FT-spectroscopy of the 12C18O rare isotopologue and deperturbation analysis of the A1Π(v = 3) level.

Research on 12C18O was carried out using two complementary Fourier-transform methods: (1) vacuum-ultraviolet absorption spectroscopy, with an accuracy ca. 0.03 cm-1 on the DESIRS beamline (SOLEIL synchrotron) and (2) visible emission spectroscopy with an accuracy of about 0.005-0.007 cm-1 by means of the Bruker IFS 125HR spectrometer (University of Rzeszów). The maximum rotational quantum number of the energy levels involved in the observed spectral lines was Jmax = 54. An effective Hamiltonian and the term-value fitting approach were implemented for the precise analysis of the A1Π(v = 3) level in 12C18O. It was performed by means of the PGOPHER code. The data set consisted of 571 spectral lines belonging to the A1Π-X1Σ+(3, 0), B1Σ+-A1Π(0, 3), C1Σ+-A1Π(0, 3) bands and several lines involving states that perturb the A1Π(v = 3) level as well as to the previously analysed B1Σ+-X1Σ+(0, 0) and C1Σ+-X1Σ+(0, 0) transitions. A significantly extended quantum-mechanical description of the A1Π(v = 3) level in 12C18O was provided. It consists of the 5 new unimolecular interactions of the spin-orbit and rotation-electronic nature, which had not been taken into account previously in the literature. The ro-vibronic term values of the A1Π(v = 3, Jmax = 55), a'3Σ+(v = 13), D1Δ(v = 4) and I1Σ-(v = 5) levels were determined with precision improved by a factor of 10 relative to the previously known values.

The spin-forbidden vacuum-ultraviolet absorption spectrum of 14N15N.

Photoabsorption spectra of 14N15N were recorded at high resolution with a vacuum-ultraviolet Fourier-transform spectrometer fed by synchrotron radiation in the range of 81-100 nm. The combination of high column density (3 × 1017 cm-2) and low temperature (98 K) allowed for the recording of weak spin-forbidden absorption bands' exciting levels of triplet character. The triplet states borrow intensity from 1Πu states of Rydberg and valence character while causing their predissociation. New predissociation linewidths and molecular constants are obtained for the states C3Πu(v = 7, 8, 14, 15, 16, 21), G3Πu(v = 0, 1, 4), and F3Πu(v = 0). The positions and widths of these levels are shown to be well-predicted by a coupled-Schrödinger equation model with empirical parameters based on experimental data on 14N2 and 15N2 triplet levels.

Fourier-transform-spectroscopic photoabsorption cross sections and oscillator strengths for the S2 BΣu-3-XΣg-3 system.

Photoabsorption cross sections and oscillator strengths for the strong, predissociating vibrational bands, v ≥ 11, in the S2 BΣu-3-XΣg-3(v,0) system are reported. Absorption measurements were undertaken on S2 vapor produced by a radio-frequency discharge through H2S seeded in helium, and also in a two-temperature sulfur furnace, at temperatures of 370 K and 823 K, respectively. S2 column densities were determined in each source by combining experimental line strengths in low-v non-predissociating B - X bands (v < 7) with calculated line f-values based on measured radiative lifetimes and calculated branching ratios. The broad-band capabilities of two vacuum-ultraviolet Fourier-transform spectrometers, used with instrumental resolutions of 0.22 cm-1 and 0.12 cm-1, respectively, allowed for simultaneous recordings of both non-predissociating and predissociating bands, thus placing the predissociating-band cross sections on a common absolute scale. Uncertainties in the final cross section datasets are estimated to be 15% for the 370-K vapor and 10% for the 823-K vapor. The experimental cross sections are used to inform a detailed predissociation model of the B(v) levels in Paper II [Lewis et al., J. Chem. Phys. 148, 244303 (2018)]. For astrophysical and other applications, this model can be adjusted simply to provide isotopologue-specific cross sections for a range of relevant temperatures.

Predissociation of the B Σu-3 state of S2: A coupled-channel model.

