  The Interface Region Imaging Spectrograph(IRIS) with its high spatial and
temporal resolution brings exceptional plasma diagnostics of solar
chromospheric and coronal activity during magnetic reconnection. The aim of
this work is to study the fine structure and dynamics of the plasma at a jet
base forming a mini flare between two emerging magnetic fluxes (EMFs) observed
with IRIS and the Solar Dynamics Observatory (SDO) instruments. We proceed to a
spatio-temporal analysis of IRIS spectra observed in the spectral ranges of Mg
II, C II, and Si IV ions. Doppler velocities from Mg II lines are computed by
using a cloud model technique. Strong asymmetric Mg II and C II line profiles
with extended blue wings observed at the reconnection site (jet base) are
interpreted by the presence of two chromospheric temperature clouds, one
explosive cloud with blueshifts at 290 km/s and one cloud with smaller
Dopplershift (around 36 km/s). Simultaneously at the same location (jet base),
strong emission of several transition region lines (e.g. O IV and Si IV),
emission of the Mg II triplet lines of the Balmer-continuum and absorption of
identified chromospheric lines in Si IV broad profiles have been observed and
analysed. Such observations of IRIS line and continuum emissions allow us to
propose a stratification model for the white-light mini flare atmosphere with
multiple layers of different temperatures along the line of sight, in a
reconnection current sheet. It is the first time that we could quantify the
fast speed (possibly Alfv\'enic flows) of cool clouds ejected perpendicularly
to the jet direction by using the cloud model technique. We conjecture that the
ejected clouds come from plasma which was trapped between the two EMFs before
reconnection or be caused by chromospheric-temperature (cool) upflow material
like in a surge, during reconnection
