Hole Burning
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In the Turkmenistan desert, a crater dubbed âThe Door to Hellâ has been burning for decades
There are places on Earth that are a little creepy, places that feel a little haunted and places that are downright hellish. The Darvaza gas crater, nicknamed by locals " The Door to Hell ," or " The Gates of Hell ," definitely falls into the latter categoryâand its sinister burning flames are just the half of it. Located in the Karakum Desert of central Turkmenistan (a little over 150 miles from the country's capital) the pit attracts hundreds of tourists each year. It also attracts nearby desert wildlifeâreportedly, from time to time local spiders are seen plunging into the pit by the thousands, lured to their deaths by the glowing flames.
So how did this fiery inferno end up in the middle of a desert in Turkmenistan? In 1971, when the republic was still part of the Soviet Union, a group of Soviet geologists went to the Karakum in search of oil fields. They found what they thought to be a substantial oil field and began drilling. Unfortunately for the scientists, they were drilling on top of a cavernous pocket of natural gas which couldn't support the weight of their equipment. The site collapsed, taking their equipment along with itâand the event triggered the crumbly sedimentary rock of the desert to collapse in other places too, creating a domino-effect that resulted in several open craters by the time all was said and done.Â
The largest of these craters measures about 230-feet across and 65-feet deep. Reportedly, no one was injured in the collapse, but the scientists soon had another problem on their hands: the natural gas escaping from the crater. Natural gas is composed mostly of methane, which, though not toxic, does displace oxygen, making it difficult to breathe. This wasn't so much an issue for the scientists, but for the animals that call the Karakum Desert homeâshortly after the collapse, animals roaming the area began to die. The escaping methane also posed dangers due to its flammabilityâthere needs to be just five percent methane in the air for an explosion to potentially take place. So the scientists decided to light the crater on fire, hoping that all the dangerous natural gas would burn away in a few weeks' time.
It's not as outlandish as it soundsâin oil and natural gas drilling operations, this happens all the time to natural gas that can't be captured. Unlike oil, which can be stored in tanks indefinitely after drilling, natural gas needs to be immediately processedâif there's an excess of natural gas that can't be piped to a processing facility, drillers often burn the natural gas to get rid of it. It's a process called " flaring ," and it wastes almost a million dollars of worth of natural gas each day in North Dakota alone.
But unlike drillers in North Dakota or elsewhere, the scientists in Turkmenistan weren't dealing with a measured amount of natural gasâscientists still don't know just how much natural gas is feeding the burning craterâso what was supposed to be a few-week burn has turned into almost a half-century-long desert bonfire.Â
After visiting the crater in 2010, Turkmenistan's president Kurbanguly Berdymukhamedov, worried that the fire would threaten the country's ability to develop nearby gas fields, ordered local authorities to come up with a plan for filling the crater in. No action has been taken, however, and the crater continues to burn, attracting unsuspecting wildlife and international tourists.
To visit the Darvaza gas crater, it's best to go at night, when the fire can be seen from miles away. The crater is located about 161 miles (about a 4 hour drive) from the Turkmen capital Ashgabat. Tours can be booked through agents in Ashgabat. Alternatively, some companies offer more structured tours of the surrounding area, with the Darvaza crater included (such as this tour , by The Geographical Society of New South Wales).
Natasha Geiling
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Natasha Geiling is an online reporter for Smithsonian magazine.
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Principles of Persistent Spectral Hole Burning
Principles of Persistent Spectral Hole Burning
Figure 1. Single molecule embedded in a transparent solid. The region enclosed in
the circle is called impurity center.
Figure 1a. An impurity center consists of one molecule and its closest surrounding
Let us consider the absorption of light by one single molecule embedded in an optically
transparent solid. An example of such solid could be a piece of plastic or polymer
material, a chunk of a laser crystal or a piece of simple window glass. The relevantÂ
quantum-mechanical system consists of the electronic and vibrational degrees of freedomÂ
of the molecule and of the vibrational motion of the surrounding solid. Figure 1 shows
a molecule surrounded by a solid. The molecule together with its closest neighbors
(region inside red circle) is called impurity center. The absorption spectrum of one
molecule is called homogeneous absorption spectrum or homogeneous line shape. Suppose
that absorption occurs because of transition from the ground electronic state of
the impurity center to it's excited electronic state. For organic dye molecules the
ground electronic sate is singlet S 0 and the excited electronic state is the lowest excited singlet S 1 . The absorption spectrum gives the probability of transition from the ground state
to the excited state as a function of frequency n (or as a function of wavelength,
l=c/n). Experimentally, the absorption spectrum is obtained by illuminating the crystal
with a beam of light of frequency n, and by measuring the ratio, I(n)/I 0 (n), where I 0 (n) and I(n) is the intensity at the input and at the output of the crystal, respectively.
Figure 2. Variation of the homogeneous absorption spectrum with temperature
In the first approximation, the absorption spectrum has a sharp maximum, where the
frequency n equals the energy difference between the states, divided by Planck's
constant, n = (E(S 1 )-E(S 0 ))/h. This is most true if the molecule is free in gas phase. In the solid, however,
the transition probability depends also on the coordinates of the surrounding atoms.
