Ozone spectroscopy in the terahertz range from first high-resolution Synchrotron SOLEIL experiments combined with far-infrared measurements and ab initio intensity calculations

Ozone is one of the important molecules in terms of the impact on the atmospheric chemistry, climate changes, bio- and eco-systems and human health. It has a strong absorption in the microwave, terahertz and far-infrared spectral ranges where a large part of the Earth's outgoing longwave radiation to space is located. In this work, the observations, and analyses of the ozone high-resolution spectra in the THz range recorded using the Synchrotron light source of the SOLEIL CNRS equipment are reported for the first time. Thanks to the exceptional brightness of the Synchrotron radiation and to the signal/noise ratio, it was possible to observe many more ozone transitions of the cold rotational band and the hot ν2-ν2 band in the range 0.9-6 THz compared to the previous works. In addition, we have carried out new measurements and assignments for the ν2 band. The simultaneous fit of the rotational band GS-GS, the hot band ν2-ν2 and the FIR ν2 band yielded an overall weighted standard deviation of 0.68 for 13,466 line positions within the experimental accuracy. This includes all previously available MW (with the best uncertainty 0.1 - 10 kHz), FIR data and the original SOLEIL measurements that provided experimental accuracy of 0.00005 - 0.0001 cm-1 for the best lines. Significant deviations in new experimental spectra compared to available spectroscopic databases were evidenced, particularly for the line positions and energy levels at high J, Ka rotational quantum numbers that are the most pronounced in the 4.5 - 6 THz range. Accurate ab initio calculations of line intensities combined with empirically fitted line positions were used to create new linelists that permit theoretical modelling of the transmittance in a good agreement with the Synchrotron spectra in the entire range of observations for various pressures and optical paths. The region near 100 cm-1 and above appears to be more sensitive to the temperature conditions that should be considered in atmospheric observation for the currently operational and future ground based and space missions.

Analysis of experimental spectra of phosphine in the Tetradecad range near 2.3 μm using ab initio calculations

Due to its major interest for the chemistry of planetary atmospheres and exobiology, accurate spectroscopy data of phosphine are required for the search of signatures of this molecule in astronomical observations. In this work, high resolution infrared laboratory spectra of phosphine were analyzed for the first time in the full Tetradecad region (3769-4763 cm-1) involving 26 rotationally resolved bands. Overall, 3242 lines were assigned in spectra previously recorded by Fourier transform spectroscopy at temperatures 200 K and 296 K, using a combined theoretical model based on ab initio calculations. The total nuclear motion Hamiltonian of PH3 including ab initio potential energy surface, was reduced to an effective Hamiltonian using the high-order contact transformation method adapted to vibrational polyads of the AB3 symmetric top molecules, followed by empirical optimization of the parameters. At this step, the experimental line positions were reproduced with a standard deviation of 0.0026 cm-1 that provided unambiguous identification of observed transitions. The effective dipole transition moments of the bands were obtained by fitting to the intensities obtained from variational calculations using the ab initio dipole moment surface. The assigned lines were used to newly determine 1609 experimental vibration-rotational levels up to Jmax = 18 with energy in the range 3896-6037 cm-1 that represents significant extension towards higher energies compared to previous works. Transitions for all 26 sublevels of the Tetradecad were identified but with noticeably fewer transitions for fourfold excited bands because of their weaker intensity. At the final step, pressure-broadened half widths were attached to each transition and a composite line list adopting ab initio intensities and empirical line positions corrected to the accuracy of about 0.001 cm-1 for strong and medium transitions was validated against experimental spectra available in the literature.

Relaxation of highly excited carriers in wide-gap semiconductors

The electron energy relaxation in semiconductors and insulators after high-level external excitation is analysed by a semi-classical approach based on a kinetic equation of the Boltzmann type. We show that the non-equilibrium distributions of electrons and holes have a customary Fermi-like shape with some effective temperature but also possess a high-energy non-Fermian 'tail'. The latter may extend deep into the conduction and valence bands while the Fermi-like component is localized within a small energy range just above the edge of the band gap. The effective temperature, effective chemical potential, and the shape of the high-energy component are governed by the process of electron-phonon interactions as well as by the rates of carrier generation and inter-band radiative recombination.

