Photodissociation of Arn−mN+2m clusters (n=2 or 3, m=0−n)

Volume 192, number 1

Photodissociation

CHEMICAL PHYSICS LETTERS

of Ar,_,N,+,

24 April 1992

clusters (n = 2 or 3, m = O--y2 )*

Thomas F. Magnera and Josef Michl Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA Received 13 January 1992

Laser-induced photodissociation cross sections of Ar: , ArN:, N4+, Ar: , Ar,N$ , ArN: , and N6+, obtained by sputtering of solid argon-nitrogen mixtures, have been measured in the 270-650 nm range. All the spectra exhibit a band in the UV (305-335 nm) and those of the trimeric ions, except for N :, also have a band in the visible (465-525 nm). The photodissociation products depend on the choice of wavelength in a readily rationalized fashion.

1. Introduction There has been considerable interest in the cluster ions of nitrogen [ l-9 1, argon [ 1O-341, and their mixtures [ 2,10,35-38 1. The larger cluster ions appear to consist of a small dimeric [ 21, trimeric [ 2,15,23 ] or tetrameric [ l&23 ] core and a solvation shell of neutral N2 or Ar units. The elucidation of their structures would be aided significantly by reliable knowledge of the absorption spectra of the dimeric and trimeric species. Only a few segments of the photodissociation (PD) spectra of Ar: [ 10-121, Ar: [ 13-16,181, ArN: [ 10,361, ArlN$ [ 381 and N: [ 2-5 ] are known, and there has been considerable debate [ 13-22,25-33 ] concerning the nature of the excited states and structure of Ar: . We now report the PD spectra and photoproduct ion distributions ofAr,_,N&,, n= 2, 3 and m = O-n in the spectral range 270-650 nm and note their relevance for the resolution of the existing controversy.

2. Experimental The photodissociation spectra were measured using a previously described [40] home-built triple quadrupole mass spectrometer. The Ar,_,N 2’, ions were generated by sputtering solid mixtures of Ar and N2 of variable composition, deposited on a cryogenic substrate, with 8 keV Ar atoms. An excimer-pumped (Lumonies HyperEx 400) dye laser (Lumonics HyperDye 300) beam was directed along the axis of the triple quadrupole. A photodissociation signal was measured in two complementary ways: as the difference between the number of parent ions detected when the laser is off and when it is on and as the difference in the sum of all daughter ions detected when the laser is off and when it is on. For any single parent ion, the mass spectrometer could always be tuned such that there was excellent agreement between the two methods except in the UV below 300 nm. This is the point at which scattered laser light saturates the Channeltron ion counter, necessitating very low laser power levels for which only the more sensitive daughter ion method can be used. When several parent ions were investigated using the same tuning, daughter ion transmission corrections were applied. The signal was time resolved and time correlated with the laser pulse [ 401, which made it possible to distinguish between photodissociation events occurring at different points along the flight path of the ions * This project was initiated at the University of Utah and continued at the Center for Structure and Reactivity at the University of Texas at Austin. 0009-2614/92/S 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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and to eliminate spurious photoinduced counts produced by the detector immediately after the laser is tired. Overlapping or nearly overlapping dye scans were done from 270 to 650 nm. An INRAD doubler was used to generate light between 270 and 350 nm. In order to avoid saturation in the visible range the energy per pulse was kept at 1 mJ or less by means of a beam attenuator. In the UV, after doubling, the energy per pulse is smaller and therefore need not be attenuated. The resulting photodissociation signal was normalized to the laser power measured with a Scientec model 35 1 power meter and by the temperature rise of the laser beam stop in the mass spectrometer, which was calibrated to laser power. Both to avoid saturation and to determine the cross section, the beam diameter was expanded in a reversed telescope until the photodissociation signal began to diminish. Crudely, this was taken to be the point at which the laser beam width exceeded the ion beam width in the second quadrupole. The actual width of the roughly rectangular laser beam was measured by moving a pinhole across a power meter in both lateral directions. The absolute cross sections were estimated from the fraction dissociated per pulse into all channels, divided by the fluence per pulse. The cross section is probably not determined with an accuracy better than within a factor of two, due to the incomplete knowledge of the degree of ion and laser beam overlap, of the lateral ion distribution, of the amount of amplified stimulated emission and of the laser beam power profile. Because of these uncertainties there was considerable day-today variation in the measured cross sections and it was necessary to use a linear scaling factor between 0.5 and 2 for each segment when assembling the individual dye-scan segments into a complete spectrum in a piecewise fashion. The spectra include data points collected over a period of years at three different locations, and the associated systematic errors. The error bars represent the amount of statistical error. Daughter ions detectable in the absence of laser light result from metastable decomposition [ 1,2] or background-gas collisions. Here, they occur under selected conditions [ 21 that minimize the amount of such decomposition and consequently the background against which the PD daughter ion signal is measured.

