Chemical Physics 399 (2012) 172–179

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Chemical Physics

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Accurate measurement of the T-shaped and linear Ar � � � I2ðX;m00 ¼ 0Þ bindingenergies using vibronic-specific I2(B, m) fragment velocity-map imaging

Jie Wei, Camille Makarem, Ashley L. Reinitz, Joshua P. Darr 1, Richard A. Loomis ⇑Department of Chemistry, Washington University in St. Louis, One Brookings Drive, CB 1134 Saint Louis, MO 63130, United States

a r t i c l e i n f o a b s t r a c t

Article history:Available online 6 July 2011

Keywords:Van der Waals complexesBinding energiesVelocity map imagingVMISFVILaser-induced fluorescenceLIFArI2

Rare gas–dihalogen complexes

0301-0104/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.chemphys.2011.06.039

⇑ Corresponding author. Tel.: +1 314 935 8534; faxE-mail address: loomis@wustl.edu (R.A. Loomis).

1 Present address: Department of Chemistry, Unive6001 Dodge St., DSC 337, Omaha, NE 68182, United St

Ion time-of-flight slow fragment velocity imaging (SFVI) is combined with laser-induced fluorescencespectroscopy measurements to accurately determine the binding energies of the T-shaped and linear con-formers of the ground-state Ar� � �I2(X;m00 ¼ 0) complex. The fluorescence-based measurements were usedto optimize the conditions for preferentially stabilizing either the T-shaped or linear conformers, and toensure proper excitation energetics for the SFVI experiments. In the ion-imaging experiments, the kineticenergy distributions of specific I2(B, m) fragments formed with very low kinetic energies via dissociationof initially prepared Ar� � �I2(B, m0) intermolecular vibrational levels were imaged to measure the bindingenergies of the linear and T-shaped conformers. The linear conformer is energetically preferred, with abinding energy of 250.3(2.7) cm�1, over the T-shaped conformer, with a binding energy of240.5(3.6) cm�1. These values are compared with previously reported data from high-level ab initio cal-culations and experiments.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Since the first spectroscopic observation of the Ar� � �I2 van derWaals complex stabilized in a supersonic expansion [1–3], theAr� � �I2 system has become a model system for experimental andtheoretical investigations aimed at better characterizing long-range interactions and a number of different intermoleculardynamics phenomena [4,5]. In the supersonic expansion, three-body collisions can stabilize the ground-state Ar� � �I2(X;m00 ¼ 0)complex with one of the collision partners, presumably a carriergas atom, departing with the excess kinetic energy required to sta-bilize the binary complex. The geometry of the ground-statecomplex is not only dictated by the long-range intermolecular po-tential, but also by the specific kinetics and thermodynamics with-in the expansion. The complex can be stabilized with a T-shapedgeometry, where the Ar atom is localized in the toroidal potentialthat is perpendicular to the I–I bond. The complex can also be sta-bilized with a linear geometry, where the Ar atom is localized at anend of the I2, along the bond axis. In the excited electronic stateassociated with Ar + I2(B, m0) there is only one minimum in theintermolecular potential, and it is in the T-shaped orientation witha geometry very similar to that of the T-shaped, ground-statecomplex.

ll rights reserved.

: +1 314 935 4481.

rsity of Nebraska at Omaha,ates.

The investigations aimed at characterizing the Ar� � �I2 systemhave made it a textbook case of how experiments can be comple-mented by theory, and then evolving theory can push the exper-iments to new standards in order to achieve accurate depictionsof the underlying physics and interactions. In the nearly 24 yearsover which efforts have focused on the Ar� � �I2 system, a largenumber of experimental groups, including Levy [1–3], Atkinson[6], Zewail [7], Klemperer [8–11], Donovan [12–14], Heaven[15,16], Pravilov [17], Neumark et al. [18], Parker [19], and Loo-mis [19,20], have provided new insights and interpretations intothe Ar + I2 interactions. The list of theory groups who have con-tributed to our understanding of this system is of comparablesize, and include the groups of Thompson et al. [21], Gray [22–26], Halberstadt [4,27–32], Beswick [27,29–31,33], Roncero[4,23,30,32,34,35], Martens [36,37], Heß [38], Buchachenko[4,17,30,32,34,35,39], Miyoshi [40], Naumkin [41], Delgado-Barrio[42,43], and Das [44].

Despite this sustained interest in Ar� � �I2, some of the basic prop-erties of the Ar + I2 interactions are still unresolved. Perhaps mostglaringly, there remain discrepancies in the binding energies ofthe two ground-state Ar� � �I2(X;m00 ¼ 0) conformers, on with a rigidT-shaped geometry and the other with a rigid linear geometry.Experimental values for the binding energies of the T-shaped con-former, DT

0, range from as low as 142 cm�1 [10] to as high as237 cm�1 [45]. The binding energies of the linear conformer, DL

0,span nearly the same range, 172 cm�1 [10] to 250 cm�1 [20]. Whilea number of spectroscopic techniques have been implemented inexperimentally estimating the binding energies within the

J. Wei et al. / Chemical Physics 399 (2012) 172–179 173

ground-state Ar + I2(X;m00 ¼ 0) potential, no single method has beenable to directly measure the binding energies of both Ar� � �I2 con-formers. Unfortunately, comparisons with theory do not provideconcrete support for identifying which of the experimental valuesare realistic, as the binding energies for these conformers calcu-lated using high-level theory have similar uncertainties, rangingfrom 140.4 cm�1 [44] to 242(11) cm�1 [41] for the T-shaped con-former and from 166 cm�1 [34] to 250(8) cm�1 [41] for the linearconformer.

