Published on Web Date: June 03, 2010

r 2010 American Chemical Society 1861 DOI: 10.1021/jz100356f |J. Phys. Chem. Lett. 2010, 1, 1861–1865

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Imaging CO2 Photodissociation at 157 nm: State-to-StateCorrelations between CO(ν) and O(3Pj=0,1,2)Zhichao Chen,†,‡ Fuchun Liu,‡,§ Bo Jiang,† Xueming Yang,*,† and David H. Parker*,‡

†State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,Dalain 116023, Liaoning, China, ‡Department of Molecular and Laser Physics, University of Nijmegen, 6525 ED Nijmegen,The Netherlands, and §Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, Jilin, China

ABSTRACT The spin-forbidden CO(ν) þ O(3Pj) channel produced by 157 nmphotodissociation of CO2 was investigated by velocity map imaging. O(3P) imageswere measured for the three 3Pj spin-orbit states ( j=0, 1, and 2), and the COvibrational-state distributions that correlate to the O(3Pj=0,1,2) spin-orbit productwere determined. Nearly all energetically allowed product CO(ν) levels (i.e., ν e 8)are observed, withminor differences in the CO vibrational distributions for the threechannels. The angular anisotropy, which also shows minor differences for the threej channels, is found to decreasewith increasing CO vibrational excitation, suggestingthat the lower vibrational states are produced by photodissociation from a morelinearOCOgeometry. Fits to the total kinetic energydistributions suggest that the COrotational energy is relatively cold. The overall results further suggest that the threespin-orbit pathways are not mixed completely in the exit channel.

SECTION Dynamics, Clusters, Excited States

C arbon dioxide is an important molecule in both theEarth's atmosphere as one of the main greenhousegases and in interstellar media; it is also the main

component of the Mars atmosphere. While CO2 photochemi-stry has received much attention, a full understanding ishampered by the complex nature of its electronically excitedstates. In the range of 120-200 nm, there are two weakabsorption bands from its 1Σg

þ ground state to the low-lying1Πg,

1Σu-, 1Δu, and 1Σg

þ excited states,1 each of whichcorrelates adiabatically to O(1D) þ CO(1Σþ) products. All ofthese transitions are electric-dipole-forbiddenbut vibrationallyallowed. Excitation of the bending mode causes the degen-erate states to split into Renner-Teller pairs along the bendingvibration coordinate; these states are stabilized in the bendinggeometry with equilibrium distances much longer than theCO2 electronic ground state for the low-lying states.2,3

The first electronic absorption band, beginning at around6 eV with a maximum near 8.4 eV, is quite irregular andspectrally diffuse.4 Our present photodissociation experimentperformed at 157 nm (7.9 eV) is associated with this band,which correspondsmost likely to an electronic transition fromthe ground state to the 1B2 surface derived from the 1Δu state.

Previous experimental studies showed that the main path-way of CO2 photodissociation following optical excitation bet-ween 120 and 170 nm is theO(1D)þ CO channel. Slanger andBlack5 established that the quantum yield for O(1D) productionis unity at 147 and 131 nm for the spin-allowed process

CO2ð1Σ þg Þþ hν f Oð1DÞþCOð1ΣþÞ

At photolysis wavelengths>174 nm, Inn and Heimerl6

observedaunit quantumyield for theO(3P)channel, suggesting

that the O(1D) channel for the CO2 photolysis is not openabove this excitation wavelength.

The spin-forbidden process

CO2ð1Σ þg Þþ hν f Oð3PÞþCOð1ΣþÞ

should be very unfavorable in a triatomicmolecule consistingonly of first row atoms. Theoretical electronic structure calcu-lations7,8 carried out for CO2 in the Franck-Condon regionnear the linear geometry, however, show the presence ofconical intersections between electronic-state surfaces in thespectral region of 120-170 nm. Nonadiabatic dynamicsinvolving curve crossing from the optically excited singletsurface to the triplet surfaces could thusplay an important rolein the photochemistry of CO2.

