High-resolution electron ionization study of CO, (CO)2 and (CO)3: appearance energies and bond dissociation energies

Chemical Physics 239 Ž1998. 409–416

High-resolution electron ionization study of CO, žCO /2 and žCO /3: appearance energies and bond dissociation energies D. Muigg, G. Denifl, A. Stamatovic 1, O. Echt 2 , T.D. Mark ¨

)

Institut fur ¨ Ionenphysik, UniÕersitat ¨ Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria Received 18 May 1998

Abstract Electron ionization cross-sections for CO, CO dimers and trimers have been investigated near the threshold with a newly constructed crossed beams apparatus using a hemispherical electron monochromator ŽHEM. to monochromatize the primary electron beam. The dissociative attachment cross-section curve for CO used to test the new set-up shows, in accordance with the high-resolution study of Stamatovic and Schulz, a vertical onset at the thermochemical threshold Ž9.63 eV. of the lowest possible DA channel OyŽ2 P. q CŽ3 P. in excellent agreement with the value of 9.63 eV derived from known thermochemical data, thereby indicating an energy scale accuracy of better than 10 meV. The appearance energies ŽAE. of some rare gases ŽAr, Kr, Xe. and molecules ŽN2 , O 2 , N2 O. measured for calibration purposes agreed with the known ionization energies ŽIE. of these compounds within 10 meV. Using a novel data handling procedure, involving a simultaneous non-linear weighted least-squares fit of two functions, the following appearance energies were obtained from the measured ionization . ŽŽ .q. cross-section curves using the AEŽCOq. as reference: AEŽŽCO.q 2 s 13.19 " 0.10 eV and AE CO 3 s 12.98 " 0.34 eV. These values are in fair agreement with ionization energies values obtained in a high-resolution photoionization experiment by Ng and co-workers yielding 13.05 " 0.04 and 12.91 " 0.04 eV, respectively. q 1998 Published by Elsevier Science B.V. All rights reserved.

1. Introduction Electron impact ionization is an important tool in the study of molecules and clusters, in particular concerning the production and identification of the corresponding ions in mass spectrometry and related studies Ždetermination of cross-sections, fragmentation patterns, reactivity. w1–3x. Details of the elec)

Corresponding author. E-mail: [email protected] Permanent address: Faculty of Physics Beograd, P.O. Box 368, 11001 Beograd, Yugoslavia. 2 Permanent address: Department of Physics, University of New Hampshire, Durham, NH 03824-3568, USA. 1

tronic and vibrational structure have, however, usually been deduced using photoionization due to the much better energy resolution available. This is particular true for the determination of appearance energies ŽAE. and related data on the energetics of positive ions w4x. Electron ionization techniques have only been used in those cases where photoionization studies appeared to be not feasible, for instance for large organic molecules and high-temperature species effusing from Knudsen cells w4x. Nevertheless, even in these cases photoionization has made major contributions with the advent of more powerful photon sources such as lasers and synchrotron radiation. A

0301-0104r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 8 . 0 0 2 9 8 - 5

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D. Muigg et al.r Chemical Physics 239 (1998) 409–416

similar situation exists in the case of cluster targets, i.e. there exist several studies from the early 1980s about appearance energies as a function of cluster size using electron impact techniques with the data obtained being accurate to roughly 0.5 eV Žfor instance w3,5x., but these studies have been superseded in the late 1980s and in the early 1990s by more accurate investigations using photoionization techniques w6x employing discharge light sources w7,8x, lasers w9,10x and synchrotron radiation w11,12x. According to Rosenstock w4x resolution of monochromators used for continuous 3 photoionization studies range from 0.04 to 0.3 nm and the wavelength scale is known typically to about 0.02 nm from calibration with known emission lines. This represents a ‘best energy resolution’ of about 5 meV at 12 eV w4x Žsee also the instrumental resolution of 1–10 meV mentioned by Ng w8x.. 4 Nevertheless, most photoionization studies have been carried out in practice with instrumentation and under experimental conditions of somewhat lower resolution. This is true in particular for the case of cluster targets. For instance, the recent synchrotron studies 5 of Hertel and co-workers of ŽN2 O. n w11x, Arn and Krn w15x and fullerenes w16,17x and by Ding and co-workers of rare-gas clusters w12x have been carried out with resolutions of about 0.2–0.4 nm corresponding to about 20–40 meV depending on the ionization energy considered. Other factors which determine the ultimate accuracy of the measured appearance energies are Ži. rotational and low-frequency vibrational excitations of polyatomic targets and Žii. problems associated with the determination of the true threshold. For instance at room temperature the FWHM of the

