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

Multi-channel photodissociation dynamics of 14N2 in its b' 1Σ+u(ν = 20) state

b' ¹Σ+u(ν = 20) is the first vibronic state above the dissociation limit N(²D_3/2,5/2) + N(²D_3/2,5/2) of ¹⁴N₂ that has been observed in the absorption spectrum. It provides a unique opportunity for studying the multi-channel photodissociation dynamics of ¹⁴N₂, particularly the competition between the spin-forbidden and spin-allowed photodissociation channels. Here, photofragment excitation (PHOFEX) and (1VUV + 1'UV) photoionization spectra of ¹⁴N₂ in the b' ¹Σ+u(ν = 20) state and the time-slice velocity-map ion (TS-VMI) images at each individual rotational levels are collected by using a vacuum ultraviolet (VUV) pump-VUV probe scheme. It is found that the spin-forbidden channels N(⁴S) + N(²D_3/2,5/2) and N(⁴S) + N(²P_1/2,3/2) are competitive with the spin-allowed channel N(²D_3/2,5/2) + N(²D_3/2,5/2) at low rotational levels, while quickly become undetectable as the rotational quantum number J increases. At high rotational levels, only the spin-allowed channel N(²D_3/2,5/2) + N(²D_3/2,5/2) can be observed, supporting previous theoretical modeling. Channel-resolved partial predissociation rate constants (PPRCs) are calculated by combining branching ratios in this study and total predissociation rate constants (TPRCs) from previous absorption spectroscopic measurements. PPRCs for dissociation into channels N(⁴S) + N(²D_3/2,5/2) and N(⁴S) + N(²P_1/2,3/2) are almost independent of J, while those of N(²D_3/2,5/2) + N(²D_3/2,5/2) show complicated rotational dependence. Possible coupling schemes between b' ¹Σ+u(ν = 20) and the high lying ¹Π_u and ³Π_u states are analyzed, which provides deep insight into the multi-channel photodissociation dynamics of ¹⁴N₂ in a high energy range.

Vacuum ultraviolet photoexcitation and photofragment spectroscopic studies of 14N15N between 109000 and 117500 cm-1

In this study, we employed a newly built time-slice velocity-map ion imaging setup, equipped with two tunable vacuum ultraviolet (VUV) laser sources, to obtain the first comprehensive high-resolution photoexcitation and photofragment excitation spectra of ¹⁴N¹⁵N in the VUV photon energy range 109 000-117 500 cm^-1. The spectroscopic simulation program PGOPHER was used to analyze the rotationally resolved spectra. Band origins, rotational constants, and isotope shifts compared with those of ¹⁴N₂ have been obtained for 31 electric-dipole-allowed vibrational states of ¹⁴N¹⁵N in the aforementioned energy range. These spectroscopic parameters are found to depend on the vibrational quantum number irregularly. Systematic perturbations of the rotational transition energies and predissociation rates within individual absorption bands have also been observed. These are proved to be caused by the strong homogeneous interactions between the valence b'¹Σ_u ⁺ state and the Rydberg c_n ^{' 1}Σ_u ⁺ states, and between the valence b¹Π_u states and the Rydberg o₃ ¹Π_u states. Heterogeneous interactions between the Rydberg c_n ¹Π_u states and c_n ^{' 1}Σ_u ⁺ states also play an important role.

An analytical formulation of isotope fractionation due to self-shielding

Isotope fractionation due to photochemical self-shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self-shielding may also contribute to sulfur isotope mass-independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodissociation rate coefficients that describe self-shielding isotope signatures for 3 and 4-isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tractable. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO₂, but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the magnitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass-dependent fractionation associated with photodissociation of optically-thick SO₂. The formulation presented here elucidates the origin of some of these important isotopic fractionation processes.

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.

Evaluation of hydrogen absorption cells for observations of the planetary coronas

Newly designed Lyman-alpha absorption cells for imaging hydrogen planetary corona were characterized using an ultra high resolution Fourier transform spectrometer installed on the DESIRS (Dichroïsme Et Spectroscopie par Interaction avec le Rayonnement Synchrotron) beamline of Synchrotron SOLEIL in France. The early absorption cell installed in the Japanese Mars orbiter NOZOMI launched in 1998 had not been sufficiently optimized due to its short development time. The new absorption cells are equipped with the ability to change various parameters, such as filament shape, applied power, H₂ gas pressure, and geometrical configuration. We found that the optical thickness of the new absorption cell was ∼4 times higher than the earlier one at the center wavelength of Lyman-alpha absorption, by optimizing the condition to promote thermal dissociation of H₂ molecules into two H atoms on a hot tungsten filament. The Doppler temperature of planetary coronas could be determined with an accuracy better than 100 K with the performance of the newly developed absorption cell.

Photochemistry on Pluto - I. Hydrocarbons and aerosols

In light of the recent New Horizons flyby measurements, we present a coupled ion-neutral-photochemistry model developed for simulating the atmosphere of Pluto. Our model results closely match the observed density profiles of CH4, N2 and the C2 hydrocarbons in the altitude range where available New Horizons measurements are most accurate (above ~ 100-200 km). We found a high eddy coefficient of 106 cm2 s-1 from the surface to an altitude of 150 km, and 3 × 106 cm2 s-1 above 150 km for Pluto's atmosphere. Our results demonstrate that C2 hydrocarbons must stick to and be removed by aerosol particles in order to reproduce the C2 profiles observed by New Horizons. Incorporation into aerosols in Pluto's atmosphere is a significantly more effective process than condensation, and we found that condensation alone cannot account for the observed shape of the vertical profiles. We empirically determined the sticking efficiency of C2 hydrocarbons to aerosol particles as a function of altitude, and found that the sticking efficiency of C2 hydrocarbons is inversely related to the aerosol surface area. Aerosols must harden and become less sticky as they age in Pluto's atmosphere. Such hardening with ageing is both necessary and sufficient to explain the vertical profiles of C2 hydrocarbons in Pluto's atmosphere. This result is in agreement with the fundamental idea of aerosols hardening as they age, as proposed for Titan's aerosols.

The high-resolution absorption spectroscopy branch on the VUV beamline DESIRS at SOLEIL

A VUV absorption spectroscopy facility designed for ultra-high spectral resolution is in operation as a dedicated branch on the DESIRS beamline at Synchrotron SOLEIL. This branch includes a unique VUV Fourier transform spectrometer (FTS) and a dedicated versatile gas sample chamber. The FTS instrument can cover a large UV-VUV spectral range from 4 to 30 eV, with an ultimate line width of 0.08 cm(-1) on a large spectral window, ΔE/E = 7%, over which all spectral features can be acquired in a multiplex way. The performance can be considered to be a middle ground between broadband moderate-resolution spectrometers based on gratings and ultra-high-spectral-resolution VUV tunable-laser-based techniques over very narrow spectral windows. The various available gaseous-sample-handling setups, which function over a wide range of pressures and temperatures, and the acquisition methodology are described. A selection of experimental results illustrates the performance and limitations of the FTS-based facility.

VIS and VUV spectroscopy of 12C17O and deperturbation analysis of the A1Π, υ = 1–5 levels

The reduced T(J) − B_AJ(J + 1) + D_AJ²(J + 1)² term values for the ¹²C¹⁷O A₁Π (υ = 5) level and for the hypothetical unperturbed crossing rovibronic levels of the perturbers.

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.

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.