Molecular Nitrogen Ions Research Articles

Short- and long-term gain dynamics in N2+ air lasing

Air lasing in the nitrogen molecular ion is not well understood because the complex physics responsible for gain is interwoven with pulse propagation in an extreme environment. Here we use a short gas jet to limit the interaction length, thereby removing the propagation effects. We report on several mechanisms that contribute to the decay of gain in different conditions, and experimentally isolate two decay timescales: the decay of long-term gain due to collisional state mixing, and short-term gain that cannot be explained by population inversion. To test the former, we control the inelastic electron scattering rate by varying the gas concentration while keeping the propagation length fixed, and predict the change of the decay using a model of collisional state mixing. We show that the same mechanism causes the decay of rotational wave packets in the states of the ion. Finally, we simulate the complex modulations of gain due to rotational wave packets and the propagation of the probe pulse through the evolving rotationally excited and inverted medium.

Application and evaluation of nitriding treatment using active screen plasma

Plasma nitriding was performed on samples of JIS SKD61 die steel, and the microstructure near the surface was analyzed. The plasma nitriding was performed using active screen plasma (ASP). In the ASP nitriding process, a dependency on the bias current applied to the sample was found.Emission spectra showed the presence of atomic nitrogen ions (N+), molecular nitrogen ions (N2+), hydrogen radicals (H), nitrogen radicals (N), and radicals consisting of nitrogen and hydrogen (NiHi). The densities of nitrogen and hydrogen radicals observed by vacuum ultraviolet absorption spectroscopy were 1.5 × 1011 and 1.0 × 1011 cm−3, respectively. Once the device current reached 0.25 A or more, the surface became cloudy. The cause of this cloudiness was investigated by X-ray diffraction and cross-sectional bright-field scanning transmission electron microscopy and was found to be a deposition layer around 50 nm thick of a mixed crystal containing the ε phase Fe23N and γ' phase Fe4N and the formation of a structure with a period of around 300 nm at the surface. Although deposition of Fe23N and Fe4N also occurred at the surface in samples without cloudiness, the deposition layer was only around 10 nm thick, and no periodic structure was formed. A chromium oxide-rich layer was found directly under the Fe23N and Fe4N deposition layer by scanning transmission electron microscopy with electron energy-loss spectroscopy. This chromium oxide-rich layer likely occurred due to selective etching of iron oxide and chromium oxide. In ASP nitriding method, the sputtering action of charged particles of N2+ and N+ was experimentally shown to play an important role in nitriding and surface morphology. Furthermore, it was discovered that there was no compound layer several micrometers thick as would form in the conventional nitriding process.

Multi-photon ionisation spectroscopy for rotational state preparation of {{bf{N}}}_{{bf{2}}}^{{boldsymbol{+}}}

In this paper we investigate the 2 + 1′ resonance enhanced multi-photon ionisation (REMPI) of molecular nitrogen via the a1Πg(v = 6) intermediate state and analyse its feasibility to generate molecular nitrogen ions in a well defined ro-vibrational state. This is an important tool for high precision experiments based on trapped molecular ions, and is crucial for studying the time variation of the fundamental constant mp/me using {{rm{N}}}_{2}^{+}. The transition is not reported in the literature and detailed spectral analysis has been conducted to extract the molecular constants of the intermediate state. By carefully choosing the intermediate ro-vibrational state, the ionisation laser wavelength and controlling the excitation laser pulse energy, unwanted formation of rotationally excited molecular ions can be suppressed and ro-vibrational ground state ions can be generated with high purity.

The sputtering yield of crystalline Si(1 0 0) surface by monoatomic and diatomic nitrogen ions impact

We have investigated the sputtering rate of thin crystalline silicon film forming silicon-on-insulator structure impinged by atomic and molecular nitrogen ions with medium energies of 25, 50, and 100 keV/(N atom) at several incident angles of 0, 30, 45, and 60°. The thickness of the surface silicon layer after ion irradiation was quantitatively estimated by Rutherford backscattering spectroscopy using 2.56-MeV 10B2+ ions with a grazing angle geometry to accurately analyze the thickness decrement in several nanometers for ultrathin Si layers. The sputtering rates were compared with a full-cascade TRIM simulation based on a binary collision approximation. We found that the sputtering yields of monoatomic N ion with higher energies (>50 keV/N) took significantly higher than that of diatomic N2 ion, probably suggesting that a wake-like effect of relatively fast NN molecular ions surviving for a long period due to a considerably strong triple bond. In addition, the observed sputtering yields for all present conditions were a few times smaller than that estimated by the full-cascade TRIM calculation using typical lattice parameters. We quantitatively discuss the details of sputtering phenomenon of crystalline silicon by nitrogen ions at medium energy region.

