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| Measurement of OH nightglow temperatures in the mesopause region using ADC detectors Michael Bittner, Kathrin Höppner, Carsten Schmidt German Aerospace Center (DLR-DFD) Themesosphere can be seen as an Early Warning System for Climate Signals. This is due to the fact that the air density decreases by five orders of magnitude from ground to about 80 km height. So it is expected that the amplitude of climate signals should be much more pronounced and is therefore earlier detectable than in the lower layers of the atmosphere. In-situ measurements of temperature in the mesopause region are, however, difficult. Aircrafts and balloons cannot reach this altitude. Hence, the use of remote sensing techniques by ground-based instruments is important for the monitoring of middle and upper atmospheric parameters. The German Remote Sensing Data Center (DFD) of the German Aerospace Center (DLR) operates the two Infrared-Spectrometers GRIPS 3 and GRIPS 4 (ground-based infrared p-branch spectrometer). Both being successors of the original instrument developed at the University of Wuppertal 30 years ago, all of which have been equipped the outstanding liquid nitrogen cooled Germanium detectors manufactured by ADC.  Fig 1: The GRIPS 3 instrument located at the Environmental Research Station “Schneefernerhaus” (47.4°N, 11.0°E). The light enters the spectrometer through the (specially designed) large window, inside the aluminium box there are a baffle and a chopper, to their left a scanning monochromator of Czerny-Turner type and finally the characteristic Germanium detector (L-model), connected to a 120 l barrel of liquid nitrogen for automatic refill. Below the actual instrument power supplies, electronics and computers are located (see also figure 3).  Fig. 2: Six raw spectra of the OH(3-1) p-branch taken with the GRIPS 4 instrument and corresponding rotational temperatures are shown. All spectra are sampled with 221 points and are gathered within three minutes each. The intensity is given in arbitrary units (nA output of the lock-in amplifier with a time constant of three seconds). Since the airglow intensity is rather weak and can also vary on very short time scales (flux: ~1000-4000 photons s-1 arcsec-2 m-2 µm-1) it is very important to use a detector with low noise levels to be able to calculate rotational temperatures of sufficient quality. The measurement technique makes use of the so-called airglow in the upper mesosphere /lower thermosphere region. A phenomenon first systematically studied by Aden B. Meinel at Lick Observatory (California) in the late 1940s, Meinel (1950). In this region atomic hydrogen reacts with ozone O3 + H → O2 + OH* (+ 3.32 eV), forming excited hydroxyl molecules (OH*) in a layer of about 8 km thickness at a peak altitude of about 87 km. Through a large number of rotational-vibrational transitions these chemically excited OH molecules emit infrared radiation between about 710 nm and 2500 nm. These emissions can be measured by ground-based instruments during nighttime (OH concentrations during daytime are small due to destruction of ozone by UV-light from the sun). The spectral range beyond 1.4 µm allows measurements even if light clouds, haze or the full moon are present. Furthermore the low energy OH transitions in this spectral region are supposed to be in local thermodynamic equilibrium with the surrounding atmosphere, e.g. Pendleton et al. (1993). In this case the intensities of the rotational emission lines of a single vibrational transition are connected via a temperature dependent Boltzmann distribution. Thus, the rotational temperature can be derived from the measured spectra and an estimator of the kinetic temperature in this height is retrieved.  Fig. 3: Sketch of the GRIPS 3 and GRIPS 4 instrumental design. The light enters the system via the tilted mirror at the right. The mirror is inclined by 22.5° resulting in a zenith angle of 45° and enabling the instrument to be operated at ordinary (infrared transparent) windows (compare figure 1). Besides the derivation of the nocturnal mean temperature for the purpose of climate studies another topic of scientific interest is the investigation of atmospheric dynamics on shorter time scales. One specific task is the quantification of the impact of smaller scale waves, such as atmospheric gravity waves, on the state of the upper atmosphere. Therefore, the GRIPS 3 instrument was set up at the Environmental Research Station “Schneefernerhaus” (47.4°N, 11.0°E) located at the south side of Mt. Zugspitze - Germany’s highest mountain – just 300m below the summit at an altitude of 2650m (http://www.schneefernerhaus.de/e-ufs.htm). This station is particularly well suited in a source region of gravity waves, since these waves are often generated when air currents have to overflow mountain ridges.  