~300 km s-1, indicating an upward expansion of the current sheet. About 30 minutes later, postflare loops seen in the Solar and Heliospheric Observatory (SOHO) EUV Imaging Telescope 195 A passband rose at ~10 km s-1. A large coronal looplike structure, observed by the SOHO Large Angle and Spectrometric Coronagraph C2 and C3 detectors, also propagated outward at ~300 km s-1. These observations are all consistent with the continued expansion of the current sheet.

Electron Bremsstrahlung Hard X-Ray Spectra, Electron Distributions, and Energetics in the 2002 July 23 Solar Flare

We present and analyze the first high-resolution hard X-ray spectra from a solar flare observed in both X-ray/γ-ray continuum and γ-ray lines. Spatially integrated photon flux spectra obtained by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are well fitted between 10 and 300 keV by the combination of an isothermal component and a double power law. The flare plasma temperature peaks at 40 MK around the time of peak hard X-ray emission and remains above 20 MK 37 minutes later. We derive the nonthermal mean electron flux distribution in one time interval by directly fitting the RHESSI X-ray spectrum with the thin-target bremsstrahlung from a double-power-law electron distribution with a low-energy cutoff. We find that relativistic effects significantly impact the bremsstrahlung spectrum above 100 keV and, therefore, the deduced mean electron flux distribution. We derive the evolution of the injected electron flux distribution on the assumption that the emission is thick-target bremsstrahlung. The injected nonthermal electrons are well described throughout the flare by a double-power-law distribution with a low-energy cutoff that is typically between 20 and 40 keV. We find that the power in nonthermal electrons peaks before the impulsive rise of the hard X-ray and γ-ray emissions. We compare the energy contained in the nonthermal electrons with the energy content of the thermal flare plasma observed by RHESSI and GOES. The minimum total energy deposited into the flare plasma by nonthermal electrons, 2.6 × 1031 ergs, is on the order of the energy in the thermal plasma.

High pressure vaporization of burning droplet with flash vapor-liquid equilibrium calculation

Unsteady burning of n-pentane liquid droplet quiescent in high-pressure oxidizing air is investigated with emphasis placed on rigorous calculation of droplet vaporization. Fully coupled conservation equations in conjunction with global reaction kinetics for chemistry are preconditioned and time-integrated by means of a dual time-stepping technique with pertinent boundary conditions. Properties of equilibrated vapor-liquid interface are evaluated by the method of Soave-Redlich-Kwong equation of state along with consistent set of mixing rules for multi-component mixtures. Envelope flame propagates persistently with gradually increasing peak flame temperature as long as droplet remains in liquid phase. At all pressures, ignition is immediate and resultant steep temperature gradient at interfacial boundary aids in the activation of vaporization, but its extent decreases at later times as the droplet surface temperature arrives at its pseudo wet-bulb value. Ignition is immediate with continuous initial temperature distribution in the vicinity of interfacial boundary and renders an impulsive rise in droplet surface temperature. Other representative pressure effects on evolving droplet combustion are also presented and discussed.

Chromospheric Evaporation and Warm Rain during a Solar Flare Observed in High Time Resolution with the Coronal Diagnostic Spectrometer aboard theSolar and Heliospheric Observatory

We present EUV light curves, Doppler shifts, and line-broadening measurements for a flaring solar active region obtained with the Coronal Diagnostic Spectrometer (CDS) aboard the NASA ESA Solar and Heliospheric Observatory (SOHO) under conditions of (1) comprehensive temporal coverage (including the quiescent preflare, impulsive, and gradual decline phases), (2) high time resolution (9.83 s), and (3) narrow field of view (4'' × 20''). The four strong lines of O III at 599.587 A, O V at 629.732 A, Mg X at 624.937 A, and Fe XIX at 592.225 A are analyzed and provide diagnostics of plasma dynamics for 4.9 ≤ log T ≤ 6.9. Wavelengths and widths measured during the preflare and late decline phases provide standards against which Doppler shifts and excess line broadening are measured during the impulsive and early decline phases. The entire profile of all four lines is blueshifted early during the impulsive rise of the flare, but only the O III, O V, and Mg X lines subsequently exhibit multiple components and downflows. These downflows provide evidence of warm rain due to cooling coronal flare plasma following chromospheric evaporation during the impulsive phase. O III and O V exhibit a pronounced precursor brightening during which the Fe XIX emission emerges above the noise; this, combined with the fact that the O III and O V intensities begin their impulsive rise earlier than do those of Mg X and Fe XIX, is consistent with the transport of coronal flare energy to the chromosphere by nonthermal particle beams.

