3Πu State Research Articles

Influence of pressure on the relative population of the two lowest vibrational levels of the C 3 Πu state of nitrogen for electron beam excitation

Continuous and pulsed 12 keV electron beams were used to excite nitrogen within a gas cell at pressures ranging from 10 to 1400 hPa. The pressure dependence of the ratio of photon fluxes for emission from vibrational levels v'=0 and 1 of the C 3Π u state has been studied. The results confirm the presence of a collisional excitation mechanism populating v'=0, 1 in addition to electron impact excitation. Rate constants of (1.27 ±0.04)×10-11 cm3s-1 [ v'=0] and (2.68 ±0.08)×10-11 cm3s-1 [ v'=1] were measured for C 3Π u quenching by ground state nitrogen. For electron beam conditions relative excitation efficiencies of 1:0.59:0.22 for vibrational levels 0, 1 and 2 were calculated. The recorded flux ratios are compared with the predictions given by a vibrational relaxation model.

The ion H +3 in a strong magnetic field

A first detailed study of the low-lying electronic states of the H 3 + molecular ion in linear configuration, parallel to a magnetic field, is carried out for B=0–4.414×1013 G in the Born-Oppenheimer approximation. The variational method is employed with a single, physically adequate trial function which includes, in particular, explicitly the electronic correlation term in the form exp (γ r12), where γ is a variational parameter. The state of the lowest total energy (ground state) depends on the magnetic field strength. It evolves from spin-singlet 1Σg for small magnetic fields B≲5×108 G to weakly-bound spin-triplet 3Σu for intermediate fields and eventually to spin-triplet 3Πu state for B≳5×1010 G.

Low lying quartet states in diacetylene, triacetylene and benzene radical cations

The restricted coupled cluster with perturbative triples (RCCSD(T)) method has been used to calculate equilibrium geometries and excitation energies of the hitherto unknown quartet states in diacetylene, triacetylene and benzene radical cations. Spectroscopic data for the doublet states obtained with the same approach are found to be in good agreement with experiment. Whereas the 2Πu and 2Πg states in diacetylene and triacetylene cations form linear–linear Renner–Teller pairs, strongly bent minima are calculated for the quartet states correlating with the 4Πg and 4Πu linear structures. In both ions there are quartets that lie close to the minimum energies of the first electronically excited doublet states. The 4Au state of benzene radical cation lies below C2E1u and could be populated during the ultrafast nonradiative decay of this doublet state.

The parameters of nonequilibrium microwave discharge in nitrogen in a tube in a rectangular waveguide

Computer codes are developed for the processing of emission spectra of nonequilibrium plasma in nitrogen for the purpose of obtaining information about the translational T g and rotational T rot temperatures, the populations of vibrational levels in the ground electron and electron-excited states, the electron energy distribution function, the electron concentration N e , and the electric field intensity E. The computer codes are used to determine the parameters of microwave-discharge plasma in nitrogen in discharge systems of two types, namely, in a discharge tube (with a radius of 1 cm), which crosses a rectangular waveguide (plasmatron on the H 10 wavelength, at a pressure of 1.7 torr and absorbed power density of 1.5 W/cm3), and in a discharge section of similar structure on the basis of prismatic resonator (at a pressure of 1.0 torr and absorbed power density of 0.4 W/cm3). The mechanisms of population of the N2(C 3Πu) state are treated.

Large-scale Ar and N2–Ar microwave plasma sources

The spatial structure, including a discharge and a remote plasma zone, of a large-scale, slot antenna excited microwave plasma source operating in Ar and N2–Ar mixtures at 2.45 GHz has been investigated. The discharge source is sustained by the electric field of pure TM surface modes. Optical emission spectroscopy, probe diagnostic techniques and radiophysics methods are applied to measure the emission intensity of radiative states, the electron density and the microwave field intensity, respectively. The spatial distribution of the electron density and the intensity of emission lines belonging to the 2nd positive and 1st negative systems of N2 follow the electric field pattern in the active-discharge zone of the plasma source. On the contrary, the populations in vibrational and rotational levels of the N2(C 3Πu) state are homogeneous irrespective of the azimuthal position.

