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Oriented Chlorine Atoms As a Probe of the Nonadiabatic Photodissociation Dynamics of Molecular Chlorine Andrew J

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Oriented Chlorine Atoms As a Probe of the Nonadiabatic Photodissociation Dynamics of Molecular Chlorine Andrew J JOURNAL OF CHEMICAL PHYSICS VOLUME 113, NUMBER 20 22 NOVEMBER 2000 Oriented chlorine atoms as a probe of the nonadiabatic photodissociation dynamics of molecular chlorine Andrew J. Alexander,a) Zee Hwan Kim, S. Alex Kandel,b) and Richard N. Zarec) Department of Chemistry, Stanford University, Stanford, California 94305 T. Peter Rakitzis Institute of Electronic Structure and Laser, Foundation for Research and Technology—Hellas, 711 10 Heraklion—Crete, Greece Yukako Asano and Satoshi Yabushita Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ͑Received 27 June 2000; accepted 1 September 2000͒ Molecular chlorine was photolyzed using circularly polarized radiation at 310 and 330 nm, and 2 orientation moments of the chlorine-atom Cl( P j) photofragment distributions were measured by resonance enhanced multiphoton ionization using circularly polarized light with Doppler resolution. The product atoms were found to be strongly oriented in the laboratory as a result of both incoherent and coherent dissociation mechanisms, and the orientation moments contributed by each of these mechanisms have been separately measured. The experimental results can be explained by 1⌸ ⍀ϭ nonadiabatic transitions from the C 1u state to higher states of 1u symmetry, induced by radial derivative coupling. Ab initio calculations indicate strong Rosen–Zener–Demkov noncrossing-type radial derivative couplings between states of 1u symmetry. The observed angular ͑␤ ͒ 2 distribution parameter indicates that 88% of Cl*( P1/2) fragments produced at 310 nm originate from a perpendicular transition to the C state. The orientation measurements suggest that 67Ϯ16% 35 2 3⌺ϩ Ϯ 3⌬ of Cl*( P1/2) atoms dissociate via the 1u( 1u) state, and 21 6% dissociate via the 1u( 1u) state. © 2000 American Institute of Physics. ͓S0021-9606͑00͒01444-6͔ 3⌸ 3 I. INTRODUCTION tion of involvement of the A 1u state has remained open. Measurements of the chlorine atom photofragment alignment The electronic states of chlorine molecules that are ac- by Rakitzis et al.6 and Bracker et al.7 are consistent with the cessed by ultraviolet radiation in the region around 330 nm dissociation proceeding mainly by adiabatic dissociation via have been of great importance to our understanding of mo- the C state, but have suggested that nonadiabatic transitions lecular bonding and the dynamics that occur when molecules involving the A state do occur. The observation of a coherent 1 dissociate. Indeed, the halogen molecules in general have alignment moment provides further support for the impor- provided a standard for theoretical and experimental studies tance of homogeneous nonadiabatic transitions between the of the electronic states and potential energy curves of di- C and A states.7 2 atomic molecules. The ultraviolet absorption spectrum of To understand the involvement of states that correlate to 2 5 Cl2 is continuous between 250 and 450 nm, with a maximum produce Cl*( P1/2), Samartzis et al. used their measure- at ϳ330 nm. Experimental measurements of the fine- ments of Cl* angular distributions, and Cl/Cl* branching ra- structure branching ratios following dissociation caused by tios to estimate the degree of nonadiabatic transition from the 3 ultraviolet radiation have been made by a number of groups. C state to states that correlate to produce ClϩCl*.Kim 4,5 Samartzis et al. have made detailed ion-imaging and et al.8 have reported measurements of the orientation of Cl* velocity-map imaging studies of Cl2 photodissociation, and atoms as a function of photolysis energy. The coherent ori- have reported angular distributions ͑parametrized by the ͓ (1) ʈ Ќ ͔ entation moment Im a1 ( , ) can be related to the inter- second-order Legendre moment of the distribution, ␤͒,and ference between dissociating pathways originating from par- branching ratios. Present understanding has it that the disso- allel ͑ʈ͒ and perpendicular ͑Ќ͒ transitions whose final states ciation around 330 nm is dominated by a perpendicular tran- correlate to the same asymptotic limit, in this case 1⌸ 8–10 sition to the C 1u state, with increasing contributions from ϩ * ϩ Cl Cl . These measurements, along with high-level ab 3⌸ 11,12 the B 0u state at longer wavelengths, although the ques- initio calculations by Yabushita, have challenged us to make a more detailed investigation of the nonadiabatic tran- a͒Present address: Department of Chemistry, University of Edinburgh, West sitions, the states involved, and the dynamics of the dissocia- Mains Road, Edinburgh, EH9 3JJ, UK. tion process. b͒ Present address: Davey Laboratory, Pennsylvania State University, Uni- In 1968, van Brunt and Zare13 predicted that the photo- versity Park, PA 16802. c͒Author to whom correspondence should be addressed; electronic mail: dissociation of a diatomic molecule could produce polarized [email protected] atomic fragments. Experimental observations of polarized 0021-9606/2000/113(20)/9022/10/$17.009022 © 2000 American Institute of Physics Downloaded 24 Feb 2012 to 171.64.124.19. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions J. Chem. Phys., Vol. 113, No. 20, 22 November 2000 Oriented chlorine atoms 9023 atomic fragments were first made in the early 1980s by photon ionization͒ at around 235 nm. The transitions used ؠ 2 52 ؠ 2 4 2 15,17 16 14,15 Vasyutinskii and by Rothe et al., Vasyutinskii, were 3s 3p 4p P1/2–3s 3p P3/2 at 236.518 nm and ؠ 2 52 ؠ 2 4 2 18 9 Siebbeles et al., and Mo et al. have presented quantum 3s 3p 4p P3/2–3s 3p P1/2 at 234.620 nm. These transi- mechanical treatments of atomic photofragment polarization. tions were specifically chosen to be sensitive to the orienta- Recently, measurements of the polarization of the electronic tion moments of the electron angular momentum distribu- angular momentum of photofragments have shown consider- tion. The atoms were probed using alternately left and right able promise for revealing details of the unbound states of circularly polarized light on a shot-to-shot basis. Thin-film molecules.19–21 Although these studies cannot match the polarizers ͑CVI Laser͒ were used to improve the quality of depth of information revealed by conventional spectroscopy, the linearly polarized ultraviolet radiation. The circular po- it is clear that they allow us to probe dynamics that were larization of the probe laser radiation was introduced by us- hitherto inaccessible. ing a 241 nm zero-order quarter waveplate ͑CVI Laser͒ and Photofragment orientation can occur by coherent disso- the handedness was changed on a shot-to-shot basis using a ciation mechanisms, but can also be induced by using circu- photoelastic modulator ͑Hinds PEM-80͒. The high quality larly polarized photolysis radiation; in this case, the direc- ͑Ͼ100:1͒ of probe laser circular polarization was confirmed ؠ 2 52 ؠ 2 4 2 tional nature of the photon angular momentum must be by REMPI using the Cl*(3s 3p 4p; P1/2–3s 3p P1/2) conserved in the laboratory frame. Zare and co-workers22,23 transition at 237.074 nm. have studied the orientation of CN fragments produced by The probe laser was arranged in a collinear counter- dissociation of ICN using circularly polarized radiation. propagating geometry with respect to the photolysis laser. Vasyutinskii and co-workers24–27 have made important theo- The dissociation and detection was carried out at the extrac- retical and experimental studies using the optical magnetic tion region of a time-of-flight mass spectrometer. Unlike pre- birefringence ͑Faraday͒ technique. Recently, Korovin et al.28 vious studies, however, the total Clϩ ion signal was col- reported the first observation of spin-oriented Rb-atom pho- lected, and the sensitivity to product velocity was achieved tofragments, produced by photolysis of RbI at 266 nm using by scanning over the Doppler-broadened line shape of the circularly polarized light. Their results indicated the presence REMPI transition. Because the chlorine atoms are traveling of both coherent and incoherent dissociation mechanisms, relatively fast ͑ϳ2kmsϪ1͒ with a well-definedspeed,no although these mechanisms could not be quantified sepa- efforts were made to further narrow the linewidth of the rately. probe dye laser ͑Spectra-Physics PDL-3; nominal fundamen- In this paper, we report the observation of electronically tal linewidth ϳ0.07 cmϪ1͒. oriented Cl-atom photofragments from the dissociation of The definition of the handedness of circular polarization Cl2 at 310 and 330 nm using circularly polarized photolysis that we use is the conventional one, i.e., left circular polar- and probe radiation. Our measurements support the presence ization ͑LCP͒ has positive handedness or helicity, and obeys of both coherent and incoherent dissociation mechanisms. the right-hand rule with the thumb pointing in the direction From our measurements we estimate the extent of nonadia- of the propagation vector.30 For LCP the electromagnetic batic radial derivative coupling that leads to production of field rotates counterclockwise in a fixed plane as viewed Cl*, and we are able to identify the states that are involved in looking into the oncoming wave, and each photon has a pro- the nonadiabatic dynamics. In what follows we present a jection of one unit of angular momentum ͑ϩប͒ with respect brief introduction to the experimental and theoretical meth- to its propagation vector.31 The absolute handedness of the ods employed ͑Sec. II͒. Results are presented in Sec. III and circularly polarized light was determined using a single a discussion in Sec. IV. A brief summary and conclusions fused-silica Fresnel rhomb ͑CVI Laser͒. The absolute phase appear in Sec. V. of the output of the rhomb can be calculated using Fresnel’s equations.30 The Fresnel rhomb was oriented in the beam path after the unknown circularly polarizing element; the ␲/4 II.
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