Ground State N2 Research Articles

An accurate analytic potential function for ground-state N2 from a direct-potential-fit analysis of spectroscopic data

Two types of combined-isotopologue analysis have been performed on an extensive spectroscopic data set for ground-state N2 involving levels up to v=19, which is bound by half the well depth. Both a conventional Dunham-type analysis and a direct-potential-fit (DPF) analysis represent the data within (on average) the estimated experimental uncertainties. However, the Dunham-type parameters do not yield realistic predictions outside the range of the data used in the analysis, while the potential function obtained from the DPF treatment yields quantum mechanical accuracy over the data region and realistic predictions of the energies and properties of unobserved higher vibrational levels. Our DPF analysis also introduces a compact new analytic potential function form which incorporates the two leading inverse-power terms in the long-range potential.

Role of electronic excited N2 in vibrational excitation of the N2 ground state at high latitudes

Vibrationally excited N2 is important in determining the ionospheric electron density and has also been proposed to play a role in the production of NO in disturbed atmospheres. We report here predictions of the absolute vibrational distributions in the ground electronic state of N2 produced by electron impact excitation, at noon and midnight under quiet geomagnetic conditions and disturbed conditions corresponding to the aurora IBCII+ and IBCIII+ at 60°N latitude and 0° longitude, at altitudes between 130 and 350 km. These predictions were obtained from a model which includes thermal excitation and direct electron impact excitation of the vibrational levels of the N2 ground state and its excited electronic states; radiative cascade from all excited electronic states to all vibrational levels of the ground electronic state; quenching by O, O2, and N2; molecular and ambipolar diffusion; and the dominant chemical reactions. Results from this study show that for both aurora and daytime electron environments: (1) cascade from the higher electronic states of N2 determines the population of the higher vibrational levels in the N2 ground state and (2) the effective ground state vibrational temperature for levels greater than 4 in N2 is predicted to be in the range 4000–13000 K for altitudes greater than 200 km. Correspondingly, the associated enhancement factor for the O+ reaction with vibrationally excited N2 to produce NO+ is predicted to increase with increasing altitude (up to a maximum at a height which increases with auroral strength) for both aurora and daytime environments and to increase with increasing auroral strength. The contribution of the cascade from the excited electronic states was evaluated and found to be relatively minor compared to the direct excitation process.

Quenching of 4He and 3He excited states by collisions with N2

A time-resolved spectroscopy method combined with pulse electron excitation are used for the experimental investigation of the quenching process of He excited states by collisions with nitrogen molecules in a He–Ne–N2 mixture at low pressure. The rate constants for quenching 4He(21S), 4He(21P), 3He(21S) and 3He(21P) states by collisions with ground state N2 molecules at Tg=600K are: k=(2.66±0.09)×10-10 and k=(1.89±0.28)×10-9cm3s-1 for 4He(21S) and 4He(21P), respectively, and k=(2.81±0.07)×10-10 and k=(3.21±0.58)×10-9cm3s-1 for 3He(21S) and 3He(21P), respectively.

Production yields of H(D) atoms in the reaction of N2(a g,ν=0) with H2 and D2

Abstract Quantum yields for the production of H(D) atoms in the reaction of singlet-excited metastable molecular nitrogen, N2(a 1 Π g, ν=0), with H2 and D2 were determined by analyzing the temporal profiles of N2(a, ν=0) and H(D)-atom densities after pulsed excitation. N2(a, ν=0) was produced by the two-photon excitation of ground-state N2, while H(D) atoms were detected by two-photon laser-induced fluorescence. N2(a, ν=0) was detected by laser-induced fluorescence utilizing the N2(b′ 1 Σ u+, ν=7) state as an upper-state. The H(D) atom yields are 0.5 ± 0.1 for H2 and 0.4 ± 0.1 for D2, suggesting that the near resonant relaxation to another metastable molecular nitrogen, N2(a′ 1 Σ u−), is also important. The production of N2(a′ 1 Σ u−) in collisions with H2(D2) was confirmed by measuring the H2(D2) pressure dependence of the two-photon laser-induced fluorescence signal of N2(a′ 1 Σ u−, ν=0) via the N2(b 1 Π u, ν=1) state.