In two-laser, action spectroscopy experiments performed onseveral different rare gas–dihalogen clusters the observation ofbound-free transitions of the linear conformers to the inner repul-sive walls of the excited-state intermolecular potentials have en-abled quite accurate DL

0 values for these systems to be measured[20,46–50]. An energy of DL

0 ¼ 250ð2Þ cm�1 was measured for thelinear Ar� � �I2 conformer in this manner [20]. Additional two-laserspectroscopy measurements performed on He� � �ICl [51] andHe� � �I2 revealed that common intermolecular vibrational levels ly-ing within the He + ICl(E0+ 3P2, v� = 11, 12) and He + I35Cl(b1 3P2, v�

= 0–2) and He + I2(E0+ 3P2, v� = 0, 1) ion-pair states could be ac-cessed. In this manner, the relative energetics of the twoconformers of the ground-state He� � �ICl and He� � �I2 complexescould be determined with high precision; the linearHe� � �ICl(X;m00 ¼ 0) conformer is 5.4 cm�1 more strongly bound thanthe T-shaped He� � �ICl(X;m00 ¼ 0) conformer [51], and the T-shapedHe� � �I2(X;m00 ¼ 0) conformer is just 0.2(1) cm�1 more stronglybound than the linear He� � �I2(X;m00 ¼ 0) conformer [49]. Unfortu-nately, for more strongly bound complexes, such as Ar� � �I2, thedensity of the intermolecular vibrational levels becomes quitehigh, and the identification of common features in the two-laserspectra recorded for the T-shaped and linear conformers is extre-mely difficult.

The results presented herein illustrate that the binding energiesfor different conformers of ground-state complexes, such as Ar� � �I2,can be measured with high accuracy using the technique of iontime-of-flight velocity-map imaging (VMI) [52]. The accuracies inthe measurements approach 1% of the binding energies, especiallywhen measuring fragments with low kinetic energies and whencoupled with fluorescence-based spectral methods. Specifically,the kinetic energy release of I2(B, m) fragments are carefully mea-sured following the dissociation of the photo-excited metastableAr� � �I2(B, m0) complexes. While the methodology shows promisefor other systems, the determination of accurate binding energiesfor the ground-state T-shaped and linear Ar � � � I2ðX;m00 ¼ 0Þ con-formers finally provides targets for theory efforts on this and sim-ilar intermolecular interactions.

2. Experimental

These experiments were undertaken on a newly built time-of-flight, VMI apparatus that has the capability of collecting fluores-cence signals from the sample while ion signals are also being re-corded. This permits laser-induced fluorescence (LIF) and two-laseraction spectra to be recorded under identical conditions used in theimaging experiments. This setup also provides a means for accu-rately tuning the excitation laser to specific transitions, of eitherfree I2 or Ar� � �I2, that will be used in the two-laser, pump–probeVMI measurements.

Ground-state, T-shaped and linear Ar � � � I2ðX;m00 ¼ 0Þ complexesare stabilized in a pulsed supersonic expansion by passing an Ar/He (2.0% Ar and 50 psi backing pressure) carrier gas over room-temperature iodine crystals. A solenoid pulsed valve (0.75 mmdiameter) is used to expand the sample into a source chamber,which is evacuated to �7.5 � 10�6 torr during an experiment usinga diffusion pump (4500 l/s). The supersonic expansion region is

crossed by a laser axis 60 mm downstream from the nozzle, andfluorescence in the intersection region of the laser and expansionis imaged orthogonally onto a photomultiplier tube (PMT) usingan optical telescope. The PMT current is amplified and recordedusing a boxcar, gated integrator. A skimmer, which has a 1 mmdiameter and is mounted 110 mm from the pulsed valve, is usedto collimate the pulsed molecular beam and to provide differentialpumping between the source and ion time-of-flight regions of theapparatus. The molecular beam enters the ionization regionthrough a 2 mm hole in the first repeller electrode of the VMI ionoptics. In general, the VMI ion optics follow the design of Eppinkand Parker [52]. The repeller electrode is located 52 mm down-stream from the skimmer. The laser beam axis for the pump–probeVMI measurements intersects the pulsed molecular beam betweenthe repeller electrode and the first of the two extraction electrodes.The ionization region of the apparatus is evacuated with a turbo-molecular pump (510 l/s) to a pressure of �4.1 � 10�8 torr duringan experiment. The ions are extracted collinearly into a time-of-flight tube evacuated by a second turbomolecular pump (1800 l/s) so that a pressure of �1.1 � 10�8 torr is held during anexperiment.

