Oscillator strength and linewidth measurements of dipole-allowed transitions in N214 between 93.5 and 99.5nm

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 ¹Π_u states of Rydberg and valence character while causing their predissociation. New predissociation linewidths and molecular constants are obtained for the states C³Π_u(v = 7, 8, 14, 15, 16, 21), G³Π_u(v = 0, 1, 4), and F³Π_u(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.

Spectroscopic study of N2(b1Πu, ν = 8) by atmospheric-pressure resonant-enhanced multiphoton ionization and fluorescence detection

A spectroscopic analysis of the strongly perturbed N2(b(1)Πu, ν = 8) state has been conducted, accounting for b(1)Πu(ν = 8) ← X (1)Σg(+)(ν = 0) transitions, for the first time, up to J' = 20. A novel laser spectroscopy technique, using a combination of resonant-enhanced multiphoton ionization and fluorescence detection at atmospheric pressure, avoids the severe effects of perturbation reported in past extreme vacuum ultraviolet absorption experiments that produced weak and unusable spectra for the ν = 8 level. The R, Q, and P branches of the three-photon absorption transition b(1)Πu(ν = 8) ← X(1)Σg(+)(ν = 0) were fit, allowing rotational term energy assignment up to J' = 20 and molecular constants to be determined. Evidence of the previously suspected perturbation in b(1)Πu(ν = 8) is clear in this data, with significant Λ-type doubling at higher J' along with an anomalous negative value determined for the centrifugal distortion coefficient.

The Physical Chemistry of Mass-Independent Isotope Effects and Their Observation in Nature

Historically, the physical chemistry of isotope effects and precise measurements in samples from nature have provided information on processes that could not have been obtained otherwise. With the discovery of a mass-independent isotopic fractionation during the formation of ozone, a new physical chemical basis for isotope effects required development. Combined theoretical and experimental developments have broadened this understanding and extended the range of chemical systems where these unique effects occur. Simultaneously, the application of mass-independent isotopic measurements to an extensive range of both terrestrial and extraterrestrial systems has furthered the understanding of events such as solar system origin and evolution and planetary atmospheric chemistry, present and past.

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.

On the strong and selective isotope effect in the UV excitation of N2 with implications toward the nebula and Martian atmosphere

Isotopic effects associated with molecular absorption are discussed with reference to natural phenomena including early solar system processes, Titan and terrestrial atmospheric chemistry, and Martian atmospheric evolution. Quantification of the physicochemical aspects of the excitation and dissociation processes may lead to enhanced understanding of these environments. Here we examine a physical basis for an additional isotope effect during photolysis of molecular nitrogen due to the coupling of valence and Rydberg excited states. The origin of this isotope effect is shown to be the coupling of diabatic electronic states of different bonding nature that occurs after the excitation of these states. This coupling is characteristic of energy regimes where two or more excited states are nearly crossing or osculating. A signature of the resultant isotope effect is a window of rapid variation in the otherwise smooth distribution of oscillator strengths vs. frequency. The reference for the discussion is the numerical solution of the time dependent Schrödinger equation for both the electronic and nuclear modes with the light field included as part of the Hamiltonian. Pumping is to all extreme UV dipole-allowed, valence and Rydberg, excited states of N(2). The computed absorption spectra are convoluted with the solar spectrum to demonstrate the importance of including this isotope effect in planetary, interstellar molecular cloud, and nebular photochemical models. It is suggested that accidental resonance with strong discrete lines in the solar spectrum such as the CIII line at 97.703 nm can also have a marked effect.

N(2) band oscillator strengths at near-threshold energies

Band oscillator strengths for 58 bands in the near-threshold region of N(2), i.e., from 116 200 to 125 400 cm(-1), are derived from measured band-integrated optical depths. The complexity of the absorption spectrum demands that the measurements be carried out on rotationally cold supersonic jet expansions. The column density N in the absorbing path of the jet cannot be measured directly. Instead, the room temperature f values of selected calibration bands are used to convert the band-integrated optical depths of the jet-cooled calibration bands to preliminary column densities [N], which, plotted as a function of jet reservoir pressure p, scatter around a straight line passing through the origin of the graph. From the slope of the line, first estimates of the effective column density N can be derived for any value of p. Second estimates are obtained by repeating the same procedure using ab initio calculated f values based on the work of Spelsberg and Meyer [J. Chem. Phys. 115, 6438 (2001)]. Depending on the jet configuration, the two estimates differ by 3%-15%; their average is accepted as the best approximation to N. The derived band oscillator strengths are compatible with ab initio results of Spelsberg and Meyer and reproduce the observations reasonably well, even where two or more transitions combine in the formation of complex band structures. They also clarify the analysis of the absorption spectrum in the region of the 7p(0) complex [Jungen, Huber, Jungen, and Stark, J. Chem. Phys. 118, 4517 (2003)] and lead to a plausible interpretation of the spectrum in the 124 680-124 880 cm(-1) range. As a result, the lowest three vibronic levels of both the 3(')d(')sigma and the 4(')s(')sigma core excited states have now been identified.