3

According to Ng w8x true continuous light sources are mandatory for very high-resolution studies of photoionization processes. 4 With the advent of sophisticated laser techniques, for instance ZEKE Žzero kinetic energy photoelectron. spectroscopy and related techniques such as MATI Žmass analyzed threshold ionization., energy resolutions have been pushed to even lower limits also for small cluster targets w13,14x. 5 In the meantime, third-generation synchrotron radiation machines which yield higher VUV fluxes at comparable optical bandwidth or substantially higher resolution at still useful fluxes have become available and should allow more accurate determinations than reported so far.

rotational envelope is approximately 25 meV, while that of polyatomic molecules may be as much as 40 meV w8x. These excitations introduce tailing structure and may shift the AEs to lower energies. For gases with low boiling points, these hot band effects can be suppressed by cooling the sample gas to lower temperatures; an ideal way to do so is to use the supersonic molecular beam method Žfor details see Ref. w18x.. These molecular-beam photoionization experiments are, however, subject to more severe sensitivity problems than are encountered in conventional gas-cell studies w8x due to the low number density of the target molecules. This and the low intensity of available VUV photons Žin the case of synchrotron radiation proper blocking of the higherorder transmissions of the monochromator has also to be taken into account. make photoionization studies of van der Waals clusters rather difficult. In particular, the determination of the threshold is hampered by the low signal intensity and large scatter of the data in this regime. 6 Finally, as the exact shape of the ionization efficiency curve close to threshold is not known, extrapolation to ‘zero ion current’ in order to determine the AE is rather difficult and ambiguous as discussed in detail by Kamke et al. w15x. All these effects limit the accuracy of AEs of clusters measured by photoionization to about "30– 50 meV. 7 As we have recently been able to construct electron monochromators which allow to produce electron beams Žwith appreciable currents in the order of nA allowing to produce plenty of ions also close to the ionization threshold. with energy resolutions as low as 5 meV when used for electron attachment

6 This problem is particularly virulent in the case of coincidence measurements such as photoion–photoelectron coincidence techniques w12,15x. In addition background counts produced by random coincidences pose another problem. 7 As mentioned already, ZEKE spectroscopy allows much higher resolutions down to a few wavenumbers; nevertheless, for certain dimers reported ionization energies are inconsistent with previous ordinary photoionization studies. In the case of the Xe dimer and the phenol dimer this discrepancy was attributed to the poor Franck–Condon factors in the case of ZEKE w13x, thus showing the difficulties of determining the adiabatic ionization energies when the spectrum is poorly resolved and the onset is weak.

D. Muigg et al.r Chemical Physics 239 (1998) 409–416

studies w19–21x, we felt that electron impact ionization studies may be an interesting alternative in determining accurate appearance energies for positively charged cluster ions. This study is the first attempt Žsee also one earlier step in this direction w22x. to apply this new generation of electron guns to the collection of quantitative data concerning the energetics of positive ions. We have investigated the electron ionization cross-sections for CO, CO dimers and trimers near the threshold with a newly constructed crossed beams apparatus using a hemispherical electron monochromator ŽHEM. to monochromatize the primary electron beam w21x. The dissociative attachment cross-section curve for CO used to test the new set-up shows, in accordance with the earlier high-resolution study of Stamatovic and Schulz w23x, a vertical onset at 9.63 eV right at the thermochemical threshold of 9.63 eV of the lowest possible DA channel OyŽ2 P. q CŽ3 P. thus indicating an accuracy of the energy scale of better than 10 meV. The appearance energies of some rare gases ŽAr, Kr, Xe. and molecules ŽN2 , O 2 , N2 O. measured for calibration purposes agree with the known ionization energies ŽIE. of these compounds within 10 meV. Using a novel data handling procedure, involving a simultaneous non-linear weighted least-squares fit of two functions w24x, the following appearance energies were obtained from the measured ionization cross-section curves using AEŽCOq. as reference: . ŽŽ .q. AEŽŽCO.q 2 s 13.194 " 0.10 eV and AE CO 3 s 12.98 " 0.34 eV. Using these values, the known IE of CO, and the estimated binding energies for ŽCO. n , the bond dissociation energies for COq–CO and ŽCO.q 2 –CO were deduced to be 0.83 and 0.22 eV, respectively. These values are in good agreement with previous measurements of the enthalpy change and in particular with a high-resolution VUV photoionization experiment of Ng and co-workers w25x.

