"This liquid is glowing!"
Toma shone UV light. Fluorescent dye glowed green.
Milia explained. "Fluorescence. Excited electrons emitting light."
Kana watched curiously. "Excited?"
Rei drew a diagram. "State where electrons rise to higher energy levels."
"Normally, electrons are in the lowest orbital. Ground state."
"But when absorbing light?"
"Jump to higher orbitals. Excited state."
Milia supplemented. "That energy difference determines absorbed light wavelength."
"E = hν," Rei wrote the equation. "Energy is proportional to frequency."
Kana asked. "What happens to jumped electrons?"
"Fall," Toma said. "But how they fall matters."
Rei explained. "In fluorescence, quickly emit light and fall."
"Nanosecond order. Very fast."
"Phosphorescence?"
"Slow. Seconds to hours," Milia answered.
"Goes through triplet state."
Kana was confused. "Triplet?"
Rei explained in detail. "Electron spin states. Singlet and triplet exist."
"Fluorescence is singlet to singlet. Allowed transition."
"Phosphorescence is triplet to singlet. Forbidden transition."
Toma did another experiment. "Shine light on chlorophyll solution."
Red fluorescence appeared.
"Chlorophyll fluorescence," Milia said.
"What happens in photosynthesis?" Kana asked.
"Converting light energy to chemical energy," Rei answered.
Milia drew a diagram. "When chlorophyll absorbs light, electrons are excited."
"Those electrons pass to electron transport chain."
"Eventually making NADPH and ATP."
Kana understood. "Excited electrons are energy carriers?"
"Correct," Rei acknowledged.
Toma asked. "Why can we see fluorescence?"
"Some excitation energy escapes as light," Milia explained.
"But most is used for chemical reactions. That's why photosynthesis is efficient."
Rei supplemented. "In photosystem II, water is oxidized."
"2 H2O → O2 + 4H+ + 4e-"
"Excited electrons have power to steal electrons from water."
Kana was surprised. "Electrons are that powerful?"
"In excited state, oxidizing and reducing power change," Milia explained.
"Reactions impossible in ground state become possible."
Toma gave another example. "Ozone formation too?"
"Yes. UV light decomposes oxygen molecules," Rei answered.
"O2 → 2O・excited oxygen atoms"
"Those react with another O2 to make O3."
Milia continued. "Vision is also photochemical reaction."
"Retinal absorbs light and isomerizes."
"Cis to trans. Shape changes."
Kana asked. "What happens with that shape change?"
"Protein structure changes. Neural signal is generated."
Rei summarized. "Light→electron excitation→chemical change→signal"
"Seeing is also chemistry."
Toma turned off the light. Fluorescence disappeared.
"Electrons returned to ground state."
Milia explained. "But gradually lose energy as heat."
"That's why emitted light has longer wavelength than absorbed light."
"Stokes shift," Rei added.
Kana thought. "The day electrons can't settle?"
"Excited state," Milia smiled. "High energy level is unstable."
"Electrons always try to return to lower state."
Toma said poetically. "But in that unstable moment, miracles happen."
"Photosynthesis, vision, fluorescence."
Rei acknowledged. "Because they're unstable, reactivity is high."
"Days when electrons can't settle are important for life."
Milia looked out the window. Sunlight.
"Right now, countless electrons are being excited."
"In leaves, in the sea, in the sky."
Kana murmured. "Electron anxiety moves the world."
"Unsettled electrons carry energy."
Toma laughed. "I have days I can't settle too."
"Humans and electrons are the same."
The four smiled. Invisible electrons are jumping even now. Days of restlessness are beautiful.