#spectroscopy
Every note tagged #spectroscopy, newest first — or browse the full archive.
One dye, one transition: how DCDHF-Me2 earns the two-level picture
The blinking-to-absorption note modeled a fluorescent dye as two levels and named what that hides. Computing the actual excited-state manifold of DCDHF-Me2 — a push-pull dye engineered for single-molecule imaging — gives a sharper answer than we planned. Its visible absorption is essentially one transition, and the rest of the manifold lives in the deep UV; benzene, the dye's own parent ring, shows the opposite arrangement for a reason symmetry makes plain.
From blinking to absorption: how one molecule becomes a spectrum
A single fluorescent molecule blinks; a cuvette full of the same molecules gives a smooth absorption band. The two pictures are the same Jablonski diagram read at different scales — one molecule versus an ensemble, one photon at a time versus a steady-state rate. This post traces the path from quantized absorption and emission events to a bulk spectrum, and extends the same diagram to two-photon absorption as the nonlinear version of the same transition.
Forbidden and allowed: what symmetry does to a spectrum
Dissolve cobalt chloride in water and the solution is pale pink; add hydrochloric acid and it turns an intense blue — same ion, same kind of transition, a hundredfold jump in intensity. The gap didn't change; the symmetry did. This post pays the pigment series' oldest promissory note and explains what "forbidden" and "allowed" actually mean — one integral, one parity argument, one character table — and why forbidden bands show up anyway.
One matrix element, two experiments: molar absorptivity and the Pockels effect
A companion to the molar-absorptivity post. The absolute height of an absorption band, the refractive index, and the electro-optic coefficient of a poled material are three readouts of one quantity — the transition dipole. Normalizing every spectrum to 1 throws that quantity away. This traces the same matrix element from Beer's law through the two-level model to the Pockels effect, with worked numbers.
Molar absorptivity is a rate constant in disguise
The molar absorptivity in Beer's law looks like a static property of a molecule — a number you read off a table, like a melting point. It is not. The integrated absorption band is proportional to the same transition dipole that fixes the spontaneous-emission rate, so an absorption measurement quietly measures a lifetime. This post follows the chain from Beer's law to the Einstein coefficients and shows why weak absorbers are always slow emitters.
Reading water off the page: geometry, orbitals, acidity, and spectra
Water is the most familiar molecule and one of the strangest. This post builds it up from the bottom — where the atoms sit, what the electrons do, why it is both an acid and a base, and how it talks to light.