Dissociative Photodetachment Studies of I2-⊙ Ar: Coincident Imaging of Two-and Three-Body Product Channels

Dissociative Photodetachment Studies of I2-⊙ Ar: Coincident Imaging of Two-and Three-Body Product Channels
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  Dissociative Photodetachment Studies of I 2 - ‚ Ar: Coincident Imaging of Two- andThree-Body Product Channels † Kathryn E. Kautzman, Paul E. Crider, David E. Szpunar, ‡ and Daniel M. Neumark*  Department of Chemistry, Uni V  ersity of California, Berkeley, California 94720, and Chemical Sciences Di V  ision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Recei V  ed: August 14, 2007; In Final Form: September 25, 2007  Van der Waals clusters serve as prototypical systems for studying processes of energy transfer. The I 2 · Arsystem has attracted particular interest due to the wide array of decay processes occurring in competitionwith one another. Here, we present systematic dissociative photodetachment (DPD) studies of the I 2 - andI 2 - ‚ Ar anions in the region 4.24 - 4.78 eV. The resulting neutral fragments are detected by time- and position-sensitive (TPS) coincident imaging. Photofragment mass distributions and translational energy distributionsfrom the DPD of I 2 - are presented and facilitate understanding of the I 2 ‚ Ar system. For the I 2 ‚ Ar complex,channels resulting from two-body dissociation leading to I 2 + Ar photoproducts are observed at all photonenergies employed. We also report the first direct observation of the previously inferred three-body dissociationchannel leading to I + I + Ar photoproducts. The relative intensities of each decay channel are investigatedin relation to the electronic state being accessed. Translational energy distributions of the I 2 ‚ Ar complex lendfurther insight into the decay mechanism for each channel. Introduction Small van der Waals (vdW) clusters are important modelsystems for understanding the nature of the weak interatomicand intermolecular forces that play a key role in chemicaldynamics and spectroscopy. 1 The I 2 ‚ Ar complex has been aparticularly important system of this type, 2 owing in part to therich variety of dynamical processes that occur upon excitationof the I 2 chromophore to the B 3 Π u (0 u + ) state, a bound state thatcorrelates to I + I*( 2 P 1/2 ) products. The complex is nowunderstood to exist in nearly isoenergetic T-shaped and linearforms, showing discrete and continuum absorption, 3 - 6 respec-tively, upon excitation of the B( 3 Π 0 u + ) state. Mechanismsinvoked to explain the decay of the electronically excitedcomplex include vibrational predissociation (VP) 7 - 11 and electronic predissociation (EP) 8,12,13 In addition, “one-atom caging” 3,9,14 - 17 (i.e., emission from intactI 2 after excitation above the B state dissociation limit) has beenobserved and is now attributed to excitation of the linear isomer.VP is the best understood and most experimentally accessibleof these processes since the I 2 / product fluoresces and is thuseasily detected. In contrast, EP is a dark channel whose existencehas been indirectly inferred. Motivated by the possibility of directly observing EP, we have undertaken a complementaryapproach to previous studies, using dissociative photodetachment(DPD) of I 2 - ‚ Ar, which is known to be T-shaped, 18 - 20 to accessthe electronic states leading to the dissociation of I 2 ‚ Ar.Much of the experimental and theoretical interest in I 2 · Ar isdriven by the observation that the intensity of the VP channeloscillates with I 2 (B) vibrational excitation. 7,8,12,13 Two explana-tions for the srcins of these oscillations have been proposed. 6,8 One of these is based on the notion that the VP yield is drivenby the nature of the intramolecular vibrational relaxation (IVR)process that transfers vibrational energy from the initially excitedI 2 (B, ν ′ ) into lower frequency I 2 ‚ Ar vdW modes until the weakbond breaks. 11 Irregularities in the VP yield result from thesparse manifold of dark states that mediate this process. Theother mechanism is based on the competition between VP andEP as a function of  ν ′ ; the products from EP do not fluoresce,and if the EP yield oscillates with ν ′ , the fluorescence from VPwill be modulated correspondingly. This latter mechanism isstrongly supported by the fluorescence quantum yield measure-ments of Burke and Klemperer. 