In this paper, we compare our measurements with predic-tions from the simple model of Gordon and co-workers,9,10

where optical excitation starts from the 1B2 surface and curvecrossing occurs to the 3B2 surface correlating with O(3P)products. The rate of transition from the singlet to tripletsurface, and hence the quantum yield for spin-forbiddenO(3P), depends on the number of times the molecule passesthrough the intersection region multiplied by the probabilityof jumping from the singlet to the triplet surface. Since thebending coordinate is highly excited in the optical excita-tion step, the molecule does not initially have a sufficientamount of energy in the dissociation coordinate to fall apart.

Received Date: March 18, 2010Accepted Date: April 16, 2010

r 2010 American Chemical Society 1862 DOI: 10.1021/jz100356f |J. Phys. Chem. Lett. 2010, 1, 1861–1865

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The excited molecule is therefore in a “complex” state whichlives for several vibrational periods. During this time, themolecule hasmultiple opportunities to cross fromone surfaceto the other. When the complex finally dissociates, one wouldexpect to obtain a statistical distributionofCO internal energy.

Dissociation that is rapid compared to rotational motionshould lead to highly anisotropic angular distributions of thefragmentswith respect to the internal direction of their opticalexcitation. The transition dipole moment in the case ofexcitation to the 1B2 state is along the long axis of the bentCO2, implying a near-parallel transition, and the fragments ofa rapid dissociation from this geometry are ejected parallel tothe electric field direction of the dissociation laser. At somedistance along the reaction coordinate, the molecule alsopasses through a spin-orbit recoupling zone, where branch-ing to the three different O(3Pj) fine structure channels isdetermined.

In this paper, we report the measurements of the tripletchannel, in particular, the correlations of the CO internalenergy distribution and the product angular distribution,and their j channel dependence on the O(3Pj) signal. Thesereveal detailed information on the relationship between theelectronic structure of the parent molecule and the dynamicsof photodissociation.

In previous experiments, Zhu and Gordon9 demonstratedin 1990 the presence of O(3P) product in CO2 photolysis at157 nm using a chemical scavenging method. The branchingratio of O(1D) and O(3P) was also determined to be 94:6 bymolecular beam photodissociation studies.11,12 In addition,Miller et al.13 measured the CO rovibrational-state populationfrom CO2 photodissociation at 157 nm using the laser-induced fluorescence technique. A nonstatistical fine structurepopulation of O(3Pj) of 0.14, 0.16, and 0.70 for j=0, 1, and 2,respectively, was also observed using the REMPI-Dopplerprofile technique byMatsumi and co-workers.10While oxygen3Pj=0,1,2 products have also been detected in the photodisso-ciation of a number of diatomic and triatomic systems,14-20

no correlated information between the product vibrationallevels and the O(3Pj) spin-orbital states for CO2 photodisso-ciation has been obtained previously. In this work, we presentexperimental results on the CO2 photodissociation using thevelocity map ion imaging method.21 The O(3Pj=0,1,2) productspin-orbit states are detected using (2þ 1) resonance-enhanced multiphoton ionization (REMPI) detection. Com-bined with imaging, this allows us to probe the spin-orbit

state-specific dissociation dynamics as well as the state-to-state correlation between CO(ν) and O(3Pj=0,1,2) for CO2

photodissociation at 157 nm.Figure 1 displays raw O(3Pj) images (quadrant-averaged)

of CO2 photodissociation at 157 nm, obtained by accumulat-ing the REMPI Oþ signals over 50000 laser shots with back-ground subtraction. The background was taken using bothlasers but without the molecular beam. The vertical arrowshows thepolarizationdirectionof thepumpandprobe lasers.No significant differences in the images were observed withthe probe laser polarization perpendicular to that of the pumplaser, indicating that the angular momentum alignment ofthe O atoms is small. Well-resolved anisotropic rings areobserved in the images, and these structures are attributedto the vibrational states of the coincident CO product. Fromthese direct images, inverted images were calculated usingthe p-Basex program.22 Angular integration of the invertedimages yields the speed distributions (Figure 2a) of theoxygen fragments for the three spin-orbit states in thecenter-of-mass (CM) frame. The CO vibrational distributioncovers almost the full range of accessible levels, supporting astatisticalmodelmentioned above. Clearly, from this data, theCO bond length in the transition state of the dissociating CO2

molecule is very different from the bond length of the ground-state CO product (1.128 Å).23