More precisely, the probability is a function of the density and of the frequency
of the vibrational states of the solid. Crystal lattice vibrations, which propagate
like waves are called phonons. Most critical is the presence of such phonons, which
are part of the ground state wave function. Because the phonon state population varies
largely with the temperature of the crystal, the whole absorption spectrum depends
strongly on the temperature. Figure 2 shows how the homogeneous absorption spectrum
of the molecule changes if the temperature is varied between room temperature (T=300
K ) and absolute zero temperature (T=0). At higher temperatures T=100 - 300 K the
spectrum is tens to hundreds of cm -1Â broad. It hardly contains any sharp structure. Since typical phonons in a solid matrix
have an energy quanta of hn ph ~10 - 1000 cm -1 , the thermal motion at around room temperature ( kT~300 cm -1 ) has enough energy to excite a wealth of phonons.  If many phonons are present,
then each time the electronic transition occurs in the impurity center, it is impossible
to predict what will be exactly the energy difference between the ground state and
the excited state. Therefore the room temperature absorption spectrum appears to
be broad and without sharp lines. It consists mostly of what is called phonon side
band. At lower temperatures, however, the number of phonons is much reduced. Then
there exists a real probability for electronic transitions where the phonons do
not participate at all. Such transitions are called zero phonon transitions. Their
important property is that they have a very well defined frequency. The corresponding
spectral feature shows up as a narrow zero phonon line (ZPL). The narrowest and most
intense zero phonon line is observed at absolute zero temperature. The width of the
ZPL is then given by the inverse value of the excited state's lifetime. The phonon
side band reduces at low temperatures to a relatively weak feature on the shorter
wavelength side of the ZPL. In some special cases the zero phonon line can be detected
already at liquid nitrogen temperature (T=77 o K), however more typical is that the sample has to be first cooled to 10 - 20 K.
Figure 3a. Random fluctuations of the crystal structure cause random changes of the
nearest environment of five chemically identical molecules;
Now let's assume that the same piece of solid contains not just one, but many molecules,
which all have the same chemical structure. Nevertheless, because no solid has a 100%
perfect regular structure, different molecules are going to find themselves in slightly
different surroundings. Figure 3a shows five chemically indistinguishable dye molecules
in a randomly fluctuating environment. Figure 3b shows that this makes the ground
and excited state energy to vary randomly from molecule to molecule, which causes
the transition frequency to change randomly as well. The probability of finding the
transition frequency in a unit frequency interval {n,n+dn} is given by inhomogeneous
distribution function, g(n). The absorption profile, which results from the superposition
of many homogeneous line shapes is called inhomogeneously broadened absorption band.
Mathematically, the inhomogeneous absorption band can be described as a convolution
of the inhomogeneous distribution function with the function describing the homogeneous
line shape. Figure 4 shows the composition of the inhomogeneous absorption band at
high (room) and at low (cryogenic) temperatures. At high
Figure 3b. The correspondence between the fluctuations of the energy levels and of
the transition frequencies.
temperatures the inhomogeneous spectrum is a superposition of many broad phonon-induced
bands. Usually the width of the phonon bands is on the same order as the width of
the inhomogeneous distribution function. In this case the absorption spectrum of molecules
within the inhomogeneous band can be hardly be distinguished from each other. Such
absorption band has practically no spectral selectivity. At low temperatures each
molecule has a sharp zero phonon line. This allows groups of molecules to be addressed
selectively based on their ZPL frequency. We say that in this case the material has
a high degree of spectral selectivity.
Figure 4. The composition of the inhomogeneously broadened absorption band at room
temperature (upper picture) and cryogenic temperature (lower picture)
In most cases, molecules and atoms always return from the excited state back to the
initial ground state. There are situation, however, when this is not always the case.
For example, some organic dye molecules can undergo a photochemical reaction, which
alters the whole chemical structure of the molecule. If such photochemically active
molecule absorbs light, then with a probability of a few % it will not return to the
initial state called educt, but rather switches over to a new ground state called
product. Often the homogeneous absorption spectrum of the product is much different
from the educt, so that the corresponding inhomogeneous bands do not even overlap.
Figure 5 shows one such example, where photochemical tautomerization at liquid helium
temperature results in the shift of the S 1 â S 0 absorption band from 634nm to 570nm. In fact, by illuminating the sample in the spectral
interval around 634nm most of the molecules can be transferred from the educt to the
product sate. If the illumination is terminated, then the initial absorption profile
is not restored unless the sample is heated up to about liquid nitrogen temperature.
Figure 5. The mechanism of photochemical tautomerization. The molecule's absorption
spectrum changes from educt (red) to photoproduct (green) as a result of illumination
in the spectral interval 620 -640nm.
To illustrate this fact, Figure 5 shows the absorption profile taken before (red)
and after (green) such illumination. Since at low temperatures the inhomogeneous absorption
band of the educt consists of narrow zero phonon lines, it is possible to produce
such photochemical transformations only in a small group of molecules, which are selected
by their ZPL frequency. Selective bleaching of the inhomogeneously broadened absorption
band consisting of narrow homogeneous absorption lines is called spectral hole burning
(SHB). Besides the photochemical tautomerization reaction shown in Fig.5, there are
many different mechanism for spectral hole burning in organic as well as in inorganic
materials. In all cases the spectral hole burning relies on three basic factors: existence
of narrow homogen
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