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Raman spectroscopy of optically trapped single aerosol particles
J. Aerosol Sci., Vol. 22, Suppl. I, pp. $427-$430, 1991.
0021-8502/91 $3.00+0.00 Pergamon Press plc
Printed in Great Britain.
RAMAN SPECTROSCOPY OF OPTICALLY TRAPPED SINGLE AEROSOL PARTICLES
G/inter Georg Hoffmann, Bernhard Oelichmann and Bernhard Schrader Institut f/ir Physikalische und Theoretische Chemie, UniversitS.t - GHS Essen, Posffach 103 764, D-4300 Essen 1, Germany
ABSTRACT Using a newly developed compact multichannel Raman spectrometer, the spectrum of an optically levitated single aerosol particle has been recorded. For the formation of the particles diethylene glycol, a liquid typically used for the generation of model aerosols, was employed.
KEYWORDS Aerosol; diethylene glycol; optical levitation; single particles; Raman spectroscopy
INTRODUCTION Single aerosol particles can be trapped stably and manipulated by focused laser beams (Ashkin, 1970). Spectroscopy of single aerosol particles is of great interest for those who are interested in the composition of. aerosols. Though Raman spectra of single particles levitated by Millikan-type electrostatic or electrodynamic balances or optically by focussed laser beams have been measured (e.g. Kiefer, 1988), there is still a need for improvement. A very fast spectrometer would be of interest for the recording of changes in the chemical composition of single aerosol particles (Schrader, 1986). For example, phenomena correlated to the formation of acid rain, i.e. the oxidation of sulfur dioxide and nitric oxides to sulfuric and nitric acid by the oxygen of air could be studied kinetically. Our approach to the solution of the problem is a low resolution Raman spectrometer with high optical throughput. It uses a CCD detector with high responsivity and dynamic range.
EXPERIMENTAL Optics The spectra shown were taken with our recently developed compact multichannel Raman spectrometer (Hoffmann et al., 1990; 1991). This consists mainly of a light collecting optic, a focusing lens, a 50/am slit, a concave holographically manufactured grating and a CCD detector. The light from the single particle is strongly filtered to remove the unwanted Rayleigh radiation. Electronics The detector used (Weston Fairchild CCD 161) features square pixels with a length of 10/am. They are arranged in a linear ("one-dimensional") array of 6000 pixels. The maximum quantum yield of the CCD reaches more than 70% in the wavelength region from 550 to 800 nm. It is cooled to about -40°C with $427
$428
G.G. HOFFMANNet
al.
a two stage Peltier cooler to reduce thermal noise. The detector is operated with a CCD191 development board at a reduced clock rate of 95.3 kHz. The output of the board is fed into a Datalog DAP 1200/4 analog-to-digital converter subsystem, where the raw data are preprocessed and converted into a spectrum. Data Processing The spectral data from the A/D-subsystem are transmitted serially to an IBM PS/2 model 80 to be inspected and stored. The programs for inspection of spectra are written in Microsoft C 6.0 running under OS/2 Vet's. 1.3. Sample preparation The droplets are generated with the aid of an ink jet print head (courtesy of Siemens AG). The samples used are of diethylene glycol (F.W. 106.12; b.p. 245°C; n:°D 1.4475; d. 1.118). The droplets have a diameter of approx. 50 /am corresponding to a volume of 6.5"104 ~m 3 (70 ng; 0.66 nMol; 4 * 1014 molecules). They can be levitated with laser powers of 0.5 to 2 Watt (Ar*-laser, 488 nm, Spectra Physics 2016-03). The particles are trapped stably for many hours. In Fig. 1 the ink jet print head mounted on a linear positioner can be seen on the left side. The short flexible pipe contains the liquid for the particle formation. The head ejects the droplet on a parabolic path into the focus of the laser beam, which can be seen coming from the lower left. The beam is reflected up from a plane mirror (middle bottom of the photo) and focused through a 80 mm focal length lens. To be save from turbulences in the air the focus of the laser beam with the droplet is contained in a housing made from Plexiglas®, which also contains the light-collecting aspheric lens. The black spectrometer compartment to the right is closed by an achromatic lens needed for the secondary filter arrangement. A close-up of the housing with a trapped particle can be seen in Fig. 2.
RESULTS AND DISCUSSION Fig. 3 shows spectra taken with our compact spectrometer. The lower spectrum is the Raman spectrum of the bulk material, the upper spectrum shows the spectrum of a droplet taken with I W of laser power and 10 minutes of integration time. The spectrum shown is a preliminary result taken with non-optimized optics and detector. It has to be taken into account that diethylene glycol is only a weakly scattering substance. Raman spectra of very small spheres or droplets are quite different from the Raman spectra of the bulk material, because they show fine structure due to resonances (Ashkin and Dziedzic, 1977; Schweiger, 1990), but these can not be resolved by our spectrometer.
