Analysis of Some New Electronic Transitions Observed using Intracavity Laser Spectroscopy (ILS) Jack C. Harms, Ethan M. Grames, 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

Electronic Transitions Observed using ILS Four transitions with strong blue-degraded bandheads were observed in the orange/red region of the visible spectrum in the plasma discharge of a copper hollow cathode Bandhead positions: (a) 16,560 cm-1 (b) 16,485 cm-1 (c) 16,027 cm-1 (d) 15,960 cm-1 The observed transitions are not consistent with any transitions reported in Pearse and Gaydon or any known transitions of Cu containing diatomic molecules

Experimental Methods Absorption spectra were collected for a molecular species produced in the plasma discharge of a Cu hollow cathode 1-1.5 torr H2 or D2 as sputter gas RF power supply to cathode: 0.22-0.60 A The hollow cathode was located within the laser cavity of a dye laser R6G/DCM Laser Dye Verdi V10 pump laser operating at 1.50 W XYZ translation of highly refractive wedge for tuning Cathode lengths ranged from 50-150 mm and generation time for experiments 90μsec Laser cavity ~1.1 m long: effective pathlength ~1 km A spectrum from an external I2 cell was collected after each plasma spectrum, and the recorded I2 positions were calibrated in PGOPHER using the reference data of Salami and Ross. The I2 calibrations were then applied to the corresponding plasma spectra. Average deviations in the calibrations were typically less than ±0.002 cm-1.

Instrument Schematic H2/D2

1st Pair of Transitions: No Observed D2 Shift

2nd Pair of Transitions: D2 Shift of ~3 cm-1 Bandheads for H2 Transitions: 15,960.1 & 16,027.6 cm-1 Bandheads for D2 Transitions: 15,957.4 & 16,024.7 cm-1

Rotational Analysis Locations of newly observed transitions: (a) 16,560 cm-1 (b) 16,485 cm-1 (c) 16,027 cm-1 (d) 15,960 cm-1 Four branches have been identified in each transition: 1 P-type, 2 Q- type, and 1 R-type Combination differences between branches were used to secure a rotational assignment. It has been found through combination differences that transitions (a) & (b) and (c) & (d) share common exited states The two sets of transitions do not seem to have any common states, however The branch patterns are consistent with a 2Σ - 2Π transition, with spin- orbit splitting of the 2Π state (a)-(b) = -71 cm-1 (c)-(d) = -65 cm-1 First lines cannot be definitively assigned due to spectral congestion Transitions were fit using PGOPHER, assuming the 2Π is inverted and the excited state is 2Σ- because of initial expectation was that the molecule is CuNH

Combination Differences 5/2 e f 3/2 e f 1/2 e J’ 2Σ Δ1F’fe (J’) = Qfe(J’+1) - Pee(J’+1) = Rff(J’) – Qef(J’) Δ1F”ef = Qef(J”) – Pee(J”+1) = Rff(J”) – Qfe(J”+1) Δ1F’fe (J’) = Rff(J’)Ω=3/2 – Qef(J’)Ω=3/2 = Rff(J’)Ω=1/2 – Qef(J’)Ω=1/2 2Π1/2 e 5/2 f 2Π3/2 e 3/2 f e 7/2 f e 1/2 f e 5/2 f J” Rff(J’)Ω=3/2 – Qef(J’)Ω=3/2 = Rff(J’)Ω=1/2 – Qef(J’)Ω=1/2 3/2 e J” f Qfe(J’+1) - Pee(J’+1) = Rff(J’) – Qef(J’) Qef(J”) – Pee(J”+1) = Rff(J”) – Qfe(J”+1)

Molecular Constants Molecular Parameters for 2Σ(-)-2Π Transition with Bandheads at 15,960 cm-1 and 16,027 cm-1 Molecular Parameters for 2Σ(-)-2Π Transition with Bandheads at 16,485 cm-1 and 16,560 cm-1

