Herzberg Continuum Research Articles

UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables

The UltraViolet and infrared Sensors at high Quantum efficiency onboard a small SATellite (UVSQ-SAT) mission aims to demonstrate pioneering technologies for broadband measurement of the Earth’s radiation budget (ERB) and solar spectral irradiance (SSI) in the Herzberg continuum (200–242 nm) using high quantum efficiency ultraviolet and infrared sensors. This research and innovation mission has been initiated by the University of Versailles Saint-Quentin-en-Yvelines (UVSQ) with the support of the International Satellite Program in Research and Education (INSPIRE). The motivation of the UVSQ-SAT mission is to experiment miniaturized remote sensing sensors that could be used in the multi-point observation of Essential Climate Variables (ECV) by a small satellite constellation. UVSQ-SAT represents the first step in this ambitious satellite constellation project which is currently under development under the responsibility of the Laboratory Atmospheres, Environments, Space Observations (LATMOS), with the UVSQ-SAT CubeSat launch planned for 2020/2021. The UVSQ-SAT scientific payload consists of twelve miniaturized thermopile-based radiation sensors for monitoring incoming solar radiation and outgoing terrestrial radiation, four photodiodes that benefit from the intrinsic advantages of Ga 2 O 3 alloy-based sensors made by pulsed laser deposition for measuring solar UV spectral irradiance, and a new three-axis accelerometer/gyroscope/compass for satellite attitude estimation. We present here the scientific objectives of the UVSQ-SAT mission along the concepts and properties of the CubeSat platform and its payload. We also present the results of a numerical simulation study on the spatial reconstruction of the Earth’s radiation budget, on a geographical grid of 1 ° × 1 ° degree latitude-longitude, that could be achieved with UVSQ-SAT for different observation periods.

Eclipses Observed by Large Yield RAdiometer (LYRA) – A Sensitive Tool to Test Models for the Solar Irradiance

We analyze the light curves of the recent solar eclipses measured by the Herzberg channel (200-220 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA-2. The measurements allow us to accurately retrieve the center- to-limb variations (CLV) of the solar brightness. The formation height of the radiation depends on the observing angle so the examination of the CLV provide information about a broad range of heights in the solar atmosphere. We employ the 1D NLTE radiative transfer COde for Solar Irradiance (COSI) to model the measured light curves and corresponding CLV dependencies. The modeling is used to test and constrain the existing 1D models of the solar atmosphere, e.g. the temperature structure of the photosphere and the treatment of the pseudo- continuum opacities in the Herzberg continuum range. We show that COSI can accurately reproduce not only the irradiance from the entire solar disk, but also the measured CLV. It hence can be used as a reliable tool for modeling the variability of the spectral solar irradiance.

Sensitivity of the Earth’s middle atmosphere to short-term solar variability and its dependence on the choice of solar irradiance data set

We simulate the time evolution of the neutral and charged species in the terrestrial middle atmosphere using a 1-D radiative-convective model with interactive neutral and ion chemistry driven by four different sets of daily spectral solar irradiance (SSI) available in the literature for the year 2000. Obtained daily time series of ozone, hydroxyl and electron densities are used to calculate their sensitivity to the short-term SSI variability at 205 nm. All applied SSI data sets possess 27-day solar rotation cycle; however, its amplitude and phase as well as the correlation between considered SSI time series differ among data sets leading to the different behavior of the atmospheric response. Contrary, the ozone and hydroxyl sensitivities to the SSI changes during solar rotation cycle are almost identical for all applied SSI data sets in the stratosphere. In the mesosphere, the difference in correlation between SSI in Herzberg continuum and Lyman-α line in considered SSI data sets leads to substantial scatter of the sensitivity estimates based on 205 nm. Our results show that for the sensitivity analysis in the stratosphere based on the SSI at 205 nm any considered SSI data sets can be applied. For the mesosphere, where the sensitivity strongly varies among applied SSI data sets more robust results can be obtained using the sensitivity calculations based on the SSI in Lyman-α line.

