Ground State N2 Research Articles

Resonance-Enhanced 2 + 2 Photon Ionization of Nitrogen: The Lyman-Birge-Hopfield Band System

High-resolution resonance-enhanced multiphoton ionization (REMPI) spectroscopy of a pulsed nitrogen beam is applied to determine the (1, 0) to (4, 0) line positions (J″ ≤ 2) of the a1Πg ← X1Σ+g transition. A strong a.c. Stark effect is observed even next to the REMPI threshold pulse energies and is tentatively correlated with certain energy levels near the three-photon energy. At the same time, the REMPI signal, which is not exactly proportional to the a ← X Franck-Condon factors, is dominated by the near-resonant enhancement due to these levels. This is particularly pronounced in the case of the (3, 0) band. In the three other cases, precise zero pulse energy values of the line positionscan be determined by linear extrapolation. The signs of the slopes for these extrapolations are opposite to those expected from the theoretical expression for the dynamical Stark shift. The obtained band origins are used to recalibrate the literature data which, so far, have exhibited major mutual disagreement. A Dunham-type least-squares fit of more than 3100 available tabulated spectral line positions yields a greatly improved mathematical description of this transition for v′ ≤ 15 and v″ ≤ 27, as well as ofa few A3Σ+u and b′1Σ+u levels. The X-state vibrational levels cannot be adequately described by a simple v″ + 12 polynomial. It is, however, found that the application of two separate polynomials for v″ ≤ 5 and v″ > 5 can remove all discrepancies. There are two conclusions from these calculations. The first is that there is an obvious onset of enhanced anharmonicity of the N2 ground state for v″ = 5 which is not observed for the isoelectronic CO molecule. This effect has already been found in earlier, shorter, and less accurate Dunham expansions. These earlier polynomials, however, do not take into account the sudden change of the vibrational frequency at v″ = 5 which must be concluded from two independent experiments by R. E. Miller. This small but distinct change suggests the mathematical treatment of the vibrational levels in the two ranges specified above. An unambiguous interpretation of the effect is not possible at present. No nearby 1Σ+g state is known which might act as a perturber, and, alternatively, a change of the electronic configuration needs to be quantified by a detailed theoretical study. Accurate measurements of the X-state vibrational levels are still rather sparse. Our work strongly suggests further experiments need to be done at a ≤0.001 cm−1 accuracy level that cover the full level range from v″ = 0 to at least v″ = 15 in order to obtain an even more unambiguous representation of this state and to reveal more details of the rotational structure.

N2(a″ 1Σg+) metastable collisional destruction and rotational excitation transfer by N2

Quenching and rotational coupling rate coefficients have been measured for the J=4–10, v=0 levels of the a″ 1Σg+ metastable state of N2 in collisions with ground state N2. Laser absorption is used to monitor the population of rotational levels of the a″ 1Σg+ state following depletion of the population of one or more levels by optical pumping to other states. The observed time dependence of the recovery of population of the perturbed level and the collision induced growth and decay of the populations of adjacent levels are interpreted in terms of quenching to other electronic levels and excitation exchange among adjacent rotational levels. For the J=6, v=0 level of the a″ 1Σg+ state the rate coefficients extrapolated to zero discharge current at 300 K are 2.3±0.1×10−16 m3/s for electronic quenching and 1.1±0.6×10−16 m3/s for excitation transfer to the J=4 and J=8 levels in collisions with N2. Very similar rate coefficients were obtained for the J=4, 7, and 8 levels.

Multireference M�ller-Plesset perturbation treatment of potential energy curve of N2

A multireference Møller-Plesset (MR-MP) perturbation method, at the second-order level, is applied to the potential energy curve of the ground state of N2, for comparison with a variety of standard ab initio methods. In spite of the drastic simplification, the MR-MP results are very reliable. The energy errors are almost independent of geometry, allowing unbiased treatment of potential energy curves. The potential efficiency and accuracy of the MR-MP approach are emphasized. © 1992 John Wiley & Sons, Inc.

