The photodissociation dynamics of state selected OD radicals has been examined at 243 and 226 nm using velocity map imaging to probe the angle–speed distributions of theD(2S) and O(3P2) products. Both experiment and complementary first principle calculations demonstrate that photodissociation occurs by promotion of OD from high vibrational levels of the ground X 2Π state to the repulsive 1 2Σ− state

Chestakov, Dmitri A.

Eppink, Andre T.J.B.

Greenslade, Margaret E.

Groenenboom, Gerrit G.

Lester, Marsha I.

Parker, David H.

Radenovic, Dragana C.

ter Meulen, J J.

van der Loo, Mark P.J.

van Roij, Andre J.A.

English

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositoryChemistry Scholarship Chemistry9-2003Photodissociation of the OD radical at 226 and243 nmDragana C. RadenovicAndre J.A. van RoijDmitri A. ChestakovAndre T.J.B. EppinkJ J. ter MeulenSee next page for additional authorsFollow this and additional works at: https://scholars.unh.edu/chemistry_facpubPart of the Physical Chemistry CommonsThis Article is brought to you for free and open access by the Chemistry at University of New Hampshire Scholars' Repository. It has been accepted forinclusion in Chemistry Scholarship by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, pleasecontact nicole.hentz@unh.edu.Recommended CitationD. Č. Radenović, A. J. A. Van Roij, D. A. Chestakov, A. T. J. B. Eppink, J. J. ter Meulen, D. H. Parker, M. P. J. Van der Loo, G. C.Groenenboom, M. E. Greenslade, M. I. Lester, “Photodissociation of the OD radical at 226 and 243 nm,” J. Chem. Phys. 119, 9341(2003).AuthorsDragana C. Radenovic, Andre J.A. van Roij, Dmitri A. Chestakov, Andre T.J.B. Eppink, J J. ter Meulen, DavidH. Parker, Mark P.J. van der Loo, Gerrit G. Groenenboom, Margaret E. Greenslade, and Marsha I. LesterThis article is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/chemistry_facpub/12Photodissociation of the OD radical at 226 and 243 nmDragana Č. Radenović, André J. A. van Roij, Dmitri A. Chestakov, André T. J. B. Eppink, J. J. ter Meulen et al.  Citation: J. Chem. Phys. 119, 9341 (2003); doi: 10.1063/1.1623175 View online: http://dx.doi.org/10.1063/1.1623175 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v119/i18 Published by the American Institute of Physics.  Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors Downloaded 07 May 2013 to 132.177.229.164. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissionsPhotodissociation of the OD radical at 226 and 243 nmDragana Cˇ . Radenovic´, Andre´ J. A. van Roij, Dmitri A. Chestakov, Andre´ T. J. B. Eppink,J. J. ter Meulen, and David H. Parkera)Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld 1, 6525ED, Nijmegen,The NetherlandsMark P. J. van der Loo and Gerrit C. GroenenboomInstitute of Theoretical Chemistry, University of Nijmegen, Toernooiveld 1, 6525ED, Nijmegen,The NetherlandsMargaret E. Greenslade and Marsha I. LesterDepartment of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323~Received 28 August 2003; accepted 11 September 2003!The photodissociation dynamics of state selected OD radicals has been examined at 243 and 226 nmusing velocity map imaging to probe the angle–speed distributions of the D(2S) and O(3P2)products. Both experiment and complementary first principle calculations demonstrate thatphotodissociation occurs by promotion of OD from high vibrational levels of the ground X 2P stateto the repulsive 1 2S2 state. © 2003 American Institute of Physics. @DOI: 10.1063/1.1623175#Experimental methods for studying molecular photodis-sociation under collision-free conditions were introduced al-most forty years ago.1 Despite its importance in atmosphericand astrochemical processes,2 no such studies have been re-ported for the hydroxyl radical. OH is experimentally chal-lenging because all of the electronic transitions originatingfrom X 2P(v50) lie in the vacuum ultraviolet region of thespectrum, except the well-known A-X transition used for la-ser induced fluorescence ~LIF! detection. Furthermore, mostsources of OH radicals also co-produce large amounts ofO(3P) and H(2D) atoms, which can overwhelm the photo-product signals. In this Communication, we report a photo-dissociation study of state-selected OD under collision-freeconditions. Using a hexapole state selector and the velocitymap imaging technique,3,4 we measure angle–speed distribu-tions for the O(3P2) atom product following photodissocia-tion at 226 nm and the D(n51, 2S) product for photodisso-ciation at 243 nm. New, fully ab initio calculations of thelower electronically excited states of OD were performed