The timing of non-thermal soft X-ray emission line broadenings in solar flares

We study 59 solar limb flares using the Bragg Crystal Spectrometer (BCS) on Yohkoh and the Burst and Transient Source Experiment (BATSE) to investigate the relative timings between the Hard X-Ray (HXR) emission and the observed non-thermal broadenings of X-ray emission lines (Vnt). We show that the duration of the HXR flux rise to maximum emission aects the relative timing of the main Vnt peak with respect to the main HXR peak. In20% of the flares studied, secondary peaks in Vnt are observed. These are always associated with a strong HXR pulse and usually occur after the associated HXR pulse. There are also flares that show a relationship between the decay times of Vnt and HXR flux. These results are conducive to a causal relationship between the HXR flux and Vnt. We divided the sample of flares into two classes, gradual rise and impulsive rise, depending on the shape of the HXR lightcurve up to maximum emission. We show that the Vnt behaviour diers in the two classes. The implications are discussed with a view to understanding the mechanism of Vnt generation.

The 1998 Outburst of XTE J1550−564: A Model Based on Multiwavelength Observations

The 1998 September outburst of the black hole X-ray binary XTE J1550-564 was monitored at X-ray, optical, and radio wavelengths. We divide the outburst sequence into five phases and discuss their multiwavelength properties. The outburst starts with a hard X-ray spike, while the soft X-ray flux rises with a longer timescale. We suggest that the onset of the outburst is determined by an increased mass transfer rate from the companion star, but the outburst morphology is determined by the distribution of specific angular momentum in the accreting matter. The companion in XTE J1550-564 is likely to be an active magnetic star, with a surface field strong enough to influence the dynamics of mass transfer. We suggest that its magnetic field can create a magnetic bag capable of confining gas inside the Roche lobe of the primary. The impulsive rise in the hard X-rays is explained by the inflow of material with low angular momentum onto the black hole, on a free-fall timescale, when the magnetic confinement breaks down. At the same time, high angular momentum matter, transferred via ordinary Roche lobe overflow, is responsible for the formation of a disk.

Post-Eruptive Flare Energy Release as Detected on AU Mic by EUVE

The long-duration emission arising after the impulsive rise and decay in a flaring event observed by the Extreme Ultraviolet Explorer on the red dwarf star AU Mic is discussed. The decay of the intensity in the Deep Survey 65–190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with the typical for the flare plasma density. The temporal behavior of the emission measure is determined for both the 65–190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1–2000 Å region over nearly 12 hrs is 3 · 1035 ergs. We first point out some difficulties with earlier explanations proposed for this event; we then propose the following physical model: the source of the prolonged emission is a system of high coronal loops, the size of which is more than the active region scale, but less than the stellar radius. Such systems are observed in soft X-rays during large solar flares after coronal mass ejections. Some additional post-flare energy input into this high coronal loop system can be caused by reconnection in a vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.Apparently, we are faced here with new kind of the surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events the subgiants components of the RS CVn binaries.

