Low energy crossed beam study of the endothermic charge transfer reaction H+2(Ar , H2)Ar+

Volume 74, number I CHEMICAL PHYSICS LETTERS 1.5 August 1980

LOW ENERGY CROSSED BEAM STUDY

OF THE ENDOTHERMIC CHARGE TRANSFER REACTION H$(Ar , H2)Ar+

R.M. BILOTTA, FN. PREUNINGER and JM. FARRAR Department of C’hemlsny. Univem~@ of Rochester, Rochester, New York 14627. US4

Recerved 20 May 1980

A crossed beam study of the title reaction if reported, from 0.45 to 7.8 eV. The reaction is predominantly transkxtioional- ly endothermic. At the lowest energy, there is evidence for two reactlon paths: a long-range electron transfer and an titi- mate coihsion mth electron transfer. Branching ratios for the competitive proton transfer reaction are presented.

1. Introduction

The study of charge transfer colhsions at low relative energies has received limited attention from ex- perimentalists, pnmanly because of the tificulties associated ~th the production and detectlon of low energy Ions [ 11. A few studies of such systems have been performed m which product angular and/or energy distributions have been measured [ 1,2], per- rmttmg an assessment of the dynamics of the charge transfer process_ Of particular interest is the recent

work of Herman and co-workers on charge transfer from Ar+ to NO [3], and Ar+ to Hz [4], the former a case of a reaction with no competition from atom

transfer reactions, the latter proceeding in the pres-

ence of competitive exothermic hydrogen atom

transfer. The Arl$ system has been the subject of

recent theoretical work [S], parucularly with respect to the reactions of H; with Ar on the lowest potential surfaces of the system.

A substantial amount of work has been done on exothennic charge transfer reactions at near thermal energies, particularly with respect to elucidatmg the roles of Franck-Condon factors and energy reso- nances 111 such reactions. The work of Bowers and co- workers [6,7] has suggested that for a wide variety of thermal energy exothermic charge transfer reactions, the combination of favorable Franck-Condon factors and energy resonance is sufficient to ensure a large rate constant. Conversely, high pressure mass

spectrometry experiments on inert gas ions and N$ undergoing charge transfer with N,O, H20, and CO2 [8], and mert gas ions with N20 [9] and Hz0 [IO], studied by ICR trapping and detection of tie emission from excited electronic states of product molecular ions, have provided evidence that reactions at thermal energies with significant exothermicities may yield product ions with large relative kinetic enew releases even in the absence of large Franck-Condon factors for production of ions in specific vibrational states.

In the case of endothermic charge transfer reactions, much less is known about the collision dynamics. Techniques applicable to the study of cxothermic rc- actions at thermal energies are clearly inappropriate; this regime calls for molecular beam studies, Such studies have not heretofore been performed on endothermic charge transfer systems to our knowledge, but are ideally suited to measuring quantities such as kinetic energy release in the post-threshold regime. Of particular interest in endothermic charge transfer is the fact that such reactions proceeding near threshold with internally cold reactants cannot satisfy the energy resonance criterion that initial relative kinetic energy equals fiial relative kinetic energy. The presence of internal excitation in the reactants may alIow

energy resonance to be achieved at the expense of converting internal energy into translational energy. Furthermore, internal excitation may increase the rate of charge transfer by facilitating a surface crossing without the internal energy being transformed into

Volume 71, number 1 CHEMICAL PHYSICS LETTERS 15 August 1980

product translatron. In systems of sufficient sunplcity, the measurement of product kmetic energy Ml pro- vide mformatlon on the role oF’~nternal energy, Fran&-Condon Factors, and eneqy resonance re- quirements m promoting endothernuc charge transfer reactions. The ArH: system has received substantial theoreucal and expenmental attenuon m recent years, particularly with regard to the role of surface crossmg in the system A? + H2 [4,5], where reactlon and charge transfer compete. We present m this work a study of the endothemuc charge transfer reaction

G_ + Ar -+ H@ 1x2 + Arf(~p~,~,~,~)

m the relative kinetic energy range from 0.4 to 7.8 eV usmg the crossed beam techruque. A report on the competltlve proton transfer reaction wll be pubhshed elsewhere [I l].