Does the "reef structure" at the ozone transition state towards the dissociation exist? New insight from calculations and ultrasensitive spectroscopy experiments

Since the discovery of anomalies in ozone isotope enrichment, several fundamental issues in the dynamics linked to the shape of the potential energy surface in the transition state region have been raised. The role of the reeflike structure on the minimum energy path is an intricate question previously discussed in the context of chemical experiments. In this Letter, we bring strong arguments in favor of the absence of a submerged barrier from ultrasensitive laser spectroscopy experiments combined with accurate predictions of highly excited vibrations up to nearly 95% of the dissociation threshold.

Electron-phonon relaxation and excited electron distribution in zinc oxide and anatase

We propose a first-principles method for evaluations of the time-dependent electron distribution function of excited electrons in the conduction band of semiconductors. The method takes into account the excitations of electrons by an external source and the relaxation to the bottom of the conduction band via electron-phonon coupling. The methods permit calculations of the non-equilibrium electron distribution function, the quasi-stationary distribution function with a steady-in-time source of light, the time of setting of the quasi-stationary distribution and the time of energy loss via relaxation to the bottom of the conduction band. The actual calculations have been performed for titanium dioxide in the anatase structure and zinc oxide in the wurtzite structure. We find that the quasi-stationary electron distribution function has a peak near the bottom of the conduction band and a tail whose maximum energy rises linearly with increasing energy of excitation. The calculations demonstrate that the relaxation of excited electrons and the setting of the quasi-stationary distribution occur within a time of no more than 500 fs for ZnO and 100 fs for anatase. We also discuss the applicability of the effective phonon model to energy-independent electron-phonon transition probability. We find that the model only reproduces the trends in the change of the characteristic times whereas the precision of such calculations is not high. The rate of energy transfer to phonons at the quasi-stationary electron distribution also have been evaluated and the effect of this transfer on the photocatalysis has been discussed. We found that for ZnO this rate is about five times less than in anatase.

Adiabatic and non-adiabatic corrections to rovibrational energies of diatomic molecules: variational calculations with experimental accuracy

Using the variational technique, eigensolutions of the radial Herman-Asgharian equation accounting for non-adiabatic terms are determined within the experimental accuracy of the high-resolution spectroscopy. This method, which is independent of the algebraic and numerical approaches currently used in the literature for a "direct-potential-fit" of diatomic rovibrational spectra, is shown to be useful for validation of available calculations and for resolving some controversial issues. Comparative discussions are reported in this paper for a dozen diatomic molecules.

Determination of the Effective Ground State Potential Energy Function of Ozone from High-Resolution Infrared Spectra

The effective ground state potential energy function of the ozone molecule near the C(2v) equilibrium configuration was obtained in a least-squares fit to the largest sample of experimental, high-resolution vibration-rotation data used for this purpose so far. The fitting is based on variational calculations carried out with the extended Morse Oscillator Rigid Bender Internal Dynamics model. The potential function is expanded in Morse-type functions of the stretching variables and in cosine of the bending angle. The present calculation produces results in significantly better agreement with experiment than previous determinations of the potential energy surface, and the energies predicted with the new surface are sufficiently accurate to be useful for the assignment of new high-resolution spectra. The rms (root-mean-square) deviation of the fit of rovibrational data up to J = 5 is 0.02 cm(-1). For the set of all 60 band centers of the (16)O(3) molecule included in the Atlas of Ozone Line Parameters, the rms deviation is 0.025 cm(-1), and for all band centers determined so far from high-resolution spectra, including those recently observed and assigned in Reims corresponding to highly excited stretching and bending vibrations (v(1) + v(2) + v(3) = 6), the rms deviation is 0.1 cm(-1). The "dark states" that produce resonance perturbations in the observed bands are described with experimental accuracy up to the (v(1)v(2)v(3)) = (080) state. Extrapolation tests demonstrate the predictive power of the potential function obtained: rotational extrapolation up to J = 10 for the 11 lowest vibrational states results in an rms deviation of 0.06cm(-1). Also, vibrational energies measured by low-resolution Raman spectroscopy (which were not included in the input data for the fit) are calculated within the experimental accuracy (rms = 1.6 cm(-1)) of the experimental values up to the dissociation limit. The statistical analysis suggests that the accuracy of the equilibrium geometry and force constants of the molecule is considerably improved relative to previous determinations. The long-range behavior of the fitted potential at the dissociation limit O(3) --> O(2) + O shows very good agreement with experimental data. The new potential energy surface was used to predict the band centers of the isotopomers (17)O(3) and (18)O(3). Copyright 1999 Academic Press.

