Name: a simple dewar/cryostat for thermally equilibrating samples at known temperatures for accurate cryogenic luminescence measurements
Uploaded: 2016-07-19T15:00:00
Description: Watch this Scientific Journal Video about A Simple Dewar/Cryostat for Thermally Equilibrating Samples at Known Temperatures for Accurate Cryogenic Luminescence Measurements at JoVE.com

It is a pleasure to acknowledge the Office of the Dean of the School of Arts and Sciences and the Office of the Provost at Concordia University for support of this research.&#160;&#160;The authors wish to thank G. A. Crosby for his many contributions to this investigation.

1. Sample Preparation and Loading for Cryogenic Spectroscopy
<ol>
	<li>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.</li>
	<li>Prepare a sample loop by twisting a length of bare copper w...

<ol>
	<li>Baker, D. C., Crosby, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectroscopic+and+magnetic+evidence+for+multiple-state+emission+from+tris(2,2'-bipyridine)ruthenium(II)+sulfate.">Spectroscopic and magnetic evidence for multiple-state emission from tris(2,2'-bipyridine)ruthenium(II) sulfate.</a> Chem. Phys. 4 (3), 428-433 (1974).</li><li>Crosby, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectroscopic+investigations+of+excited+states+of+transition+metal+complexes.">Spectroscopic investigations of excited states of transition metal complexes.</a> Acc. Chem. Res. 8 (7), 231-238 (1975).</li><li>Hager, G. D., Watts, R. J., Crosby, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Charge+transfer+excited+states+of+ruthenium(II)+complexes+II.+Relationships+of+level+parameters+to+molecular+structure.">Charge transfer excited states of ruthenium(II) complexes II. Relationships of level parameters to molecular structure.</a> J. Am. Chem. Soc. 97 (24), 7037-7042 (1975).</li><li>Elfring, W. H., Crosby, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Excited+states+of+mixed-ligand+chelates+of+ruthenium(II)+and+rhodium(III).">Excited states of mixed-ligand chelates of ruthenium(II) and rhodium(III).</a> J. Phys. Chem. 80 (20), 2206-2211 (1976).</li><li>Elfring, W. H., Crosby, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Excited+states+of+mixed-ligand+chelates+of+ruthenium(II).+Quantum+yield+and+decay+time+measurements.">Excited states of mixed-ligand chelates of ruthenium(II). Quantum yield and decay time measurements.</a> J. Am. Chem. Soc. 103 (10), 2683-2687 (1976).</li><li>Fordyce, W. A., Rau, H., Stone, M. L., Crosby, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiple+state+emission+from+rhodium(I)+and+iridium(I)+complexes.">Multiple state emission from rhodium(I) and iridium(I) complexes.</a> Chem. Phys. Lett. 77 (2), 405-408 (1981).</li><li>Pankuch, B., Crosby, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectroscopic+measurements+of+solutions+and+rigid+glasses.">Spectroscopic measurements of solutions and rigid glasses.</a> Chem. Instrum. 2 (3), 329-335 (1970).</li><li>Landee, C. P., Greeney, R. E., Lamas, A. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improved+helium+cryostat+for+a+vibrating+sample+magnetometer.">Improved helium cryostat for a vibrating sample magnetometer.</a> Rev. Sci. Instrum. 58 (10), 1957-1958 (1987).</li><li>Fairman, R. A., Spence, K. V. N., Kahwa, I. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+liquid+nitrogen+gas-flow+cryostat+for+variable+temperature+laser+luminescence.">A simple liquid nitrogen gas-flow cryostat for variable temperature laser luminescence.</a> Rev. Sci. Instrum. 65 (2), 503-504 (1994).</li><li>Meyer, G. D., Ortiz, T. P., Costello, A. L., Kenney, J. W., Brozik, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+fiber-optic+coupled+luminescence+cryostat.">A simple fiber-optic coupled luminescence cryostat.</a> Rev. Sci. Instrum. 73 (12), 4369-4374 (2002).</li></ol>

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...

