Abstract

The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO2 laser (tunable within the 9–11-µm region) provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.

© 2000 Optical Society of America

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References

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  1. A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
    [CrossRef]
  2. V. E. Gusev, A. A. Karabutov, Laser Optoacoustics (Institute of Physics, New York, 1993).
  3. A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980).
  4. Y.-H. Pao, ed., Optoacoustic Spectroscopy and Detection (Academic, New York, 1977).
  5. W. P. Mason, R. N. Thurston, eds., Physical Acoustics (Academic, London, 1988), Vol. 18.
  6. P. Hess, ed., Photoacoustic, Photothermal and Photochemical Processes at Surfaces and in Thin Films, Vol. 46 of Topics in Current Physics (Springer-Verlag, Berlin, 1989).
    [CrossRef]
  7. C. B. Scruby, L. E. Drain, Laser Ultrasonics: Techniques and Applications (Hilger, Bristol, UK, 1990).
  8. M. W. Sigrist, Photoacoustic Monitoring of Polymerization Processes (Springer-Verlag, Berlin, 1990).
  9. S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
    [CrossRef]
  10. G. N. Pearson, M. Harris, D. V. Willetts, P. R. Tapster, P. J. Roberts, “Differential laser absorption and thermal emission for remote identification of opaque surface coatings,” Appl. Opt. 36, 2713–2720 (1997).
    [CrossRef] [PubMed]
  11. Type 704 oil contains tetramethyl tetraphenyltrisiloxane and pentaphenyl trimethyltrisiloxane, Vacuum Products1997–1998 (Edwards High Vacuum, Crawley, UK).
  12. D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
    [CrossRef]
  13. A. Wood, Acoustics (Blackie, Glasgow, 1960).
  14. R. W. B. Stephens, A. E. Bate, Acoustics and Vibrational Physics, 2nd ed. (Arnold, London, 1966).

1998 (1)

S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
[CrossRef]

1997 (1)

1986 (1)

A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
[CrossRef]

1982 (1)

D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
[CrossRef]

Bate, A. E.

R. W. B. Stephens, A. E. Bate, Acoustics and Vibrational Physics, 2nd ed. (Arnold, London, 1966).

Brassington, D. J.

D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
[CrossRef]

Drain, L. E.

C. B. Scruby, L. E. Drain, Laser Ultrasonics: Techniques and Applications (Hilger, Bristol, UK, 1990).

Freeborn, S. S.

S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
[CrossRef]

Greig, F.

S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
[CrossRef]

Gusev, V. E.

V. E. Gusev, A. A. Karabutov, Laser Optoacoustics (Institute of Physics, New York, 1993).

Hannigan, J.

S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
[CrossRef]

Harris, M.

Karabutov, A. A.

V. E. Gusev, A. A. Karabutov, Laser Optoacoustics (Institute of Physics, New York, 1993).

Mackenzie, H. A.

S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
[CrossRef]

Pearson, G. N.

Roberts, P. J.

Rosencwaig, A.

A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980).

Scruby, C. B.

C. B. Scruby, L. E. Drain, Laser Ultrasonics: Techniques and Applications (Hilger, Bristol, UK, 1990).

Sigrist, M. W.

M. W. Sigrist, Photoacoustic Monitoring of Polymerization Processes (Springer-Verlag, Berlin, 1990).

Stephens, R. W. B.

R. W. B. Stephens, A. E. Bate, Acoustics and Vibrational Physics, 2nd ed. (Arnold, London, 1966).

Suttie, R. A.

S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
[CrossRef]

Tam, A. C.

A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
[CrossRef]

Tapster, P. R.

Willetts, D. V.

Wood, A.

A. Wood, Acoustics (Blackie, Glasgow, 1960).

Appl. Opt. (1)

J. Phys. D (1)

D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
[CrossRef]

Rev. Mod. Phys. (1)

A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
[CrossRef]

Rev. Sci. Instrum. (1)

S. S. Freeborn, J. Hannigan, F. Greig, R. A. Suttie, H. A. Mackenzie, “A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water,” Rev. Sci. Instrum. 69, 3948–3952 (1998).
[CrossRef]

Other (10)

Type 704 oil contains tetramethyl tetraphenyltrisiloxane and pentaphenyl trimethyltrisiloxane, Vacuum Products1997–1998 (Edwards High Vacuum, Crawley, UK).

A. Wood, Acoustics (Blackie, Glasgow, 1960).

R. W. B. Stephens, A. E. Bate, Acoustics and Vibrational Physics, 2nd ed. (Arnold, London, 1966).

V. E. Gusev, A. A. Karabutov, Laser Optoacoustics (Institute of Physics, New York, 1993).

A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980).

Y.-H. Pao, ed., Optoacoustic Spectroscopy and Detection (Academic, New York, 1977).

W. P. Mason, R. N. Thurston, eds., Physical Acoustics (Academic, London, 1988), Vol. 18.

P. Hess, ed., Photoacoustic, Photothermal and Photochemical Processes at Surfaces and in Thin Films, Vol. 46 of Topics in Current Physics (Springer-Verlag, Berlin, 1989).
[CrossRef]

C. B. Scruby, L. E. Drain, Laser Ultrasonics: Techniques and Applications (Hilger, Bristol, UK, 1990).

M. W. Sigrist, Photoacoustic Monitoring of Polymerization Processes (Springer-Verlag, Berlin, 1990).

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Figures (10)

Fig. 1
Fig. 1

Theoretical dependence of PA signal (defined as the peak pressure of the acoustic pulse) on the dimensionless parameter β. The data are plotted versus β-0.5: This parameter is proportional to the absorption coefficient α. We evaluated the pressure using the parameter values of the test material (type 704 diffusion pump oil), and the results were normalized to unit range and laser-pulse energy by expressing the vertical axis in units of microbars times meters per joules. The dashed curve represents a linear extrapolation of the limiting behavior at large β.

