Ionized Nitrogen Molecules Research Articles

Generation of radiation on forbidden transitions in the region of 654-658 nm in an acoustoplasmic medium

In the region of the first positive system (FPS) band of spectrum of nitrogen in acoustoplasma, under laboratory conditions, the generation of intense lines was experimentally obtained at forbidden transitions of a singly ionized nitrogen molecule at 654.81 and 658.36 nm. The intense lines obtained correspond to the stimulated emission mechanism. The intensity of the forbidden lines significantly (up to 17 times) exceeded the intensity of neighboring lines in the spectrum of the first positive system band of nitrogen. Spectroscopic studies did not allow resolution of the rotational spectrum. However, the spectral width for both lines at forbidden transitions is much smaller than the FPS bandwidth of nitrogen. The result obtained is explained by an analogue of Raman scattering and an acoustoplasmic medium, which remove the prohibitions characteristic of dipole radiation.

Calculation study of selective ion extraction from ion source with Paul-trap-type laser cooling device

A single-ion implantation (SII) system combining a laser-cooled linear-Paul-trap ion source (LPT-IS) and a two-stage acceleration lens is being developed to produce an array of nitrogen vacancy (NV) centers, which have recently attracted significant attention as potential qubits. The fabrication of the NV center array requires repeated implantations of a single nitrogen molecule ion (N2+) from SII into a diamond specimen with an energy on the order of a 10-keV and spatial precision of several tens of nanometers or less. To satisfy this requirement, N2+ ions with very low emittance must be selectively extracted from the LPT-IS. In this study, the selective extraction of N2+ ions is investigated using a three-dimensional multiparticle simulation by varying magnitudes and temporal profile of the voltage applied to the end plate electrodes of a conventional LPT. The emittance of the N2+ extracted from the LPT-IS is the lowest when N2+ ion is at the leading end of an array of N2+ and Ca+ ions in the string-like crystalline structure formed with sympathetic cooling. We show that a single N2+ ion with required energy can be extracted from the array of N2+ and Ca+ ions passing through the end plate electrode of the LPT-IS, without spoiling the emittance, by increasing the voltage applied to the end plate electrode at the precise required moment.

Laser emission at 428 nm in N2+: Competition between two- and three-level amplification

Lasing of N2+ at 428 nm is studied as a function of the delay dt between two 800 nm ultrashort pumping pulses from a Ti:sapphire laser. This lasing corresponds to a population-inverted transition between levels B2Σu+(0) and X2Σg+(1) of a singly ionized nitrogen molecule. An important erasing of laser emission is observed under pumping with two consecutive pulses separated by large delays −20 ps < dt < +20 ps. From an analysis of the quenching results, we conclude that the mechanism of lasing corresponds to a coherent two-photon amplification scheme. Our results show that a two-photon process can dominate over a population-inverted one-photon amplification.

Temperature dependence of divacancy spin coherence in implanted silicon carbide

Spin defects in silicon carbide (SiC) have attracted increasing interest due to their excellent optical and spin properties, which are useful in quantum information processing. In this paper, we systematically investigate the temperature dependence of the spin properties of divacancy defects in implanted 4\emph{H}-SiC. The zero-field splitting parameter $D$, the inhomogeneous dephasing time $T_2^{*}$, the coherence time $T_2$, and the depolarization time $T_1$ are extensively explored in a temperature range from 5 to 300 K. Two samples implanted with different nitrogen molecule ion fluences ($\rm {N_2}^{+}$, $1\times 10^{14}/\rm cm^{2}$ and $1\times 10^{13}/\rm cm^{2}$) are investigated, whose spin properties are shown to have similar temperature-dependent behaviors. Still, the sample implanted with a lower ion fluence has longer $T_{2}$ and $T_{1}$. We provide possible theoretical explanations for the observed temperature-dependent dynamics. Our work promotes the understanding of the temperature dependence of spin properties in solid-state systems, which can be helpful for constructing wide temperature-range thermometers based on the mature semiconductor material.

