Photodissociation of oriented HXeI molecules generated from HI–Xe n clustersN. Hendrik Nahler, Reinhard Baumfalk, Udo Buck, Zsolt Bihary, R. Benny Gerber, and Bretislav Friedrich Citation: The Journal of Chemical Physics 119, 224 (2003); doi: 10.1063/1.1577311 View online: http://dx.doi.org/10.1063/1.1577311 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/119/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Orientation of dipole molecules and clusters upon adiabatic entry into an external field J. Chem. Phys. 129, 024101 (2008); 10.1063/1.2946712 Controllable generation of highly stripped ions with different charges by nanosecond laser ionization of clusters atdifferent wavelengths Appl. Phys. Lett. 87, 034103 (2005); 10.1063/1.1997281 Photodissociation of hydrogen iodide on the surface of large argon clusters: The orientation of the librationalwave function and the scattering from the cluster cage J. Chem. Phys. 120, 4498 (2004); 10.1063/1.1643895 Photodissociation of oriented HXeI molecules in the gas phase J. Chem. Phys. 114, 4755 (2001); 10.1063/1.1354144 The interaction of gold clusters with methanol molecules: Infrared photodissociation of mass-selected Au n + (CH3 OH) m J. Chem. Phys. 112, 752 (2000); 10.1063/1.480718

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Photodissociation of oriented HXeI molecules generatedfrom HI–Xe n clusters

N. Hendrik Nahler, Reinhard Baumfalk, and Udo Bucka)

Max-Planck-Institut fu¨r Stromungsforschung, Bunsenstrasse 10, 37073 Go¨ttingen, Germany

Zsolt BiharyDepartment of Chemistry, University of Califorina, Irvine, California 92697-2025

R. Benny GerberDepartment of Chemistry, University of Califorina, Irvine, California 92697-2025 and Department ofPhysical Chemistry, The Hebrew University, Jerusalem 91904, Israel

Bretislav FriedrichDepartment of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138

~Received 22 November 2002; accepted 3 April 2003!

We report the production in the gas phase of ionically bound HXeI molecules. The molecules aregenerated by the photodissociation of HI molecules in large Xen clusters and are identified from theasymmetry of the detected H atom fragments arising from the dissociation of oriented HXeI. Theorientation, resulting from a synergistic action of a pulsed laser field with a weak electrostatic field,is quite pronounced, due to a large ratio of the polarizability anisotropy to the rotational constant ofHXeI. © 2003 American Institute of Physics.@DOI: 10.1063/1.1577311#

I. INTRODUCTION

The class of chemical compounds containing rare-gasatoms has recently been considerably expanded by the dis-covery of HRgX molecules.1–5 Here Rg stands for a rare-gasatom and X is either a halogen atom or a radical group suchas CN or OH. The HRgX molecules were first observedamong the photolysis products of HX hydrides embedded inrare-gas matrices.Ab initio calculations demonstrated thatthese molecules were strongly bound, with partial ionic in-teractions between the positively charged Rg atom and thenegatively charged X group.6,7 The minimum separationswere found to be much shorter than the corresponding vander Waals radii. Although metastable with respect to a decayinto the HX1Rg channel, the dissociation energy of the Hatom with respect to the RgX species varies between 0.4 and1.4 eV.4 In a matrix, the HRgX molecules are formed asfollows: the H atoms, produced by the photodissociation ofthe HX molecules, are trapped by the RgX fragments. Thisoperation takes place in a fast, direct process after the Hatoms have been reflected from close-lying sites8 or afterthey start migrating through the crystal structure of the ma-trix. The HRgX molecules are usually detected by IR spec-troscopy. Here one makes use of their theoretically predicted,distinct H–Rg stretching frequencies.6

Recently, the Go¨ttingen group published a brief commu-nication on the first observation of an HRgX molecule in thegas phase.9 The HXeI molecules were generated by the pho-tolysis of HI molecules embedded in the outer shells of largeXen clusters.10 The production mechanism is a direct one aswas observed in a recent simulation:11 the H atoms are scat-tered back from the next shell of Xe cage atoms, slowed

down, and captured by one of the XeI units. To detect theHRgX molecules, they are first photodissociated back intothe H1RgX fragments. The H atoms are then photoionized,passed through a time-of-flight spectrometer, and detected.The laser photoionization takes place in an electrostatic ex-traction field. The combined laser and electrostatic fieldsstrongly orient the HRgX molecules, by virtue of their largepolarizability anisotropy, and give rise to a highly asymmet-ric spatial distribution of the product H atoms. In this way, itis possible to distinguish between the H atoms arising fromthe oriented HRgX molecules and the H atoms formed fromunoriented, isotropic molecules, such as HI, whose polariz-ability anisotropy and dipole moment are small.