12 However, the exact mechanismof EP is still under investigation, 2,21,22 and it has in fact neverbeen directly observed from the excitation of I 2 ‚ Ar. Detectionschemes capable of simultaneously analyzing both two-bodyand multibody decay events 20,23 permit further investigation of these competitive dissociation channels, as is demonstrated here.In this work, I 2 and I 2 ‚ Ar are formed through photodetachmentof the corresponding anions. We examine the DPD of I 2 - andthen compare these results with the dissociative photodetachmentof I 2 - ‚ Ar. DPD creates I 2 and I 2 ‚ Ar in excited electronic states,which then go on to dissociate by the following schemes:The relevant potential energy curves for I 2 - and I 2 in theinternuclear distance range of interest are shown in Figure 1. † Part of the “Giacinto Scoles Festschrift”.* Corresponding author. E-mail: ‡ Present address: Department of Biological, Chemical, and PhysicalSciences, Roosevelt University, Chicago, IL 60605. Ar ‚ I 2 (B, ν ′ ) f  Ar + I 2 / ( ν e ν ′ ) (1)Ar ‚ I 2 (B, ν ′ ) f  Ar + I( 2 P 3/2 ) + I( 2 P 3/2 ) (2)I 2 - + h ν f  I 2 / + e - f  I + I + e - (3)I 2 - ‚ Ar + h ν f  I 2 / ‚ Ar + e - f  I 2 + Ar + e - f  I + I + Ar + e - (4) 12795  J. Phys. Chem. A 2007, 111, 12795 - 1280110.1021/jp0765401 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 11/21/2007  The neutral repulsive curves and neutral ground state are fromthe work of de Jong et al. 24 The B 3 Π u (0 u + ) state potential wasconstructed using a Morse potential with literature values forthe well depth and equilibrium internuclear distance. 25 Theneutral curves have also been extensively studied by Teichteiland Pelissier. 26 The potential energy curve for the I 2 - X 2 ∑ u + ground state was obtained through a combination of femtosec-ond stimulated emission pumping (fs-SEP) and photoelectronspectroscopy (FPES). 27 - 29 This curve has a minimum at R e ) 3.205 Å, which locates the center of the Franck - Condon (FC)region accessed by photodetachment.Figure 2a shows a magnified view of relevant neutral curvesnear R e (I 2 - ). Photodetachment transitions to these curves wererecently mapped out by Parsons et al. 30 via photoelectronspectroscopy of I 2 - at 5.83 eV; Figure 2b shows the portion of the photoelectron spectrum relevant to the work discussed here.The photoelectron spectrum of I 2 - ‚ Ar was recorded by Asmiset al. 31 at a photon energy of 4.657 eV (266 nm) and comparedwith previous spectra of bare I 2 - . From the shift in the spectruminduced by the Ar (24 meV) and the binding energy of T-shapedI 2 ‚ Ar, 29 meV, 11 the dissociation energy of the anion complexwas found to be 53 meV. Other than this shift, the photoelectronspectra for bare and complexed I 2 - were very similar, asexpected since the Ar only weakly perturbs the anion andneutral. In particular, vertical detachment energies to the excitedstates of I 2 - ‚ Ar were all simply shifted relative to those of I 2 by the value of the solvent shift. The other notable differencewas that the I 2 - ‚ Ar complexes were vibrationally cooler thanthe bare anions, as evidenced by the reduced contribution fromhot bands in the I 2 - ‚ Ar spectrum because complexes formedwith vibrational energy exceeding the binding energy dissociateprior to mass selection.In the work described here, a multiparticle time- and position-sensing detector was used to analyze two- and three-body decaychannels from the DPD of I 2 - and I 2 - ‚ Ar. These data wereobtained at a series of photon energies, indicated in Figure 2,enabling us to map out the dynamics as progressively more I 2 excited states, up to and including the B 3 Π u (0 u + ) state, areaccessed. For the I 2 system, a large increase in photofragmentyield is observed as the photon energy is increased. Thetranslational energy distributions indicate direct dissociationfrom repulsive potential energy surfaces for all but the highestphoton energy. Similar trends are observed with the I 2 ‚ Arsystem. Mass distributions indicate that DPD to I 2 + Ar and I + I + Ar occurs at all photon energies used. The I + I + Archannel grows in accordance with the increased photofragmentyield of I 2 . Translational energy distributions for the three-bodydissociation channel are similar to the I 2 distributions, indicatingdirect dissociation at all but the highest photon energy. Two-body decay events (i.e., I 2 + Ar) show that little energy isdeposited into translation of the photofragments and that thesedistributions change little with increasing photon energy,consistent with a VP mechanism in which the vibrational energyis channeled into the vdW modes until the weak bond breaks. Experimental Procedures The experimental apparatus shown in Figure 3 has beendescribed in detail previously and will only be briefly describedhere. 