toyield accurate wave functions and internuclear dependenttransition dipole functions for the states involved. These areused to explain the observed dissociation dynamics.OD is produced by an electric discharge in a D2O/Armixture expanded through a pulsed valve.5 The molecularbeam passes through a skimmer followed by a 12 cm longhexapole electrostatic lens which focuses OD in theX 2P3/2 J53/2, f lambda doublet level.3 The enlargement ofthe distance between the source and photodissociation areaby the insertion of the state selector decreases the contribu-tions from all other species in the discharge beam, while thehexapole increases the concentration of the state-selectedcomponent by a factor of ;8 at the crossing point with thephotodissociation laser. Using both laser induced fluores-cence and resonance enhanced multiphoton ionization~REMPI! detection techniques,6,7 the only species in thebeam that was found to focus with the hexapole was OD. Acorresponding increase in signal for the photodissociationprocesses with the hexapole on is thus direct proof that thephotodissociation signal arises from OD.At about 5 cm from the end of the hexapole both photo-dissociation of OD and photoionization of the atomic frag-ments is induced by pulsed radiation from a frequency-doubled dye laser with a pulse energy of ;1.5 mJ. The laserwavelength is chosen for two-photon resonant three-photonionization @~211! REMPI# of either O(3P2) atom products at226 nm or D(2S) atom products at 243 nm. We will reportcomprehensive studies of O(3P2,1,0) detection from OD andOH at 226 nm in a later article. There are no known one- ortwo-photon transitions of OD or D2O at these atomic REMPIwavelengths. The O1 or D1 atoms produced by the REMPIprocess are velocity mapped onto the two-dimensional imag-ing detector using an electrostatic inversion lens. A CCDcamera records the 2-D image, which is presented as a slicethrough the reconstructed 3-D image using an inverse Abeltransformation.4Raw D1 and O1 images ~hexapole on–hexapole off! arepresented in Fig. 1. These images are a summation from50 000 laser shots with the hexapole on, corrected by a sub-sequent background image taken with the hexapole off. Boththe D1 and O1 images show a strong peak in the middle ofthe image, which corresponds to zero-velocity fragmentO(3P2) or D(2S) atoms formed in the discharge source.These atoms are not fully eliminated by the hexapole on–offsubtraction scheme and cause a slight overload of the CCDcamera at the image center. Signals at larger distances fromthe image center arise from atoms with increasingly higherphotofragment velocities. The images are calibrated byO(3P2) from O2 photodissociation at 226 nm and by D(2S)from DI photodissociation at 243 nm. Two rings appear inthe O1 image corresponding to O(3P2) atoms with velocitya!Author to whom correspondence should be addressed. Electronic mail:parker@sci.kun.nlJOURNAL OF CHEMICAL PHYSICS VOLUME 119, NUMBER 18 8 NOVEMBER 200393410021-9606/2003/119(18)/9341/3/$20.00 © 2003 American Institute of PhysicsDownloaded 07 May 2013 to 132.177.229.164. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissionsof 1670 and 1765 m/s with an uncertainty of 625 m/s. Con-verting these O atom velocities to total kinetic energy release@TKER5(mOD /mD)3KERO# yields values of 2.08 and 2.3260.06 eV. For D1, at least three rings are seen, correspond-ing to D atom velocities of 11 800, 12 800, and 13 800 m/sand TKER values of 1.62, 1.91, and 2.2160.06 eV, respec-tively. The TKER curves obtained from integrating over theangular distribution of the images are shown in Fig. 2. Thewidths of the peaks in the TKER curves are determined byelectron recoil from the ~211! REMPI process, as will bediscussed in a future publication.All of the observed angular distributions for D and Oatom signals peak in the horizontal plane, indicating a per-pendicular transition (2S1 ,2←2P or 2D←2P) to a rapidlydissociating state. For the stronger rings in the D atom imagea fully perpendicular angular distribution is measured(sin2 u,b521.060.2). A quantitative discussion of the an-gular distributions including the effects of O atom angularmomentum alignment will be given in a following paper.The observed peaks in the TKER distributions can onlybe assigned to one-photon dissociation of OD in high vibra-tional levels of the ground 2P electronic state. Using theenergy balance equation TKER5hn1E(vib)OD2D0 , withOD vibrational energies E(vib)OD and bond energy D0 fromRefs. 