Since the early 1800's, the makers of fireworks have been adding metal salts to gunpowder to produce awe-inspiring colors. Intense heat from gunpowder's pyrotechnic flame energizes electrons in the metal atoms and metal-containing molecules.    When these electrons fall back to their ground-state orbital, they emit colored light at characteristic wavelength's: yellow-orange from sodium, red from strontium, yellow-green from barium, and blue-green from copper.

    Today, scientists are using the same physical process for another purpose. Bill Flower, of Sandia National Laboratories in Livermore, California, is leading an effort to measure light from metal pollutants and identify them as they escape into the air. Called laser spark spectroscopy, the technique uses a focused, high-energy laser beam (instead of the pyrotechnic flame from ignited gunpowder) to excite the metals, causing their electrons to emit light at characteristic wavelengths as they "relax" and return to ground state.

    Light spectra of metals energized by gunpowder and laser beams are produced by the same principle; there is, however, a difference. Instead of fireworks' yellows, reds, greens, and blues, the laser- excited light consists of a large amount of light at wavelengths shorter than those of blue light. It's called ultraviolet (beyond violet) light, which humans cannot see.

    Whereas visible light from a fireworks display pleases the eye, ultraviolet light activated by a laser provides extremely accurate, timely information about metal content. Because each metal emits unique wavelengths of light, laser spark spectroscopy instantaneously provides a "fingerprint" that distinguishes each metal present, and the intensity of the light can even indicate metal concentration.

    A modern incinerator does a lot more than just burn trash. It carefully controls air flow to burn the trash completely. The heat is not wasted but is used to make steam that drives a turbine that generates electricity. Unfortunately, toxic metals escape out the incinerator's smokestack into the atmosphere. Monitoring these metals is a challenge. "Today, metal emission monitoring relies on manual collection of samples followed by chemical analysis. This means you're getting only a snapshot of what is happening when the sample is being taken," says Flower. "It is difficult to determine how much of the metal escapes measurement by adhering to the walls of the test chamber. Even if a sample is truly representative, laboratory analysis typically takes two to four weeks."

    In contrast, laser fingerprinting allows accurate, continuous, monitoring of metals while they are passing through the emission stacks of power plant incinerators. Continuous monitoring means that fluctuations in metal emissions can be detected and corrected as they occur, instead of two to four weeks after the fact.

    "Laser spark spectroscopy's continuous, accurate feedback ultimately will allow operators of incinerators to save money by better using their equipment to comply with air pollution regulation," notes Flower. "This approach will also allow operators to assure neighbors that they are remaining in compliance each day of the year."

    More than a century ago, fireworks makers discovered that metal salts make beautiful colors. Today, Bill Flower and his co-workers have shown that light emissions from energized metal to make for more than a pretty picture.

Figure 1. The working end of the laser spark spectroscopy monitor. The monitor is mounted on an industrial exhaust stack where it fires a high-power laser into the exhaust gases. The laser is focused in a region half as wide as a human hair where it heats the gas into a plasma, which emits light of certain wavelengths. The light is collected and focused onto an optical fiber that leads to a spectrometer, which identifies each metal by its wavelength. Every half-second the laser fires, the gases spark, and the spectrometer measures the metal contaminants.



 

Laser versus Gunpowder. Compared to fire works, a lot more energy is concentrated in the laser pulse. The temperature of a typical pyrotechnic flame produced by burning gunpowder ranges between 1,500 and 2,000 °C. The laser pulse creates temperature five or six times hotter. The higher energy level is the reason why laser spark spectroscopy produces ultraviolet instead of only visible light.

    "Light emissions from metals in fireworks come from metal atoms or metal-containing molecules in the vapor state whose electrons are thermally excited to higher energy states," says chemist John Conkling, executive director of the American Pyrotechnics Association. "When the electrons return to the ground state, protons of light are emitted. The difference in energy between the ground state electrons and the thermally excited electrons are low enough, however, for visible light to be emitted and perceived by the audience as beautiful colors."



    "With laser spectroscopy, the higher energy laser beam dissociates the metal-containing molecules and particles into a plasma of atoms and ions," notes Sandia Lab's Bill Flower. "In this case the difference in energy between the excited electrons is 50 great that higher energy ultraviolet light predominates." .