Rotational spectrum of formaldehyde reinvestigated using a photomixing THz synthesizer

► We develop a cw-THz spectrometer by photomixing able to measure rotational transitions on a large frequency range with a sub-MHz spectral purity and frequency accuracy. ► 60 new rotational transitions of H2CO are measured for the first time by photomixing cw-THz spectroscopy. ► A new set of ground state molecular parameters with reduced uncertainties is proposed for the spectroscopic databases. ► Coupled to a multipass cell, the cw-THz spectrometer is also able to probe transitions within the 31 41 and 61 states of H2CO.