The technique of orienting molecules by the combinedaction of radiative and electrostatic fields was recently pro-posed by Friedrich and Herschbach.12,13 However, the appli-cation of the technique to the HRgX molecules studied in ourexperiment requires a careful consideration of the initial ro-tational state of the molecules.14 In the present contributionwe, therefore, present a full account of the experimental re-sults for the HXeI molecules. We also describe the pendularstates formed in the combined fields and show that, in orderto observe a strong anisotropy of the H atoms, the HXeImolecules must enter the fields in their rotational groundstate. We corroborate this by a calculation of the zero-pointmotion of the HXeI molecule in the Xen cluster, using po-tentials developed for this system based on a recent modelingof spectroscopic data.15,16 Finally, we provide an importantpiece of experimental evidence to prove that the HXeI mol-ecules are indeed oriented: by changing the direction of thestatic electric field we shift the signal originating from theoriented HXeI molecules into another part of the time-of-flight spectrum.

The paper is organized as follows. In Sec. II we brieflya!Electronic mail: ubuck@gwdg.de

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describe the experimental setup. The mechanism of the ori-entation and the calculated pendular states are presented inSec. III. Section IV describes a simulation of the motion ofthe HXeI molecule in the Xen cluster. The experimental re-sults are presented in Sec. V, and the work is discussed andsummarized in Sec. VI.

II. EXPERIMENT

The experiments were carried out in the Go¨ttingen appa-ratus which is typically used for photodissociation experi-ments of molecules in different cluster environments.17–19

We produce a xenon cluster beam by a supersonic expansionof neat xenon gas into the source chamber through a nozzleof conical shape with a diameter ofd560 mm, an openingangle of 30°, and a length of 2 mm. By varying the pressurebetween 1.0 and 5.0 bar and the temperature between 203 Kand 218 K averaged cluster sizes between^n&5107 and2017 were obtained. The beam conditions for three typicalsizes are summarized in Table I. Next the cluster beampasses a pickup cell containing HI molecules at the partialpressure of 431022 mbar.20 The conditions are chosen suchthat mainly single HI molecules are picked up by the xenonclusters. Recent molecular dynamics simulations for similarsystems show that, in spite of the relatively large attractionbetween HI molecules and the Xe atoms of the cluster, HIwill stay in a substitutional position in the surface layer.21

After passing several differentially pumped vacuumchambers, the molecular beam enters the detection chamberhosting a two-stage time-of-flight mass spectrometer~TOFMS! of the Wiley–McLaren type. It is surrounded by acopper shield mounted on a high-pressure helium compres-sor which keeps the temperature at 20 K and helps to sup-press the background of unwanted H atoms. Here the HImolecules are dissociated by linear polarized laser light of243.06 nm which is focused into the mass spectrometer by a400 mm lens on a spot of a radius of 14mm. This gives at alaser energy of 1.76 mJ over a pulse length of 5 ns a laserintensity of 2.231011 W/cm2 assuming a Gaussian beamprofile. The actual value of the laser intensity which effec-tively interacts with the molecules inside the apparatus is afactor of 4 smaller caused by the inhomogeneous beam pro-files in different directions.

