Pump-probe time-dependent interferometry performed using an infrared pump and soft $x$-ray probe in a COLTRIMS geometry

We have developed an apparatus for performing laser-pump/$x$-ray-probe experiments in a COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) geometry. A soft $x$-ray beam is produced by focusing an intense (2$^{.}$10$^{14}$ W/cm$^{2 }$-- 2$^{.}$10$^{15}$ W/cm$^{2})$ and fast (6 fs -- 50 fs) laser beam into a thick (5 Torr -- 80 Torr) gaseous medium using three different techniques: an effusive jet, a gas cell and a hollow fiber. This beam is crossed with a room-temperature effusive (10$^{-3}$ Torr) gaseous target of Ar or Ne. The $x$-ray beam is analyzed in terms of its flux, harmonic energies and angular divergence by measuring full three dimensional photoelectron momentum images in coincidence with the recoil-ions. This apparatus will be used to perform time-dependent interferometry of dissociating H$_{2}^{+}$ (D$_{2}^{+})$ ions. Neutral H$_{2}$ (D$_{2})$ will be ionized and excited using an infrared pump pulse and probed after a time delay up to 240 fs with the soft x-ray pulses. The $x$-rays will be focused onto the target, and delayed relative to the pump pulse, using a two-component coaxial coated mirror to select a photon energy band in the 36-48 eV range. Preliminary results will be presented. *This work was supported by Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy.