The main research activity of the Frankfurt group in the late seventieth was the investigation of inner shell vacancy production and their decay (quasimolecular x rays) by measuring angular dependent x-ray emission probabilties. Thus to perform measurements at very small angles in gas targets the detection of the recoiling target ion (transverse kinematics) instead of the slightly scattered projectile was obvious. This idea was discussed between Lew Cocke and Horst Schmidt-Böcking during Lews visit in 1978 in Heidelberg. First experiments on the He target were performed in 1978-9 at the Tandem accelerator at Kansas State University, however, these experiments were not really successful and in principle to difficile with the simple technique used at KSU and not worth to continue.

In 1982 the Frankfurt group developed a new approach and applied for funding of this project to GSI (but never received any answer or funding). It was the PHD thesis of Joachim Ullrich (1982-1987) to develope and test a new generation of spectrometers to measure at least the transverse momentum of the recoiling target ion. To perform such experiments also new generation position sensitive detectors (MCP with wedge and strip anodes) were developed and successfully tested. The steady progress of this project was published in the annual reports of the IKF-University Frankfurt in the early eightteeth. At the GSI UNILAC successful experiments could be performed and the first publication on the new recoil ion momentum spectrometer with reliable small angle data appeared in Phys.Lett. A125 (1987) 193 by Ullrich and Schmidt-Böcking. In these experiments in MeV heavy ion rare gas collisions very large impact parameters could be investigated for the first time. During this time Lepera and Ivan Sellin measured with a very different technique (non coincident method) the momenta of low energetic recoil ions too. They presented first results on the angular dependence at Denton 1986. But they did not continue with this work.

However, testing the resolution of the technique the correspondence of recoil and projectile transverse momentum was measured in the low 10-5 rad regime. The expected correspondence was broken below such angles and one first concluded that this is the principle limitation of the recoil ion momentum technique. However, with the help of CTMC calculations of Ronald Olson and numerous discussions with him (from 1986 -87 RO was Humboldt Award fellow in Frankfurt) the Frankfurt group ( J.U., Reinhard Dörner and HSB) realized, that the method (recoil ion momentum spectroscopy) was not limited to angles above 10-5 rad , but that the method was at lower angles even sensitive to the momenta of the involved electrons. It was immediately clear that the thermal motion of the target was the limiting resolution barrier and one had to cool the target gas as much as possible. Thus measurements beyond Born-Oppenheimer Approximation were possible and through a coincidence with the emitted electrons the correlated motion of nuclei and electrons could be revealed. This was the birth of the COLTRIMS microscope idea.

In 1986 Reinhard Dörner had joined the recoil group and in his PHD thesis (1986 - 91) cooling of the target (windowless gas cell) to a few Kelvin temperatur was inplemented into the apparatus, thus the first generation COLTRIMS system was born. RD could thus successfully measure the angular dependence of the ionization of He by fast proton impact down to very small angles (below 10-5 rad). He used a spectrometer which had no extraction field. During these years the cooperation with Lew Cocke on these recoil work started to deepen again and in the Frankfurt and the KSU labs recoil ion momentum spectroscopy got the major research project. Information and also man power was exchanged frequently between both groups. In 1991 Lew Cocke and Horst Schmidt-Böcking received together for this work the Max-Planck Research Award. The KSU group used a diffusive jet with a small recoil ion extraction field. Here Rami Ali and later Vicki Frohne in their PHD work measured successfully transverse, but Rami Ali succeeded for the first time to measure also longitudinal momenta of the recoil ion.

Frankfurt started in the PHD work of Ottmar Jagutzki (1989- 94) the development of a super sonic jet. Parallel to these work the Caen group of Jean Piere Gardin and Amin Cassimi bought a super sonic jet, to use the in heavy ion collisions created low energetic recoils as a very cold ion source. They also reallized in the arly ninteeth (partly in discussions with their collegues from Frankfurt and KSU) that they had to implement just a position sensitive detector to get a perfect COLTRIMS system.

Using the experience made in Frankfurt and KSU Volker Mergel started in 1992 to build the first high resolution COLTRIMS imaging system. He implemented first a super sonic jet (internal gas temperature a few 10 milli K) where the He gas was precooled to about 15°Kelvin. Second he implemented an electrostatic extraction system with a space focusing lense that even for a large target region a recoil momentum resolution of below 0.1 a.u. with an recoil detection efficiency of nearly 100% could be obtained. Thus for the first time (1993-4) the capture of electrons from He to excited states could be measured and could be resolved in the different longitudinal momentum in fast proton He collisions. His apparatus incorporated all important features of the COLTRIMS dynamic microscope imaging the recoil ion momenta by measuring the ion time of flight and the impact position on the recoil ion detector.

Since 1989 Joachim Ullrich was employed at GSI but continued in extrem close contact the cooperation with the Frankfurt group thus he was fully involved in all further Frankfurt developement. In the nineteeth he began (from 1994 with Robert Moshammer) to develope a COLTRIMS system for the GSI heavy ion accellerators and storage rings. Therefor the GSI system needed a completely different approach. They introduced for their very open spectrometer magnetic fields and could obtain also for high energetic electron detection a nearly 100% detection efficiency. He received for his very important contributions to the COLTRIMS development the 1999 Leibniz-Award (3 Mil. DM research funding).

This is only a short review of the COLTRIMs history seen with Frankfurt eyes. Since about 1992 many students (PHD, Master thesis and Diplomwork) at Frankfurt, KSU, GSI and Freiburg have succesfully performed COLTRIMS experiments.