Spectroscopy

On an August evening in 1864, the English amateur astronomer William Huggins pointed his spectroscope at the planetary nebula NGC 6543 and saw, instead of the continuous spectrum of a star, three bright emission lines on a dark background. The nebula was glowing gas, not unresolved stars — yet the lines resisted easy reading: Huggins could match one to hydrogen, while the brightest two went unidentified for decades, ascribed to a supposed element "nebulium" and shown only in 1928 to be forbidden lines of doubly-ionized oxygen. Two years earlier Gustav Kirchhoff and Robert Bunsen in Heidelberg had announced that each element produces its own characteristic pattern of spectral lines — a fingerprint in light. Cecilia Payne-Gaposchkin's 1925 PhD thesis used the new quantum theory of atomic spectra to read stellar absorption lines and concluded that the Sun is overwhelmingly hydrogen and helium — a result so contrary to prevailing belief that her advisor Henry Norris Russell pressured her to soften it, then later conceded she was right.

Different frequencies of electromagnetic radiation probe different physical processes in matter, and quantum mechanics dictates that atoms and molecules have discrete energy states: transitions between states absorb or emit photons of energy equal to the gap (ΔE = hν). Each kind of transition occupies a different energy range, giving each spectroscopic method its characteristic information. Rotational transitions sit in the microwave, vibrational in the infrared, valence-electronic in the UV-visible, core-electronic in the X-ray, nuclear in the gamma. IR identifies functional groups from their characteristic stretches; UV-Vis is the workhorse for chromophores and transition-metal complexes; X-ray fluorescence identifies elements; X-ray diffraction determines crystal structures brief 281; radio astronomy has detected over 250 distinct molecules in interstellar clouds through their microwave rotational lines. Two methods stand out for organic chemistry and biology. NMR spectroscopy — nuclear-spin transitions in a strong magnetic field — is the most powerful single tool for determining molecular structure in solution; the 1991 Nobel to Richard Ernst recognized Fourier-transform NMR, and the 2002 Nobel to Kurt Wüthrich extended it to protein structure. Mass spectrometry — ionization produces gas-phase ions, mass analyzers separate them by m/z — won the 2002 chemistry Nobel for John Fenn and Koichi Tanaka's ESI and MALDI methods that made it routine for biological molecules. Astronomical spectroscopy extends the same physics to extreme distances: the Doppler shift reveals exoplanet radial velocity; transmission spectroscopy of transiting exoplanets identifies atmospheric molecules.