Angular distributions and angular momentum alignment of O(3PJ) atoms formed in the photolysis of O2via the Herzberg continuum

Sliced velocity-map imaging has been used to measure photofragment scattering distributions for the O((3)P(2)) and O((3)P(1)) products of O(2) photolysis following laser excitation into the Herzberg continuum between 205 and 241 nm. The images were analysed to extract the photofragment spatial anisotropy parameter, β, together with the alignment parameters a(∥), a(⊥), a(⊥), and Re[a(∥, ⊥)]. Our alignment measurements bridge the gap between the recent 193 nm measurement of Brouard et al., Phys. Chem. Chem. Phys., 2006, 8, 5549 and those of Alexander et al., J. Chem. Phys., 2003, 118, 10566 at 222 and 237 nm, and extend out to the threshold at 241 nm. Our measured parameters show no strong dependence on photolysis wavelength. Near the threshold we were able to separate the contributions from the O((3)P(2)) + O((3)P(2)) and O((3)P(2)) + O((3)P(1)) channels, and found significantly different photofragment alignments for the two cases.

Photodissociation Imaging of Diatomic Sulfur (S2)

The photodissociation of diatomic sulfur, S(2), in the region of the first dissociation limit is studied with velocity map imaging. Correlated fine structure distributions P(J1,J2) for the two S((3)P(J)) fragments are determined at selected photolysis wavelengths. Image analysis of the speed distributions of the atomic fragments following product-state-specific detection results in a revision of the bond energy to D(0) = 35636.9 +/- 2.5 cm(-1) with respect to the lowest rovibrational level. This value arises from reinterpretation of previous spectroscopic data showing onset of predissociation in the B(3)Sigma(u)(-) state, as the measurements presented here demonstrate that the long-range correlation of the excited state invoked as causing the dissociation is S((3)P(2)) + S((3)P(2)) rather than S((3)P(2)) + S((3)P(1)). The wavelength dependence of data for the S((3)P(2)) + S((3)P(2)) channel suggests involvement of photoexcitation through the optically forbidden Herzberg continuum bands in addition to dissociation initiated via the optically allowed B(3)Sigma(u)(-)-X(3)Sigma(g)(-) and B''(3)Pi(u)-X(3)Sigma(g)(-) bands. Changes in product recoil velocity angular distributions and atomic angular momentum polarization were also measured as a function of dissociation wavelength. The results are compared with predictions from an adiabatic model for dissociation, which provides a basis for interpretation but does not explain quantitatively the experimental results.

Reconstruction of the solar spectral UV irradiance for nowcasting of the middle atmosphere state on the basis of LYRA measurements

Abstract. The LYRA instrument onboard ESA PROBA2 satellite will provide 6-hourly solar irradiance at the Lyman-alpha (121.6 nm) and the Herzberg continuum (~200–220 nm wavelength range). Because the nowcasting of the neutral and ionic state of the middle atmosphere requires the solar irradiance for the wide spectral range (120–680 nm) we have developed the statistical tool for the reconstruction of the full spectrum from the LYRA measurements. The accuracy of the reconstructed irradiance has been evaluated with 1-D transient radiative-convective model with neutral and ion chemistry using the daily solar spectral irradiance measured with SUSIM and SOLSTICE instruments onboard UARS satellite. We compared the results of transient 1-year long model simulations for 2000 driven by the observed and reconstructed solar irradiance and showed that the reconstruction of the full spectrum using linear regression equation based on the solar irradiance in two LYRA channels can be successfully used for nowcasting of the middle atmosphere. We have also identified conditions when the proposed approach does not yield spectral reconstruction with sufficient accuracy.