The photo-generated ions are directed onto a position sensi-tive detector comprised of a chevron type multi-channel platedetector (MCP), which is gated for detection at the ion arrivaltimes, and a P20 phosphor screen. The third VMI lens electrodeand the front face of the MCP detector are grounded. In orderto measure fairly small kinetic energies of the ion fragments,the voltages on the VMI ion optics are kept low, 700 V and488 V, for the repeller and the extractor electrodes, respectively,and a relatively long field-free ion flight path, 1.57 m, is used; de-creased MCP detection efficiency prevents the use of even lowervoltages. In these slow fragment velocity imaging (SFVI) experi-ments, ion fragments with kinetic energies down to 10 cm�1, orless, can be measured. With these low kinetic energies and longion flight times, ion-ion repulsion can alter the flight path ofthe ion fragments and distort the ion image. As a result, ion den-sities are kept low by reducing the pulse energy of the probe, orionizing, laser so that only a few ions are formed on each laserpulse. The ion images are acquired using a charged-coupled de-vice (CCD) camera (1024 � 768 resolution) and summed over atotal of 200,000 laser pulses. The ion images are analyzed postacquisition using the Basex inversion program [53]. Ion-yieldspectra are acquired using a PMT in place of the CCD camera.As in reference [19], images are calibrated with I-atom fragmentsformed from the photodissociation of molecular I2 that yieldsI(2P1/2) + I(2P3/2) and I(2P3/2) atoms. These atom products are ion-ized and detected through resonant 2 + 1 ionization. Calibrationimages are acquired for laser wavelengths between 495 nm and497 nm. The I2(B, m) fragment molecules are ionized via 1 + 1 res-onance with the I2(E) ion-pair state [52].

The pump (or excitation) laser pulses are from a commercialnanosecond Nd:YAG pumped dye laser, operating with pyrro-methene 597 or pyrromethene 580 dye. The pump laser beampath traverses the fluorescence region of the source chamberthrough two Brewster windows and is steered back through theionization region using two mirrors, external to the vacuumchamber. The doubled output of a second nanosecond Nd:YAGpumped dye laser, operating with LDS698 (Pyradine I) dye is usedfor the probe (or ionization) laser pulses. The probe laser beam isfocused by a 50 mm focal length lens and counter-propagatedwith the pump laser beam through the ionization region. Thetriggering of the probe pulse is delayed by 50 ns from the pumppulse, and the pump and probe laser pulse energies are kept be-tween 1–2 mJ and 0.2–0.8 mJ, respectively. Both the pump andprobe lasers are linear polarized and oriented parallel to theimaging detector face.

(a)I2(E,v=55)

I2(E,v=54)

I2(E,v=53)ount

s

174 J. Wei et al. / Chemical Physics 399 (2012) 172–179

3. Results and data analysis

3.1. Ar� � �I2 excitation spectra

The LIF spectra recorded throughout the I2 B–X, m0–0 spectral re-gion when using an Ar/He expansion contain discrete featuresassociated with transitions of the T-shaped He� � �I2 and Ar� � �I2

complexes. As observed in the spectra recorded in the I2 B–X,16–0 and 21–0 regions, Fig. 1, the features associated with transi-tions of the T-shaped conformers are shifted to slightly highertransition energies than the corresponding I2 B–X, m0–0 monomerfeature [1,2,45,49]. Additional features can also be observed tohigher transition energies [49], but these are much weaker andtypically have more complicated contours depending on theamount of I2(B, m0) vibrational excitation. The feature at17991.8 cm�1 in Fig. 1(b) is such a feature, and it is attributed totransitions of the linear conformer to delocalized bending levelswithin the He + I2(B, m0 = 21) potential; lifetime broadening ob-scures the rotational structure within this feature. There are alsosimilar weak features attributed to transitions of Ar� � �I2 [20], butthere is also a non-zero continuous background fluorescence signalin the I2 B–X, m0–0 spectra that obscures these features in LIF spec-tra. The continuum fluorescence signal is attributed to the excita-tion of the linear He� � �I2(X;m00 ¼ 0) and Ar� � �I2(X;m00 ¼ 0)complexes to the inner, repulsive walls of the excited-state inter-molecular potentials at energies above the He + I2(B, m0) andAr + I2(B, m0) asymptotes [8,20]. As a result, the bound-free transi-tions associated with each complex are observed to turn-on atenergies equal to DL

0 for that complex above each I2 B–X, m0–0 bandorigin, and to form a broad continuum extending to even highertransition energies. In these experiments, the intensity of the con-tinuum signal follows predominantly with the percentage of Arused in the carrier gas, and we conclude that most of this contin-uum signal is from bound-free transitions of the Ar� � �I2(X;m00 ¼ 0)complex [20]. The intensity of the continuum signal also increasesrelative to the intensities of the discrete T-shaped Ar� � �I2 featureswith decreasing temperature, consistent with a larger DL

0 over DT0

for the Ar� � �I2(X;m00 ¼ 0) complex [8,10,20]. Note that the oppositetrend was observed for He� � �I2 with the intensity of the He� � �I2

Fig. 1. LIF spectra recorded in the I2 B–X, 16–0, (a), and I2 B–X, 21–0, (b), spectralregions. The arrows show the excitation wavenumbers used to excite the T-shapedand linear Ar� � �I2(X;m00 ¼ 0) conformers in the VMI measurements.

continuum signal decreasing relative to that of the T-shapedHe� � �I2 features with decreasing temperature in the expansion[49]. This further suggests that most of the continuum signal canbe attributed to transitions of the linear Ar� � �I2 complex.