3. Results The cross sections are plotted against wavelength to give the PD spectra of A$, Ar,N$ and N: (fig. 1). The spectrum of each dimer ion is characterized by a broad and featureless band with a near-UV maximum at 317-t7 nm for Ar:, 331+6 nm for ArN: and 328It6 nm for N$. Included in fig. 1 are earlier data for N: [ 3,4], ArN: [ 361 and Ar: [ 121. The results presented here are in good agreement with the somewhat fragmentary earlier data, however, they appear to have slightly larger maximum cross sections and narrower absorption profiles. In fig. 2 the PD spectra of Ar: , ArzN:, ArN: , and N6+ are shown over the same wavelength range. Each trimer ion, except for Nz, has two intense absorption bands in the region under investigation: one in the nearUV centered at 309 + 6 nm for Ar: , 3 14 2 6 nm for Ar2N$, and 320 f 6 nm for ArN: , and the other in the visible, at 524 + 5 nm for Arc, 482? 9 nm for Ar,N: and 466 + 8 nm for ArN$ . The absolute magnetide of o,,, for Ar: is = 73 x 1O- ‘* cm2, twice smaller [ 14,16,17 ] and the width of this band is 4000 cm-‘, z 150% larger [ 141, than reported previously. A comparison of cross sections and linewidths determined here for ArZN: and the relative cross sections [ 381 reported previously previously can be made qualitatively in fig. 2. Finally, for Nz only one broad maximum at 3 19 f 8 nm is found within the region searched, and the spectrum is very similar to that of N$ and the larger N& cluster ions [2]. There is close similarity between the spectral location of the higher-energy UV band for the trimers and those of the corresponding dimers, but the absorption cross sections for the trimer UV bands are 30°h larger and slightly blue-shifted. This is in partial disagreement with an earlier report [ 181 of the UV absorption of A$, according to which Ar: has a larger cross section in the UV than Ar: . The location of the A,,,,, of the visible bands shifts towards the blue as the number of N2 molecules in a trimer increases, and the bands become broader and weaker. The most pronounced changes occur after the substitution of the second Ar by N2. 100

Volume 192, number 1

” t(a)r;

CHEMICAL PHYSICS LETTERS

1 .

.

.

.

.

.

.

.

.

,

.

.

.

.

.

24 April 1992

_

F

IS :

10 5: r

r,

10

20

Wavenumber

30

x3

40

/ lo3 cm-l 10

Fig. I. Photodissociation spectra: (a) Ar; , (a’ ) A$ (ref. [ 12 ] ), (b) ArN:, (b’) ArN: (ref. [36]), (c) N4+, (c’) N.$ (ref. [3]) and (c” ) Nz (ref. [ 41). The lines are least-squares Gaussian tits.

20

Wavenumber

30

40

/ lo3 cm-l

Fig. 2. Photodissociation spectra: (a) A$, (a’ ) Ar: (refs. [13,14]), (b) Ar,N:, (b’) Ar,N: (ref. [38]), (c) ArN: and (d) N,+ . The lines are least-squares Gaussian fits. 101

Volume 192, number I

CHEMICAL PHYSICS LETTERS

24 April I992

0.8

.

.

.

.