New Analysis of 2nu1 + nu2, nu1 + nu2 + nu3, and nu2 + 2nu3 Bands of Ozone in the 2600-2900 cm-1 Region

The 2600-2900 cm-1 spectral range is revisited for an accurate determination of line intensities of the 2nu1 + nu2, nu1 + nu2 + nu3, and nu2 + 2nu3 bands of ozone. The fit on 1702 energy levels of (012), (111), and (210) states determined from observed transitions with Jmax </= 61 and Ka max </= 17 gives a rms = 5.6 x 10(-4) cm-1 and provides a satisfactory agreement between calculated and observed line positions. Line intensities have been measured and fitted, leading to the determination of transition moment parameters for the three bands. Using these parameters, we have obtained the following estimations for the integrated band intensities S(nu1 + nu2 + nu3) = 2.509 x 10(-20), S(nu2 + 2nu3) = 0.330 x 10(-20), and S(2nu1 + nu2) = 0.0802 x 10(-20) cm-1/mol cm-2 at 296 K, with a cutoff of 10(-26) cm-1/mol cm-2. The value of the µ(210)-(000)1 parameter associated with the &phi;x operator for 2nu1 + nu2 band obtained in our analysis appears to be eight times smaller than the previously determined value of this parameter [M. A. H. Smith, C. P. Rinsland, J. M. Flaud, C. Camy-Peyret, and A. Barbe, J. Mol. Spectrosc. 139, 171-181 (1990)]. The general line listing of the 2nu1 + nu2, nu1 + nu2 + nu3, and nu2 + 2nu3 bands (up to Jmax = 70 and Ka max = 23) has been generated with an intensity cutoff 10(-26) cm-1/mol cm-2. The perturbations of (111) and (012) states by the (040) state are discussed. Copyright 1999 Academic Press.

First Study of the v2 = 3 Dyad {(130), (031)} of Ozone through the Analysis of Hot Bands in the 2300-2600 cm-1 Region

Hot bands nu1 + 3nu2 - nu2 and 3nu2 + nu3 - nu2 of 16O3 in the region 2300-2600 cm-1 and the cold band 3nu2 + nu3 in the region 3050-3110 cm-1, corresponding to the v2 = 3 dyad {(130), (031)}, have been observed for the first time, using the Fourier Transform Spectrometer (FTS) at Reims and the usual experimental setup providing a large product pressure x path length, p x l. Three hundred sixty-five rovibration energy levels of the upper states were obtained with J and Ka up to 46 and 9, respectively. The fit of these data gives a r.m.s. deviation of 1.93 x 10(-3) cm-1. The v2 dependence of the rotational parameters A, B, and C for the (1v20) and (0v21) states is discussed. Copyright 1998 Academic Press. Copyright 1998Academic Press

Infrared Spectrum of Ozone in the 4600 and 5300 cm-1 Regions: High Order Accidental Resonances through the Analysis of nu1 + 2nu2 + 3nu3 - nu2, nu1 + 2nu2 + 3nu3, and 4nu1 + nu3 Bands