Angular distributions were also obtained for the threespin-orbit oxygen channels by integrating the imaging sig-nals over the radius or product speed (Figure 2b). The overallangular distributions for the three spin-orbital O(3Pj=0,1,2)microchannels are similar. The overall anisotropy parameterswere determined by fitting the angular distributions inFigure 2b. The averaged anisotropy parameters over theproduct velocity distribution are 1.07, 1.15, and 1.01 for theoxygen 3Pj=0,1,2 product channels, respectively. Within ourerror (Δβ ≈ (0.1), they are quite similar, and the j=2 and0 channels are closer in value to each other compared to thej=1 channel. On average, our results are consistent withthose of previous experiments (ref 12, β=1.25). These highanisotropy parameters show that the O(3Pj=0,1,2) channel is afast (compared to rotation) dissociationprocess fromaparallelelectronic excitation. The transition state is only slightly dis-placed from linearity.

The speed distributions in Figure 2a were then convertedinto the CM total kinetic energy distributions, and the resultswere shown for the three individual spin-orbit levels of

Figure 1. Raw images of the spin-orbital O(3Pj=2,1,0) products from the photodissociation of CO2 at 157 nm. The ring features correspondto the vibrational states of the coincident CO product.

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O(3Pj=0,1,2) in Figure 3. The ÆEintæ are about 52, 54, and 48%of the available energy (∼19500 cm-1)11,24 partitioned intoCO internal energy for the j=2, 1, and 0, respectively. Theseresults are consistent with previous experiments (ref 10).

To abstract more information, a qualitative simulation ofthe energy distribution was carried out. Each rotational peakwas simulated in energy space using a Gaussian line shape offixedenergywidth (600cm-1) that reflectedour instrumentalcontribution. Within one vibrational peak, the CO rotational-state distribution was assumed to be a Boltzmann distribu-tion. The rotational temperature and the intensitywere variedto fit each vibrational peak. The fitting results obtained werealso shown in Figure 3. The fitting error is estimated to beabout 10%, which is also reflected in the varying (rotational)widths of the vibrational peaks.

The simulation reveals that the CO is not highly rotationallyexcited in the O(3Pj) þ CO(ν) channel. The relatively coldrotational temperature (∼900 K) of the CO products for allthree oxygen spin-orbit channels suggests that the transitionstate is quite close to a linear geometry for the dissociationprocess. The results are that the CO rotational distributions forall three O(3Pj=0,1,2) spin-orbit levels are similar, suggestingthat the three spin-orbit oxygen channels have similartransition states in the dissociation.

From the simulations, the CO vibrational distributions forthe three oxygen product 3Pj spin-orbit states are obtained(Figure 4a). One can see clearly that the CO product vibra-tional-state distributions are somewhat different for the pro-duct channels. In Figure 4a, one observes again that COvibrational distributions for j=0 and 2 are more similar toeach other than to the j=1 distribution. There is obviously

more vibrational excitation for j=1. Anisotropy parametersfor the individual CO vibrational-state products that corre-spond to the three O(3Pj=0,1,2) spin-orbit states were also

Figure 2. (a) O(3Pj=0,1,2) product speed distributions derivedfrom the images shown in Figure 1. (b) Integrated (all speeds)product angular distributions for the O(3Pj=0,1,2) products.

Figure 3. The product total kinetic energy distributions (blackempty circles) derived from the speed distributions in Figure 2 forthe O(3Pj=0,1,2) þ CO(ν) channels. The red lines are the fittingresults, and the dashed lines are the individual CO vibrationalcomponents.

Figure 4. (a) Vibrational-state distribution of the CO coproductsfor different j states of the O(3Pj) product. (b) Relative anisotropyparameter for individual CO vibrational states correlated to thethree O(3Pj=0,1,2) spin-orbit states.

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obtained (Figure 4b). The anisotropy parameters for the threespin-orbit channels are generally similar but not exactly thesame. The anisotropy parameters are translational-energy-dependent and decrease slightly as the CO internal energyincreases. These results imply that the coupling between thesinglet and triplet states that induces different spin-orbitpathwaysmight not be exactly the same, thus causing slightlydifferent dynamics for the different spin-orbit dissociationchannels.

