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We report the results of a high temporal resolution study on the formation of superradiance (SR) in plasma on the first negative system of molecular nitrogen ions in plasma of an air filament. The filament is formed by a femtosecond laser pulse with a wavelength of λ = 950 nm. For the first time, the actual SR pulse duration at λ = 428 nm is measured, which constitutes 1.15 ps (FWHM). According to measurements of a spectral width equal to 0.26 nm, this pulse duration is close to transform-limited (1 ps). It is shown that there is a delay by 6 ps in the SR pulse maximum relative to the pump.

A comprehensive study of electron collisions with the X 1Σg + ground state as well as the metastable A 3Σu + and a 1Πg excited states of the N2 molecule is reported using the fixed-nucleus R-matrix method. Integral elastic scattering and electronic excitation cross sections from the X 1Σg + ground state to the eight lowest electronic states, A 3Σu +, B 3Πg, W 3Δu, B′ 3Σu −, a 1Πg, a′ 1Σu −, w 1Δu and C 3Πu, overall agree well with the available experimental and theoretical results although updates of some recommended values are suggested. Accurate electron impact electronic transition cross sections starting from the A 3Σu + and a 1Πg metastable excited states are reported. The total summed electronic transition cross sections from the a 1Πg state is dominant: an order of magnitude higher than those of the X 1Σg + ground state. The de-excitation cross sections generally show a downward trend with increasing incident electron energy, which is different from the elastic and electronic excitation cross sections which generally increase with collision energy. There is a prominent 2Πu symmetry resonance peak at 2.8 eV for electronic de-excitation scattering of a 1Πg → B 3Πg, which significantly contributes to the total summed cross sections from the a 1Πg excited state. The present results provide a new insight which will aid understanding of electron spectra in the atmosphere of the Earth and Titan.

We investigated the vibrational temperature of molecular nitrogen in the downstream of helicon-wave excited helium and argon-based plasmas. It was confirmed by optical emission spectroscopy that the major part of the helium plasma was at a recombining state and it had an ultralow electron temperature of approximately 0.1 eV. In spite of the ultralow electron temperature, the vibrational temperature of molecular nitrogen, which was added into the helium plasma, was higher than that in the argon-based plasma at an ionizing state with an electron temperature of 1.7 eV. According to the relationship between the rate coefficient of electron impact vibrational excitation and the electron temperature, the higher vibrational temperature in the helium plasma is not attributable to the more efficient vibrational excitation. Therefore, the higher vibrational temperature is owing to the less efficient destruction of vibrational excited states in the helium plasma with the ultralow electron temperature.

Triple differential cross sections (TDCSs) are reported for the positron and electron impact ionization of molecular nitrogen at a projectile energy of 250 eV. The TDCSs are calculated in the distorted wave Born approximation formalism using the orientation-averaged molecular orbitals. The present attempt is helpful for analyzing recent measurements (de Lucio and DuBois 2016 Phys. Rev. A 93 032710) and studying the effect of projectile charge on the ionization of a molecular target. The TDCS trends are compared for positron and electron impact ionization in terms of binary and recoil intensities and binary lobe positions for different values of energy loss. The binary emission of electrons is enhanced for positron impact; however, nearly the same recoil emission is observed for positrons as well as electron impact. Significant discrepancies are observed in the measurements in terms of the relative binary intensity for positron and electron impact.

The efficiency for conversion of a positron beam into a positronium (Ps) beam by charge exchange from molecular nitrogen has been investigated in the range of Ps kinetic energy, EPs = 30–250 eV. Measurements have also been made for the production efficiency of collimated Ps from molecular hydrogen in order to make a direct comparison with previous work. The results show that while H2 remains the best converter at lower energies, N2 becomes more efficient from 90 eV upwards. Using N2 as a production gas for collimated Ps, the accessible energy range for Ps scattering is increased by at least 100 eV. A measure of the differential Ps formation cross-section from N2, as well as indirect estimates of Ps–N2 total cross-sections, have also been obtained.

The ordinary and coincident electron-energy-loss spectra of N2 have been measured in an expected energy range for the formation of doubly excited states to investigate their formation and decay processes in electron-N2 collisions. The present results are compared with those obtained by photoexcitation experiments. Existence of the optically forbidden doubly excited states and their dissociative character have been revealed clearly for the first time.

The paper presents the results of researched processes in the interelectrode gap of High-current low-inductance vacuum spark (HLIV) on the "PION" system [1] by means of active laser diagnostics with visualisation of radiation field. The influence of the cathode geometry and position of the trigger system on the plasma dynamics of HLIV is considered. The regularities of the discharge for different initial conditions are established. The conditions under which the most stable plasma point is formed in space and time are determined.