The near-threshold absorption spectrum of N2

A new comprehensive multichannel quantum defect study of the near-threshold absorption of N214 has been carried out over the energy range 118 720–125 425 cm−1. A nearly complete understanding of the rotationally cold spectra reported earlier [K. P. Huber and Ch. Jungen, J. Chem. Phys. 92, 850 (1990); K. P. Huber et al., ibid. 100, 7957 (1994)] has been achieved in the region where core-excited s and d Rydberg levels built on the A 2Πu state of the ion interact with the series of p and f complexes converging to the lowest vibrational levels of X 2Σg+. The interactions reduce to a purely electronic quantum defect matrix which, after suitable transformations, accounts for the observed perturbed structures and intensities arising from vibronic coupling, rotational l uncoupling, and the different geometries of the X and A ion cores. The final calculations converged with 42 nonzero quantum defect parameters reproducing the 597 upper-state rovibronic levels with a standard deviation of 1.12 cm−1. The results have been used to calculate the R(0) line oscillator strengths in terms of eight nonvanishing electronic dipole transition moments, the latter treated as parameters that were fitted to photoelectrically measured band absorption f values. The calculations satisfactorily reproduce the observed oscillator strength distribution. Using ab initio calculated core properties for ground state N2+, the long-range model for a nonpenetrating Rydberg electron interacting with a quadrupolar and polarizable ion core predicts the diagonal f quantum defects in reasonable agreement with the results of the least-squares fits. Similar to NO, deviations from predictions by the same model for the diagonal d quantum defects arise primarily from the strong sσ∼dσ interchannel coupling and from the intrachannel interaction of the dπg Rydberg with the 1πg valence orbital, which, in contrast to 2π of NO, is occupied not in the ground state of N2, but in the electronically excited precursor states a′ 1Σu−, w 1Δu, and b′ 1Σu+.

Production Yields of H(D) Atoms in the Reactions of N2(a′ 1Σu−) with H2O, D2O, and HOD

Abstract The deactivation processes of N2(a′ 1Σu−, v = 0), the lowest metastable singlet-state molecular nitrogen, by H2O, D2O, and HOD were investigated. The overall rate constants as well as the quantum yields for the production of H(D) atoms were determined. N2(a′, v = 0) was produced by energy transfer from N2(a 1Πg, v = 0), while N2(a, v = 0) was produced by a two-photon excitation of the ground-state N2. The rate constants for the deactivation by H2O and D2O were determined to be (4.22 ± 0.27) × 10−10 and (4.21 ± 0.11) × 10−10 cm3 s−1, respectively, by measuring the decay profiles of N2(a, v = 0) under equilibrated conditions. The quantum yields for the production of H(D) atoms were determined to be 0.9 −0.2+0.1 both for H2O and D2O under the assumption that the only exit for H2(D2) is the production of two H(D) atoms. In a reaction with HOD, the yield ratio of [H]/[D] was measured to be 1.0 ± 0.1. This lack of the isotope effect suggests that the decomposition proceeds by forming bound intermediate complexes, such as HNNOD and DNNOH.

Selective production and kinetic analysis of thermally equilibrated N2(B3Πg, v = 0) and N2(W3Δu, v = 0)

The kinetic behavior of triplet-state molecular nitrogen, N2(B 3Πg, v = 0) and N2(W 3Δu, v = 0), was examined. These molecules were produced by excitation transfer between Xe(6s[3/2]1) and ground-state N2 followed by collisional vibrational relaxation. Xe(6s[3/2]1) was produced by two-photon laser excitation of Xe(6p[1/2]0) followed by concomitant amplified spontaneous emission (ASE). The weighted averages of the rate constants for the quenching of N2(B, v = 0) and N2(W, v = 0) were determined for 15 molecules by measuring the decay profiles of N2(B, v = 0). The observed rate constants are all one order of magnitude smaller than those reported for N2(B, v ≥ 1). The literature values for N2(B, v ≥ 1) must be the rate constants to produce neighbouring N2(W), from which N2(B) can be reproduced, and do not represent those for net quenching. In the reaction with H2(D2), the production of H(D) atoms was confirmed to be one of the major exit channels, while the relaxation to N2(A 3Σ+u) was found to be minor.

