Chemical Applications of Lasers.

The principle advantages of laser light over ordinary light is that it is monochromatic (very narrow range of wavelengths in the laser beam), it is coherent (all photons in the same phase), it has high power and it can be collimated over long path lengths. A laser can be pulsed, i.e., short bursts of laser light can be generated for durations of pico seconds and femtoseconds and this feature has enabled chemists to probe processes at very very short time scales. The high power and minute focusing has made lasers a powerful tool in surgery as well as physiotherapy. Applications of lasers in spectroscopy and chemical reaction dynamics are wide spread (we have already considered some aspects earlier). We will first consider a practical application in isotope separation.

Species that have the same molecular structure but differ only in their isotopic composition are called isotopomers. Because of their differing mass, their vibrational energy levels are different. If a mixture of two isotopomers is ionized directly, both members get ionized and it is difficult to separate either one of them by deflecting them by electromagnetic fields. However, if we first excite one of the two species selectively by a tunable laser and then ionize the excited state of this isotopomer, the ions of this isotopomer can easily be separated from the other unionized isotopomer by electric fields. Thus, the separation is done in two steps by two laser beams. The example of the separation of ²³⁵U from ²³⁸U is shown in Figure 31.7. The separation of ³²SF₆ and ³⁴SF₆ by selective vibrational excitation of one of them by IR laser photons and then dissociating this with another UV photon is shown in Figure 31.8.