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&#39;-bipyridine, and other ligands illustrate well the inherent power of temperature-dependent spectroscopy to elucidate the structures, symmetries, energetics, and chemical behaviors of a manifold of emissive excited electronic states.1-6 
However, to do temperature-dependent cryogenic spectroscopy well is not a trivial matter. It is all too easy for the sample under spectroscopic interrogation not to be thermally equilibrated and thus to manifest a wide range of temperatures across a thermal gradient. The resulting measured spectrum is, in effect, a superposition of emissions over a range of temperatures. Moreover, even the average temperature over this range of temperatures might be quite different from the readout of the temperature probe (e.g., a thermocouple or resistance temperature device) placed on or near to the sample. Thus, to do temperature-dependent cryogenic spectroscopy correctly requires the establishment of experimental conditions under which the sample temperature is known, stable, uniform, and, when the time comes, adjustable. These conditions can be achieved with extremely modest apparatus comprised of a CCD spectrograph, excitation source, optical Dewar, and thermocouple operating under simple, straightforward experimental protocols (see Figure 1).
<img alt="Figure 1" fo:display-align="center" src="/files/ftp_upload/54267/54267fig1.jpg" /> 
Figure 1. Luminescence Spectrograph Setup for Low Temperature Spectroscopy. The system as shown in this top view includes: (a.) CCD detector, (b.) spectrograph, (c.) entrance slit and filters, (d.) luminescence collection optics, (e.) laser or arc lamp excitation source, (f.) excitation beam, (g.) a fused-silica optical Dewar on xyz translation mount, (h.) thermocouple sample junction, (i.) sample, (j.) thermocouple reference junction: 0 &#176;C = 273.15 K ice/water bath, (k.) digital voltmeter. <a href="https://www.jove.com/files/ftp_upload/54267/54267fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Undesirable thermal gradients in the sample and erroneous average sample temperatures are almost sure to result if one side of a sample is placed in physical contact with a cryogenic &#34;cold finger&#34; surface while the other side of the sample is in vacuum. The most practical way to ensure that the complete sample is at uniform measurable temperature T is to totally immerse the sample and the temperature probe in a cryogenic liquid at temperature T (e.g., liquid nitrogen or liquid helium) or in a cryogenic vapor at temperature T (e.g., cold nitrogen or cold helium vapor). Variable-temperature cryostats achieve a constant temperature sample environment by balancing cryogen flow with electrical resistance heating to achieve the desired cryogenic sample temperature.7-9 A thermal exchange gas may be employed to ensure the sample temperature is uniform. The idea is to have the sample in thermal equilibrium with the exchange gas which in turn is in thermal equilibrium with the cryostat. Cryostat designs have emerged that achieve thermal equilibration of the sample at various temperatures simply by adjusting the sample height h above the liquid level of the cryogen in a storage Dewar.10 Samples are excited and luminescence is detected via fiber optic cables or lens systems. At a given sample/probe height h, the cryogen vapor temperature is T(h) and this temperature increases as h increases (i.e., the Dewar provides a smooth thermal gradient dT/dh &#62; 0 in the vapor). The cryogen gas above the liquid in effect becomes the exchange gas. Positioning a small sample and temperature probe at h ensures thermal equilibration of the sample at T(h). To increase sample temperature, h is increased. To decrease sample temperature, h is decreased. The low temperature limit of such a cryostat is the temperature of the liquid cryogen at h = 0. This low temperature limit can be decreased further by reducing the pressure. In a large storage Dewar (e.g., a 100-L liquid helium Dewar or a 10-L liquid nitrogen Dewar), the cryogen boil-off rate is negligible during the time frame of a series of spectroscopic measurements thus allowing an adjustment in sample height h above the liquid cryogen to become a known adjustment in sample temperature.
Spectroscopic investigations in this laboratory of the temperature dependence of oxygen-induced quenching of the luminescence from transition metal complexes led to the adaptation of a small fused silica optical Dewar for variable-temperature spectroscopic investigations with liquid nitrogen in the 77-300 K range (see Figure 2).
<img alt="Figure 2" fo:display-align="middle" src="/files/ftp_upload/54267/54267fig2.jpg" /> 
Figure 2. Fused Silica Optical Dewar Setup for Variable Temperature (77-300 K) Cryogenic Luminescence Spectroscopy. This diagram of the optical Dewar illustrates the complete variable temperature system. (a.) liquid nitrogen, (b.) transparent (4.0 cm) unsilvered optical access region of Dewar, (c.) copper sample loop, (d.) thermocouple junction, (e.) silvered region of Dewar, (f.) alligator clip, (g.) wood dowel, (h.) distance between liquid nitrogen level and sample, (i.) evacuated region between inner and outer Dewar walls, (j.) cork stopper, (k.) nitrogen gas vent hole, (l.) thermocouple wires, (m.) thermocouple wires separated and secured to wood dowel with PTFE tape. <a href="https://www.jove.com/files/ftp_upload/54267/54267fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Fused silica is non-emissive and provides high optical transmission from the near ultraviolet, through the visible, and out to the near infrared (~200-2,000 nm). The basic concepts operative in the large storage Dewar system described previously10, where sample height above the liquid cryogen determines sample temperature, were successfully carried over on a small scale using this small optical Dewar. However, instead of mechanically adjusting the sample height h above a stationary liquid cryogen level to adjust the sample temperature T, the sample position with respect to the Dewar itself is fixed (Figure 2). The slow boil off of the liquid nitrogen in the optical Dewar over a period of several hours gradually increases the distance h of the sample above the falling liquid nitrogen level (Figure 3).