On the 1(A)1Σ+u → 1(X)1Σ+g emission of Na2 observed following the laser excitation of the 1(B)1Πu state

An emission clearly assigned to the 1(A)1Σ+u → 1(X)1Σ+g band system of Na2 has been recorded following the excitation of the 1(B)1Πu state by using different Ar+ laser lines. A whole simulation of the 1(A)1Σ+u → 1(X)1Σ+g emission profile, including bands with 0 ⩽ ν′ ⩽ 30, 0 ⩽ ν″ ⩽ 55 and J extended up to 50, shows that the 1(A)1Σ+u excited state is in a Boltzmann distribution owing to a rovibrational relaxation within this electronic state. This simulation suggests that the zone of maximum spectral emission does not correspond to the most intense transition but to a superposition of many individual bands for high vibrational levels of the 1(A)1Σ+u state. The 1(A)1Σ+u → 1(X)1Σ+g emission is similar whatever the laser wavelength used to excite the 1(B)1Πu state. Collisional energy transfer processes among different species are discussed. Further work is needed for the complete identification of the competitive energy transfer processes.

Integral cross sections for the direct excitation of the A 3Σu+, B 3Πg, W 3Δu, B′ 3Σu−, a′ 1Σu−, a 1Πg, w 1Δu, and C 3Πu electronic states in N2 by electron impact

Integral cross sections for electron impact excitation out of the ground state (X 1Σg+) to the A 3Σu+, B 3Πg, W 3Δu, B′ 3Σu−, a′ 1Σu−, a 1Πg, w 1Δu, and C 3Πu states in N2 are reported at incident energies ranging between 10 and 100 eV. These data have been derived by integrating differential cross sections previously reported by this group. New differential cross section measurements for the a 1Πg state at 200 eV are also presented to extend the range of the reported integral cross sections for this state, which is responsible for the emissions of the Lyman‐Birge‐Hopfield band system (a 1Πg → X 1Σg+). The present results are compared and critically evaluated against existing cross sections. In general, the present cross sections are smaller than previous results at low impact energies from threshold through the excitation function peak regions. These lower cross sections have potentially significant implications on our understanding of UV emissions in the atmospheres of Earth and Titan.

Nitrogen: a possible substitute for mercury as a UV-emitter for mercury-less low-pressure discharge fluorescent lamps using Penning-like energy transfer

Nitrogen is well known as a UV-emitter because of its 2nd positive band lying around 350 nm. However, it is a molecular gas and when energy is stored in rotational or vibrational levels, the gas temperature can become high enough to melt even the glass tube. To avoid such problems the authors tried to use the Penning-like effect between the argon metastables and nitrogen C 3Πu states with very low partial pressure of nitrogen compared with that of the argon. By adding a very small amount of nitrogen to argon, the discharge changes drastically. Though pure argon discharge and pure nitrogen discharge are both unstable and easy to constrict, a small amount of nitrogen improves the stability of the discharge and emits the nitrogen 2nd positive band UV light by energy transfer from the argon metastable to the nitrogen molecule C 3Πu state. Nitrogen-added argon fluorescent lamps operated at 30 kHz of alternative square voltage wave achieve about 4500 cd m−2 at 3.75 W and at 9.3 kPa of pressure with good dimming characteristics. The maximum luminance should be higher than this because the input power is limited by the power supply. At this condition the luminous efficacy achieves 7.1 lm W−1. These results show a possibility of argon–nitrogen fluorescent lamps as mercury-less fluorescent lamps.

Double resonance spectroscopy of several highly excited rovibronic states of H2

The energies of several highly excited levels of the B1Σ+u, B′1Σ+u and C 1Πu states of molecular hydrogen, located several hundred wave numbers below the second dissociation limit, have been measured using two-colour, resonantly enhanced multi-photon ionization. The states were probed by excitation from the double-well E, F 1Σ+u state, populated by two-photon excitation from the ground state. Ion production was detected as a function of wavelength using a time-of-flight mass spectrometer. Non-adiabatic couplings extensively mix the configurations of the B 1Σ+u, B′1Σ+u and C 1Πu states leading to perturbations that are predicted to vary considerably as a function of rotational and vibrational excitation. Term energies are compared to previous measurements and to ab initio theoretical calculations which include non-adiabatic effects. Several observed discrepancies with the calculations and previously reported energies are discussed. Term energies for two rovibronic levels are reported for the first time.