A coupled-channel Schrödinger equation model of predissociation in the B Σu-3 state of S2 is developed and optimized by comparison with recent photoabsorption spectra of the B Σu-3-X Σg-3(v,0) bands for 11 ≤ v ≤ 27, covering the energy range 35 800-41 500 cm-1. All bands in this range exhibit varying degrees of diffuseness, with corresponding predissociation linewidths Γ ≈ 4-60 cm-1 full-width at half-maximum. Model comparisons with both low-temperature (T = 370 K) and high-temperature (T = 823 K) spectra indicate, for many bands, significant dependence of the linewidth on both the rotational quantum number J and the fine-structure component Fi. Just as in the analogous case of O2, the B(v)-state predissociation in S2 is caused principally by spin-orbit interaction with 3Πu, 1Πu, 5Πu, and Σu+3 states. The inner-limb crossing with B″3Πu is responsible for the predissociation of B(v = 11) and provides a significant slowly varying contribution for B(v ≥ 12). The outer crossings with the 1Πu, 5Πu, and 2 Σu+3 states are responsible for oscillatory contributions to the predissociation widths, with first peaks at v = 13, 20, and 24, respectively, and the 5Πu contribution dominant. Prior to the photodissociation imaging study of Frederix et al. [J. Phys. Chem. A 113, 14995 (2009)], which redefined the dissociation energy of S2, the prevailing paradigm was that only the 1Πu interaction was responsible for the B(v = 11-16) predissociation: this view is not supported by our model.

Observation of a new electronic state of CO perturbing W ¹Π(v=1).

We observe photoabsorption of the W(1) ← X(0) band in five carbon monoxide isotopologues with a vacuum-ultraviolet Fourier-transform spectrometer and a synchrotron radiation source. We deduce transition energies, integrated cross sections, and natural linewidths of the observed rotational transitions and find a perturbation affecting these. Following a deperturbation analysis of all five isotopologues, the perturbing state is assigned to the v = 0 level of a previously unobserved (1)Π state predicted by ab initio calculations to occur with the correct symmetry and equilibrium internuclear distance. We label this new state E″ (1)Π. Both of the interacting levels W(1) and E″(0) are predissociated, leading to dramatic interference effects in their corresponding linewidths.

Experimental verification of strong rotational dependence of fluorescence and predissociation yield in the b1Πu(v = 1) level of 14N2.

New, rotationally resolved fluorescence-excitation spectra confirm coupled-channel Schrödinger-equation predictions of strong rotational dependence of the fluorescence and predissociation yields in the b(v = 1) level of (14)N(2).

High-resolution Fourier-transform extreme ultraviolet photoabsorption spectroscopy of 14N15N.

The first comprehensive high-resolution photoabsorption spectrum of (14)N(15)N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet and span 100 000-109 000 cm(-1) (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N(2) that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schrödinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.

Oscillator strengths and line widths of dipole-allowed transitions in (14)N2 between 86.0 and 89.7 nm.

Oscillator strengths of 23 electric-dipole-allowed bands of (14)N(2) in the 86.0-89.7 nm (111,480-116,280 cm(-1)) region are reported from synchrotron-based photoabsorption measurements at an instrumental resolution of 6.5x10(-4) nm (0.7 cm(-1)) full width at half maximum. The absorption spectrum comprises transitions to vibrational levels of the c(n) (1)Pi(u) (n=3,4), o(3) (1)Pi(u), and c(n+1)(') (1)Sigma(u)(+)(n=3,4) Rydberg states as well as the b (1)Pi(u) and b(') (1)Sigma(u)(+) valence states. The J dependences of band f-values derived from the experimental line f-values are reported as polynomials in J(J+1) and are extrapolated to zero nuclear rotation in order to facilitate comparisons with the results of coupled Schrodinger equation calculations. Many bands in this study are characterized by a strong J dependence of the band f-values and display anomalous P-, Q-, and R-branch intensity patterns. Predissociation line widths are reported for six bands. The experimental f-value and line-width patterns inform current efforts to develop comprehensive spectroscopic models for N(2) that incorporate rotational effects and predissociation mechanisms, and are critical for the construction of realistic atmospheric radiative-transfer models.

A coupled-channel model of the 3Pi(u) states of N2: structure and interactions of the 3ssigma(g)F3 3Pi(u) and 3ppi(u)G3 3Pi(u) Rydberg states.

New and existing spectroscopic data on N(2), obtained using a wide variety of experimental techniques, are interpreted using a coupled-channel Schrodinger-equation (CSE) model of the structure and predissociation dynamics for the interacting Rydberg and valence states of (3)Pi(u) symmetry. As a result, v>0 levels of the 3ppi(u)G(3) (3)Pi(u) Rydberg state are assigned correctly for the first time, leading to the identification of very strong perturbations in the G(3)-state vibrational structure. A four-channel CSE model, which includes the 3ssigma(g)F(3) (3)Pi(u) and 3ppi(u)G(3) (3)Pi(u) Rydberg states and the C(') (3)Pi(u) and C (3)Pi(u) valence states, indicates strong Rydberg-Rydberg coupling between the F(3) and G(3) states, strong Rydberg-valence coupling between the G(3) and C(') states, and weaker coupling between the F(3) and C(') states.

Sign reversal of the spin-orbit constant for the C 3Pi(u) state of N2.