Cluster-enhanced X–O2 photochemistry (X=CH3I, C3H6, C6H12, and Xe)

The effect of a local environment on the photodissociation of molecular oxygen is investigated in the van der Waals complex X-O(2) (X=CH(3)I, C(3)H(6), C(6)H(12), and Xe). A single laser operating at wavelengths around 226 nm is used for both photodissociation of the van der Waals complex and simultaneous detection of the O((3)P(J),J=2,1,0) atom photoproduct via (2+1) resonance enhanced multiphoton ionization. The kinetic energy distribution (KED) and angular anisotropy of the product O atom recoil in this dissociation are measured using the velocity map imaging technique configured for either full ("crush") or partial ("slice") detection of the three-dimensional O((3)P(J)) atom product Newton sphere. The measured KED and angular anisotropy reveal a distinct difference in the mechanism of O atom generation from an X-O(2) complex compared to a free O(2) molecule. The authors identify two one-photon excitation pathways, the relative importance of which depends on IPx, the ionization potential of the X partner. One pathway, observed for all complexes independent of IPx, involves a direct transition to the perturbed covalent state X-O(2)(A'(3)Delta(u)) with excitation localized on the O(2) subunit. The predominantly perpendicular character of this channel relative to the laser polarization detection, together with data on the structure of the complex, allows us to confirm that X partner induced admixing of an X(+)-O(2) (-) charge transfer (CT) state is the perturbing factor resulting in the well-known enhancement of photoabsorption within the Herzberg continuum of molecular oxygen. The second excitation pathway, observed for X-O(2) complexes with X=CH(3)I and C(3)H(6), involves direct excitation into the (3)(X(+)-O(2) (-)) CT state of the complex. The subsequent photodissociation of this CT state by the same laser pulse gives rise to the superoxide anion O(2) (-), which then photodissociates, providing fast (0.69 eV) O atoms with a parallel image pattern. Products from the photodissociation of singlet oxygen O(2)(b (1)Sigma(g) (+)) are also observed when the CH(3)I-O(2) complex was irradiated. Potential energy surfaces (PES) for the ground and relevant excited states of the X-O(2) complex have been constructed for CH(3)I-O(2) using the results of CASSCF calculations for the ground and CT states of the complex as well as literature data on PES of the subunits. These model potential energy surfaces allowed us to interpret all of the observed O((3)P(J)) atom production channels.

The photodissociation dynamics of O2 at 193 nm: an O(3PJ) angular momentum polarization study

In the following paper we present translational anisotropy and angular momentum polarization data for O((3)P(1)) and O((3)P(2)) products of the photodissociation of molecular oxygen at 193 nm. The data were obtained using polarized laser photodissociation coupled with resonantly enhanced multiphoton ionization and velocity-map ion imaging. Under the jet-cooled conditions employed, absorption is believed to be dominated by excitation into the Herzberg continuum. The experimental data are compared with previous experiments and theoretical calculations at this and other wavelengths. Semi-classical calculations performed by Groenenboom and van Vroonhoven [J. Chem. Phys, 2002, 116, 1965] are used to estimate the alignment parameters arising from incoherent excitation and dissociation and these are shown to agree qualitatively well with the available experimental data. Following the work of Alexander et al. [J. Chem. Phys, 2003, 118, 10566], orientation and alignment parameters arising from coherent excitation and dissociation are modelled more approximately by estimating phase differences generated subsequent to dissociation via competing adiabatic pathways leading to the same asymptotic products. These calculations lend support to the view that large values of the coherent alignment moments, but small values of the corresponding orientation moments, could arise from coherent excitation of (and subsequent dissociation via) parallel and perpendicular components of the Herzberg I, II and III transitions.