1. Experimental

The elpenments reported here have been carned out on our crossed beam Ion-neutral apparatus de- scribed m earher work from our laboratory [ 121. The H?; beam is prepared by electron unpact on H2 and possesses a detnbution of ~bratlon~ states aven by Fran&-Condon factors For formation of $(u’) From H2(u = 0) [ 131: the mean Hs vibrational exclta- tion IS 0.9 eV. The beam is momentum analyzed, focused and decelerated to the desired collision energy; the energy spread from the source 1s 0.3 eV fwhm with an angular spread rangmg from 3O at 0.4 eV to lo at 8 eV. This Hz reactant beam intersects a modu- lated neutral beam of Ar atoms at 90* in a coflislon chamber held at 5 X low7 Torr with a hqurd mtrogen trapped oil dlffuslon pump. The beam is prepared by supersonic expansion of a rmxture of 2% Ar m H, through a 0.1 mm hameter nozzle and IS coUuna;ed to ?,“.

Product ions are detected in the plane of the beams by an energy anaiyzer-quadrupole mass spectrometer detector which rotates m the plane of the beams; the low energy detection capabiLitIes of this detector have been characterized thoroughly. The energy scale 1s cahbrated by detection of H: formed by resonant charge transfer to the H, car>er gas in the neutral beam; thus measurement-is made at 90” to the &ec- tion of the Ion beam. T_ab energy profiles of Ar’ pro-

96

duced by charge transfer From Hz are measured by a home-built muluchannel scaler controlled by a mini- computer.

In order to deduce the barycentric doubly differential cross sectlon I@, 0) From the expenmental data, we have employed Slska’s iterative procedure [14] to deconvolute the initial spreads 111 beam energy and intersection angle From the data and transform the data to the barycentric system. The global propertzes of the scattering are presented by plotting barycentric (ccm.) polar flux at a particular energy. The angIe averaged barycentric kmehc energy distribution is obtained from

P(E;) = x lc-II(tr, 6,) sin Of , 11

whde the product translational energy averaged angular dtstnbution g(e) may also be obtamed from the polar flus through the relation

g(8) = P(*+, 8) *

3. Results and discussion

Fig. 1 shows barycentnc polar flux contour maps for the charge transfer reaction H’;_(k, H,)Ar+ at three relative energies of O-45,2.86, and 7.8 eV. Also shown m fig. 1 are the barycentnc energy and angular ~stnbutions P(.E~) and g(8) extracted from our data by averaging I(zr, 0) over 6 and ~1, respectlveIy_ The contour maps show quite clearIy that the predommate mechanism for charge transfer is that of the Ar+ product scattering backward wth respect to H$ m the barycentric system, I.e., charge transfer takes place with very little change m the uutial ciirectron of Ar. Such colhsions are characterized by large impact parameters, the presence of an avoided crossmg seam in the entrance channel of the reaction * at RH_H of -0.9 b, correspondmg to an f?Ar_H distance 24 A confirms the long-range nature of the surface crossing leadmg to charge transfer.

The barycentnc angular distnbutions are quite narrow at the highest two energes, wth virtually no intensity for scattermg angles smaller than 140”. How-

* At tfic lot\est co&ion energr. the *PI,, state of i\r+ is closed WC do not mclude It m the Fran&-Condon c&cu- 1JtlOIl at 0 45 Cv.

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Volume 74, number 1 CHLhlICAL PHYSICS LE-I-I’ERS 15 August 1980

ever, at the lowest kmetic energy of 0.45 eV, the dominant backward peak becomes quite broad and a rather flat plateau of intensity roughly 75% of the maxunum appears XI the fonvard direction with respect to the mcommg H;. The angular dlstrlbution IS quite similar to that obtamed at a comparable energy by Herman et al. [3] on the related system Ar+ + H,, we \vlll discuss this simrlanty below.

The translatlonal energy flus dlstnbutlons of the products. P(E;), are also shown in fig. 1 and mdlcate the Important conclusion that at all colhslon energes, the charge transfer process 1s predommantly transla- tlonally endothermic. An endotherrmc process near threshold, takmg place wtth eternally cold reactants, must be translationally endothermic (I.e., the quantity Q=Ei-E+mustb 2 negative); howevrr, the presence of substantial Internal excltatlon III the reactants allows Q to become posltlve if a large enough fraction of the reactant Internal energy is conwerted mto product relative translatlonal energy. The translational energy dlstnbutlons of fig. 1 are marked with arrows at the point where Q = 0, I.e., the energy resonance con- ltion, clearly, the most probable Q value is always negative and ranges from -0.3 to -0.8 2V LII these es- periments. The energy resonance cntenon IS most

closely obeyed for the low2st energy experiment,

where th2 most probable Q value is -0.3 eV, wme d larger net conversion of untial relative translational energy inro product Internal excltatlon occurs as the relative colbslon energy mcreases, leading to most probable Q values of -0.6 to -0.8 eV. The most probable Q values for these ehperunents are hsted m table 1. The negative ulues of Q mdlcate that the reactant

internal excitation (essentially all vlbratlonal excltatlon [ 111) 1s tzof transformed mto product translation, but conversely, the mltlal translational energy IS chan-

Tabla I H: + Ar - products

-_-

Colhslon hlost probable Q Charge tr.msfcr/ energy (2V) (2v) a) proton tnnsrer

---_I__ __

0-U -0.3 = 0.05 0.3 _c 0.05 2 86 -0.8 = 0.1 0.8 5 0.2 7.75 -0.6 *- 0.15 7_8 +5

a) hlost probable Q for charge transkr.

98

nelled mto product internal excitation. The reaction

$(u = 0) + Ar 4 H2(u = 0) + ~r”(‘p~,~)

is endothernuc by 0.33 eV while the production of Ar+(‘PI,l) IS endothemuc by 0.5 1 eV. The most probable product translational energy of 0.17 eV at a colhsion of 0.45 eV shows that translational energy is consumed in overcommg the reaction endothenniclty; the reactant internal excitation is approximately equal to that of the Hz product. At hgher energes, the average internal excitation of the products exceeds that of rh2 reactants; agam, no net converslon of vibrational excitation m the reactants into product excitation occurs.

Further mslght mto the translatronal energy dntri- butlons of reactlon products may be obtained by con- sldermg the Franck-Condon factors for recombination of H$ in a rfistribution of vlbratlonal states with an argon atom to yield Ar’(zP,,2, “PI,,) m a 2 . 1 ratlo. As a first approximation to the H3 product vibrational dstribution, we may convolut< the Franck- Condon factors for the lomzatlon process H~(u = 0) --f G(u) with those for the subsequent recombmation Hi(u) + H,(u’). We have used Franck-Condon factors deduced from the vibrational overlap integrals of Moran et al. 1161 in their computation of H&-Hz charge transfer, their computation Includes Franck- Condon factors for the overlap of H;(u = O-5) with H,(IJ’ = O-3). Stnce the rotational excitation of the resctant Ions is expected to be small [l 1] and evidence etists for the predommance of small AJ transItions in charge transfer [ 171, we have not included rotational energy m our computatloq. Usmg recombination en-

2rges for Ar+(‘P,,~,1,~) of 15.755 and 15.932 eV respectively [IS] and the structures of $ [ 121 and H, [19], we have computed a translational energy distnbutlon for the products Ar+(zP,,,,,,,) + H2 (u’ = O-3) ansmg from reactants H;(u = O-5) + Ar. The calculation IS performed by assuming a statlstlcal ratio of 2 . 1 for Ar+ zP3,z : 3-P1,, and mcludes an average over the 0.1 eV bandpass of our detector.