A Comparison of PE Surfaces Modified by Plasma Generated Neutral Nitrogen Species and Nitrogen Ions

The surface modification of polyethylene (PE) by neutral nitrogen species (ground and excited state N2 as well as atomic N; modified nitrogen plasma treatment) has been compared to the effect of nitrogen ion bombardment using X-ray Photoelectron Spectroscopy (XPS) and contact angle measurements. XPS results indicate that a greater nitrogen concentration was grafted during the modified nitrogen plasma treatment of PE, an effect that was attributed to surface sputtering during ion beam modification. The distribution of nitrogen-containing functionalities was strongly dependent upon the treatment strategy; the modified nitrogen plasma treatment lead predominantly to imine groups being formed at the PE surface, while amine groups were the dominant species produced during ion beam modification. The presence of electron irradiation during the modified nitrogen plasma treatment of PE did not modify the rate of nitrogen incorporation or change the nature of N-containing functional groups produced but did lead to a systematic decrease in contact angle.

Reactions of N2(a′ 1Σu−) with H2, CH4, and their isotopic variants: Rate constants and the production yields of H(D) atoms

The reactions of N2(a′ 1Σu−, v=0) with H2, CH4, and their isotopic variants were examined. N2(a′, v=0) was produced by energy transfer from N2(a 1Πg, v=0), while N2(a, v=0) was produced by two-photon excitation of ground state N2. The rate constant for the deactivation of N2(a′,v=0) can be determined by measuring the decay profiles of N2(a, v=0) under the conditions that equilibration between N2(a, v=0) and N2(a′, v=0) can be assumed. The detection of N2(a, v=0) was accomplished by a laser-induced fluorescence technique by utilizing the N2(b′ 1Σu+, v=7) state as an upper state. The rate constants for the quenching of N2(a′, v=0) by N2, H2, D2, CH4, CH2D2, and CD4 were determined to be (2.0±0.1)×10−13, (2.8±0.1)×10−11, (1.7±0.1)×10−11, (2.9±0.2)×10−10, (2.4±0.3)×10−10, and (2.6±0.2)×10−10 cm3 molecule−1 s−1, respectively. H(D) atoms were identified as reaction products by a two-photon laser-induced fluorescence technique. The yields for the production of H(D) atoms from CH4 and CD4 were both determined to be 0.7±0.2 under the assumption that the only exit for H2(D2) is the production of two H(D) atoms. No preferential production of H or D atoms was observed in the reaction with CH2D2, suggesting that the reaction proceeds via bound intermediate complexes.

Kinetics of N2(A 3Σu+) molecules and ionization mechanisms in the afterglow of a flowing N2 microwave discharge

To gain an understanding of the processes responsible for theformation of the well-known short-lived afterglow (SLA) or pink afterglowof nitrogen, different diagnostic techniques are implemented in theafterglow of a 440 Pa microwave nitrogen discharge in a 3.8 cm diameterflow tube. Using the intracavity laser absorption spectroscopy technique,we measure the space-dependent absolute density of N2(A 3Σu+; v = 0) metastable molecules, as well as their rotationaltemperature, which in fact corresponds to the gas temperature, Tg. The density of N2(A 3Σu+) molecules is about5×1017 m-3 at the end of the discharge zone (z = 4 cm). Itthen continuously decays by almost two orders of magnitude to reach aminimum around z = 12 cm before monotonically increasing to a secondarymaximum of 5×1016 m-3 located around z = 19 cm. It thenslowly decays at longer distances. The space-dependentN2(B 3Πg) fluorescence intensity evolves in exactly thesame way. A simple kinetic model is developed and we conclude thatmetastable molecules are locally formed in the SLA and not carried to thisregion by the gas flow. Contributions to the creation ofN2(A 3Σu+) molecules from N-N atom recombination, aswell as from high vibrational levels of the ground-state N2(X, v)molecules are analysed. To account for the high density ofN2(A 3Σu+) molecules, despite the (2-3)×1021 m-3 density of N atoms in the afterglow, we proposethat N(2P) metastable atoms, resulting from the efficient quenching ofN2(A 3Σu+) molecules by N(4S) atoms, reactimmediately with vibrationally excited N2(X 1Σg+;v''⩾10) molecules to again form N2(A 3Σu+) molecules. The absolute electron density in the SLA was alsomeasured by microwave interferometry. Its axial dependence also shows apronounced minimum at z = 12 cm before reaching a maximum of6×1015 m-3 at z = 19 cm. The possible processes for thislocal ionization, in the absence of any electric field, could be binarycollisions of the electronically excited molecules, and/or of thevibrationally excited ground-state molecules in v''⩾30levels.

