30.5 Applications of Atomic Excitations and De-Excitations

• The double-helix structure of DNA was discovered in 1953 by an international team of scientists working at the Cavendish Laboratory.
• They were the first to discern the structure of DNA using x-ray data.
• The 1962 Nobel Prize in Physiology or Medicine was awarded to Crick, Wilkins, and Watson.
• There is a lot of debate over whether or not Rosalind Franklin was included in the prize.

• The figure shows a pattern of x rays in a crystal.
• x-ray crystallography is a process that gives information about crystal structure, and it was the type of data that Rosalind Franklin supplied to Crick for DNA.
• x rays give information on the atomic arrangements in materials and confirm the size and shape of atoms.
• Current research in high-temperature superconductors involves complex materials whose lattice arrangements are crucial to obtaining a superconducting material.
• x-ray crystallography can be used to study these.

• The interference pattern was created by the X-ray diffraction from the hen egg lysozyme.
• The German Max von Laue convinced two of his colleagues to scatter x rays from crystals after he discovered x rays in 1895.
• If a pattern of waves is obtained, the x rays could be determined.
• Von Laue was awarded the 1914 Nobel Prize in physics for suggesting that x rays are waves.
• The father and son team of Sir William Henry Bragg and his son Sir William Lawrence Bragg were awarded a joint Nobel Prize in 1915 for inventing the x-ray analyzer.
• After graduating from mathematics, the elder Bragg moved to Australia.
• He studied physics and chemistry at the University of Adelaide.
• The younger Bragg was born in Australia but went back to England to work in x-ray and neutron crystallography, and he supported the work of James Crick and Max Perutz for their work on unraveling the mysteries of DNA.
• This time, we see the enabling nature of physics--establishing instruments and designing experiments as well as solving mysteries in the biomedical sciences.

• Other uses for x rays will be studied later in the chapter.
• X rays have an effect on cell reproduction and are useful in the treatment of cancer.
• X rays from outer space can be used to determine the nature of their sources, such as black holes.
• x rays can be used to detect atmospheric tests of nuclear weapons.
• X rays can cause the fluoresce of atoms, which makes them a valuable analytical tool in a range of fields from art to archaeology.

• The properties of matter and phenomena in nature are related to atomic energy levels.
• The transparency of air, the color of a rose, and the output of a laser are a few examples.

• It may not seem like they have much in common, but glow-in-the-dark pajamas and lasers are different applications of atomic de-excitations.

• Light from a laser is based on atomic de-excitation.

• The color of a material is determined by the ability of its atoms to absorb certain wavelengths.
• The levels of the lycopene's atoms are separated by a variety of energies, which correspond to all visible photon energies except red.
• Another example is air.
• It is transparent because there are few energy levels visible to the naked eye.
• The light cannot be absorbed.
• The visible light is scattered weakly by the air because the visible wavelength is larger than the air atoms.
• To cause red sunsets and blue skies, light must pass through kilometers of air.

• The atomic energy levels of a material are related to its ability to emit light.
• Some rocks glow in black light because of their mineral composition.
• Posters with black lights make them glow.

• When illuminated by a black light, objects glow in the visible spectrum.
• Emissions are related to the mineral's energy levels.
• In the case of scorpions, the blue glow is due to the presence of proteins near the surface of their skin.
• This is a colorful example of fluorescent activity in which de-excitation occurs in the form of visible light.

• An atom is excited to a level several steps above its ground state by the absorption of a relatively high-energy UV photon.
• One way the atom can de-excite is to re-emit a photon of the same energy as excited it, a single step back to the ground state.

• Smaller steps in which lower-energy (longer wavelength) photons are emitted are all other paths of de-excitation.

• There are many types of energy input.
• The use of fluorescent paint, dyes, and soap in clothes makes the colors look brighter in the sun.
• X rays can be used to make visible images.
• Neon lights and gasdischarge tubes that produce atomic and molecular spectrum can be caused by electric discharges.
• Atomic emissions from mercury atoms are caused by an electric discharge in mercury vapor.
• The inside of a fluorescent light is coated with a fluorescent material that emits visible light over a broad spectrum of wavelength.

• The fluorescent lights are four times more efficient in converting electrical energy into visible light than the incandescent lights are.

• The atom is excited by the UV photon.
• It can de-excite in a single step, re-emitting a photon of the same energy, or in several steps.
• If the atom de-excites in smaller steps, it will emit different energy than if it excited it.
• UV, x-rays, and electrical discharge are some of the energy inputs that can causeescence.

• glow-worms can be found in the Waitomo caves on New Zealand's North Island.
• The glow-worms hang up to 70 silk threads each to catch prey that fly towards them in the dark.
• The process of turning energy into light is very efficient.