Fig. 3 shows the computed Franck-Condon translatlonal distnbutlon P(Q), plotted as a function of Q = EG - ET. compared with the expenmental translational distnbuhons, also plotted as a function of Q. Of particular interest is the fact that the maximum m the calculated distnbunon 111 the range Q = -0.6 to -0.8 eV arises from a superposnion of Franck-

Volume 74, number 1 CHEhlICAL PHYSICS LETTERS

IO-

P(Q)

w

-2 -I 0 -I

Q= ES E, (eV)

Fg. 2. Center of mass translational energy fluzz dlstrlbutlons P(Q) plotted as a function of Q = ET - ET, compared wth I-ran&-Condon model at mdlcated colhslon energtes. Lines at the top of the figure mdtcate the Q values correspondmg to productlon of Hz m vibrarlonal states u = O-3 from Hz III states O-5, ~9th Ar’ m the ‘P3/, state. Q values for Ar* (zPt/~) production for correspondmg molecular trangtlons are more pontivc by 0.18 eV.

Condon tranntions, the malonty of which originate in

excited vibrational states of H:. The presence of ex-

cited wbrational states m the reactant beam creates a

local maximum m the P(Q) dlstnbution near Q = -0.8 eV, but the model predicts substantial intensity for

values of Q both more exothermic and more endo-

therrmc than the observed maxunum. At lower colli-

sion energies, Q = -0.3 eV 1s the most probable trans-

lational exothermicity, substantially smaller than vaI- ues seen in the higher energy experiments because the available energy IS insufficient to place an additional

0.5 eV into internal excitation. Smaller Franck-

Condon factors near Q = 03 eV suggest a smaller charge transfer cross section, as we observ2 experi-

mentally. The observed P(Q) distribution at 7.8 2V is quite broad, exhibiting none of the structure apparent III the Franck-Condon calculation. Part of the width of the distribution anses from apparatus r2solutio11,

but the data sugg2st clearly that a broader rang2 of product states is populated at higher collision energies.

This charge transfer system is competitivz with proton transfer [ 1 I], the El&Q, H)EIAr+ reaction being exothermic by 1.3 2V. As reactants approach in the entrance channel, vibrational energy facihtates crossing

the seam onto the charge-transferred surface. A co& sion system which undergoes a transition to the surface correlating wth Ar+ + H, will certainly be re- flected back into the entrance channel and if the system remains on that surface through the r2gion of the seam, charge transfer occurs. One must also consider the probability that a system remains on the reactive $ + Ar surface, reaches the inner repuIsiv2 wall of the reactive surface, and, rather than “turning the comer” and becoming proton transfer products, is re- flected back into the entrance channel where crossing the seam leads to charg2 transfer. W2 have measured the ratio of charge trasfer to proton transfer by in-

tegration of our measured lab differential cross set- tions over energy and angle; the results ar2 shown in table I. The processes are quit2 competitive at lower

energies, while the charge transfer process becomes increasmgly dominant as the collision enerw incr2ases. Clearly at low energies, the details of the lowest po-

tential surface in the reactive region as w2ll as in the repon of the seam must contribute to the branching ratio.

A comparison of our results with those of Herman et al. [4] on the reverse reaction Ar+ +- H2 + Ar -I- E$ IS m order. Their investigations over a comparable energy range show that the charge exchange occurs by two distinct mechanisms, on2 an electron transfer reaction taking place at long rang2 yielding a narrow dstribution of product fiial states with Q = 0, the second intrmate cotion mechanism yielding a wide distribution of product final states and most probabIe Q values slightly positive. The angular distribution for

our experiment at 0.45 2V compares quite w2l.l with

that for the Herman experiment at a comparable energy, lendmg credence to an “intimate collision” mech-

99

Volume 74, number 1 CHEhlICAL PHYSICS LETTERS 15 August 1980

._Lp- -- _

.- l

-=e__ -__

. c

. c- =.._ - : _

. = -_ c .

0 1 . - .

