Podcast
Questions and Answers
What does the process of photoassociation involve?
What does the process of photoassociation involve?
- Absorption of energy from a photon
- Breaking a molecule into two products
- Formation of a molecular complex (correct)
- Transition between vibrational states
In a Jablonski Diagram, which of the following statements accurately represents singlet states?
In a Jablonski Diagram, which of the following statements accurately represents singlet states?
- S1 has lower energy than S0
- S0 is the minimum energy state
- Each singlet state corresponds to increasing energy levels (correct)
- There are only two singlet states
What role does internal conversion (IC) play in photophysical processes?
What role does internal conversion (IC) play in photophysical processes?
- It indicates a process of energy loss through radiation
- It describes the emission of light from a ground state
- It facilitates the formation of more complex molecules
- It represents a transition between excited states without photon emission (correct)
Which of the following processes results in the creation of new products from an excited molecule?
Which of the following processes results in the creation of new products from an excited molecule?
What is the primary significance of photochemical processes in biology?
What is the primary significance of photochemical processes in biology?
What happens to molecules when they absorb photons?
What happens to molecules when they absorb photons?
What is the nature of excited states in molecules?
What is the nature of excited states in molecules?
Which process can lead to the deactivation of an excited state?
Which process can lead to the deactivation of an excited state?
What notation is used to represent a molecule in an excited state?
What notation is used to represent a molecule in an excited state?
What is one example of a photochemical process?
What is one example of a photochemical process?
Which process converts an excited state into an ion and an electron?
Which process converts an excited state into an ion and an electron?
Which of the following is a non-radiative process?
Which of the following is a non-radiative process?
What does the term 'excimer' refer to in molecular photochemistry?
What does the term 'excimer' refer to in molecular photochemistry?
What is the reason the emitted photon has a lower energy than the transmitted photon in fluorescence?
What is the reason the emitted photon has a lower energy than the transmitted photon in fluorescence?
Which transition is indicated as possible for emission in the discussed fluorescence process?
Which transition is indicated as possible for emission in the discussed fluorescence process?
What does the shape similarity between the absorption and fluorescence spectra suggest?
What does the shape similarity between the absorption and fluorescence spectra suggest?
In what time frame does fluorescence typically occur after initial excitation?
In what time frame does fluorescence typically occur after initial excitation?
What is one of the primary processes through which molecules return to the ground state after excitation?
What is one of the primary processes through which molecules return to the ground state after excitation?
Which statement correctly describes the process of absorption?
Which statement correctly describes the process of absorption?
What must occur for a photon to cause absorption in a molecule?
What must occur for a photon to cause absorption in a molecule?
Which type of light interaction is primarily involved in rotating molecular transitions?
Which type of light interaction is primarily involved in rotating molecular transitions?
What happens if the energy of the photon does not match the energy difference between two states during absorption?
What happens if the energy of the photon does not match the energy difference between two states during absorption?
Which scattering technique involves the inelastic interaction of light with molecular vibrations?
Which scattering technique involves the inelastic interaction of light with molecular vibrations?
At what energy levels do vibrational movements occur in molecules?
At what energy levels do vibrational movements occur in molecules?
Which process is a result of a molecule being excited by absorbed light energy?
Which process is a result of a molecule being excited by absorbed light energy?
What is the primary reason for the complexity of spectra as light energy increases?
What is the primary reason for the complexity of spectra as light energy increases?
What do the vertical squiggly lines on the Jablonski diagram represent?
What do the vertical squiggly lines on the Jablonski diagram represent?
How is excess vibrational energy converted in excited species?
How is excess vibrational energy converted in excited species?
What does internal conversion involve?
What does internal conversion involve?
What is the typical duration of internal conversion?
What is the typical duration of internal conversion?
Which of the following statements about fluorescence is true?
Which of the following statements about fluorescence is true?
What happens to the energy as a molecule steps down the vibrational ladder in fluorescence?
What happens to the energy as a molecule steps down the vibrational ladder in fluorescence?
Quinine in tonic water absorbs light at which wavelengths?
Quinine in tonic water absorbs light at which wavelengths?