The npσ,π to EF emission systems in D2 studied by selective excitation

The npσ 1Σ+u and npπ 1Πu states in D2 have been selectively excited using monoenergetic synchrotron light in the range of 13.97–15.84 eV and the subsequent dispersed emission to the EF 1Σ+g state was observed using a grating spectrometer. In total, 18 emission bands from the levels n = 3–6 were studied and rotationally analysed. The intensities of the P and R branches relative to the Q branch were found to vary strongly in the npπ 1Π+u–EF1Σ+g bands indicating the existence of predissociations of npπ 1Π+u levels above the dissociation limit D(1s) + D(2ℓ).

Rotational and Vibrational Temperature Measurements in a High‐Pressure Cylindrical Dielectric Barrier Discharge (C‐DBD)

AbstractThe rotational (TR) and vibrational (Tv) temperatures of N2 molecules were measured in a high‐pressure cylindrical dielectric barrier discharge (C‐DBD) source in Ne with trace amounts (0.02 %) of N2 and dry air excited by radio‐frequency (rf) power. Both TR and Tv of the N2 molecules in the C 3Πu state were determined from an emission spectroscopic analysis the 2nd positive system (C 3Πu → B3Πg). Gas temperatures were inferred from the measured rotational temperatures. As a function of pressure, the rotational temperature is essentially constant at about 360 K in the range from 200 Torr to 600 Torr (at 30W rf power) and increases slightly with increasing rf power at constant pressure. As one would expect, vibrational temperature measurements revealed significantly higher temperatures. The vibrational temperature decreases with pressure from 3030 K at 200 Torr to 2270 K at 600 Torr (at 30 W rf power). As a function of rf power, the vibrational temperature increases from 2520 K at 20 W to 2940 K at 60 W (at 400 Torr). Both TR and Tv also show a dependence on the excitation frequency at the two frequencies that we studied, 400 kHz and 13.56 MHz. Adding trace amounts of air instead of N2 to the Ne in the discharge resulted in higher TR and Tv values and in a different pressure dependence of the rotational and vibrational temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiative Characteristics of N2 Second Positive Bands in High-temperature Plasmas (Analysis with Predissociation and Non-Boltzmann Rotational Population Distribution)

The N2 second positive bands observed in high-temperature air plasmas are considered to be useful to determine internal states of high-temperature plasmas. In the experiments with a micro-air plasma-jet, the N2 second positive bands with higher vibrational levels (v′≥2) were found to show a characteristic intensity distribution. The intensity distribution could not be explained by a usual equilibrium radiation theory. In order to investigate the phenomenon, theoretical spectra were calculated by taking account of two effects, that is, predissociation of N2 C 3Πu state through C′3Πu state and non-Boltzmann rotational population distribution. This calculated spectra with such effects agreed very well with the experiments.

Ab Initio calculations and vibrational energy level fits for the lower singlet potential-energy surfaces of C3

Ab initio multireference configuration interaction potential energy surfaces are computed for the eight lowest singlet surfaces of C(3). These reveal several important features, including several conical intersections in linear, nonlinear, and equilateral triangle geometries. These intersections are important because, particularly for the excited A (1)Pi(u) state, reasonable ab initio results could only be obtained by including nearby, near degenerate, (1)Sigma(u) (-) and (1)Delta(u) states that cross the A (1)Pi(u) state around 4500 cm(-1) above the equilibrium geometry, and a (1)Pi(g) state whose potential in turn crosses the other states about 2000 cm(-1) further up. These states are probably responsible for the complexity of the shorter wavelength UV absorption spectrum of C(3). The computed potential energy surface for the ground, X (1)Sigma(g) (+), state and for the lowest two excited singlet surfaces (which both correlate with the A (1)Pi(u) state in a collinear geometry) are fitted to analytic functional forms. Vibrational energy levels are calculated for both states, taking account of the Renner-Teller coupling in the excited A (1)Pi(u) state. The potential parameters for both states are then least-squares fitted to experimental data. The ground-state fit covers a range of approximately 8500 cm(-1) above the lowest level, and reproduces 100 observed vibrational levels with an average error of 2.8 cm(-1). The A (1)Pi(u) state surfaces cover a range of 3250 cm(-1) above the zero-point level, and reproduce the 44 observed levels in this range with an average error of 2.8 cm(-1).

