Uncertainties in temperature measurements of optically levitated single aerosol particles by Raman spectroscopy

doi:10.1016/S0022-2860(98)00704-2

Journal of Molecular Structure

Volumes 480–481, 4 May 1999, Pages 311-316

https://doi.org/10.1016/S0022-2860(98)00704-2 Get rights and content

Abstract

Temperature is one of the key parameters in determining the rate of heat and mass transfer in an evaporating or growing aerosol single particle. In particular, stratospheric aerosol particles consisting of H₂O, H₂SO₄ and HNO₃ play a pivotale role by activating and deactivating reactions of reservoir gases (N₂O₅, ClONO₂, HCl), which are responsible for the depletion of stratospheric ozone. The technique of optical levitation allows a contactless examination of single aerosol particles. Raman spectroscopy offers the possibility to examine simultaneously the temperature, the chemical composition, as well as the phase of organic or inorganic substances.We present an experimental setup allowing optical trapping of single aerosol particles by a focused argon-ion laser beam. Pressure and temperature in the reaction cell can be reduced to stratospheric conditions. An optimized compact Raman spectrometer registers the inelastically scattered light. Comparison of theoretically predicted uncertainties in temperature measurements to experimental data is shown, and experimental limitations are discussed.

Introduction

The acceleration and trapping of microparticles by radiation pressure dates back to 1970, as first reported by Ashkin [1]. Subsequently, Thurn and Kiefer [2] have been the first to analyze the Raman spectrum of optically levitated single particles, using glass spheres and quartz crystals. Vehring and Schweiger [3] presented a noncontact temperature-sensing method for microdroplets of water, which relies on temperature dependence of the OH stretching band in the Raman spectrum. Rassat and Davis [4] were the first to apply the method of using Stokes/anti-Stokes Raman intensity ratios for the determination of temperature to single microparticles. LaPlant et al. [5] used the statistics of photon counting to set theoretical limits on the accuracy of the Stokes/anti-Stokes-technique and developed an expression whereby the theoretical temperature uncertainty for any Raman band at a given temperature can be predicted. Finally, Popp et al. [6] elucidated the temperature behaviour of a microdroplet that is optically levitated by a focused laser beam.

The purpose of this article is to demonstrate the applicability of Raman spectroscopy to temperature measurements at stratospheric conditions ( $T=180–200 K$ ). Bulk and single particle temperatures are identified by Stokes/anti-Stokes Raman intensity ratios and by the position of the maximum from the OH stretching band in the Raman spectrum (see designated regions in Fig. 1). To our knowledge this is the first Raman spectroscopic investigation of binary sulfuric acid/water-aerosol-droplets at low temperatures using the optical levitation technique.

Section snippets

Theory

The intensity of Raman-scattered radiation is proportional to the fourth power of the frequency of the scattered light [7]. For a nonabsorbing sample in thermal equilibrium, a Boltzmann relationship relates the temperature of the material to the ratio of Stokes and anti-Stokes Raman scattering intensities, I_S/I_AS: $T= h.c. k · ν ̃_{R} In I_{S} /I_{AS} −4 ln ν ̃_{0} − ν ̃_{R} / ν ̃_{0} + ν ̃_{R},$ where h is the Planck constant, k is the Boltzmann constant, c is the light velocity, $ν ̃_{0}$ is the frequency of the excitation source, $ν ̃_{R}$ is the

Experimental

A coherent model Innova 90 P-3 argon ion laser operating in the TEM₀₀ mode at 488 nm is used as the light source for the levitation of single microdroplets and simultaneously for the excitation of the light-scattering spectra (see Fig. 4). An interference filter is used to supress the plasma lines of the argon ion laser. The beam is focused by a f = 65 mm lens, which results in a focal beam waist of about 25 μm. The microdroplets are generated by means of an ultrasonic nebulizer in a cubic

Results and discussion

The line shape of the OH stretching band is affected by temperature and concentration (Fig. 5, A). The relative wavenumber decreases linearly with decreasing temperature. The slope of the calibration line for a 57% sulfuric acid is 1.2 K cm⁻¹. The standard deviation of the regression curve as a measure for the accuracy has a value of ± 3 K. As a result of the exothermicity of the reaction HSO₄⁻+H₂O⇔SO₄²⁻+H₃O⁺, the SO₄²⁻ band gains and the HSO₄⁻ band loses intensity with decreasing temperature (

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Uncertainties in temperature measurements of optically levitated single aerosol particles by Raman spectroscopy

Abstract

Introduction

Section snippets

Theory

Experimental

Results and discussion

J. Mol. Structure

Phys. Rev.

Appl. Opt.

Appl. Spectrosc.

Appl. Spectrosc.