Which transitions contribute to the absorption spectrum in fluorescence?
Which transitions contribute to the absorption spectrum in fluorescence?
What characterizes spontaneous emission?
What characterizes spontaneous emission?
How does stimulated emission differ from spontaneous emission?
How does stimulated emission differ from spontaneous emission?
What is the role of energy gaps in stimulated emission?
What is the role of energy gaps in stimulated emission?
Which statement is true regarding photon absorption?
Which statement is true regarding photon absorption?
What does the diagram of potential energy curves illustrate?
What does the diagram of potential energy curves illustrate?
Which aspect of stimulated emission is critical for laser operation?
Which aspect of stimulated emission is critical for laser operation?
What occurs during spontaneous emission?
What occurs during spontaneous emission?
In what way is stimulated emission dependent on original photons?
In what way is stimulated emission dependent on original photons?
What is the common feature of photons emitted through spontaneous and stimulated emission?
What is the common feature of photons emitted through spontaneous and stimulated emission?
What is one key characteristic of photons emitted through spontaneous emission?
What is one key characteristic of photons emitted through spontaneous emission?
Flashcards
Light interaction with matter at a molecular level
Light interaction with matter at a molecular level
Light interacting with the electric charges in matter causes them to move, resulting in various energy levels. Examples include rotational movements for microwaves, vibrational movements for Infrared light, and electronic movements for visible and UV light.
Absorption of light by matter
Absorption of light by matter
The process where a molecule transitions from a lower energy state to a higher energy state by absorbing a photon whose energy matches the energy difference between the states.
Non-resonant absorption
Non-resonant absorption
When a photon's energy doesn't match the energy difference between states, it can still be absorbed but is immediately re-emitted, lasting only 10 femtoseconds.
Light interaction with bulk matter
Light interaction with bulk matter
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Emission of light by matter
Emission of light by matter
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Fluorescence
Fluorescence
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Rayleigh scattering
Rayleigh scattering
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Raman scattering
Raman scattering
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Photoassociation
Photoassociation
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Photodecomposition
Photodecomposition
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Jablonski Diagram
Jablonski Diagram
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S0
S0
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Vibrational Relaxation (VR)
Vibrational Relaxation (VR)
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Excited State
Excited State
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Deactivation
Deactivation
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Radiative Process
Radiative Process
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Non-radiative Process
Non-radiative Process
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Excited Molecule
Excited Molecule
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Photochemistry
Photochemistry
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Ground State
Ground State
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Electronic Excitation
Electronic Excitation
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Spontaneous Emission
Spontaneous Emission
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Stimulated Emission
Stimulated Emission
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Cloned Photon
Cloned Photon
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Stimulated Emission: Laser Operation
Stimulated Emission: Laser Operation
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Potential Energy Curves
Potential Energy Curves
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Vibrational States
Vibrational States
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Photon Absorption
Photon Absorption
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Photon Emission
Photon Emission
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Vibrational Relaxation
Vibrational Relaxation
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Non-radiative Decay
Non-radiative Decay
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Internal Conversion
Internal Conversion
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S0 (Singlet Ground State)
S0 (Singlet Ground State)
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Vibrational Transitions
Vibrational Transitions
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Sn (Excited Singlet State)
Sn (Excited Singlet State)
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Study Notes
Module Structure
- Fundamentals of light
- Propagation of light in waveguides
- Light interaction with matter
- Lasers
- Photobiology basics
- Biophotonics applications