Fig. 4: The “Schneefernerhaus”, a former hotel officially opened 1931, has been a US Army Recreation Facility between 1945 and 1952 and is since 1998 the home of the Environmental Research Station “Schneefernerhaus”. Subset a) shows the southward view from the GRIPS laboratory onto the skiing area beneath the station and further onto the Austrian Alps. Subset b) shows the station itself, it contains a total of eleven floors, a two track cogwheel train station inside the mountain and its own small cable car. The GRIPS laboratory a) is located in the middle of the main wing (grey face of the building), second row and third column of windows. Subset c) shows one nights time series of OH(3-1) temperatures (upper panel) and OH(3-1) intensities (lower panel) measured at the “Schneefernerhaus” during a measurement campaign in 2006. The variation of the parameters clearly shows the presence of an atmospheric wave. Note that the fluctuation in intensity is of the order of 10%, while it is only of the order of 1% for the temperatures – this is a typical modulation of airglow by atmospheric waves. The GRIPS 4 is designed as a mobile instrument. Besides frequent co-located measurement campaigns with GRIPS 3 at the “Schneefernerhaus” the most fascinating place of operation of the mobile GRIPS 4 so far has been a cruise onboard the German Research Vessel “Polarstern” from Bremerhaven (53.5°N, 8.6°E), Germany, to Cape Town (33.9°S, 18.4°E), South Africa, during the trip ANT XXIII/1. The primary objective of this campaign was to validate mesopause temperature measurements of the satellite instrument SCIAMACHY onboard the European satellite ENVISAT. The cruise allowed simultaneous measurements of both instruments in different climate regions.  Fig. 5: The German Research Vessel “Polarstern”, the icebreaker built in 1982 is Germany’s largest ship dedicated to scientific research (subset b)). In 2005 on cruise ANT XXIII/1 form Bremerhaven to Cape Town (see subset a)) the GRIPS 4 instrument was installed onboard the ship to compare its results with the results of the satellite borne instrument SCIAMACHY. Subset c) shows the temperatures derived from both instruments whenever the respective fields of view of both instruments matched to a sufficient degree. Selected literature: Bittner, M., Offermann, D. and Graef, H.H., 2000. Mesopause temperature variability above a midlatitude station in Europe. Journal of Geophysical Research 105, 2045-2058. Bittner, M., Offermann, D. and Graef, H.H., Donner, M. and Hamilton, K., 2002. An 18-year time series of OH rotational temperatures and middle atmosphere decadal variations. Journal of Atmospheric and Solar-Terrestrial Physics 64, 1147-1166. Bittner, M., and Höppner, K., 2007. Temperature measurements in the mesopause region (~87 km). – In: The expedition ANTARKTIS-XXIII/1 of the research vessel “Polarstern” 2005, M.R. van der Loeff (eds.), Berichte zur Polar- und Meeresforschung 556, 122-124. Bracher, A., M. Weber, H. Bovensmann, S. Noёl, C. von Savigny, K. Höppner, M. Bittner und J.P. Burrows, 2006. 1st final report in phase A for the ESA project “Long term validation of SCIAMACHY data (SciLoV)”. Projektbericht (Contract No. 18809/05-I-LG) an die ESA vom Institut für Umweltphysik (IUP), Universität Bremen, Deutschland. Höppner, K., and Bittner, M., 2007. Evidence for Solar signals in the mesopause temperature variability? Journal of Atmospheric and Solar-Terrestrial Physics 69, 431-448. Höppner, K. and Bittner, M., 2009. Detection of solar activity signatures in OH* temperature fluctuations possibly related to the differential rotation of the sun. Journal of Atmospheric and Solar-Terrestrial Physics, doi:10.1010/j.jastp.2009.04.008 Meinel, A.B., 1950. OH Emission Bands in the Spectrum of the Night Sky. Astrophysical Journal 111, 555-564. Pendleton, W. R., Jr., P. J. Espy, and M. R. Hammond 1993. Evidence for Non-Local-Thermodynamic-Equilibrium Rotation in the OH Nightglow, Journal of Geophysical Research 98(A7), 11567–11579. Pilger, C. and Bittner, M., 2009. Infrasound from tropospheric sources: Impact on mesopause temperature ? – Journal of Atmospheric and Solar-Terrestrial Physics 71, 816-822. von Savigny, C., K.-U. Eichmann, E. J. Llewellyn , H. Bovensmann, J. P. Burrows, M. Bittner, K. Höppner, D. Offermann, M. J. Taylor, Y. Zhao, W. Steinbrecht, and P. Winkler, 2004. First near-global retrievals of OH rotational temperatures from satellite-based Meinel band emission measurements. Geophysical Research Letters 31, L15111, doi: 10.1029/2004GL020410, 1-5. | |||||