3.2. Resonant 1+1 ionization of I2(B, m) fragments

Because of difficulties reported by Zhang et al. [19] encounteredwhen using resonant 1 + 1 ionization via the I2(E) and I2(F) states toprobe I2(B, m) fragments formed from the dissociation of excited-state Ar� � �I2(B, m0), we spent considerable time identifying isolatedoptical transitions that do not have overlap with other I2 transi-tions and calibrating the energies of the ionization lasers. Byaccessing resonant levels lying to lower energy than utilized previ-ously [19], the problems associated with spectral congestion canbe partially avoided and higher selectivity in ionizing the productchannels can be achieved. The ion-yield spectra obtained withthe excitation laser fixed on the I2 B–X, 13–0 band head and scan-ning the ionization laser through the I2 E–B region indicate high1 + 1 ionization selectivity, as shown in Fig. 2(a). Only three reso-nances are observed in this spectrum, and they are assigned asthe I2 E–B, m–13 transitions with m = 53–55 based on publishedenergies [54]. The relative strengths of the ion signals associatedwith the different resonances are dictated by the Franck–Condonfactors of the I2 E–B, m–13 transitions. Based on the I2 ionization po-tential [55], only I2 E–B, m–13 transitions with m P 52 are energet-ically open for the resonant 1 + 1 ionization detection ofI2(B, m = 13).

An expansion of the energy region about the I2 E–B, 54–13 fea-ture in the 1 + 1 ion-yield spectrum, shown in Fig. 2(b) bottom(blue) spectrum, indicates there is little rotational excitation, as ex-pected, since the energy of the excitation laser was fixed on therotational band head of the I2 B–X, 13–0 transition, thereby access-ing low rotor states. The ion-yield spectrum obtained by scanning

29080 29120 29160 29200 29240

Ion

C

Excitation to I2(B,v′=13)

29140 29150 29160

(b)

T-shaped Ar···I2

Linear Ar·· ·I2

Ion

Coun

ts

Probe Wavenumber (cm–1)

I2(B,v′=13)

Fig. 2. (a) Resonant 1 + 1 ion-yield spectrum obtained by scanning the probe laserthrough the I2 E–B, 54–13 region with the excitation laser fixed on the I2 B–X, 13–0band head at 17213.2 cm�1. (b) Resonant 1 + 1 ion-yield spectra obtained byscanning the probe laser through the I2 E–B, 54–13 region. The I2(B, m = 13) ion-yieldspectrum (bottom) is that shown in (a). The T-shaped Ar� � �I2 ion-yield spectrum(top) was obtained with the excitation laser fixed on the band head of the T-shapedtransition at 17527.9 cm�1 in the I2 B–X, 16–0 region. The linear Ar� � �I2 ion-yieldspectrum (middle) was obtained with the excitation laser fixed on the linear Ar� � �I2

bound-free transition at 17530.4 cm�1, which is just 2.5 cm�1 to higher energy thanthe T-shaped band accessed. The spectra are offset for clarity, and the intensity ofthe molecular I2(B, m = 13) spectrum has been reduced for comparison.

Fig. 3. Raw VMI images of I2(B, m = 13) fragments obtained with excitation at: (a)17527.9 cm�1, which is the band head of the T-shaped Ar� � �I2 transition in the I2 B–X, 16–0 region; (b) 17530.4 cm�1, which accesses the continuum associated withbound-free transitions of the linear conformer to just higher transition energy thanthe 16–0 T-shaped transition. VMI images of I2(B, m = 18) fragments obtained withexcitation at: (c) 17995.8 cm�1, which is the band head of the T-shaped Ar� � �I2

transition in the I2 B–X, 21–0 region; (d) 17998.3 cm�1, which accesses thecontinuum associated with bound-free transitions of the linear conformer to justhigher transition energy than the 21–0 T-shaped transition.

J. Wei et al. / Chemical Physics 399 (2012) 172–179 175

across the same probe spectral region, but with the excitation laserfixed on the band head of the T-shaped Ar� � �I2 transition in the I2

B–X, 16–0 region, is shown in Fig. 2(b) as the top (black) spectrumlabeled T-shaped Ar� � �I2. The I2(B, m = 13) fragments detected in theT-shaped Ar� � �I2 ion-yield spectrum are formed via vibrational pre-dissociation, either directly to the product states or through intra-molecular vibrational energy redistribution, of the metastable T-shaped Ar� � �I2(B, m = 16) level initially prepared [50,56]. As willbe mentioned below, some I2(B, m = 13) fragments can also be gen-erated by bound-free transitions of the linear conformer. The mid-dle (red) spectrum in Fig. 2(b), labeled Linear Ar� � �I2, was obtainedwith the excitation laser fixed to just higher energy, +2.5 cm�1,than the T-shaped Ar� � �I2 transition in the I2 B–X, 16–0 region. Thisexcitation accesses the continuum of states lying above theAr + I2(B, m0) dissociation limits with m0 6 13. The broadening ofthe main peak and the additional signals observed to lower probewavenumbers in the T-shaped and Linear Ar� � �I2 ion-yield spectraindicate there is some rotational excitation beyond that observedwhen preparing I2(B, m0 = 13). Burroughs and Heaven [16] reportedbimodal I2(B, m, j) product rotational distributions for the vibra-tional predissication of most of the T-shaped Ar� � �I2(B, m0) intermo-lecular vibrational levels. In those two-color, fluorescence-basedexperiments, they observed a cold Boltzmann distribution and asecondary distribution consisting of higher rotational states. Thedistributions observed here are similar, although our energy reso-lution is not sufficient to observe the individual peaks associatedwith the higher rotor states.

