This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys. Cite this: DOI: 10.1039/c3cp53507j 268 nm photodissociation of ClN 3 : a femtosecond velocity-map imaging study D. Staedter, a N. Thire ´, ab E. Baynard, a Peter C. Samartzis c and V. Blanchet* ad We report the first time-resolved study of the photochemistry of chlorine azide (ClN 3 ) by femtosecond velocity-map imaging (fs-VMI). The dissociation dynamics are initiated at 4.6 eV and the photofragments are detected by multiphoton ionization using an intense laser field centered at 803 nm. A dissociation time of 262  38 fs was measured from the rising time of the co-fragments N 3 and Cl. The time dependency of the angular distribution of N 3 , which converges from b 2 B 2 to b 2 = 1.61  0.07 in 170  45 fs, reveals the parallel nature of the transition dipole moment. A Introduction The ultraviolet (UV) photolysis of ClN 3 has been studied exten- sively in the nanosecond time regime. 1–4 It proceeds through two channels: ClN 3 + hv - Cl( 2 P 3/2,1/2 )+N 3 (linear, cyclic) (a) ClN 3 + hv - NCl(X, a, b) + N 2 (X, A) (b) Here, we focus on the radical bond rupture channel (a) which has been investigated by femtosecond time-resolved spectro- scopy at 4.6 eV. The branching ratio of the bond rupture channel (a) relative to the molecular elimination channel (b) at 5 eV has been measured to be 95  3%. 5 The aim of the present work is to gain insight into ClN 3 photolysis at ultrafast timescales in order to understand the mechanism of linear and cyclic (or high-internal-energy form of) N 3 production. ClN 3 being an asymmetric top molecule with a rotational timescale in the picosecond domain (ps), 6 the conservation of momenta between the two fragments has to be monitored on a shorter time scale, in order to probe time- resolved dynamics of the electronic states involved, to measure lifetimes and to evaluate the role of the parent molecule rotation and vibration in the dissociation process. The ClN 3 absorption spectrum shows four absorption bands in the UV range, centered at 350, 250, 210 and 170 nm. 2 For excitation energies larger than B5 eV the production of N 3 in a high- internal-energy form has been observed. Based on energetic considerations it is very likely that this form is the theoretically predicted 7–9 cyclic N 3 but definitive evidence for that is yet to appear. 10,11 Using high-resolution synchrotron radiation and a quadrupole mass spectrometer, Quinto-Hernandez et al. deter- mined the bond energy experimentally to be 1.86  0.05 eV for the production of the linear N 3 fragment. 12 This compares well with ab initio electronic structure calculations, which yield a value of D 0 (Cl–N 3 ) = 1.87 eV. 12 From the angular dependency of the Cl fragment parallel to the laser polarization, the absorption band at 250 nm was assigned to A 0 ’ A 0 transition, with the transition dipole moment within the molecular plane. Indeed, the anisotropy parameter was measured to be b 2 = 1.88  0.1 for Cl and 1.71  0.1 for Cl* for dissociation at 266 nm. 10 The deviation of these values from the b 2 = 2 limit for a parallel transition was used to estimate the dissociation time in the range of a few hundred femtoseconds. 13 The excitation process at 268 nm is shown schematically in Fig. 1, along with the main energy thresholds. Due to the high ionization potentials of the fragments of 11.03 and 12.96 eV for N 3 and Cl, respectively, the photofragments are detected by multiphoton ionization with an intense 803 nm probe pulse. 14 The remainder of the paper is organized as follows. A description of the experimental setup is given in Section B. The mass spectra and the time-resolved ion transients recorded for all fragmentation species of ClN 3 are presented in Section C.1. The energy balance of the co-fragments N 3 and Cl recorded by velocity-map imaging is discussed in Section C.2. The time dependencies of these two co-fragments as a function of their energies are summarized in Section C.3, before concluding with the time dependence of the N 3 fragment angular distribu- tion in Section C.4. a University of Toulouse, CNRS, Laboratoire Collisions Agre ´gats Re ´activite ´, IRSAMC, F-31062 Toulouse, France. E-mail: blanchet@celia.u-bordeaux1.fr b Institut National de la Recherche Scientifique - Centre E ´ nergie Mate ´riaux et Te ´le ´communications-EMT, 1650 Boulevard Lionel-Boulet, Varennes, Qc, J3X1S2, Canada c IESL-FORTH Institute of Electronic Structure and Laser Foundation of Research and Technology Hellas, Vassilika Vouton, Heraklion, Crete 71110, Greece d University of Bordeaux - CNRS - CEA, Centre des Etudes de Laser Intenses et Applications (CELIA), UMR5107, F-33400 Talence, France Received 17th August 2013, Accepted 4th October 2013 DOI: 10.1039/c3cp53507j www.rsc.org/pccp PCCP PAPER Published on 07 October 2013. Downloaded by Institut nationale de la recherche scientifique (INRS) on 22/10/2013 14:55:09. View Article Online View Journal