Optical observation of the C, 3ssigma(g)F3, and 3ppi(u)G3 3Pi(u) states of N2

High-resolution laser-based one extreme-ultraviolet (EUV)+one UV two-photon ionization spectroscopy and EUV photoabsorption spectroscopy have been employed to study spin-forbidden (3)Pi(u)-X (1)Sigma(g) (+)(v,0) transitions in (14)N(2) and (15)N(2). Levels of the C (3)Pi(u) valence and 3ssigma(g)F(3) and 3ppi(u)G(3) (3)Pi(u) Rydberg states are characterized, either through their direct optical observation, or, indirectly, through their perturbative effects on the (1)Pi(u) and (1)Sigma(u) (+) states, which are accessible in dipole-allowed transitions. Optical observation of the G(3)-X(0,0) and (1,0) transitions is reported for the first time, together with evidence for six new vibrational levels of the C state. Following the recent observation of the F(3)-X(0,0) transition at rotational resolution [J. P. Sprengers et al., J. Chem. Phys. 123, 144315 (2005)], the F(3)(v=1) level is found to be responsible for a local perturbation in the rotational predissociation pattern of the b(') (1)Sigma(u) (+)(v=4) state. Despite their somewhat fragmentary nature, these new observations provide a valuable database on the (3)Pi(u) states of N(2) and their interactions which will help elucidate the predissociation mechanisms for the nitrogen molecule.

Fluorescence excitation spectra of the bΠu1, b′Σu+1, cnΠu1, and cn′Σu+1 states of N2 in the 80–100nm region

Fluorescence excitation spectra produced through photoexcitation of N(2) using synchrotron radiation in the spectral region between 80 and 100 nm have been studied. Two broadband detectors were employed to simultaneously monitor fluorescence in the 115-320 nm and 300-700 nm regions, respectively. The peaks in the vacuum ultraviolet fluorescence excitation spectra are found to correspond to excitation of absorption transitions from the ground electronic state to the b (1)Pi(u), b(') (1)Sigma(u) (+), c(n) (1)Pi(u) (with n=4-8), c(n) (') (1)Sigma(u) (+) (with n=5-9), and c(4) (')(v('))(1)Sigma(u) (+) (with v(')=0-8) states of N(2). The relative fluorescence production cross sections for the observed peaks are determined. No fluorescence has been produced through excitation of the most dominating absorption features of the b-X transition except for the (1,0), (5,0), (6,0), and (7,0) bands, in excellent agreement with recent lifetime measurements and theoretical calculations. Fluorescence peaks, which correlate with the long vibrational progressions of the c(4) (') (1)Sigma(u) (+) (with v(')=0-8) and the b(') (1)Sigma(u) (+) (with v(') up to 19), have been observed. The present results provide important information for further unraveling of complicated and intriguing interactions among the excited electronic states of N(2). Furthermore, solar photon excitation of N(2) leading to the production of c(4) (')(0) may provide useful data required for evaluating and analyzing dayglow models relevant to the interpretation of c(4) (')(0) in the atmospheres of Earth, Jupiter, Saturn, Titan, and Triton.

Rotational effects in the band oscillator strengths and predissociation linewidths for the lowest Πu1–XΣg+1 transitions of N2

A coupled-channel Schrodinger equation (CSE) model of N2 photodissociation, which includes the effects of all interactions between the b, c, and o 1Pi u and the C and C' 3Pi u states, is employed to study the effects of rotation on the lowest-upsilon 1Pi u-X 1Sigmag+(upsilon,0) band oscillator strengths and 1Pi u predissociation linewidths. Significant rotational dependences are found which are in excellent agreement with recent experimental results, where comparisons are possible. New extreme-ultraviolet (EUV) photoabsorption spectra of the key b 1Pi u<--X 1Sigmag +(3,0) transition of N2 are also presented and analyzed, revealing a b(upsilon=3) predissociation linewidth peaking near J=11. This behavior can be reproduced only if the triplet structure of the C state is included explicitly in the CSE-model calculations, with a spin-orbit constant A approximately 15 cm(-1) for the diffuse C(upsilon=9) level which accidentally predissociates b(upsilon=3). The complex rotational behavior of the b-X(3,0) and other bands may be an important component in the modeling of EUV transmission through nitrogen-rich planetary atmospheres.