Focal points: Two-photon effect with laser light causes a specific chemical reaction for first time

For the first time the two-photon effect achieved with light from a laser has been used to cause a specific chemical reaction. At the Bell Telephone Laboratories, Y.H. Pao and P.M. Rentzepis irradiated a sample of distilled styrene monomer with light from a pulsed ruby laser and found that they had created polystyrene. The simultaneous In the experiments, freshly distilled styrene monomer was subjected to a succession of some 20 laser pulses. The sample was kept at liquid nitrogen temperature to stabilize free radicals released during the two-photon process. Following the irradiation, the sample was warmed to room temperature and the polymer was precipitated. An infrared spectrum analysis showed that Yo-Han Pao (left) and Peter Rentzepis of Bell Labs are shown irradiating a sample of distilled monomer styrene (in the flask) with light from a pulsed ruby laser (not shown). Simultaneous absorption of two photons by the monomer causes polymerization. absorption of two photons by the monomer molecule was found to have caused this polymerization. The success of these experiments may open a potentially vast area of molecular phenomena to exploration with the laser's highly intense and monochromatic beam of light. Pao and Rentzepis believe they have demonstrated a general phenomenon and believe that many other multiphoton photochemical reactions may be induced and studied. the precipitate was identical to that of the known polymer. Monochromatic light from a ruby laser occurs at a wavelength of 6940 A. Photons at this wavelength have an energy equivalent of 1.8 eV. Styrene monomer, however, cannot absorb such light, and-even if it could-this energy would not be sufficient to induce a chemical reaction. However, in the interaction of the intense laser beam with the molecular system, there is a process by which two photons are absorbed almost simultaneously to excite the monomer by 3.6 eV. This, in turn, causes the formation of free radicals and induces polymerization.