OUTLOOK The discussed results have been acquired with a spectrometer originally designed for on-line process control with Raman- and fluorescence spectroscopy. It therefore uses a detector which is not optimized for fastest aquisition of spectra. The replacement of the linear detector (with 10 lam * 10/am pixels) by a two-dimensional CCD detector (with 23 /am * 23 /am pixels) is planned and would reduce (as preliminary experiments show) the time necessary to record one spectrum two to three orders of magnitude corresponding to an improvement of the signal to noise ratio by one to two orders of magnitude.
ACKNOWLEDGEMENTS Financial help from the "Bundesministerium f/Jr Forschung und Technologie" (BMFT) and the "Fond der Chemischen Industrie" is gratefully acknowledged.
Raman spectroscopy of single aerosol particles
Fig. 1: Photograph of the spectrometer's sample compartment I
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$429
Fig. 2: Close-up of the sample compartment I
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3000
3500
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500
1000
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2000
2500 cm-1
Fig. 3: Raman spectra of a diethylene glycol single particle (upper spectnlm) and the corresponding bulk material (lower spectrum)
4000
G.G. HOn:M^NNet al.
$430 REFERENCES
Ashkin, A. (1970). Acceleration and trapping of particles by radiation pressure. Phys. Rev.Lett., 24, 156-159 Ashkin, A. and J. M. Dziedzic (1977). Observation of resonances in the radiation pressure on dielectric spheres. Phys.Rev.Lett., 38, 1351-1354 Hoffmann, G. G., B. Oelichmann and B. Schrader (1990). Compact multichannel spectrometer for measurement of Raman and fluorescence spectra. In: Xllth International Conference on Raman Spectroscopy (Columbia) (J. R. Durig and J. F. Sullivan, eds.), pp. 862-863, Wiley Hoffmann, G. G., H.-U. Menzebach, B. Oelichmann and B. Schrader (1991). Combined Raman and fluorescence spectroscopy with the same compact CCD-based instrument. Appl. Spectrosc., 45, to be published Kiefer, W. (1988). Micro-Raman spectroscopy of particles in the Mie-size range: a short review. Croatica Chemica Acta ~ 473-486 Schrader, B. (1986). Micro Raman, fluorescence, and scattering spectroscopy of single particles. In: Physical and Chemical Characterisation of individual Airborne Particles (K. R. Spumy, ed.), pp. 358-379, Ellis Horwood / John Wiley, Chichester, New York Sehrader, B. (1989). Neue Anwendungsm6glichkeiten der IR- und Raman-Spektroskopie, GIT FachL /,ab. ~ 981-985 Schweiger, G. (1990). Observation of morphology dependent resonances caused by the input field in the Raman Spectrum of microdroplets, J. Raman Spectrosc., 21, 165-168
0021-8502/91 $3.00+0.00 Pergamon Press plc
Printed in Great Britain.
RAMAN SPECTROSCOPY OF OPTICALLY TRAPPED SINGLE AEROSOL PARTICLES
G/inter Georg Hoffmann, Bernhard Oelichmann and Bernhard Schrader Institut f/ir Physikalische und Theoretische Chemie, UniversitS.t - GHS Essen, Posffach 103 764, D-4300 Essen 1, Germany
ABSTRACT Using a newly developed compact multichannel Raman spectrometer, the spectrum of an optically levitated single aerosol particle has been recorded. For the formation of the particles diethylene glycol, a liquid typically used for the generation of model aerosols, was employed.
KEYWORDS Aerosol; diethylene glycol; optical levitation; single particles; Raman spectroscopy
INTRODUCTION Single aerosol particles can be trapped stably and manipulated by focused laser beams (Ashkin, 1970). Spectroscopy of single aerosol particles is of great interest for those who are interested in the composition of. aerosols. Though Raman spectra of single particles levitated by Millikan-type electrostatic or electrodynamic balances or optically by focussed laser beams have been measured (e.g. Kiefer, 1988), there is still a need for improvement. A very fast spectrometer would be of interest for the recording of changes in the chemical composition of single aerosol particles (Schrader, 1986). For example, phenomena correlated to the formation of acid rain, i.e. the oxidation of sulfur dioxide and nitric oxides to sulfuric and nitric acid by the oxygen of air could be studied kinetically. Our approach to the solution of the problem is a low resolution Raman spectrometer with high optical throughput. It uses a CCD detector with high responsivity and dynamic range.
EXPERIMENTAL Optics The spectra shown were taken with our recently developed compact multichannel Raman spectrometer (Hoffmann et al., 1990; 1991). This consists mainly of a light collecting optic, a focusing lens, a 50/am slit, a concave holographically manufactured grating and a CCD detector. The light from the single particle is strongly filtered to remove the unwanted Rayleigh radiation. Electronics The detector used (Weston Fairchild CCD 161) features square pixels with a length of 10/am. They are arranged in a linear ("one-dimensional") array of 6000 pixels. The maximum quantum yield of the CCD reaches more than 70% in the wavelength region from 550 to 800 nm. It is cooled to about -40°C with $427
$428
G.G. HOFFMANNet
al.