3.3. Imaging of I2(B, m) fragments

VMI experiments, similar to those undertaken recently by theParker group [19], were performed to characterize the kinetic energyrelease (KER) and angular anisotropy of the I2(B, m) product frag-ments. As stated, a distinct difference in these experiments is theability to precisely excite on specific transitions using complemen-tary fluorescence and ion detection. For the data reported here,two different probe wavenumbers were used to detect eitherI2(B, m = 13) or I2(B, m = 18) products. The probe laser was fixed at29159.0 cm�1 to detect rotationally cold I2(B, m = 13) fragments viathe 1 + 1 resonantly enhanced ionization through the I2(E, m = 54) le-vel. In order to detect I2(B, m = 18) fragments, the probe laser wasfixed at 28745.0 cm�1, thereby accessing the I2(E, m = 55) level inthe 1 + 1 ionization scheme. Ion-yield spectra, similar to the oneshown in Fig. 2(a), were recorded to identify the probe transitionsand to verify that the I2 E–B, 55–18 resonant transition was not over-lapped by other ion-producing transitions. These spectral regionsand product channels were selected largely because of the lack ofspectral congestion near the T-shaped features in the I2 B–X, 16–0and 21–0 regions. In addition, the excited-state T-shapedAr� � �I2(B, m0 = 16, 21) intermolecular vibrational levels have a rela-tively high efficiency for vibrational predissociation into theDm = �3 channel forming I2(B, m = 13, 18) fragments in comparisonto the propensity for electronic predissociation [9].

The raw I2(B, m = 13) and I2(B, m = 18) VMI images acquired withthe excitation laser fixed on the band heads of the T-shaped fea-tures in the I2 B–X, 16–0 and 21–0 region at 17527.9 and17995.8 cm�1 are shown in Fig. 3(a) and (c), respectively. The high-est ion counts are along the laser polarization axis, vertical, indicat-ing a positive anisotropy parameter for formation of the I2(B, m)fragments. For a prompt dissociation of the excited-state T-shapedlevels, a perpendicular angular distribution, or a negative anisot-ropy parameter, is expected since the transition dipole momentwithin the complex is nearly along the I–I bond axis while therepulsive intermolecular forces are on average perpendicular tothe I–I bond. The vibrational predissociation is not prompt, how-ever, with the complexes having excited-state lifetimes that are

comparable to the rotational periods of the complexes [10,57]. Asa result of the long lifetimes, an isotropic angular distribution ofthe fragments would be expected [58].

Since there are continuum signals underlying these I2 B–X, m0–0spectral regions, VMI data were also acquired with the excitationlaser fixed to just higher wavenumber, +2.5 cm�1, than the bandheads of the T-shaped features in order to identify the contributionfrom the direct dissociation of the linear conformer in the T-shapedimages. The images obtained with excitation to the continuum inboth spectral regions, Fig. 3(b) and (d), have positive anisotropyparameters, which is consistent with a prompt, direct dissociationof the linear conformer [19]. The intensities of the ion images forthe continuum excitations are significantly weaker than those forthe T-shaped excitations. These intensity differences are expectedsince the T-shaped features are much more intense than the con-tinuum signals in the LIF spectra. In addition, the excited-state T-shaped intermolecular vibrational levels preferentially formI2(B, m) products in the Dm = m � m0 = �3 channel, which are beingdetected. In contrast, the weak continuum signals in the LIF spectrahave contributions from I2(B, m) products in many different vibra-tional levels energetically below the Dm = �3 level. After subtract-ing the contributions from the direct dissociation of the linearconformers, albeit from +2.5 cm�1 to higher transition energy, theVMI images obtained for the vibrational predissociation of the T-shaped Ar� � �I2(B, m0 = 16, 21) intermolecular vibrational levels,Figs. 3(a) and (c), are nearly isotropic. Continued investigationsaimed at characterizing anisotropies of the I2(B, m) product frag-ments are ongoing.

The VMI images in Fig. 3 provide information about the KER ofthe Ar and I2(B, m) product fragments. In these experiments, theexcitation scheme follows:

Fig. 4. Binding energy distributions obtained in SFVI measurements. (a) Ionizationof I2(B, m = 13) fragments detected with excitation in the I2 B–X, 16–0 spectralregion on the T-shaped Ar� � �I2 band head and on the linear Ar� � �I2 continuum to justhigher energy. (b) Ionization of I2(B, m = 18) fragments detected with excitation inthe I2 B–X, 21–0 spectral region on the T-shaped Ar� � �I2 band head and on the linearAr� � �I2 continuum to just higher energy. The ion counts for the linear signals werenormalized in order to facilitate comparison of the energetics. Since the T-shapedtransitions are overlapped by the linear continuum signals, the linear signal wassubtracted from the T-shaped signal to obtain an adjusted T-shaped signal.

