Correlating Excitation and Emission Wavelengths Yields Analysis of Complex Chemical Environments

Lynn M. Savage

Spectroscopically scrutinizing chemicals within complex environments — such as inside cometary matter or combustible materials — is challenging because many molecules absorb and emit at similar wavelengths, confounding spectral analysis. Laser-induced fluorescence spectroscopy is frequently used because of its high sensitivity and resolution, but it is limited in that it provides only a single dimension of analysis, based on either excitation or emission spectra.

Now researchers at the University of Sydney in Australia, led by Timothy W. Schmidt and Scott H. Kable, have developed a technique that acquires excitation and emission spectra simultaneously, providing a two-dimensional spectrum from which common elements can be intuitively identified and unknown materials more readily reported.

To test their method, the scientists measured the output from a stream of cooled argon gas infused with 0.1 percent benzene, igniting the gas with pulsed electrical discharges that fragmented the benzene and that effected a series of chemical recombination events. They collected the emissions from the gas with a 308-nm excimer laser pumping a dye laser tuned at 0.05-nm intervals between 445 and 480 nm. Both lasers were made by Lambda-Physik AG (now part of Coherent Inc. of Santa Clara, Calif.).

Kable said that they worked with benzene because of its significance to their investigations into molecules of astrophysical importance. “Benzene is a good source of carbon and hydrogen and potentially a good precursor for polycyclic aromatic hydrocarbons, which are believed, but not proven, to be present in the interstellar medium.”

The collected fluorescence emissions were imaged onto the slits of a Princeton Instruments spectrograph, dispersed across the device’s CCD array as a series of images of the entrance slit. The 2-D array itself was then imaged by an intensified CCD camera, also made by Princeton Instruments, which summed the slit images at each step across the laser’s tunable range.

The summed images provided a 2-D spectrum of excitation versus emission wavelengths (Figure 1). Each pixel in the graph represents the sum of intensities of each CCD element multiplied by the 60 rows in the CCD array and multiplied again by the number of laser pulses used — in this case, 200. Every individual feature in the gas is representable as a coordinate on the chart.

Figure 1. Acquiring spectra from both excitation and emission wavelengths has enabled researchers to visualize the components of a benzene-argon discharge. C₂ bands appear as cometlike heads and tails, while C₃ bands appear as a series of tight spots ~2 nm apart on the emission wavelength axis. Each component in the discharge can be represented as a coordinate on the array. Reproduced with permission of The Journal of Physical Chemistry A Letters.

The researchers found that the fluorescence generated at the C₂ and C₃ transitions (known as the Swan and comet bands, respectively) were clearly distinguishable within the benzene-argon mixture. They also discovered that, although the majority of the 2-D fluorescence structure was identifiable as either C₂ or C₃, there were weaker structures that correlated with neither carbon form.

Using a scanning monochromator made by Spex Instruments (now part of Horiba Jobin Yvon in Madison, N.J.), they investigated one such structure at the (476, 476) coordinate. They laid the spectrum acquired with the scanning technique over the 2-D array, revealing a molecule that appeared to be a polyatomic hydrocarbon with several active vibrational modes.

They are working to further identify the mystery molecule, along with other unknown spectral features on the benzene-argon map, and are trying to optimize the technique by improving the data-analysis process.

The Journal of Physical Chemistry A Letters, Nov. 16, 2006, pp. 12355-12359.