Third harmonic from air breakdown plasma induced by nanosecond laser pulses

Harmonic generation is a nonlinear optical effect consisting in frequency up-conversion of intense laser radiation when phase-matching conditions are fulfilled. Here, we study the mechanisms involved in the third harmonic (TH) generation process, the conversion efficiency, and the properties of TH radiation generated in air by focusing infrared linearly polarized nanosecond laser pulses at intensities of the order of TW/cm2. By analyzing the emission from the air breakdown plasma, we demonstrate that filamentary breakdown plasma containing molecular nitrogen ions acts as an optical nonlinear medium enabling generation of TH radiation in the axial direction. The data reveal important properties of the TH radiation: maximum conversion efficiency of 0.04%, sinc2 dependence of the TH intensity on the square root of the pump intensity, and three times smaller divergence and pulse duration of TH as compared to the pump radiation.

Free-space Ν2+ lasers generated in strong laser fields: the role of molecular vibration.

We investigate free-space lasing actions from molecular nitrogen ions (N2+) at the wavelengths of ~391 nm and ~428 nm. Our results show that pronounced gain can be measured at either 391 nm or 428 nm laser wavelength with a pump laser centered at 800 nm wavelength, whereas the gain at 391 nm laser wavelength completely disappears when the wavelength of the pump laser is tuned to 1500 nm. Our theoretical analysis reveals that the different gain behaviors can be attributed to the vibrational distribution of populations in X2Σg+(v=0) and X2Σg+(v=1) states as the N2+ ions are generated by photoionization in the laser fields, giving rise to more robust (i.e., less sensitive to the pump laser wavelength) population inversion for generating the 428 nm laser.

An anatomy of strong-field ionization-induced air lasing

It is known that in an intense laser field of a sufficiently high strength combined with a sufficiently long wavelength, i.e., in the regime of the Keldysh parameter γ < 1, photoionization of atoms and molecules can be realized through a quantum tunnel process. The tunnel ionization preferentially occurs from the orbital with the lowest ionization energy, thus the majority of the generated ions will stay on the ground state. It is surprising that tunnel ionization of nitrogen molecules with mid- and near-infrared intense laser fields can initiate strong laser-like emissions, indicating generation of stimulated emissions in molecular nitrogen ions. The physical mechanism behind the observation is still under debate. Here, we review the major progresses we made in the past a few years. The focus is placed on investigations on the lasing action at 391 nm wavelength initiated by either mid-infrared strong laser fields in the wavelength range from 1.2 to 2 µm or near-infrared intense laser fields around 800 nm wavelength. We reveal that the mechanisms of lasing actions are different for the pump lasers in the above two spectral regions. We also show that the coherent wavepackets of molecular nitrogen ions generated in the intense laser fields uniquely allow for efficient nonlinear interaction with light at resonance frequencies.

Ion beam genetic-technology for modification of rice phenotypes

Ion beam genetic-technology (IBGT) extends ion beam technology to genetic modification of biological living materials aiming at improving biological species properties by ion-beam-induced mutation breeding. In this work, ion beam implantation of rice seeds was operated using a home-developed vertical compact ion implanter which was featured with simple structure, easy operation, high beam intensity and broad beam irradiation, particularly for IBGT applications. Many ten thousands of seeds of two selected local rice varieties were bombarded with 30-kV and 50-kV accelerated mixed molecular and atomic nitrogen ions to fluences ranging from 1 × 1016 to 4 × 1016 ions/cm2. The ion bombardments in 34,000 rice seeds of the two famous Thai rice varieties resulted in broad-spectrum mutation induction found in 91 rice mutants with phenotypic modifications and grain qualities such as photoperiod insensitivity, short in stature and changes in size of rough seed and brown rice, color of brown rice, proximate compositions and antioxidant capacities. This paper reports details of the results and exhibits evidence of the rice developments.

