DIODE-LASER AND FOURIER-TRANSFORM DIODE-LASER AND FOURIER-TRANSFORM SPECTROSCOPY OF SPECTROSCOPY OF 1414NHNH33 AND AND 1515NHNH33

IN THE NEAR-INFRARED (1.5 µm)IN THE NEAR-INFRARED (1.5 µm)

Nofal IBRAHIM, Pascale CHELIN, Johannes ORPHALLaboratoire Interuniversitaire des Systèmes Atmosphériques (LISA)

Université de Paris-XII, 91405 Créteil Cedex, France

CARLOS E. FELLOWSLaboratorio de Espectroscopia e Laser

Universidade Federal Fluminense, Niteroi, Rio de Janeiro, Bresil

CNRS-LISACNRS-LISA MSS 2006, Columbus, June 19-23, 2006MSS 2006, Columbus, June 19-23, 2006 22

NH3: strong bands in the near IRo With a short cell (30 cm) and a few mbar of NHWith a short cell (30 cm) and a few mbar of NH33, one can , one can

achieve optical densities higher than 1 for the strongest achieve optical densities higher than 1 for the strongest lines. lines.

→ Useful for calibration of NIR spectra NH3 is also an atmospheric trace gas and present in

the atmospheres of the giant planets

→ NIR useful for concentration determination

1.1. MotivationMotivation 1.1. MotivationMotivation

Precise spectral line parameters are needed!

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Goal: Photo-Acoustic Spectroscopy (PAS) and detection limits of NH3 around 1.5 µm

However: significant problems with line positions and intensities wrt. the literature (Lundsberg-Nielsen et al., JMS 1993)

Solution: new FTS spectra recorded at Orsay

New NH3 linelist has already been used by other groups (R. Lees, M. Okumura, …)

2. This work2. This work2. This work2. This work

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External Cavity Diode Lasers: available from telecommunication

3. ECDL3. ECDL 3. ECDL3. ECDL

Fast: one scan (0.5-1.0 cm-1) in a few seconds

And: Relatively cheap !

• Linewidth 1 MHz (0.00003 cm-1)

• Power 30 mW

• Tunable range: 1535 – 1565 nm(6390 – 6515 cm-1)

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Photo-Acoustic Spectroscopy (PAS)

4. PAS 4. PAS 4. PAS 4. PAS

Thanks to Prof. Th. Huet, University of Lille, France

Photoacoustic effect:Collisional energy transfers

Laser tuned to a molecular transition:

Excited molecules

Partly, non radiative de-excitation

Temperature changes

Pressure changes

Acoustic wave

Detection by microphone

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Simultaneous measurement of NH3 by direct absorption using a White-type cell

5. Our PAS set-up5. Our PAS set-up 5. Our PAS set-up5. Our PAS set-up

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Simultaneous measurement of NH3 by direct absorption using a White-type cell

5. Our PAS set-up5. Our PAS set-up 5. Our PAS set-up5. Our PAS set-up

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6. Results6. Results 6. Results6. Results

Photo-acoustic spectroscopy (PAS)

Direct absorption spectroscopy

FTS spectra (14NH3 and 15NH3)

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6. Results : 1. PAS6. Results : 1. PAS 6. Results : 1. PAS6. Results : 1. PAS S/N > 1000

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6. Results : 6. Results : 1. PAS1. PAS 6. Results : 6. Results : 1. PAS1. PAS

Spectral calibration using the line list (FTS) of Lundsberg-Nielsen et al. (1993)

Problem: different relative line intensities: lines 2 and 3 should

be very similar (60% of the line 1 !!!)

Check with direct absorption !

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6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy 6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy

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6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy 6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy

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6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy 6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy

P = 0.01 Torr

(pure Doppler lines)

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6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy 6. Results : 6. Results : 2. Direct absorption spectroscopy2. Direct absorption spectroscopy

Spectral calibration using the line list (FTS) of Lundsberg-Nielsen et al. (1993)

Problem: relative line intensities still do not agree: lines 2 and 3

should be very similar (60% of the line 1) – so what is

going on here ???

The Doppler widths are not the same !

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7. Results : Solving the puzzle7. Results : Solving the puzzle 7. Results : Solving the puzzle7. Results : Solving the puzzle

Spectral calibration using the line list (FTS) of Lundsberg-Nielsen et al. (1993)

The Doppler widths are not the same !

? Record new FTS spectra !

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8. New spectra using FTS 8. New spectra using FTS 8. New spectra using FTS 8. New spectra using FTS

6300 6400 6500 6600 6700 6800 6900 7000 7100 7200 7300 7400 75000.0

1.0

2.0

wavenumber in cm-1

inte

nsi

ty in

a.u

.

Experimental conditions:• Bruker IFS 120 HR Fourier spectrometer (Orsay)

• Absorption cell 25 cm, CaF2 windows

• NH3 pressure 30 mbar

• Spectral range 5950 – 7850 cm-1

• Spectral resolution 0.02 cm-1

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8. 8. 1414NHNH33 vs vs 1515NHNH33 8. 8. 1414NHNH33 vs vs 1515NHNH33

14NH3

15NH3

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8. Comparison of line positions8. Comparison of line positions 8. Comparison of line positions8. Comparison of line positions

14NH3

15NH3

Line No.Lundsberg-

Nielsen et al.JMS 162 (1993)

This work (FTS) (*)

Difference in cm-1

1 6528.764 6528.7585 0.0055

2 6528.894 6528.8828 0.0112

3 6529.184 6529.1766 0.0074

(*) calibrated using the IUPAC recommended H2O lines of Toth, accuracy (RMS) 0.0005 cm-1

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One problem remains (PAS)One problem remains (PAS)One problem remains (PAS)One problem remains (PAS)

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9. Conclusion9. Conclusion9. Conclusion9. Conclusion

New list of 14NH3 and 15NH3 line positions (and intensities) in the NIR around 1.5 microns*

NIR photoacoustic spectroscopy can be used for quantitative measurements but needs calibration for each individual line (may be molecule dependent)

* The list is available in digital format upon request to the authors.

We wish to thank Professors Ron Lees and Li-Hong Xu from New Brunswick (Canada) for very helpful discussions.