Fourier Transform Infrared Spectrometry Coupled with Cryogenic Trapping for the Study of Laser-Desorbed Species

The feasibility of using cryogenic trapping techniques combined with infrared spectrometry to study the products of laser desorption is demonstrated. A prototype system for cryogenically trapping transient species desorbed during the laser desorption process is described. The products produced by a high-power pulsed carbon dioxide laser (10.6 μm) are cryogenically trapped on an infrared transmitting substrate attached directly to a closed-cycle helium refrigerated coldhead via an OFHC copper holder whose temperature is maintained at approximately 18 K. The sample is then rotated into the beam of a Fourier transform infrared (FT-IR) spectrometer, and conventional transmission spectra are obtained. System performance is described, and representative results obtained for a sample of rhodamine B dye and a polyethylene glycol polymer are presented. The rhodamine B dye is shown to significantly fragment as a result of laser desorption (consistent with the known behavior of the ionic products studied by laser desorption mass spectrometry), while the polyethylene glycol sample produces almost exclusively intact neutral species (also in agreement with laser desorption mass spectrometry results).