F. Rostas

Photoabsorption and photodissociation cross sections of CO between 88.5 and 115 nm

Abstract Cross sections for photoabsorption and photodissociation of CO between 88.5 and 115 nm have been measured at a spectral resolution of 0.015 nm using a synchrotron radiation source. Most of the photodissociation is found to occur in rotationally resolved bands and not in continua. The region below 100 nm predominates in the photodissociation rate. The band absorption cross sections of the first Rydberg states are considerably increased with respect to previous studies following a better appreciation of pressure saturation effects. The integrated photodissociation cross section above 91.2 nm, which is of interest in many astrophysical situations, is measured to be 300 ± 40 × 10 −18 cm 2 nm, sharply increased with respect to usually accepted values.

High-Resolution HUBBLE SPACE TELESCOPE STIS Spectra of C I and CO in the β Pictoris Circumstellar Disk

High resolution FUV echelle spectra showing absorption features arising from CI and CO gas in the Beta Pictoris circumstellar (CS) disk were obtained on 1997 December 6 and 19 using the Space Telescope Imaging Spectrograph (STIS). An unsaturated spin-forbidden line of CI at 1613.376 A not previously seen in spectra of Beta Pictoris was detected, allowing for an improved determination of the column density of CI at zero velocity relative to the star (the stable component), N = (2-4) x 10^{16} cm^{-2}. Variable components with multiple velocities, which are the signatures of infalling bodies in the Beta Pictoris CS disk, are observed in the CI 1561 A and 1657 A multiplets. Also seen for the first time were two lines arising from the metastable singlet D level of carbon, at 1931 A and 1463 A The results of analysis of the CO A-X (0-0), (1-0), and (2-0) bands are presented, including the bands arising from {13}^CO, with much better precision than has previously been possible, due to the very high resolution provided by the STIS echelle gratings. Only stable CO gas is observed, with a column density N(CO) = (6.3 +/- 0.3) x 10^{14} cm{-2}. An unusual ratio of the column densities of {12}^CO to {13}^CO is found (R = 15 +/- 2). The large difference between the column densities of CI and CO indicates that photodissociation of CO is not the primary source of CI gas in the disk, contrary to previous suggestion.

A model of the B 1Σ+–D ’ 1Σ+ Rydberg–valence predissociating interaction in the CO molecule

The strong interaction between the B 3sσ 1Σ+ Rydberg state and the D’ 1Σ+ valence state of the CO molecule is shown to cause large changes in the vibrational and rotational constants of the B state, as well as predissociation of all rotational levels of B (v’=2) and a breaking off in the emission of B (v’=1) levels at J=36 in 12C 16O and J=37 in 13C 16O. A two‐state diabatic model of the Rydberg–valence interaction is constructed and vibrational term values, widths, and intensities are calculated by close coupling in order to account for the strong mixing. The model separates the differences between the spectroscopic constants of the B state and those of the ground state molecular ion into two components, one due to the R‐dependent quantum defect of the B state and another due to the strong Rydberg–valence perturbation. The perturbation is characterized by a constant coupling matrix element of 2900 cm−1 inside the crossing point of the two diabatic potentials, decaying to zero at long internuclear distanc...

Vibrational band oscillator strengths and dipole transition moment of the A 1Π–X 1Σ+ system of CO

Rotationally integrated vibrational absorption cross sections of the A 1Π−X 1Σ+ (v’−0) bands of CO have been measured in the wavelength range 125–155 nm using monochromatized synchrotron radiation as background light source. Absorption cross sections for a few singlet‐triplet transitions are also obtained. Vibrational band oscillator strengths are determined for 0≤v’≤12 and compared to other experimental and theoretical values. It is found that the new data are well reproduced by extrapolating to shorter internuclear distances the parabolic dipole transition moment function determined by De Leon [J. Chem. Phys. 89, 20 (1988); 91, 5859 (1989)]. Deperturbed lifetimes for levels 0≤v’≤13 are calculated from the parabolic dipole moment function and, except for v’=13, are found in good agreement with available experimental results.