At this interaction point, the molecular beam, the disso-ciation laser beam, and the TOFMS collection axis are ori-ented mutually perpendicular to each other. The TOFMS isoperated in the so-called low-field mode with a small electricextraction field of 4.2 V/cm. In this way also the velocity ofthe particles can be measured including those with zero

velocities which would not be detected in a time-of-flightarrangement without fields. The experimental arrangement inthis region is schematically shown in Fig. 1. In the course ofthis work, the drift region of the TOFMS was enlarged toimprove the resolution of the instrument.22 In addition, adevice was introduced to change the direction of the staticelectric field with respect to the detector and the polarizationof the laser field.22

Following the dissociation, some of the H atoms are di-rectly reflected from the next shell and combine, in a con-certed reaction, with the I atoms left from the dissociationand one of the Xe atoms of the cluster to form the newmolecules HXeI. After the photoexcitation, not only the 12different potential surfaces of the neutral reactants, but alsothe potential manifold of the charge transfer states becomesaccessible.8,11,23This process is sketched in Fig. 2. In fact, inmatrices the trapping of positively and negatively chargedmolecules has been experimentally observed for a similartype of reaction.24,25Once the molecule HXeI is formed, it isadiabatically turned around in the direction of the plane-polarized laser field. This still allows the molecule to point intwo directions like a double arrow. By the interaction withthe weak electric field, only one direction is selected~themolecular axis then behaves like a single arrow!, namely, theone in which the H atom points towards the detector. Thedetails of this process are presented in the next two sections.We note that during the process of the deceleration of the H

TABLE I. Beam data.

Xe Xe Xe

Diameter of conical nozzle~mm! 60 60 60Expansion pressure~bar! 3.0 4.0 5.0Nozzle temperature~K! 218 223 203Pickup cell pressure (1022 mbar) 2.0 2.0 4.0Average cluster size 668 983 2017

FIG. 1. Schematic view of the apparatus used in the present experiment.

FIG. 2. Schematic view of the potential surfaces that are involed in theproduction of HXeI molecules.

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atoms and the formation of the HXeI molecule, the Xen clus-ter starts to evaporate so that finally a nearly free, orientedHXeI molecule results.

Subsequently, the HXeI molecule is dissociated and theH fragments are detected by one-color, resonance-enhancedmultiphoton ionization ~REMPI! in a 211 excitationscheme. For this ionization, the same laser pulse as for thedissociation is used. The 243.06 nm light is generated bymixing the fundamental of an injection-seeded Nd:YAG laser~Quanta Ray GCR-5! with the frequency-doubled output ofa dye laser ~LAS, LDL 20505! operating at 630 nmand pumped by the second harmonic of the Nd:YAG laserat 532 nm.

The main observable in the present study is the kineticenergy of the hydrogen atoms. It is obtained from a carefulanalysis of the measured time-of-flight spectra. The transfor-mation from time to energy variables is carried out by acomplete simulation of the particle trajectories, taking intoaccount the photodissociation process, the molecular beamdata, the finite interaction volume, the geometry of theTOFMS, and the electronic response of the detector.18,19

III. ORIENTATION OF HXeI

It has been well established that polar molecules can beoriented by electrostatic fields.26 The orientation effect scaleswith the dimensionless parameterv[m«S /B, wherem is the~body-fixed! permanent electric dipole moment of the mol-ecules,«S is the electric field strength, andB is the rotationalconstant.27 Since a sizable effect occurs forv>10, electricfields on the order of 100 keV are required for typical valuesof the ratio,m/B'10 D/cm21. In the basis of the field-freestates,uJ,M &, the permanent dipole potential couples stateswith sameM but with J’s that differ by61. As a result, theeigenstates arehybrids of field-free rotor states for a fixedvalue ofM but a range ofJ’s. BecauseuJ,M & states of botheven and odd parity contribute to a given linear superposi-tion, the hybrid states haveindefinite parity. The eigenstatesare labeled byM and the nominal valueJ of the angularmomentum for the field-free rotor state that adiabatically cor-relates with the high-field hybrid function.

It is also well known that molecules can be aligned by anonresonant radiative field~a static field would work too, butthe requisite field strength can only be obtained in a laserfield!.28–30 A nonresonant radiative field interacts with theanisotropic molecular polarizability and produces a double-well induced-dipole potential. As a result, all states bound bysuch a potential occur as tunneling doublets. The interactionis characterized by a dimensionless anisotropy parameterDv[v i2v' , with v i ,'[2pa i ,'I /(Bc) proportional tothe componentsa i anda' of the polarizability parallel andperpendicular to the molecular axis and to the laser intensityI ; the laser intensity is related to the electric field vector«L

by I 5 (c/4p) «L2 ~Ref. 31!. Since the induced-dipole poten-

tial couples states with sameM but with J’s that differ by 0or 62, the resulting hybrid states are superpositions of field-free rotational states of either even or odd parity, and so havea definite parity. The components of a tunneling doublet havethe sameM but J’s that differ by61 and so have opposite

parities.32 The splitting between two members of a tunnelingdoublet decreases as exp@2Dv1/2#. Therefore, by increasingDv, their eigenenergies can be drawn arbitrarily close to oneanother.

If the nonresonant radiation is delivered as a pulse ofintensity I (t)5I 0g(t/t), with I 0 the peak intensity,g(t/t)the pulse time profile, andt the pulse duration, the anisot-ropy parameterDv5Dv(t) and the induced dipole potentialbecome a function of time. The corresponding time-dependent Schro¨dinger equation, with a time dependence de-termined by the time profileg(t/t) of the pulse, can be castin a dimensionless form by dividing throughB. As a result,the equation clocks the time in units of\/B which defines a‘‘short’’ and a ‘‘long’’ time for any molecule and pulse dura-tion. The analysis33,34 of the time dependence shows that inthe long-pulse limit (t'5\/B), the interaction is adiabaticand the pendular states faithfully follow the field as if it werestatic at any instant.

Recently, Friedrich and Herschbach proposed a generaltechnique of orienting molecules, amenable to a wide varietyof species and applications.12,13The key aspect is to endow apolar molecule with a pseudo-first-order Stark effect. Suchmolecules, whether linear or asymmetric, in effect can bemade to act almost like a symmetric top. The pseudo-first-order Stark effect arises from the combined action of a staticelectric field and a nonresonant radiative field: while the la-ser field creates quasidegenerate tunneling doublets of oppo-site parity ~see above!, the electrostatic field couples themvia the permanent electric dipole interaction and so createsoriented states of indefinite parity. Thus often a very weakstatic electric field can convert second-order alignment by alaser into a strong first-order orientation. The effect occursfor a wide range of polar molecules, in fact for any, as onlyan anisotropic polarizability is required.

Congruent~i.e., parallel or antiparallel! combined fieldsenhance the hybridization of theJ states, whileM remains agood quantum number. If the fields are not collinear,Mstates are also hybridized. The orientation of the molecularaxis is then no longer azimuthally symmetric about eitherfield. In the high-field limit, the axis becomes localized intwo or more directions with respect to the plane defined bythe field vectors. These and other features of the doubly hy-bridized pendular states offer further useful tools for the con-trol and manipulation of molecular trajectories.

The orientation of the molecular axis in a given state ischaracterized by itsorientation cosine, ^cosuS&, whereuS isthe polar angle between the molecular axis and the directionof the electrostatic field«S . Similarly, the alignment of themolecular axis is characterized by thealignment cosine,^cos2 uL&, with uL the polar angle between the molecular axisand the polarization vector of the laser field«L . Note thatuS

05arccoscosuS& anduL05@arccoscos2 uL&#

1/2 are the angu-lar amplitudes of the molecular axis in the fields; the greaterthe orientation or alignment cosine, the smaller the angularamplitude. Figure 3 shows the dependence of the^cosuS& forthe members of the lowest tunneling doublet, theJ50, M

50 and J51, M50 states, as functions ofDv at fixed~small! values ofv for congruent~i.e., parallel or antiparal-lel! fields. The two states are oriented opposite with respect

226 J. Chem. Phys., Vol. 119, No. 1, 1 July 2003 Nahler et al.

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to one another, with the consequence that their mixturewould exhibit alignment only. One can also see that at highenoughDv, the orientation always becomes nearly ‘‘perfect’’~i.e., as good as the uncertainty principle allows!, with anangular amplitude of less than620° for either state. Sincethe angular amplitudeuL

0 is always smaller thanuS0 , the

strong orientation effect can also be regarded as arising fromthe restriction on the angular amplitude of the molecule im-posed by the intense radiative field: once the radiative fieldcouples the molecular axis to its field vector«L , the perma-nent dipole, which lies along the same molecular axis, has nochoice but to remain within the range preordained by theradiative field; the parallel static field just serves to define apreferred direction. Note that forv!Dv>10 and theJ

50, M50 or J51, M50 doublet, the valuesvp andDvp

of the interaction parameters which yield a ‘‘perfect orienta-tion’’ are approximately related byvp'23exp(20.113Dvp). So, for instance, ifDvp5100 can be attained~which is the case for most molecules, even in a laser pulselong enough to warrant adiabaticity!, vp51025 would cor-respond, for a molecule withm/B51 D/cm21, to a trulypuny electrostatic field of just 0.5 mV/cm.

