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Air-lasing from single ionized N$_2^+$ molecules induced by laser filamentation in air has been intensively investigated and the mechanisms responsible for lasing are currently highly debated. We use ultrafast nitrogen K-edge spectroscopy to follow the strong field ionization and fragmentation dynamics of N$_2$ upon interaction with an ultrashort 800 nm laser pulse. Using probe pulses generated by extreme high-order harmonic generation, we observe transitions indicative of the formation of the electronic ground X$^2\Sigma_{g}^{+}$, first excited A$^2\Pi_u$ and second excited B$^2\Sigma^+_u$ states of N$_2^+$ on femtosecond time scales, from which we can quantitatively determine the time-dependent electronic state population distribution dynamics of N$_2^+$. Our results show a remarkably low population of the A$^2\Pi_u$ state, and nearly equal populations of the X$^2\Sigma_{g}^{+}$ and B$^2\Sigma^+_u$ states. In addition, we observe fragmentation of N$_2^+$ into N and N$^+$ on a time scale of several tens of picoseconds that we assign to significant collisional dynamics in the plasma, resulting in dissociative excitation of N$_2^+$.

In this work we present an experimental demonstration of the Contextual Subspace Variational Quantum Eigensolver on superconducting quantum hardware. In particular, we compute the potential energy curve for molecular nitrogen, where a dominance of static correlation in the dissociation limit proves challenging for many conventional quantum chemistry techniques. Our quantum simulations retain good agreement with the full configuration interaction energy in the chosen STO-3G basis, outperforming all benchmarked single-reference wavefunction techniques in capturing the bond-breaking appropriately. Moreover, our methodology is competitive with several multiconfigurational approaches, but at a considerable saving of quantum resource, meaning larger active spaces can be treated for a fixed qubit allowance. To achieve this result we deploy an error mitigation/suppression strategy comprised of dynamical decoupling, measurement-error mitigation and zero-noise extrapolation, in addition to circuit parallelization that not only provides passive averaging of noise but improves the effective shot-yield to reduce the measurement overhead. Furthermore, we introduce a modification to previous adaptive ansatz construction algorithms that incorporates hardware-awareness into our variational circuits to minimize the transpilation cost for the target qubit topology., Comment: 22 pages, 12 figures

Nitrogen represents an archetypal example of material exhibiting a pressure driven transformation from molecular to polymeric state. Detailed investigations of such transformations are challenging because of a large kinetic barrier between molecular and polymeric structures, making the transformation largely dependent on kinetic stimuli. In the case of nitrogen, additional complications occur due to the rich polymorphism in the vicinity of the transition. Here, we report the observation of both molecular (\theta) and polymeric (BP) phases, crystallized upon temperature quenching of fluid nitrogen to room temperature at 97-114 GPa. Synchrotron single-crystal X-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations have been used for diagnostics of the phases and determination of their structure and stability. Molecular \theta-nitrogen is the most stable among molecular phases bordering the stability field of polymeric phases, partially settling a previously noted discrepancy between theory and experiment concerning the thermodynamic stability limit of molecular phases., Comment: 23 pages, 11 figures, 3 tables

The potential energy curves for molecular ions up to $N_2^{4+}$ are calculated in an ab initio manner using the multi configurational self-consistent field method. Specifically, we implement in an automatic way a previously used double loop optimisation scheme within the multi configurational self-consisted field method. We obtain the potential energy curves up to $N_2^{4+}$ ions with any combination of core, inner valence, and outer valence holes. Finally, we provide the code used to generate these potential energy curves., Comment: 20 pages, 16 figures

Soft x-ray radiation from the sun is responsible for the production of high energy photoelectrons in the D and E regions of the ionosphere, where they deposit most of their ionization energy. The photoelectrons created by this process are the main drivers for dissociation of Nitrogen molecule ($N_2$) below 200 km. The dissociation of N2 is one of main mechanisms of the production of Nitric Oxide (NO), an important minor constituent at these altitudes. In order to estimate the dissociation rate of N2 we need its dissociation cross-sections. The dissociation cross-sections for N2 by photoelectrons are primarily estimated from the cross-sections of its excitation states using predissociation factors and dissociative ionization channels. The lack of cross-sections data, particularly at high electron energies and of higher excited states of $N_2$ and $N_2^+$, introduces uncertainty in the dissociation rate calculation, which subsequently leads to uncertainties in the NO production rate from this source. In this work, we have fitted updated electron impact cross-sections data and by applying predissociation factors obtained, updated dissociation rates of N2 due to high energy photoelectrons. The new dissociation rates of N2 are compared to the dissociation rates obtained from Solomon and Qian [2005]. The new dissociation cross-sections and rates are estimated to be about 30% lower than the Solomon and Qian [2005] model. Simulations using a parameterized version of the updated dissociation rates in the Atmospheric Chemistry and Energetics (ACE1D) model leads to a $20%$ increase in NO density at the altitudes below 100 km is observed.

We photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition (molecular phase) for different states of the ion. We observe a 0.76 $\pi$ shift for the electrons produced in the ionization channels leading to the X$^2 \Sigma_g^+$, $v'=1$ and $v'=2$ states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short lived states, which is hardly accessible otherwise.

