Transient Ionization Research Articles

Some new aspects of the transient ionization layer of comet Siding Spring origin in the Martian upper atmosphere

AbstractThe close encounter of comet Siding Spring with Mars resulted in the formation of a dense transient ionization layer in the Martian upper atmosphere at altitudes between 80 and 120 km. Instruments on three spacecraft orbiting Mars detected the presence of this layer, as reported in previous publications. In this study, we reanalyzed the ionograms of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express to get further insight about the recurrence of the layer. For this purpose, data from three periapsis passes of MARSIS that took place 5 h, 12 h, and 19 h after peak dust deposition are used. We found that the transient ionization layer was sustained at least for 19 h on the nightside and 12 h on the dayside. While the peak density of the layer on the nightside gradually decreases from orbit to orbit, it does not change much on the dayside. Some ionograms in all three orbits show two transient ionization layers that are separated by ~60 km in apparent altitude. These double layers occur preferentially in regions of strong vertical magnetic fields. The bottom layer of the double structure is probably an oblique echo due to reflections from ionization bulges (formed in regions of vertical magnetic fields) at altitudes of the transient ionization layer. Horizontal bifurcation of the original layer is considered as another plausible mechanism for explaining the double‐layer structure.

Current-Driven Hydrogen Desorption from Graphene: Experiment and Theory.

Electron-stimulated desorption of hydrogen from the graphene/SiC(0001) surface at room temperature was investigated with ultrahigh vacuum scanning tunneling microscopy and ab initio calculations in order to elucidate the desorption mechanisms and pathways. Two different desorption processes were observed. In the high electron energy regime (4-8 eV), the desorption yield is independent of both voltage and current, which is attributed to the direct electronic excitation of the C-H bond. In the low electron energy regime (2-4 eV), however, the desorption yield exhibits a threshold dependence on voltage, which is explained by the vibrational excitation of the C-H bond via transient ionization induced by inelastic tunneling electrons. The observed current independence of the desorption yield suggests that the vibrational excitation is a single-electron process. We also observed that the curvature of graphene dramatically affects hydrogen desorption. Desorption from concave regions was measured to be much more probable than desorption from convex regions in the low electron energy regime (∼2 eV), as would be expected from the identified desorption mechanism.

Diagnosing transient ionization in dynamic events

The present study aims to provide a diagnostic line ratio that will enable the observer to determine whether a plasma is in a state of transient ionization. We use the Atomic Data and Analysis Structure (ADAS) to calculate line contribution functions for two lines, Si IV 1394 A and O IV 1401 A, formed in the solar transition region. The generalized collisional-radiative theory is used. It includes all radiative and electron collisional processes, except for photon-induced processes. State-resolved direct ionization and recombination to and from the next ionization stage are also taken into account. For dynamic bursts with a decay time of a few seconds, the Si IV 1394 A line can be enhanced by a factor of 2-4 in the first fraction of a second with the peak in the line contribution function occurring initially at a higher electron temperature due to transient ionization compared to ionization equilibrium conditions. On the other hand, the O IV 1401 A does not show such any enhancement. Thus the ratio of these two lines, which can be observed with the Interface Region Imaging Spectrograph, can be used as a diagnostic of transient ionization. We show that simultaneous high-cadence observations of two lines formed in the solar transition region may be used as a direct diagnostic of whether the observed plasma is in transient ionization. The ratio of these two lines can change by a factor of four in a few seconds owing to transient ionization alone.

Total ionizing dose and single-event effect in vertical channel double-gate nMOSFETs

In this paper, the total ionizing dose (TID) and single-event effect (SEE) in vertical channel double-gate (DG) nMOSFETs are comprehensively investigated. Due to the vertical channel structure and the excellent gate control capability, the vertical channel DG transistor is relatively resistant to TID and transient ionization effect. However, the dc characteristics of vertical channel DG device are very sensitive to permanent damage induced by a few ions hitting the device. The on-state current and transconductance of the vertical channel DG MOSFETs show significant degradation after exposure to heavy ions, which is attributed to the formation of displacement damage in the channel. As the device feature size scales down to the deca-nanometer regime, the influence of permanent damage induced by a few ions striking the device static performance cannot be ignored and should be seriously considered in radiation-hardened technologies.