In summary, the CO(ν) þ O(3Pj=0,1,2) channel from CO2

photolysis at 157 nmhas been investigated using the velocitymap imaging technique. Well-resolved broad CO coproductvibrational distributions with relatively cold rotational distri-butions are observed, with minor but consistent differencesfor each spin-orbit channel, where j=0 and 2 show similarbehaviorcompared to j=1.Theoverall anisotropyparametersare consistent with a fast dissociation process by a parallelexcitation. For each j state, the anisotropy parameters dec-rease slightly as the CO internal energy increases.

These detailed correlated product-state distributions ofCO(ν) and O(3Pj=0,1,2) should provide a very good testingground for theoretical studies on this spin-orbit nonadiabaticdissociation process. The multiple surfaces crossing modelreferred to above is clearly too simple to explain all of the databut does qualitatively account for the vibrational distributions.The similar energy and recoil anisotropydistributions indicatethat the couplings that control the intersystem crossingbetween the singlet and triplet surfaces that produce thethree final spin-orbit levels should occur in generally thesame area of the potential energy surfaces and are not mixedcompletely in the exit channel. Further interpretation awaits amore advanced ab initio theoretical treatment of the CO2

molecule.

EXPERIMENTAL METHODS

We have performed CO2 photodissociation at 157 nmusing a single molecular beam velocity map ion imagingapparatus. The experimental setup used in this work has beendescribed previously.25,26 Briefly, the CO2 beam was pro-duced by expanding a neat CO2 sample through a Jordanvalve with a 0.5 mm nozzle. The CO2 sample used here wasrated 99.9% and was used without purification. At a distanceof 20 mm downstream from the nozzle, the expandedbeam passed through a skimmer with a 1 mm diameteraperture that separated the source and detection chambers;60 mm downstream from the nozzle, the collimated beampassed through a 2 mm hole in a repeller electrode plateand propagated further along the axis of the 360 mm longtime-of-flight tube of the ion imaging detector.

The CO2 molecular beam was intercepted by a 157 nmlaser beam,whichwasgeneratedbya commercial F2 excimerlaser (PSX-100) with a 10 Hz repetition rate. In order to avoidmultiphoton complications, only about 0.1 mJ/pulse of the157 nmphotolysis lightwas focused into theCO2 beamwith aCaF2 lens of 15 cm focal length. A commercial 157 nm thin-filmpolarizer (LaseroptikGmbH)was used to produce linearlypolarized 157 nm light. The resulting oxygen fragments werephotoionized about 20 ns later by a probe laser beam

produced by doubling the output of a tunable dye laser(Radiant Narrowscan), which was pumped by the thirdharmonic of a Continuum Surelite Nd:YAG laser. The three3Pj=0,1,2 spin-orbit levels of the oxygen atom products weredetected via a (2 þ 1)REMPI scheme27 at the probing laserwavelengths of 226.234, 226.058, and 225.654 nm, respec-tively. About 0.2 mJ of the probing laser was focused into theCO2 photolysis region using a lens of also 15 cm focal length.During image acquisition, the probe laser was scanned overthe Doppler profile of the chosen REMPI transition.

The two laser beams were spatially overlapped at the CO2

beam at a position in the middle between the repeller and theextractor electrode. The electric field polarization directions ofboth lasers were set to be perpendicular to the time-of-flightaxis. The images were collected by an imaging detector withdual 40 mm diameter multichannel plates coupled to a phos-phor screen. A cooled charge coupled device (CCD) camerawas used to record the ion signals on the phosphor screen.

AUTHOR INFORMATION

Corresponding Author:*To whom correspondence should be addressed. E-mail:xmyang@dicp.ac.cn (X.Y.); parker@science.ru.nl (D.H.P.).

ACKNOWLEDGMENT Thisworkwas supportedby theNederlandseOrganisatie voor Wetenschappelijk Onderzoek and partially by theNational Science Foundation of China, the Ministry of Science andTechnology, and the Chinese Academy of Sciences. F.L. was supportedby the National Science Foundation of China under Grant No.10704028, Basic research funds of Central Colleges under Grant No.200903373, and the State Scholarship fund by the China ScholarshipCouncil.

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