(b’)

*me.

tie

0

..=:

’

0.2

0.0 . . . . , . . . , . , . . . . . . . , . . . 10 20 30

.. ., 40

Wavenumber / lo3 cm’] Fig. 3. Product ion abundances: (a) ArN:, (b) ArN:, (b’ ) ArN: (ref. [36] ), (c) Ar,N:.

The product ion fractions are shown in fig. 3 as a function of wavelength for the photodissociation of ArN: , Ar,N: , and ArN: . The fractional abundance of the N: product ion from ArN$ is slightly less than 0.2 and is nearly invariant with wavelength. This compares well with a previously measured value of slightly more than 0.2 [ 361. The N: abundances for Ar,N$ and ArN :, however, are strongly wavelength dependent. At visible wavelengths Ar+ is the dominant photoproduct ion from the dissociation of Ar2N:. This dominance is diminished by almost half at the shorter UV wavelengths. For ArN: the behavior is similar but inverse the Ar+ product is twice as heavily favored in the UV than in the visible. The three ions appear to have converging product ion distributions at the short wavelengths. Metastable decomposition product ion abundances were measured for Ar2N: : ArN$ ( 1.2O/6), Ar: ( 1.O%), NC (0.03%) and Ar+ (0.09%), and for ArN$ : ArN$ (3.4%), N: (O.l%), N$ (0.13%) and At-+ (0.13%). The loss of a single constituent to a dimeric product ion is always observed [ 1,2] and is attributed to energy, slightly above the dissociation limit, stored randomly in the internal modes of the cluster. The much smaller fraction of ions that disintegrate to Ar+ or N: is attributed to a slow electronic relaxation process. Dimeric product ion abundances lower than l-3% were not detectable against the metastable decomposition background over the wavelength range examined and are consistent [ 15,381 with previously reported dimer abundances.

4. Discussion The PD spectrum absorption from 350 has now been found The PD spectrum 102

of A$ was previously known from 350 to 860 nm [ 10-121. The gradually decreasing to 500 nm has been assigned to a ‘C: +-‘C: transition [ 12 1. The maximum for this band at 3 17 nm and is in excellent agreement with a calculated [ 27 ] vertical value of 308 nm. of N 4’ was previously known between 270 .and 306 nm [ 41 and between 350 and 650

