Luminescent charge transfer reactions in the N+ + CO system

C3emica.l Physics 55 (1981) 313-329

North-I-loltand Publishing Company

D. NEUSC&ER, Mnx-Plnck-Znstirutf

Ch. OTIINGER and S. ZIMhiERhiAblN SSnLimungsforschung.Cotingen.

West Germany

Received 11 August 1980

Optical emission from N’ + CO co&ions io a beam-gas arrangement was studied ia the energy range 3-1000 evtab. The total luminescence cross sectioq has a maximum of 1 AZ at 20-100 eVlab_ At 20 eV it amounts to S?S% of the total charge transfer cross section. The measured spectra consist of wektructured CO’(A-XJ emission at h > 3500 A and, in the mediumenergy range, a quasicontinuum between 2500 and 3500 A. This is due to CO*(A) emission froni high o levels (10 to > 26). A computer simulation yielded detailed rotational and vibrational distriiutions. SevereI distinct reaction mechanisms appear to contribute, one of&em possibly involving CO*(a4Z’) as a precursor.

1. Introduction The system N’ + CO is composed of atoms from the middle of the periodic table, which therefore each have a very open electronic shell structure. This manifests itself by the fact that these three atoms as well as their positive ions ali have several low-lying excited states, in the energy range up to about 5 eV. This in turn gives rise to a rich manifold of electronically excited states of the three-atom system, making it a very promising candidate for studies of electronically non-adiabatic transitions, such as charge. transfer, between the various electronic energy hypersurfaces. These points were fim emphasized by Schlier and his collaborators [ 121. Some of the electronic energy surfaces were calculated in ref. ]2] ab initlo using the SCF.approximation as well as, in a few cases, CI methods. The experiments accompanying these calculations [ 11 gave very precise cross section data for all five possible reaction channels, yielding CO*, C’, e, C&, NO*, inthe energy range 0.2-20 eVl&. Perhaps surprisingly, in view of the multitude bf electronicaIiy’.ex&ed states available, the measured cross &ion curves showed very little struc.tnre at the energies corr&ponding to_@ onset of formation ofthe &ous possii@ excited products. This .Rn&ng~ssone of the.~.motivations Of the-present work; in whit%8 syst&tic_&& w& p&rrncd for optical t&l&ion from N+ -+.CO c%i$lcms in a beam -. -._. : -,:-. d 30l~l~/8l~OOOO-ClOOO~olt

experiment over a wide collision energy range. By these means the formation of excited products which radiate into lower states wi+&lifetimes on the order of microseconds or less can be detected with great sensitivity (down to cross sections of =lW3 AZ) and at the same time, from the spectral analysis. with high specificity regar&ng the internal energy states of the products. The optical ion-impact experiments are thus complementary to the total cross section measurements. While they provide only data on products formed in radiating states, they are capable of particularly high sensitivity and energy resolution. Previous experiments of this type done in our laboratory have demonstrated the usefulness of this approach (see, e.g., ref. [3]). In the present case it turned out that the most important luminescent product charme& for emission in the 2000-8OOQ A range, was formation of electronically excited CO+, i.e. luminescent charge transfer. In addition, some atomic lines (0, N) were observed in the long-wavelength part of the spectrum. VUV emission was not searched for, although NO+(A), emitting near 13001500 A, is a possiile reaction product. The CO* results are also interesting in relation to o*er luminescent charge transfer reactions with CO, as studied earlier by ~_using rare gas ions as projectiles [4,5] ; Additional information on the.p + CO system has re&ntly_beenobtained by kinetic+mergy measurementa &a&ii& products which were done in this

02.50 0 North-Holland Pnbiishing Company

laboratory using the same instrument as in the present work, but in a different configuration [6]. The charge transfer reaction h*+CO+N+CO+

(1)

is exoergic by 052 eV if both reactants and products are in their electronic ground states. Formation of CO*(A’ H) and COi(B* Z’) is endoergic by 2.00 and 5.14 eV, respectively. Furthermore, particularly in beam experiments like the present work, the participation of electronically excited, me&table reactant ions N+(’ D) and N(‘S) must be considered. These reactants would make-the reactions more exoergic by 1.90 and 4.05 eV; respectivelyRate constants for charge transfer (1) at thermal energy were measured by Fehsenfeld et al_ ]7,8] and Anicich et al. [9] _ Their results correspond to effective cross sections of about 100 A2 _Drift tube experiments [lo] showed a drop of the total charge transfer cross section to about 10 A2 at 1 eVc_,_ coilision energy. This is about three times greater than the very precise guided bear results of Frobin et al. [I], the discrepancy probably being due to the collision energy spread in the drift tube work [lo], in a region where the cross section drops sreeply with increasing energy. In ref. [l] at 3.7 eV,_,_ a cross section minimum of 1.99 A2 for reaction (1) was measured (the total cross section.being 3.56 A*)_ At 133 eV,.,_ CO+ formation and total cross sections of 334 A2 and 6.9 A2, respectively, were measured [I]. The reactions-of me&&able N(‘D) iona with CO were studied by Rutherford and Vroom [ll] between 2 and 330 eV,,_. Below 15 eV=_,_ N(rD) was found to have a total charge transfer cross section of about 40 A2, much greater than ground state Nt3 P). However, the latter cross section increases strongly with energy, while the N(’ D) cross section drops slightly, so that at 330 eV,,_ the ratio u(’D) : o(’P) is about 2.5. We will show below that in the present work’ excited N’ ions do not contribute noticeably to our results.