LYRA, a solar UV radiometer on Proba2

LYRA is the solar UV radiometer that will embark in 2006 onboard Proba2, a technologically oriented ESA micro-mission. LYRA is designed and manufactured by a Belgian–Swiss–German consortium (ROB, PMOD/WRC, IMOMEC, CSL, MPS and BISA) with additional international collaborations. It will monitor the solar irradiance in four UV passbands. They have been chosen for their relevance to Solar Physics, Aeronomy and Space Weather: (1) the 115–125 nm Lyman-α channel, (2) the 200–220 nm Herzberg continuum range, (3) the Aluminium filter channel (17–70 nm) including He II at 30.4 nm and (4) the Zirconium filter channel (1–20 nm). The radiometric calibration will be traceable to synchrotron source standards (PTB and NIST). The stability will be monitored by onboard calibration sources (LEDs), which allow to distinguish between potential degradations of the detectors and filters. Additionally, a redundancy strategy maximizes the accuracy and the stability of the measurements. LYRA will benefit from wide bandgap detectors based on diamond: it will be the first space assessment of a pioneering UV detectors program. Diamond sensors make the instruments radiation-hard and solar-blind: their high bandgap energy makes them insensitive to visible light and, therefore, make dispensable visible light blocking filters, which seriously attenuate the desired ultraviolet signal. Their elimination augments the effective area and hence the signal-to-noise, therefore increasing the precision and the cadence. The SWAP EUV imaging telescope will operate next to LYRA on Proba2. Together, they will establish a high performance solar monitor for operational space weather nowcasting and research. LYRA demonstrates technologies important for future missions such as the ESA Solar Orbiter.

Photodissociation of O2 via the Herzberg continuum: Measurements of O-atom alignment and orientation

Irradiation of molecular oxygen O2 in the region of the Herzberg continuum between 218 nm and 239 nm results in the production of open-shell photofragments O(3P)+O(3P). Product O(3Pj; j=0,1,2) atoms were ionized using resonantly enhanced multiphoton ionization (2+1 REMPI) near 225 nm and the ions collected in a velocity-sensitive time-of-flight mass spectrometer. By controlling the polarization of the photolysis and ionization radiation, we have measured alignment and orientation parameters of O-atom electronic angular momentum (j) in the molecule frame. The results show alignment from both parallel and perpendicular transitions that are cylindrically symmetric about the velocity (v) of the recoiling O atom. We also observe electronic alignment that is noncylindrically symmetric about v, resulting from coherence between multiply excited dissociative states. Photodissociation with linearly polarized light is shown to produce O atoms that are oriented in the molecule frame, resulting from interference between parallel and perpendicular dissociative states of O2. Semiclassical calculations that include spin–orbit coupling between six excited states reproduce closely the observed polarization.

Photodissociation of O2 in the Herzberg continuum. II. Calculation of fragment polarization and angular distribution

Parallel and perpendicular components of the Herzberg I, II, and III transitions contribute to the photodissociation of O2 in the Herzberg continuum. The photodissociation dynamics determines the O(3Pj), j=0,1, and 2 atomic fine-structure branching ratios and angular distributions, which were determined in ion imaging experiments at λ=236, 226, and 204 nm by Buijsse et al. [J. Chem. Phys. 108, 7229 (1998)]. In the preceding paper we presented potential energy curves for all eight ungerade O2 states that correlate with the O(3P)+O(3P) dissociation limit, and the R-dependent spin–orbit and the nonadiabatic radial derivative couplings between these states. Here, we employ these potentials and couplings in a semiclassical calculation of the fine-structure branching ratios, atomic polarizations, and fine-structure resolved anisotropy parameters. We discuss the adiabaticity of the dissociation by comparing the results with adiabatic and diabatic models. The O(3Pj) 2+1 REMPI detection scheme used in the experiment is sensitive to the polarization of the atomic fragments. We predict an important effect of the polarization on the anisotropy of the j=1 and j=2 ion images at low energies (λ>236 nm). The agreement between the semiclassical calculations and experiment is reasonable, possible explanations for the remaining differences are discussed.

Photodissociation of O2 in the Herzberg continuum. I. Ab initio calculation of potential energy curves and properties

We present ab initio complete active space self-consistent-field plus multireference configuration interaction (CASSCF+MRCI) potential energy curves for the eight electronically excited ungerade states of oxygen (A 3Σu+, c 1Σu−, A′ 3Δu, Πu1, Πu3, Πu5, Πu-5, and 2 3Σu+) that correlate with the O(3P)+O(3P) dissociation limit. We also report the R-dependent spin–orbit couplings between these states and the R-dependent radial derivative coupling matrix element 〈2 3Σu+|∂/∂R| A 3Σu+〉. The near degeneracy in the long range of the same-symmetry states 2 3Σu+ and A 3Σu+ may result in unphysical mixing of these states in a CASSCF calculation. We derive the correct asymptotic behavior of these states as dictated by the quadrupole–quadrupole interaction and we show how a correct long range description of these states can be achieved numerically by employing undistorted molecular orbitals. Bound state calculations using Herzberg I, II, and III potentials show excellent agreement with all available spectroscopic data. In the accompanying paper the potentials and couplings will be employed in a semiclassical study of the photodissociation of O2 in the Herzberg continuum.