The formations of molecular nitrogen and hydrogen complexes (η6-C6H6)-Cr(CO)2-L(L=N2 and H2) were observed via time-resolved infrared spectroscopy following 355nm laser photolysis of (η6-C6H6)Cr(CO)3 in the presence of N2 and H2 in gas phase. The rate constants for reactions of (η6-C6H6)-Cr(CO)2 with N2 with N2 and H2 were measured to be (1.4±0.1)×105 Torr-1s-1 and (2.6±0.1)×105 Torr-1s-1, respectively. The complexes (η6-C6H6)Cr(CO)2L (L=N2 and H2) have lifetimes longer than 1 ms in our experimental conditions implying the N2 and H2 binding energies in (η6-C6H6)Cr(CO)2 L (L=N2 and H2)>17 kcal mol-1.

Photoabsorption of molecular nitrogen (N2) is investigated near the first ionization threshold using an R-matrix, multi-channel quantum defect (MQDT) approach.

We investigate nonlinear interaction of nitrogen molecules with a two-color laser field composed by an intense 800 nm laser pulse and a weak 400 nm laser pulse. It is demonstrated that the spectrum of 400 nm pulses is dramatically broadened when the two beams temporally overlap. In comparison, the spectral broadening in argon is less pronounced, although argon atoms and nitrogen molecules have comparable ionization potentials. We reveal that the dramatic spectral broadening originates from the greatly enhanced nonlinear optical effects in the near-resonant condition of interaction between the 400 nm pulses and the nitrogen molecular ions.

The dissociation of nitrogen in a 2.45 GHz microwave plasma has been significantly enhanced by the application of a magnetic field perpendicular to the oscillating electric field. The degree of dissociation measured in the emergent beam was found to be strongly dependent on the magnetic field strength, nitrogen pressure and microwave power. These results have been interpreted in terms of microwave absorption resonances. Dissociation fractions up to 0.67 have been measured.

We observed the appearance of a broad feature at 360 nm in spectra obtained during the destruction of ensembles of molecular nitrogen nanoclusters containing stabilized nitrogen, oxygen, hydrogen and deuterium atoms. We attribute this broad feature with a maximum at 360 nm to the N4 polynitrogen molecules which are the product of the neutralization reaction of ions with electrons in solid N2.

The lifetime of the metastable A3Σu + state of molecular nitrogen was examined in the afterglow of inductively-coupled nitrogen plasma, and it was compared with the diffusion time constant which was determined by the geometry of the vacuum chamber, the gas pressure, and the gas temperature. As a result, we found that the decay time constants observed at almost all the discharge conditions were longer than the diffusion time constants. We discussed the possibilities of non-relaxed reflection on the chamber wall and gas-phase production of N2(A3Σu +), in order to explain the long decay time constants observed experimentally.

Triply differential cross sections are presented for 250 eV positron impact single ionization of molecular nitrogen. Data are presented for binary and recoil interactions resulting from projectiles being scattered between 0 and 5 degrees and energy losses up to approximately 50 eV.

We present a spectroscopic investigation of molecular nitrogen using a self-resonating co-axial dielectric barrier plasma reactor. The properties of the nitrogen plasma generated were recorded as functions of radio frequency (27.12 MHz), power (20–50 W) and gas pressure (0.1–60 mbar). The intensity distribution of the transitional structure of the electronic bands C3Πu → B3Πg, B2Σu+ → X2Σg+ and D2Πg → A2Πu was measured as a function of gas pressure. Relative energy efficiency (total emitted intensity from the plasma It divided by total electric power to the discharge P), as high as 14% can be obtained at a dissipated electrical power level of 20 W and a pressure of 10 mbar.

At the new free-electron laser (FEL) for vacuum ultraviolet (VUV) and soft x-ray radiation FLASH of the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, multi-photon double ionization of molecular nitrogen has been observed and studied by ion time-of-flight spectroscopy. The experiments have been performed at the microfocus beamline BL2 with photon pulses of 25 fs duration and irradiance levels up to 2 × 1013 W cm−2 at a photon energy of 38 eV, i.e. well above the first ionization/dissociation thresholds of the target. A new and important aspect of our experiments has been the reliable determination of absolute photon numbers per pulse with the help of a calibrated setup allowing the investigation of nonlinear effects by quantitative measurements. Results obtained are discussed in terms of a sequential two-photon ionization scheme.

