JOURNAL OF CHEMICAL PHYSICS VOLUME 118, NUMBER 23 15 JUNE 2003 Photodissociation of O2 via the Herzberg continuum: Measurements of O-atom alignment and orientation Andrew J. Alexander,a) Zee Hwan Kim,b) and Richard N. Zarec) Department of Chemistry, Stanford University, Stanford, California 94305-5080 ͑Received 29 January 2003; accepted 24 March 2003͒ Irradiation of molecular oxygen O2 in the region of the Herzberg continuum between 218 nm and 3 ϩ 3 3 239 nm results in the production of open-shell photofragments O( P) O( P). Product O( P j ; jϭ0,1,2) atoms were ionized using resonantly enhanced multiphoton ionization ͑2ϩ1 REMPI͒ near 225 nm and the ions collected in a velocity-sensitive time-of-flight mass spectrometer. By controlling the polarization of the photolysis and ionization radiation, we have measured alignment and orientation parameters of O-atom electronic angular momentum (j) in the molecule frame. The results show alignment from both parallel and perpendicular transitions that are cylindrically symmetric about the velocity (v) of the recoiling O atom. We also observe electronic alignment that is noncylindrically symmetric about v, resulting from coherence between multiply excited dissociative states. Photodissociation with linearly polarized light is shown to produce O atoms that are oriented in the molecule frame, resulting from interference between parallel and perpendicular dissociative states of O2 . Semiclassical calculations that include spin–orbit coupling between six excited states reproduce closely the observed polarization. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1574511͔ I. INTRODUCTION ties of the product O atoms with respect to the electric field vector ⑀ of the linearly polarized photolysis radiation. The Oxygen plays a key role in the chemistry of the Earth’s angular distribution can be written using the expression4 atmosphere. The splitting of the oxygen bond by ultraviolet ͑UV͒ solar radiation is the primary step in the formation of 1 I͑␪⑀͒ϭ ͓1ϩ␤P ͑cos ␪⑀͔͒, ͑1͒ ozone (O3), and is responsible for the existence of the ozone ␲ 2 1 4 layer. The UV photodissociation of O2 can be loosely ••• ␪ classed in three different wavelength regions: the Herzberg where P2( ) is the second order Legendre polynomial, ⑀ continuum ͑242–200 nm͒, the Schumann–Runge bands is the polar angle between v and ⑀, and the spatial distribu- ͑200–176 nm͒, and the Schumann–Runge continuum ͑176– tion of velocities is parameterized by ␤. The ␤ parameter 100 nm͒. takes the limits ϩ2 for a parallel (ʈ) transition and Ϫ1 for a The Herzberg continuum is the region of near-threshold perpendicular (Ќ) transition. At 226 nm, ␤ϭ0.61 for 3 ϩ 3 3 ʈ Ќ dissociation of O2 to give O( P) O( P). The electronic O( P2), indicating a mixture of 54% ( ) with 46% ( ). configuration of the O atoms (1s22s22p4) leads to a consid- Buijsse et al. have developed a detailed photoabsorption erable number of electronic states of the O2 molecule, and a model to directly compare with the experimental results. The 3⌺Ϫ Herzberg I transition was found to be the most significant, ground state X g that is unusual in that it has two un- paired electrons. In the Herzberg continuum, absorption oc- accounting for 86% of the total cross section at 242 nm, and 3⌺ϩ Ј 3⌬ 1⌺Ϫ rising to 94% at 198 nm. The resulting electric dipole tran- curs from the X state to the A u , A u , and c u states: these are called the Herzberg I, II, and III transitions sition includes both parallel and perpendicular components respectively. These Herzberg transitions are electric dipole ʈ SO forbidden, and the absorption cross sections (␴) are weak: 3 Ϫ 3 Ϫ 3 ϩ X ⌺ Ϯ !B ⌺ Ϯ $A ⌺ Ϯ , ͑2͒ for example, ␴Ϸ10Ϫ24 cm2 at 240 nm.2 Oscillator strength g, 1 u, 1 u, 1 for the Herzberg transitions is borrowed from electric dipole SO Ќ ͑ ͒ 3⌺Ϫ $ 3⌸ ! 3⌺ϩ ͑ ͒ allowed transitions, mainly through spin–orbit SO interac- X g,0ϩ 1 g,0ϩ A u,Ϯ1 , 3 tions between ground and excited states. Buijsse et al.,3 have recently published results of a detailed velocity-map imaging SO Ќ 3⌺Ϫ $ 3⌸ ! 3⌺Ϫ ͑ ͒ study of photodissociation in the Herzberg continuum. The X g,Ϯ1 1 g,Ϯ1 A u,0Ϫ . 4 velocity map experiments measured the directions of veloci- Here, we have given the projection ⍀ϵ⌳ϩ⌺ of total angu- lar momentum on the bond axis as an additional subscript on ⌺ϩ Ϯ a͒ 2 1⌳ Ј 3⌬ Present address: School of Chemistry, University of Edinburgh, Edinburgh the term symbol, g/u,⍀ . The transitions to A u and Ϫ EH9 3JJ, UK. c 1⌺ are purely perpendicular in character. b͒ u Present address: College of Chemistry, University of California, Berkeley Adiabatically, the A, AЈ, and c states correlate to give O CA 94720-1460. ͒ ϭ 3 ϩ 3 c Author to whom correspondence should be addressed. Electronic mail: atoms with j 2, i.e., O( P2) O( P2). Several studies have 3 ϭ 5–7 [email protected] shown significant populations of O( P j) with j 0,1. 