• There are many uses for florescence.
• It is used to follow a molecule in a cell.
• One can study the structure of genes.
• The emission of visible light is observed when the molecule is illuminated with UV light and tagged with fluorescent dyes.
• Identification of elements within a sample can be done this way.

• The fluorescent dye shown in Figure 30.32 is called fluorescein.
• Figure 30.33 shows the dispersion of a fluorescent dye in water.

• The dye is used in the laboratory.
• A beaker of water has fluorescent powder added to it.
• Under ultraviolet light, the mixture gives off a bright glow.
• These are small single-crystal molecules.
• The indicators are small and provide improved brightness.
• All colors can be excited with the same wavelength.

• Conventional phosphors have a longer lifetime than organic dyes.
• They are an excellent tool for long-term studies of cells.

• Chicken cells are being imaged using a fluorescent dye.
• Cell nuclei are blue while neurofilaments are green.
• Spontaneous de-excitation has a very short lifetime.
• Some levels have lifetimes of up to minutes or even hours.
• The energy levels are slow in de-exciting because their quantum numbers are different from the lower levels.
• phosphorescent substances are used to make glow-in-the-dark materials, such as Luminous dial on some watches and clocks and on children's toys and pajamas.
• The stored energy is released partially as visible light when the atoms or molecules decay slowly.
• After the ceramic has cooled from its firing, atomic energy can be frozen in.
• Since the release is slow, thermoluminescence can be used to date antiquities.
• The older the ceramic, the less light it emits.

• The Chinese ceramic figure can be stimulated to de-excite and emit radiation by heating a sample of the ceramic, a process called thermoluminescence.
• Since the slowly states de-excite over centuries, the amount of thermoluminescence decreases with age, making it possible to use this effect to date and authenticate antiquities.
• The figure is from the 11th century.
• Today's lasers are commonplace.
• Lasers are used to read bar codes at stores and libraries, laser shows are staged for entertainment, laser printers produce high-quality images at a relatively low cost, and lasers send large numbers of telephone messages through optical fibers.
• Lasers are used in a number of things, including surveying, weapons guidance, tumor eradication, and for reading music CDs and computer CD-ROMs.

• The answer is that lasers emit single-wavelength EM radiation that is very coherent.
• Laser output can be more precisely manipulated than other sources.
• Laser output is so pure and coherent because of how it is produced, which depends on a metastable state in the lasing material.
• electrons are raised to all possible levels when energy is put into a large collection of atoms The electrons that originally excited the metastable state and those that fell into it from above are included.

• A population inversion has been achieved if a majority of electrons are in the metastable state.

• An electron falls from the metastable state.
• A second photon of the same wavelength and phase with the first is emitted when this photon finds another atom in the metastable state.
• An excited atom with an electron in an energy orbit higher than normal releases a photon of a specific Frequency when the electron drops back to a lower energy orbit.
• If this photon strikes another electron in the same high-energy space, another photon of the same frequency is released.
• The emitted and triggering photons are both in the same phase and travel in the same direction.
• A majority of atoms must be in the metastable state to produce energy because the probability of absorption of a photon is the same as the probability of stimulated emission.
• Einstein was the first to realize that stimulated emission and absorption are equally probable.
• The laser acts as a temporary energy storage device that produces a massive energy output.

• One atom in the metastable state spontaneously decays to a lower level, producing a photon that stimulates another atom to de-excite.
• The second photon is in phase with the first and has the same energy and wavelength.
• The emission of other photons is stimulated by both of them.
• A net production is necessary for there to be a net absorption.

• The process was developed after advances in quantum physics.
• The development of lasers was one of the reasons why the Soviet Union and the United States won a joint Nobel Prize in 1964.
• Arthur Schawlow won the 1981 Nobel Prize for his work in laser applications.
• The devices were called masers because they produced microwaves.
• T. Maiman created the first working laser in 1960.
• The red light was produced by using a flash lamp and a rod.
• The name laser is used for all of the devices that produce a variety of wavelengths.
• In a process called optical pumping, energy input can come from a flash tube, electrical discharge, or other source.
• A large percentage of the original pumping energy is dissipated in other forms.
• Mirrors can be used to enhance stimulated emission by multiple passes of the radiation back and forth.
• Some of the light can be seen through one of the mirrors.
• A laser's output is 1% of the light passing back and forth in a laser.

• Laser construction uses a method of pumping energy into the lasing material.