Treatment of the Cathode The observed transitions have been seen under a variety of conditions, all centered around treatment of the cathode surface prior to installation Best results when cathode surface is polished abrasively and inside of cathode is smoothed with metal file Also good results when cathode is soaked in ammonia based cleaning solution Transitions fade after extended plasma discharge Some success regenerating transitions by running NH3 plasma to “treat” cathode before resuming H2 discharges Also some success using small flow of gas from the headspace of a degassed vial of ammonia cleaner Use of ammonia as a reagent gas results in many interferent lines in region of transitions, making data collection with NH3 present impractical Limited success at enhancing the red-most transitions with the addition of compressed air to the gas mixture

Reaction Conditions Transitions were first observed in Ar discharges when dye laser system was initially configured and vacuum system had a leak rate of 5 mtorr/min The A-X transition of CuO overlaps transitions and is very strong in Ar discharges H2 greatly enhanced the strength of the transitions and suppresses the CuO transitions Once the vacuum system was improved, we were unable to reproduce the transition without treatment of the cathode surface On several occasions, the transitions were observed with shockingly strong intensity in discharges initiated just after cathode was installed in vacuum system Transition not reproduced in spectra collected the next day after vacuum chamber had been thoroughly evacuated Transitions are strongest when plasma forms within the hollow cathode, but persist for much longer time when plasma is diffuse and not within hollow cathode

Implications from Experimental Conditions Surface conditions of the cathode play major role in molecular production Molecule contains hydrogen because of observed shift in transition energy when deuterium is used as the sputter gas Molecule likely contains oxygen or nitrogen based on enhanced transition intensity with exposure of cathode to atmosphere or aqueous ammonia solutions

Interpretation of Rotational Structure The J dependence of Δ1F combination differences is given by: Δ1F ( J ) = 2B J + 2B The slope and intercept of Δ1F vs. J will only be equal for either integer or half-integer values of J The half-integer values of J that satisfy this relationship for the identified branches indicate that the molecule has even multiplicity The observed branch structure fits very well to a 2Σ - 2Π transition The 2 “missing” branches are predicted to be weak and the intensity and density of the identified branches could easily mask weak spectral features The relative simplicity of the spectrum seems to suggest a linear molecule

Molecular Candidates Initially considered both CuNH and HCuN as possible sources for the observed transitions Both possibilities eliminated through communications with Wenli Zou: CuNH is predicted to be bent in its ground state with A, B, C values of 16.34 cm-1, 0.4415 cm-1, and 0.4299 cm-1 Lowest lying linear state is 2Π, but B value is predicted to be 0.4096 cm-1 Bond lengths for Cu-N and N-H would have to be 1.95 Å and 0.71 Å to reproduce H2/D2 shift in rotational constants HCuN is predicted to be linear in its ground state, but the B value for this 4Σ- state is predicted to be 0.4945 cm-1 Bond lengths would both need to be 1.95 Å in order to reproduce shift in B values CuH has B-value of 7.8 cm-1, CuH2 of 1.5 cm-1 No indication of splitting of lines that would result from the 63Cu:65Cu isotopic abundance ratio of 69%:31% The molecule likely does not contain copper If the molecule is not metal containing, it must be composed of several atoms to result in such a small rotational constant Several possibilities have been considered, but no matches have yet been found Least unreasonable match to date is HCCO: Quasilinear in is ground state: X 2A” [2Π] Renner-Teller complex with A 2A’ state (B+C)/2 for HCCO and DCCO are 0.3613 and 0.3283 cm-1 Experimental B values for H2 and D2 species: 0.3600 and 0.3262 cm-1

Conclusions Four new electronic transitions have been observed in the orange-red region of the visible spectrum The transitions are identified by strong blue-degraded bandheads Four rotational branches have been identified in each transition, and they have been analyzed as two 2Σ(-) – Hund’s Case (a) 2Π transitions Spin-Orbit Splitting: -65 and -71 cm-1 B”: 0.344480 (27) and 0.350348 (25) cm-1 B’: 0.351377 (26) and 0.355255 (25) cm-1 One of the 2Σ - 2Π transitions undergoes a shift of - 3 cm-1 when D2 is used as a sputter gas instead of H2 The molecule to which these transitions belong has not yet been identified

Acknowledgements National Science Foundation UMSL Department of Chemistry and Biochemistry & Center for Nanoscience