Diagnostics of inductively coupled chlorine plasmas: Measurements of the neutral gas temperature

We report measurements of the bulk, neutral gas temperature in a chlorine transformer-coupled plasma. A trace amount (2%–5%) of N2 was added to the discharge and the rotational temperature of the C3Πu state was determined from the C3Πu→B3Πg emission in the ultraviolet. This temperature has been shown by others to be equal to the rotational temperature of ground-state N2, which is the thermally equilibrated (translational and rotational) gas temperature (Tg). The gas temperature 3 cm above the wafer is equal to, or only slightly above the wall temperature (300 K) throughout the low-power, capacitively coupled regime (<60 W, 0.024 W/cm3). Between the lowest (130 W, 0.053 W/cm3) and highest (900 W, 0.36 W/cm3) inductively coupled mode powers investigated, Tg increases sublinearly with power (and electron density). The high-power (900 W) Tg increases with increasing pressure (650, 750, 900, and 1250 K at 2, 5, 10, and 20 mTorr, respectively). Mechanisms of neutral gas heating are discussed. The energy released in dissociation of Cl2 appears to be the dominant heating mechanism.

Renormalized CCSD(T) and CCSD(TQ) approaches: Dissociation of the N2 triple bond

The recently proposed renormalized and completely renormalized CCSD(T) and CCSD(TQ) methods, which can be viewed as generalizations of the noniterative perturbative CCSD(T) and CCSD(TQf) schemes and which result from the more general method of moments of coupled-cluster equations, are applied to the dissociation of the ground-state N2 molecule. It is shown that the renormalized and completely renormalized CCSD(T) and CCSD(TQ) methods provide significantly better results for large N–N separations than their unrenormalized CCSD(T) and CCSD(TQf) counterparts.

Diagrammatic complete active space perturbation theory: Calculations on benzene, N2, and LiF

Second-order diagrammatic complete active space perturbation theory (D-CASPT2) is designed to have the best qualities from multireference Mo/ller–Plesset (MRMP) theory, complete active space perturbation theory CASPT2, and the effective valence shell Hamiltonian Hv method, including possessing a linked diagram theorem (LDT) and an efficient second-order formalism. The accuracy of this method is illustrated through calculations of the electronic excitation energies of N2 and benzene, the spectroscopic constants of the N2 ground state, and the ionic-neutral avoided crossing in the potential curves of LiF.

Gas-phase luminescence of aromatic carbonyl compounds in excited nitrogen at atmospheric pressure

Certain types of aroyl compounds such as benzaldehyde, acetophenone, benzophenone, and anthraquinone produce intense gas-phase luminescence in excited nitrogen at atmospheric pressure. This luminescence was measured in pressure ranges of 1.00–1.67 atm and temperature ranges of 343–473 K. A novel, radioactively (Ni-63) stimulated, high-voltage( [Formula: see text]1750 V/mm), low-current ( [Formula: see text]35 nA) discharge in high-purity nitrogen was used for gas chromatographic detection and spectral excitation. The gas-phase luminescence spectra of about sixty aroyl compounds - introduced as gas chromatographic peaks - could thus be measured and compared with literature spectra obtained by conventional excitation in condensed phases. Only a few gas-phase spectra are available from the literature, and these did agree well with the spectra of this study. A speculative luminescence mechanism is proposed, in which ground-state N2 becomes excited by collision with fast electrons. This is followed by efficient triplet-triplet energy transfer from N2(A 3Σu+) to the n–>π* excited aroyl compound. Key words: aroyl phosphorescence, excited-nitrogen excitation, aroyl triplet-triplet excitation, aroyl luminescence detection, gas-phase phosphorescence spectra, gas-phase aroyl detection.

