Free-running ultraviolet dual comb spectroscopy enabling absolute electronic fingerprinting

Abstract Ultraviolet (UV) spectroscopy reveals electronic transitions in matter that underpin atmospheric photochemistry and molecular dynamics. While dual-comb spectroscopy (DCS) has revolutionized precision measurements in the infrared region and expanded to the THz to the visible regions, its extension into the UV has been accomplished only very recently with demanding experimental efforts. Here we introduce free-running ultraviolet dual-comb spectroscopy (UV-DCS), a straightforward, high-fidelity method for absolute absorption cross sections and rapid molecular fingerprinting in the atmospheric UV window. The approach delivers high spectral resolution (1 GHz), broad bandwidth (12 THz), and fast acquisition (500 ms) without active stabilization, resulting in a UV-DCS quality factor that exceeds previously reported values.. Applied to formaldehyde (HCHO), a key atmospheric pollutant and photochemical driver, the technique yields an unprecedented count of rovibrational transitions in this window, enriching the molecular line list used for atmospheric chemistry and remote sensing. The measurements produce refined rotational constants, enabling high-accuracy quantum simulations of molecular eigenstates and informing ab initio models. Beyond robust absolute cross sections, UV-DCS provides a universal, rapid fingerprinting tool for transmissive species, offering fast atmospheric sensing and a rigorous benchmark for quantum theory without stabilization requirements. Coupled with advanced formaldehyde synthesis, these results support improved atmospheric monitoring, more reliable retrievals of HCHO abundances, and enhanced validation of fundamental molecular physics. Overall, this work realizes a free-running UV-DCS platform combining GHz resolution, multi-terahertz bandwidth, and sub-second acquisition, and applies it to refine formaldehyde ultraviolet spectroscopic parameters relevant to atmospheric and molecular science.