The apparatus has been described previously [3]. It consists basically of an ion source, a mass selector, a reaction chamb,er, and an optical spectrometer with associated photon counting detection electronics_

Instead of the spectrometer used in ref. [3], in the present work the particularly fast v/2.0) Jobin-Yvon model HL grating monochromator was employed, as is also described in some detail in ref. 1121. In the wavelength range mostly studied, A d 5000 A, an EMl6256 photomultiplier was used, while at X > SO00 A an RCA C 31034 tube was employed. Spectra were recorded with 25,12 and 6 A resolution (fwhm). Data collection times were between 1 and 19 hour per spectrum, at a CO target gas pressure of 10 mT0r-r. Measurements of the light intensity as a function of target gas pressure, done at various collision energies and a different wavelengths, showed a perfectly linear relationship up to at least 20 mTorr, indicating that only bimolecular collision processes are responsible for the light pmduction. In these pressure dependence experiments corrections were applied for the attenuation of the ion beam in its passage through the reaction chamber. The attenuation amounted to 47% at 10 mTorr. The N’ ions were generated from N2 in a plasma ion source run at 0.1 Torr. A low cathode-toanode voltage, VA = 45 V; was chosen in order to ._. mmmnze the metastable N’ ion content of the beam and to prevent the formation of Nz*. The residual fraction of metastable ions in the beam was determined using the differential attenuation method [3,13,14]. Fig. 1 shows the intensity of the ion beam measured behind the reaction chamber, as a function of CO pressure in the chamber. Two components of the fall-off can be distinguished. The more strongly attenuated component is filtered completely out of the beam as soon as the pressure exceeds 3 mTorr. I

I

Fig. 1;SpmIIog pIBt of tie N’ ion beam iniensitytrak&d

throughth_e z&ion p-..

+axhber, as a fuktiod of t&get g

i

--

-.-

1.

D. Narschiifer et al. /Luminescentcharge transfer reactions

315

The filtered beam is then attenuated purely exponentially. it consists of ground state Nfcl?) ions, while the proportion of the excited N’ ions, which constitute the more strongly attenuated component, can be determined from the extrapolation top = 0 of the exponential fall-off (dashed Iine in fig. 1) to be 6%. In our experiment the optical spectrometer views a section of the ion beam located centmhy between the entrance and exit slits of the reaction chamber. In order to remove the Nf (‘D) component at the center of the chamber, twice as high a pressure is necessary as for removing it at the chamber exit. Fig. 1, referring to the beam intensity after traversal of the full length of the chamber, shows that 3mTorr removes N+(’ D) at the exit slit, so that in the center of the chamber Nf(t D) will be filtered out at pressures of 6 mTott

or higher. The results.shown

below

were taken at 10 mTorr, so that they should be due to N+(3P) reactions only.

3. Results Fig. 2 gives a survey of all the low-resolution (25 d fwhm) spectra measured in this work. The spectra shown are uncorrected for the spectral response cf the apparatus, which is included as a dashed line with the 2 eV spectrum_ Comparison with the known band head wavelengths of the CO+(A’ II-X* Z’) transition as marked shows immediately that nearly all of the structure is due to this transition. Only at 666 eV=.,_ do bands of the CO’@%’ Z”-X2 2’) and CO(b 3 C+-a 3 II) transitions play a role. Also, at E _. > 100 eV, an atomic line (‘rcr.3 appears, which isduetothe3s’PO+3s1Stransitionofcarbonat 2478.6 A. As well as these immediately identifiable features, there appears a fairIy unstructured broad intensity ‘hump” in the spectra between 8 and IO0 or 200 eV,,_. At some energies, e.g. 20 and 30 eV, this quasi-continuum contains most of the total emission intensity_ In view of the variety of possible emitters in luminescent NC .+ CO collisions, some of which are spectroscopicahy not very web known (e.g. m), it &i&perative to positively identify this part of the spectrumThis analysis, by means of a veryc&e’fiiI dtimputer simulation of the~spectrr, is one_of the maihyiopios of _this_work. ._ -~. & ve+ Iojv erier& (2 Ad 3 eV,&_)~ fig; 2 shivs.

.,_..I

I....,...

.

.

.

2(*3-‘mmommnnomxm

A

IA1

A IA1

Fig. 2. Spectra of luminescence from N‘ + CO &oUisionsas a funckn of energy- Most of the emission is due to CO*(A), including the “homp”at 2500-3500 A (see text). The rela-_ tive spectral response of the apparatos is =gIvenby the dashed line. The oidinate scales of the individualpanels have heen multiplied by the factors given.

some CO&-a) and CO’i@-m bands. These features are spurious. They do not arise from ions, but from electron impact excitation of CO. The electrons are released by impact of stray ions on electrodes outside the reaction chamber entrance slit, from where they are accelerated into the chamber. Electron impact excited emissions can be distinguished from ion impact luminescence by raising the reaction chamber to a potent@ more positive thau the ion source. While ali emission due to ions then disapPears, the spectn due to efectron-gas coIIi.5ionspersist. In any case the electron-jnduced background is so weak that it never was a problem at higher colhsion energies where luminescent charge tran&ei bc&mes strong. A detailed analysis, as presented in *therest of this

316

D. Neuscizfer

er a~ /Luminescent

charge transfer

rarctib~

’

.:

LL”-,-“‘,‘~.~

N'+CO .-CO-(A-X) Ec.q =3leV .25A

FWHM

$ 1

LOO0 -

# 5 2

J

2

-

2000.