Excitation of N2(B 3g) in the nitrogen short-lived afterglow

The excitation of the N2(B 3g) state in microwave discharges ignited in flowing nitrogen and in their short-lived afterglow is investigated. Simultaneous measurements in the post-discharge on this species and the N2+(B 2u+) ion through emissions, and also on the electronic ground state of N2 by spontaneous Raman scattering, provide information of their vibrational distribution function. A spatial study has been carried out, showing a quite slow evolution of the vibrational populations along the short-lived afterglow, and in particular the first proof of a steady vibrational distribution of N2(X 1g+) from the beginning to the bulk of this ionized region. The N2(B 3g vibrational distribution is analysed by a steady-state kinetic model taking into account all the B state excitation processes, i.e. electron collisions and reactions between molecular excited states, and quenching. The reaction between the vibrationally excited N2(X 1g+) and N2(A3u+) is shown to be the pre-eminent excitation mechanism in the short-lived afterglow.

Studies of state-selective electron capture using double translational energy spectroscopy

An apparatus developed in this laboratory which utilises double translational energy spectroscopy (DTES) is being used to study state-selective electron capture by state-prepared multiply charged ions. This experimental approach avoids the ambiguities inherent in many previous measurements carried out with primary ion beams containing unknown fractions of metastable ions. In this review, the effectiveness and future potential of DTES is illustrated by a discussion of some recent measurements of one-electron capture by 1S ground state and 3P metastable C2+ ions and by 2P ground state N2+ ions in collisions with a number of different gas targets.

Does N2− exist? A coupled-cluster study

Potential energy curves of the ground-state N2 molecule and its doublet N2− anion are calculated at the coupled-cluster level with single and double excitations and with noniterative triples [CCSD(T)] as well as with the multireference averaged-quadratic coupled-cluster (MR-AQCC) method. The N2− anion is shown to be temporary and decays to its neutral parent plus a free electron at bond lengths shorter than ≈1.4 and larger than ≈2.5 Å. Thus, the N2− anion exists within the 1.4⩽R(N–N)⩽2.5 Å range at the Born–Oppenheimer approximation.

Auroral N2 emissions and the effect of collisional processes on N2 triplet state vibrational populations

Previous model results have shown that the N2 triplet vibrational level populations in the aurora are strongly affected by cascade and quenching by atomic and molecular oxygen. As the aurora penetrates to lower altitudes (less than 100 km) the role of quenching by atomic oxygen becomes less important and processes involving N2 collisions begin to play a more prominent part. We are developing a model which will yield steady state vibrational level populations for both the singlet and triplet valence states of N2. The model currently provides results for the seven low‐lying N2 triplet states (A 3Σu+, B 3IIg, W3Δu, B′ 3Σu−, C3IIu, D3Σu+, and E3Σg+). These states are responsible for auroral emissions from the UV (Vegard‐Kaplan (VK), second positive (2PG)) through the visible to the infrared (first positive (1PG), infrared afterglow (IRA), Wu‐Benesch (WB)). We have included two additional collisional processes in the current model which were not treated previously. These are the intersystem collisional transfer of excitation (ICT) between the B state and the A, W, and B′ states and vibrational redistribution within the A state vibrational manifold, both due to collisions with ground state N2. The present work compares our current model results with those of a previous model, as well as ground, airborne, and rocket observations. The comparison between N2(A) (VK) and N2(B) (1PG) vibrational level populations predicted by our model and a number of auroral observations indicate that the current model achieves a significant improvement in the fit between calculation and observation. In addition, the current model predicts a shift in the band intensity distribution of the 1PG Δν = 3 sequence from the infrared into the visible red at the lower altitudes (less than 90 km) as well as an overall enhancement in the entire 1PG system. Consequently, this provides a possible explanation of a dominate feature of type b aurora, the auroral red lower border.

