A Lab Measures a Second Infrared Fingerprint for the Mothball Molecule

A laboratory group led by Arvind Paunikar reports the first measurement of the infrared absorption spectrum of naphthalene in its lowest excited triplet state, in a preprint posted to arXiv on Aug. 25, 2026.

Naphthalene is the smallest polycyclic aromatic hydrocarbon, or PAH, and the compound in traditional mothballs. It is one of the flat carbon-and-hydrogen molecules usually credited with the aromatic infrared bands, emission features the paper describes as observed "ubiquitously across a wide variety of interstellar objects." Those bands are commonly attributed to PAHs shedding energy as infrared light after ultraviolet radiation excites them.

The authors write that neutral PAHs have a second way to shed that energy: crossing into the triplet manifold, a longer-lived excited state that radiates its own infrared spectrum. That route "is expected to be a competing relaxation channel," they write, but neutral PAHs in triplet states "have remained largely unexplored, primarily because of the lack of direct spectroscopic access."

According to the paper, the measured triplet spectrum is "markedly different" from the vibrational spectrum of the singlet ground state, and the authors report that anharmonic frequency calculations reproduce their measurements closely.

The authors present the spectrum as "a stepping stone for detecting triplet-state PAHs in the interstellar medium" and as a basis for building more accurate infrared emission models.