For the parameters of our experiment~summarized inTable II! we obtainDv570 for HXeI. The value ofDa wascalculated from the polarizabilitya of the constituents usinga relation betweenDa anda derived from similar ionically

bound systems. The long laser pulse duration in our experi-ment, t'5 ns, corresponds tot/(\/B)'26, and so amplyensures adiabaticity.33,36

The value ofDv570 is large enough to align theJ50, M50 and J51, M50 states of the HXeI moleculewith an angular amplitude of just620°. Our electric field«54.2 V/cm gives rise tov50.016; however, evenv'1023 would be enough to strongly couple theJ50, M

50 andJ51, M50 states and thus ‘‘perfectly’’ orient them~for Dvp'70, vp'1023). The orientation cosines of thetwo states atv50.016 andDv570 are^cosuS&0,050.932~along the static field! and ^cosuS&1,0520.932 ~opposite tothe static field!.

While theJ50, M50 state correlates with the rotationalground state~whoseJ50) of the HXeI molecule, theJ51,M50 correlates with the first excited rotational state~whoseJ51). Since the energy difference between theJ50, M

50 and J51, M50 states is quite small, about 0.03B atDv570 andv50.016, a separation between them cannot beattained by cooling the beam. Therefore, in order for theHXeI molecule to be oriented along the static field, it has tobe formed in its ground rotational state, leaving theJ51state empty. That this is indeed the case is ensured by thecluster environment, as will be described in the next section.

IV. PROPERTIES OF HXeI IN XENON CLUSTERS

Several processes can, in principle, affect the orientationof HXeI and should be considered for the interpretation ofthe experiments. For that purpose, the motion of HXeI in thexenon cluster was simulated. HXeI has been shown to oc-cupy a disubstitutional site in Xe matrices, in a relativelyrigid cage.15,16 Ab initio calculations at the MP2 level showstrong anisotropy of the guest–host interaction for this sys-tem, which also indicates strong hinderance of molecular ro-tations in the solid. We used the analytic potentials, fitted toab initio points, that we developed for the matrix-isolatedspecies also for the clusters.15 In our computations, the HXeImolecule was surrounded by 18 Xe atoms, and the clusterwas relaxed using free boundary conditions. The resultingsystem is depicted in Fig. 4. This cluster provides a reason-able representation also in the case of HXeI embedded inlarger clusters, but near their surface. We obtained a solva-tion energy of about 1.2 eV for the molecule in this cluster.Next we calculated the normal modes of the system. We listthe local molecular modes with their frequency in Table III.We also show the zero-point amplitudes for the tilt motion ofHXeI with respect to the equilibrium axis and that of the Hbending. The stretching and translational modes are obvi-ously not connected with possible rotations of the molecule.

If we assume that HXeI, after its formation, remains in acluster environment, one of the possibilities to consider isthat after relaxation, HXeI is stabilized in a loose cage withinthe Xe cluster. In principle, it may be possible that preces-sion of the molecular axis takes place about another axis inthe cage. In the simulations of the present study and in thoseof Ref. 15, we searched extensivly for such a possiblity butfound no evidence for it: Loose cage structures of HXeI in

FIG. 3. The dependence of thecosuS& for the members of the lowest

tunneling doublet, theJ50, M50 and J51, M50 states, as functions ofDv at fixed ~small! values ofv for congruent fields. The two states areoriented opposite with respect to one another.

TABLE II. Characteristic data for the orientation of different molecules:Daanisotropy of dipole polarizabilities,B rotational constant,m dipole mo-ment, and cosus& orientation cosine. The dimensionless parametersv andDv are defined in the text.

Molecule Da (Å 3) B (cm21)a m ~D!b v Dv ^cosus&

HXeI 3.4 0.0268 6.4 0.016 70 0.932HI 0.43c 6.551 0.45 0.005 0.072 0.002

aGeometries from Ref. 4.bReference 4.cReference 35.

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Xe clusters, with internal precession of the HXeI axis, do notseem to be stable.

Another relevant process that, in principle, may affectthe experiment is the~degenerate! bending vibration ofHXeI. However, according to our calculations the bendingmode of HXeI is very stiff, and the bending amplitude is ofthe order of 10° or smaller, which is insufficient to affect theexperimental results discussed later. The twofold degenerateorientational tilting~libration! modes, the best candidates torepresent rotations, have very small zero-point amplitudes,below 2°. Our results thus clearly demonstrate that the mol-ecule is held very rigidly in the cold cluster. Excited rota-tional levels of the molecule, as it leaves the cluster, can beexpected to be scarcely populated.

Rotation of HXeI in Xe clusters may occur if the clusteris melted. In our view, melting of the cluster upon formationof the HXeI molecule seems unlikely. Simulations of theformation of HXeCl by photolysis of HCl in a Xe matrixshowed no sign of melting.11 In fact, the evidence availablesuggests that the ‘‘heating’’ of the cluster upon photodisso-ciation is likely to result in sequential evaporation, with boil-ing off of rare-gas atoms. A study of photolysis of HCl inArn clusters37 shows substantial evaporation after 1 ps. After

50 ps, only clusters with six argon atoms or fewer survive toa significant extent. Similar results were obtained forHF–Arn clusters.38 It seems most consistent with the data toassume that extensive evaporation of Xe atoms takes placeafter the formation of HXeI. When the molecule is photo-lyzed, it is probably not clustered at all, or just a few Xeatoms remain. The structure of these HXeI(Xe)n complexeswas determined and the moments of inertia for a relaxednonlinear HXeI-Xe complex calculated. The rotational con-stants are 0.026 cm21, 0.011 cm21, and 0.008 cm21 ~ascompared to the rotational constant of 0.026 cm21 of thelinear HXeI molecule!. Complexes with more than onechemically unbound xenon atom have rotational constantsthat are smaller than 0.008 cm21. For these larger com-plexes, the rotational constants along different axes start tobecome more and more similar, indicating their more or lesscompact, spherical structure.

V. EXPERIMENTAL RESULTS

Experiments have been carried out for the dissociation ofHI on a xenon cluster of the average sizes from^n&5107 to2017. In a new series of experiments the polarization of thelaser field and the direction of the static electric field wereparallel. The results of the measured time-of-flight distribu-tion of the H atom products for two selected average sizes^n&5668 and 2017 and parallel polarization are shown in thelower part of Fig. 5 by the gray lines.

The present TOF distributions are characterized by anasymmetry in the intensity with increasing amplitudes forincreasing cluster sizes. This is caused by the higher prob-

FIG. 4. Calculated geometrical arrangement of the HXeI molecule in theXen cluster.

FIG. 5. Measured time-of-flight distributions of H atoms from the dissocia-tion of the system HI–Xen at 243 nm in arbitrary units for different averagecluster sizes n&. Lower panel: polarization angle 0°. Upper panel: polar-ization angle 90°. Gray points: measured values. Black line: symmetric part.Gray shaded area: asymmetric part of the spectrum.

TABLE III. Frequencies of local modes for HXeI in the xenon cluster. Thezero-point amplitudes in the direction of the H–Xe bond are also shown.

ModeFrequency

(cm21)Amplitude

~deg!

H–Xe stretch 1356H bending (23degenerate) 479 8.5I–Xe stretch 133Molecular tilting (23degenerate) 39 1.8Perpendicular translation (23degenerate) 35Parallel translation 21