Femtosecond laser-induced nitrogen fluorescence emission at different air pressures

In this paper, we experimentally investigate femtosecond laser-induced nitrogen fluorescence emission at different air pressures (0.1 Pa–100 kPa). The variation of nitrogen molecule (N2) and molecular nitrogen ion (N2+) characteristic fluorescence spectral lines' intensity as a function of air pressure is observed. When the air pressure is between 10 Pa and 100 kPa, the intensity of fluorescence signals from N2+ and N2 shows opposite variation as a function of air pressure. We deduce that dissociative recombination is the main pathway for the production of the C3∏u state of N2. This research helps elucidate the physical emission mechanism of femtosecond laser-induced nitrogen fluorescence emission.

Model-based reproduction and validation of the total spectra of a\xa0solar flare and their impact on the global environment at the X9.3 event of September 6, 2017

We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15–35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside Atmosphere and Ionosphere model for Aeronomy), it was found that EUV radiation of approximately 15–35 nm affects a wide altitude range of 120–300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules.

Populations of B2Σu+ and X2Σg+ electronic states of molecular nitrogen ions in air determined by fluorescence measurement

We carry out a combined experimental and numerical investigation on the population distribution between the ground state and the excited state of molecular nitrogen ions in an 800-nm laser field. Efficient population transfers between the ground state ${\mathrm{N}}_{2}^{+}(X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+},\ensuremath{\nu}=0)$ and the excited state ${\mathrm{N}}_{2}^{+}(B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+},{\ensuremath{\nu}}^{\ensuremath{'}}=0)$ occur with a 400-nm time-delayed pulse, resulting in an enhanced fluorescence at 391 and 428 nm. The population distribution between the ground state ${\mathrm{N}}_{2}^{+}(X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+},\ensuremath{\nu}=0)$ and the excited state ${\mathrm{N}}_{2}^{+}(B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+},{\ensuremath{\nu}}^{\ensuremath{'}}=0)$ is extracted from the fluorescence enhancement factor. The results show that the population of ${\mathrm{N}}_{2}^{+}(X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+},\ensuremath{\nu}=0)$ decreases when the laser exceeds 2.1 mJ in our experiment. Our study provides a method to check the population information of both excited and ground states experimentally in a strong-field-induced plasma of ionized nitrogen molecules.

Theory of femtosecond strong field ion excitation and subsequent lasing in

Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent experiments in air or pure nitrogen pumped with an intense femtosecond laser pulse at wavelength of 800 nm. The mechanism responsible for the lasing is highly controversial. In this article we propose a model explaining the delayed lasing, which contains two parts: (i) ionization of neutral nitrogen molecules and subsequent excitation of nitrogen ions in a strong pump laser pulse, and (ii) coherent emission of excited ions due to the presence of long-lived polarizations maintained by a weak laser post-pulse and coupling simultaneously ground state to states A2Π u and of singly ionized nitrogen molecules . Two regimes of signal amplification are identified: a signal of a few picosecond duration at low gas pressures and a short (sub-picosecond) signal at high gas pressures. The theoretical model compares favorably with results obtained by different experimental groups.

Optical amplification from high vibrational states of ionized nitrogen molecules generated by 800-nm femtosecond laser pulses.

We experimentally investigated the interaction between nitrogen molecules and intense femtosecond laser pulses. When irradiated by an 800-nm pump laser and a delayed 355-nm seed laser, the spectral lines around 353.3 nm and 353.8 nm are observed to be greatly amplified, no matter whether the pump laser is circularly or linearly polarized. The two spectral lines correspond to the transition of N2+ (B, ν' = 5 → X, ν = 4) and N2+ (B, ν' = 4 → X, ν = 3), respectively. In comparison with the spectral lines related with ground vibrational states of nitrogen molecular ion, the observed amplification exhibits different polarization dependence of the pump laser. This distinctive change can be explained by the population variation of high vibrational states caused by the pump laser with different polarizations.