We have calculated triple differential cross sections (TDCS) for the ionization of the nitrogen molecule by electron impact. The measured recoil-to-binary peak ratio for the outer valence orbitals (3σg, 1πu, 2σu) of the target is well reproduced by our models. One of our models is better for the description of the binary region of the TDCS, while the other reproduces a better recoil peak, similar to our previous observation for the CH4 molecule.

Non-molecular solid nitrogen, metastable even at zero pressure and low temperature, was recently manufactured by Eremets et al. (Nature 411 (2001) 173) and investigated with respect to its pressure stability. In this study we present analytical potential molecular-dynamics simulations that allow reproducing all the stages of the experiment. The bonding structure and energetics of the N polymeric state obtained from the dynamical simulations are verified to be reasonable using a plane-wave ab initio method. We predict that the metastable low-pressure phase has predominantly 3-folded bonded atoms in a disordered configuration. The energy accumulated in the metastable phase is about 1 eV/atom.

Parametric computations of the electron energy distribution function and of the electron mobility in a photoplasma of a lithium vapour mixture with argon and molecular nitrogen are presented. The possibility of the occurrence of a steady-state negative electron mobility when using a certain ratio of mix components under optical resonant excitation of the Li(2S-2P) transition is considered. The critical plasma parameters (the lithium excited atomic density, the temperature of the vibrational distribution of nitrogen molecules, the plasma ionization degree, the external electric field strength) determining this effect are revealed.

A self-consistent plasmachemical model describing the dynamics of the non-equilibrium microwave discharge in molecular nitrogen with consideration for kinetic, photochemical and electrodynamic phenomena is proposed. The photochemical block of the model accounts for the processes of photoexcitation and photoionization of nitrogen molecules in the ground and excited electronic states. Radiative emittance of the discharge plasma is conditioned by the processes of photorecombination of electrons and positive ions as well as by the processes of spontaneous radiation of electronically excited molecules. Solution of radiation transfer equations and calculation of photochemical constants were made with allowance for the vibrational-rotational structure of the corresponding radiative transitions. The calculations performed have shown that the velocities of the ionization front propagation in the microwave discharge that were observed experimentally may be explained without invoking considerations concerning the existence of easily ionizable admixtures in nitrogen. The phenomenon of electron generation in the photohalo region of the discharge is mainly conditioned by the processes of stepwise photoionization of nitrogen molecules. The mechanisms leading to the development of kinetic instabilities in the discharge region are analysed, and a satisfactory agreement between calculation and experimental results is noted.

Using a technique applied previously to vibrationally excited molecular nitrogen (N2*) in the region of daytime and nighttime aurora, the emission intensity of the N2 second positive band system in an inductively coupled plasma (ICP) has been analysed and the vibrational temperature of nitrogen molecules in the ICP is thus determined. The result shows that the vibrational temperature increases with the increase of the neutral gas pressure from 0.04Pa to 10Pa, then decreases with the further increase of the pressure from 10Pa to 100Pa. Also, this is explained by using the Boltzmann relation between the vibrational temperature and the concentration of the vibrationally excited N2*(X1?g+) molecules.

The coupled negatively charged nitrogen-vacancy (NV−) center system is a promising candidate for scalable quantum information techniques. In this work, ionized nitrogen molecules are implanted into bulk diamond to generate coupled NV− center pairs. The two-photon autocorrelation measurement and optically detected magnetic resonance measurement are carried out to confirm the production of the NV− center pair. Also, both 1.3 μs decoherence time and 4.9 kHz magnetic coupling strength of the NV− center pair are measured by controlling and detecting the spin states. Along with nanoscale manipulation and detection methods, such coupled NV− centers through short distance dipole-dipole interaction would show high potential in scalable quantum information processes.

Electron energy loss spectra for gaseous molecular nitrogen have been measured with an improved resolution of 30 meV at an impact energy of 35 eV and for scattering angles between -30 and +80'. The 10.6-13.25 eV energy loss range has been studied. The differential cross sections for excitation of the C 3 Pi u, E 3 Sigma g+ and a" 1 Sigma g+ optically forbidden states have been measured relative to those of optically allowed transitions as a function of the scattering angle. The measurements are compared with lower resolution data reported previously in the literature.

Tunalbe coherent extreme ultraviolet radiation has been produced in nitrogen by third harmonic generation (THG) processes enhanced by two-photon resonance with the a1Πg(ν' = 1) state and three-photon resonance with the c'41Σu+(ν' = 1) state, respectively. The highly resolved THG spectra imply that THG can be used to probe the higher excited states and to detect the rotational population of a molecular gas.