a two stage Peltier cooler to reduce thermal noise. The detector is operated with a CCD191 development board at a reduced clock rate of 95.3 kHz. The output of the board is fed into a Datalog DAP 1200/4 analog-to-digital converter subsystem, where the raw data are preprocessed and converted into a spectrum. Data Processing The spectral data from the A/D-subsystem are transmitted serially to an IBM PS/2 model 80 to be inspected and stored. The programs for inspection of spectra are written in Microsoft C 6.0 running under OS/2 Vet's. 1.3. Sample preparation The droplets are generated with the aid of an ink jet print head (courtesy of Siemens AG). The samples used are of diethylene glycol (F.W. 106.12; b.p. 245°C; n:°D 1.4475; d. 1.118). The droplets have a diameter of approx. 50 /am corresponding to a volume of 6.5"104 ~m 3 (70 ng; 0.66 nMol; 4 * 1014 molecules). They can be levitated with laser powers of 0.5 to 2 Watt (Ar*-laser, 488 nm, Spectra Physics 2016-03). The particles are trapped stably for many hours. In Fig. 1 the ink jet print head mounted on a linear positioner can be seen on the left side. The short flexible pipe contains the liquid for the particle formation. The head ejects the droplet on a parabolic path into the focus of the laser beam, which can be seen coming from the lower left. The beam is reflected up from a plane mirror (middle bottom of the photo) and focused through a 80 mm focal length lens. To be save from turbulences in the air the focus of the laser beam with the droplet is contained in a housing made from Plexiglas®, which also contains the light-collecting aspheric lens. The black spectrometer compartment to the right is closed by an achromatic lens needed for the secondary filter arrangement. A close-up of the housing with a trapped particle can be seen in Fig. 2.
RESULTS AND DISCUSSION Fig. 3 shows spectra taken with our compact spectrometer. The lower spectrum is the Raman spectrum of the bulk material, the upper spectrum shows the spectrum of a droplet taken with I W of laser power and 10 minutes of integration time. The spectrum shown is a preliminary result taken with non-optimized optics and detector. It has to be taken into account that diethylene glycol is only a weakly scattering substance. Raman spectra of very small spheres or droplets are quite different from the Raman spectra of the bulk material, because they show fine structure due to resonances (Ashkin and Dziedzic, 1977; Schweiger, 1990), but these can not be resolved by our spectrometer.
OUTLOOK The discussed results have been acquired with a spectrometer originally designed for on-line process control with Raman- and fluorescence spectroscopy. It therefore uses a detector which is not optimized for fastest aquisition of spectra. The replacement of the linear detector (with 10 lam * 10/am pixels) by a two-dimensional CCD detector (with 23 /am * 23 /am pixels) is planned and would reduce (as preliminary experiments show) the time necessary to record one spectrum two to three orders of magnitude corresponding to an improvement of the signal to noise ratio by one to two orders of magnitude.
ACKNOWLEDGEMENTS Financial help from the "Bundesministerium f/Jr Forschung und Technologie" (BMFT) and the "Fond der Chemischen Industrie" is gratefully acknowledged.
Raman spectroscopy of single aerosol particles
Fig. 1: Photograph of the spectrometer's sample compartment I
'
I
I
'
$429
Fig. 2: Close-up of the sample compartment I
'
I
I
3000
3500
?
500
1000
1500
2000
2500 cm-1
Fig. 3: Raman spectra of a diethylene glycol single particle (upper spectnlm) and the corresponding bulk material (lower spectrum)
4000
G.G. HOn:M^NNet al.
$430 REFERENCES
Ashkin, A. (1970). Acceleration and trapping of particles by radiation pressure. Phys. Rev.Lett., 24, 156-159 Ashkin, A. and J. M. Dziedzic (1977). Observation of resonances in the radiation pressure on dielectric spheres. Phys.Rev.Lett., 38, 1351-1354 Hoffmann, G. G., B. Oelichmann and B. Schrader (1990). Compact multichannel spectrometer for measurement of Raman and fluorescence spectra. In: Xllth International Conference on Raman Spectroscopy (Columbia) (J. R. Durig and J. F. Sullivan, eds.), pp. 862-863, Wiley Hoffmann, G. G., H.-U. Menzebach, B. Oelichmann and B. Schrader (1991). Combined Raman and fluorescence spectroscopy with the same compact CCD-based instrument. Appl. Spectrosc., 45, to be published Kiefer, W. (1988). Micro-Raman spectroscopy of particles in the Mie-size range: a short review. Croatica Chemica Acta ~ 473-486 Schrader, B. (1986). Micro Raman, fluorescence, and scattering spectroscopy of single particles. In: Physical and Chemical Characterisation of individual Airborne Particles (K. R. Spumy, ed.), pp. 358-379, Ellis Horwood / John Wiley, Chichester, New York Sehrader, B. (1989). Neue Anwendungsm6glichkeiten der IR- und Raman-Spektroskopie, GIT FachL /,ab. ~ 981-985 Schweiger, G. (1990). Observation of morphology dependent resonances caused by the input field in the Raman Spectrum of microdroplets, J. Raman Spectrosc., 21, 165-168