Enhanced Coherent Emission from Ionized Nitrogen Molecules by Femtosecond Laser Pulses.

The forward emission spectra were experimentally measured for ionized nitrogen molecules by an 800 nm pump laser and a delayed seed laser. It was found that emission lines around both 428 and 391 nm are greatly enhanced upon use of a 391 or 428 nm seed laser. The emission lines around 391 and 428 nm can be assigned to the rotational transitions of N2+ [B2Σu+(v' = 0) → X2Σg+(v = 0)] and N2+ [B2Σu+(v' = 0) → X2Σg+(v = 1)], respectively. They originate from seed-induced superfluorescence and resonant stimulated Raman scattering. The genetic algorithm was utilized to simulate the experimental observations and determine the relative population of B2Σu+(v' = 0), X2Σg+(v = 1), and X2Σg+(v = 0). The result verifies that vibrational population inversion is achieved between B2Σu+(v' = 0) and X2Σg+(v = 0) by the 800 nm pump laser. Our finding provides new insights into controlling the coherent emission of ionized nitrogen molecules, which has promising application in filamentation-based remote atmospheric sensing.

Subfemtosecond-resolved modulation of superfluorescence from ionized nitrogen molecules by 800-nm femtosecond laser pulses.

Superfluorescence emission around 391 nm is generated when nitrogen molecules are irradiated by a strong 800-nm pump laser and a delayed seed laser. The emission corresponds to the transition between N2+(B2Σu+,ν″=0) and N2+(X2Σg+,ν=0). When another weak 800-nm probe laser is injected and scanned after the pump laser, the superfluorescence intensity is observed to exhibit periodical modulation. The period is determined to be ~2.63 fs, corresponding to the transition frequency between N2+(A2Πu,ν'=2) and N2+(X2Σg+,ν=0). Based on theoretical derivation, these observations can be attributed to the laser-induced population transfer and polarization variation between the relevant electronic states of ionized nitrogen molecules.

Terahertz control of air lasing

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission could be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed light on the role of intense transient fields in the coherent emission from photoexcited gas molecules.

Coherent modulation of superradiance from nitrogen ions pumped with femtosecond pulses

Singly ionized nitrogen molecules in ambient air pumped by 800 nm femtosecond laser give rise to superradiant emission. Here, we study this superradiance by injecting a pair of resonant seeding pulses at different intensity ratios inside the nitrogen gas plasma. Strong modulation of the 391.4 nm superradiant emission with a period of 1.3 fs is observed when the delay between the two seeding pulses is finely tuned. The modulation contrast is increased and then decreased with the delay time when the second seed pulse is stronger than the first one, and the maximum modulation contrast occurs at longer delay time when the second seeding pulse is stronger. This reveals the increase of the macroscopic polarization with time after the seeding pulse. Moreover, these observations provide a new level of control on the "air lasing" based on nitrogen ions, which can find potential applications in optical remote sensing.

Stimulated-Raman-scattering-assisted superfluorescence enhancement from ionized nitrogen molecules in 800-nm femtosecond laser fields

We have observed collective spontaneous emission (superfluorescence) from ionized nitrogen molecules generated by 800-nm laser pulses. The superfluorescence intensity around 391 nm was found to be greatly enhanced when injecting an external seed with various wavelengths. Our finding provides insights into controlling the coherent emission from air filamentation induced by intense femtosecond laser fields, which has great potential application in remote atmospheric sensing.

Optimizing the 391-nm lasing intensity from ionized nitrogen molecules in 800-nm femtosecond laser fields

We investigate the 391-nm lasing dynamics from ionized nitrogen molecules in 800-nm femtosecond laser fields. By comparing the radiation intensity, spectrum shape, and temporal profile of the 391-nm lasing at various experimental conditions, we conclude that the lasing dynamics contains not only the generation and the decay of ionized nitrogen molecules, but also the seed-built coherence among emitters as well as the propagation effect in the plasma filamentation. These results provide reliable guidance for optimizing the 391-nm lasing from ionized nitrogen molecules in 800-nm femtosecond laser fields, which have potential applications for remote sensing in the atmosphere.