O[~y)‘yyy,

2oO0

I

2500

3Ow

35OO A

,,.I,

35w

I1

,..I

.

.

ACAI

Fig- 3. H&h-tesolution (6 A fnlun) from N+ + CO collixions taken at 1000 eVm (666 eV,a. Nearly all features are due to the CO+(A-X) transition. The dotted line is a computergenerated CO’(A-X) spectrum. The ordinate scale is in photon munts per channe1. the nwnber of channeIs is 1020. The dashed line is the zero after dark current (50 cts/charmel) SUbtIXtiOIl.

paper, was cmly made for spectra taken at three representative energies, namely 666,36 and 5 eve_,_. Fig. 3 shows a spectrum taken at E = 666 eV,,_ with higher resobrtion (6 A fwbm)_ The heads of the CO+(A--X) subbands are now restrIvedand are marked pairwise by brackets and the (u’. LJ”)symbol. Some other weak features can be identified as atomic lines, CO+@-X) bands in second order and NH(c r H-b r Xi) emission from reaction of w with impurity hydrocarbon molecules_ The dotted line is a computer-geneated spectrum which is discussed below. in f&s. 4 and 5 spectra taken at 30 eV,,_ are shown, again in coq&arison with siuuuh&?dspectra (dotted). The purpose of.fe. 4 is to demonstrafe the agreement of the synthetic tiththe observed spectruq (w&h is identical with the.39 eV spectrum. : show in figY2)_in $e.region of theweh-resolved ~C@(~~~ bands from.@, 0))to (7,O); Fig_ S empha: -i&s * computer simula~on-m-the qii-conti&rm pqt,of~th~ 3O.eV spec@m,-under the much more -: -_ : @iranding conditions of 6 II fwhm resolu&on. I

-In fig.6 a-higher-r&ohio&ectriuitaken

at 5..

AxlO

5OOO

Fig. 4. The 30 eV spectrom from fg. 2 shown together with a simulated spectrum (dotted line).

.

4500

4000

LOO0

[Al

_

eV,,_ is compared with its simulation.

At this energy,

12 A resolution was the limit set by the low intensity_ Some extremely weak spurious CO and CO” emissions (see above) are marked, as is a weak W emission resubing from chemihr&inescent El- exchange reactions [ 161 between p and .impurity hydrocarbons. Fig. 7_givesone exampIe of a spectrum measured in the long-wavelength region. Because of the low

D_ Na*Mfer

r.*o.r-ro

CO (b-a) 1 2 3

et al_ /Luminescent

&

5m-

0 ,,,,,,.,,,,r,,,,,.,,,,,,,, 2ouJ

2soLl

3lm

3sw

UUJ

f.M

5axJ

A CA1

6. Spectrum of luminescence from N* + CO collisions (resolution12 A fwhm),togetherwith a simulated spectrum (dotted). Fig.

at 5 eV,,

detection sensitivity this part of the spectrum was only explored at a few selected energies. Fig. 7 shows the continuationof the CO+(A--x) bands as well as some atomic lines. The latter appear only at high collision energy; at e.g. 20 eV,,_ they are absent. These lines arise partly from dissociative N’ + CO

chae

transfm rm3iont

317

collisions, especially the unresoIved 01 triplet near 7775 A, and partly from electron capture by the @ ions into excited N states, e.g. the resolved NI triplet at 7424-7468 A. The intensities in this wavelength range relative to those at X < 5000 A can be appreciated by noting that the (1,l) to (1,O) intensity ratio, calculated from the Franck-Condon factors and the relative spectmi response, is 0.84. In the h > 5000 A range the optics used to focus the iigbt from the reaction zone onto the spectrometer entrance slit was different from that ordinarily used. Instead of the set of two quartz lenses employed in *he h < 5000 A range;for A > 5000 A the second lens was exchanged for a glass acbromat [ 151. The relative spectral response curve for this system, in combination with the RCA C 31034 photomu!tiplier (incident number of photons per AA and second), is also given in fig. 7. 3.1. Luminescence cross section

The total cross section for collisions leading to light emission was measured as a function of energy. At each colbsion energy the spectraI intensity distribution, after correction for the wavelengg dependence of the detection system, was integrated over the whole wavelength range covered (2000-5000 A). L&M

emitted at wavelengths greater than 5000 A was in&ded in the integration making an extrapolation based on the computer simulation of the spectra (see next section). It turned out that, depending on the energy, 50-60% of the total emission occurs at A > 5000 A, although the actually measured signal drops rapidly towards this limit as a result of strongly diminishing photomultiplier response. The integrated light intensity was then divided by the ion current as measured at the etit electrode of the reaction chamber, giving a relative measure of the emission cross section:

(the CO pressure was kept the same at all collision energies). A certain problem in applying eq. (2) is the measurement of the effective current ii,,- Because of the increasing spreading of the ion beam inside the reaction chamber with decreasing ion energy, the portion of the ion beam viewed by the optical spectrometer may be energy dependent. In order to check this we measured the luminescent charge transfer reactions I-Ie++Ar-,Ar+*

+He

Ar+’ + 4610 A and 4765 A emission,

(3)

for which reliable relative luminescence cross sections in the energy range 15 to 1800 eVm, have been determined by two independent groups (their results were published jointly in ref. [17]). Eq. (2) gave in this case agreement with the results of ref. [17] within 520%. At least to within this accuracy, therefore, the ion current passing through the spectrometer field of view and the ion current measured on the reaction chamber exit plate are proportional to each other over the whole energy range. Consequently eq. (2) gives &heenergy dependence-of the Iuminescence cross section to witbin*20%. Absolute Cross sections were obtained by no&alining the relative values to the absolute cross [email protected] .for the luminescent charge transfer Ar++Nr

+@(B)+AL

(4)

1000 eV. The spectrometer employed in the present work was calibrated by repeating Simonis’ exieriments and using his value for ~(0,0). The error limit of the absolute emission cross section as resulting from the calibration procedure is 30%. An additional uncertainty may be introduced by the intensity. of the emission at X > SOO!lA. This contribution was derived from computer-generated spectra which were fitted to the experimental results in the region A < 5000 A. Beyond th& limit, therefore, some extrapolation of the simulations is involved. However, this would lead to an appreciable error only in the unlikely event that the bands with u’= 0 and t have a considerably different higlM rotational population (giving rise to 5000A)thanbandswitho'2 2 emissionath> whose rotational distriiution falls almost entirely witbin the observed range X < SC00 A. The luminescence cross section determined in this way is only identical with the cross section for production of CO+(A) by the direct process N+ +CO+N+CO+(A)

(5)

if (a) population of CO’(A) by other parallel reactions, e.g. radiative cascades or non-radiative transitions from higher states can be excluded, and (b) all CO+(A) ions, once formed, contribute to emission, ifee.are not removed from the observation region or quenched by gas collisions. AIthough CO+(B -+ A) transitions are ahowed, they can be ignored here on account of the small branching ratlo of 8% of B + A relative to B -+ X and the ve;y small B + X intensity observed. SimilarlytheCO’(ZA+AZll)system [18] canplay no role since the spectra showed no evidence of it in the 2200-2800 .&range (for an example of excitation of this emission by ch&ge transfer, see refi [5]). Other cas+de transitions are not known; although we may have evidence of CO+c C-A? Il) transitions above 8 eV,.,. (&e below)_ In this case, for&ration of CO”e Z) would be included in the cross section determined -. ’ here. : Loss of ionglived CO*(A) by drift out-of the observation region is unimportant, becausethe COf .. io.&are predo xninantl~ formed by glancing &arge transfer collisions &r~‘have therefore l&v kinetic : eriergji: CO~(A)~Ioti by quenching, however, must +e co&i&red. .TI&&osS &tion.for ~enching.of

For this reaction the cross section for emission of. the @(Ii) (0,Oj l&&as beeu deter&&e& by Sims-:. ~&&ij;>t$&&,.-. C@(A)$y-Cd;&&&:&&& :; .:: . r$f5] tobe.u(O;C$= 1.17 x lQG*.8em* at.&, = -1.: ,. aa &&,gly &ith ;_ From &~~p&~&i&f,$

D. NeuscMfer et aL /Luminescent charge transfer reacths

Fig. 8. N” + CO chargetransfer excitation spectra measured at three different CO target gas pressures. to establish absence of quenching effects. These spectra were taken with e different snectrometer - McPherson 218 + E&5 6256 multiplier - which explains the different intensity distribution compared to f=. 4.

II’=‘5--8, up to 50% quenchingmighthave been expected at 10 mTorr CO, while for very high u’ levelsno quenchingmeasurementsexist- We have therefore taken spectraat three cli%erentCO pressures,2,5 and 10 mTorr, see fig. 8. They agreewiti