20,32,33 A vibrationally and rotationally cooled beam of I 2 - or I 2 - ‚ Ar was created by flowing Ar (25 psig) over I 2 crystalsat room temperature and expanding the resulting mixture througha pulsed piezoelectric valve operating at 60 Hz. The beam wassubsequently crossed by a 1 keV electron beam produced froman electron gun. The resulting anions were accelerated to alaboratory beam energy of 6.5 keV and mass selected using aBakker time-of-flight mass spectrometer. 34,35 I 2 - ‚ Ar anionsformed utilizing this type of source have been estimated to havea vibrational temperature of  ∼ 70 K. 31 Anions of the desiredmass were dissociatively photodetached by the output of anexcimer (Lambda Physik LPX 210, 308 nm) pumped dye laser(Lambda Physik Scanmate 2E). The arrival times and positionof the recoiling photofragments were then detected in coinci-dence by a time- and position-sensitive (TPS) detector located2.15 m downstream from the dissociation region.Two methods were used to prevent the undissociated parention beam from reaching the detector: either a beam blockintercepted the undissociated parent beam or a pulsed electricfield ( ∼ 200 V) deflected parent (and daughter) ions out of thepath of the detector. The beam block blocks a greater area of the detector than the deflecting field, discriminating againstdissociation events where little energy is partitioned intotranslation. Results for I 2 were collected using the beam blockto discriminate against contributions from I 2 - that were detachedbut not dissociated. Results presented here for the I 2 ‚ Ar systemwere collected using the pulsed electric field to detect the low-energy I 2 + Ar channel and low-energy I + I + Ar events.While using the pulsed electric field method allows for moreefficient detection of low-energy events than obtained whenusing the beam block, not all low-energy events are detected inthis method, perhaps owing to a “dead spot” in the center of the detector. The dead spot is found to be approximately 6 mmacross and 3 mm high, which is slightly smaller than the 7 mm × 5 mm beam block. It was found that for the I + I + Archannel, a minimum translational energy of 30 - 50 meV isrequired for the Ar to successfully clear the dead spot.The TPS detector, based on the design by Zajfman andAmitay, 36 consists of a standard imaging quality 75 mm Z-stackmicrochannel plate (MCP) assembly coupled to a phosphorscreen (Burle Spec. S9739, Rev. 0). A dichroic beam splitterpositioned at 45 ° to the phosphor screen transmits 50% of thephosphorescence to an image intensifier and CCD camera(Dalsa, CA-D6-D512). The remaining emission is reflected toa 4 × 4 PMT anode array (Hamamatsu H 6568-10) that recordsprecise timing information. The beam splitter preferentiallyreflects bluer light and transmits redder light, in accordance with Figure 1. I 2 anion ground state and low-lying I 2 neutral potential energycurves. The anion ground state appears as the lower solid black line.The 1 Σ g + (0 g + ) ground state of the neutral is the higher-lying black line.The bound A ′ 3 Π u (2 u ) and A 3 Π u (1 u ) states appear as solid red and greenlines, respectively, while the unbound B ′ 3 Π u (0 u - ) and B ′′ 1 Π u (1 u )curves are dark and light blue, respectively. The repulsive 3 Π g 1(2 g )state appears as a solid magenta line, while the close-lying repulsive 3 Π g (1 g ) state is orange. The B 3 Π u (0 u + ) state appears as the dark purplecurve leading to the I + I* products. 