Fourier Transform Spectroscopy of the O2 Herzberg Bands. III. Absorption Cross Sections of the Collision-Induced Bands and of the Herzberg Continuum

Absorption spectra of molecular oxygen were measured in the laboratory under temperature and pressure conditions prevailing in the Earth's atmosphere. Spectra of pure O2, O2 + N2, and O2 + Ar were recorded in the 41 700 to 33 000 cm−1 region (240–300 nm) at a maximal optical path difference of 0.45 cm using a Fourier transform spectrometer and a multiple reflection gas cell. The different components of the spectra, namely the discrete bands of the three Herzberg systems, the Herzberg continuum, and the collision-induced diffuse Wulf bands, were separated. The contribution of the Herzberg bands was first subtracted using the line parameters determined previously [A. Jenouvrier, M.-F. Mérienne, B. Coquart, M. Carleer, S. Fally, A. C. Vandaele, C. Hermans, and R. Colin, J. Mol. Spectrosc. 198, 136–162 (1999)] from high-resolution data. Spectra recorded at various pressures then made it possible to determine by linear regression the intensity of the Wulf bands and the Herzberg continuum. The characteristics of the Wulf bands have been investigated in details: vibrational analysis, pressure effect, foreign gas effect, and a simulated spectrum are reported. The Herzberg continuum cross section is determined below the dissociation limit. A comparison with literature data is given. The new O2 absorption cross sections and O2–O2 collision-induced absorption cross sections are useful in connection with atmospheric measurements of ozone and other trace gases in the UV spectral region.

Photophysics of O2 excited by tunable laser radiation around 193 nm

The photodissociation/ionization dynamics of O2 around 193 nm have been studied using a narrowband tunable ArF excimer laser and the velocity map imaging technique. Angular and kinetic energy distributions of the product O+ ions and O(3P2) atoms are recorded and analyzed. The production of O(3P2) atoms is resonance enhanced on the one-photon level by the B 3Σu−(v=4) state, which is part of the B 3Σu−←X 3Σg− Schumann–Runge bands. Angular distribution measurements for individual rotational levels of the B state yield values for the anisotropy parameter, βSR, which are in good agreement with the values predicted by independent measurements of predissociation lifetimes from spectral linewidths. An average value of β=0.48 is found for the underlying Herzberg continuum at 193 nm. O2+ production is enhanced on the two-photon level via members of the nsσg(n−1)dπg1Σg+ Rydberg series terminating at higher vibrational levels of the ion. The high Rydberg states autoionize into the O2+ X 2Πg ground state or absorb one more photon and then autoionize into the A 2Πu and b 4Σu− states of O2+, which subsequently fluoresce. Production of O+ from one- and two-photon dissociation of the O2+ formed after two-photon absorption is also observed and characterized.

The Wulf bands of oxygen

The Wulf bands of oxygen in the 240–290 nm spectral region are caused by collision-induced absorption of the Herzberg III ( A′ 3 Δ u – X 3 Σ − g ) system. These bands had been previously attributed to the oxygen dimer, (O 2) 2. Under atmospheric conditions the Wulf bands are thus the long-wavelength extension of the Herzberg continuum. Absorption of solar radiation by the Wulf bands may be an additional source of NO in the stratosphere.