2. Experimental 2.1. Apparatus The apparatus used for the present experiments, schematically shown in Fig. 1, consists of an electron gun, a collision chamber and an electron collec-

411

Fig. 1. Schematic diagram of the instrument. Electrons are emitted from a hot filament and focused into a beam. They pass the hemispherical energy selector at a constant energy of about 2 eV and are focused and brought to the final collision energy before they interact with the neutral beam and are collected at a Faraday cup.

tion system. Clusters are produced by a non-seeded adiabatic expansion of CO through a thin orifice of 20 mm diameter. Stagnation temperature and pressure are 107 K and 2 bar, respectively. At a distance of 2 cm, the cluster beam is skimmed Žcommercial skimmer from Beam Dynamics, 1 mm diameter.. 8 cm further downstream, shortly before it interacts with the electrons, the beam is collimated to a diameter of 3 mm. The expansion chamber and the interaction region are separately evacuated by turbomolecular pumps Žpumping speeds 500 and 200 lrs, respectively.. The monochromatized electrons Žwith typical currents of about 5 nA. are produced by a standard home-built hemispherical electron analyzer whose performance has been improved by careful attention to a number of technical details. The hemispheres, the sample inlet system and all electron–ion–optical elements are made of a single material Žstainless steel. to improve uniformity of surface potentials. Differential pumping Žusing turbomolecular pumps. between the different parts and frequent bake-outs are invoked to reduce contamination of the surfaces by the sample gas. Residual magnetic fields in the whole instrument are kept below 0.003 Gauss with Helmholtz coils compensating the Earth’s magnetic field. Ferromagnetic materials were avoided in the vicinity of the electron beam. All voltages applied to the electron–ion–optical elements are supplied by a

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D. Muigg et al.r Chemical Physics 239 (1998) 409–416

specially constructed power supply with a ripple of ( 1 mV. Ions formed in the collision chamber are extracted by a weak electric field. Usually, an extraction voltage of about 0.1 V was used, for more details on the influence of this extraction voltage on the energy resolution and energy scale calibration see Ref. w21x. The extracted ions are analyzed by a quadrupole mass spectrometer with a nominal mass range of 2000 amu. The mass-selected ions are detected by a channeltron multiplier operated in single-ion counting mode. The energy scale Žachieving an accuracy of better than 10 meV. and energy resolution Žachieving an energy resolution of approximately 50–100 meV at electron currents large enough to study ionization cross-sections close to threshold. was characterized and determined by measuring the s-wave attachment cross-section of CCl 4 near zero energy w19,20x, the Oy onset for DA to CO at about 9.6 eV w21x and appearance energies for the production of cations of various test gases Žincluding rare and molecular gases..

Fig. 2. Formation of Oy from CO by electron impact. The ClyrCCl 4 cross-section curve near 0 eV is used to calibrate the electron energy scale and to determine the electron energy resolution Ž80 meV.. The arrow points to the threshold energy for the lowest possible DA channel COqe™Oy Ž2 P.qCŽ3 P. Žwhich is in excellent agreement with the thermochemical value of 9.63 eV., the arrow at 10.88 eV w23x points to the theoretical onset for the first excited channel COqe ™ Oy Ž2 P. qC ) Ž1 D .. The OyrCO result of Stamatovic and Schulz w23x Ždesignated by the dashed line. are normalized to the maximum of the peak of the present curve.