319

each other to within MO!% or better, so that quenching effects could be ignored under our conditions. The cross section for CO+(A) formation, as obtained from eq. (2) and normalized,is shown in fig. 9. For comparison, the massspectrometricresultsof Frobm et al. [l] are included. The dashedcurve, giving the difference between the two, shows that a large part of the increaseof total CO+ production above 4 eVc_,_ is due to the CO*(A) channel. At energiesbelow 4 eVc.m., the cross section as calculatedfrom eq. (2) goes through a minimum and increasesagaintowardsthe lowest energy attainable. This is due to an increasingproportion of light produced by secondary electrons (see above)_Furthermore in this region some bnninescencecaused by the smahmetastableion content of the N+ beam cannot be excluded. For these reasonsluminescencecross section dataat EC,_ < 4 eV have been omitted from fig. 9. The extrapolatedonset of CO+(A) production (dotted line) agreeswith the theoreticalthreshold of 2 eV,,_ for N'(3P)+CO+NcS)+CO*(A). Above the energyrangeshown in fig. 9, the CO+(A) cross section goes through a broad maximum of about 1 A2 at 30-50 eVcm-, then goes through a wide minimumof 05 II2 at 200 eVc_,_, beyond which it increasesagainsteadilyto O-8A’ at 666 eV,,_3.2. Specmun simulation The synthetic band contours shown in figs. 3-6 were calculatedusingthe methods describedin previous publicationsfrom this laboratory [21,22] The reso!ution profile of the spectrometerwas taken to be triangularas in the earlierwork, except for the 6Aspectra.Hereagaussian profile, adaptedin width to measuredHg fines,wz~used for the first t&e and resultedin noticeable improvement. The prkrcipalresultwhich finally emergedfrom a systematicdevelopmentof successivelyimproved fits was that it is’@sible to representthe observed spectrasolely on the basisof CO’(A-X) transitions... This i@udes, as wiJlbe shown in detailbelow, even the quaskoritinuous broad ‘Tnnnp”which appearsin the medium-energy spectra.Therefore only the simti-

D_ Xeuschiifw et aL / Luminescmr charge tmnrfer reactions

320

Fii_ 10. Potential energy diagram of the CO’ states, together witt the CO ground state. The a4ZC state has not been observed, but may play a role as a precusor to popoIate high v’CO+(A) levels in this experiment. Its approximate position has b-em inferred from the isoelectronic molecules CN and N;.

potentiai curves considered in this work, together with the CO ground state curve. Table 1 lists the CO’ molecular constants employed 1231. FranckCcndon factors for the CO’(A-X) transitions are given in the hterature only up to the vibrational level u’= 11 of the upper state [24]_ Since this was not suff%ient to describe the observed spectra, we calculated Franck-Condon factors (FCFs) up to u’= 26 and v” = 6, based on RKR potential energy curves, using a program by Zare f25]. Where compari-

TabIe 1 MoIecoIar constants of co+ =) A%

X2$

Te

0

we

w&z %Ye Be Lld6

a? re

-:

2214.24 15.164 0.0007 1.9772 0.0190 637

1.1152

--a) re~-2n A,hlLothet constants in cm”.

207333 1562.06 13532 0.0131 15894 0.0194. 6.6 -1175 1.2438

sons are possible the-agreement of our FCFs with the literature values is generally within 1%. It was necessary to assume that the constants listed in table 1 are valid up to the very high vibrational lwels of CO+(A) which under ordinary circumstances are not observed. This assumption could not be verified directly because bands involving these high levels were not individually resolved. However, the relative population distriiution over the u’levels derived below will be rather insensitive against the exact choice of molecular constants and hence of the FCFs since it is found that the distribution is quite flat over a wide U’range. Owing to the centrifugal tenn in the effective potential, FCFs can in certain cases depend markedly on the rotational state of the molecule. In the present case the CO*(A) is highly rotatisnahy excited, but. the J dependence of the FCFs was found to be quite small. For example, for the (3,O) band the FCF was 0252 and 0.253 for J’ = 1 and 35, respectively. The largest absolute changeof any FCF between these J limits was 0.05. Table 2 lists the FCFs for the higher u’ levels, calculated for J’ = 35. These were used in the simulation of the 30 eV spectrum. For the 666 eV spectrum FCFs calculated for J’ = 20 were used, because in this case the rotational excitation of CO*(A) was found to be Iess (see below). The London-Hlinl factors appropriate for the twelve branches of a 2 Ii-* Z transition have been calculated by Earls [26]. They apply for any coupling intermediate between Hund’s cases a and b. The electronic transition moment was taken to be independent of the internuclear distance. This is in agreement with several earlier etridiee on this point [27-291. The relative population of the * Ifrp and * IIs, states of CO’(A) was taken to be equal. The two corresponding heads of subba&Is~ separated by 27 A [(l; 0) band] to 14-A [(& 0) bar$]. [30] are each composa~ofaQaridRheadcmly3tol~apart (unresoIved in fg. 3): The reIa%e iatensi~es of the partly resolved subband heads depend therefore not only on-the *lTIp~ flTsy;Lpopulation ratio, but also in a complicated Way on the lotatioqal pO@lation ,... distribution. The.c&rallXitdf.the aisin&ted spectrum as shopin ii&& 3.does ti&v&rant con&sions as to a pd@ibly +$htly-dif@rant .eII tpz a& 2 RJrz pdj$.&q~ 1-Lr ;..I ..:..(‘,:_.:::f::..: ._;_ ._ ^ ~_@k$.i~@&?&ial aiid~vib,r&o& &ulation _ ‘&tcjbu&rp(j’) G&tin? &&mi&&e most &$ .::

9.

NeustSfm et 0L / Luminercentchargetransferreactions

321

Table2 CO' (A2n-Xz~+)F~ck--Condon~~orsandbandorigiaaraveleagths~~~) ’ d

0”

IO

13

1.12- 2 2894 6.75-3 2792 3.70- 3 2698 2.00-3

14

2612 1.07 - 3

11 12.