Power loss due to Corona on High Voltage Transmission Lines

The effect of corona on high-voltage transmission lines is one of the causes power loss and this is uneconomical and undesirable. The study of corona and factors responsible for the loss is investigated through sample calculation based on Peek's formula and the results are then simulated through matlab program. This paper presents the relations between power loss due to corona to corona parameters through matlab programs When in an insulation system, the voltage gradient (voltage stress) exceeds a critical voltage, the air molecules surrounding the high voltage transmission line conductors become ionized (transient gaseous ionization) resulting in partial discharges. Corona loss occurs if the line to line voltage exceeds the corona threshold. The conductive region is not high enough to cause electrical breakdown or arcing to any nearby objects (1). Corona can occur within voids of an insulator, at the conductor or at the insulator interface. Rough surfaces are more liable to corona because the unevenness of the surface decreases the value of the breakdown voltage. It can be detected due to its visible light in form of purple glow consisting of micro arcs and its sound can be heard through its hissing and cracking sound (2). The smelling of the presence of ozone production is noticed during corona activity (3). The effects of corona are cumulative and permanent and the failure can occur without warning. In insulation system, corona discharges result in voltage transients. The effects of corona associated with the operation of high voltage transmission lines include radio interference, audible noise, gaseous effluents (Ozone and Nitrogen oxide) and shock potential. Conductor voltage, diameter and shape, dusts, water drops, and surface irregularities such as scratches are factors that affect the performance and conductor's electrical surface gradients. The energy loss due to corona is transformed into sound, radio noise, chemical reactions of the air components and heat (4). Corona reduces the reliability of insulation system thereby degrading insulation and causes system failure due to dielectric breakdown. The study becomes necessary because of the prevailing negative effect of corona parameters on the power loss of high voltage transmission lines. The power loss under fair weather conditions and under stormy weather conditions when investigated, analyzed and simulated using matlab programs gives the results which help us to take necessary measures to minimize the power loss under fair and stormy weather conditions. High quality insulated and good service design underground high transmission cables can help to eliminate the loss due corona effect The advantage of corona is that the sound generated during corona activity can be used to build high accuracy audio speakers and also it has no mass to be moved to create the sound so that transient response is improved. The controlled corona discharge can be used for filtrations and printing.

Ultrafast dynamics of the ns (n = 3,4) and 3d Rydberg states of O2

The ultrafast dynamics of the lowest optically accessible Rydberg states of molecular oxygen (O2) has been studied by time resolved transient ionization. The process was triggered by the absorption of two pump photons in the 304 nm-220 nm range which corresponds to an energy range of 8.2 eV-11.3 eV, and probed by ionization with photons at 800 nm. From the analysis of the experimental data different time constants ranging from a few femtoseconds to hundreds of picoseconds were found. The identification of the states responsible for the measured lifetimes revealed important facts about the dynamics of the Rydberg states in this energy range, underlying the importance of the coupling between Rydberg and valence states in the relaxation mechanisms of the molecule.

The Diagnostic Potential of Transition Region Lines Undergoing Transient Ionization in Dynamic Events

We discuss the diagnostic potential of high cadence ultraviolet spectral data when transient ionization is considered. For this we use high cadence UV spectra taken during the impulsive phase of a solar flares (observed with instruments on-board the Solar Maximum Mission) which showed excellent correspondence with hard X-ray pulses. The ionization fraction of the transition region ion O V and in particular the contribution function for the O V 1371A line are computed within the Atomic Data and Analysis Structure, which is a collection of fundamental and derived atomic data and codes which manipulate them. Due to transient ionization, the O V 1371A line is enhanced in the first fraction of a second with the peak in the line contribution function occurring initially at a higher electron temperature than in ionization equilibrium. The rise time and enhancement factor depend mostly on the electron density. The fractional increase in the O V 1371A emissivity due to transient ionization can reach a factor of 2--4 and can explain the fast response in the line flux of transition regions ions during the impulsive phase of flares solely as a result of transient ionization. This technique can be used to diagnostic the electron temperature and density of solar flares observed with the forth-coming Interface Region Imaging Spectrograph.

MAGNETIC RECONNECTION IN NON-EQUILIBRIUM IONIZATION PLASMA

We have studied the effect of time-dependent ionization and recombination processes on magnetic reconnection in the solar corona. Petschek-type steady reconnection, in which model the magnetic energy is mainly converted at the slow-mode shocks, was assumed. We carried out the time-dependent ionization calculation in the magnetic reconnection structure. We only calculated the transient ionization of iron; the other species were assumed to be in ionization equilibrium. The intensity of line emissions at specific wavelengths were also calculated for comparison with {\it Hinode} or other observations in future. What we found is as follows: (1) iron is mostly in non-equilibrium ionization in the reconnection region, (2) the intensity of line emission estimated by the time-dependent ionization calculation is significantly different from that with the ionization equilibrium assumption, (3) the effect of time-dependent ionization is sensitive to the electron density in the case that the electron density is less than $10^{10}$ cm$^{-3}$, (4) the effect of thermal conduction lessens the time-dependent ionization effect, (5) the effect of radiative cooling is negligibly small even if we take into account time-dependent ionization.