2.2. Energy scale and energy resolution Fig. 2 shows the results for the energy dependence of the Oy cross-section for DA to CO. Also shown in this figure is the DA signal for Cly from CCl 4 , indicating an energy resolution of about 80 meV in the low energy region close to zero eV for this experimental run. The OyrCO cross-section curve shows, in accordance with the earlier measurements of Stamatovic and Schulz w23x, a sharp onset near 9.6 eV. However, the trailing edge of the present Oy resonance does not agree with that reported in Ref. w23x. This is probably due to the weak extraction field which we are using in our current work, leading to the loss of ions formed with larger kinetic energy. Similar discrimination effects have been observed and discussed in detail in a previous investigation with this apparatus concerning electron attachment to NO w21x. As can be seen from Fig. 2, the FWHM of the derivative of the apparent ‘step function’ near threshold is, with 118 meV, only slightly larger than the FWHM of the ‘zero energy peak’ of CCl 4 under the same experimental conditions of the monochromator, thus indicating the true vertical character of this

onset corresponding to a particular type of transition in the potential energy diagram as discussed in detail in Ref. w23x. Chantry w26x has pointed out that for a DA cross-section which features such a step function near threshold, the true onset corresponds to the steepest part of the observed curve, provided that the energy scale corresponds to the most probable electron energy Žthe latter is fulfilled due to the energy calibration used here.. Deriving the onset in this way we obtain a value of 9.63 eV for the onset of OyrCO in excellent agreement with the benchmark value of 9.63 eV 8 reported by Stamatovic and Schulz w23x using a trochoidal monochromator with a nominal resolution of 70 meV Žfor earlier measurements see Ref. w23x.. These experimental onset values compare very favorably with a value of 9.63 eV derived from the difference between the bond dissociation energy DŽC–O. s 11.09 eV w27x and the electron affinity EAŽO. s 1.461 eV w28x thus demon-

8 It is interesting to note that we obtain an onset value of 9.67 eV from the data of Stamatovic and Schulz w23x if using the same method as applied to the analysis of our data.

D. Muigg et al.r Chemical Physics 239 (1998) 409–416

strating the reliability of the experimental technique used and at the same time giving an estimate for the upper limit of the energy scale accuracy of about 10 meV. Similar results have also been obtained for the DA to NO w21x. 2.3. Appearance energy determination For electron impact ionization of atoms and small molecules the ionization efficiency, I Ž E ., rises in the vicinity of the threshold, Eo , with the excess energy raised to the power of the corresponding charge state z, i.e. I Ž E . s Ž E y Eo . z w29,30x. This so-called z th power law w30x Žfor singly charged ions usually expressed as linear dependence law w22x. holds for each ionic state of the atom or molecule populated in the process of ionization. In the case of large molecules and especially clusters many different ionic states can be reached by electron impact ionization and therefore the onset region of the ionization efficiency curve consists of the sum of many individual cross-sections Žone for each ionic state.. This leads to threshold regions curved more strongly than predicted by the z th power threshold law Žsee for instance results on fullerenes w24,31x., and makes it extremely difficult to extract appearance energies from the experimental data. Taking into account this complication, these data have been usually analyzed as follows w31,32x. The data points are raised to the power 1rp, where the value of 1rp is varied until the part close to threshold approximately follows a straight line. One expects that the optimum value of p exceeds z, as explained above. A straight line is then fitted to this part. Its intersection with the background, which is estimated from the ion yield below threshold, defines the appearance energy. This method leads to a systematic overestimate of p and Eo if the data contain a large amount of background. Although one can, and should, subtract the Žestimated. background b before raising I Ž E . to the power 1rp w33x, this does not solve the problem that one is trying to ‘optimize’ two parameters, p and b, without being able to objectively assess the quality of the linearization. The procedure may introduce a large bias, and the statistical error Žstandard deviation. of the resulting ‘best’ value of Eo can only be estimated.

413

Following the procedure outlined in a recent study by Matt et al. w24x, we avoid these shortcomings by rigorously applying a weighted non-linear leastsquares fitting procedure to the raw spectra, using the Marquardt–Levenberg algorithm. A function f Ž E . is fitted over an energy range which fully coÕers the threshold range. The weights are given by 1rŽ N q 1. where N is the total number of counts per energy bin. The modified power-law is, of course, mathematically undefined below Eo . The function which we fit, instead, is f Ž E. s

½

if E ( Eo

b b q c Ž E y Eo .

p

if E ) Eo

.