Volume

192, number

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CHEMICAL

PHYSICS

LETTERS

24 April 1992

nm [ 2,3 ] but remained unconnected through the maximum by a commonly normalized set of measurements. The position of the absorption maximum at 330 nm observed here agrees well with calculations [ 9,4 1] that predict a linear ground state geometry and assign the band to a ‘Cl t2ZC: transition. Previous PD experiments [5] combined with kinetic energy analysis of the products have confirmed that the terminating state of the observed transition is repulsive and polarized parallel to the long molecular axis, and that the molecule is linear. The PD spectrum of ArN: is very similar to that of N$ but it is weaker. Except for measurements at the lines of an Ar-ion laser [ 361 and between 570 and 660 nm [ 101 the spectrum was not previously known. A single ab initio calculation on the ArN$ system is known to us [ 3 5 ] and it focuses on diabatic charge transfer, but it does predict that the linear structure is the most stable. An analysis of the photoproduct kinetic energies (361 proves that ArN: is linear and also dissociates from a purely repulsive excited state along a coordinate that mixes the NC (X 2Z) /Ar ( ‘S) and Ar+ ( 2P3,2) /N,( X ‘C ) product states. The dramatic differences in the spectral location and linewidth of Ar+ and N: unsolvated or solvated by a single Ar or N2 stand in contrast to the minor changes in linewidth and location observed in the vibrational predissociation spectra of NC solvated by He (421 or as many as 4 Ne [ 43,441. The strong ion-solvent interactions, D(N,-N,+ ) = 1.0 eV (61 D(Ar-N: )= 1.06 eV (451 and D(Ar-Ar+)= 1.14 eV (451, clearly make Arc, ArN: and N$ separate chemical entities from simple solvated Ar+ and N :. Further solvation of the dimeric ions usually proceeds with much weaker ion-solvent interactions ( z O.l03. eV) (461 and it is not so obvious as to when a trimeric ion will behave as a moiety chemically distinct from that of its dimeric cousin. The Ar-containing trimeric ions, Ar: , Ar2N $ and ArN 2, have a very strong new band in the visible in addition to a UV band whose spectral location is similar to those of the UV bands found for the dimers. These visible bands are several times more intense and therefore appear to be unrelated to any band observed for the dimeric series. The exceptional species in the trimeric series is N 6’) which is not observed to have a strong visible band (fig. 2). A comparison between the known bond strengths for D(Ar:-Ar) (4.89kcal/mol [25]),D(Ar$-Ar) (1.67 kcal/mol[25]),andD(Ar:-Ar) (1.62kcal/mol[25]) on the one hand, and for D(N:-N,) (2.76 kcal/mol (61) and D(N,$-N,) (2.71 kcal/mol [6]), and D(Nsf -N2) (2.52 kcal/mol (61) on the other hand, underscores the difference between the Ar,+ and the N&, series. Unlike the Ar: -Ar bond strength, the value for N4+-N2 does not differ significantly from those found in larger homologous clusters, and there seems to be no special interaction between N: and N2 beyond ordinary solvation. It has been noted elsewhere [ 471 that (CO,),’ spectra and bond strengths follow a similar pattern, and we note here that the large blue-shift in the A,,,,, for (NO),+ (481 relative to Iz,,, for (NO): [ 31 also coincides with a strong D( (NO):-NO) interaction (7.4 kcal/mol (491) compared to D( (NO):-NO) (3.7 kcal/mol (491). A preliminary assignment of the bands for the trimeric ions Ar,N: and ArN: can be made by analogy with those for A$. Justification for this follows from the similarity in the dimer PD spectra of A$, ArN: and N: , in their photoproduct kinetic energy distributions [ 36 1, and in their bond strengths [ 451. This has been discussed previously [ 36 ] and it was concluded that the similarity implies a rough chemical equivalence between Ar and N2. It can probably be safely assumed that the bond strengths and potential energy surfaces for the three Ar-containing trimeric ions are also very similar. Most ab initio calculations on Ar: (25-311 yield a linear symmetric structure and predict a single strong absorption in the visible near 500 nm, corresponding to a ‘Z: e’C,+ transition. Recent results [ 181 and those reported here show that Ar: has a PD band in the UV near 3 15 nm, very close to where A$ does, and that the observed visible band at 524 nm is not just a shifted absorption, but an entirely new one. The presence of an allowed absorption band in the UV is not consistent with a linear symmetrical structure and has been postulated to be due to a forbidden 2 “C: c2Z,’ transition induced by ground-state vibrational excitation in the asymmetric stretching mode [ 22 1, Alternative explanations, which however contradict the calculated geometries, are that Ar: is linear but not symmetrical, with a double minimum ground state potential energy surface [ 17 1, or that it is bent [ 191. Either would be consistent with the presence of two allowed absorptions, one in

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the visible and one in the UV. Experiments in which a kinetic energy (KE) analysis of the photodissociation product ions is done have led to different conclusions about the structure of Arc, including proposed Dooh [ 161, C,, [ 17,201 and Czv [ 19 ] structures. One early ab initio calculation [ 341, but no others, predicts a bent structure for Ar:. Although the exact structure of Ar: is still uncertain, if is clear at this point that Ar: is best viewed as a single chemical entity and not as solvated A$, and that ArZN: [ 381 and ArN: should be viewed in the same manner. On the other hand, N6+ is best viewed as N: solvated by NZ. We expect that in Nl the closest analogue to the visible absorption band of the other trimers would be a very weak ion-to-solvent charge transfer band [ 18 ] in the near-IR. The probable isomeric structures of Ar2N: and ArN: can be deduced from the thermal metastability products. The nearly equal probability for the loss of N2 or Ar from ArzN: is only consistent with the asymmetric [ Ar-Ar-Nz ] + isomer, whereas the nearly exclusive loss of N2 from ArN: is only consistent with the symmetric [ NZ-Ar-N2] + isomer. These isomers are also consistent with the wavelength-dependent PD product ion abundances as demonstrated below. From the analysis of the Ar: PD product ion KE distribution [ 16- 18,2 1,22 ] and from calculated potential energy surfaces [ 2 1,22,29-3 11, it has been proposed that in the ‘Cz state of Ar: the positive charge is shared by the outer Ar atoms, that visible excitation results in a symmetrical linear dissociative fragmentation with an 800/6probability [ 17,2 1,221 of the charge being carried by one or the other fast moving end Ar atoms (adiabatic dissociation), and only a 20% probability of the charge ending up on the slow central Ar atom (nonadiabatic dissociation) [ 3 l-33 1, [ Ar-Ar-Ar ] + [