Photoionization and photodissociation dynamics of the B 1Σu+ and C 1Πu states of H2 and D2

The photoionization and photodissociation dynamics of H(2) and D(2) in selected rovibrational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states have been investigated by velocity map ion imaging. The selected rotational levels of the B (1)Sigma(u) (+) and C (1)Pi(u) states are prepared by three-photon excitation from the ground state. The absorption of fourth photon results in photoionization to produce H(2)(+) X (2)Sigma(g)(+) or photodissociation to produce a ground-state H(1s) atom and an excited H atom with n >or= 2. The H(2) (+) ion can be photodissociated by absorption of a fifth photon. The resulting H(+) or D(+) ion images provide information on the vibrational state dependence of the photodissociation angular distribution of the molecular ion. The excited H(n >or= 2) atoms produced by the neutral dissociation process can also be ionized by the absorption of a fifth photon. The resulting ion images provide insight into the excited state branching ratios and angular distributions of the neutral photodissociation process. While the experimental ion images contain information on both the ionic and neutral processes, these can be separated based on constraints imposed on the fragment translational energies. The angular distribution of the rings in the ion images indicates that the neutral dissociation of molecular hydrogen and its isotopes is quite complex, and involves coupling to both doubly excited electronic states and the dissociation continua of singly excited Rydberg states.

Hyperfine interactions and perturbation effects in the B0_u^+(^3Π_u) state of ^127I_2

We report new measurements of the hyperfine spectra of B←X transitions in the wavelength range 500–517 nm. Four effective hyperfine parameters, eqQB, CB, dB, and δB, are determined for an extensive number of rovibrational levels spanning the intermediate region 42≤v′≤70 in the electronically excited B0u+(3Πu) state. Second-order perturbation accounts for most of the observed rovibrational dependence of the hyperfine interactions. In addition, it was found that, near vibrational levels v′=57–60, the 1g(1Πg) electronic state strongly perturbs the B state through rotational coincidence, leading to effects such as abnormal variations in the hyperfine parameters and strong u–g mixing recorded for the transition P(84)60–0. Various perturbation effects in the B state identified so far are summarized. Also, the radial dependence of the hyperfine interactions was examined by removal of the vibrational average in the hyperfine parameters.

Rovibrational distribution determination of H2 in low temperature plasmas by Fulcher-α band spectroscopy

It is proposed that the rotational and vibrational temperatures of H2 can be derived from the fitting to the H2 Fulcher band line emissions. However, the fitting relies on the employed spontaneous radiative transition probabilities and the excitation rate coefficients, which are dependent on the vibrational and rotational quanta in each electronic state. From our study, it is found that, in low temperature plasmas both the Fulcher-α radiative transition and the electron impact excitation to the d 3 Πu state from the ground electronic state deviate significantly from the Franck–Condon approximation. The vibrationally resolved complete sets of the cross sections are calculated based on the semi-classical Gryzinski theory, and the Fulcher band transition probabilities are obtained based on the newest published data on the electronic transition moment and the solution to the radial Schrödinger equation. The fitting to experimental line emissions shows that Boltzmann's Law can be employed for the distribution of vibrational (in lower level), and particularly for the rotational, populations in the ground electronic state of H2 in low temperature plasmas.

The open-shell interaction of He with the B 3Piu(0+) state of Br2: an ab initio study and its comparison with a diatomics-in-molecule perturbation model.