12796 J. Phys. Chem. A, Vol. 111, No. 49, 2007  Kautzman et al.  the spectral sensitivities of the camera and PMT. This detectionscheme permits the detection of both anionic and neutralfragments. Data sets taken with the deflecting field were notfound to alter the arrival positions of the photofragments or thecenter of the dissociation events, indicating that all photofrag-ments are neutral. Additionally, both two- and three-bodyphotofragment events can be detected, enabling the investigationof the two types of dissociation on an approximately equalfooting. The coincident time and position information obtainedfor each event were then used to calculate photofragment massdistributions to determine the products of the dissociation.Translational energy distributions (P(  E  T )) were also calculatedfor each product channel. Unlike the triple coincident experi-ments performed by Hanold et al., 23 we do not detect the electronand thus do not measure the correlation between the electronkinetic energy (eKE) distribution and the P(  E  T ) distribution. ResultsI 2 - : Photofragment Yield and Translational EnergyDistributions. To establish a baseline for the role of thesolvating Ar, the dissociative photodetachment of I 2 - wasinvestigated at several photon energies between 4.24 and 4.75eV to determine how the dissociation of I 2 changes withincreasing photon energy. Photofragment yields and translationalenergy distributions are displayed in Figure 4. All products werefound to be neutral. As shown in Figure 4a, the I 2 dissociationyield grew rapidly between 4.37 and 4.50 eV. The individualdata sets have been normalized for laser power, number of lasershots, and ion production. Data sets were taken at the samephoton energies on different days to determine the fluctuationsin dissociation due to day-to-day fluctuations of experimentalparameters. Despite normalization, a difference in the absoluteintensity of dissociation (up to 40%) is still apparent betweendata sets taken with the same photon energies due to fluctuationsin laser power, laser alignment, and ion production. However,the qualitative trend of increasing dissociation with increasingphoton energy far exceeds the fluctuations between data setstaken at the same photon energy.The P(  E  T ) distributions obtained from I 2 - DPD are plottedin Figure 4b. In general, the maximum value of  E  T increaseswith photon energy. Moreover, the distributions are structured,particularly the one at 4.75 eV, which shows three peaks and atail extending to high energy. Figure 2. (a) Potential energy curves relevant to the current study. (b) Closeup of photoelectron spectrum. The black line is the photoelectronspectrum. The gray fill is the fit to the photoelectron spectrum using the curves from panel a. Vertical dashed lines represent the photon energiesused in the current study. Figure 3. Schematic of the fast beam coincident imaging apparatus.Detail of the interaction region showing the pulsed field used to deflectundissociated parent ions from striking the detector is also shown. Figure 4. (a) Photofragment mass distributions for I 2 - DPD. (b)Translational energy distributions for I 2 - DPD. Dissociative Photodetachment Studies of I 2 - ‚ Ar J. Phys. Chem. A, Vol. 111, No. 49, 2007  12797  I 2 - ‚ Ar Mass Distributions and P(  E T ) Distributions. Figure5a,b shows the mass distributions corresponding to channels inwhich two and three fragments, respectively, were detected fromthe DPD of I 2 - ‚ Ar at several photon energies. To facilitatecomparison with the DPD of bare I 2 - , the photon energies inFigure 5 were increased by 24 meV, the difference betweenthe electron affinities of I 2 ‚ Ar and bare I 2 . 31 In Figure 5a, thepeaks corresponding to 40 and 254 amu represent dissociationto Ar + I 2 products. The central peak corresponds to two equalmass fragments, each with a mass of 147 amu. This channel isassigned to dissociation events producing I + I + Ar productswhere the two I fragments are detected but the Ar does notpossess enough translational energy to clear the dead spot inthe center of the detector. Because the third fragment was notdetected, our analysis program assigned the channel to a two-body event producing two particles of equal mass (147 amu).Figure 5a shows that the intensity of the central peak rises withincreasing photon energy, correlating with the total I + I signalseen from bare I 2 - . Figure 5b shows that three-body dissociationevents in which all three fragments are directly observed alsooccur at each photon energy, with a peak at 40 amu and a peakof roughly double the intensity at 127 amu.Figure 6 shows P(  E  T ) distributions for three-body decayevents observed at the three highest photon energies, with theI 2 - DPD at the corresponding photon energy shown as a dashedline for comparison. The fraction of three-body events