Excited-state lifetime of adenine near the first electronic band origin

The excited-state lifetime of supersonically cooled adenine was measured in the gas phase by femtosecond pump-probe transient ionization as a function of excitation energy between 36 100 and 37 500cm(-1). The excited-state lifetime of adenine is ∼2ps around the 0-0 band of the (1)L(b) ππ(∗) state (36 105cm(-1)). The lifetime drops to ∼1ps when adenine is excited to the (1)L(a) ππ(∗) state with the pump energy at 36 800cm(-1) and above. The excited-state lifetimes of (1)L(a) and (1)L(b) ππ(∗) states are differentiated in accordance with previous frequency-resolved and computational studies.

Scaling behavior of ultrafast two-color terahertz generation in plasma gas targets: energy and pressure dependence

Ultrafast terahertz emission from two-color generated laser plasma gas targets is studied using air and the noble gases (neon, argon, krypton, and xenon) as the generation media. Terahertz output pulse energy and power spectra are measured as function of gas species, gas pressure, and input pulse energy up to 6 mJ per pulse using a 40-fs 1-kHz Ti:sapphire laser system as the drive source. Terahertz pulse energies approaching 1 microJ per pulse with spectral content out to 40 THz and pulse duration of 35 fs is reported. A simple one dimensional transient photocurrent ionization model is used to calculate the spectra showing good agreement with experiments.

Photophysics of 1-Aminonaphthalene: A Theoretical and Time-Resolved Experimental Study

The photophysics of 1-aminonaphthalene (1-napthylamine, AMN) has been investigated on the basis of a constructive experimental-theoretical interplay derived from time-resolved measurements and high-level quantum-chemical ab initio CASPT2//CASSCF calculations. Transient ionization signals at femtosecond resolution were collected for AMN cold isolated molecules following excitation from the vibrationless ground level to a number of vibrational states (within the pump resolution) in the lowest accessible excited state and further multiphoton ionization probing at 500, 800, and 1300 nm. Theory predicts two pipi* states, (1)L(b) and (1)L(a), as the lowest singlet electronic excitations, with adiabatic transitions from S(0) at 3.50 and 3.69 eV, respectively. Since the associated oscillator strength for the lowest transition is exceedingly small, the (1)L(b) state is not expected to become populated significantly and the (1)L(a) state appears as the main protagonist of the AMN photophysics. Though calculations foresee a surface crossing between (1)L(a) and the lower (1)L(b) states, no dynamical signature of it is observed in the time-dependent measurements. In the relaxation of (1)L(a), the radiant emission competes with the intersystem crossing and internal conversion channels. The rates of these mechanisms have been determined at different excitation energies. The internal conversion is mediated by a (1)L(a)/S(0) conical intersection located 0.7 eV above the (1)L(a) minimum. The relaxation of a higher-lying singlet excited state, observed above 40 000 cm(-1) (4.96 eV) and calculated at 5.18 eV, has been also explored.

Cyclohexadiene ring opening observed with 13 fs resolution: coherent oscillations confirm the reaction path

The third harmonic (270 nm, 11 fs), produced in a short argon cell from Ti-sapphire laser pulses (810 nm, 12 fs), was used to excite 1,3-cyclohexadiene to its lowest pipi* state (1B). Probing was done by transient ionization by the 810 nm pulses, measuring the yields of the parent and a fragment ion. As previously found with 10 times longer pulses, the molecule leaves in two steps (time constants tau(1), tau(2)) from the spectroscopic (1B) to a dark (2A) state and from there (within tau(3)) to the ground-state surface. In addition to slightly improved values for tau(1)-tau(3), we found in all three locations (L(1)-L(3)) on the potentials coherent oscillations, which can be assigned to vibrations. They are stimulated by slopes (driving forces) of the potentials, and the vibrational coordinates indicate the slope directions. From them we can infer the path following the initial excitation: the molecule is first not only accelerated towards CC stretching in the pi system but also along a symmetric C[double bond, length as m-dash]C twist. The latter motion-after some excursion-also erects and stretches the CH(2)-CH(2) bond, so that Woodward-Hoffmann interactions are activated after this delay (in L(2)). On leaving L(2) (the 1B minimum) around the lower cone of the 1B/2A conical intersection, the wave packet is rapidly accelerated along an antisymmetric coordinate, which breaks the C(2) symmetry of the molecule and eventually leads in a ballistic path to (and through) the last (2A/1A) conical intersection. The ring opening begins already on the 1B surface; near the 2A minimum it is already far advanced, but is only completed on the ground-state surface.