Ar-Ar-Ar ] + -

VIS VIS

[Ar+1/2_Ar-Ar+‘/2]* [Art’/2_Ar_Ar+‘/2]*

adiab.

Ar + (fast ) + 2Ar

(80%)

)

(1)

non-ad.

-

Ar+Ar+(slow)

+Ar

(20%) .

(2)

The UV band is assigned [ 221 to the forbidden ‘C: -‘Z: transition activated by coupling to the asymmetric stretching mode of Ar: with a Dmh ground state geometry. If the molecular symmetry is C,,, the UV band is assigned to an allowed *C++*C+ transition. The calculated potential energy curve for the *XC:state has not been discussed in detail [ 2 1,22,29-3 1] but clearly suggests that adiabatic non-symmetrical linear dissociation should produce a neutral atom from one of the outer argons and a positively charged dimer in its excited “Cz state from the other two argons. The dimer would then dissociate further [ 27 1: [

Ar-Ar-Ar] + 2

Ar: (‘Zz )...Ar -

Ar:(2X,+) adlab. Ar++Ar.

adiab.

Ar:(2C,+)+Ar,

(3) (4)

This sequential adiabatic Ar: (‘C: ) dissociation mechanism seems reasonable theoretically, but so far has not been examined experimentally, and it is not known how important the non-adiabatic alternatives are. At the moment analogous calculations are not available for the clusters Ar2N:, ArN: and Nl . In view of the similarities between N2 and Ar, it is perhaps sensible to assume that in the first approximation the results for Ar2N: and ArN: are similar to those for Ar3+ *. Adiabatic dissociation from the state excited with visible light is concerted, with one of the outer constituents carrying the positive charge. Adiabatic dissociation from the state excited with UV light produces a neutral end constituent plus a charged excited dimer that subsequently dissociates. The first-order picture is then modified by non-adiabatic process, as in Ar: . Applying this crude model to the above isomers of Ar2N: and ArN: , wavelength-dependent product ion distributions are predicted from 270 to 650 nm. At the longest wavelengths, by analogy with ( 1) and (2), the observed product ion distributions result from

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Volume 192, number

1 VIS

[N2-Ar-N2]+

VIS

[N,-Ar-N2]+

-

CHEMICAL

[ [ 1 [NZ+1/2_Ar_N$i/2

[

]+-

Ar-Ar-N,

LETTERS

1* _,adiab’N,+Ar+N$(fast)

(65%)

non-ad.

[N2+1/2_Ar_N;i/2

1* -,

N2 +Ar+(slow)

adiab. Ar+Ar+N$

]*

Ar-Ar-N:

VIS

PHYSICS

+N2

(35%)

(fast)

)

(8%))

non-ad: Ar+Ar+(slow)+N2

5

Ar+-Ar-N2]

* -

Ar+(fast)+Ar+N2

(6)

(5)

(7) (8)

adiab.

