Photodissociation of HBr in and on Arn clusters: the role of the position of the molecule

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  • Photodissociation of HBr in and on Arn clusters: the role of the

    position of the molecule

    N. Hendrik Nahler,a Reinhard Baumfalk,a Udo Buck,*a Holger Vach,b Petr Slavcekc and

    Pavel Jungwirthc

    a Max-Planck-Institut fur Stromungsforschung, Bunsenstrasse 10, 37073, Gottingen, Germany.E-mail: ubuck@gwdg.de

    b Laboratoire de Physique des Interfaces et des Couches Minces CNRS-Ecole Polytechnique,91128, Palaiseau Cedex, France

    c J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic andCenter for Complex Molecular Systems and Biomolecules, Dolejskova 3, 18223, Prague 8,Czech Republic

    Received 24th April 2003, Accepted 23rd June 2003First published as an Advance Article on the web 9th July 2003

    Photodissociation experiments are carried out for single HBr molecules which are embedded in the interioror absorbed on the surface of large Arn clusters. For the embedded case the size dependence is measured for theaverage sizes hni 51, 139, 230, 290 and 450. For the surface case and the average size hni 139 the sourcetemperature is varied from T 163 K to 263 K. The measured kinetic energy of the H atom fragments exhibitspeaks at zero and 1.3 eV which mark completely caged and unperturbed fragments, respectively. The results arecompared with Molecular Dynamics simulations which account for the quantum librational delocalizationof the HBr molecule. The location of the molecule in/on the cluster is obtained from a trajectory study of thepick-up process under realistic conditions. For the embedded case corresponding to a co-expansion experiment,three argon layers are sufficient to completely hinder the H atom, in perfect agreement with the calculations.For the pick-up experiment, the large change of the source temperature leads to very small changes ofcluster temperature dependent properties. Events starting from the second shell have a higher exitprobability than those coming from the surface.

    I. Introduction

    Photodissociation of molecules in condensed media is theobject of extensive experimental and theoretical efforts in che-mical reaction dynamics.1 Recently these activities have alsobeen extended to clusters of finite size ranging from singlesolvent atoms to hundreds of particles.2,3 The advantage ofworking with clusters is that the finite size simplifies the theo-retical treatment and a direct comparison with measurementsbecomes possible. On the experimental side, new observablesare accessible like the kinetic energy distributions of the dis-sociation products. They directly probe the probability of thecage exit and the cage effect depending on fast or slow velocityof the photo-fragments.The systems among the solvated molecules that attracted

    most interest by theoreticians are hydrogen halides interactingwith rare gas clusters. They have been treated using varioustheoretical approaches.416 They exhibit a rich behavior inthe electronically excited states leading asymptotically to theexcited and ground spinorbit states. Depending on the influ-ence of the couplings, the behavior changes significantly whengoing from HCl to HI.1719 The results of the calculationsdemonstrate that in small systems the influence of the raregas atoms on the dissociation process is quite weak.9,15,20

    The situation changes when the first and second shell of theicosahedral structures close at n 12 and n 54.4,6,8,11 Wenote that even for larger sizes a small amount of cage exit pro-cesses occurs. A new perspective was introduced when thehydrogen halide molecule was placed in the outer shell of therare gas cluster. For the n 12 case the cage exit probability

    was smaller than for the embedded case.10 This trend contin-ued for chromophores with two solvation layers and it becameclear that caging probability depends strongly on the specificsurface site and on the librational motion of the HXmolecule.13,14

    On the experimental side, experiments with well defined sizedistributions and an elaborate analysis of the product state orvelocity are still quite rare. The most detailed results are avail-able for molecular ions embedded in inert atomic or moleculargases.21,22 For neutral systems, the OClO and HNO3 moleculeshave been investigated in different cluster environments.23,24

    For hydrogen halides, either small systems2527 or neat clustershave been studied.28 The Gottingen group systematicallyinvestigated HBr and HI molecules and their complexes in dif-ferent rare gas clusters using the pick-up technique for placingthem on the surface and coexpansion to embed them into theinterior of the clusters.13,2931 This effort indeed led to the firstdirect comparison of measured and calculated kinetic energydistributions for HBrArn clusters.

    14 For the embedded caseof HBrAr97 the agreement was quite convincing. In orderto verify also the theoretical predictions of the strong sizedependence of this behavior, we have carried out new measure-ments for the average sizes hni 57 and hni 130450. Thecomparison for the surface case indicated that some intensityat the cage exit was missing in the calculation. In addition,the cage exit probability for HIAr140 was smaller than thatfor HBr, and preliminary calculations were not in agreementwith the measurements. Therefore, we suggested that perhapsin the pick-up process the molecule penetrates a bit furtherinside thus leaving the outer shell. This idea was inspired by

    3394 Phys. Chem. Chem. Phys., 2003, 5, 33943401 DOI: 10.1039/b304511k

    This journal is # The Owner Societies 2003

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    http://dx.doi.org/10.1039/b304511khttp://pubs.rsc.org/en/journals/journal/CPhttp://pubs.rsc.org/en/journals/journal/CP?issueid=CP005016

  • the thorough study of Vach of the pick-up process,32 where asa rule-of-thumb it was found that the probability of goinginside increases with smaller size and larger attraction of themolecule. The latter is definitely the case when going fromHBr to HI. Therefore we decided to first simulate by Molecu-lar Dynamics the pick-up process and then carry out the calcu-lations for the dissociation process starting from the calculatedinitial conditions. Here, it came as a surprise that the probabil-ity for cage exit events starting from the second shell is largerthan for molecules in the surface. For HBrArn we added alsonew measurements for different source temperatures of the Arncluster beam. In this way, we hoped to test the theoreticalresult that the excitation of the librational motion completelychanges the cage exit probability. In this paper we concentrateon the HBrArn systems, while the results for HIArn will bepresented in a separate study.33

    II. Experiments

    In these experiments the HBr doped argon clusters are gener-ated in a molecular beam apparatus. The HBr molecules arephotodissociated with UV light deriving from a pulsed ns lasersystem. Within the same laser pulse the H atoms are ionizedvia a (2+1)REMPI (resonance enhanced multi-photon ioni-zation) scheme and are detected energy-sensitively in a WileyMcLaren time-of-flight mass spectrometer (WM-TOFMS)operating in the so called low-field mode. Details of this experi-mental setup have been described elsewhere.34,35 Here only ashort summary of the major components and the various tech-niques of cluster preparation is given, including the implemen-ted experimental improvements.13

    To prepare the HBr molecules in the surface region of theAr cluster, the cluster beam is first produced by a supersonicexpansion of neat Ar through a nozzle with conical shape witha diameter of 60 mm, an opening angle of 30 and a length of2 mm. The average cluster size of the Arn host clusters ishni 139. Increasing the nozzle temperature from 163 to263 K and adjusting the expansion pressure in the range from2 to 6 bar according to the relation provided by Hagena36

    should cause a slight rise of the internal energy of the Arn clus-ter at similar cluster size. Between the second and third cham-ber, which are both differentially pumped, the cluster beampasses through a scattering cell filled with HBr molecules ata partial pressure of 4 102 mbar. In the resulting pick-upprocess under these conditions, it is most probable that onlyone HBr molecule is adsorbed by the Arn cluster.

    13

    HBrArn clusters with the HBr molecule embedded in thecluster are produced by supersonic expansion of very dilutemixtures of 0.050.2% HBr in Ar through the above men-tioned nozzle. These conditions ensure that only one singleHBr molecule is embedded inside each cluster.13 The actualbeam data used in the present experiments are given in Table 1.Following mixed cluster preparation the cluster beam enters

    the chamber containing the two-stage Wiley-McLaren typeTOFMS.37 To suppress the H atom background from

    hydrocarbons, the WM-TOFMS is surrounded by a coppershield mounted on a high-pressure helium compressor whichresults in a vacuum pressure of 6 109 mbar. In order to dis-sociate the clusters, the polarized laser light is focused into theWM-TOFMS by a 400 mm lens. At their interaction point, thecluster beam, the dissociation laser, and the WM-TOFMS col-lection axis are oriented mutually perpendicular to each other.Thus, Doppler effects are eliminated in the photodissociationmeasurements. In all the present experiments the laser is line-arly polarized in the plane formed by the laser and the clusterbeam, at 90 to the collection axis. The H atoms from thedissociation process are ionized in the same laser pulse usingone-color (2+1)REMPI. The 243.06 nm light is gener