Post-flare Coronal Activity on AU Mic Detected by EUVE

Data of long-duration emission arising after the impulsive rise and decay in a flaring event on the red dwarf star AU Mic are discussed. Intensive EUV emission in the band 65-190 Å was registered by the Extreme Ultraviolet Explorer (EUVE) after both impulses during half a day. A similar behavior of the flux in the Fe XVIII 93.9 Å line is detected after the first powerful impulse. The decay of the intensity in the 65-190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with a typical flare plasma density. Some difficulties with two explanations of this event proposed earlier are discussed. Explanation (i) - the radiation of dense loops at main phase of the flare, and (ii) - the emission of the low-dense plasma of coronal transients (CME). The temporal behavior of the emission measure is determined for both the 65-190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1-2000 Å region for the long-duration event lasting almost 12 hours is 3 · 1035 ergs. The following physical model is proposed to explain the prolonged event (Fig. 1): the source of emission is the system of high coronal loops, the size of which is more than the active region scale, but is less than the stellar radius. The temperature of the plasma in the loops decreases from 107 K slowly, during a few hours. The densities in these loops are in the range 1013 cm−3 to 5 · 109 cm−3. Such systems, when the plasma therein becomes cool, are observed in the Hα line during large solar flares (for instance, June 15, 1991) after CME. Some additional post-flare energy input into this high coronal loop systems can be caused by the reconnection in vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.Apparently, we are faced here with a new kind of surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events on subgiants, which are components of RS CVn binaries.The full version of this contribution will be published in Astronomicheskij Zhurnal (Astronomy Reports), 1995, Vol. 72.

Heat transfer characteristics of a partially insulated tube with an impulsive temperature rise

The heat transfer for a laminar forced convection inside a two dimensional planar symmetric duct is analyzed. The fluid passage is formed by two parallel plates, and flow is fully developed and incompressible. Flow is isothermal to a position x o = 0, where the wall temperature jumps impulsively to T 1 > T 0 and remains at this value up to the position x 1, where it jumps back T 0. The problem is considerably simplified by introducing a transformation to reduce the heat transfer problem to the standard thermal entrance region problem for flow between parallel plates. Various heat transfer characteristics for different values of Prandtl and Nusselt numbers are analyzed and found to be physically realistic.

The solar white-light flare of 1989 March 7 - Simultaneous multiwavelength observations at high time resolution

Abstract : We present observational data for the 1989 March 7 white-light flare (WLF), including SMM/HXRBS hard X-ray measurements and CCD optical images in 5000 A continuum and at 3 A in the red and blue wings of the H alpha line. The optical data were acquired at a 0.5 s rate under good seeing and were further processed to remove image motion and distortion. The flare kernel shows an impulsive rise in both H alpha and continuum that is well correlated with impulsive hard X-rays. The Hot emission shows a red asymmetry which we interpret as Doppler redshift associated with explosive heating of the chromosphere. These results are consistent with previous observational descriptions which traditionally have been interpreted in terms of heating and ionization of the flare chromosphere by a nonthermal electron beam. However, new observational results which have not been reported previously include (1) the separation of the flare kernel into a bright inner core and a fainter outer region, where the two components display distinctly different temporal behavior and amount of Hot red asymmetry, and (2) a delay, relative to hard X-rays, of approx. 1 s in the impulsive rise of the H alpha wing emission, followed by an additional 1-2 s delay in the 5000 continuum. It is concluded that the observed fluxes and timing of the hard X-ray and WLF optical emissions are consistent with chromospheric heating by nonthermal electrons, with additional visible light continuum possibly being contributed by a backwarmed photosphere irradiated by intense chromospheric recombination continua (principally Balmer continuum) .... Sun: flares, Sun: X-rays, Gamma rays.

Time-resolved reflectance studies of surface melting during laser-assisted deposition with a modulated laser source

Time-resolved reflectance measurements have been used to study the time evolution of the surface reflectance during deposition of gold from dimethylgold hexafluoroacetylacetonate with both modulated and unmodulated laser light. The observed change in reflectivity found during deposition with a modulated laser source is due to surface melting or sintering of the gold deposit. The change in reflectance observed upon undergoing a transition from a rough to a smooth surface is theoretically calculated for a surface with a random distribution of facet shapes and sizes. The observed changes in reflectance are consistent with the rough polycrystalline surface layer becoming smooth in a transient molten state. An impulsive temperature rise due to a rapid exothermic reaction taking place at the leading edge of the laser pulse has been estimated theoretically and is consistent with the observed time-dependent data.