<img alt="Figure 3" fo:display-align="middle" src="/files/ftp_upload/54267/54267fig3.jpg" /> 
Figure 3. Close up of Sample Region of Optical Dewar. Temperatures: sample immersed in liquid nitrogen to level h0, to give T0 = 77 K; sample immersed in cold nitrogen vapor at levels h1 &#60; h2 &#60; h3 above the liquid nitrogen level to give sample temperatures T1 &#60; T2 &#60; T3. <a href="https://www.jove.com/files/ftp_upload/54267/54267fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
This allows for a slow, controlled increase in the sample temperature over time (up to several hours) while maintaining both the sample and the temperature probe, a copper-Constantan thermocouple junction, in thermal equilibrium with the cold nitrogen vapor. Luminescence spectra spanning the visible and near-infrared regions are acquired in just a few milliseconds per spectrum (or hundreds of spectra per second) with a CCD-equipped luminescence spectrograph during which sample temperature is virtually constant (&#916;T &#60; 0.1 K) as each spectral dataset is acquired. Typical wait times between spectra at temperatures ~5 K apart are ~5-15 min. Moreover, the effects of sample heating or photochemical degradation of the sample by the exciting light are minimized since the excitation light is only allowed to strike the sample a few seconds per spectrum. In the interest of simplicity, portability, and quickness of sample loading, fiber optic cables are not employed. Samples are excited directly with either the 365 nm band from a mercury arc lamp or the 405 nm line of a diode laser. Emitted light from the samples is picked up directly from the emitting sample in the Dewar by a collection lens and impinged on the entrance slit of the spectrograph by a focusing lens. Samples of the ruthenium and rhodium complexes under investigation are prepared for spectroscopic study as thin films of ~10-3-10-4 M solute in oxygen-saturated solutions. The solutions are held by surface tension in small copper wire loops (~3 mm loop diameter formed out of 0.0150 in. dia. copper wire). The thermocouple junction height is then adjusted so it is equal to the sample height (hthermocouple = hsample) and in close proximity to the sample loop as shown in Figures 2 and 3. Temperatures are determined by measuring the voltage difference between the thermocouple sample junction and a 0 &#176;C water/ice thermocouple reference junction using a high-impedance digital voltmeter and comparing to a temperature vs. voltage table for a Type T copper/Constantan thermocouple. The thin-film sample solutions captured in the wire loops are flash frozen by quick immersion in liquid nitrogen in the optical Dewar. Then the frozen solutions are allowed to warm up very gradually over time, remaining frozen, while their luminescence spectra are measured as a function of temperature. The luminescence intensity versus temperature data are analyzed according to the following model.
The total luminescence intensity of the sample at temperature T is given as the sum of the intensities arising from oxygenated and unoxygenated complexes:
<img alt="Equation 2" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq2.jpg" />.&#160;&#160;&#160; (1)
The luminescence intensity from the complexes without oxygen is assumed to be temperature independent. However, the luminescence intensity of the oxygenated complexes will decrease exponentially with increasing temperature due to oxygen quenching. This can be described by an Arrhenius equation of the form
<img alt="Equation 3" fo:vertical-align="middle" src="/files/ftp_upload/54267/54267eq3.jpg" />.&#160;&#160;&#160;&#160; (2)
In Equation (2), Ea is the quenching activation energy and k is the Boltzmann constant. The maximum luminescence intensity will be observed in the low temperature region (see Figure 5), where there is insufficient thermal energy to overcome the quenching activation barrier (i.e., energy transfer from the complex to oxygen). If Equation (2) is substituted into Equation (1), the expression
<img alt="Equation 5" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq5.jpg" />&#160;&#160;&#160;&#160; (3)
is obtained. In Equation (3), <img alt="Equation 6" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq6.jpg" /> is the intensity arising from oxygenated complexes in the low temperature region. Rearrangement of Equation (3) yields
<img alt="Equation 7" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq7.jpg" />.&#160;&#160;&#160;&#160; (4)
Taking the natural logarithm of the both sides of Equation (4) gives the expression
<img alt="Equation 8" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq8.jpg" />.&#160;&#160;&#160;&#160; (5)
From Equation (5) it is evident that a plot of <img alt="Equation 9" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq9.jpg" />versus&#160;<img alt="Equation 10" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq10.jpg" /> will give a straight line with <img alt="Equation 11" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq11.jpg" />, from which the luminescence quenching activation energy is obtained as
<img alt="Equation 12" fo:display-align="middle" src="/files/ftp_upload/54267/54267eq12.jpg" />.&#160;&#160;&#160;&#160; (6)

