Time-resolved coincidence imaging of ultrafast molecular dynamics Arno Vredenborg, Wim G. Roeterdink and Maurice H.M. Janssen Laser Centre and Department of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands E-mail: mhmj@chem.vu.nl Abstract. We report on femtosecond molecular dynamics experiments in NO 2 using a novel photoelectron-photoion coincidence imaging apparatus. Introduction Ultrafast photon induced molecular dynamics can be probed in great detail with time- resolved coincidence imaging of ions and electrons [1-3]. The three-dimensional energy- and angle-resolved detection of ions and the correlated electrons gives full information about the energy and angular distribution of the photofragments over the various ac- cessible reaction channels. By varying the pump-probe delay time, the transition state properties of the reaction can be elucidated. Moreover, the coincident angular detection of electrons and ionic fragments enables the determination of the molecular frame an- gular distribution of the electron. These distributions directly reflect the contributions of the molecular orbitals involved in the photoionization process. Although the time re- solved coincidence imaging technique is experimentally very involved, the results can be very intuitive. In this contribution we present the dissociation pathways in NO 2 observed after excitation with 400 nm femtosecond laser pulses. Experimental Methods In our laboratory in Amsterdam a new time-resolved photoelectron-photoion coinci- dence imaging machine has become operational for the study of femtosecond molecular photodynamics. The novel apparatus has been recently described in full detail [2]. In the detection chamber, two position- and time-sensitive particle detectors are mounted perpendicular to the molecular beam. The electrons and ions are detected in a velocity map imaging configuration with additional lenses to provide low extraction fields. The electron detector uses small pore micro-channel-plates (5 μ m) to obtain high resolution electron images. The new apparatus has an electron time-of-flight (TOF) resolution of 18 ps on a typical TOF of 15 ns. The electron energy resolution is nearly laser band- width limited with ∆E/E ≈ 3.5 % for electron energies near 2 eV. The commercial laser system (Spectra Physics) consists of a Titanium-Sapphire os- cillator (Mai-Tai) which seeds the chirped regenerative amplifier (Spitfire Pro) running at 5 kHz. The pulse duration at 800 nm of the amplified regen pulses is approximately 130 fs with a pulse energy of 500 μ J. In the experiment reported here we used the second harmonic at 399.8 nm with a typical pulse energy of 15 μ J. Furthermore, two multi-stage noncollinear opto-parametric amplifiers (NOPA) are available to generate tunable short (30 fs) laser pulses in the visible wavelength region.