Fig. 2
Fig. 2

Theoretical acoustic pulse shapes for the test material at different values of α corresponding to the range of values examined experimentally in Section 3. Time t = 0 corresponds to the arrival time of the center of the laser pulse at the liquid surface.

Fig. 3
Fig. 3

Experimental setup. The infrared optical pulses are emitted from a carbon dioxide laser and are directed onto the surface of the sample by lens L and fold mirror M. The microphone detects the resulting PA pulses, and these are averaged in a digital oscilloscope to reduce background noise levels.

Fig. 4
Fig. 4

Measured PA pulse profiles. The two pulses shown here are separated by 1.04 ms, corresponding to a pulse repetition frequency of 961 Hz and are the result of averaging 256 measurements. The PA pulse is detected 6.6 µs after the laser pulse, corresponding to the acoustic time of flight over the 2.2-cm range. We define the PA signal as the height of the initial acoustic peak, shown here for the second on-resonance pulse. Note that there is still an observable signal for the off-resonance illumination.

Fig. 5
Fig. 5

Dependence of the PA signal on laser-pulse energy (pulse length τ = 3 µs, wavelength λ = 9.51 µm). Note that the linear increase of signal amplitude with pulse energy implies a more effective conversion of optical to acoustic energy for larger laser pulses.

Fig. 6
Fig. 6

Dependence of the PA signal on laser-pulse length. Here we normalized the signal by dividing by the laser-pulse energy. The laser photons emitted in the tail of the pulse can be seen to have little effect on the measured signal. The laser energy for the different pulse lengths is plotted for comparison.

Fig. 7
Fig. 7

Range dependence of the PA signal. The PA signal can be seen to reduce with a 1/R dependence, indicating a good approximation to emission of a spherical wave.

Fig. 8
Fig. 8

Temperature dependence of PA signals. Note that their relative strength was scaled arbitrarily. In contrast to the signals from low-volatility oil and glass, ethanol shows a large increase in signal with temperature. Also plotted is the saturated vapor pressure for ethanol. The ethanol is unlikely to be in a state of equilibrium with its vapor because of airflow and convection currents, and this accounts for the discrepancy between the data and the ethanol vapor density variation based on the saturated vapor pressure curve.

Fig. 9
Fig. 9

PA spectrum for oil superimposed on the measured absorption spectrum. We measured the absorption spectrum with a conventional absorption spectrometer, using both a thin cell (thickness 5 µm) and a thicker (35 µm) cell, and the combined result is plotted here. The results show good agreement with the theoretical prediction that PA amplitude should be roughly proportional to the absorption coefficient α.

Fig. 10
Fig. 10

Ethanol PA spectrum, together with the measured absorption spectra of vapor and liquid phases. The values were scaled arbitrarily to roughly align the peak values. Note the close correspondence of the PA and vapor spectra and the substantial discrepancy with the liquid spectrum.

Tables (1)

Tables Icon

Table 1 Thermal and Other Parameter Values used in the Direct Comparison of Theory and Experiment for Type 704 Diffusion Pump Oil

Equations (23)

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Q=κ Tz, Tt=1cQz=κc2Tz2.
T=T0z<0, T=T0+αcl exp-αzz>0.
Tn=T-T0,
x=caκa z z<0, x=clκl z z>0.
Tn=0 x<0, Tn=αcl exp-xακlclx>0,
Q=κcTnx, Tnt=1κcQx=2Tnx2.
Tke=coskxexp-k2t,  Tko=sinkxexp-k2t/κc.
Tnx, t=Tex, t+Tox, t/κlcl x>0,  Tn-x, t=Tex, t-Tox, t/κaca x>0.
Tex, t=0dk coskxexp-k2t2κlα2πκlα2+k2clκlcl+κaca,
Tox, t=0dk sinkxexp-k2t2kακlclκacaπκlα2+k2clκlcl+κaca.
Tn0, t=Te0, t=0dk exp-k2t2κlα2πκlα2+k2clκlcl+κaca=c expt/τerfct/τ,
τ=clα2κl,  c=κlcl αclκlcl+κaca.
Tfω=12π0dt expiωtTn0, t=κlcl1+-i/ωτακl2πκlcl+κaca1+iωτ.
It=ετl4 ln 2π exp-4 ln 2t2τl2.
Iω=ε2π exp-ω22σl2.
rω=exp-ω22σm2.
Tfω=εκlcl1+-i/ωτ2πσlακlκlcl+κaca1+iωτ exp-ω22σl2.
Afω=ερ0νsκaiωτ+12πT0caκlcl+κaca1+iωτ exp-ω22σl2.
Afω, d=Afω, 0iωA/2πνsR.
Afω=Aερ0iωκaiωτ+14π2RT0caκlcl+κaca1+iωτ×exp-ω22σl2.
Att=Eρ0σ2κa4π2RT0βcaκlcl+κaca×-iγdγiβγ+12π1+iβγ exp-γ22expiγσt.
Att=Eρ0σ3/2ακaκl4π2RT0clcaκlcl+κaca fσt,
PA signal=BαE/R,

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