8 and 9, the two main peaks seen for O atom detectionat 226 nm are attributed to photodissociation of OD in v53 and v54, as marked in Fig. 2. For D atom detection at243 nm, the three main peaks correspond to dissociation ofOD (v53, 4, and 5!.First principle calculations also demonstrate that thephotodissociation processes at 226 and 243 nm occur by one-photon excitation of OD X 2P(v) to the repulsive 1 2S2state. For these calculations, ab initio potentials for the X 2Pand 1 2S2 states are computed at the MRCI level of theorywith the MOLPRO package.10–12 The aug-cc-pV6Z basis set13is used and the molecular orbitals are obtained from aCASSCF calculation14,15 in which the lowest s orbital is keptdoubly occupied. The active space consists of four s, twopx , and two py orbitals. A grid of 30 points from 1 to 16 a0is employed. The quality of the X 2P potential was evaluatedby computing the lowest seven vibrational energy levels us-ing the sinc-function discrete variable representationmethod,16 and the energy spacings were found to agree withexperiment to within 0.1%. The R-dependent transition di-pole moment between the two states was also calculated atthe MRCI level with orbitals from a state-averaged CASSCFcalculation. The photodissociation cross sections for OD at226 and 243 nm were then evaluated using17s~E !5pv«0cu^X 2Px~v !umx~R !u1 2S2,E2&u2,where \v is the photon energy and u1 2S2,E2& is the con-tinuum wave function at total energy E with photodissocia-tion boundary conditions,18 which were obtained using therenormalized Numerov method.17Figure 3 illustrates the process of photodissociation fromthe OD X 2P(v55) state based on the calculated potentials,transition dipole function, and wave functions. As seen in thefigure, when using 243 nm light OD(v55) can be excited tothe 1 2S2 repulsive curve leading to the first dissociationlimit, but poor overlap of the bound and continuum wavefunctions prevents absorption from OD(v55) at 226 nm.More quantitatively, the calculations show that the ODX 2P(v55) photodissociation cross section is ;30 timeslarger at 243 nm than at 226 nm. This same effect is foundexperimentally, where a peak corresponding to v55 is ob-served in the TKER curve for D atoms at 243 nm in Fig. 2,while this peak is not detectable above the background in theO atom TKER curve at 226 nm.The relative intensity of the peaks in the TKER curves ofFig. 2 can be qualitatively understood based on the photodis-sociation cross sections computed for various vibrationalstates. The calculations predict that at a vibrational tempera-ture of ;1700 K the v53 and 4 states ~and v52 to a lesserFIG. 1. Velocity map images of ~a! O1 and ~b! D1 ions detected followingphotodissociation of OD and subsequent 211 REMPI of the atomic prod-ucts at 226 and 243 nm, respectively, with insets showing the velocity scale.Darker areas correspond to more signal, as coded by the gray-scale bar onthe left side of the figure. The central spots originate from zero-velocityO(3P2) or D(2S) atoms produced in the discharge source. The outer ringsarise from photodissociation of vibrationally excited OD X 2P radicals. Thearrow at the right indicates the direction of the laser polarization.FIG. 2. Total kinetic energy release ~TKER! derived from velocity mapimages of O(3P2) and D(2S) fragment atoms following photodissociation ofOD at 226 and 243 nm, respectively. The initial vibrational state of OD isdetermined from energy balance with TKER5hn1E(vib)OD2D0 . The bargraphs show the calculated photodissociation yields for OD X 2P(v) at avibrational temperature of 1700 K.9342 J. Chem. Phys., Vol. 119, No. 18, 8 November 2003 Radenovic´ et al.Downloaded 07 May 2013 to 132.177.229.164. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissionsextent! result in the largest photodissociation yield at 226nm, while a distribution of states about v54 ~primarily v53, 4, and 5! contribute to the yield at 243 nm. The pre-dicted yields are shown in the bar graphs of Fig. 2. A Bolt-zmann population distribution in the vibrational states is as-sumed, and the temperature is adjusted to approximate theexperimental TKER distributions. Note that such a tempera-ture distribution is probably too simple for the description ofthe complicated excitation and cooling mechanisms in thepulsed discharge source. The quality of the experimental datais limited by problems in background subtraction and doesnot allow the extraction of a more accurate population dis-tribution. A Boltzmann distribution of 1700 K for OD yieldsa fractional population in v55, for example, of 331025.Independent measurement of the v51 population in the mo-lecular beam using laser induced fluorescence from the A-Xtransition