We perform a combined theoretical and experimental investigation of the superradiance in the quantum coherent system generated by strong laser fields. The semiclassical theory of superradiance that includes the superradiant temporal profile, character duration, time delay, intensity is derived. The experimental data and theoretical predictions of 391-nm forward emission as a function of nitrogen gas pressure are compared and show good agreement. Our results not only demonstrate that the time-delayed optical amplification inside the molecular nitrogen ions is superradiance, but also reveal the quantum optical properties of strong-field physics., Comment: 11 pages, 5 figures

A route to break molecular nitrogen (N2) under mild conditions is demonstrated by N2 gas cracking on, and incorporation into, lanthanide films. Successful growth of lanthanide nitride thin films, made by evaporation of lanthanides in a partial N2 atmosphere at room temperature and pressure as low as 10-4 Torr, is confirmed using X-ray diffraction. In-situ conductance measurements of pure lanthanide thin films exposed to N2 gas show an immediate surface reaction and a slower bulk reaction. Finally, we report partial reversal of the nitrogen incorporation in a lanthanide nitride by cycling vacuum and nitrogen conditions in the sample chamber.A route to break molecular nitrogen (N2) under mild conditions is demonstrated by N2 gas cracking on, and incorporation into, lanthanide films. Successful growth of lanthanide nitride thin films, made by evaporation of lanthanides in a partial N2 atmosphere at room temperature and pressure as low as 10-4 Torr, is confirmed using X-ray diffraction. Insitu conductance measurements of pure lanthanide thin films exposed to N2 gas show an immediate surface reaction and a slower bulk reaction. Finally, we report partial reversal of the nitrogen incorporation in a lanthanide nitride by cycling vacuum and nitrogen conditions in the sample chamber.

We performed sensitive observations of the N15ND+(1-0) and 15NND+(1-0) lines toward the prestellar core L1544 using the IRAM 30m telescope. The lines are not detected down to 3 sigma levels in 0.2 km/s channels of around 6 mK. The non-detection provides the lower limit of the 14N/15N ratio for N2D+ of ~700-800, which is much higher than the elemental abundance ratio in the local ISM of ~200-300. The result indicates that N2 is depleted in 15N in the central part of L1544, because N2D+ preferentially traces the cold dense gas, and because it is a daughter molecule of N2. In-situ chemistry is unlikely responsible for the 15N depletion in N2; neither low-temperature gas phase chemistry nor isotope selective photodissociation of N2 explains the 15N depletion; the former prefers transferring 15N to N2, while the latter requires the penetration of interstellar FUV photons into the core center. The most likely explanation is that 15N is preferentially partitioned into ices compared to 14N via the combination of isotope selective photodissociation of N2 and grain surface chemistry in the parent cloud of L1544 or in the outer regions of L1544 which are not fully shielded from the interstellar FUV radiation. The mechanism is the most efficient at the chemical transition from atomic to molecular nitrogen. In other words, our result suggests that the gas in the central part of L1544 already went trough the transition from atomic to molecular nitrogen in the earlier evolutionary stage, and that N2 is currently the primary form of gas-phase nitrogen., Comment: 5 pages, 2 figures, 2 tables, Accepted for publication in A&A Letters

We investigate the formation of multiple-core-hole states of molecular nitrogen interacting with a free-electron laser pulse. We obtain bound and continuum molecular orbitals in the single-center expansion scheme and use these orbitals to calculate photo-ionization and Auger decay rates. Using these rates, we compute the atomic ion yields generated in this interaction. We track the population of all states throughout this interaction and compute the proportion of the population which accesses different core-hole states. We also investigate the pulse parameters that favor the formation of these core-hole states for 525 eV and 1100 eV photons.

We compute molecular continuum orbitals in the single center expansion scheme. We then employ these orbitals to obtain molecular Auger rates and single-photon ionization cross sections to study the interaction of N2 with Free-Electron-Laser (FEL) pulses. The nuclei are kept fixed. We formulate rate equations for the energetically allowed molecular and atomic transitions and we account for dissociation through additional terms in the rate equations. Solving these equations for different parameters of the FEL pulse, allows us to identify the most efficient parameters of the FEL pulse for obtaining the highest contribution of double core hole states (DCH) in the final atomic ion fragments. Finally we identify the contribution of DCH states in the electron spectra and show that the DCH state contribution is more easily identified in the photo-ionization rather than the Auger transitions.

In this research, we employ accurate time-dependent density functional calculations for ultrashort laser spectroscopy of nitrogen molecule. Laser pulses with different frequencies, intensities, and durations are applied to the molecule and the resulting photoelectron spectra are analyzed. It is argued that relative orientation of the molecule in the laser pulse significantly influence the orbital character of the emitted photoelectrons. Moreover, the duration of the laser pulse is also found to be very effective in controlling the orbital resolution and intensity of photoelectrons. Angular resolved distribution of photoelectrons are computed at different pulse frequencies and recording times. By exponential growth of the laser pulse intensity, the theoretical threshold of two photons absorption in nitrogen molecule is determined., Comment: 7 pages, 6 figures, 2 tables