[

)

24 April 1992

i

’

(92%)

(9)

In the absence of information on the KE distribution of products, we cannot separate the dissociation of Ar,N : into adiabatic and diabatic contributions. UV excitation of Ar,N T and ArN : generates a state assumed to be analogous to the upper 2 2Z: state of Ar: and sequential dissociation can be expected. In ArN$ , the two stepwise dissociation paths are equivalent by symmetry, Along either path, the UV excited state correlates with the same excited 2 2X+ state of ArNZ that is involved in the photodissociation of ArN:: ArN:

uv[ ArN: (2 2z+ ) - adiab’ Ar++N2

ArNT x

ArNl(z2xC+)

non-ad: Ar+N$

(82Oh),

(10)

(18%) .

(11)

Our experimental

results for ArN:

are accommodated

by the scheme

[N2-Ar-N2]+

x

N2Ar+(2Z+)...N

2- adiab. ArN:(22C+)+N2,

(12)

[N2-Ar-N2]+

x

N2Ar+(2C+)...N

2- non-ad. ArN2+N:.

(13)

If the unsymmetrical dissociation of ArN: were purely adiabatic, ArN: (2 2C+) would be the product, and it would decay according to ( 10) and ( 11) to yield Ar+ and NT in the ratio 82: actually observed in the experiment on ArN: is 70: 30, and the excess abundance of N$ can be the non-adiabatic process ( 13). The branching ratio of ( 12) to ( 13) derived in this fashion is In Ar,N: , the two stepwise dissociation paths yield distinct products: [Ar-Ar-N,]+

xAr;(22:)...N

2- adiab’ Ar$(2Z:)+N2,

sole primary 18. The ratio attributed to 85: 15.

(14)

non-ad.

Ar-Ar-N2]

+2

[Ar-Ar-N,]+

x

[

[ Ar-Ar-N,]

+x

Ar: (‘Xc ) ...N2 Ar...ArN$(2 Ar...ArNz

Ar2 +N$

,

2C+) - adiab’ Ar+ArN:(22Z+), (2 2C+ ) -“Onead.Ar+ + ArN2 .

(15) (16) (17)

In the second step, A$ (‘Zz ) will yield Ar+ +*Ar and ArN: (2 2E+) will decay according to ( 10) and ( 11) to yield Ar+ and N: in the ratio 82: 18. The experimentally observed ratio Ar+ : N$ is 82O/6:18%, the same as in the decay of ArN: , ( 10) and ( 11). In the absence of information on ICE release, this cannot be quantitatively dissected unequivocally in terms of the individual process ( 14)-( 17), but qualitatively, the preponderance of Ar+ makes sense. The simplest way to account for the result is to say that of the two competing adiabatic dissocation paths for Ar2N:, the one that produces the more stable set of products, ArN$ (2 2X+ ) predominates to the exclusion of the other, and also to the exclusion of the potentially competing non-adiabatic paths. However, the data do not exclude a participation by processes ( 14) and ( 15 ) as long as they occur to a similar extent. Note that we have ignored the anticipated low-lying II states of the trimeric ions completely in the above discussion of spectra and dissociation, and that this is not necessarily correct. However, we have no experimental indications that they are of importance for the phenomena discussed here.

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5. Conclusions ( 1) Photodissociation will be a very useful means for the identification of the ionic core of larger Ar,,_,N,:, clusters. (2) A model in which the trimers Ar: , ArZN: and ArN$ are considered as single molecules, and not as solvated dimers, and according to which UV excitation leads to sequential decomposition, is in agreement with all the new data. (3) The fourth trimeric species, Nz , stands apart from the first three and behaves spectrally as would be expected for N$ very weakly perturbed by an N2 molecule. (4) There appears to be a correlation between ground-state energetics and the spectra of the ions: the shift in L, between the dimeric and trimeric ions is large only if D( X: -X) is large relative to D( X: -X).

Acknowledgement This work was supported by the National V. Balaji for many helpful discussions.

Science Foundation

(CHE-9000202

and 9020896).

We thank Dr.

