Gustav Schweiger

PHYSICAL AND CHEMICAL (RAMAN) CHARACTERIZATION OF BIOAEROSOLS—POLLEN

The chemical characterization of pollen is explored using fluorescence spectroscopy and two Raman spectroscopic techniques. The on-line Raman technique involves trapping single pollen particles in an electrodynamic balance coupled to a Raman spectrometer. The off-line method requires the pollen samples to be deposited on a substrate and analyzed using a Raman microprobe. It is shown that fluorescence spectra of various grass and tree pollens do not show sufficient differences to provide an adequate method for characterizing them. Furthermore, the strong fluorescence masks any underlying Raman spectra. Although photochemical bleaching can reduce the fluorescence signal, it is shown that use of a near-IR laser provides Raman signals that can be used to characterize the pollen particles. In the on-line system, the particle density was determined from electrodynamic springpoint measurements and the geometric pollen size measured with a video microscope.

Raman Investigation of Photopolymerization Reactions of Single Optically Levitated Microparticles

Raman spectroscopy was used to investigate polymerization reactions in a single micrometer-sized monomer droplet. An Ar+ laser levitated the microparticles and simultaneously excited the Raman scattering. The polymerization reaction was initiated by exposing the monomer droplets to the UV radiation of a mercury arc excitation lamp. The Raman spectrum of the reacting particle was investigated on-line. The results demonstrate that the combination of the technique of optical levitation and Raman spectroscopy allows nondestructive in situ measurements of single particles and is therefore very useful for the study of fundamental processes.

Optical Determination of the Temperature of Transparent Microparticles

We present a noncontact temperature-sensing method for microdroplets of water, which relies on temperature dependence of the OH stretching band of the spontaneous Raman spectrum. Temperatures of freely moving evaporating droplets with mean particle diameters of approximately 35 µm have been measured with an accuracy of ±1°C in the range from 10 to 65°C. The method allows the observation of droplet cooling during the initial unsteady phase of evaporation.

ELECTRODYNAMIC TRAPPING AND MANIPULATION OF PARTICLE CLOUDS

Apparatus and techniques were developed to electrodynamically trap and manipulate groups of microparticles. The equipment consists of a vibrating orifice aerosol generator, an inductive particle charger, a plenum chamber, and a double-ring electrodynamic balance. Salt particles (NaNO3) of controllable and measurable mass and charge were produced and introduced into the balance in nitrogen at flow rates up to 25 cm3/min. Ordered arrays of any number of particles up to 26 were assembled and manipulated. Methods for compressing the arrays are presented, and controlled ejection of single particles from a trapped array is demonstrated. Particles of opposite polarity were successfully levitated and kept apart, and aggregation of these particles was then induced by changing the electric field. Raman spectra were recorded for multiple salt particles, each having a diameter of 3.5 μm, by aligning them in a laser beam. The enhanced Raman signal is compared with that from a single particle isolated from the array. F...

Structural resonances in a dielectric sphere illuminated by an evanescent wave

Mie scattering of an evanescent wave by a dielectric sphere has been theoretically treated by Chew et al. [Appl. Optics 18 (1979) 2679]. We found some discrepancies between our expansion coefficients and that of Chew. The correct analytic expressions are given by reviewing the theory shortly. Furthermore, we investigate the resonance structure and the source function for different decay rates of the evanescent wave. Numerical results are given for the differential scattering cross section and the source function. Source functions for resonant and nonresonant cases are presented for different penetration depths of the evanescent wave.

The characterization of fine particles originating from an uncharged aerosol: Size dependence and detection limits for Raman analysis

A new experimental method for chemical in situ analysis of ambient aerosol particles is presented. Aerosol particles from the atmosphere with diameters > 1 μm were charged and subsequently captured in an electrodynamic balance. Raman scattering from the particles was excited with an argon ion laser. Raman spectra were taken with a CCD detector through a spectrograph and used to identify chemical substances in the particles. Test particles of sodium sulfate and diethyl sebacate (DES) were employed to determine the detection limit of the method and the size dependence of Raman scattering. The detection limit for sodium sulfate was 0.27 pg, corresponding to a particle diameter of 580 nm. The size-averaged Raman scattering was found to be approximately proportional to volume for particles with diameters > 500 nm using excitation in the visible region.

Analysis of the particle stability in a new designed ultrasonic levitation device

The use of acoustic levitation in the fields of analytical chemistry and in the containerless processing of materials requires a good stability of the levitated particle. However, spontaneous oscillations and rotation of the levitated particle have been reported in literature, which can reduce the applicability of the acoustic levitation technique. Aiming to reduce the particle oscillations, this paper presents the analysis of the particle stability in a new acoustic levitator device. The new acoustic levitator consists of a piezoelectric transducer with a concave radiating surface and a concave reflector. The analysis is conducted by determining numerically the axial and lateral forces that act on the levitated object and by measuring the oscillations of a sphere particle by a laser Doppler vibrometer. It is shown that the new levitator design allows to increase the lateral forces and reduce significantly the lateral oscillations of the levitated object.

Linear Raman Spectroscopy on Droplet Chains: A New Experimental Method for the Analysis of Fast Transport Processes and Reactions on Microparticles

A new experimental method for the analysis of mass and energy transport and reactions on microparticles is presented. A chain of microdroplets from a vibrating orifice generator was injected into a quiescent gas phase. Linear Raman spectra from the microparticles and the surrounding gas were taken at different distances from the generator. Concentration changes were measured as a function of droplet lifetime. A period of time of up to 20 ms could be studied with a resolution of 10 μs. An argon-ion laser in a 90° scattering geometry was used for excitation. Spectra were taken through a modified double monochromator with a two-dimensional charge-coupled device (CCD) detector, one axis of which was used for spatial resolution. Profiles of gaseous components near the droplets could be measured with a resolution of 50 μm. The method has been applied to analysis of absorption, dissociation, and isomerization in the SO2–H2O system and to the investigation of the desorption process of CO2 from water droplets. Chemical components in gas and liquid phase could be separated. The detection limit in aqueous media was 1 mmol/L.

Enhancement of the Raman spectrum of optically levitated microspheres by seeded nanoparticles

Optical levitation experiments were performed with diethyl sebacate and diethyl hexylsebacate particles with a diameter range of 20–30 μm. The particles were seeded with polystyrene–latex particles with a diameter of 90 nm. Elastically and inelastically scattered light was recorded simultaneously. Although no change in the Mie scattering could be observed, the nonresonant part of the Raman spectrum was raised by a factor of 2. The C–H stretching region between 2800 and 3100 cm−1 was investigated, and the time dependence of the Raman intensity was measured at a fixed wave number of Δν = 2920 cm−1.

Temperature sensing by using whispering gallery modes with hollow core fibers

We describe temperature sensing by hollow core fibers using whispering gallery modes of a spherical microresonator. Light from a tunable laser was coupled into the input end of the hollow core fiber. Optical resonances were excited in a microsphere inserted in the modified output end. Part of the light was coupled back from the resonator into the hollow core fiber and transported back to the input end. This light was recorded via a beam splitter by a diode. The sensing principle is based on the shift of the optical resonances by changing the temperature of the resonator. This shift is monitored and leads to the temperature of the resonator and surrounding respectively.