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 Rachel A. Harris and Leah C. O'Brien Department of Chemistry Southern Illinois University, Edwardsville, IL 62026

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 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

3 MO Diagram NiF NiF 3d 4s 2p σ δ π π σ σ

4 Energy levels of NiF with T e < cm -1. Left: Calculated electronic states [Zou and Liu, JCP (2006)] Right: Known electronic states [Krouti, Hirao, Dufour, Boulezhar, Pinchemel, Bernath (JMS 214, (2002)]

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

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)]

7 ILS Schematic Diagram

8 Intracavity Laser Chamber

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 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, (2002)]  The ground state is known from microwave study [Tanimoto et al., JMS 207, (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

14 The (1,0) band of the [11.1] 2 Π 3/2 – X 2 Π 3/2 transition  Bandhead at cm -1 ( 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

15  140 lines  Isotopologue structure for Ni ( 58 Ni, 60 Ni) was not observed  J″ min = 1.5  J″ max = 55.5

16 Molecular Parameters  ∆G ½ = 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

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

18 Acknowledgements  Funding from NSF (JJOB and LCOB) and PRF (LCOB)  Undergraduate student Rachel Harris at SIU Edwardsville

19 Bond Length from [11.1] – X data for 58 Ni 19 F