Jet reconnection base (UV burst or mini-flare) and jet evolution between 01:57 UT to 02:09 UT (from top to bottom). First column: images in AIA 304 Å. Second, third, and last columns: IRIS spectra of the jet reconnection site (UV burst) at slit 1 in the Mg II k 2796.35 Å line, C II doublet, and Si IV 1393.76 Å, respectively. The vertical cyan arrows and inclined dashed lines (panel a) in the first column indicate the position of the slit and the length of the spectra in the three other columns (30 < y(pixel) < 150). The white dashed line in panels h, m, and r shows the location for the profiles at y = 79 pixel, presented in Fig. 4.