176 J. Wei et al. / Chemical Physics 399 (2012) 172–179

Ar � � � I2ðX;m00 ¼ 0Þ!hvAr � � � I2ðB; m0Þ ! Arþ I2ðB; m; jÞ þ KER: ð1Þ

There is an observed propensity for forming I2(B, m, j) that mini-mizes the KER of the dissociation fragments, and typically theDm = �3 level is the dominant product channel [16,45]. The KERcan be used to determine the binding energies, DL

0 and DT0, of the

ground-state Ar� � �I2(X;m00 ¼ 0) complex. The KER is:

KER ¼ EðAr � � � I2ðB; m0ÞÞ � EðI2ðB; m; jÞÞ � D0; ð2Þ

where, EðAr � � � I2ðB; mÞ ¼ hm for excitation from the jet-cooled vibra-tional ground levels. The rotational excitation was not explicitly in-cluded in this term as the rotational temperature in the expansionwas measured to be �1 K. Furthermore, the rotational energy inEðI2ðB; m; jÞ is neglected since rotationally cold products are probedwhen fixing the ionization laser on the band heads in the I2 E–B,54–13 and 55–18 transitions. Following the conservation ofmomentum, the kinetic energy of the I2(B, m) fragment, KE(I2) isjust:

KEðI2Þ ¼ MAr=Mcomplex � KER: ð3Þ

The ion images can then be used to measure KE(I2) since the radiusof the image is proportional to the speed of the photo-fragment.

Qualitatively, the images in Figs. 3(a) and (b) are larger thanthose in (c) and (d) because of higher available energies in (a)and (b) that result from slight differences in the binding energiesof the complexes within the Ar + I2(B, m0 = 16, 21) potentials andthe anharmonicity of the I2(B) potential. Comparing the size ofthe images acquired in each spectral region, Figs. 3(a) and (c) ver-sus (b) and (d), the images resulting from vibrational predissocia-tion of the T-shaped excited-state levels are larger than thosefrom the direct dissociation of the linear complexes. Consideringthat the excitation energy for the linear conformer is 2.5 cm�1

higher than for the T-shaped, which would result in larger imagesin (b) and (d) if the binding energies of the conformers were thesame, the images indicate that DL

0 > DT0.

Kinetic energy distributions of the I2(B, m) fragments were de-rived from the ion images by reverse transformation of the two-dimensional velocity projections to three-dimensional distribu-tions using the BASEX program [53]. The slice through midpointof this three-dimensional distribution is then integrated aboutthe angular coordinate to obtain the kinetic energy distributionof the I2(B, m) fragments, KE(I2). The energetic resolution in KE(I2)achieved in these SFVI measurements is estimated to be 2.2 cm�1

based on a fit of the intense, low-energy peak in the distributionto a Gaussian distribution. This high kinetic energy resolution isachievable by imaging fragments with very low kinetic energies.Experimentally, SFVI utilizes very low voltages in the ion opticsand a fairly long ion flight path, 1.57 m. A similar strategy forobtaining high-resolution images was implemented by the Neu-mark group in their technique of slow electron velocity imaging(SEVI) [59]. In the SFVI and SEVI imaging approaches, the breadthof a kinetic energy distribution, DKE, is:

DKE ¼ 2ffiffiffiffiffiffiffiffiffiffiffiffi

A � KEp

� Dr; ð4Þ

where A is a constant of the experimental setup dictated largely bythe ion optics and length of the ion flight path. The image resolutionin terms of pixels, Dr, is largely determined by focusing conditionsand is approximately the same for different amounts of fragment ki-netic energy, KE. As a result, a higher fragment kinetic energy reso-lution, i.e., a smaller DKE, can be achieved when the ion fragmentsare imaged with lower amounts of KE. The binding energies andKE(I2) are related to each other through Eqs. (2) and (3).

The KE(I2) distributions obtained when exciting the T-shapedand linear Ar� � �I2(X;m00 ¼ 0) conformers in the I2 B–X, 16–0 and21–0 spectral regions are plotted in Fig. 4(a) and (b), respectively.The binding energies of the ground-state conformers are plotted on

the bottom axis, and the KE(I2) of the ion fragments are on the topaxis. As the excitation energies are different for the T-shaped andlinear continuum excitation, the I2(B, m) fragment kinetic energiesobtained from the continuum excitations have been shifted to low-er kinetic energy by the amount 2.5 cm�1 MAr/Mcom-

plex = 0.34 cm�1, which is virtually negligible in Figs. 3 and 4.Note that the binding energy scale is larger than the I2(B, m) frag-ment kinetic energy scale because of momentum conservation.Lastly, the ion count distributions for the linear conformers are sig-nificantly weaker than for the T-shaped distribution, but have beennormalized in Fig. 4 to emphasize the differences in the energeticsof the distributions.

