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FM Spectroscopy of Metal Radicals

High resolution spectroscopy of small, metal containing molecules in the gas phase is one means of obtaining precise experimental measurements of chemical bonding and electronic structure in transition metals. These can provide valuable calibration data for increasingly sophisticated quantum chemistry calculations. The transition metals play an important role in catalysis, and the potential for quantum chemistry to advance our understanding is only beginning.

Preliminary experiments at BNL in collaboration with Prof. Tim Steimle of Arizona State University have demonstrated the utility of transient FM spectroscopy for detecting small metal- containing molecules made in a laser ablation/chemical reaction source.

Ablation source A pulsed UV laser is focused on a metal rod, creating an ablation plume of metal atoms. A pulsed flow of gas reacts with the metal atoms, quenches the reaction, and cools the reactive intermediates to cryogenic temperatures in a supersonic expansion. In this example, CH4 reacts with Pt to make PtC, which is detected by FM absorption spectroscopy.

Normally absorption spectroscopy is considered a low-sensitivity technique, and in this case, only small amounts of PtC are produced, and the pathlength for observation is only a few mm. With FM spectroscopy, the sensitivity is high enough to see good spectra with minimal effort.

A 2D FM spectrum of PtC illustrates the velocity structure of the expanding shell of PtC through the probe beam. This figure shows one rotational line, split into four components, one for each of the major Pt isotopes. Red and blue colors indicate positive and negative mixer output. The "v" -shaped structures reveal the increasing Doppler shifts of the PtC plume as it passes the probe laser. Transforming the FM spectra to the underlying absorption spectra (approximately an integral transformation) shows this time-dependent splitting of each absorption line.

The first sample arriving at the probe beam has been travelling with minimum transverse velocity, and has a narrow, unsplit Doppler line shape. The central, downward-moving sample leaves the probe beam and is replaced by sample with larger transverse velocity components. The lines broaden and develop minima in the center.

FM spectroscopy holds promise for the measurement of high resolution spectra of larger metal-containing radicals, many of which will be difficult to detect by LIF, as the fluorescence quantum yields are likely to be smaller. Simple modification of the source to a short slit jet should further enhance the sensitivity and resolution of the transient FM measurements of similar molecules.

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Rev: 29 Sep 99