15 16 17 18 19 20 21 22 23 24 2s 26

cc

Q

2533 5.69-4 2461 3.01-4 2394 1.59- 3 2332 8.34-S 2214 4.39-5 2220 2.31-S 2170 1.22- 5 2124 6.40- 6 2080 3.38- 6 2039 1.79-6 2001 9.63-7 1965 491-7 1931

u”=l

$

5.75 - 2 3089

6.61 - 2

4.04-2 2973 2.69 - 2

2867 1.71--2 2770 1.06-2 2682 6.39- 3 2600 3.78-3 2526 2.21-z 2457 1.28-3 2393 7.31-4 2333 4.16-4 2278 2.36-4 2227 134-4 2179 7.53-5 2134 4.24-s 2!l93 2.38-5 2054 1.33--5 2017

= 2

33G9 6.89-2 3176 6.20-2 3055 5.03-2 2946 3.80-2 2846 2.71-2 2755 1.86-2 2671 1.23-2 2594 7.98-3 2522 5.07-3 2457 3.17-3 2396 1.95-3 2339 1.20-3 2286 727-4 2237 4.39-4 2191 2.64-4 2149 158-4 2109

0” = 3

v"=4

u"=S

u-=6

3.69-3 3560 2.11-2 3406 4.02-2 3267 532-2 3142 5.53-2 3029 5.12-2 2926 4.33-2 2831 3.43-2 2745 258-2 2665 1.86-2 2592 131-3 2524 8.94 - 3 2461 6.01-3 2403 397-3 2349 2.60- 3 2298 1.68- 3 2251 1.08- 3 2207

352-2 3846 I.@$-2 3667 4.48-S 3507 6.13-3 3364 2.06-2 3234 3.46-2 3117 4.34-2 3010 458-Z 2912 4.31-2 2823 3.76-2 2741 3.08-2 2665 2.41-2 2595 1.82-2 2530 134-2 2470 959-3 2435 6.76-3 2363 4.69-3 2315

3.02-2 4111 4.66-2 3967 3.83- 1 3781 182-2 3614 3.14-3 3465 5.11-4 3331 8.09-3 3209 198-Z 3098 3.03- 2 2997 3.68-2 2904 387-2 2820 3.70-2 2742 3.30-2 2670 2.78-2 2603 2.25-2 2541 1.77-2 2484 135-2 2430

1.03 - 2 456.5

features of the spectra, namely, the extent of the shadirig‘towards the red of eac& individual baud and th&rel@e intensities of the.ba&s respectively. Where the bands tie not individually resolved, such as iu theSpe&um showu in fig. 5 ; both P(J) and P(u’determine j the oVe&ll baud contour.P(J? and P(u’) were in this wosk found by tCal&d+rror. This method works especially @l in the present case owing to the: &ar .tipqativn of many bgds. Figs. 1 l-l 3 give the r&t& rotatiqnal distributions P(J3 arrived tit in this way, forthecm. energies 666,30 and 5 eV, respectively. Fig, 14 shows the .$h&&al distributionP(u~ for~all three energies. It is on these racteristic

1.11-3 4315 195-2 4095 3.73-2 3901 3.14-2 3728 2.37-2 3573 8.46-3 3433 5.36-3. 3307 1.72-3 3192 9.10-3 3087 1.85-2 2991 2.64-2 2904 3s4 -2 2823 3.31-2 2748 331-2 2679 2.92-2 2616 253-2 2557

_.

d&r&ions

P(J) and P(v’) that the synthetic spec(&tied curves) are based. For ~E+XUIS of computational convenience the rotational distriiutions P(J) were defined by three separate ~sectidus:(a) A thermal distribution tra &own in figs. 3-6

P(f)

- (2J’ + 1) exp [-&kc

J’(J’ l l)/kT)]

(61

(see ref. [3lj, p. 1@) with au adjustable “tempemWe” parameter T. This part of PQ’) extends up to J’=J,;(b)Bet~~euJ~ andJ* >Jl,P(~=Cwas asmmedconsiant.(c)FromJa to& >J&‘{J?was tal& tofall oh l&&ly. The five adjustable param-- e;ers&, Jz, J, , -TVC-g& P(J@)sufhient &xibiliiy

D. Akuschiiferet 4L

322

chargetmnsfer reac;ions

/Luminescent

1.0

pm

0.5

, . . i . . , ,

0

50

0

J’

100

FE_ 11. Relative rotational level population of CO*(A,

J

150 0

u’) as

produced by 666 eV,, N” + CO charge traosfei. from spectrum simulation shown in fg. 3. These model distributions approximate the trae distriiutions quite closely in the IowJ’ regime (J’ < 25). while at medium and high I’ the simolatian determines mainIy the area under the P(Y) curves.

for a good fit with the observed spectra at all energies. Fig. 11 shows the distribution P(J’) as used for the simulation of “Je highestenergy spectrum_ Slightly different rotational temperatures T were used for the seven vibrational levels contributing to the spectrum, ffie corresponding curves fw within the shaded band of fig. 3_ The average rotational excitation, as given in table 3, decreases with increasing u’, mostly on account of the monotonica& decreasing “tail” in the 1’ > J1 section. The accuracy of the simulation in this part of the distribution is comparatively Iow, because the corresp&ling waielen,@s are spread out from the band head. For J’>J, the quantity determined best by the simuIation is the area under the P(J’) curve and its trends with U’(e&ted accuracy ~lS%)), wbileP(J’j itselfmay deviate from the model distributions shown by up to ~30% at certain J’. C’I’he“tail” section for u’= 0 (dashed) is based on ti extrapolation u’= 3 + 2 * 1+ 0, because the corresponding rotational lines of the (0,O) band

EC, (ev) 5

-.

