  Spectroscopy has attracted much attention in molecular detection,
biomolecular identification, and chemical analysis for providing accurate
measurement. However, it is almost unable to distinguish different sources with
overlapped resonances in mixed analytes. Here, we present dual-comb delay
spectroscopy to overcome this problem. The introduction of group delay
spectroscopy provides a new tool to identify sources that would lead to
overlapped resonances in intensity or phase spectroscopy. To obtain
sufficiently high spectral resolution and signal-to-noise ratio for achieving
reliable group delay spectrum, a probe comb with the wavelengths precisely
scaned by a microwave source is applied, leading to attometer-level resolution
and million-level signal-to-noise ratio. In an experiment, spectroscopy with an
optional resolution up to 1 kHz (8 attometer), an average signal-to-noise ratio
surpassing 2,000,000, and a span exceeding 33 nm is demonstrated. Two
overlapped resonances from two different sources are clearly differentiated.
Our work offers a new perspective for exploring the interaction between matter
and light.