Photoluminescence of Silicon with ADC Germanium Detectors Nelson Rowell
The performance of of silicon based electronic devices is strongly influenced by small concentrations of impurities and crystal defects. On one hand, devices would not even be possible without intentional impurities - also known as dopants - while, on the other hand, unintentional impurities and crystal imperfections at concentrations as low as 1015 cm-3 can cause devices to cease to function. Low temperature photoluminescence (PL) being sensitive to impurities/defects over a large range of concentrations has been an enabling technique for the development of new materials such as SiGe. In the method we use liquid helium to cool the sample, a visible laser to excite free charge carriers, and a Fourier transform spectrometer to analyse the longer wavelength (1 - 2 µm) emitted radiation with a high sensitivity Germanium detector. A highly sensitive detector is needed since the internal radiative conversion efficiency in materials like silicon, an indirect gap material, is less than 0.01%. The excited carriers consist of negatively charged electrons and positively charged holes which, at low temperature, couple in pairs to form excitons which are analogous to hydrogen atoms. Neutral excitons can diffuse throughout a Si crystal until they encounter an impurity or defect where the electrons and holes self-annihilate when there is a small, but finite, probability that photons will be emitted. The energy of the emitted photons depends fairly uniquely on the type and configuration of the recombination center so the emitted spectrum is a very useful measure of the electronic quality of the material.
Fourier transform photoluminescence apparatus Due to limited measurement sensitivity, photoluminescence method was rarely applied to Silicon before 1990, notably by Paul Dean and Michio Tajima. It has only been with the development of high sensitivity Germanium detectors by Louis Wang (Applied Detector Corporation) and their combination with Fourier transform spectrometers that the method has become truly feasible. This new PL methodology was pioneered by Ed Lightowlers of Kings College, London, Mike Thewalt of Simon Fraser University, Vancouver, and ourselves of the National Research Council, Ottawa for the study of a range of impurities in bulk Silicon as well as for the development of SiGe epilayer and multiple quantum well systems on Silicon substrates. Through the use of PL measurements, the development of high quality SiGe epilayers has been possible. In recent years such epilayers have enabled higher performance Si-based electronic devices to be manufactured. The spectrum below illustrates the use of PL in evaluating layer thickness and concentration. This data was taken at low temperature for a multiple quantum well sample grown by ultra high vacuum chemical vapor deposition methods. The three SiGe quantum wells with 22% Ge fraction had thicknesses of 1.3, 2.5, and 6.3 nm leading to significant and different quantum confinement shifts of the basic PL spectral lines for the three wells. These three sets of lines are highlighted in color. Other lines in the spectrum are due to the Silicon substrate.
Photoluminescence spectrum of a SiGe quantum well sample on a Silicon substrate. Each of the three wells provides two PL lines. Due to spatial confinement effects (inset), these emission lines have different energies among the different thickness SiGe wells. We can obtain the wavelength by dividing the energy into 1240.
Beyond a basic measure of material quality, there are many other examples of the use of low temperature PL in Si-based compounds which include Ge fraction in SiGe, defect densities in SiGe, SiGe quantum dots and islands, dopant concentration and location, and atomic diffusion after thermal annealing.
Photoluminescence apparatus using ADC detector.  | |||||
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