at 4.26eV was too small to produce a meaningful distribution. Notethat events are counted as three-body events only if the Ar atomhas sufficient recoil energy to clear the dead spot in the middleof the detector, which means, for example, that three-bodyevents that occur in the plane parallel to the detector are favored,as are higher translational energy events. Distributions calculatedfrom the events where only the I fragments were detected (mass147 peak in Figure 5) yield distributions peaked at slightly lowertranslational energies, as shown by the gray line at 4.39 eV inFigure 6. This shift is expected since the contribution to thetotal recoil energy from the Ar atom is neglected. In any case,comparison of the two types of distributions shows that anybias in the true three-body distributions is small and that theAr atom contributes relatively little to E  T . Comparison to theI 2 - DPD shows that at the two lower photon energies, thedistributions match quite well at high E  T , but at the highestenergy, the tail at high E  T in the bare I 2 - distribution issuppressed in the DPD of I 2 - ‚ Ar.Figure 7 shows the translational energy distributions for two-body decay events of I 2 - ‚ Ar. All two-body distributionspresented have been normalized with a detector acceptancefunction (DAF), which accounts for geometric factors thatprevent the collection of all of the dissociation events. Thedetails of the DAF have been described previously. 32 The energypartitioned into photofragment translation for the two-bodydissociation channel is found to peak near zero for all photonenergies. Within the limitations of our data collection andanalysis programs, these distributions are found to be the sameat all photon energies. Discussion In this section, we first analyze the results for the bare I 2 molecule by examining the changes in the photofragment massdistributions as the photon energy increases. The translationalenergy distributions are then utilized to consider the DPDpathways of I 2 - . Using insights gained from the analysis of I 2 - ,the mass distributions and translational energy distributions fromDPD of I 2 - ‚ Ar are then examined. I 2 - Photofragment Yield. By examining the photoelectronspectrum and relativistic potentials provided by de Jong et al.(see Figure 2), it can be determined that at a photon energy of  Figure 5. Photofragment mass distributions for I 2 - ‚ Ar. (a) Photo-fragment mass distributions for two-body events. (b) Photofragmentmass distributions for three-body events. Figure 6. Translational energy distributions for the I + I + Arphotoproducts channel. Translational energy distributions for I 2 - areshown as dashed lines. The 4.39 eV panel also displays the translationalenergy distribution calculated using events where Ar is undetected asa gray line. 12798 J. Phys. Chem. A, Vol. 111, No. 49, 2007  Kautzman et al.  4.24 eV, we are accessing the bound X, A ′ , and A states of I 2 ,with slight contributions from the unbound B ′ and B ′′ states.At higher photon energies (4.37, 4.46, and 4.50 eV), as moreof the repulsive translational states supported by the 3 Π curvesbecome accessible, the dissociation yield rises sharply. Contri-butions from the B state, which is known to undergopredissociation, 37 - 40 are only observed at the highest photonenergy of 4.75 eV. The increase in photofragment yield between4.50 and 4.75 eV is slightly greater than the average intensityfluctuations and is consistent with the occurrence of predisso-ciation; further evidence for this process is presented in I 2 - Translational Energy Distributions. I 2 - ‚ Ar Mass Distributions. The results for I 2 - provide aframework for the photofragment mass distributions from I 2 - ‚ Ardissociative photodetachment. The key result is that thecompetition between two- and three-body DPD mirrors thephotofragment yield from I 2 - . At the lowest photon energy, 4.26eV, I 2 + Ar production appears to dominate, but the contributionfrom three-body decay increases significantly as the photonenergy is raised. The two-body channel presumably results fromVP from Ar complexed to vibrationally excited I 2 in its boundX, A ′ , and A states. As the photon energy is raised to 4.48 eV,three-body dissociation increases, owing to the progressivelygreater contribution from directly repulsive states of I 2 . At thehighest energy, 4.78 eV, where the excited