The interaction of He with Br2 in electronically excited B 3Piu state is investigated using spin-unrestricted single and double coupled-cluster approach with noniterative perturbative treatment of triple excitations. Internal electrons of the Br atom are described by effective core pseudopotentials. The validity of this approach is analyzed by comparing the lowest 2Sigma+ and 2Pi electronic states of the HeBr molecule with those obtained in all electron calculations [J. Chem. Phys. 115, 10438 (2001)]. In this context, we examine the performance of different basis sets and saturation with bond functions. The comparison of theoretical blue-shifts with the experiment provides confidence about the present ab initio calculations. In addition, He-Br results of ab initio calculations at the same level are used to obtain approximate He-Br2 (3Piu) interactions in the framework of the diatomics-in-molecule first order perturbation theory (IDIM-PT1) [J. Chem. Phys. 104, 9913 (1996)]. Overall, the IDIM-PT1 model results show a good agreement with the ab initio ones, being the main difference the sensitivity to the elongation of the Br-Br bond.

Ab initio investigations of the unimolecular decay of CS+2 for internal energies lower than 10 eV

One-dimensional potential energy curves of the lowest doublet and quartet states of CS+2 and their mutual spin–orbit interactions have been calculated using the complete active space self-consistent field and internal contracted multi-reference configuration interaction methods. Between the A2Πu and the C2Σ+g states, four doublet satellite states (two 2Πu, one 2Πg and one 2Φu) in addition to the valence B2Σ+u state are found. The two 2Πu states have been already observed in the photoelectron spectra where they were associated with a long vibronic progression. Several quartet states have been also located in the 3–10 eV internal energy range. The avoided crossing regions and the conical intersections for the doublets and the quartets are located. The predissociation mechanisms of the doublet states of CS+2 leading to the first dissociation limit (S+(4Su) + CS(X1Σ+)) are also discussed. They are found to involve vibronic and/or spin–orbit interactions between the doublet and the quartet states.

Potential energy, Λ doubling and Born–Oppenheimer breakdown functions for the B 1Πu “barrier” state of Li2

The potential energy curve for the B 1Πu state of Li2 has a rotationless barrier which protrudes above its energy asymptote. A direct fit to spectroscopic data for all three isotopomers of this species, including Λ-doubling splittings and tunneling predissociation line widths, is used to determine an accurate analytic potential energy function plus Born–Oppenheimer breakdown and Λ-doubling perturbation radial strength functions for this system. This analysis introduces an analytic model for representing a potential function with a rotationless barrier, and shows that a radial perturbation function treatment can determine the symmetry of the perturbing state giving rise to Λ-doubling splittings.

Extreme ultraviolet laser excitation of isotopic molecular nitrogen: The dipole-allowed spectrum of 15N2 and 14N15N

Extreme ultraviolet+ultraviolet (XUV+UV) two-photon ionization spectra of the b 1Πu(v=0–9), c3 1Πu(v=0,1), o 1Πu(v=0,1), c4′ 1Σu+(v=1) and b′ 1Σu+(v=1,3–6) states of 15N2 were recorded with a resolution of 0.3 cm−1 full-width at half-maximum (FWHM). In addition, the b 1Πu(v=1,5–7) states of 14N15N were investigated with the same laser source. Furthermore, using an ultranarrow bandwidth XUV laser [∼250 MHz (∼0.01 cm−1) FWHM], XUV+UV ionization spectra of the b 1Πu(v=0–1,5–7), c3 1Πu(v=0), o 1Πu(v=0), c4′ 1Σu+(v=0), and b′ 1Σu+(v=1) states of 15N2 were recorded in order to better resolve the band-head regions. For 14N15N, ultrahigh resolution spectra of the b 1Πu(v=0–1,5–6), c3 1Πu(v=0), and b′ 1Σu+(v=1) states were recorded. Rotational analyses were performed for each band, revealing perturbations arising from the effects of Rydberg-valence interactions in the 1Πu and 1Σu+ states, and rotational coupling between the 1Πu and 1Σu+ manifolds. Finally, a comprehensive perturbation model, based on the diabatic-potential representation used previously for 14N2, and involving diagonalization of the full interaction matrix for all Rydberg and valence states of 1Σu+ and 1Πu symmetry in the energy window 100 000–110 000 cm−1, was constructed. Term values for 15N2 and 14N15N computed using this model were found to be in good agreement with experiment.