Many conventional spectrometric techniques have been effectively used in the detection of solids, liquids, and gases. However, newer laser spectroscopy techniques have emerged as the tools of choice in many distinct scientific applications.
Technological advances, in conjunction with the decreased cost and complexity of laser devices, make laser spectroscopy more accessible and easier to use. As a result, laser spectroscopy is increasingly being used in tracking air quality; process control; medical research, national security; agriculture and artwork authentication.
Three of the most common uses of lasers in spectroscopy are Raman spectroscopy, laser-induced fluorescence spectroscopy, and cavity ring-down spectroscopy.
Named for the Indian scientist who created it, Raman spectroscopy is a category of methods that involve determining the dispersion of monochromatic laser light caused by a sample. Raman spectroscopy is one of many spectroscopic solutions used to gain insight into the molecular vibrations and crystal structures of samples.
While almost all of the light reflecting off a sample is the identical wavelength as the incoming light, a small amount of light does reflect at different wavelengths. This occurs because the laser beam interacts with phonons, natural vibrations contained in the molecules of most solids and liquids. These oscillations force the photons from the laser to accumulate or lose energy. The detected change in energy can provide data on the phonon modes in the system and by extension, about the molecules contained in the sample.
In the typical Raman test, a laser light source is used to irradiate a sample, which generates a minuscule amount of 'Raman scattered' light. This scattered light is sensed using a CCD camera. The distinctive pattern in a Raman spectrum allows for the recognition of substances, the assessment of regional crystallinity, determination of orientation and identification of stress in a sample.
Raman spectroscopy has some distinct advantages over other techniques, including being a non-contact test, having the spatial resolution to sub-micron scale, being capable of evaluating transparent samples in gas, liquid, solution, solid, crystal and emulsion forms. Raman spectroscopy also has the benefit of not needing any sample preparation.
Fluorescence is visible radiation released by particular materials after receiving incident radiation at a shorter wavelength of light.
In laser-induced fluorescence, a sample is irradiated with a laser, pushing sample's electrons an excited state with higher energy levels. This excitation sustains for just a handful of nanoseconds before the electrons come back down to their ground state. As they shed energy, these electrons release light, at a wavelength longer than the wavelength of the laser light. Since the energy states are distinctive for each atom, fluorescence emissions are distinct and can be used for detection. The typical system used to conduct this test includes a nitrogen laser, a sensor, and a spectrometer. These instruments can all be placed into a small, easily-transportable system.
Laser-induced fluorescence is a commonly used tool with many applications in industry and research. For example, it has been used to protect consumers from fruits and vegetable that have been tainted by pesticides.
Cavity Ring-Down Spectroscopy
Cavity ring-down spectroscopy is commonly used to measure free radicals in the atmosphere, particularly those with little to no fluorescence quantum yield that cannot be identified via laser-induced fluorescence spectroscopy.
In CRDS, the exponential loss (“ring down”) of laser light radiated into a highly-calibrated optical cavity is assessed to detect various gas species. The decay of laser light will occur in a vacuum at a known rate, and when a gas that absorbs the laser light is put in the cavity, a second source of loss speeds up the ring down time. The composition of a gas sample can then be determined based on the ring down speed.