  As part of the first Potsdam Echelle Polarimetric and Spectroscopic
Instrument (PEPSI) key-science project, we aim to provide well-exposed (viz.
deep) high-resolution spectra of representative stellar targets. These spectra
will be made available in form of (electronic) atlases. The first star in this
series of papers is our Sun. It also acts as a system-performance cornerstone.
The deep spectra in this paper are the results of combining up to 100
consecutive exposures per wavelength setting and are compared with other solar
flux atlases. Our software for the optimal data extraction and reduction of
PEPSI spectra is described and verified with the solar data. Three deep solar
flux spectra with a spectral resolution of up to 270,000, a continuous
wavelength coverage from 383 nm to 914 nm, and a photon signal to noise ratio
(S/N) of between 2,000-8,000:1 depending on wavelength are presented.
Additionally, a time-series of 996 high-cadence spectra in one cross disperser
is used to search for intrinsic solar modulations. The wavelength calibration
based on Th-Ar exposures and simultaneous Fabry-Perot combs enables an absolute
wavelength solution within 10 m/s (rms) with respect to the HARPS laser-comb
solar atlas and a relative rms of 1.2 m/s for one day. For science
demonstration, we redetermined the disk-average solar Li abundance to
1.09+/-0.04 dex on the basis of 3D NLTE model atmospheres. We detected
disk-averaged p-mode RV oscillations with a full amplitude of 47 cm/s at 5.5
min. Comparisons with two solar FTS atlases, as well as with the HARPS solar
atlas, validate the PEPSI data product. Now, PEPSI/SDI solar-flux spectra are
being taken with a sampling of one deep spectrum per day, and are supposed to
continue a full magnetic cycle of the Sun.