Ab initio calculations are performed at the multireference configuration-interaction level of theory on the diagonal spin-orbit functions for the lowest non-Rydberg states of (3)Pi(u) symmetry in molecular nitrogen. Spin-orbit constants deduced from the ab initio results confirm the recent suggestion, based on new experimental results, that the C (3)Pi(u) state of N(2), long known to be regular in the region of its potential-energy curve minimum, becomes inverted at higher energies. By removing the effects of the crossing C(') (3)Pi(u) state, it is shown that A(v) for the C state changes sign from positive to negative near v=8, corresponding to a change in principal molecular-orbital configuration from (1sigma(g))(2)(1sigma(u))(2)(2sigma(g))(2)(2sigma(u))(3sigma(g))(2)(1pi(u))(4)(1pi(g)) to (1sigma(g))(2)(1sigma(u))(2)(2sigma(g))(2)(2sigma(u))(2)(3sigma(g))(1pi(u))(3)(1pi(g))(2) at an internuclear distance near 1.4 A.

Structure and predissociation of the 3psigma(u)D (3)Sigma(u) (+) Rydberg state of N(2): first extreme-ultraviolet and new near-infrared observations, with coupled-channels analysis.

The 3psigma(u)D (3)Sigma(u) (+) Rydberg state of N(2) is studied experimentally using two high-resolution spectroscopic techniques. First, the forbidden D (3)Sigma(u) (+)-X (1)Sigma(g) (+) transition is observed for the first time via the (0,0) band of (14)N(2) and the (1,0) band of (15)N(2), using 1 extreme-ultraviolet +1 ultraviolet two-photon-ionization laser spectroscopy. Second, the Rydberg-Rydberg transition D (3)Sigma(u) (+)-E (3)Sigma(g) (+) is studied using near-infrared diode-laser photoabsorption spectroscopy, thus extending the previous measurements of Kanamori et al. [J. Chem. Phys. 95, 80 (1991)], to higher transition energies, and thereby revealing the (2,2) and (3,3) bands. The combined results show that the D(v=0-3) levels exhibit rapidly increasing rotational predissociation as v increases, spanning nearly four orders of magnitude. The D-state level structure and rotational predissociation signature are explained by means of a coupled-channels model which considers the electrostatically coupled (3)Pi(u) Rydberg-valence manifold, together with a pure-precession L-uncoupling rotational interaction between the 3psigma(u)D (3)Sigma(u) (+) and 3ppi(u)G (3)Pi(u) Rydberg p-complex components.

Interactions of the 3ppi u c1Pi u(v=2) Rydberg-complex member in isotopic N2.

The 3ppi u c1Pi u-X 1Sigmag+(2,0) Rydberg and b' 1Sigmau+-X 1Sigmag+(7,0) valence transitions of 14N2, 14N15N, and 15N2 are studied using laser-based 1 extreme ultraviolet (XUV)+1' UV two-photon-ionization spectroscopy, supplemented by synchrotron-based hotoabsorption measurements in the case of 14N2. For each isotopomer, effective rotational interactions between the c(v=2) and b'(v=7) levels are found to cause strong Lambda-doubling in c(v=2) and dramatic P/R-branch intensity anomalies in the b'-X(7,0) band due to the effects of quantum interference. Local perturbations in energy and predissociation line width for the c(v=2) Rydberg level are observed and attributed to a spin-orbit interaction with the crossing, short-lived C 3Pi u(v=17) valence level.

Oscillator strengths and line widths of dipole-allowed transitions in (14)N(2) between 89.7 and 93.5 nm.

Line oscillator strengths in the 20 electric dipole-allowed bands of (14)N(2) in the 89.7-93.5 nm (111480-106950 cm(-1)) region are reported from photoabsorption measurements at an instrumental resolution of approximately 6 mA (0.7 cm(-1)) full width at half maximum. The absorption spectrum comprises transitions to vibrational levels of the 3p sigma(u) c(4)' (1)Sigma(u)(+), 3p pi(u) c(3) (1)Pi(u), and 3s sigma(g) o(3) (1)Pi(u) Rydberg states and of the b' (1)Sigma(u)(+) and b (1)Pi(u) valence states. The J dependences of band f values derived from the experimental line f values are reported as polynomials in J'(J'+1) and are extrapolated to J'=0 in order to facilitate comparisons with results of coupled Schrodinger-equation calculations. Most bands in this study are characterized by a strong J dependence of the band f values and display anomalous P-, Q-, and R-branch intensity patterns. Predissociation line widths, which are reported for 11 bands, also exhibit strong J dependences. The f value and line width patterns can inform current efforts to develop comprehensive spectroscopic models that incorporate rotational effects and predissociation mechanisms, and they are critical for the construction of realistic atmospheric radiative-transfer models.