1. High sensitivity CRDS of the a 1 ∆ g ←X 3 Σ − g band of oxygen near 1.27 μm: magnetic dipole and electric quadrupole transitions in different bands of six isotopologues High sensitivity CRDS of the a 1 ∆ g ←X 3 Σ − g band of oxygen near 1.27 μm: magnetic dipole and electric quadrupole transitions in different bands of six isotopologues Samir Kassi a, Olga Leshchishina a, c, Le Wang a, Iouli E. Gordon b, Laurence S. Rothman b, Alain Campargue a a Laboratoire de Spectrométrie Physique (associated with CNRS, UMR 5588), Université Joseph Fourier de Grenoble, B.P. 87, 38402 Saint-Martin-d’Hères Cedex, France b Harvard-Smithsonian Center for Astrophysics, Atomic and Molecular Physics Division, Cambridge MA 02138, USA c Laboratory of Theoretical Spectroscopy, V.E. Zuev Institute of Atmospheric Optics SB, Russian Academy of Science, 1, Academician Zuev sq., 634021, Tomsk, Russia ABSTRACT: The CW-Cavity Ring Down Spectroscopy (CW-CRDS) technique has been used to record the high sensitivity absorption spectrum of a 1 ∆ g ←X 3 Σ − g band of molecular oxygen near 1.27  m. The spectra were obtained between 7640 and 7917 cm −1 with “natural” oxygen and with samples highly enriched in 18 O and 17 O. The measured transitions belong to the a 1 ∆ g ←X 3 Σ − g (0-0) bands of 16 O 2, 16 O 18 O, 16 O 17 O, 17 O 18 O, 18 O 2 and 17 O 2. The (0-0) bands of 16 O 2 and 18 O 2 show (previously undetected) electric quadrupole transitions with line intensities ranging from 1×10 −30 to 1.9×10 −28 cm/molecule. Lines of the isotopologues containing 17 O atom show partly resolved hyperfine structure, especially in the 17 O 2 spectrum. Accurate spectroscopic parameters for the observed bands were derived from a global fit of the experimental line positions, combined with microwave and Raman measurements available in the literature. Overview of the a 1 Δ g - X 3 Σ g − band of oxygen recorded by CW-CRDS (P= 50.0 Torr, 30.0 Torr, T= 300.2 K, 296K). The upper and two lower panels correspond to O 2 with an isotopic composition near natural abundance sample, a highly 18 O-enriched and 17 O-enriched samples, respectively. Our fibered CW-CRDS spectrometer Spectral region (30 DFB diodes) 7650-7918 nm (7123-7917 cm -1 ) Routine sensitivity: 10 -10 cm -1 ie 1 % absorbance for 300 km path length High dynamics on the intensities: absorption coefficients from 10 -5 to 10 -10 cm -1 are measured on a single spectrum A 10 cm -1 wide section of the spectrum of the 18 O-enriched sample near 7747 cm -1, where transitions belonging to seven bands of four isotopologues were assigned. Differences between the wavenumbers values measured in this work and those provided in the HITRAN database for 16 O 18 O and 16 O 2 versus the line intensity. The 16 O 18 O wavenumber values were measured in the spectrum of the 18 O enriched sample and the line intensities correspond to the 4.9 % relative abundance of 16 O 18 O in this sample. The SPFIT software [2] was used to fit all the measured transitions. The ground X 3 Σ g − electronic state is represented by the following effective Hamiltonian: where B, and  are rotational, spin-spin and spin-rotation interaction constants, respectively, while the other constants are their first and second order centrifugal distortion terms. The rotational energies in the a 1 Δ g state were fit to a simple expression: BAND-BY-BAND FIT The results of the fit are summarized in Table 1, 2. Due to the hyperfine splitting affecting the transitions of the 16 O 17 O and 17 O 18 O species, the ground state rotational constants were fixed to the values obtained in the separate fit of MW data for these two species. The MW lines from Cazzoli et al [1] were fit together with electronic transitions. The hyperfine structure resolved in the ground state was fit to where b F is a Fermi contact parameter, c is a dipolar parameter, and eQq is electric quadrupole interaction parameter. We determined the centrifugal distortion constant D in the ground state. We then fixed that constant to its fitted value and fit only the data from Ref. [1] with the same amount of constants as in that work. The results of the fit are given in Table 3. We then fixed the new ground state constants and performed the fit of only electronic transitions measured in this work. In such a way previously unavailable constants for the a 1 Δ g state of 16 O 17 O and 17 O 18 O were determined (see Table 2). Table 1 Spectroscopic parameters of the v= 0 and 1 levels of the X 3 Σ g − and a 1 Δ g states of 16 O 2 and 18 O 2 Table 2 Spectroscopic parameters of the v = 0 level of the X 3 Σ g − and a 1 Δ g states of 16 O 18 O, 16 O 17 O and 17 O 18 O Table 3 Spectroscopic parameters of the v = 0 level of the X 3 Σ g − state of 16 O 17 O and 17 O 18 O from new fit of the data from Ref. [1] REFERENCES 1. Cazzoli G, Degli Esposti C, Favero PG, Severi G. Microwave spectra of 16 O 17 O and 18 O 17 O. Nuovo Cimeto B Serie 1981; 62:243-54. 2. Pickett HM. The fitting and prediction of vibration-rotation spectra with spin interactions. J Mol Spectrosc 1991; 148:371-7. Overview of the absorption spectrum of 16 O 2 (lower panel) and 18 O 2 (upper panel) (P= 50.0 Torr) on the left hand side and of 16 O 17 O (lower panel) and 17 O 2 (upper panel) (P= 30.0 Torr) on the right hand side. Full circles, open stars and full stars correspond to the a 1 Δ g - X 3 Σ g − (0-0) band, the (1-1) hot band and quadrupole transitions, respectively. For completeness, a few lines of 16 O 2 (0-0) band (light full circles), which were saturated in the CRDS spectrum, have been replaced by the corresponding HITRAN values. The 17 O atom has a nuclear spin I = 5/2. Coupling of the nuclear spin to electron spin in the X 3 Σ g − state [1] and to the electronic angular momentum in the a 1 Δ g state gives rise to a magnetic hyperfine structure in case of the 17 O- containing isotopologues. As a result of the Doppler broadening, the magnetic hyperfine structure cannot be resolved but it clearly shows up as a broadening of the transitions of the 16 O 17 O, 17 O 2 and 17 O 18 O species. Partly resolved hyperfine structure of 16 O 17 O and 17 O 2 transitions in the 17 O-enriched sample. Spectra recorded at room and liquid nitrogen temperatures are shown on the upper and lower panel, respectively. laser ON -50050 100 Laser diode Photodiode Lambdameter Optical isolatorCoupler AO Modulator Laser OFF threshold =f(T,I) 6nm/diode 30 DFB diodes The spectrum of the 17 O sample was also recorded at liquid nitrogen temperature allowing a better resolution of the hyperfine structure of the 17 O isotopologues Hyperfine structure of the 16 O 17 O and 17 O 2 R1R1 transition at different temperatures.