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1 Intracavity Laser Absorption Spectroscopy of Nickel Fluoride in the Near-Infrared James J. O'Brien Department of Chemistry & Biochemistry University of Missouri, St Louis, MO 63121 Rachel A. Harris and Leah C. O'Brien Department of Chemistry Southern Illinois University, Edwardsville, IL 62026
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2 Previous Work High-resolution spectroscopy of NiF started over 30 years ago in the UV region by Bernard Pinchemel More recently, Pinchemel and Bernath groups have studied the visible and near-IR region by laser induced fluorescence spectroscopy (LIF) and FT emission spectroscopy Energy level diagram (presented later) based on their work Additionally, Chen et al. have examined transitions in the 435-570 nm region by LIF of NiF in a jet source Calculations by Zou and Liu (2006) on Ni halides, and by Koukounas and Mavridis (2008) on diatomic fluorides
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3 MO Diagram 0 -4 -2 -6 -8 -10 -12 -14 -16 NiF NiF 3d 4s 2p σ δ π π σ σ
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4 Energy levels of NiF with T e < 15000 cm -1. Left: Calculated electronic states [Zou and Liu, JCP (2006)] Right: Known electronic states [Krouti, Hirao, Dufour, Boulezhar, Pinchemel, Bernath (JMS 214, 152-174 (2002)]
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5 Intracavity Laser Spectroscopy (ILS) Technique Gaseous absorber contained INSIDE resonator cavity of multimode laser that is operated in a time- modulated fashion Absorption lines act as wavelength dependent loses, which are enhanced as the laser evolves in time Amplified absorption lines appear superimposed on the spectrally broad output of the laser Laser’s output is directed to a high-resolution spectrograph
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6 ILS details (Beer-Lambert relationship) ILS laser observed at well defined time after the onset of laser operation, the averaged time-resolved spectrum (for initial ~500 μs) is given by: Absorbance = ln [I 0 (ν)/I(ν)] = (ν) N [c t g l/L], I 0 (ν), I(ν) is intensity of laser without and with absorption at frequency ν, (ν) is the absorption coefficient at ν N is the number density [ pressure or concentration] c is the speed of light, 3 x 10 8 meter/second t g is the generation time (≈ 100 µs) l/L is the fraction of cavity occupied by the absorber i.e., Effective absorption pathlength = [c t g l/L] t g determines sensitivity (L eff = 20 km for t g = 100 µs, l/L = 2/3), permits high dynamic range t g ~ 500 µs relatively easy for standing wave lasers; longer times possible with ring configured systems 1000’s miles of pathlength! “World record” effective absorption pathlengths (L eff ) is 70,000 km [V.M. Baev and coworkers, Applied Physics B 69, 171 (1999)]
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7 ILS Schematic Diagram
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8 Intracavity Laser Chamber
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9 Recorded (1,0) band of [11.1] 2 Π 3/2 – X 2 Π 3/2 transition of NiF using ILS Molecular source, a Nickel-lined, 2-inch long hollow cathode located inside the cavity of a Ti:sapphire laser Laser beam carries the signal to a 2m McPherson with 1024 channel diode-array detector SF 6 as oxidant in Argon; 1.6–1.7 Torr pressure Set 0.6 Amp plasma discharge current Recorded 11680-11725 cm -1 region; 3 cm -1 per scan For each discharge scan also record background with discharge off and divide the pair Calibrate all spectra using I 2 lines observed in an extracavity oven using ILS laser as light source
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13 The (0,0) band of the [11.1] 2 Π 3/2 – X 2 Π 3/2 transition The (0,0) band of this transition is known [Pinchemel et al., JMS 215, 262-268 (2002)] The ground state is known from microwave study [Tanimoto et al., JMS 207, 66-69 (2001)] The [11.1] 2 Π 3/2 v=0 state required an extra parameter, a, to separate the e/f levels: E = BJ(J+1) − DJ 2 (J+1) 2 ± a/2 ± p/2 (J+½) ± p J /2 J(J+1)(J+½) Nearby perturbing electronic state
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14 The (1,0) band of the [11.1] 2 Π 3/2 – X 2 Π 3/2 transition Bandhead at 11722.27 cm -1 (8528.43 Å) Two R-branches and two P-branches Lines assigned using microwave parameters for ground state energy levels and Δ 2 F values A Hund’s case (c) Ω=3/2 polynomial was used to represent the energy levels for the excited and ground states: E = BJ(J+1) − DJ 2 (J+1) 2 ± p/2 (J+½) ± p J /2 J(J+1)(J+½) Inclusion of the “a” parameter in the excited state did not improve the fit, nor was it determined by the fit Perturber not affecting the v=1 level of the excited state
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15 140 lines Isotopologue structure for Ni ( 58 Ni, 60 Ni) was not observed J″ min = 1.5 J″ max = 55.5
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16 Molecular Parameters ∆G ½ = 620.2 cm -1 for [11.1] 2 Π 3/2 From calculations: ω e ' = 633 [Zou and Liu] ω e ' = 657 [Koukounas and Mavridis] X 2 Π 3/2 and [11.1] 2 Π 3/2 v=0 values from Pinchemel et al. [JMS 2002] Ground state parameters held fixed in the fit
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17 Conclusions The (1,0) band of the [11.1] 2 Π 3/2 – X 2 Π 3/2 transition of NiF has been recorded by intracavity laser absorption spectroscopy and analyzed to obtain the molecular parameters of the upper state Excited state v=1 levels do not require additional “a” parameter First metal-fluoride molecule from our lab
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18 Acknowledgements Funding from NSF (JJOB and LCOB) and PRF (LCOB) Undergraduate student Rachel Harris at SIU Edwardsville
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19 Bond Length from [11.1] – X data for 58 Ni 19 F
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