<table border="0" cellpadding="0" cellspacing="0" width="819">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Diode laser 405 nm</td>
			<td style="width:189px;">Generic</td>
			<td style="width:119px;"></td>
			<td style="width:295px;">Generic pencil-type laser pointer for luminescence excitation: 5 mW at 405 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Quartz optical dewar</td>
			<td style="width:189px;">Custom fabrication</td>
			<td style="width:119px;"></td>
			<td>3.5 cm id. X 25.0 cm length with 4.5 cm unsilvered region for optical access</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Programmable 5 1/2 digit DMM</td>
			<td style="width:189px;">Keithley</td>
			<td style="width:119px;">Model 192</td>
			<td>High impedence DMM for reading thermocouple voltages</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Copper thermocouple wire</td>
			<td style="width:189px;">Omega Engineering</td>
			<td style="width:119px;">SPCP-010</td>
			<td>0.010 in. diameter bare copper thermocouple wire</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Constantan thermocouple wire</td>
			<td style="width:189px;">Omega Engineering</td>
			<td style="width:119px;">SPCC-010</td>
			<td>0.010 in. diameter bare Constantan (copper/nickel) thermocouple wire</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Polychromator/Spectrograph</td>
			<td style="width:189px;">Jarrell-Ash</td>
			<td style="width:119px;">82-415</td>
			<td>0.25 m Ebert monochromator with back slit assembly removed to enable operation as a polychromator</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">CCD camera</td>
			<td style="width:189px;">Andor</td>
			<td style="width:119px;">DV-401-UV</td>
			<td>Thermoelectrically cooled (-35 C) CCD camera for detecting emitted light</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Copper wire for sample loop</td>
			<td style="width:189px;">Generic</td>
			<td style="width:119px;"></td>
			<td>0.0150 in. diameter bare copper wire for sample loop</td>
		</tr>
	</tbody>
</table>

a simple dewar/cryostat for thermally equilibrating samples at known temperatures for accurate cryogenic luminescence measurements

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 (&#916;T &#60; 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.

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.
<table border="1" fo:keep-together.within-page...

Watch this Scientific Journal Video about A Simple Dewar/Cryostat for Thermally Equilibrating Samples at Known Temperatures for Accurate Cryogenic Luminescence Measurements at JoVE.com

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

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.

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and ...

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