Monte Carlo simulation of nonequilibrium shock fronts

A direct simulation of a shock-tube experiment carried by AVCO (pressure of 1 Torr and velocity of 6.4 km/sec), repeated by Sharma and Gillespie (1990), and used by Park (1988) to develop his two-temperature model, is presented. Results show that the electronic ground state of N2 is not in rotational nor vibrational equilibrium with that of N2(+). Moreover, a two-temperature model is inadequate to describe nonequilibrium flows behind shocks. The role of impurities is examined. It is shown that the effects of a small fraction of H2O are insignificant. Good agreement with the measurements of Sharma and Gillespie is indicated.

Plasma preconditioning and the role of elevated vibrational temperature in production of excited N2 vibrational distributions

We have undertaken a series of investigations of excitation and decay processes in molecular nitrogen in pulsed electric discharges under conditions relevant to those of the aurora. Previously, we had observed the continuous evolution of the spectrum of the N2(B³Πg) → N2(A³Σu+) (1 PG) with time and under the influence of collisions after the initial direct electron excitation. In the present investigation, we find that there are progressive changes in the condition of the gaseous medium which occur during the time interval between pulses. At all but the very lowest repetition rates, the sample atmosphere is found to be “preconditioned” at the beginning the excitation pulse by virtue of its previous excitation‐deexcitation history. One possible component of a preconditioned medium is the presence of electronically excited metastables. Another is the retention of excitation in the ground state vibrational manifold. Our observations indicate N2 ground state vibrational temperatures of greater than 3000 K just prior to the beginning of the excitation pulse when conditions for preconditioning are favorable. In addition, our model results indicate that currently accepted N2(B) quenching coefficients are not adequate to explain our observations during the afterglow following the discharge. Ground‐based and rocket measurements of the auroral and airglow spectra have revealed alterations in the distribution of nitrogen molecular band emission intensities only very recently. We are led to believe that most of these phenomena being uncovered in the laboratory are likewise occurring in the aurora, to varying extents depending on conditions and will become evident as observational technology advances. We include a discussion of some conditions under which deviations in the distribution of excited N2 band intensities have been observed in the aurora.

Vibrationally resolved photoelectron angular distributions and branching ratios for the ground state of N2+

The branching ratios and photoelectron angular distributions for the ground state of N2+ have been measured over the photon energy range 22-44 eV. The angular distribution measurements for nu '=0 blend in quite well with earlier data, extending them to higher energies, and, for nu '=1, agreement with earlier branching ratio data is good; the results for nu '=2, for which no measurements were available previously, agree best with calculations by Lucchese and McKoy (1981). In the case of the authors' data for the angular distribution parameters for the nu '=1 and nu '=2 levels, combined with those of other authors at lower photon energies, agreement with theory is less satisfactory.

Energetics and spin- and Λ-doublet selectivity in the infrared multiphoton dissociation HN3(X̃ 1A′)→N2(X 1Σ+g)+NH(X 3Σ−,a 1Δ): Theory

An investigation of the energetics and mechanism of the dissociation of ground state HN3(X̃ 1A′) into ground state N2(X 1∑+g)+NH(X 3∑−) products is presented. This process, which can be induced by multiphoton infrared pumping, occurs through a crossing between the lowest-energy singlet potential energy, which correlates asymptotically with electronically excited NH products (a 1Δ), and the lowest triplet surface. By means of ab initio CASSCF and MCSCF-CI calculations we have determined that the geometry at the minimum singlet–triplet crossing corresponds to an approximately linear N3 backbone with a perpendicular NH bond. The interior N–N distance is ∼3.6 bohr. This transition state lies ∼12 500 cm−1 above the energy of X̃ 1A′ state of HN3 at the experimental equilibrium geometry. Since the N–N and N–H bonds are perpendicular at this transition state, there will be no torques tending to twist the system out of a planar geometry. The crucial singlet–triplet coupling occurs because the HN3 wave function in the region of this transition state can be considered an equal admixture of N2(X)⋅NH(a 1Δ) and N2(X)⋅NH(b 1∑+). Since the ground state HN3 wave function as well as the relevant spin–orbit Hamiltonian are symmetric with respect to reflection of the spatial and spin components of all the electrons in the plane of the molecule, and since the NH fragment must rotate in the plane of the initital HN3 molecule if the dissociation is planar, NH products can be formed only in states in which the wave function (electronic+rotational) is also symmetric with respect to this operation. For a molecule in a 3∑− electronic state the wave functions in only the F1 and F3 multiplets will be symmetric so that one would expect population in only the F1 and F3 levels. A similar symmetry argument implies that the NH products formed in the lowest spin-allowed channel [N2(X 1∑+g)+NH(a 1Δ)] will be found predominantly in the Δ(A′) Λ-doublet state, which is symmetric with respect to reflection of the spatial coordinates of the electrons in the plane of rotation. This spin- and Λ-doublet selectivity has been found experimentally by Stephenson, Casassa, and King (accompanying article). The implications of similar spin selectivity in other photodissociation processes leading to molecules in 3∑− states, e.g., SO(X 3∑−) and O2(X 3∑−g), are also considered.