• Many types of lasing materials are used to make lasers.
• The existence of a metastable state or phosphorescent material is what determines lasers.
• Some lasers produce continuous output while others are short-lived.
• The more common lasers produce something on the order of.
• The red light that comes from the laser is very common.
• The number of atoms of helium is ten times greater than the number of neon atoms.
• The first excited state of helium stores energy.
• Neon atoms have an excited state that is nearly the same as that in helium, which makes it easy to transfer this energy.
• The neon state that produces the laser output is also metastable.
• There are so many more helium atoms in neon that it can produce a population inversion.
• The population can be maintained even while lasing occurs because helium-neon lasers have continuous output.
• The most common lasers in use today are made of Silicon.
• The energy is pumped into the material by passing a current into the device.
• Light bounces back and forth and a tiny fraction emerges as laser light thanks to special coating on the ends and fine cleavings of the material.
• The lasers can produce outputs in the range of a few hundredths of a watt.

• The gas mixture has more neon atoms than helium.
• In a collision, excited helium atoms can de-excite by transferring energy to neon.
• Neon allows lasing by the neon to occur.

• There are many uses of lasers.
• Lasers can focus on a small spot.
• They have a wavelength that is defined.
• There are many types of lasers that provide the same wavelength of light.

• One needs to be able to pick a wavelength that will be preferentially absorbed by the material of interest.

• The objects appear a certain color because they absorb all other colors.
• The wavelength absorbed depends on the energy spacing between the electrons in the molecule.
• Unlike the hydrogen atom, biologicalmolecules are complex and have a variety of absorption lines.
• In the selection of a laser with the appropriate wavelength, these can be determined.
• Water absorbs light in the UV and IR regions.
• hemoglobin absorbs most of the UV light.

• Laser surgery uses a wavelength that is absorbed by the tissue it is focused upon.
• Total loss of vision can be caused by a detached retina.
• scar tissue that can hold the retina in place, salvaged the patient's vision after Burns made by a laser focused to a small spot on the retina.
• Refractive dispersion of different wavelength light sources can't be focused as precisely as a laser.
• Laser surgery in the form of cutting or burning away tissue is more accurate because the laser output can be very precisely focused and is preferentially absorbed.
• Depending on what part of the eye needs repair, the appropriate type of laser can be selected.
• The repair of tears in the eye can be done with a green laser.
• The light absorbed by tissues containing blood can be used to "weld" the tear.

• A laser is used to focus on a small spot on the retina, causing scar tissue to hold it in place.
• The light is focused by the lens of the eye and the laser is brought to the eye.

• Lasers are being used in dentistry.
• The soft tissue of the mouth is the most common place where lasers are used.
• They can be used to heal wounds.
• The erbium YAG laser is used to cut into bones and teeth.

• Since lasers can produce very high power in short bursts, they can be used to focus a lot of energy on a small glass sphere.
• The incident energy increases the fuel temperature so that fusion can occur and it also increases the density of the fuel.
• The implosion is caused by the impinging laser.

• Nuclear fusion can be achieved using a system of lasers.
• A burst of energy is focused on a small fuel pellet, which is imploded to the high density and temperature needed to make the fusion reaction proceed.
• CDs and DVDs have larger storage capacities than vinyl records.
• The encyclopedia can be stored on a single CD.
• The CD can be used to record digital information because the pits are very small.
• They are read by having a cheap solid-state laser beam scatter from pits as the CD spins, revealing their digital pattern and the information on them.

• Laser-created pits on a CD's surface hold digital information.
• The laser light scattered from the pit can be read.
• The precision of the laser makes it possible for large information capacity.

• holograms are used for amusement, decoration on novelty items and magazine covers, security on credit cards and driver's licenses, and for serious three-dimensional information storage.
• When viewed from different angles, a hologram is a true three-dimensional image.

• Holography uses light interference, whereas normal photography uses wave optics.
• The light from a laser is split into two parts by a mirror.
• The reference beam shines on a piece of film.
• Light from the object is interfering with the reference beam.
• The exposed film looks foggy, but close examination shows a complicated interference pattern on it.
• The film is darkened where the interference was constructive.
• Holography uses the wave characteristics of light as compared to normal photography, which requires a lens.

• A piece of film is interfered with by a single wavelength coherent light from a laser.
• A partially silvered mirror splits the laser beam into two parts, one illuminating an object and the other shining directly on the film.

• Light falling on a hologram can create a three-dimensional image.
• The film's exposed regions are dark and block the light, while less exposed regions allow light to pass.
• The film acts like a collection of gratings.
• Light passing through the hologram is diffracted in various directions, producing both real and virtual images of the object used to expose the film.
• The interference pattern is the same as the object.
• The interference pattern gives you different perspectives when you move your eye to different places.
• The image is three-dimensional like the object.