0021-9606/2003/118(23)/10566/9/$20.0010566 © 2003 American Institute of Physics Downloaded 21 Apr 2004 to 171.64.124.7. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp 10567 J. Chem. Phys., Vol. 118, No. 23, 15 June 2003 Photodissociation of O2 3 ϭ ϳ Buijsse et al. measured the j 0:1:2 branching ratio as of 400 Torr. The O2 was photolyzed using linearly polar- 1.00:3.33:9.00. Clearly a significant amount of nonadiabatic ized light in the 218–239 nm wavelength range. The second interaction takes place during the dissociation. There are a harmonic ͑532 nm͒ ofaNd3ϩ:YAG laser ͑Continuum total of eight states of ungerade symmetry correlating to the PL9020͒ was used to pump a dye laser ͑Continuum ND6000, 3 ϩ 3 O( P) O( P) dissociation limit, these include three rhodamine 590ϩ610 dye͒ to produce tunable radiation in the Ј 1⌸ Herzberg states (A, A , and c) and five others (1 u , region 550–616 nm, which was doubled to 275–308 nm 3⌸ 3⌺ϩ 5⌸ 5⌺Ϫ 1 u ,2 u ,1 u , and 1 u ). The ungerade states using a KDP crystal. The resulting doubled light was then may couple through long-range SO interaction, and the ͑ ͒ 3 ϩ 3 ϩ mixed with the residual fundamental 1064 nm of the A ⌺ and 2 ⌺ states may also interact by radial deriva- ϩ u u Nd3 :YAG laser in a second KDP crystal to produce tunable tive coupling due to the radial kinetic energy operator. Un- photolysis radiation from 218 to 239 nm. Measurements of fortunately, angular distribution and branching ratio mea- product O(3P ; jϭ1,2) atom alignment were made at pho- surements do not reveal clues to the specific states involved j tolysis wavelengths of 221.667 and 237.049 nm; for brevity, in the nonadiabatic dissociation. we shall refer to these wavelengths as 222 and 237 nm, re- Recently, van Vroonhoven and Groenenboom8 ͑VG͒ cal- spectively. Measurements of O(3P ) atom orientation were culated ab initio potentials for the eight ungerade states, and 2 made at several photolysis wavelengths in the range the SO and radial derivative couplings between them. These 3 ϭ potentials were used in semiclassical calculations of the elec- 218.521–238.556 nm. The O( P j ; j 0,1,2) atom products ϩ ͑ tronic state-resolved branching ratios and ␤( j) parameters as were ionized by 2 1 REMPI resonantly enhanced multi- ͒ 3 3 a function of the dissociation energy, and the calculations photon ionization via the 2p 4p( P jЈ) intermediate state were compared with the experimental results of Buijsse using wavelengths of 226.233, 226.059, and 225.656 nm for ϭ et al.9 Significantly, VG also calculated electronic state re- j 0, 1, and 2, respectively. Polarization measurements are 3 ␳(k) ϭ ϭ not possible for O( P0) because this state has zero total an- solved alignment moments 0 ( j)(k 2,4; j 1,2), but at that time no experimental data were available for compari- gular momentum. Tuneable probe radiation around 226 nm son. The alignment moments are sensitive to nonadiabatic was produced by frequency doubling the 450 nm output of a 3ϩ ͑ transitions between states, and their experimental determina- second Nd :YAG pumped dye laser Spectra Physics DCR- tion would provide a useful test of ab initio and dynamical 2A, PDL-3, coumarin 450 dye͒. The probe and photolysis calculations. laser beams were counterpropagated and both loosely fo- In this article, we present measurements of electronic cused ͑ϩ50 cm focal length͒ to overlap at the center of the ϩ 3 pulsed expansion in the TOF. The resulting O ions were orientation and alignment of O( P j) atoms following disso- ciation via the Herzberg continuum in the region 218–239 collected with the TOF spectrometer operating under 12 nm. The O-atom polarization is obtained in terms of mol- velocity-sensitive conditions. (k) The direction of the linear polarization of the photolysis ecule frame polarization parameters aq (p) that distinguish between atoms produced by parallel (ʈ) or perpendicular (Ќ) beam was controlled using a double Fresnel rhomb ͑Optics transitions, or coherence due to mixed transitions.10 The For Research͒, and the direction of the probe linear polariza- measured alignment parameters allow a detailed comparison tion was switched on a shot-by-shot basis using a photoelas- with the recent semiclassical calculations of VG.