High-resolution photoelectron angular distribution measurements of N2

By use of a deconvolution technique to analyse experimental data, it is possible to obtain approximate values of rotationally resolved relative cross sections and asymmetry parameters for the photoionisation of the ground state of N2 to the N2+(X 2 Sigma +g) state with vibrational quantum numbers v'=0 and 1 by He I radiation. The difference of vibrationally resolved asymmetry parameters between the v'=0 and 1 states can be explained by a large variation of rotationally resolved relative cross sections and asymmetry parameters.

Full CI benchmark calculations on N2, NO, and O2: A comparison of methods for describing multiple bonds

Full configuration interaction (CI) calculations on the ground states of N2, NO, and O2 using a DZP Gaussian basis are compared with single-reference SDCI and coupled pair approaches (CPF), as well as with CASSCF multireference CI approaches. The CASSCF/MRCI technique is found to describe multiple bonds as well as single bonds. Although the coupled pair functional approach gave chemical accuracy (1 kcal/mol) for bonds involving hydrogen, larger errors occur in the CPF approach for the multiple bonded systems considered here. CI studies on the 1Σ+g state of N2, including all single, double, triple, and quadruple excitations show that triple excitations are very important for the multiple bond case, and accounts for most of the deficiency in the coupled pair functional methods.

The a b i n i t i o model potential method. Main group elements

In this paper we present the ab initio core model potential method, in which the Coulomb and exchange core operators Jc and Kc are represented as accurately as possible using adequate local and nonlocal potentials, and the valence basis set is optimized in atomic MP SCF calculations following the variational principle. Nonrelativistic model potential parameters and valence basis sets are presented for the main group elements from Li to Xe. The pilot SCF molecular calculations on the ground states of N2, P2, As2, Sb2, and CaO show a good agreement between all-electron and model potential results, in particular the changes observed in molecular results due to improvements of the valence part of the all-electron basis set are reproduced by the model potential calculations.

An a b i n i t i o study of the molecular electric polarizabilities of N2, HCN, acetylene, and diacetylene

The independent components of the α, β, γ, A, and B tensors were computed for the ground states of N2, HCN, acetylene, and diacetylene molecules from electric moments induced in a perturbing static electric field. The results are used to comment on trends in nonlinear susceptibilities of polyacetylenes and polydiacetylenes.

Electric polarizabilities and hyperpolarizabilities for the ground state of the nitrogen molecule

The static polarizabilities of the ground state of N2 were calculated at three different internuclear separations in the LCAO-SCF-MO approximation. All independent components of electric polarizability tensors up to the fourth rank were included. The isotropy and anisotropy of the γ tensor are γ = 766, Δ1γ = 2264 and Δ2γ = -76e 4 a 4 0 E -3 h respectively (at the experimental internuclear separation of 2·0744 a 0).

Vibronic excitations of N2(a1Πg) and CO (A1Π) in collisions of H+with N2(X) and CO (X) in the 60-1000 eV energy range

Vibronic excitation into N*2 (a 1 Pi g) and CO* (A 1 Pi ) states from collisions of protons with ground-state N2 and CO is studied in the 60 eV<or=E<or=1 keV energy range using a high-resolution energy loss technique. The gross energy dependences of the vibrational distributions display similar trends to those found previously with He+ projectiles. These data are complemented by measurements of angular differential cross sections for excitation of N2 in various electronic states in order to provide an overall view of the excitation processes in the collisional systems considered. Analysis of the data is attempted within a quasi-molecular framework that is derived from analogies of H++N2, CO with H++rare gas systems together with information provided by available ab initio calculations. The deviations of the vibrational distributions from the Franck-Condon prediction that are observed for CO* (A) throughout most of the investigated energy range are attributed to long-range charge-dipole interactions. The departure from Franck-Condon distributions at low energies both for N*2 (a) and CO* (A) is analysed in terms of bond length dependences of the electron transition mechanisms revealed by the H+-N2, CO correlation diagrams.

