A Reexamination of the Red Band of CuO: Analysis of the [16 A Reexamination of the Red Band of CuO: Analysis of the [16.5] 2Σ- - X 2Πi Transition of 63CuO and 65CuO Jack C. Harms, Ethan M. Grames, Bushra Ahmed+, SirkHoo “Kai” Yun+, Leah C. O’Brien*, and James J. O’Brien University of Missouri – St. Louis, Department of Chemistry and Biochemistry *Southern Illinois University Edwardsville, Department of Chemistry +STARS High School Research Program

Why CuO? Copper monoxide has been widely studied for various reasons: The dominant electron configuration for low-lying molecular orbitals is Cu+(3d10) O-(2p5), making it simpler to model than other transition metal oxides with open d-orbitals CuO has been identified in the atmosphere of cool stars Many superconducting materials have been made from CuO doped frameworks, making the electronic structure of the diatomic molecule of particular interest Why are we examining CuO? Strong interferent present in plasmas produced with a Cu cathode Most recently published line positions date to 1974 and lack precision necessary to identify CuO features in regions where new transitions have been identified -Steimle, T. C. & Azuma, Y. J. Mol. Spec. 118, 237 (1986). -O’Brien, L. C. et al. J. Mol. Spec., 180, 365 (1996)

Electron Configuration for X 2Π Ground State of CuO -Merer, A. J. Annu. Rev. Phys. Chem. 40, 407 (1989)

Previous Spectroscopic Investigations CuO was first detected in 1893 Defied correct analysis until 1967 Since then, CuO has been studied quite extensively More than 18 publications since 1967 A comprehensive summary of the work on CuO published in 1989 by A. J. Merer The e/f parity of the ground state was assigned definitively by O’Brien et al. in the analysis of the high resolution FT emission spectrum of the Y 2Σ+ - X 2Πi transition, solidifying the (+/-) assignments of the excited 2Σ states Most accurate rotational constants for the ground state of 63CuO come from mw absorption work of Steimle et al. 1997 -Merer, A. J. Annu. Rev. Phys. Chem. 40, 407 (1989) -O’Brien, L. C., Kubicek, R. L, Wall, S. J., Koch, D.E., Friend, R.J., and Brazier, C.R., J. Mol. Spec., 180, 365 (1996) -Steimle, T., Namiki, K, and Saito, S., J. Chem. Phys., 107(16), 22 (1997)

What has been learned? The X 2Πi ground state is not completely described by the Cu+(3d10)O-(2p5) configuration The spin-orbit splitting of 276 cm-1 is too large compared to the predicted 150 cm-1 for a predominantly O- orbital Magnetic hyperfine interactions indicate ground state has contribution from two other electron configurations The magnitude of the dipole moment of the molecule indicates that the bonding in the molecule is somewhat covalent in nature, not purely ionic Many states have yet to be observed/rotationally analyzed

Focusing on the red band of CuO Rotationally analyzed in 1974 as 2Σ+ - X 2Πi Q-Bandheads at 16222.1 cm-1 and 16499.1 cm-1 Included rotational analysis of the blue 2Πi – X 2Πi transition in determination of ground state constants Emission onto photographic plate: ±0.05 cm-1 confidence in peak positions Analyzed 0-0 and 1-1 bands In 1977 and 1982, the symmetry of the 2Σ+ state was suggested and proven to be inverted (actually a 2Σ- state) and that the Λ-doubling parameter, p, of the ground state was also inverted as a result In 2002, the (0,0), (1,0), (2,0), and (3,0) bands of the A 2Σ- - X 2Π3/2 transition were recorded using LIF and rotationally analyzed. Lifetime measurements were also performed. However, line positions were less well determined than the 1974 work and were not provided. In our study, the red band of CuO was recorded at high resolution in absorption using ILS to correct the rotational assignment, better determine molecular constants, and more accurately measure line positions of the transition. The transition was observed with enough intensity for both 63CuO and 65CuO to be rotationally analyzed. -O. Appelblad and A. Lagerqvist, Phys. Scriptra, 10, 307 (1974) -Y. Lefebvre, B. Pinchemel, and J. Schamps, J. Mol. Spec., 68, 81 (1977) -Y. Lefebvre, B. Pinchemel, J. M. Delaval, and J. Schamps, Phys. Scripta, 25, 329 (1982) -J. Jin, R. Qin, Z. Xiao-Peng, C. Yang, and C. Cong-Xiang, Chinese Physics, 11(5), 481 (2002).