30. ,’666

--

~-_. ._

..

of CO+(A) product from N+ + COixGsioas

--

3).

12);66(~=18):70(~~20) 1);41(u = 2); 38(u = 3); 3?@ = 5)

-,_I _. _. <‘.I __.[__

Sl(v=O-z);57(~=?>;til@r

-- 46(u =6);+3(zi=

D

1 . ...

48(r,=o1_2.>-4):-57(v=

I

were largely beyond the long wavelength limit of the measurements.) In the “thermal” region, J’ J, regionP(J‘)isnowmuch mter thqinfig. l,l,_arouad 5O%of itsmaximum

G)

.

100

3’

Fig. 12. Relative rotational level population of CO*{-4. v’) as produced by 30 eV,, N’ + CO charge transfer, fmm spectrum simulations shown in fe. 4 and 5. For error limits, see tig_ Il.

Table3 Mean angulax momentum

50

._

. . _ _: _

:

.;:.

_._

-

.. _. ~._

D. NeuschZfer et QL /Luminescent chargerratzsfzrreactim

value. Thus the average rotational excitation has increased greatly compared to the high-energy colli~ons,asaresultofboththeJ’ Jr regions. The trend of rotational excitation as a function of u’is reversed compared to fig. Il. In the thermal part T is now increasing from 1800 K for u’= 0 to 2300 K for u’= 8. As before, however, u’= 3 is an exception, having an especially high T(3) = 3000 K. The u’= 4 temperature is elevated to a lesser extent, T(4)= 2500 IL The error limits are here about 100 K. The main complication at 30 eve_,_ is that very high vibrational levels contribute to the emission (see fig. 13), which give rise to the “hump” in the spectrum, fig. 4, between 2500 and 3500 A. Because of severe band overlap, rotational distributions cannot be derived for U’> 10 with the same precision as for lower u’. For example, trial simulations were done in which for v’2 10 P(J> was taken to be thermal for all J’, with a value of T even higher than those given in tig. 12. These distributions average out the “kinks’ in P(J’). The result was almost, though not quite as good as that shown in fig. 5. Fig. 13 shows P(J> for 5 eV,.,. collision energy. The rotational excitation is higher still than at 30 ~Vc.m_,both in the “themxd” part and in the “tail”. In the former, T is now 2500 K for the low u’levels, but again with marked exceptions at v’= 3 and 4,

323

has now increased to about 75% of the maximum of P(J’). The shaded ar~indicate the error lhnits. Witbin these boundaries a change of P(J> has little effect on the shape of the synthetic spectrum. The vibrational dissbutions for the three coIlision energies are shown in fig. 14. Error limits are indicated at a few representative points. A few comments are in order: Firstly, near u’= 3 all three curves lie somewhat above the average falI_off drawn through the lowenergy part of flu’). Thus not only the rotational, but also the viirational distributions have an anomaly here. Secondly, the 666 eV simulation is in the v’= 7-10 range only based on a 25 A spectrum and is therefore iess reliable than the 5 eV simulation, particularly on account of the overlapping C!O(a-b) emission. The difference between these two distributions indicated in fig_ 14 in the high u’ range may therefore not be real. At low u’(O-3), however, the 666 eV distriiution dearly exceeds the others well outside the error limits. Thirdly, at 30 eV,,_ P(J) extends to much higher vibrational levels than at high and low collision energy. The dir+ tribution as shown breaks off at u’= 26 because FCFs were only calculated up to that point. It is likely, however, that levels d > 26 are also populated. They would mainly contribute to the emission around 2600 iX, where in fact the simulation as

z3)=5W)(3KandT(4)=40001dThe’~‘,J’>JI, 1

L

1.0

_-____

PDI

Em

= 666eV

-E,

=3oev

--E,

=

5-v

.O

:/-so

0

I?@ 13:RefativerotationalIeyelr~+&~n of kO+(A, u3 as produced by 5 eYcm. N’ + CO’charg~ transf+ from spectmm simulatidn shown in f+ 6. The shaded areas indicate

Grmrlimits.

‘.

.:

-

_. -:

.:

~.

.._~..

:

:.

;.

_ .^

:

:.I-:

i

.