Since the excitations at +2.5 cm�1 to higher energy than theband heads of each of the T-shaped features only access transitionsof the linear conformer, the ‘‘Linear’’ distributions, red distribu-tions in Fig. 4, reveal the binding energies for the linearAr� � �I2(X;m00 ¼ 0) conformer. Excitation on the band heads of theT-shaped features, however, also includes signals from the overlap-ping linear continuum transitions. Consequently, the binding en-ergy distributions of the ‘‘pure’’ T-shaped conformer, blackdistributions, were determined by subtracting the unnormalizeddistributions of the linear continuum, red distribution, from thoseof the T-shaped band head, gray distributions, in each spectral re-gion. Both the ‘‘Linear’’ and ‘‘adjusted T-shaped’’ distributions havea dominant ion signal at high binding energy values and much

J. Wei et al. / Chemical Physics 399 (2012) 172–179 177

weaker tails that extend to lower binding energies. The ion signalsin the tail regions of the distributions most likely result from colli-sions of the ions with either carrier gas atoms or interactions withother ions in the molecular beam since its contribution decreaseswith lower expansion pressures and iodine concentrations. Asmentioned, the main peaks in the I2(B, m) fragment distributions,shown in Fig. 4(a) and (b), were fit to Gaussian profiles. The centersof the fitted Gaussians obtained for the T-shaped and linear ion dis-tributions are taken to be the binding energies of the correspond-ing ground-state conformers. The binding energies measured forexcitation in the 16–0 and 21–0 spectral regions are the samewithin error, and the averages of the two measurements yieldDL

0 ¼ 250:3ð2:7Þ cm�1 and DT0 ¼ 240:5ð3:6Þ cm�1. The uncertainty

represents three times the standard deviation of the fit and in-cludes the reliability in calibrating the images.

4. Discussion and conclusion

A wide range of experimental and theoretical methods havebeen used to find the binding energies of the T-shaped and linearconformers of the Ar� � �I2(X;m00 ¼ 0) complex. Some of the reportedvalues, as well as those obtained from these VMI measurements,are included in Table 1. A review published in 2003 [4] contains amore complete summary of the theoretical efforts on the Ar� � �I2

system up to that time. As mentioned in the introduction, thetheoretical values (upper five) vary significantly and there doesnot appear to be a clear trend in the values converging to asymp-totic energies with time. The experimental values (lower six) alsovary, but most of the experimental methods only measure thebinding energy of one of the conformers, not both at the sametime.

By monitoring the closing of the Dm = �3 vibrational predissoci-ation channel that occurs because of the anharmonicity of the I2(B)potential, DT

0 was estimated to be 237(3) cm�1 by Blazy et al. [45].Burroughs and Heaven [16] measured the rotational product-statedistributions of the I2(B, m, j) vibrational predissociation fragments,and suggested DT

0 ¼ 234 cm�1, in support of the value reported byBlazy et al. [45]. In other experiments, however, Stevens Milleret al. [10] reported a significantly different value for DT

0. They mea-sured the vibrational distributions of the photofragment productsformed via direct dissociation of the linear conformer and deter-mined DL

0 ¼ 172ð4Þ cm�1. Using this DL0 value and the assumption

that the populations of the two conformers are in thermodynamicequilibrium in the supersonic expansion, they estimated DT

0 to be142(15) cm�1. This value for DT

0 is significantly smaller than theother values, but was supported by ab initio calculations [38],which found DT

0 and DL0 to be 160 cm and 173 cm�1, respectively.

Stevens Miller, et al. [10] proposed that the discrepancy in the DT0

Table 1Binding energies, in cm�1, of the T-shaped and linear conformers of the ground-stateAr� � �I2(X;m00 ¼ 0) complex, DT

0 and DL0.

DT0 DL

0Method Year [Reference]

160 173 ab initio MP4 1998/9 [10,38]209 166 DIM PT1 2000 [34]242(11) 250(8) Empirically modified 2001 [41]212.0 237.8 CCSDT(T) 2002 [42]140.4 182.7 CCSD(T)/BS2 2011 [44]237(3) – Anharmonicity 1980 [45]142(15) 172(3.5) Vibrational distribution 1999 [10]234 – Rotational distribution 2001 [16]– 250(2) Action spectroscopy 2005 [20]– 207(18) VMI 2009 [19]240.5(3.6)a 250.3(2.7)a VMI w/LIF This work

a Values are the averages of two independent measurements, as described in thetext. The uncertainty is three times standard deviation.

values could be a result of electronic predissociation,Ar� � �I2(B, m0) ? Ar + I + I, that yields I-atoms that do not fluoresce.The electronic quenching could efficiently compete with vibra-tional predissociation forming the highest energetically openvibrational product channel. This would lead to a falsely low esti-mate of DT

0. Furthermore, Burroughs and Heaven [16] suggestedthat the assumption of thermal equilibrium in the jet expansionby Stevens Miller et al. [10] may not be valid and that assumptionled to erroneous values of DT

0 based on the measured DL0 value. Re-

cent work on He� � �ICl [51,60] has shown that while the popula-tions of the two conformers can be tuned by varying thetemperature in the expansion, the populations are not in equilib-rium and slightly incorrect binding energies for the conformerswould be estimated.

In 2005, Darr et al. [20] reported a much larger value of DL0,

250(2) cm�1, than reported by Stevens Miller et al. [10]. Darret al. [20] performed action spectroscopy experiments where theydirectly probed the formation of specific I2(B, m0) vibrational levelswhile scanning through and above the region of the Ar + I2(B, m0)intermolecular potential. By identifying the turn-on of the contin-uum of states lying above the asymptote, the binding energy ofthe linear conformer was directly measured. Most recently, DL

0

was estimated in photofragment VMI experiments by Zhanget al. [19]. While focus of that work was on characterizing thedissociation dynamics of the linear conformer energetically highon the repulsive intermolecular potential and beyond the dissoci-ation limit of the I2(B) state, they also used the KER of the I2(B, m)photofragments to estimate DL

0 as 207(18) cm�1. Nevertheless,their VMI experiments form the foundation for the work pre-sented here.