Measurement of the vibrational temperature of the ground state of N2 according to the emission of the N2 + molecular ion

Measurement of the vibrational temperature of the ground state of N2 according to the emission of the N2 + molecular ion

Excitation of vibrational and electronic states in a glow discharge column in flowing N2

The excitation and ionisation number densities of different states of N2 have been measured in a long positive column of a glow discharge in flowing N2 when the residence time of excited species in the column was varied between 5*10-4 and 2.5*10-1 s. The vibrational population of the ground state N2 (X 1 Sigma g+, nu ) and the electron energy distribution function (EDF) have been calculated for different values of gas pressure, electric field and electron densities. For residence times greater than 10-2 s the N2(X, nu ) and the EDF tails are strongly overpopulated. The vibrational population of the excited states N2(C3 Pi u, nu ') and N2+(B2 Sigma u+, nu ') have been experimentally determined by measuring the second positive and first negative band intensities. The measured N2(C, nu ') distributions are in good agreement with the calculated populations assuming that the C3 Pi u state is created by electron collisions and destroyed by radiative emission. The ionic lines emitted from N2+(B, nu ') have large intensities for long residence times t>10-2 s suggesting that the heavy particle reaction: N2(X, nu >or=12)+N2+(X) to N2(X)+N2+(B) is the main process.

Guaranteed convergence in ground state multiconfigurational self-consistent field calculations

We show how an optimization constraint algorithm of Fletcher that guarantees convergence to the lowest state of a given symmetry may be practically implemented in a multiconfigurational self-consistent field (MCSCF) calculation. Other MCSCF procedures in current use have not been proven mathematically to guarantee convergence. Calculations on the ground states of N2 and CO show that rapid and efficient convergence is obtained with the Fletcher restricted step size algorithm.

The Influence of ground-state vibrationally excited N2 molecules on the vibrational excitation of N2, N 2 + electronic states

The vibrational distributions of the electronic states N2 (C3IIu, ν′), N2 (B3 IIg, ν″) and N 2 + (B2Σ u + , ν′) have been calculated from the self-consistent electron energy distribution function (e.d.f.) and the vibrational distribution of the ground-state N2 (X1Σ g + , ν) molecules in nonequilibrium electrical discharges. The results show a temporal evolution of these distributions following the corresponding evolution of e.d.f. and N2 (X, ν) with the residence time in the discharge.

Electron impact ionization cross section of metastable N2(A Σ+u)

The nondissociative electron impact ionization cross section of the metastable N2(A 3Σ+u) state is measured from threshold to 240 eV. Rising from an appearance potential of 10.1±0.6 eV, the cross section peaks at about 40 eV with a value of (1.14±0.36)×10−16 cm2. These values are extracted from the apparent ionization cross section of an N2 beam composed of ∼50% N2(A 3Σ+u) and ∼50% N2(X 1Σ+g). This mixed state beam is formed by charge transfer neutralization of a 1 keV N+2 beam with NO. The N2(A 3Σ+u) cross section is normalized by measuring it relative to the ionization of a ground state N2 beam formed by charge transfer neutralization of N+2 with N2.

SCF-CI calculations of the potential energy curve and one-electron property curves of theX 1∑g+ ground state of N2

The energy and important one-electron properties of the nitrogen molecule are evaluated in the1∑g+ ground state for various internuclear separations near the equilibrium geometry, at the SCF-CI level. Both our Roos CI and Graphical Unitary Group (GUG) programs were used in performing single and multiple root CI computations. The one-electron properties included are the quadrupole and hexadecapole moments (both with respect to the centre of mass) and the electric field gradient at the N nucleus. Dependence of the properties upon the vibrational quantum number are evaluated using the Dunham analysis scheme.

SCF-CI calculations of the potential energy curve and one-electron property curves of theX 1∑g+ ground state of N2

SCF-CI calculations of the potential energy curve and one-electron property curves of theX 1∑g+ ground state of N2