Ž 1.

In other words, a pair of functions 9 , coupled through the parameter b which describes the background, is simultaneously fitted to the complete set of data points. The fit involves four parameters. Note that the comparison of E with Eo involves the unknown parameter Eo . This method removes any arbitrariness, except for the choice of the energy range E low , E high over which the non-linear fit is executed. We find that the choice of E low is not crucial, provided that it is well below Eo Žproblems will, of course, arise if the background is not truly constant, for example as a result of interference from other ions of similar mass-to-charge ratio.. The value of Ehigh , however, has a significant effect on the results. Generally, high values tend to produce larger values of Eo and p combined with small statistical errors s , while low E high values result in smaller values of Eo and p combined with large uncertainties. A systematic analysis of the effect of E high suggests a solution. Very large values of E high can be rejected on the grounds that the quality of the fit, assessed through the value of x 2 which is reported by the algorithm, becomes poor. For reduced values of E high , which do produce excellent fits, although with somewhat varying results for Eo and sE o Ž sE o s standard deviation of Eo ., we find that the upper

9

Alternatively, one could fit a single function g Ž E . defined as y1 g Ž E . s b q  1qexp w Ž Eo y E . r ´ x 4 c Ž< E y Eo <. p . For all practical purposes, g Ž E . is identical to f Ž E . ŽEq. Ž1.., provided the Žfixed!. parameter ´ is kept sufficiently small.

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D. Muigg et al.r Chemical Physics 239 (1998) 409–416

bound of the threshold energy, Eo q sE o, remains approximately constant. In other words, our experimental data analyzed through this method provide a good estimate for the upper bound of the appearance energy, while the lower bound is more uncertain. This is tolerable, because the appearance energy itself cannot provide more than an upper bound to the Žadiabatic. ionization energy. The ‘best’ values of Eo reported in this work are obtained using a value of Ehigh Žin the present study all of the ions have been analyzed up to approximately 3 eV above Eo . which produces a ‘reasonable’ statistical error sE o, commensurable with the observed effect of Ehigh on Eo .

Fig. 3 Žcontinued..

Fig. 3. Ionization efficiency curves for various atoms and molecules. Curves on the left-hand side are plotted on a linear intensity scale. On the right-hand side, a logarithmic intensity scale is chosen. The line through the data points are the results of the non-linear weighted least-squares fit described in the text from which the appearance energies and their standard deviations can be extracted.

Fig. 3 shows as an example, on the left-hand side, a linear plot of the experimental data for some rare-gas atoms and some molecules used as test targets. A more detailed view of the crucial threshold region is afforded by the semi-logarithmic plot, log Ž I . versus E, shown on the right-hand side of Fig. 3. The solid lines represent the results of the non-linear fit Žwith the corresponding p values given in Table 1., they also indicate the energy range Elow , E high which was ultimately chosen. The agreement with the data is excellent. The electron energy scale has been calibrated Žand accordingly adapted in Fig. 3. by comparing our value for the threshold of Xeq, which has an uncertainty Žstandard deviation. of about 0.0116 eV, with the best value of 12.12987 eV available in the literature w34x. The appearance energies and standard deviations obtained for the other test targets are given in Table 1. Comparison of the present results with the best known values taken from the data compilation w34x gives an average deviation between the present AE values and the previous values of about 10 meV Žnot

D. Muigg et al.r Chemical Physics 239 (1998) 409–416 Table 1 Appearance energies and concomitant statistical errors determined in the present study using the fit function Ž1. with p values given in the last column. Also shown the best values given in the NIST tables w34x and the deviation between the present values and the NIST values

D

AE ŽeV. Xeq Neq Krq Arq Nq 2 Oq 2 N2 Oq a

NIST w34x

Present

12.12987 a 21.56454"0.00001 13.99961"0.00001 15.759"0.001 15.581"0.008 12.0697"0.0002 12.889"0.004