Intracavity Laser Absorption Spectroscopy (ILS) of CuO CuO Molecules produced in the discharge of a Cu-hollow cathode 0.6 torr Ar with periodic addition of O2 to oxidize cathode surface Radio-pulse plasma generation at various discharge currents 0.22-0.60 A Hollow cathode placed within resonator chamber of dye laser system R6G Laser Dye in Ethylene Glycol Pumped by 1.50 W of a Verdi-V10 Laser Laser wavelength was adjusted using a tuning wedge on an xyz translation stage Effective pathlength of method, Leff, proportional to generation time, tg, and speed of light. Leff for these experiments was 250-500 m Plasma spectra were collected in conjunction with spectra from an external I2 cell for absolute wavenumber calibration of the laser output using the Iodine Atlas reference. Spectra are processed and calibrated using an in-house software program, ANEW. Peak positions are determined from 1st and 2nd derivative zero crossings, and smooth unblended lines can be determined to an accuracy of ±0.003 cm-1 when properly calibrated.

[16.5] 2Σ- - X 2Π3/2 transition R21 Branch Structure

Rotational Analysis of the [16.5] A 2Σ- - X 2Πi transition of CuO 11 of 12 expected branches were identified The R12 branch was obscured by the intense Q2 bandhead 464 lines were identified for the (0,0) band of 63CuO 147 unique lines were resolved for the (1,1) band of 63CuO 173 unique lines were resolved for the (0,0) band of 65CuO Line positions from the Y 2Σ+ - X 2Πi analysis by O’Brien et al. (797 lines) and from the pure rotational spectrum of the X 2Πi ground state from Steimle et al. (114 lines) were included in the fit of the (0,0) band of the [16.5] A 2Σ‐ ‐ X 2Πi transition of 63CuO for a total of 1375 data points fit to 21 parameters The (1,1) band of the [16.5] A 2Σ- - X 2Πi transition of 63CuO was rotationally fit using only our experimental data The [16.5] A 2Σ- - X 2Πi transition of 65CuO was fit using two methods: (a) Rotational constants for the X 2Πi ground state were calculated from the constants determined in the fit of the (0,0) band of 63CuO and the dependence of the constants upon the molecular reduced mass and held fixed in the fit (b) Only parameters not expected to depend on the reduced mass (A, p, q) were held fixed in the fit, while the Origin, B, D, AD, and pD were allowed to float

Molecular Constants for the X 2Πi State of 63CuO

Molecular Constants for the [7.8]Y 2Σ+ & [16.5] A 2Σ- States of 63CuO )

Molecular Constants for 65CuO

Comparison of Current Work to Appelblad and Lagerqvist, 1974 In general, the rotational assignment is in good agreement with the previous literature The e/f parity assignments of the rotational branches have been inverted consistent with a 2Σ- state The line positions of the strong Q-bandhead transitions were shown to be more consistent with low J transitions going into the bandhead than the high J transitions used in the original fit All molecular constants obtained from this study are more precisely determined by an order of magnitude, consistent with the improved spectral resolution

Conclusion The line positions for the [16.5] A 2Σ- - X 2Πi transition of CuO have been better determined by an order of magnitude A global fit of the data for the A, Y, and X states of 63CuO provides improved molecular constants Rotational constants for the [16.5] A 2Σ- - X 2Πi transition of 65CuO have been reported for the first time The symmetry assignments of the involved transitions are absolute due to the inclusion of the pure rotational spectrum of the X 2Πi state and the [7.8] Y 2Σ+ - X 2Πi transition in the fit

Ongoing Work Currently, two high school students working in our lab through the UMSL STARS program are analyzing the CuO absorption spectrum between 14,500 and 15,700 cm-1 The β 2Δ3/2 – X 2Πi transition has been identified (not previously reported at high resolution) Several other previously observed bands have also been identified and will be included in this comprehensive analysis of CuO

Acknowledgements National Science Foundation University of Missouri-St. Louis Southern Illinois University – Edwardsville UMSL M.S./undergraduate student Ethan M. Grames High School STARS Students Kai Yun (Summer 2016) Bushra Ahmed (Summer 2016) Ryan Russell (current) Trisha Nair (current)