yield a population of 4%, which agrees reasonablywell with a 1700 K distribution. REMPI studies of OH viathe 3 2S2 Rydberg state also detect population in higher vi-brational levels of the ground electronic state from this dis-charge source, as will be described in a future publication.In studies of other diatomic molecules, such as O2 undersimilar conditions,19 two-photon dissociation is often foundto be the dominant channel. In this study, the laser intensityis suitable for two-photon resonant three-photon ionizationof OH, O and H atoms. It is thus striking that the dominantsignal is due to one-photon dissociation of OD to the repul-sive 1 2S2 curve, starting from the very weakly populatedhigher vibrational levels of ground state OD. No evidencefor two-photon dissociation is observed. While it is knownthat discharge beams produce vibrationally excitedradicals,20 it is remarkable that the imaging experiment isable to detect signal from vibrational levels which containsuch small fractions ~as low as 1025) of the population. Nei-ther one-photon nor two-photon dissociation is detected fromOD X 21(v50) at 243 or 226 nm. Stepwise two-photon dis-sociation via the repulsive 1 2S2 curve, starting from vibra-tionally excited OD is also not observed. The apparent weak-ness of two-photon excitation is most likely due to thetransition dipole and unfavorable Franck–Condon overlap.D.H.P. and M.I.L. thank the Dutch National ScienceFoundation ~NWO! for a visiting professor fellowship B70-316. The experimental work in the Netherlands is part of theresearch program of the FOM-NWO organization. The abinitio work has been financially supported by the Council forChemical Sciences of the Netherlands Organization for Sci-entific Research ~CW-NWO!. M.E.G. and M.I.L. gratefullyacknowledge support from the Air Force Office of ScientificResearch.1 Molecular Photodissociation Dynamics, edited by M. N. R. Ashfold andJ. E. Baggott ~Royal Society of Chemistry, London, 1987!.2 Progress in Atmospheric Physics, edited by R. Rodrigo, J. J. Lo´pez-Moreno, M. Lo´pez-Puertas, and A. Molina ~Kluwer Academic, Dordrecht,1988!.3 K. Schreel and J. J. ter Meulen, J. Phys. Chem. A 101, 7639 ~1997!.4 A. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477 ~1997!.5 M. C. van Beek and J. J. ter Meulen, Chem. Phys. Lett. 337, 237 ~2001!.6 J. Luque and D. R. Crosley, ‘‘LIFBASE: Database and Spectral SimulationProgram ~Version 1.5!,’’ SRI International Report No. MP 99-009 ~1999!.7 E. de Beer, M. P. Koopmans, C. A. De Lange, Y. Wang, and W. A.Chupka, J. Chem. Phys. 94, 7634 ~1991!.8 M. C. Abrams, S. P. Davis, M. L. P. Rao, and R. Engleman, Jr., J. Mol.Spectrosc. 165, 57 ~1994!.9 B. Ruscic, D. Feller, D. A. Dixon, K. A. Peterson, L. B. Harding, R. L.Asher, and A. F. Wagner, J. Phys. Chem. A 105, 1 ~2001!.10 H.-J. Werner and P. J. Knowles, J. Chem. Phys. 89, 5803 ~1988!.11 P. J. Knowles and H.-J. Werner, Chem. Phys. Lett. 145, 514 ~1988!.12 MOLPRO is a package of ab initio programs written by H.-J. Werner and P.J. Knowles, with contributions from R. D. Amos, A. Berning, D. L. Coo-per et al.13 T. H. Dunning, J. Chem. Phys. 90, 1007 ~1989!.14 H.-J. Werner and P. J. Knowles, J. Chem. Phys. 82, 5053 ~1985!.15 P. J. Knowles and H.-J. Werner, Chem. Phys. Lett. 115, 259 ~1985!.16 G. C. Groenenboom and D. T. Colbert, J. Chem. Phys. 99, 9681 ~1993!;D. T. Colbert and W. H. Miller, ibid. 96, 1982 ~1992!.17 R. Schinke, Photodissociation Dynamics ~Cambridge University Press,Cambridge, 1993!.18 B. R. Johnson, NRCC Proceedings 5, 86 ~1979!.19 B. Buijsse, W. J. van der Zande, A. T. J. B. Eppink, D. H. Parker, B. R.Lewis, and S. T. Gibson, J. Chem. Phys. 108, 7229 ~1998!.20 G. B. Courreges-Lacoste, J. P. Sprengers, J. Bulthuis, S. Stolte, T.Motylewski, and H. Linnartz, Chem. Phys. Lett. 335, 209 ~2001!.FIG. 3. Photodissociation of OD X 2P(v55) at 226 and 243 nm via therepulsive 1 2S2 curve depicted with ab initio potentials, bound state andcontinuum wave functions, and transition dipole moment function ~arbitraryunits!. The photodissociation cross section is large at 243 nm, but pooroverlap of the wave functions when starting from v55 results in minimalproducts at 226 nm, as seen experimentally.9343J. Chem. Phys., Vol. 119, No. 18, 8 November 2003 Photodissociation of ODDownloaded 07 May 2013 to 132.177.229.164. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

Photodissociation of the OD radical at 226 and 243 nm

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Photodissociation of the OD radical at 226 and 243 nm

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