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ability of the larger clusters to capture HI molecules and thusgenerate a larger number of HXeI molecules. The asymmetryis a clear indication that part of the intensity originates fromdissociation processes of oriented molecules. In our experi-mental arrangement the dissociation of an unoriented mol-ecule, like HX, always results in a symmetric time-of-flightspectrum of the H atoms.19,39 First the detector sees the Hatoms which were emitted in the direction of the detector andfinally those H atoms which were initially flying in the op-posite direction and then turned around by the weak electricfield. This behavior enables us to easily separate the asym-metric part of the spectrum from the remaining part. Wesubstract such a distribution from the measured spectrum sothat the two symmetric halves of the remaining spectrumgive, when transformed to the kinetic energy distribution,identical results. We note that the remaining time-of-flightdistributions need not be necessarily perfectly identical,since the atoms see different electric field distributions ontheir way to the detector. The result of such a procedure forthe two average cluster sizes is also displayed in Fig. 5 wherethe black lines represent the symmetric part and the shadedgray areas the asymmetric part of the spectrum. Comparedwith the published results,9 the new data are more structuredand the asymmetry is smaller. The former result is tracedback to the higher resolution of the TOF mass spectrometerand the latter one to the lower nominal intensity of the laserwhich reduces the alignment.

When the polarization of the laser field is changed to beperpendicular to the electric field, the results are nearly iden-tical. One example for the cluster size^n&5668 is depictedin the upper part of Fig. 5.

The underlying kinetic energy distributions for the sym-metric part of the spectra are within the experimental uncer-tainties the same. This is demonstrated in Fig. 6 for the twosizes in the parallel arrangement. These results are clearlyattributed to the dissociation of HI molecules in Xen clusters.The kinetic energy distributions exhibit the typical patternexpected for such an event. The arrows mark the energies ofthe unperturbed H atoms at 1.0 and 2.0 eV, which correspondto the direct cage exit and leave the I atom in the excited andground spin orbit state, respectively. The maximum intensityis reached around zero energy. By the interaction of the H

atoms with the surrounding Xe atoms of the cluster, the Hatoms are completely slowed. Similar results have been ob-tained for HI on Krn and HBr on Xen .10 This is in contrast tothe results obtained for the lighter Nen and Arn clusters forwhich the peak observed at zero kinetic energy is much morepronounced.10

The result for the asymmetric part of the spectra is de-picted in Fig. 7. Since the second, slower part of the time-of-flight distribution is missing, the detected H atoms have tocome from a molecule which is oriented in such a way thatthe H atom points in the direction of the detector. It exhibitsa distribution which essentially covers the energies from zeroto about 3500 cm21 which corresponds to 0.43 eV withsome peaks in between.

To verify that the asymmetric signal in the energy lossspectrum of the H atoms originates from oriented HXeI mol-ecules, we carried out an experimental test. We changed thedirection of the weak static electric field. The experimentalpulse sequence and the expected pattern of the TOF spectrumare depicted in Fig. 8. During the laser pulse the field«S ispointing in the direction away from the detector. In this pe-riod no H atom is accelarated in the direction of the detector,but the field interacts with the subgroup of aligned HXeImolecules and selects those with the H atom pointing awayfrom the detector. Immediately when the field is turnedaround, the H atoms from the HI background spectrum fly inthe direction of the detector. The H atoms from orientatedHXeI molecules are also turned around so that the asymmet-ric part of the spectrum is shifted to much longer flight times.The result of the TOF spectrum for^n&5983 is shown inFig. 9. Now the asymmetry appears in the second part of thespectrum. The subtracted part after the simulations is shownas an inset in the same panel. This is clear evidence that wedeal in our experiments with oriented HXeI molecules. The

FIG. 6. Measured kinetic energy distributions of H atoms from the disso-ciation at 243 nm in arbitrary units: HI molecules close to the surface of Xen

clusters for two different cluster sizes: crosses^n&5668, triangles^n&52017. The nominal cage exit energies which are related to the two spinorbit states of I are marked.

FIG. 7. Measured kinetic energy distributions of H atoms from the disso-ciation at 243 nm in arbitrary units: HXeI molecules originating fromHI–Xen clusters for different cluster sizes:^n&52017 ~triangles!, 983~circles!, and 668~crosses!. The middle curve is obtained from the measure-ment of the reversed field direction shown in Fig. 9.

229J. Chem. Phys., Vol. 119, No. 1, 1 July 2003 Photodissociation of HXeI

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resulting kinetic energy distribution is also displayed in Fig.7. It is quite similar to ones already observed earlier.