Vibrational and electronic excitation of ionized nitrogen molecules in intense laser fields

Vibrational and electronic excitation of ionized nitrogen molecules in intense laser fields

The fast decay of ionized nitrogen molecules in laser filamentation investigated by a picosecond streak camera

A column of bright filament is generated when a strong 800 nm femtosecond laser propagates in pure nitrogen gas, from which some fluorescence emission lines are observed. These emission lines can be assigned to the transitions of and N2 (C3Πu → B3Πg). By using a picosecond streak camera, we investigated the fluorescence decay dynamics of It was found that the decay time is in the sub-nanosecond range when the gas pressure is several millibar, more than two orders magnitude shorter than its spontaneous emission lifetime. Through monitoring the population evolution of as a function of free electron density, we concluded that the fast decay originates from the collision between and free electrons in the plasma filament.

Changes in Properties of Dielectric Barrier Discharge Plasma Jets for Different Gases and for Insulating and Conducting Transfer Plates

Dielectric Barrier Discharge (DBD) plasma jets has been studied extensively in recent years because of its wide range of applications. DBD plasmas can be produced using many different gases and can be applied to a broad variety of surfaces and substrates. In this work, we provide a comparison of DBD plasmas generated using argon (Ar), helium (He) and nitrogen (N2), as well as their mixtures with water vapor in order to know how some plasma properties are affected by the use of different gases. All plasmas were studied in two different conditions, using a transfer plate made of a conductive material and using a transfer plate made of an insulating one. We observed that the processes of excitation and ionization of nitrogen molecules by direct collisions with Ar or He are more evident and significant in He plasmas, which means that He atoms in metastable states have greater ability to transfer energy to molecules of nitrogen in the plasma. The collisions of He atoms in metastable states with N2 molecules determine the vibrational temperature (Tvib) values in He plasmas, while in Ar and N2 plasmas the Tvib values are determined mainly by collisions of electrons with N2 molecules.

Field-Free Permanent Alignment of Ionized Nitrogen Molecules

Field-Free Permanent Alignment of Ionized Nitrogen Molecules

Characteristics of sub-atmospheric pressure glow discharge plasmas for preparation of a-C:H films

A new method of synthesis of a-C:H films using a pulsed glow discharge plasma with a mixed gas of helium and nitrogen at sub-atmospheric pressure of 2 kPa was proposed. Characteristics of the pulsed glow discharge plasma produced with a pair of parallel plate electrodes by applying high-repetition bipolar voltage pulses were investigated using optical emission spectroscopy. The ratio of intensities of the 0–0 transitions of the second positive system of nitrogen molecule (337.1 nm) and the first negative system of ionized nitrogen molecule (391.4 nm) was measured to investigate the electric field strength in the discharge plasma. The spatial structure of the plasma emission was similar with that of a typical glow discharge. It was also found that an intensive electric field appeared near the cathode electrode during negative ripple pulse voltages induced by an inductor added between the electrodes and the power supply. A uniform a-C:H film on a silicon substrate with hardness of 7 GPa was successfully prepared with the pulsed glow discharge plasma with the ripple pulse voltage application, where acetylene was used for the precursor gas. The deposition rate is 0.2 μm/min which is approximately 10 times higher than that of the conventional plasma CVD method under the low gas pressure. It is discussed that a rough estimate of incident ion energy onto the negatively biased substrate using a collisional plasma sheath model indicates the possibility of increase in the incident ion energy that is needed for enhancement of fraction of sp3 carbon bond due to decrease in the neutral gas pressure from 100 kPa to 2 kPa.