Generation of Raman lasers from nitrogen molecular ions driven by ultraintense laser fields

Atmospheric lasing has aroused much interest in the past few years. The ‘air–laser’ opens promising potential for remote chemical sensing of trace gases with high sensitivity and specificity. At present, several approaches have been successfully implemented for generating highly coherent laser beams in atmospheric condition, including both amplified-spontaneous emission, and narrow-bandwidth stimulated emission in the forward direction in the presence of self-generated or externally injected seed pulses. Here, we report on generation of multiple-wavelength Raman lasers from nitrogen molecular ions (), driven by intense mid-infrared laser fields. Intuitively, the approach appears problematic for the small nonlinear susceptibility of ions, whereas the efficiency of Raman laser can be significantly promoted in near-resonant condition. More surprisingly, a Raman laser consisting of a supercontinuum spanning from ∼310 to ∼392 nm has been observed resulting from a series near-resonant nonlinear processes including four-wave mixing, stimulated Raman scattering and cross phase modulation. To date, extreme nonlinear optics in molecular ions remains largely unexplored, which provides an alternative means for air–laser-based remote sensing applications.

Near-Resonant Raman Amplification in the Rotational Quantum Wave Packets of Nitrogen Molecular Ions Generated by Strong Field Ionization.

The generation of laserlike narrow bandwidth emissions from nitrogen molecular ions (N_{2}^{+}) generated in intense near- and mid infrared femtosecond laser fields has aroused much interest because of the mysterious physics underlying such a phenomenon. Here, we perform a pump-probe measurement on the nonlinear interaction of rotational quantum wave packets of N_{2}^{+} generated in midinfrared (e.g., at a wavelength centered at 1580nm) femtosecond laser fields with an ultrashort probe pulse whose broad spectrum overlaps both P- and R-branch rotational transition lines between the electronic states N_{2}^{+}(B^{2}Σ_{u}^{+},v^{'}=0) and N_{2}^{+}(X^{2}Σ_{g}^{+},v=0). The results indicate the occurrence of highly efficient near-resonant stimulated Raman scattering in the quantum wave packets of N_{2}^{+} ions generated in strong laser fields in the midinfrared region, of which the underlying mechanism is different from that of the air lasers generated in atmospheric environment when pumping with 800nm intense pulses.

Optical gain in rotationally excited nitrogen molecular ions

We pump low-pressure nitrogen gas with ionizing femtosecond laser pulses at 1.5 $\ensuremath{\mu}\mathrm{m}$ wavelength. The resulting rotationally excited ${\mathrm{N}}_{2}^{+}$ molecular ions generate directional, forward-propagating stimulated and isotropic spontaneous emissions at 428 nm wavelength. Through high-resolution spectroscopy of these emissions, we quantify rotational population distributions in the upper and lower emission levels. We show that these distributions are shifted with respect to each other, which has a strong influence on the transient optical gain in this system. Although we find that electronic population inversion exists in our particular experiment, we show that sufficient dissimilarity of rotational distributions in the upper and lower emission levels could, in principle, lead to gain without net electronic population inversion.

Excitation of nitrogen molecular ions in a strong laser field by electron recollisions

A semiclassical model of electron rescattering in a strong laser field is applied to the evaluation of the probability of nitrogen molecular ion excitation. Depending on the ionization phase, a group of electrons oscillate around the parent ion for several laser periods and rescatter at large angles. The effect of ion attraction enhances significantly the probability of ion excitation by electron–ion collisions. The total probability of ion transfer from ground to excited ionic molecular state and the corresponding dipolar moment are evaluated for typical laser filament parameters.

The electron temperature in the plasma of a DC discharge created in a supersonic airflow

The plasma parameters of a pulsating DC discharge created in a supersonic airflow with a Mach number of M = 2 are determined. It is revealed that along with the intense bands of CN and the molecular nitrogen ion, as well as the spectral lines of atomic oxygen, nitrogen, hydrogen and copper, an intense continuous spectrum is observed in the spectrum of the gas-discharge plasma radiation, which is caused by the deceleration of electrons on ions. The dependences of the electron temperature on the discharge current and longitudinal coordinates are determined. It was revealed that the studied plasma is nonequilibrium, with the electron temperature being much higher than the gas temperature.

Influence of nitrogen admixture to argon on the ion energy distribution in reactive high power pulsed magnetron sputtering of chromium

Reactive high power impulse magnetron sputtering (HiPIMS) of metals is of paramount importance for the deposition of various oxides, nitrides and carbides. The addition of a reactive gas such as nitrogen to an argon HiPIMS plasma with a metal target allows the formation of the corresponding metal nitride on the substrate. The addition of a reactive gas introduces new dynamics into the plasma process, such as hysteresis, target poisoning and the rarefaction of two different plasma gases. We investigate the dynamics for the deposition of chromium nitride by a reactive HiPIMS plasma using energy- and time-resolved ion mass spectrometry, fast camera measurements and temporal and spatially resolved optical emission spectroscopy. It is shown that the addition of nitrogen to the argon plasma gas significantly changes the appearance of the localized ionization zones, the so-called spokes, in HiPIMS plasmas. In addition, a very strong modulation of the metal ion flux within each HiPIMS pulse is observed, with the metal ion flux being strongly suppressed and the nitrogen molecular ion flux being strongly enhanced in the high current phase of the pulse. This behavior is explained by a stronger return effect of the sputtered metal ions in the dense plasma above the racetrack. This is best observed in a pure nitrogen plasma, because the ionization zones are mostly confined, implying a very high local plasma density and consequently also an efficient scattering process.