Application of complete space multireference many-body perturbation theory to N2: Dependence on reference space and H0

We study the wide latitude available in choosing the reference space and the zeroth order Hamiltonian H0 for complete reference space multireference perturbation theory. This effective Hamiltonian Heff method employs a general one-body form of H0 which is varied by using different molecular orbitals and orbital energies. An energy gap is imposed between the zeroth order reference and secondary space states by forcing the valence orbitals to be degenerate. The forced valence orbital degeneracy removes the occurrence of detrimentally small perturbation energy denominators. Extensive computations are provided for the nitrogen molecule, where calculated ground state spectroscopic constants are compared with full configuration interaction computations and calculated vertical excitation energies are compared with multireference coupled cluster computations. It is demonstrated that the forced reference space degeneracy can lead to certain perturbation denominators becoming too small for practical convergence. This characteristic is illustrated by a simple two-orbital model which stresses the need for important zeroth order excitation energies (equivalent to the factors appearing in the perturbation energy denominators) to exceed those in an Epstein–Nesbet perturbation partitioning. This simple model illustrates the general behavior found in all the more extensive Heff computations. In many cases where zeroth order excitation energies are too small for satisfactory third order results, improvements are obtained by using an H0 which redefines the orbital energies in order to increase problematic zeroth order excitation energies. The necessary orbital energy shifts are identified by examining the first order wave functions from larger reference spaces and the zeroth order energies. Frequently, fractional occupancy Fock-type operators are employed to provide the requisite orbital energy shifts. Some of the reference spaces investigated deviate extremely from quasidegeneracy and, thus, appear to be beyond the range of applicability of the forced degeneracy Heff method. Novel techniques are employed for properly treating some of these cases, including the use of orbitals which optimize the quasidegeneracy of the reference space and minimize energy denominator problems. By considering reference spaces of varying sizes, we describe the tradeoff between employing large reference spaces, which provide excellent first order descriptions, and the difficulties imposed by the fact that larger reference spaces severely violate the quasidegeneracy constraints of the Heff method. The same tradeoff exists when the optimal first order CASSCF orbitals are compared with orbitals generated by a VN−1 potential. The VN−1 potential orbitals, which produce relatively quasidegenerate reference spaces, are equivalent to the sequential SCF orbitals used in previous Heff computations, but are more simply obtained by a unitary transformation. The forced degenerate valence orbital energy εv̄ is computed from an averaging scheme for the valence orbital energies. The ground state N2 computations contrast two averaging schemes—populational and democratic. Democratic averaging weighs all valence orbitals equally, while populational averaging weighs valence orbitals in proportion to their ground state populations. Populational averaging is determined to be useful only in situations where core–core and core–valence correlation are unimportant. A Fock-type operator used by Roos and co-workers is employed to uniquely define CASSCF orbitals within their invariant subspaces. This operator is found to be more compatible with populational than democratic averaging, especially when the reference space contains high lying orbitals.

Characterization of a molecular beam containing metastable nitrogen and its use in scattering experiments with xenon

A thermal molecular beam containing metastable nitrogen, N2*, produced in a microwave discharge in He–N2 mixtures, is characterized by magnetic analysis and, after velocity selection, is used for scattering experiments with Xe. The magnetic analysis reveals that the excited N2 molecules are essentially in the A 3Σ+u metastable state but also evinces the presence of a small concentration of A′ 5Σ+g molecules, increasing with beam velocity. This allows us to estimate the mean collision free lifetime of the 5Σ+g metastable molecules by an experiment completely different from those usually performed to this purpose. Absolute total cross sections are measured both for the metastable and the ground state N2 by passing the molecular beam through a cooled scattering chamber containing Xe. From the analysis of the scattering data, the characterization of the intermolecular potential for both systems is obtained and from the potential parameters the average molecular polarizability of metastable N2 is obtained.