A Simple Dewar/Cryostat for Thermally Equilibrating Samples at Known Temperatures for Accurate Cryogenic Luminescence Measurements

The overall goal of this experiment is to measure temperature dependent luminescence spectral intensities. This method can help answer key questions on the topic of excited state energy migration in metal complexes. Such as, ruthenium diamine complexes.

The main advantage of this technique is that by using a simple apparatus and procedure, accurate temperature dependence spectral intensities can be measured. Demonstrating the procedure will be Cameron Portune, a technician from my laboratory. First, prepare about three millimeters of low molarity luminescent chromophore solution in water or in alcohol for the best combination of surface tension and solubility.

Next, prepare a sample loop. Twist a length of bare copper wire around a nail or screw to give a single loop, followed by about 30 millimeters of twisted wire. Then rinse the loop with nitric acid, followed by distilled water, and finally with 95%ethanol.

Be sure to put on protective gear for the next step. Dip the loop, once dry, into the sample solution. The surface tension of the solution should create a thin film of the solution in the loop.

Then, immediately dip the loaded sample loop into liquid nitrogen to freeze and stabilize the thin film sample solution. Begin with preparing a copper constantan, or Type T thermocouple from two lengths of copper wire and one length constantan wire which is a copper nickel alloy. Next, clamp the copper and contstantan wires together at a 90 degree angle.

Sauter or twist together two copper constantan junctions. One is for the sample and the other is for a standard reference. It really helps for two people do to this step.

Then, pull the two wires firmly and twist them together five or six times. The insulation ensures that the only contact made will be at the thermocouple junction. Next, connect the two copper thermocouple wires from the sample and reference junctions to the input terminals of a volt meter.

Then place both the sample and reference junctions in a zero degrees Celsius ice water bath and zero the volt meter. Next, the loaded sample loop and thermocouple junction are aligned in the liquid nitrogen-filled optical dewar. This requires a cork that fits snugly into the top of the optical dewar.

Prepare a standard grip to hold the loop using a clip, a dowel, and some plumber's tape. Then, align the sample loop of the thermocouple junction with the path of the excitation light beam. At least an hour in advance, turn on all the electronics on the CCD spectrograph.

Also, turn on the pal te cooled CCD camera to reach a stable operating temperature. When conducting the test, monitor the temperature on the volt meter while the nitrogen slowly boils off. The volt meter accurately reports the temperature because the sample is fully immersed either in liquid nitrogen or cold vapor.

Take spectra at every five Kelvin increment. To do this, momentarily turn on or unblock the excitation light and use the CCD spectrograph to acquire a luminescent spectrum. This takes just a few seconds.

Then, turn off or reblock the excitation light to minimize photochemical degradation or sample heating errors. The thermocouple voltage should not change appreciably from beginning to the end of the acquisition interval. For the analysis, convert the reported voltage to temperature.

Taking these measurements, make no changes to the optics, electronics, or excitation light intensity. Later, intensity correct the CCD spectral data sets. The complex tris-four-seven-dimethyl-one-tenth-ananthroleme rodium three was dissolved in oxygen-saturated glycerol and investigated using the described method at temperatures between 77 and 200 Kelvin.

The luminescence intensity remained essentially constant from 77 Kelvin to 175 Kelvin, then progressively diminished as the temperature increased from 175 Kelvin to 240 Kelvin. A plot was made of a parameter that represents the extent of quenching versus the reciprocal Kelvin temperature. An oxygen luminescence quenching activation energy of 31.5 kilojoules per mole was thus calculated.

The quenching activation energy for several other related complexes was also obtained by this method. The internal consistency of these results suggests that the sample temperature is known and constant while spectra are being acquired. This technique can be done in one to two hours if it is performed properly.

While attempting this procedure, it's important to remember that the sample and thermocouple cannot be moved once the experiment starts or intensity readings will not be consistent.