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We analyze spectra of a slipping flare kernel observed during the 2015 June22 M6.5-class flare by the Interface Region Imaging Spectrograph (IRIS). Duringthe impulsive and peak phases of the flare, loops exhibiting an apparentslipping motion along the ribbons were observed in the 131{\AA} channel ofSDO/AIA. The IRIS spectrograph slit observed a portion of the ribbons,including a moving kernel corresponding to a flare loop footpoint in Si IV, CII, and Mg II at a very-high 1 s cadence. The spectra observed in the kernelwere mostly redshifted and exhibited pronounced red wings, as typicallyobserved in large flares. However, in a small region in one of the ribbons, theSi IV 1402.77{\AA} line was partially blueshifted, with the correspondingDoppler velocity |v_{D}| exceeding 50 km s$^{-1}$. In the same region, the C II1334.53{\AA}, 1335.66{\AA} and 1335.71{\AA} lines were weakly blueshifted(|v_{D}| < 20 km s$^{-1}$) and showed pronounced blue wings, which wereobserved also in the Mg II k 2796.35{\AA} as well as the Mg II triplet2798.75{\AA} and 2798.82{\AA} lines. Using high-cadence AIA observations wefound that the region where the blueshifts occurred corresponds to theaccelerating kernel front as it moved through a weak-field region. The IRISobservations with high resolution allowed us to capture the acceleration of thekernel under the slit for the first time. The unique observations ofblueshifted chromospheric and TR lines provide new constrains for currentmodels of flares.

Blueshifted Si IV 1402.77{\AA} line profiles in a moving flare kernel observed by IRIS