Fig. 5. Schematic diagram of the potential energy curves for the T-shaped (solid)and linear (dotted) geometries. In the SFVI experiments, the T-shaped conformer isexcited at hmT-shaped, and the excited-state level within the Ar + I2(B, m0 = 16)potential undergoes vibrational predissociation forming Ar + I2(B, m0 = 13) fragmentswith a measurable kinetic energy release (KER). The linear conformer is excited athmLinear, and is promoted to the inner, repulsive wall associated with theAr + I2(B, m0 = 13) intermolecular potential. The excited-state linear complex under-goes direct dissociation forming the separate Ar + I2(B, m0 = 13) fragments with ameasurable KER. The amounts of the KER for the two conformers is differentbecause of dissimilar ground-state binding energies and excitation wavenumbers.

178 J. Wei et al. / Chemical Physics 399 (2012) 172–179

While the dissociation mechanisms for the T-shaped and linearconformers are different, the VMI experiments are able to accu-rately measure the binding energies of both conformers. A sche-matic of the ground- and excited-state potentials are shown inFig. 5. In our experiments, the T-shaped conformer was excitedat hmT-shaped to the lowest intermolecular vibrational level withinthe Ar + I2(B, m0 = 16 or 21) potential energy surface. The excited-state complex undergoes vibrational predissociation preferentiallyforming the Ar + I2(B, m = 13 or 18) separate fragments with littlerotational excitation and a well-defined KER that is partitioned be-tween the fragments by their relative masses. The ionization laseris tuned to only detect the lowest rotor states of the I2(B, m = 13 or18) fragments. The KE(I2) distribution and the conservation of en-ergy is then used to determine DT

0. The linear conformers are exitedat hmLinear and are promoted to the continuum of states lying abovethe dissociation limit for the Ar + I2(B, m0 = 13 or 18) potential eventhough the excitation energy is in close proximity to the monomerI2 and T-shaped Ar� � �I2 bands in the B–X, 16–0 and 21–0 regions.The excited-state complexes promptly dissociation into separatefragments, again with a well-defined KER. Similar selectivity inthe ionization step and analysis yields an accurate measurementof DL

0.The value of DT

0 for Ar � � � I2ðX; m00 ¼ 0Þ obtained in our SFVIexperiments is in excellent agreement with the original experi-mental value obtained using the potential anharmonicity [45]and the more recent value obtained using the product-state rota-tional distribution [16]. The DL

0 for Ar � � � I2ðX;m00 ¼ 0Þ agreesremarkably well with the action spectroscopy measurements, alsoperformed in our laboratory [20]. Both DT

0 and DL0 determined in

our SFVI measurements also agree remarkably well with the mod-ified ab initio calculations reported in 2001 [41,42]. In that work,Naumkin [41] performed ab initio calculations using CCSD-T withan extended basis set, and incorporating relativistic effective corepotentials for the inner shells of the iodine atoms. They then inter-polated the ab initio points using empirical Ar + I potentials to ob-tain the potential energy surface. Using this modified potential, thebinding energies were calculated to be DT

0 ¼ 242 cm�1 andDL

0 ¼ 250 cm�1.The binding energy of the Ar� � �I2 complex within the excited

state, which also has a rigid T-shaped geometry, can be reportedwith high accuracy using the ground-state binding energy andthe spectral shift of the T-shaped features observed in LIF spectra.Using DT

0 ¼ 240:5 cm�1 and a spectral shift of +13.3 cm�1 [45], abinding energy of 227.2 cm�1 is obtained for the Ar + I2(B, m0 = 21)intermolecular potential. There is a dependence of the excited-state binding energy on I2 vibrational excitation, m0, with lowerintermolecular potentials having slightly larger binding energies;the binding energy for the Ar + I2(B, m0 = 18) intermolecular poten-tial is 227.6 cm�1 [45]. These values are just outside the range re-ported by Blazy et al., 220–226 cm�1 [45], and a bit larger thanobtained by Burroughts et al. <220 cm�1, using rotational prod-uct-state distributions [16].

There are several strengths of our VMI measurements that sup-port the accuracy the binding energies of the Ar� � �I2 conformers re-ported. First, this approach enables the binding energies of bothconformers to be measured under the same experimental condi-tions with few assumptions required. The use of fluorescence mea-surements and narrow bandwidth lasers enables the expansionconditions to be optimized and the spectral regions to be carefullycharacterized so that there is a high confidence in the excitationand ionizing transitions being utilized. By creating and detectingthe I2(B, m) fragments with low kinetic energies, the SFVI measure-ments achieve high kinetic energy resolution. As a result, the bind-ing energies reported here should provide accurate targets forfuture theory efforts, which may subsequently be used for calculat-ing other dynamical processes.

Acknowledgements

This work was supported by the National Science Foundationthrough a CAREER Award, CHE-0346745. The authors would liketo thank S.W. North and M.P. Grubb for providing their VMI acqui-sition software.

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