New infrared undulator beamline at FLASH M. Gensch a* , L. Bittner a , A. Chesnov b , H. Delsim-Hashemi c , M. Drescher c , B. Faatz a , J. Feldhaus a , U. Fruehling a , G.A. Geloni a , Christopher Gerth a , O. Grimm c , U. Hahn a , M. Hesse a , S. Kapitzki a , V. Kocharyan a , O. Kozlov b , E. Matyushevsky b , N. Morozov b , D. Petrov b , E. Ploenjes a , M. Roehling a , J. Rossbach c , E.L. Saldin a ,B. Schmidt a , P. Schmueser a , E.A. Schneidmiller a , E. Syresin b , A. Willner c , M.V. Yurkov a a Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany b Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia c Institute for Experimental Physics, University of Hamburg, Luruper Chaussee 149, 22671 Hamburg, Germany Elsevier use only: Received date here; revised date here; accepted date here Abstract At the vacuum ultraviolet (VUV) free electron laser in Hamburg (FLASH) an infrared (IR) beamline is being built to allow novel pump-and-probe experiments combining coherent IR pulses with the FEL radiation in the VUV spectral range. It will provide useful IR radiation generated by a purpose built undulator over the wavelength range from 200 μm to 10 μm and possibly even shorter. The commissioning of the beamline has started this summer and first light will be delivererd to the experimental hall by autumn 2007. Another important application of the beamline will be electron diagnostics of the longitudinal charge distribution of the electron bunches. © 2001 Elsevier Science. All rights reserved Synchrotron radiation; pump-probe; infrared; THz; VUV; FEL; coherent * Corresponding author. Tel.: +49-40-8998-4526; fax: +49-40-8998-4475; e-mail: michael.gensch@desy.de. 1. Introduction In this paper we announce the start of commissioning of a new IR beamline at the vacuum ultraviolet free electron laser in Hamburg (FLASH). The beamline is transporting the radiation of an IR undulator in the free electron laser (FEL) accelerator tunnel into the experimental hall of FLASH. The main purpose is to provide IR radiation for VUV/IR pump probe experiments with the VUV pulses from FLASH. One diagnostic port of the beamline shall furthermore be used for studies of the longitudinal charge distribution of the electron bunches in the FEL accelerator. 2. FLASH FLASH is a world wide unique source of extremely brilliant, intense, femtosecond (fs) VUV