Ar ‚ I 2 (B) complexis accessible, both channels are accessible via VP and EP. Thereappears to be relatively more three-body dissociation at 4.78eV than at 4.48 eV, again mirroring the results for bare I 2 - andpossibly reflecting the contribution from EP. I 2 - Translational Energy Distributions. We now examinethe P(  E  T ) distributions for the dissociative photodetachment of I 2 - . The general trend seen in Figure 4 is that as the photonenergy increases, the P(  E  T ) distributions shift toward highertranslational energy. This trend can arise from several effects.With increasing photon energy, transitions to higher lyingrepulsive states become accessible (see Figure 2a), resulting ina larger translational energy release since all the repulsive statesaccessed in this experiment correlate to the same asymptote.Moreover, for dissociative detachment to a particular repulsiveelectronic state, higher translational energy levels of that stateare accessed with increasing photon energy until the entire FCrange of levels is covered. These points are clarified bysuperimposing the photon energies used onto the fit for the I 2 - photoelectron spectrum, as shown in Figure 2b.Thus, at 4.24 eV, the entire FC profile of the B ′ 3 Π u (0 u - )state and most of the profile of the B ′′ 1 Π u (1 u ) state can bereached. The photon energies 4.37, 4.46, and 4.5 eV additionallyaccess part, nearly all, and all of the translational energy levels,respectively, associated with the close-lying 3 Π g (2 g ) anda 3 Π g (1 g ) states. Hence, the faster distribution at 4.37 eV relativeto 4.24 eV results primarily from accessing higher electronicstates, whereas the progressively faster distributions at 4.46 and4.50 eV as compared to 4.37 eV result from higher-lyingtranslational energy levels associated with the same electronicstates. At 4.50 eV, lower vibrational levels of the B 3 Π u (0 u + )state are also available as shown by the photoelectron spectrum,but this is a bound rather than repulsive state. Finally, at 4.75eV, all translational and vibrational levels of the electronic stateslisted previously are accessible, and it is of interest to comparethe resulting P(  E  T ) distribution directly to the photoelectronspectrum of I 2 - as discussed next.In dissociative photodetachment, the energy available to bepartitioned into translation corresponds towhere E  T is the translational energy of the photofragments, h ν is the photon energy, and E  int and E  int - are the internal energy of the products and parent anion beam, respectively. Since theproduct of I 2 dissociation produces atoms, and the channel toproduce I* is not energetically accessible, E  int is zero. E  int - isalso assumed to be negligible for these studies. D 0 is thedissociation energy of I 2 - , EA is the electron affinity of I, andeKE is the kinetic energy of the departing electron. Insertingthe known values for D 0 , 1.007 eV, 41 and EA(I), 3.059 eV, 42 reduces eq 5 toIn any non-resonant photodetachment process, the photo-electron kinetic energy distribution represents, to first-order, aFC mapping of the initial anion wavefunction onto the energeti-cally accessible neutral electronic states, regardless of whetherthey are bound or repulsive states. For a purely repulsive stateof a diatomic, the photoelectron spectrum is necessarilyunstructured, and if several repulsive states are accessed, thephotoelectron spectrum will be comprised of a series of broadpeaks, each corresponding to a different repulsive neutral state.According to eq 6, there is a one-to-one correspondence betweeneKE and E  T , and the P(  E  T ) distribution should resemble thephotoelectron (PE) spectrum.In Figure 8, the I 2 - PE spectrum 30 has been overlaid withthe P(  E  T ) distribution at 4.75 eV, with the energy scale shiftedaccording to eq 6. While the PE spectrum shows four peaks,the P(  E  T ) distribution shows only three, with a tail extendingto high energy instead of the fourth peak. This observationreflects the fact that the B state is bound, so that eqs 5 and 6 no Figure 7. Translational energy distributions for the I 2 + Ar photo-products channel.  E  T ) h ν +  E  int - -  E  int -  D 0 - EA(I) - eKE (5)  E  T ) h ν - 4.066 - eKE (6) Dissociative Photodetachment Studies of I 2 - ‚ Ar J. Phys. Chem. A, Vol. 111, No. 49, 2007  12799
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