SHE - Wave Particle Duality

Fluorescence

• occurs when an atom or molecule absorbs a photon, promoting an electron to a higher, quantised energy level. The electron almost immediately (within nanoseconds) falls back down — often via an intermediate energy level — emitting a new photon

• because some energy is lost, the emitted photon has less energy and therefore a longer wavelength than the absorbed photon

  • STOKES SHIFT

• (this is why fluorescent dye absorbs invisible light and glows visible light)

Phosphorescence

• excited electron becomes trapped in a metastable state - a higher energy level from which the quantum-mechanically ‘allowed’ transition back to the ground state is much less probable

• reaches meta-stable states and eventually falls back down from there

• delays re-emission from nanoseconds to seconds, minutes/even hours → produces glow in the dark effect long after light source is removed

X-rays

• produced by two mechanisms inside an X-ray tube both grounded in the photon model of light

  • CONCEPTUALLY THE REVERSE OF THE PHOTOELECTRIC EFFECT

  • KE of a charge converts into photon energy rather than photon energy freeing a charge

• Characteristic X-rays → high-speed electrons knock an electron out of an inner shell of a target atom

  • outer shell electron drops down to fill the vacancy, releasing a photon whose energy equals the gap between those two quantised inner-shell energy levels

  • because inner-shell energy gaps are large, the emitted photon falls in the x-ray region of the spectrum

BASICALLY

• xrays are smashed into a metal target, where electrons slow down and release photons, and electrons knock out other electrons, which are filled

X-ray intensity

• corresponds to peaks

Bremsstrahlung (braking radiation)

• fast electrons are decelerated in the electric field of the target nuclei. The kinetic energy lost appears as a photon, producing a continuous spectrum of X-ray energies up to a maximum set by the electron’s full KE converting into a single photon

• due to the emission of photons as electrons decelerate on collision with atoms

Medical sciences

• X-ray radiography and CT scanning → uses X-ray photon absorption → depends on density and atomic number of tissue which gives contrast between bone and soft tissue

  • fluorescent dyes and tags are used to visualise cells and structures

Forensic sciences

• UV light is used to induce fluorescence in biological fluids, fibres and latent fingerprints making invisible evidence visible

Astronomy - e.g. space telescopes

• Space-based X-ray telescopes detect X-rays from extremely energetic sources - black-hole accretion disks, neutron stars, supernova remnants - that are absorbed by Earth’s atmosphere and invisible from the ground

• Spectral analysis of starlight → (emission and absorption lines) allows astronomers to identify the elements present in a star or nebula and to determine temperature, motion, and composition

Industry - food irradiation

• high-energy X-rays/gamma rays sterilise food by ionising molecules in bacteria and pests

  • depends on energy of each individual photon, not only on intensity of radiation

Industry - airport scanners

• image luggage and cargo - image contrast again comes from differential absorption of X-ray photons by different materials

Industry - x-ray fluorescence

• a sample is bombarded with X-rays, ejecting inner shell electrons. As outer electrons fall to fill vacancies, sample re-emits characteristic X-ray photons whose energies identify elements present

Industry - fluorescent labelling, dyes and biological markers

• a fluorophore is attached to a target molecule (e.g. protein, antibody, DNA probe), and excited with light of one wavelength. The longer-wavelength fluorescent light it re-emits is then imaged, allowing biologists to track structures and processes

• these all show why photon energy not just intensity governs ionisation and excitation

domestic and industrial lighting

• fluorescent tubes and compact fluorescent lamps - an electrical discharge excites mercury vapour atoms, emitting UV phons as electrons fall between quantised energy levels

  • UV light strikes a phosphor coating which absorbs it and fluorescenes, re-emitting as visible light

  • sodium and mercury vapour street/industrial lighting rely on the characteristic,

Pharmaceutical and cosmetic industries

• UV and X-ray sterilisation of pharmaceutical equipment, packaging and products uses the same ionising photon physics as food irradiation

• Fluorescence-based bioassays and tracers are used in drug discovery and testing, to track binding, absorption or metabolism of a drug.

  Cosmetic 'optical brighteners' absorb UV light and fluoresce visible blue light, making fabrics, papers or cosmetic products appear brighter or whiter; sunscreen testing also relies on UV photon-absorption physics.

The Laser

• light amplification by stimulated emission of radiation

• a photon is emitted whenever an electron drops between two quantised atomic energy levels

• creates a ‘popular inversion’ so more atoms are in an excited stage than in the ground state

• incoming photon with energy exactly matching that energy gap can then stimulate an already-excited atom to emit a second photon, which cascades into an intense, coherent, monochromatic beam

Photovoltaic cells

• convert light directly into electricity through a solid-state version of the photoelectric effect

• photons striking the semiconductor transfer their energy to electrons

• if a photon’s energy exceeds the energy needed to free an electron into the conduction band (work function), that electron can contribute to an electric current

• photovoltaic cells only work because light arrives as discrete photon 'packets' of energy E = hf, with a frequency threshold below which no current is produced regardless of intensity. A wave-only model of light cannot explain this threshold behaviour.

LEDS

• light emitting diodes

• conceptually the reverse of a photovoltaic cell

• at the junction of a semiconductor diode, electrons and holes recombine and an electron drops from a high conduction band to a lower valence band, quantised energy level, releasing the energy difference as a single photon

• size of energy gap determines photon’s energy and therefore the colour of light

• electrons are given energy to jump that gap by applying a voltage. If v is at threshold, electrons can now conduct and fall back down the band gap, releasing photons

LED equation

• eVth = hf = Eg

WHERE

• Eg = band gap

• E = electron charge

• Vth = threshold voltage

Australian synchrotron

• a particle accelerator that accelerates electrons in a ring

• coming off the ring are experimental stations - electrons ‘brake’ as they go around the ring, emitting photons equal in energy. Those photons are ‘shone’ on a sample to image the sample

• Synchrotron light → When electrons are accelerated, they produce emr which is extremely bright, highly polarised, emitted in short pulses.

spectral analysis

●      “atoms of an element emit and absorb specific wavelengths of light that are unique to that element; this is the basis of spectral analysis.” This is the direct mechanism behind X-ray fluorescence (XRF) analysis: a sample is excited and the specific characteristic wavelengths it re-emits identify which elements are present — exactly the forensic and industrial ‘XRF’ application named in the SHE point.

photovoltaic cell by analogy with the photoelectric effect equation,

Ek = hf − φ   (the photoelectric effect equation)   —   a very close conceptual analogue for a photovoltaic cell: φ, the work function in the photoelectric effect, plays the same threshold-energy role as the semiconductor's band gap in a solar cell. A photon must supply at least that much energy before any usable (photo)current is produced; any extra energy above the threshold appears as the kinetic energy of the freed charge carrier, exactly as in Ek = hf − φ.

quantisation

●      Quantisation — “atomic phenomena and the interaction of light with matter indicate that states of matter and energy are quantised into discrete values” is again the umbrella idea: a laser produces one specific wavelength (not a spread of colours) and an LED's colour is fixed by its material, precisely because the underlying energy transitions are discrete, not continuous.