  We report generation of ultrashort UV pulses by soliton self-compression in
kagom\'e-style hollow-core photonic crystal fiber filled with ambient air. Pump
pulses with energy 2.6 uJ and duration 54 fs at 400 nm were compressed
temporally by a factor of 5, to a duration of ~11 fs. The experimental results
are supported by numerical simulations, showing that both Raman and Kerr
effects play a role in the compression dynamics. The convenience of using
ambient air, and the absence of glass windows that would distort the compressed
pulses, makes the setup highly attractive as the basis of an efficient
table-top UV pulse compressor.
