Raman Scattering Microscope

Jasmine MacKenzie

University of Minnesota - School of Physics and Astronomy

Minneapolis, MN 55455

Abstract

In this project we attempt to make a simple Raman spectrometer from non-specialized equipment. It was found that the spectrometer produces noise a factor of

times greater than the expected signal. Data was taken with a dedicated confocal Raman microscope for three minerals and their spectrums were found to all be correct within 1 sigma with the exception of two peaks in fluorapatite 1031.8

4.5 , off by 1.82 sigma and 1061.3 4.5 , off by 2.29 sigma) and one peak in calcite (159.58 4.5 , off by 1.24 sigma)

Goals

• • Build a simple Raman spectrometer.

  • If the above is successful, test that the intensity of the peaks are proportional to the amount of a certain atom present in a mineral.

Introduction

In many aspects of science and industry it's necessary to identify the chemical makeup of a sample without destroying or altering the sample at all, such as when identifying contaminates in a silicon chip. Raman spectroscopy takes advantage of the fact that when a photon is scattered inelastically the change in frequency is unique to each molecular bond. This not only allows scientists to identify the chemical composition in a substances but also to characterize the substance itself.

Only 1 in 10^{6}-10^{8} photons that scatter off of a molecule will scatter inelastically. The rest are scattered elastically which makes detecting the effects Raman scattering much more difficult in practice. Much of the difficulty in Raman spectroscopy has been in developing ways to effectively filter out as much of the Rayleigh light (photons scattered elastically) as possible and making detectors sensitive enough to detect the Raman shifted photons. The Rayleigh photons overwhelm the Raman signal, not only in the peak itself but in the noise it can generate. This is the primary challenge when assembling a Raman spectrometer from non-specialized equipment. Not only will filtering need to be done to remove the elastically scattered light but even the equipment itself can generate enough noise to overwhelm any potential signal.

A Raman spectrum is made by plotting the intensity of the shifted light to its corresponding shifted wavenumber. The location of the peaks functions as a unique fingerprint for the sample and allows one to identify the chemical make-up. But while this will provide clues about the bonds present, it doesn't say how many are there. The relative intensity of the peaks is roughly proportional to the relative number of bonds. This allows more quantified analysis beyond the identification of atoms in an sample. By taking the spectrum of a known sample, identifying the peaks and their bonds then comparing the intensities we hope to confirm that this is a viable way to find the chemical formula.

Theory

When a photon comes into contact with an atom it can interact in one of two ways: absorption or scattering. Absorption involves exciting an electron to a higher energy level. Because electronic states are quantized, this requires that the incoming photon be of a specific frequency. Scattering, on the other hand, deals with vibrational states. When the photon is scattered by the bond it is elevated to what are called virtual states, a term for non-quantized states. In the case of elastic scattering, molecule will return to its original energy state with no shift in frequency. For Raman scattering, better known as inelastic scattering, the molecular bond will be in a new vibrational mode, resulting in a change in frequency. This shift in frequency is unique to each molecular bond and the resulting spectrum of Raman shifts is essentially a fingerprint for that bond.

At room temperature, a photon will usually scatter off a molecular bond and excite it. The scattered photon will then have a lower energy than it originated with. This is called Stokes scattering. Light can also scatter off of a vibrationally excited molecule. The photon will then have gained energy. Appropriately this is called anti-Stokes scattering. Figure 1 provides a visual diagram of the changes in energy a molecule will exhibit for the different types of scattering.