Cryostats, matrix isolation

If one intends to use Ne or H2 as a host gas, one needs to take recourse to cryostats that are able to attain temperatures of 4 K or less. Until recently, ow cryostats, where liquid He is guided in a controlled fashion to the cold end, were usually required for such experiments. However, two-stage 4 K closed-cycle cryostats have recently become available that appear to obviate the need for liquid He. ° The author knows of two prominent matrix-isolation research groups who have been using such devices for a little while now and have no complaints about them. 

Two providers currently offer closed cycle cryostats for matrix isolation with 0.5-1 W of cooling power at 4 K. The first is Janis Reserach (http //www.janis.com/p-a4k.html), which uses hardware from Sumitomo, the other is Cryomech (http //www.cryomech. com/cryostats.html). 

The technique of matrix isolation has been shown to produce highly characteristic spectra of individual components of complex samples combination of MI spectroscopy with separation techniques promises to increase further the analytical capabilities of the technique. While our research to date has emphasized FTIR and molecular fluorescence spectrometry, MI as a sampling procedure is not limited to these two forms of spectrometry. For example, some interesting preliminary analytical results by MI Raman spectrometry recently have been described (32). It should also be stressed that the cryogenic procedures required for the vast majority of MI spectral studies are neither difficult nor unduly expensive except in very special cases, closed-cycle cryostats requiring no cryogenic liquids (and no prior experience in low-temperature techniques) are entirely satisfactory for MI... 

Figure 4.4-1 Basic composition of an apparatus for matrix-isolation experiments a) Rotatable cryostat with gas-handling system, b) Sectional view in the level of the matrix support, (1) matrix support, (2) refrigerator, 4-40 K, (3) radiation shield, 77 K, (4) vacuum shroud, (5) infrared window, X KBr, y PE, z quartz glass, (6) spray-on nozzle, (7) synthetic device, e.g., Knudsen cell, (8) turbomolecular pump, p < 10 mbar, (9) to backing pump, (10) transfer line, quartz or stainless steel capillary, (11) needle valve, (12) inert gas inlet, Ne, Ar, N2,..., (13) bulb for gas mixtures, (14) capacity manometer, (15) sample, (16) to high-vacuum system.

Gas Introduction and Matrix Formation. For introduction of gases for condensation and formation of matrices in the cryostat system, we employed two types of deposition technique SSO (Slow Spray-On) and PMI (Pulsed Matrix Isolation). In the SSO run, matrix gas (pure nitrogen, or argon) and sample were introduced slowly and separately into the setup via fine needle valves with micrometers. In the PMI (Pulsed Matrix Isolation) run, a mixture of matrix gas and sample(s) was introduced via electromagnetic valves controlled by a micro-computer. In PMI runs, not only was the deposition rate easily controlled over a wide range with good reproducibility, but a stratified matrix could also be prepared if two kinds of gas samples are introduced alternately and repeatedly. 

For matrix-isolation studies there is a minimum of necessary equipment. The essentials include a) a refrigeration system (cryostat), b) a sample holder, c) a vacuum chamber (shroud) to enclose the sample, d) means of measuring and controlling the sample temperature, e) a vacuum-pumping system, f) a gashandling system, g) devices for generating the species of interest, h) spectrometers for analysis of the matrices. 

For infrared spectroscopic measurements at temperatures lower than 77 K, liquid helium is needed as the coolant instead of liquid nitrogen. The lowest temperature attainable is about 4 K. For matrix-isolation spectroscopy, a cryostat combined with a helium compressor for circulating cooled helium is commonly used. The lowest temperature attainable with such a system is about 15K, which enables the use of argon, krypton, and xenon, as well as nitrogen for matrix materials. 

Cryostat. A cryostat designed for MBssbauer measurements of species isolated in a low temperature matrix was cooled down by a Cryomini D closed-cycle helium refrigerator (Osaka Sanso Kogyo Co. Ltd.) (Fig. 5). The sample holder in the cryostat was maintained at a desired temperature between 15 and 100 K by means of a 40-flanganin heater and a DTC-2 digital temperature controller... 

---