References [ I] T.F. Magnera, D.E. David and J. Michl, Chem. Phys. Letters 123 ( 1986) 327. [2] T.F. Magnera, D.E. David and J. Michl, J. Chem. Sot. Faradaty Trans. 86 ( 1990) 2427. [3] G.P. Smith and L.C. Lee, J. Chem. Phys. 69 (1978) 5393. [4] S.C. Ostrander and J.C. Weisshaar, Chem. Phys. Letters 129 (1986) 220. [5] M.F. Jarold, AI. Illies and M.T. Bowers, J. Chem. Phys. 81 (1984) 214. [6] K. Hiraoka and G. Nakajima, J. Chem. Phys. 88 (1988) 7709, and references therein. [ 71 T. Leisner, 0. Echt, 0. Kandler, X.J. Yan and E. Rechnagel, Chem. Phys. Letters 148 ( 1988) 386. [ 81 P. Scheier and T.D. Mark, Chem. Phys. Letters 148 ( 1988) 393. [ 91 S.C. DeCastro, H.F. Schaefer III and R.M. Pitzer, J. Chem. Phys. 74 ( 198 1) 550. [lO]T.M.Miller,J.H.Ling,R.P.SaxonandJ.T.Mosely,Phys.Rev.A 13 (1976)2171. [ 111 J.T. Moseley, R.P. Saxon, B.A. Huber, P.C. Cosby, R. Abouaf and M. Tadjeddine, J. Chem. Phys. 67 [ 121 L.C. Lee and G.P. Smith, Phys. Rev. A 19 ( 1979) 2329, and references therein. [ 131 C.R. Albertoni, R. Kuhn, H.W. Sarkis and A.W. Castleman Jr., J. Chem. Phys. 87 ( 1987) 5043. [ 141 N.E. Levinger, D. Ray, K.K. Murray, A.S. Mullin, C.P. Schultz and W.C. Lineberger, J. Chem. Phys. [ 151 N.E. Levinger, D. Ray, M.L. Alexander and W.C. Lineberger, J. Chem. Phys. 89 (1988) 5654. [ 16 ] Z.Y. Chen, C.R. Albertoni, M. Hasigawa, R. Kuhn and A.W. Castlemah Jr., J. Chem. Phys. 9 1 ( 1989 [ 171 J.T. Snodgrass, C.M. Roehl and M.T. Bowers, Chem. Phys. Letters 159 (1989) 10. [ 181 M.J. Deluca and M.A. Johnson, Chem. Phys. Letters 162 ( 1989) 445. [ 19) C.A. Woodward, J.E. Upham, A.J. State and J.N. Murrell, J. Chem. Phys. 91 (1989) 7612. [ 201 N.G. Gotts, R. Hallett, J.A. Smith and A.J. State, Chem. Phys. Letters 181 ( 1991) 491. [ 2 I] T. Nagata, J. Hirokawa, T. Ikegami and T. Kondow, Chem. Phys. Letters 17 1 ( 1990) 433. [ 221 T. Nagata, J. Hirokawa and T. Kondow, Chem. Phys. Letters 176 ( 199 1) 526. [23] H. Haberland, T. Kolar, C. Ludewigt, A. Risch and M. Schmidt, Z. Physik D 20 (1991) 33. [ 241 T.D. Mark, P. Scheier, K. Leiter, W.R. Hei, K. Stephan and A. Stamotovic, Intern. J. Mass Spectrom. 281. [25] K. Hiraoka and T. Mori, J. Chem. Phys. 90 (1989) 7143. [26] A. Ding, Z. Physik D 12 (1989) 253. [27] W.R. Wadt, Appl. Phys. Letters 38 (1981) 1030. [28] H.U. Bijhmer and SD. Peyerimhoff Z. Physik D 3 (1986) 195. [ 291 J. Hesslich and P.J. Kuntz, Z. Physik D 2 ( 1986) 25 1. [30] P.J. Kuntz and J. Valldorf, Z. Physik D 8 (1988) 195. [ 3 I] F.X. Gadea and M. Amarouche, Chem. Phys. 140 ( 1990) 385.

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( 1977) 1659.

89

( 1988) 71.

) 40 19.

and Ion Processes

74 ( 1986)

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CHEMICAL PHYSICS LETTERS

24 April 1992

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