Diode laser probe of CO2 vibrational excitation produced by collisions with hot deuterium atoms from the 193 nm excimer laser photolysis D2S

Chemical Cartography: Finding the Keys to the Kinetic Labyrinth

Very high resolution lasers allow spectroscopic pictures to be taken following a collision between two molecular reactants. The features of these "pictures" are the electronic, vibrational, rotational, and translational motions of the atomic particles, which relate the quantum states of the reactants to the quantum states of the products. Such state-to-state kinetic information can be used to test the shape and nature of the interaction potential that controls the collision process. The potential itself is akin to a map of the terrain through mountains and valleys where elevation is a measure of energy instead of height. Accurate mapping of this potential surface leads to an understanding of the forces which control rates and mechanisms of chemical reactions. The application of four different advanced laser techniques to the study of collisions between "hot" hydrogen(H) atoms and carbon dioxide(CO(2)) molecules has provided a wealth of information about both reactive and nonreactive collisions for this system. The availability of data for rotationally, vibrationally, and translationally inelastic excitation of CO(2) by H atoms, when compared with data for reactive events producing OH + CO, provides insights into the dynamics of collisions between H and CO(2), and illustrates the future promise of these powerful techniques for elucidating features of potential energy surfaces.

High sensitivity detection of transient infrared absorption using tunable diode lasers

A novel technique is described for monitoring transient infrared absorptions with a tunable diode laser (TDL). The technique involves modulating the TDL at frequencies as high as 250 kHz and using both analog and digital subtraction of background signals. Transient absorptions as small as 0.001% can be detected with a response time of a few microseconds. The technique provides excellent discrimination between transient absorption lines and spurious effects such as beam deflection. Transient detection of the CF(2) radical in a photolysis cell is used to illustrate the technique, and a comparison is made with more conventional TDL detection methods. The modulation technique is clearly a very powerful tool for transient infrared spectroscopy.