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Intracavity Laser Absorption Spectroscopy ofNickel Fluoride

in the Near-InfraredJames J. O'Brien

Department of Chemistry & BiochemistryUniversity of Missouri, St Louis, MO 63121

Rachel A. Harris and Leah C. O'BrienDepartment of Chemistry

Southern Illinois University, Edwardsville, IL 62026

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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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MO Diagram0

-4

-2

-6

-8

-10

-12

-14

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Ni FNiF

3d

4s

2p

σ

δ

π

π

σ

σ

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Energy levels of NiF with Te < 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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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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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 [I0(ν)/I(ν)] = (ν) N [c • tg• l/L], I0(ν), I(ν) is intensity of laser without and with absorption at frequency ν,

(ν) is the absorption coefficient at νN is the number density [ pressure or concentration]c is the speed of light, 3 x 108 meter/secondtg is the generation time (≈ 100 µs)l/L is the fraction of cavity occupied by the absorber

i.e., Effective absorption pathlength = [c • tg • l/L]

tg determines sensitivity (Leff = 20 km for tg = 100 µs, l/L = 2/3), permits high dynamic range

tg ~ 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 (Leff) is 70,000 km [V.M. Baev and coworkers, Applied Physics B 69, 171 (1999)]

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ILS Schematic Diagram

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Intracavity Laser Chamber

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Recorded (1,0) band of [11.1] 2Π3/2 – X 2Π3/2

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

SF6 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 I2 lines observed in an extracavity

oven using ILS laser as light source

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The (0,0) band of the [11.1] 2Π3/2 – X 2Π3/2 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] 2Π3/2 v=0 state required an extra parameter, a, to separate the e/f levels: E = BJ(J+1) − DJ2(J+1)2 ± a/2 ± p/2 (J+½) ± pJ/2 J(J+1)(J+½)

Nearby perturbing electronic state

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The (1,0) band of the [11.1] 2Π3/2 – X 2Π3/2 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 Δ2F 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) − DJ2(J+1)2 ± p/2 (J+½) ± pJ/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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140 lines Isotopologue

structure for Ni (58Ni, 60Ni) was not observed

J″min = 1.5 J″max = 55.5

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Molecular Parameters

∆G½ = 620.2 cm-1 for [11.1] 2Π3/2

From calculations: ωe' = 633 [Zou and Liu] ωe' = 657 [Koukounas and Mavridis]

X 2Π3/2 and [11.1] 2Π3/2 v=0 values from Pinchemel et al. [JMS 2002] Ground state parameters held fixed in the fit

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Conclusions

The (1,0) band of the [11.1] 2Π3/2 – X 2Π3/2 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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Acknowledgements

Funding from NSF (JJOB and LCOB) and PRF (LCOB)

Undergraduate student Rachel Harris at SIU Edwardsville

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Bond Length from [11.1] – X data for 58Ni19F