Ion Pair Formation in Multiphoton Excitation of NO 2 Using Linearly and Circularly Polarized Femtosecond Light Pulses: Kinetic Energy Distribution and Fragment Recoil Anisotropy † C. Elkharrat, ‡ Y. J. Picard, ‡ P. Billaud, ‡ C. Cornaggia, § D. Garzella, § M. Perdrix, § J. C. Houver, ‡ R. R. Lucchese, ⊥ and D. Dowek* ,‡ Institut des Sciences Mole ´culaires d’Orsay, UMR8214 UniV Paris-Sud et CNRS, Bat. 350, F-91405 Orsay Cedex, France, SerVice Photons Atomes & Mole ´cules, CEA IRAMIS, SerVice des Photons, Atomes et Molécules, Saclay, Bat. 522, F-91191 Gif-sur-YVette, France, and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3255 ReceiVed: April 23, 2010; ReVised Manuscript ReceiVed: June 30, 2010 The NO 2 ion pair photodissociation dynamics leading to NO + (X 1 Σ + ,V) + O - ( 2 P 3/2 or 2 P 1/2 ), induced by a 1 kHz femtosecond laser with wavelengths near 400 nm, has been characterized using the coincidence vector correlation method. The ion pair production after four-photon absorption reaches more than 15% of the primary ionization. The kinetic energy release of the fragments demonstrates a significant vibrational excitation of the NO + (X 1 Σ + ,V) molecular fragment. Recoil ion fragment emission is strongly aligned along the polarization axis of linearly polarized light or preferentially emitted in the plane perpendicular to the propagation axis of circularly polarized light. The formalism describing the recoil anisotropy for bound-to-bound n-photon transition inducing prompt axial recoil dissociation of a nonlinear molecule has been developed to interpret the measured anisotropies in terms of excitation pathways via near-resonant intermediate states of specific symmetries. Possible reaction pathways are discussed that are consistent with the data and supported by calculations of potential energy surfaces and transition moments. I. Introduction The NO 2 (X 2 A 1 ) molecule has a bent equilibrium geometry with C 2V symmetry in its open-shell ground electronic state. The corresponding valence-shell electronic structure is (4a 1 ) 2 (3b 2 ) 2 - (1b 1 ) 2 (5a 1 ) 2 (1a 2 ) 2 (4b 2 ) 2 (6a 1 ) 2 . NO 2 is of fundamental importance in chemistry, and the properties of its neutral and ionic states have attracted much interest. Despite its apparent simplicity as a triatomic molecule, NO 2 has varied and complex behavior involving unimolecular reaction dynamics, intramolecular vi- brational redistribution, vibronic coupling, and short-time nuclear and electronic dynamics. Consequentially, NO 2 has been the subject of many experimental and theoretical studies in recent years that have investigated the photodissociation and photoionization reactions of NO 2 in a wide variety excitation schemes (see, e.g., refs 1–22 and 23 for a recent review). The literature is too vast to be reviewed extensively, and in the following, we will only give references to papers directly relevant to the processes considered here. In this work, we focus on the new information which can be gained by combining the use of ultrashort light sources with coincident three-dimensional (3D) imaging techniques providing direct measurements of differential energetic and angular observables. In the course of a recent study of one-color multiphoton ionization of NO 2 initiated by the absorption of 405-396 nm photons delivered by a femtosecond laser source, we have observed significant production of the NO + -O - ion pair. This paper is dedicated to the discussion of the ion pair formation results, obtained using the electron-ion velocity vector cor- relation method, 22,24 which is focused here on the (V NO + ,V O - ,e ˆ ) vector correlation, where the polarization vector e ˆ represents the polarization axis of linearly polarized light, P, or the propagation axis of circularly polarized light, k. The results for dissociative and nondissociative photoionization will be reported in another publication. 25 To the best of our knowledge, this is the first observation of the NO + -O - ion pair in multiphoton excitation of the NO 2 molecule. The thermochemical threshold of the NO + -O - ion pair process lies at 10.918 eV; then, using 405-396 nm photons, its formation requires the absorption of at least four photons. The explored range corresponds to an excitation energy ranging between 12.24 and 12.52 eV, which includes the threshold for the NO + (X 1 Σ + ,V) 0) + O( 3 P 2 ) dissociative ionization channel at 12.37 eV. In the experimental conditions considered in this study, the NO + -O - ion pair is observed with an abundance larger than 15% relative to the total photoionization (PI) yield induced by linearly polarized light. This value is significantly larger than the value of ∼5 × 10 -3 relative to the primary ionization found in a study of the O - ion yield in one-photon photoexcitation of NO 2 26 using photoion mass spectrometry, 27 which demonstrates that this dissociation channel plays a significant role among the multiphoton excitation schemes and may therefore be an important intermediate state in the pathways of other reaction channels. Since the ion pair threshold lies 1.3 eV above the adiabatic PI threshold of the NO 2 + (X 1 Σ g + ) ground-state parent molecular cation at 9.6 eV, this channel corresponds to a superexcited state of NO 2 , embedded in the ionization con- tinuum; apart from neutral dissociation, other channels to be † Part of the “Reinhard Schinke Festschrift”. * To whom correspondence should be addressed. Tel: 33 (0)1 6915 7672. Fax: 33 (0)1 6915 5811. E-mail: danielle.dowek@u-psud.fr. ‡ Universite ´ Paris-Sud. § CEA IRAMIS. ⊥ Texas A&M University. J. Phys. Chem. A 2010, 114, 9902–9918 9902 10.1021/jp103672h  2010 American Chemical Society Published on Web 08/12/2010