12.12987 a 21.5012"0.0331 62 13.9902"0.0145 8 15.749"0.0117 10 15.5899"0.0113 8 12.0730"0.0212 3 12.8655"0.0087 23

p

ŽmeV. 1.12 1.30 1.22 1.30 1.18 1.24 1.28

415

the present values are in fair agreement with the previous high-resolution VUV photoionization experiments of Ng and co-workers w25x reporting values of 13.05 " 0.04 and 12.91 " 0.04 eV, respectively. Following the procedure outlined in Ref. w25x, using the present ionization energies, the known ionization energy of CO and the estimated dissociation energy for ŽCO. 2 and ŽCO. 3 Žassuming the binding energy of the trimer to be the same as that of the dimer, i.e. 0.008 eV; see Ref. w25x. the bond energies for COq–CO and ŽCO.q 2 –CO are calculated to be 0.83 " 0.10 and 0.22 eV, respectively. The corresponding values derived in the photoionization

Used as reference.

taking into account the value of Ne which shows a larger deviation indicating a slight deviation from linearity of the energy scale at these higher electron energies.. From this comparison we conclude in accordance with the results obtained in the negative ion study discussed above that we are able to determine appearance energies for molecular targets within errors of about "10 meV in the electron energy range below 20 eV.

3. Results and discussion Applying this new data analysis procedure Žnonlinear weighted least-squares two-functions fit, NWLT. to the monomer, dimer and trimer ion produced by electron impact ionization of a ŽCO. n cluster beam Žsee Fig. 4., we obtain after calibrating the energy scale in these experimental runs to the known CO monomer ionization energy of 14. 0142 " 0.0003 eV w34x the following appearance energies: . ŽŽ .q . AEŽŽCO.q 2 s 13.194 " 0.10 and AE CO 3 s 12.98 " 0.34 eV. The errors given are statistical errors allowing also for the slight deviation at very small signals close to threshold between the data points and the fit for the monomer ion Žsee Fig. 4.. Clearly, as can be seen from the large scatter in the data for the dimer and trimer ion, an improved signal level would lead to a much lower statistical error and thus more accurate appearance values Žas in the case of ordinary molecular ions, see above.. Nevertheless,

Fig. 4. Ionization efficiency curves for the production of COq, ŽCO.q Ž .q 2 , and CO 3 by electron impact ionization of a neutral CO cluster beam. Curves on the left-hand side are plotted on a linear intensity scale, on the right-hand side, a logarithmic intensity scale is used. The line through the data points are the results of the non-linear weighted least-squares fit described in the text Žwith p values of 1.43, 1.53 and 2.37, respectively. from which the appearance energies and their standard deviations can be extracted.

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D. Muigg et al.r Chemical Physics 239 (1998) 409–416

study of Ng and co-workers w25x are 0.97 " 0.04 and 0.16 " 0.08 eV, respectively. Moreover, because of the high degree of cooling in the supersonic expansion, these bond energies derived from the electron impact ionization and photoionization study can be taken to be the enthalpy changes for the corresponding ion molecule association reactions at about 0 K. The enthalpy change for the association reaction COqq CO m ŽCO.q has been determined previ2 ously w35,36x at elevated temperatures. In order to compare these values with the ionization experiments these data have to be converted to yD H0 K values using arguments similar to those used in Ref. w25x thereby yielding for the reaction COqq CO m ŽCO.q w x 2 values of 1.1 " 0.3 eV 35 and 0 0.91 eV w36x. Good agreement can be seen between the values obtained here, the photoionization values and those of Chong and Franklin w35x and Meotner and Field w36x.

Acknowledgements Work carried out in the Association Euratom– ¨ ¨ OAW and partially supported by the FWF, ONB, BMWV, Wien, Austria. It is a pleasure to acknowledge valuable discussions with Dr. Nigel Mason, UCL, London, UK.

References w1x L.G. Christophorou ŽEd.., Electron Interactions and Their Applications, Academic Press, Orlando, FL, 1984. w2x T.D. Mark, ¨ G.H. Dunn, Electron Impact Ionization, Springer, Wien, 1985. w3x T.D. Mark, ¨ Int. J. Mass Spectrom. Ion Proc. 79 Ž1987. 1. w4x H.M. Rosenstock, Int. J. Mass Spectrom. Ion Phys. 20 Ž1976. 139. w5x T.D. Mark, ¨ A.W. Castleman Jr., Adv. Atom. Mol. Phys. 20 Ž1985. 66. w6x H. Haberland ŽEd.., Clusters of Atoms and Molecules, Springer, Berlin, 1994. w7x P.M. Dehmer, S.T. Pratt, J. Chem. Phys. 76 Ž1982. 843. w8x C.Y. Ng, Adv. Chem. Phys. 52 Ž1983. 263.