I I -*-* _i

2 4 6 e IO E;(N)

Fs. 3. Center of mass translational energy flu\ dlstnbutlons for long-range electron trarsferI $T < 0 < n (closed cucles). and mtunate transfer, 0 < 0 C zn (open cucles), at a colhslon encrg of 0.45 eV.

amsm for small impact parameters m our case as well. We have computed translational energy Distributions for our data at 0.45 eV, both for the long-range electron jump channel (fn < 8 < z) and for the “mtlmate collision” channel (0 < 0 < in) by integrations over the appropnate rallges of 0. These distributions are compared m fig. 3, where one sees that the energy &s- tnbution for the intunate colhsion mecharusm 1s substantially broader than that of the long-range electron Jump channel, the most probable translatIonal energy somewhat larger m the mtimate colhslon case. The shapes of the two dlstnbutions mdlcate that the mtimate colhslon mechanism is less selective in the fmal vIbrational states of Hz formed m the reaction. Herman et al. [3] reached a sunilar conclusion m their work on Ar+ + Hz.

A major factor in the lfferent dynamics observed here and reported for the reverse reaction by Herman arises from the high vlbratlonal excitation of the reactants m our case. The dynamics of competitive charge transfer and rearrangement on the lowest reactive surface must depend qmte sensitively on the internal excitation of the reactants. The theoretical work of Baer and Beswuzk [5] on this system, usmg close cou- phng techniques demonstrates also that the vlbrational- ly adiabnbc surfaces for the approach of H;(J) to Ar depend sensiuvely on the vlbratlonai quantum number LJ of the reactant H?;. One thus expects dynarmcal ef- fects ansmg from the potential surface in the reactions of \nbrationally excited reactants which are not present for vlbratlonless reactants. The role of vlbrational excitation in the endothemuc charge transfer

system reported here 1s clearly of cruc~.I importance m interpreting the reaction dynamics and suggests that studies conducted with partial state selection [20] in the s_ beam are in order; such studies are currently under way m our laboratory_

Acknowledgement

731s work has been supported by the US Depart- ment of Energy. Support of early stages of this research by Research Corporation and the American Chemical Society Petroleum Research Fund IS also gratefully acknowledged_

References

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[2] JI. Paulson, F. Dale and S.A. Studnlarz, Intern. J. hlass. Specuom. Ion Phys. 5 (1970) 113.

[3] Z. Herman, V. Pa&k, A.J. Yencha and J.H. FutreU, Chem. Phys. Letters 37 (1976) 329.

[4] P.M. H~crl, V. Pacik and Z. Herman, J. Chem. Phys. 67 (1977) 2678.

[S] hf. Bscr and J.A. Besaick, Phys. RCV. A19 (1979) 1559. [6] MT. Bowers and D.D. EUeman, Chem Phys. Letters 16

(1972) 486. [7] J.B. Laudenslager, W.T. Huntress and h1.T. Bowers, J.

Chem. Phys. 6 l(1974) 4600. [8] P-W_ Harland and K.R. Ryan, Intern. J. hIass Spectrom.

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(1979) 247. [IO] R. Derm, S. Fenatem, hl. Gerard-Am, T.R. Covers, R.

hfan, G. hIauclaire, C.Z. Profous and C. Sounsseau, Chem. Phb s. 44 (1979) 65.

[ 1 l] R-M. Bdotta, F.N. Preunmger and J.M. Farrar. J. Chem Phys., to be published.

1121 R.hl. Bllotta, F.N. Preunmger and J-hi. Farrar, J. Chem. Phys. 72 (1980) 1583.

[ 131 D. VLlIareJo, J. Chem. Phys. 49 (1968) 2523. [14] P_E. Saks, J. Chem. Phys. 59 (1973) 6052. 1151 S. Cluprnan and R.K. Preston, J. Chem. Phys. 60 (1974)

650. [16] T.F. Moran, MR. nannery and D.L. Albntton, J. Cbem.

Phys. 62 (1975) 2869. [17] K. Bukmsbaw and J.B. Hasted, J. Phys. B4 (1971) 1711. [ 181 C.E. Moore, Atormc Enera Levels, NatL Bur. Std. US

Cucula~ 467 (1948). [ 191 G.P. Huber and G. Henberg, Constants of dmtomic mol-

ecules (Van Nostrand, Prmceton, 1979). [20] Z. Herman and V. Pac&,Intern. J. Mass Spectrom. Ion

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Documents

Low energy crossed beam study of the endothermic charge transfer reaction H+2(Ar , H2)Ar+