Population inversion in the rotational levels of the superradiant N2 + pumped by femtosecond laser pulses.

Nitrogen molecular ions (N2 +) in air plasma pumped by femtosecond laser pulses give rise to superradiant emission at 391.4 nm in the presence of an external seed pulse at proper wavelength. Due to the transient alignment of the nitrogen molecular ions, the superradiance signal presents a strong modulation as a function of the temporal delay between the pump and the seed pulses. Through Fourier transformation with high frequency resolution, we distinguished the contribution of the finely separated rotation levels of the upper and lower states. It was found that the population density of certain rotational levels in the upper state is higher than that in the lower one, indicating that population inversion of the rotation levels of the two involved states is a key enabling factor for this superradiant emission.

Carbon dust particles in a beam-plasma discharge

This paper focuses on dynamics of micro-sized carbon dust grains in beam-plasma discharge (BPD) plasmas. It was demonstrated that injected dust particles can be captured and transported along the discharge. Longitudinal average velocity of the particles in the central area of the plasma column was 17 m/sec, and 2 m/sec in the periphery. Dust injection caused a decrease of emission intensity of metastable nitrogen molecular ion. This effect is suggested for a spectroscopy method for particles’ potential measurements. Five-micron radius carbon dust grains obtained potential above 500 V in the experiments on PR-2 installation, proving the feasibility of BPDs for the charging of fine dust particles up to high potential values, unattainable in similar plasma conditions.

Superradiance by molecular nitrogen ions in filaments

Results of the experimental study of population inversion in the resonant electronic transition \({B^3}{\pi _g} - {A^3}\sum _u^ + \) of nitrogen ions by optical pumping of atmospheric air and pure nitrogen by a femtosecond laser pulse at a wavelength of 950 nm are presented. It is shown that the inversion results from selective population of the \(N_2^ + \left( {{B^2}\sum _u^ + ,v' = 0} \right)\) excited state during multiphoton excitation of the autoionization state of the nitrogen molecule with an energy of 18.7 eV. Seed photons for superradiance at transitions of molecular nitrogen ions are photons of the axial supercontinuum that occurs in a filament at the corresponding wavelengths. The superradiance mode is implemented at the wavelength λ = 358.4 nm referred to the transition of the CN molecule.

Population Redistribution Among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields.

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel-ionized nitrogen molecules at various driver wavelengths in the near- and midinfrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground N_{2}^{+}(X^{2}Σ_{g}^{+}) state and the excited N_{2}^{+}(A^{2}Π_{u}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states occur in strong laser fields. The couplings result in a population inversion between the N_{2}^{+}(X^{2}Σ_{g}^{+}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states at wavelengths near 800nm, which is verified by our experimental observation of the amplification of a seed at ∼391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.

Generation of elliptically polarized nitrogen ion laser fields using two-color femtosecond laser pulses.

We experimentally investigate generation of nitrogen molecular ion () lasers with two femtosecond laser pulses at different wavelengths. The first pulse serves as the pump which ionizes the nitrogen molecules and excites the molecular ions to excited electronic states. The second pulse serves as the probe which leads to stimulated emission from the excited molecular ions. We observe that changing the angle between the polarization directions of the two pulses gives rise to elliptically polarized laser fields, which is interpreted as a result of strong birefringence of the gain medium near the wavelengths of the laser.

Contribution of nitrogen atoms and ions to the luminescence emission during femotosecond filamentation in air

During femtosecond filamentation in air, nitrogen molecules and corresponding molecular ions undergo dissociation due to the high intensity of laser pulses, generating nitrogen atoms and atomic ions. The generated atoms and atomic ions emit luminescence in the UV range, which superposes on those emissions for the neutral and ionic nitrogen molecules. Here we report on a significant difference between the emission behavior of the 391-nm line and the other spectral lines under different pump laser polarizations. We attribute this difference to the contribution of the atomic ions to the luminescence emission around 391 nm. The difference becomes more evident in tightly focusing cases, providing an indirect but effective evidence for the dissociation of nitrogen molecular ions.