A time resolved study of S 1/S 2 electronic coupling in naphthalene

The relaxation dynamics of jet cooled naphthalene (NPH) has been studied by mass-resolved transient ionization, following excitation to the S 1(L b) and S 2(L a) states and further single/multi photon ionization at a variety of probe wavelengths. Time-dependent signals were collected from the parent C 10 H 8 + and the C 10 H 7 + fragment ions. The relaxation transients include two lifetimes of 30 and 800 fs. The short decay suggests the presence of a conical intersection nearby the L a state surface minimum that is crossed in one single passage by most part of the population, whereas the long lifetime is tentatively assigned either to internal vibrational redistribution (IVR) within the L b state, or to the decay of coherent recurrences.

Theoretical analysis and simulations of strong terahertz radiation from the interaction of ultrashort laser pulses with gases

Terahertz (THz) radiation from the interaction of ultrashort laser pulses with gases is studied both by theoretical analysis and particle-in-cell (PIC) simulations. A one-dimensional THz generation model based on the transient ionization electric current mechanism is given, which explains the results of one-dimensional PIC simulations. At the same time the relation between the final THz field and the initial transient ionization current is shown. One- and two-dimensional simulations show that for the THz generation the contribution of the electric current due to ionization is much larger than the one driven by the usual ponderomotive force. Ionization current generated by different laser pulses and gases is also studied numerically. Based on the numerical results we explain the scaling laws for THz emission observed in the recent experiments performed by Xie et al. [Phys. Rev. Lett. 96, 075005 (2006)]. We also study the effective parameter region for the carrier envelop phase measurement by the use of THz generation.

Tbulk-BICS: A Built-In Current Sensor Robust to Process and Temperature Variations for Soft Error Detection

This paper presents a parameterized current sensor able to detect transient ionization in the silicon substrate. Each sensor is controlled by a set of trimming bits that can be used to attune the sensitivity of the sensor compensating process and temperature variations. By choosing different configurations in the trimming bits, it is possible to adjust the performance of the sensor, which can increase the number of transistors monitored by a single sensor reducing the area overhead. Monte Carlo simulations are used to evaluate the sensor behavior. Results from a case-study circuit with embedded Tbulk-BICS confirm the efficiency of the technique.

Transient quantum evolution of two-dimensional electrons under photoexcitation of a deep center

Dpto. Fisica Basica, Universidad de La Laguna, La Laguna, 38206-Tenerife, Spain(Dated: February 1, 2008)We have considered the ballistic propagation of the 2D electron Wigner distribution, which isexcited by an ultrashort optical pulse from a short-range impurity into the first quantized subband ofa selectively-doped heterostructure with high mobility. Transient ionization of a deep local state intoa continuum conduction c-band state is described. Since the quantum nature of the photoexcitation,the Wigner distribution over 2D plane appears to be an alternating-sign function. Due to a negativecontribution to the Wigner function, the mean values (concentration, energy, and flow) demonstratean oscillating transient evolution in contrast to the diffusive classical regime of propagation.

Kα X-ray emission spectra from highly charged Fe ions in EBIT

A detailed spectral model has been developed for the computer simulation of the 2p → 1s Kα X-ray emission from highly charged Fe ions in the electron beam ion trap (EBIT). The spectral features of interest occur in the range 1.84–1.94 Å. The fundamental radiative emission processes associated with radiationless electron capture or dielectronic recombination, inner-shell electron collisional excitation, and inner-shell-electron collisional ionization are taken in account. For comparison, spectral observations and simulations for high-temperature magnetic-fusion (tokamak) plasmas are reviewed. In these plasmas, small departures from steady-state corona-model charge-state distributions can occur because of ion transport processes, while the assumption of equilibrium (Maxwellian) electron energy distributions is expected to be valid. Our investigations for EBIT have been directed at the identification of spectral features that can serve as diagnostics of extreme nonequilibrium or transient ionization conditions, and allowance has been made for general (non-Maxwellian) electron energy distributions. For the precise interpretation of the high-resolution X-ray observations, which may involve the analysis of blended spectral features composed of many lines, it has been necessary to take into account the multitude of individual fine-structure components of the Kα radiative transitions in the ions from Fe XVIII to Fe XXV. At electron densities higher than the validity range of the corona-model approximation, collisionally induced transitions among low-lying excited states can play an important role. It is found that inner-shell-electron excitation and ionization processes involving the complex intermediate ions from Fe XVIII to Fe XXI produce spectral features, in the wavelength range from 1.89 to 1.94 Å, which are particularly sensitive to density variations and transient ionization conditions. PACS Nos.: 52.72.+v, 32.80.Dz, 32.70.Fw, 32.30.Rj

Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization

4-Aminobenzonitrile derivatives have two excited states of similar energy: besides the benzene-like L(b) state (also termed "locally excited" or LE state) one with charge-transfer (CT) character that is slightly higher in the isolated molecules. The CT state can be lowered by solvents of suitable polarity, so that dual fluorescence can be observed in them. It is controversial along which coordinate this state is displaced, although the amino-group twist is a wide-spread assumption. We investigated a number of such compounds by transient ionization in the gas phase, initially exciting the higher-lying L(a) state (S(2)). Here we briefly review the previous results on 4-(dimethylamino)benzonitrile (the prototype of this class of molecules), 4-piperidino-, pyrrolidino- and pyrrolyl-benzonitrile and compare them with new results on 4-aminobenzonitrile and on the bridged derivative N-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6). Although in the latter two molecules the CT state has never been detected before, we find the same relaxation path for all compounds: From S(2), the wave packet passes through a conical intersection (CI); from there part of it reaches the S(1) (L(b)) state directly, whereas another part temporarily populates the CT state (also in NMC6), from where it goes around the CI also to the L(b) well. The wave packet directly reaching the L(b) well oscillates there along coordinates involving amino-group twist and wagging or molecular arching and a quinoidal distortion. These coordinates must be components of the CI displacement vector. A vibration involving bond-length alternation of the benzene ring is ascribed to a momentum caused by the electronic symmetry change in the CI, i.e., to the nonadiabatic coupling vector. Also the CT state involves amino-group twist, as to conclude from the anisotropy of the corresponding signal. The six-membered aliphatic ring in NMC6 hinders the twist and raises the CT state to an energy that is, however, still below the L(a) state, so that it can be temporarily populated in a barrierless process. Also in aminobenzonitrile the CT state is between L(a) and L(b) and is reached from L(a) without a barrier. The twist is rationalized by vibronic interaction with a higher state that is pi-antibonding between the amino group and the aromatic ring.

Ionospheric effects of the cosmic gamma ray burst of 29 March 2003

We present evidence for ionospheric effects caused by the gamma ray burst that originated at a cosmological distance. At the time of the strong cosmic gamma‐ray burst of 29 March 2003 (GRB030329) that took place in the nighttime in Japan we observed a transient decrease in the strength of the radio noise coming from extraterrestrial sources (cosmic noise) at 38 MHz. We also observed a sudden field‐amplitude decrease of an 8.006 MHz transmission signal recorded at a distance of 690 km from the transmitter. These phenomena are interpreted as a result of an ionospheric absorption enhancement due to transient ionization caused by GRB030329. We also report no appreciable effect on the ionospheric electron column content derived using GPS (Global Positioning System) microwave signals.

Photoionization dynamics of glycine adsorbed on a silicon cluster: “On-the-fly” simulations

Dynamics of glycine chemisorbed on the surface of a silicon cluster is studied for a process that involves single-photon ionization, followed by recombination with the electron after a selected time delay. The process is studied by "on-the-fly" molecular dynamics simulations, using the semiempirical parametric method number 3 (PM3) potential energy surface. The system is taken to be in the ground state prior to photoionization, and time delays from 5 to 50 fs before the recombination are considered. The time evolution is computed over 10 ps. The main findings are (1) the positive charge after ionization is initially mostly distributed on the silicon cluster. (2) After ionization the major structural changes are on the silicon cluster. These include Si-Si bond breaking and formation and hydrogen transfer between different silicon atoms. (3) The transient ionization event gives rise to dynamical behavior that depends sensitively on the ion state lifetime. Subsequent to 45 fs evolution in the charged state, the glycine molecule starts to rotate on the silicon cluster. Implications of the results to various processes that are induced by transient transition to a charged state are discussed. These include inelastic tunneling in molecular devices, photochemistry on conducting surfaces, and electron-molecule scattering.