A 2+1 REMPI study of the E-X transition in CO. Indirect predissociations in the E 1Π state

Abstract In this study 2+1 resonance enhanced multiphoton spectra of the E 1 Π( v =1, 0)-X 1 Σ + ( v =0) transitions in 12 C 16 O, 13 C 16 O, 12 C 18 O and 13 C 18 O have been recorded in the laser wavelenght range, 210–216 nm, and rotationally analysed. Perturbations have been observed in both the e and f parity levels of the E 1 Π( v =1) state which have the characteristics of an indirect predissociation similar to the well-known accidental predissociation in the J e =31 level of 12 C 16 O (E 1 Π, v =0). The present results favour 3 Π valence perturbing state which is itself predissociated by a 3 Π repulsive state correlating to the lowest dissociation limit; C( 3 P)+O( 3 P). Lambda doubling constants for all four isotopes in both the v =0 and v =1 vibrational levels of the E 1 Π state have also been obtained.

Photodissociation of H2O and D2O below 132 nm

A complete determination of the rotational and vibrational distributions in the OH/OD(A 2Σ+) fragments which result from the VUV photodissociation of H2O/D2O as a function of excitation energy is presented. VUV excitation was performed at eight different wavelengths (eleven for D2O) in the range between 132 and 124 nm. The vibrational branching ratios show a relatively fast rise of the vibrational excitation at threshold followed by a plateau. The general trend of the experimental results is well reproduced by a phase space theory calculation and by ab initio calculations reported by van Hemert and van Harrevelt. Rotational distributions seem to depend only on the available excess energy and, for a given excess energy appear to be similar for all vibrational levels in OH and OD.

Modelling the VUV emission spectrum of N2: preliminary results on the effects of rotational interactions on line intensities.

A theoretical model of the excited singlet ungerade electronic states of the nitrogen molecule is presented. This work is an extension of a previous study (D. Stahel, M. Leoni and K. Dressler, J. Chem. Phys. 79, 2541-2558, 1983) with the addition of rotational interactions between states of different symmetry. These rotational interactions together with the homogeneous couplings frequently lead to extreme mixing of the states. This model is being used to analyse the high resolution vacuum ultraviolet emission spectrum which is currently being recorded photoelectrically. The ultimate goal of this work is the reliable interpretation of the low resolution emission spectra observed from planetary atmospheres, notably that of Titan, and the transitions assigned as being important in the Voyager 1 spectra of Titan are discussed in detail.

Photodissociation of H2O in the “second continuum”

Abstract The photodissociation of H2O near the threshold for production of excited OH(A 2Σ+) fragments has been studied by two-photon excitation of the B 1A1 state at 266 nm. The rotational population distribution is found to peak between N′ = 6 and 9 with a strong contribution in the N′=2 to 5 range. The long-lived “trapped” trajectories proposed by Segev and Shapiro are not positively confirmed by this test but cannot be excluded either.

Lifetimes of the A 1Π and low-lying triplet states of CO

Abstract The available experimental data concerning the lifetimes of the A 1 Π, a′ 3 Σ + , e 3 Σ − and d 3 Δ is reassessed in the light of a recently completed perturbation analysis of the ν=0–4 levels of the A 1 Π state of CO. This work provides an extensive set of spin-orbit and rotation-electronic parameters characterizing the interactions of the A 1 Π state with adjacent triplet and singlet states. Some of the resulting mixing coefficients are used to analyze the available lifetime measurements both of short-lived levels, of essentially 1 Π character, and long-lived triplet levels. In several cases, the intrinsic lifetimes of the A 1 Π (ν) and of the perturbing triplet state are determined. The strong J dependence of lifetimes are perfectly reproduced by the model and predictions are made for yet unmeasured levels.

Oscillator strengths for transitions to Rydberg levels in 12 C 16 O, 13 C 16 Oa nd 13 C 18 O between 967 and 972 Å

Absorption oscillator strengths have been determined from high-resolution spectra in the 967-972 A region of three CO isotopomers for transitions to the Rydberg levels 4pπ(0), 3dπ(1) and 4pσ(0), as well as to the mixed E(6) level recently characterized by Eidelsberg et al. (2004). Synchrotron radiation from the Super-ACO electron storage ring at Orsay (LURE) was used as a light source. Oscillator strengths were extracted from the recorded spectra by least-squares fitting of the experimental profiles with synthetic spectra taking into account the homogeneous and heterogeneous interactions of the four levels. Column densities were derived from fits to the 3pπ(0) absorption band whose oscillator strength is well established. These are the first reported measurements for 13 C 18 O. For 12 C 16 O, our results are consistent with the larger values obtained in the most recent laboratory and astronomical studies.