23.8Laser spectroscopy

A laser 50 is a light source emitting waves of light that not only share the exact same frequency (color), but are also in-phase with each other. A beam of light consisting of just one frequency (color) is called monochromatic. A beam of light consisting of waves in perfect synchronization with each other is called coherent. Laser light is both monochromatic and coherent51. Modern solid-state lasers are constructed similarly to light-emitting diodes (LEDs), emitting light when energized with direct current electricity.

Certain types of solid-state lasers are capable of emitting light over a limited range of frequencies. That is to say, the light emitted by one of these laser types will always be of a consistent color at any given moment in time, but that color may be made to change over time. This modern class of solid-state lasers is therefore useful as light sources for absorption spectroscopy because the color of the emitted light may be adjusted to align with the absorption frequencies of certain chemical compounds.

As mentioned in the “Introduction to optical analyses” section, some molecules exhibit optical absorption over a band of frequencies (colors), and these absorptive bands are actually comprised of many fine lines clustered together on a spectrum diagram. Each line within an absorption band represents a particular resonant mode which a molecule dissipates incident light energy in the form of heat, and these resonant modes may be highly specific to the molecular species.

The following absorption spectrum illustrates this concept. Magnifying one of the bands found in this absorption spectrum reveals a closely-spaced series of individual absorption lines:

Each absorption band is comprised of sharply-defined lines

Wavenumber (cm-1)

50The term “laser” is actually an acronym, standing for Light Amplification by Stimulated Emission of Radiation.

51It is this coherence of laser light that enables the beam to remain highly focused, unlike light from other sources which tends to spread.

One of the challenges of absorptive spectroscopy is the problem of distinguishing one molecular species from another when their respective absorption bands happen to overlap. Laser spectroscopy avoids this problem by focusing on just one absorption line at a time, rather than attempting to measure light absorption over a broad spectrum. Even if multiple compounds have overlapping bands, a great many of the individual lines comprising those bands will be unique to each compound. A diode laser “tuned” to a single absorption line is therefore able to distinguish one compound from another even if other lines within the absorption band are shared by interfering species.

The frequency range of a laser diode is quite narrow, and so any spectroscopic analyzer employing this technology requires a laser light source custom-tuned for the species of interest. If multiple species having widely di ering absorption spectra are to be measured, multiple laser sources will be required (each laser tuned to the very narrow range associated with each species’ absorption lines). One design of multi-laser gas analyzer is shown here:

Multi-source laser spectroscopy gas analyzer

Beam splitter

In order for the one detector to discriminate between light received from each laser source, the sources must be sequentially energized. A computer controlling the laser sources analyzes the detector’s signal and correlates that against the known frequency emitted by each laser source at any given time in order to determine gas concentration. Like all gas analyzers, this design must be calibrated against known concentrations of gas (i.e. “zero” and “span” calibration gases) in order to establish “baseline” light intensities needed for accurate calculations of gas quantities.