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

Summary

A simple liquid nitrogen Dewar/cryostat apparatus comprised of a small fused silica optical Dewar, a thermocouple, and a charge-coupled device (CCD) spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, freezing, and alignment, accurate and stable sample temperatures, and small size/portability.

Abstract

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and a CCD spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, fast sample freezing, fast alignment of the sample, accurate and stable sample temperatures, and small size and portability of the Dewar/cryostat cryogenic unit. When coupled with the fast data acquisition rates of the CCD spectrograph, this Dewar/cryostat is capable of supporting cryogenic luminescence spectroscopic measurements on luminescent samples at a series of known, stable temperatures in the 77-300 K range. A temperature-dependent study of the oxygen quenching of luminescence in a rhodium(III) transition metal complex is presented as an example of the type of investigation possible with this Dewar/cryostat. In the context of this apparatus, a stable temperature for cryogenic spectroscopy means a luminescent sample that is thermally equilibrated with either liquid nitrogen or gaseous nitrogen at a known measureable temperature that does not vary (ΔT < 0.1 K) during the short time scale (~1-10 sec) of the spectroscopic measurement by the CCD. The Dewar/cryostat works by taking advantage of the positive thermal gradient dT/dh that develops above liquid nitrogen level in the Dewar where h is the height of the sample above the liquid nitrogen level. The slow evaporation of the liquid nitrogen results in a slow increase in h over several hours and a consequent slow increase in the sample temperature T over this time period. A quickly acquired luminescence spectrum effectively catches the sample at a constant, thermally equilibrated temperature.

Introduction

Within the cryogenic temperature domain, temperature-dependent investigations of the electronic luminescence spectra and luminescence lifetimes of light emitting molecules provide a wealth of information about the excited electronic states of these molecules and the photochemical and photophysical phenomena that arise from these states. The pioneering temperature-dependent photophysical investigations of Crosby and co-workers on ruthenium(II), rhodium(I), and rhodium(III) complexes of 1,10-phenanthroline, 2,2'-bipyridine, and other ligands illustrate well the inherent power of temperature-dependent spectroscopy to elucidate the structures, symmetries, energetics, and ....

Protocol

1. Sample Preparation and Loading for Cryogenic Spectroscopy

1. Prepare ~3 ml of a ~10^-3-10^-4 M solution of luminescent chromophore in an appropriate solvent.
   Note: While many solvents can be used, water and various alcohol solvents (e.g., ethanol, ethanol/methanol mixtures, ethylene glycol, and glycerol) provide an excellent combination of solubility and surface tension characteristics for cryogenic work.
2. Prepare a sample loop by twisting a length of bare copper wire ~0.4 mm in diameter around a nail or screw to give a single loop ~3 mm in diameter followed by an ~30 mm straight length of braided copper wire.

Results

Representative results obtained in the above described apparatus for a temperature-dependent luminescence quenching study in the 77-200 K region of the luminescent compound Tris(4,7-dimethyl-1,10-phenanthroline)rhodium(III), [Rh(4,7-Me₂-1,10-phen)₃]³⁺, dissolved in oxygen-saturated glycerol are listed in Table 1 and plotted in Figures 4, 5, and 7.

Discussion

The development of this apparatus for low temperature luminescence spectroscopy arose out of necessity. It was critical that solutions containing the chromophore of interest and also supersaturated with oxygen could be loaded, frozen, and positioned for spectroscopy all in an instant in a Dewar/cryostat design in which sample temperature was well defined, stable, and slowly changeable. Virtually all commercial cryostats take much more time to load with sample than these experimental constraints would allow. It was also i.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Diode laser 405 nm	Generic		Generic pencil-type laser pointer for luminescence excitation: 5 mW at 405 nm
Quartz optical dewar	Custom fabrication		3.5 cm id. x 25.0 cm length with 4.5 cm unsilvered region for optical access
Programmable 5 1/2 digit DMM	Keithley	Model 192	High impedence DMM for reading thermocouple voltages
Copper thermocouple wire	Omega Engineering	SPCP-010	0.010 in. diameter bare copper thermocouple wire
Constantan thermocouple wire	Omega Engineering	SPCC-010	0.010 in. diameter bare Constantan (copper/nickel) thermocouple wire
Polychromator/Spectrograph	Jarrell-Ash	82-415	0.25 m Ebert monochromator with back slit assembly removed to enable operation as a polychromator
CCD camera	Andor	DV-401-UV	Thermoelectrically cooled (-35 °C) CCD camera for detecting emitted light
Copper wire for sample loop	Generic		0.0150 in. diameter bare copper wire for sample loop