Angular distributions for photodissociation of O2 in the Herzberg continuum

Photodissociation in the Herzberg continuum of molecular oxygen has been studied at 236, 226 and 204 nm. Using ion-imaging and monitoring of O(3Pj), j=0, 1, and 2 product-atom angular distributions, the amount of parallel character of the transition was measured. In order to interpret these data, analyses of the photoabsorption oscillator strengths and the parallel-perpendicular nature of the Herzberg I, II and III bands, and extrapolation of these properties into the Herzberg-continuum region have been performed. Our measured fine-structure-averaged angular distributions are found to be consistent with this photoabsorption model. In addition, the dynamics of the dissociation process is discussed, based on the O-atom fine-structure distributions.

Middle atmospheric ozone and temperature responses to solar irradiance variations over 27‐day periods

A one‐dimensional (1‐D), photochemical‐radiative, time‐dependent model with vertical diffusion is developed to investigate the response of ozone in the middle atmosphere to solar UV irradiance variations over 27‐day solar rotation periods. The model, based on equinoctial conditions at the equator, extends from 10 to 100 km with a vertical resolution of 0.5 km and employs an approximation based on photochemical families. The amplitudes of the wavelength‐dependent solar UV variation over 27‐day periods as observed by the Upper Atmosphere Research Satellite (UARS) are adopted in the model. The results of the 1‐D model show that at altitudes of 60–80 km, ozone decreases with increasing solar irradiance at Lyman α. The amplitude of the ozone variation is about 1% of its equilibrium value at about 70–75 km. Below 50 km an ozone increase is found due to increased photolysis of molecular oxygen by increased solar irradiance over the Herzberg continuum. The peak of this ozone increase (about 0.3%) is located at about 35 km. The peaks of the ozone oscillation in the upper stratosphere precede those of the solar oscillation as a result of the strong influence of the temperature feedback on the ozone response through the temperature dependent chemical reaction rates. A modified NCAR 2‐D model is also used to study the short‐term ozone response. Comparison of the 2‐D and 1‐D results shows that horizontal transport increases the phase lag of the temperature response. This large phase lag is essential to simulate the observed ozone response given by Nimbus 7 solar backscattered ultraviolet observations. The positive temperature response around 70 km observed by Nimbus 7 stratospheric and mesospheric sounder data is confirmed by the UARS microwave limb sounder temperature observation, but neither 1‐D nor 2‐D models are able to reproduce this observed temperature response at this level. The effects of the exothermic chemical heating and vertical diffusion coefficient on the mesospheric temperature and ozone responses are discussed. The relative response of ozone and temperature to normalized solar variations over different spectral regions and the roles of different photochemical families in the ozone response are presented. Finally, the seasonal and latitudinal (up to middle latitudes) variations in the ozone response are evaluated with the use of the 2‐D model.

Oxygen absorption cross-section in the Herzberg continuum

A new set of measurements of the absorption cross-section of oxygen in the wavelength range 208–240 nm is reported. These new data, obtained using a 90 m long cell, are about 15% lower than the most recently published.

Oxygen dimerization and pressure dependence of the absorption cross-section in the Herzberg continuum

Using experimental data available on the dependence of the absorption cross-section of O 2 on pressure in the Herzberg continuum, the equilibrium constant of oxygen dimerization has been estimated. The results show that the formation of the oxygen dimer, if it occurs according to the equilibrium constant calculated from the second virial coefficient, cannot account for the pressure dependence of the absorption cross-section of oxygen.

Sensitivity of stratospheric composition to oxygen absorption of solar radiation (175?210 nm)

We have used a two dimensional radiative-chemical-transport model of the stratosphere to investigate the sensitivity of trace gas distributions to absorption of oxygen in the wavelength region 175–210 nm. Two different formulations for the Herzberg continuum absorption cross sections are used. The calculated transmission of ultra-violet light in the stratosphere is lower and higher than observed, depending on the choice of absorption cross section. For the higher transmission O3, ClO, and HO2 are found to be significantly increased in the lower stratosphere. Calculated O3 in the upper stratosphere, chlorofluorocarbons, N2O and odd-nitrogen are lower. The photolysis of oxygen is considerably faster at high latitudes implying that the photochemical recovery of depleted polar ozone is faster than currently assumed.