Laser experiments in undergraduate physical chemistry laboratory

Richard G. Laylon and Edward M. Eyring University of Utah Salt Lake City

Laser Experiments in Undergraduate Physical Chemistry Laboratory

Since they were first envisaged by Schawlow and Townes in 19.58 ( I ) , solid state optical masers (lasers) have become comparatively inexpensive so that their use is now practicable in an undergraduate physical chemistry laboratory to illustrate several fundamental chemical concepts. We have horrowed a laser from our Physics Department and have had a group of juniors in chemistry and chemical engineering perform two experiments that permit a wide range of instructive calculations. The necessary components of an improved laser are shown schematically in Figure 1. Three oil-filled 100 microfarad capacitors wired in parallel are charged to 3000 v by a dc power supply. Using a Tessla coil (vacuum leak detector) as a trigger, the capacitor bank is then discharged through five 150 watt xenon discharge tubes (EG R. G 1 X l 0 0 ) all of which are in contact with and parallel to a 3/s-in. diameter X 3'/4-in. long pink ruby rod, i.e., corundum (A1203), doped with Cr203to the extrnt of 0.05% by weight. An additional inductance ( 2 ) would have limited the peak current to

are placed at the two foci of a specularly reflecting cylindrical ellipse (3). The -100 microsecond (2) flash of "green" light from the xenon tubes "pumps" the hulk of the Cr+3ions out of the ground IA2 electronic state and into a band of excited electronic states such as the 'TIand "2 states (4). This is not the familiar spectroscopic notation of gaseous monatomic ions but rather that appropriate to the description of the states of an ion in a crystal field ( 5 ) . By either radiative or radiationless transitions many of the excited Cr+3ions drop back down to the metastable %Estate from which a radiative transition to the ground state is eomparatively improbable. Rubies have long been prized for the red fluorescence produced by the ?E UA, transition. If the pumping to excited states and subsequent reversion of a considerable number of Cr+a ions to the ZEstate has occurred rapidly enough, the population of the 2E state is greater than that of the ground &A2state and a "population inversion" is said to exist. This is a colorful way of saying that the Boltzmaml statistical distribution

-

h',/Ko XENON LAMP