| Several thermo-optical spectroscopic methodologies have been developed based upon the non-radiative relaxation process of excited molecules. These techniques are generically classified as "Photothermal Spectroscopy". When solids are analyzed, the thermal energy produced causes a change in the density within the medium as well as a structural change in the surface geometry.;Development of an analytical sensor utilizing this thermo-optical approach has been accomplished by adhering polymer films (cellulose) to glass microscope slides. These films were chemically modified to allow covalent attachment of a pH indicator dye (phenol red). The sensor was placed adjacent to a solution at some known pH. A modulated Argon Ion laser (514 nm) is used to excite the polymer surface. A Helium Neon Laser (632 nm, no absorbance within the polymer) was employed to optically sense the change in the polymer structure due to thermal de-excitation.;During the analysis, both laser beams were vertically polarized and collinearly aligned. The beams were directed normal to the sensor by reflection from a polarizing cube. The light first struck the sensor at the glass/air interface and then at the polymer/solution interface. A quarter wave plate was used to spherically polarize the beams before the light impinge the sensor. The light, reflected from both interfaces, passed back through the quarter wave plate. It was then horizontally polarized and transmitted through the polarizing cube. |
