My time in Dr. Arnold’s physical chemistry lab has been spent exploring Raman Spectroscopy under the supervision of PhD. student Christopher Cooper. This characterization technique involves bombarding a sample—contained in a cuvette – with laser light in an effort to observe the Raman effect. When a sample is subjected to high intensity light, the energy associated with the light may be transferred to a molecule. In scattering, when this transfer of energy occurs, the molecule is excited from a ground state to a virtual state. Transitions to virtual states are short-lived and the molecule quickly releases the energy by emitting photons. In instances where the molecule returns to its ground state, the released photons are at the same frequency, v0, as the incident radiation. This phenomenon is known as Rayleigh scatter. A second possibility is for the molecule to return to a vibrationally excited state and the emitted photon has less energy. In this case the molecule undergoes a vibrational transition and the decrease (or increase) in photon energy reflects this change in vibrational state. A detector is used to collect the photons and determine their frequency. From this information, a Raman spectrum can be generated. Raman spectra report the Raman shift, (), on the x-axis and the intensity of the transition on the y-axis.
REU student Michael Matrona (right, front) with graduate student mentor Christopher Cooper (middle, back) and faculty mentor Chemistry and Biochemistry Professor Brad Arnold in the Arnold laser spectroscopy laboratory.
Raman spectroscopy shows potential as a standoff detection technique. However, detecting a sample’s Raman signal is not an easy matter since the Raman effect is a relatively weak phenomenon. Furthermore, the Raman effect is in competition with fluorescence; in fact, the Raman signal is on the order of a billion times less intense than a fluorescent signal. Certain measures must be taken to mitigate this difference in intensities. We have employed 213-nm light when developing our Raman spectra since UV radiation scatters 40 to 50 times more efficiently than visible light (which Raman spectroscopy is typically conducted with). Ultraviolet Raman spectroscopy also boasts the potential for resonance enhancement, which may prove to intensify the Raman signal enough for it to become a viable in-field technique.
Below are generated Raman spectra for cyclohexane taken at distances of 3m and 4m. We have found that atmospheric constituents begin to appear in the Raman spectra acquired at these distances. Take note of the O2 band at approximately 1555 cm-1 and the N2 band observed at approximately 2331 cm-1. Also note the relative intensity of these bands increases with distance from the collection optic.
Figure 1– Raman spectra of cyclohexane taken with the sample 3m from the collection optic.
Figure 2 – Raman spectra of cyclohexane taken with the sample 4m from the collection optic.
Raman spectroscopy conducted at a distance boasts promise for in-field applications as a standoff detection technique. For instance, if this technique were perfected, detection of hazardous materials could occur at a safe distance from the hazard site. Institutions such as the military and waste management facilities would be benefited because the technique would limit human exposure to explosives and unidentified hazardous materials.