Ivan E. Den Besten
ond James W. Tracy Bowling Green Stote University Bowling Green, Ohio 43403
II
flectrodelessly Discharged Photochemical Lamps
Very many chemical processes are carried out by the ultraviolet irradiation of reactants. A photochemist who irradiates a sample has need for a light source, of sufficient output, providing radiation of wavelengths absorbed by the sample being irradiated. The most frequently used light source is a low-pressure mercury resonance lamp, of either the cold cathode design or the heated cathode design. - . .orovidine useful output a t 1849 and 2537 A. We describe here an electrodelkss light source; this source is especially useful and convenient for small-scale laboratory photolysis of organic compounds in solution and for irradiation of gases at pressures greater than about 20 ton. The technique is also readily adaptable t o reduced temperature photolysis; additionally one does not need to use quartz photolysis vessels. The usual types of mercury vapor lamps require electrode connections directly to the lamp, and it is not possible to place the light source directly within the solution or gas mixture which is to be irradiated. This alternative method uses a radio-frequency or microwave transmitter, appropriately coupled to an antenna, to achieve excitation of the light source. The apparatus is illustrated in the figure. The light source (E) consists of a quartz tube containing a droplet of mercury, and filled with 5-10 torr of helium or neon gas. The lamp is surrounded by the sample and cooling jacket (D),all of which are contained within an antenna coil (A). This antenna, made from Y4-in. copper tubing, typically is a 4-in. diameter, 10-turn coil. A variable capacitor (C) is used for tuning and matches the impedance of the antenna with that of the transmitter. The antenna is driven by a radio-frequency transmitter (B). We have used a Heathkit DX-GOB transmitter at
Calvert, J. G. and Pitts, J. N., "Photochemistry," John Wiley and Sans, Inc., New York, 1966, p 686-695. 2Moore, W. M., Hammond, G. S., and Foss, R., J. Arner. Chem. Soe., 83,2789 (1961).
13.56 MHz with a maximum output of 100 W. A commercial diathermy unit, operated at typically 2450 MHz could also be used. The quantum output of the lamp may be varied by changing the power output of the transmitter, then retuning the transmitter and the antenna (with CI). The lamp intensity may also be controlled by using a dilute ionic solution circulating through the cooling jacket. For maximum lamp output, it is necessary to use a nonconducting solution as the coolant; a distilled water-ethylene glycol mixture is satisfactory. We find that the lamp output decreases exponentially with increasing ionic strength of the coolant. In this arrangement, the lamp is contained directly within the sample or solution being irradiated; consequently the full quantum output of the lamp is utilized. Considerable heat is given off by the discharged mercury lamp; therefore i t is necessary to use a cooling jacket around the sample. We have easily maintained a sample temperature of -20°C by circulating an ethylene glycol-water coolant, while maintaining a useful output of 2537 A mercury resonance radiation. Below this temperature the lamp output a t 2537 A rapidly decreases, and the system becomes impractical for routine photolysis. The lamp output is probably most conveniently determined by using the henzophenonehepzyhydrol chemical actinometer developed by Apparatus for elactrodeless Hammond and co-workphotochemical irradiation. A, aners.2 Actinometers using tenna: 8 , transmitter; C,. ionic solutions, of course, CaPaCilOr: C2, variable cap: D. jacketed flask; E, lamp. cannot he used.
Volume 50, Number 4, April 7973 / 303