w9x S. Leutwyler, M. Hofmann, H. Harri, E. Schumacher, Chem. Phys. Lett. 77 Ž1981. . w10x J.B. Hopkins, P.R.R. Langridge-Smith, M.D. Morse, R.E. Smalley, J. Chem. Phys. 78 Ž1983. 1627. w11x W. Kamke, B. Kamke, H.U. Kiefl, I.V. Hertel, J. Chem. Phys. 84 Ž1986. 1325. w12x P. Gantefor, E. Holub-Krappe, A. Ding, J. Chem. ¨ G. Broker, ¨ Phys. 91 Ž1989. 7972. w13x K. Muller Dethlefs, O. Dopfer, T.G. Wright, Chem. Rev. 94 ¨ Ž1994. 1845, and references therein. w14x F. Merkt, Annu. Rev. Phys. Chem. 48 Ž1997. 669. w15x W. Kamke, J. deVries, J. Krauss, E. Kaiser, B. Kamke, I.V. Hertel, Z. Phys. D 14 Ž1989. 339. w16x J. DeVries, H. Steger, B. Kamke, C. Menzel, B. Weisser, W. Kamke, I.V. Hertel, Chem. Phys. Lett. 188 Ž1992. 159. w17x H. Steger, J. Holzapfel, A. Hielscher, W. Kamke, I.V. Hertel, Chem. Phys. Lett. 234 Ž1995. 455. w18x G. Scoles ŽEd.., Atomic and Molecular Beam Methods, Oxford University Press, New York, 1988. w19x S. Matejcik, A. Kiendler, P. Stampfli, A. Stamatovic, T.D. Mark, ¨ Phys. Rev. Lett. 77 Ž1996. 3771. w20x S. Matejcik, G. Senn, P. Scheier, A. Kiendler, A. Stamatovic, T.D. Mark, ¨ J. Chem. Phys. 107 Ž1997. 8955. w21x G. Denifl, D. Muigg, A. Stamatovic, T.D. Mark, ¨ Chem. Phys. Lett. 288 Ž1998. 105. w22x C. Winkler, T.D. Mark, ¨ Int. J. Mass Spectrom. Ion Proc. 133 Ž1994. 157. w23x A. Stamatovic, G.J. Schulz, J. Chem. Phys. 53 Ž1970. 2663. w24x S. Matt, O. Echt, R. Worgotter, V. Grill, P. Scheier, C. ¨ ¨ Lifshitz, T.D. Mark, ¨ Chem. Phys. Lett. 264 Ž1997. 149. w25x S.H. Linn, Y. Ono, C.Y. Ng, J. Chem. Phys. 74 Ž1981. 3342. w26x P.J. Chantry, Phys. Rev. 172 Ž1968. 125. w27x K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules, van Nostrand, New York, 1979. w28x D.M. Neumark, K.R. Lykke, T. Anderson, W.C. Lineberger, Phys. Rev. A 32 Ž1985. 1890. w29x T.D. Mark ¨ in: L.G. Christophorou ŽEd.., Electron Interactions and Their Applications, Academic Press, Orlando, FL, 1984. w30x T.D. Mark, ¨ J. Chem. Phys. 63 Ž1975. 3731. w31x R. Worgotter, B. Dunser, P. Scheier, T.D. Mark, ¨ ¨ ¨ ¨ J. Chem. Phys. 101 Ž1994. 874. w32x J. Laskin, J.M. Behm, K.R. Lykke, C. Lifshitz, Chem. Phys. Lett. 252 Ž1996. 277. w33x H. Steger, J. deVries, B. Kamke, W. Kamke, T. Drewello, Chem. Phys. Lett. 194 Ž1992. 247. w34x NIST Tables, http:rrwebbook.nist.govrchemistryrformser.htm. w35x S.L. Chong, J.L. Franklin, J. Chem. Phys. 54 Ž1971. 1487. w36x M. Meotner, F.H. Field, J. Chem. Phys. 61 Ž1974. 3742.