VI. DISCUSSION AND CONCLUSION

The attribution of the observed results to oriented HXeImolecules is justified as follows.~i! HXeI can be generatedunder our experimental conditions after the photolysis of HIin an environment of xenon atoms as was shown in a verydetailed manner for matrices and also in simulations forclusters.11 ~ii ! HXeI has a very large anisotropy of the dipolepolarizabilityDa with respect to the rotational constantB sothat it can be easily aligned even in our laser field with mod-erate intensities.~iii ! It has a dipole moment of 6.4 D whichpoints in the direction of the H atom.~iv! The orientationtakes place by a combined action of the laser field and theweak electric field for a nonrotating molecule in the groundstate.~v! The latter behavior has been demonstrated in thesimulation of HXeI in the Xen cluster.~vi! The orientationhas also been nicely demonstrated experimentally, by chang-ing the direction of the electric field. Without the describedmechanisms, no H atoms would otherwise have reached thedetector producing an asymmetric time-of-flight distribution.~vii ! The results for the kinetic energy exhibit a distributionwhich essentially covers the energies from zero to about 0.4

eV with some structure in between. This corresponds exactlyto the predicted well depth of the HXeI molecule. The ex-planation of the structure and the detailed mechanism of thephotodissociation process will be treated in a forthcomingpaper.

From the properties of the molecules presented in TableII it is clear that HI cannot be oriented under our experimen-tal conditions. There is, however, a chance to orient othermolecules in the series like HXeBr, HXeCl, and HKrCl. InRef. 9 we erroneously thought that also HKrI is a possiblecandidate. This molecule, however, was never observed orcalculated to be stable so that the failure to observe it is notsurprising. The same is valid for HKrBr. We note that thestability of these molecules usually increases with decreasingmass of the halogen atom since their electron affinity islarger and smaller X2 stabilize the molecule by a larger Cou-lomb interaction. In contrast, the probability to orient them inthe combined laser and electric field increases with increas-ing mass of the halogen atom because of the larger polariz-abilities and smaller rotational constants. This trade-off is thereason that conditions become less and less favorable to ori-ent these molecules. Nevertheless, we were able to detectHXeCl of this family;22 these results will be presented in adifferent study.

We note that in the calculation of Sec. III, which predictsthe complete orientation of HXeI, it is assumed that thepulsed laser field and the static electric field are parallel toone another. In our experiments, however, also the perpen-dicular arrangement of these two fields gave similar results.A possible mechanism is the aligment of the asymmetric topmolecule Xe–HXeI perpendicular to the HXeI axis along anintense linearly polarized laser field as was recently demon-strated for a similar system in Ref. 40.

In conclusion, we note that based on a combined pulsedlaser field in the nanosecond range and a weak static electricfield, we were able to orient HXeI, a molecule with a largeanisotropy of the dipole polarizability and a permanent di-pole moment, as was predicted recently by Friedrich andHerschbach. We used this ability to identify this moleculeHXeI in the gas phase by detecting the asymmetric distribu-tion of the H atom dissociation product. It was generated bythe photodissociation of HI molecules in large Xen clusters.

ACKNOWLEDGMENTS

The authors thank Ara Apkarian for illuminating discus-sions on the formation of HXeI and Mika Pettersson for im-portant comments on the stability of these molecules. Thework in Gottingen was financially supported by the DeutscheForschungsgemeinschaft in SFB 357 and a travel grant fromthe DAAD-NSF exchange program. The work in Irvine wassupported by Grant No. CHE-010 1199 from the Chemistrydivision of NSF~to R.B.G.!. Work at the Hebrew Universitywas supported by the DFG, Germany in SFB 450.

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FIG. 8. Schematic experimental arrangement and expected TOF pattern forchanging the direction of the electric field~gray arrow! to prove the orien-tation of HXeI. Left: usual arrangment. Right: inverted static electric field.

FIG. 9. Measured time-of-flight distributions of H atoms from the dissocia-tion of HXeI molecules originating from HI–Xen clusters for the cluster size^n&5983 at 243 nm in arbitrary units with changed direction of the electricfield. Measured values~gray points! and symmetric~black line! and asym-metric ~gray shaded area! parts of the spectrum.

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231J. Chem. Phys., Vol. 119, No. 1, 1 July 2003 Photodissociation of HXeI

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