The very weak bands nu1 + 2nu2 + 3nu3 and 4nu1 + nu3 of 16O3 have been observed for the first time, using the Fourier transform spectrometer (FTS) of Reims and the usual experimental setup providing a large product p x l of approximately 38 Torr x 36 m. The upper levels of these A-type bands which are rather close in energy (they appear respectively at 5291.722 and 5307.790 cm-1) belong to two different sets of interacting polyads. To correctly reproduce the rotation-vibration energy levels and account for the observed perturbations, both bands are treated in a dyad approximation: the (123) state in the Coriolis resonance with the (330) state, and the (401) state in the Coriolis resonance with the (024) state. The assignments of the rotation-vibration levels of the (123) state are confirmed by measurements of line positions of the hot band nu1 + 2nu2 + 3nu3 - nu2 which has also been observed for the first time. The fits are very satisfactory: the r.m.s. deviation for 249 energy levels of the (123) state is 2.4 x 10(-3) cm-1 and is 2.0 x 10(-3) cm-1 for 266 levels of the (401) state. These r.m.s. are near experimental accuracy. Transition moments for the three observed bands are determined from measured line intensities. Copyright 1997 Academic Press. Copyright 1997Academic Press

Analysis of the 2nu1 + nu2 + 2nu3 Band of Ozone

The 2nu1 + nu2 + 2nu3 band of ozone, occurring in the 4780 cm-1 region, has been observed for the first time, using a Fourier transform spectrometer, at 0.008 cm-1 resolution and using a large path length pressure product. Assignments of vibration-rotational transitions have been made up to J = 48 and Ka = 9. As a few levels with Ka = 1 or 2 are perturbed, it has been necessary to take into account the Coriolis resonance between the (212) and (141) vibrational states. With the effective Hamiltonian explicitly accounting for the interaction between these two states, the fit on 165 energy levels leads to the rms deviation of 1.9 x 10(-3) cm-1, which is near the experimental accuracy. Line intensities of the 2nu1 + nu2 + 2nu3 band have been measured and calculated. The set of spectroscopic parameters for interacting bands, as well as transition moment constants, is given. A complete list of line positions and intensities, with a cutoff of 1 x 10(-26) cm-1/molecule.cm-2 at 296 K up to J = 65 and Ka = 15, has been generated, which leads to the integrated band intensity Sv (2nu1 + nu2 + 2nu3) = (5.1 +/- 2.0) x 10(-23) cm-1/molecule.cm-2. Copyright 1997Academic Press

Line Positions and Intensities of the nu1 + nu2 + 3nu3, nu2 + 4nu3, and 3nu1 + 2nu2 Bands of Ozone

Using a Fourier transform spectrometer, we have recorded the spectra of ozone in the region of 4600 cm-1, with a resolution of 0.008 cm-1. The strongest absorption in this region is due to the nu1 + nu2 + 3nu3 band which is in Coriolis interaction with the nu2 + 4nu3 band. We have been able to assign more than 1700 transitions for these two bands. To correctly reproduce the calculation of energy levels, it has been necessary to introduce the (320) state which strongly perturbs the (113) and (014) states through Coriolis- and Fermi-type resonances. Seventy transitions of the 3nu1 + 2nu2 band have also been observed. The final fit on 926 energy levels with Jmax = 50 and Kmax = 16 gives rms = 3.1 x 10(-3) cm-1 and provides a satisfactory agreement of calculated and observed upper levels for most of the transitions. The following values for band centers are derived: nu0(nu1 + nu2 + 3nu3) = 4658.950 cm-1, nu0(3nu1 + 2nu2) = 4643.821 cm-1, and nu0(nu2 + 4nu3) = 4632.888 cm-1. Line intensities have been measured and fitted, leading to the determination of transition moment parameters for the two bands nu1 + nu2 + 3nu3 and nu2 + 4nu3. Using these parameters we have obtained the following estimations for the integrated band intensities, SV(nu1 + nu2 + 3nu3) = 8.84 x 10(-22), SV(nu2 + 4nu3) = 1.70 x 10(-22), and SV(3nu1 + 2nu2) = 0.49 x 10(-22) cm-1/molecule cm-2 at 296 K, which correspond to a cutoff of 10(-26) cm-1/molecule cm-2.