Improved Experimental Line Positions for the (1,1) Band of the b 1 Σ + g - X 3 Σ - g Transition of O 2 by Intracavity Laser Absorption Spectroscopy Leah C. O'Brien, Southern Illinois University, Edwardsville, IL ; Emily C. O'Brien and James J. O'Brien, University of Missouri, St. Louis, MO 63121

The Atmospheric A Band of O 2  The weak b 1 Σ + g - X 3 Σ – g transition in the far red is designated as the atmospheric A band in the solar spectrum  This transition is spin forbidden and electric-dipole forbidden.  As early as 1927 this transition was described as atmospheric absorption in the solar emission spectrum, where the long atmospheric path length enabled the detection of these weak spectral features (Dieke and Babcock; Babcock).  The most comprehensive spectroscopic study to date on the atmospheric A bands is still that by Babcock and L. Herzberg (1948): this work presents the line positions, assignments and molecular constants observed from the analysis of the (0,0), (1,0), (2,0), (3,0), (1,1), (2,1), and (3,1) bands of 16 O 2, and the (0,0) and (1,0) bands of 16 O 18 O and 16 O 17 O.

Recent work on the (0,0), (1,0) and/or (2,0) bands  A.J. Phillps, F. Peters, and P.A. Hamilton, J. Mol. Spectrosc. 14 (1997)  S.-L. Cheah, Y.-P. Lee, and J.F. Ogilvie, JQSRT 64 (2000)  L.R. Brown, C. Plymate, J. Mol. Spectrosc. 199 (2000)  S.F. Yang, M.R. Canagaratna, S.K. Witonsky, S.L. Coy, J.I. Steinfeld, R.W. Field and A.A. Kachanov, J. Mol. Spectrosc. 201 (2000)  L.C. O’Brien, H. Cao and J.J. O’Brien, J. Mol. Spectrosc. 207 (2001)  D.J. Robichaud, J.T. Hodges, P. Maslowski, L.Y. Yeung, M. Okamura, C.E. Miller, and L.R. Brown, J. Mol. Spectrosc. 251 (2008)  D. Lisak, P. Maslowski, A. Cygan, K. Bielska, S. Wojtewica, M. Piwinski, J.T. Hodges, R.W. Trawinsky and R. Ciurylo, Phys. Rev. 81 (2010) :1-10.  D. A. Long, D. K. Havey, M. Okumura, C. E. Miller3 and J. T. Hodges, Phys. Rev. A 81, (2010).  I.E. Gordon, L.S. Rothman, G.C. Toon, JQSRT 112 (2011)

Previous experimental work on the (1,1) band  In 1927 Babcock estimated the intensity of the (1,1) band of the b 1 Σ + - X 3 Σ – transition was 1/2500 the intensity of the (0,0) band of this transition.  H.D. Babcock, Phys. Rev. 35 (1930) 125.  R. Mecke and W. Baumann, Zs. f. Phys. 73 (1932)  H.D. Babcock and L. Herzberg, Ap. J. 108 (1948)  N. J. van Leeuwen, H.G. Kjaergaard, D.L. Howard and A.C. Wilson, J. Mol. Spectrosc. 228 (2004)  Prediction of (1,1) line positions by I.E. Gordon, L.S. Rothman, G.C. Toon, JQSRT 112 (2011)  Based on (1,0) band measurements and IR/microwave work of Rouillé et al.

Intracavity Laser Absorption Spectroscopy  Absorption spectra recorded using intracavity laser spectrometer at the University of Missouri – St. Louis  Configured with a hollow cathode plasma discharge, 3 A from RPG power supply  Plasma discharge was used to enhance absorption from v=1 in the ground state of oxygen to record the (1,1) band the b 1 Σ + - X 3 Σ – transition of oxygen: absorption for the (0,0) band features also were enhanced  Pressures of 4-8 Torr of oxygen were employed, so that the plasma discharge formed primarily outside of the hollow cathode  Generation times (t g ) up to 200 µs were used, the copper hollow cathode was 50 mm long, the overall laser cavity length was 2.30 m and the section of the cavity containing oxygen was 1.81 m  Spectra are recorded as a series of overlapping spectral segments, each segment being ̴ 6 cm −1 wide

Calibration  Calibration is accomplished by alternatively measuring the spectrum of the intracavity O 2 species and an I 2 absorption spectrum recorded from an extra-cavity iodine cell heated to approximately 590  C  Part 1 (11400 – cm -1 ) of the widely used Iodine Atlas served as reference  Used relationship between the dispersion determined for the diode array detector at the central channel and the central channel position in wavenumbers for a given grating order: for the specific iodine spectra used to calibrate the oxygen lines reported herein, iodine reference lines were selected such that the dispersion determined for the diode array detector was consistent with the established linear relationship for this region  In total, spectra were collected at 33 separate spectrometer grating positions, and the dispersion ranged from cm -1 /channel at cm -1 to cm -1 /channel at cm -1 : this method provided a self-consistent check that the most appropriate Iodine reference lines were selected from the Iodine Atlas  Peak positions are determined from the zero crossing-points of the first derivative spectra using Savitzky–Golay polynomial smoothing: the procedure enables the positions for isolated, unblended lines to be determined to an accuracy of better than ±0.005 cm −1  Spectra of the (1,1) band of the b 1 Σ + - X 3 Σ – transition were collected on 3 separate days, and the peak positions used in the fit were the average of the measured line positions

Dispersion vs. Wavenumber at Channel 512

Plasma-enhanced absorption from v″=1  4 spectra are collected at every monochromator position  Background: no oxygen, no plasma  Iodine: no oxygen, no plasma  Oxygen-no-plasma: ~4 torr oxygen  Oxygen-with-plasma: ~4 torr oxygen  Oxygen-with-plasma showed increased absorption for oxygen for v″=0,1  New!

Analysis of the (1,1) band  Assignments were straightforward based on the work of van Leeuwen et al.  A nonlinear least-squares program was used to fit the line positions to standard energy level expressions. For the b 1 Σ + g state, the energy levels are given by E(J) = T v + B v J(J + 1) – D v J 2 (J + 1) 2 + H v J 3 (J + 1) 3.  For the v″=1 in the X 3 Σ – g ground state, the energy level expressions given in Rouillé et al. were employed. 5. H.D. Babcock and L. Herzberg, Ap. J. 108 (1948)

J" P P(J")obs-calc R R(J")obs-calc * * Line list for (1,1) band *blended line, deweighted in fit

J" P Q(J")obs-calc R Q(J")obs-calc 2* * * * Linelist for (1,1) band *blended line, deweighted in fit

Summary  The (1,1) band of the b 1 Σ + - X 3 Σ – transition of O2 was recorded by ILS  The rms residual for the strong, unblended lines obtained in the fit is cm -1  The accuracy of the previous experimental data set by van Leeuwen et al. was estimated at 0.02 cm -1, and thus our work represents a significant improvement in the accuracy and precision of experimental line positions.  Comparison to the (1,1) band line list that was calculated by Gordon et al. shows:  Average discrepancy in line position was found to be cm -1  RMS deviation in line position was found to be cm -1  A new method of producing vibrational hot molecules for absorption spectroscopy is described

Acknowledgements  National Science Foundation  Emily O’Brien, summer undergraduate researcher (from University of Missouri – Columbia)  Thank you for your attention!