- Bioimaging
- Tissue engineering
Topics to be Covered
- Atoms and Molecules
- Molecular level interactions
- Absorption, spontaneous emission, stimulated emission
- Fate of excited molecules
- Fluorescence
- Light scattering
- Rayleigh, Mie, Raman, spectroscopy
Introduction
- Light interaction with bulk matter includes reflection, refraction, and diffraction
- At a molecular level, time-varying electric fields of light interact with matter's electric charges and dipoles
- Forces cause charges and dipoles to accelerate
- Rotational - microwaves or far infrared light
- Vibrational - infrared light
- Electronic - visible and ultraviolet light
- As energy increases (from microwaves to UV), more movements become possible, making spectra more complex
- Absorption and emission involve quantized electronic and vibrational states of molecules
Absorption
- Absorption is a transition from a lower energy state to a higher one, powered by a photon
- The energy difference between the states (En-Em) must equal the photon energy (hf)
- The photon is annihilated in the process
- If the photon energy doesn't match the energy difference, the photon might be re-radiated (e.g., in 10 femtoseconds) causing phenomena like Rayleigh scattering
Spontaneous Emission
- Spontaneous emission is a random process
- An excited state returns to a stable lower energy state by emitting a photon
- The phase and direction of the emitted photon are random and independent of other photons
Stimulated Emission
- A photon with energy equal to the energy gap triggers an excited state to return to a lower energy state, emitting a second photon
- The second photon has identical frequency, phase, direction, and polarization to the original photon
- This is how lasers work, with the original photon being retained
Potential Energy Curves
- Diagrams show ground and excited electronic energy states of a molecule
- Both have vibrational states (labeled v')
- Photon absorption generally starts from v"=0 ground state
- Photon emission generally starts from an excited electronic state with v'=0
Excited States of Molecules
- Molecules (or aggregates) enter excited states when absorbing photons
- Excited states are unstable and return to ground states as quickly as possible
- Energy release can be radiative or non-radiative
- Several processes compete for deactivation
- Excited states are denoted by A* and ground state by A
Excited States of Molecules - 2
- Photoinduced electron transfer
- Photoionization (A* → A+ + e-)
- Electron transfer (D* + A → D+ + A)
- Energy transfer (A* + B → A + B*)
- Excited state complexes
- Excimer (A* + A → (A-A)*)
- Exciplex (A* + B → (A-B)*)
- Various processes like state-to-state crossing (internal conversion), vibrational relaxation.
Photochemistry
- Photochemistry describes the absorption of light by molecules and resulting chemical reactions leading to stable compounds
- Excited molecules can release energy through photoassociation (A* + B → A-B) or photodecomposition (A* → B + C)
- These processes play roles in biology (e.g., photosynthesis, vision)
Photophysical Process
- Jablonski diagrams show energy transitions
- Singlet states (e.g., S0, S1, S2) with increasing state number, minimum energy increases
- So represents the ground state
- Short lines represent quantized vibrational states
- Straight lines represent transitions connected to photon absorption or emission
- Vertical 'squiggly' lines on Jablonski diagrams indicate vibrational relaxation
Vibrational Relaxation
- Energy loss through vibrational relaxation occurs on vertical 'squiggly' lines
- Unless between zero-point vibrational states, excess vibrational energy exists and is converted into translational energy through collisions (heating)
- Energy can also be lost through emission in the infrared range when collisions are infrequent
State-to-State Crossing
- Horizontal lines represent changes in quantum states without energy changes
- A non-radiative process is internal conversion (IC)
- Internal conversion is rapid and involves vibrational relaxation to the lowest vibrational level of the excited state (S1)
Tonic Water
- Quinine absorbs light at 250nm and 350nm
- Quinine emits light at 450nm
Fluorescence
- Fluorescence is the process where an excited singlet state (S1) returns to the singlet ground state (S0) by emitting a photon
- The absorption spectrum has a higher wavelength than fluorescence spectra
- Fluorescence is rapid (nanoseconds) and used in numerous applications, such as environmental monitoring, clinical chemistry, and DNA sequencing
Fluorescence - 2
- Initial photon absorption takes the molecule from the zero vibrational state of the ground state to an excited electronic state with vibrational energy
- Energy loss through collisions leads to vibrational relaxation
- The excited molecule returns to the ground state by emitting a photon, resulting in a fluorescence spectrum
- Vibrational states play significant roles in the absorption and emission spectra
Fluorescence - 3
- 0-0 absorption and fluorescence transitions can occur simultaneously (same wavelength)
- Emitted photon has lower energy than the absorbed photon, leading to a higher wavelength in the fluorescence spectrum
- Fluorescence is a rapid process, occurring within nanoseconds
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