0 FK_ 14. Relative viitioxtal

10

Ym

20

k&l p~puhtions

producedtry N++ CO chargetrsnrfer.

of CO+(Al

D. Neuschaferet al. /Luminescent chargetmnsfc reactions

324

shown in fig. 5 falls somewhat short of the observed spectrum. Inclusion of levels u’> 26 would require less population of the levels at about u’= 20-26 than shown in fig. 14, so that the complete vibrationai distribution is expected to fall off monotonically towards the highest bound CO+(A) levels (fig. 14, dotted line). Although the exact amount of the contribution of high u’levels is contingent upon the validity of the calculated FCFs up to u’> 20 (see above), the general success of the simulation of the 30 eV,.,_ spectrum in the 2500-3500 A region proves beyond any doubt that the emission is due to CO*(A) in very high vibrational states. Even without simulation, it is then evident from the spectra shown in fig. 2 that l Jlese high u*levels are appreciably populated in the whole energy range between 8 and 100 eV,,_. This identitication of the “hump” in the 2500-3500 A region demonstrates well the pcwer of the spectrum simulation technique. Without such detailed analysis the hump might have been ascribed to NO fl band emission. However, the obse_ved pronounced structure around 3300 _&can deftitely not be explained in this way. Also the fl band system extends below 2500 A, and in the 4500 A range it is in conflict with the clearly developed CO*(A) band structure. 4. Discussion 4.1. C@(A)

rotational excitation

The product rotational excitation in luminescent charge transfer reactions has been studied thoroughly onIyinthecaseofNt *. In the keV collision energy range, g(B 2 Z,‘) rotational temperatures on the order of 500-3000 K were usually measured. In some experiments an increase of ‘Tmt with decreasing collision energy was found [33] _Some+&es deviations from a Boltzrnann distribution were observed 1341. In comparison, charge transfer with CO to give CO+ (A2 II) has been studied relatively little [4,33,3541], ami then mostly with emphasis on the vibrational state distriiution. Simonis found in Iow-resolution experiments 1421 that the rotational excitation of CO+(A), formed by charge transfer with Ar’, was considerably higher at 50 eVnb than at loo0 eVJ& _ energy, the same trend as in the s experiments. With *

For referencessee ref. 1321.

Ne+ + CO, however, the band contours indicated little rotational excitation, both at 50 and 1000 eV. The Ati + CO case was studied more quantitatively and with higher resolution by Ottinger and Simonis [2 1] _Here computer simulations showed that the CO+ rotational distributions were approximately thermal, at Ieast in the IowJ range, with T’,, = 1000 K and 5000 K at ,!?I&,= 1000 eV and 87.5 eV, respectively (although the deviations from a thermal distributlon were considerable at the lower energy [43])_ By way of contrast, the rotational excitation of CO+(A) formed reactively by Cc + 0s collisions was found to be extremely high, the distribution being that for T,r = 45000 K with a high J cut-off PIThe

present work appears to be the first accurate study of the rotational excitation of CO’(A) produced by charge transfer. The general trends of the results are the same as in the A? t CO and some of the I$ experiments, with the mean rotational excitation decreasing towards higher collision energy, and at the same time the distribution becomicg more thermal. The decreasing trend is as would be expected: Analogous to the classical Franck-Condon argument for the vibration, the rotation of the nuclei, too, should be little perturbed in the limit of a very fast encounter. The rotational distribution should then be thermal with T,, equal to the gas temperature in the reaction-chamber (WOO K)_ For the vibrational distribution it has been found in many charge transfer experiments that this Fmnck-Condon limit is not reached ilntil the collision energy is well above 1 keV 1441. It is therefore plausible that in our experiment Tmt = 650-850 K at 1000 eVt& is still somewhat elevated. With decreasing collision energy the projectile-target interaction becomes strange;, as is reflected by the increasing rotational excitation. A simple model has been proposed by Liu [45] which gives the amount of angular momentum transfer AL in a charge tra&fer reaction which is either endoergic or exoergic by AE. Assuming the atomic projectile of m to follow a straight-line path with impact parameter b and a lab system energy E, one fmds in a good approximation for the change of the orbital angular momentum of the .&stern ,

LX = b -A&/ti)1, AL = 1 lb M(h/E)1’2

:

(7)

D. NeuschZfer et al. /Luminescent charge transfer reacrkwts (E, AE in eV, AL in fr, m in au, b in A), independent of the target mass. The change in orbital ar&ar momentum is compensated by an equal amount of internal angular momentum of the target molecule after the collision. AL is in this model a consequence of the energy change AE associated with the reaction, and tends to zero at high collision energies_Note that this model violates energy conservation, because the rotational energy transferred to the target is ignored. However, the rotational energy at J = 20 of a CO” ion is only 0.1 eV or 5% of AE. The rotational state distributions measured m this work clearly consist of two components which apparently result from two different mechanisms. The “the&’ component at low J is responsible for the main band structure of the spectra, while the “tail” at highJ produces the unstructured background intensity from the zero line in figs. 3-6 (which corresponds to the Ijhotomultiplier dark current of Gl cps) to the level of the valleys between the band heads. Without this tail, e.g. with a purely thermal 800 K distriiution, one obtains a synthetic spectrum which drops a!most to zero between the bands [e-gbetween the (2,O) and (3,0) bands to ~1% of the peak heights] and is clearly unsatisfactory. We attribute the “thermal” component to the Liu-mechax&m, i-e, to glancing collisions, while the ‘Yail” at high J results from more intimate collisions. With the latter, the angular momentum transfer is accompanied by a momentum transfer and deflection of the trajectory, and AL is determined not just by the asymptotic energy balance LIE- The most probable angular momentum values of the distributions shown in figs. 11-13 are, for IJ’= 0, Jm+_ = 12,19,21 fi at E = 1000,45,75 eVtarr,respectively. Using these J m.p. values for &L in (7), (the initial Jmop_= 8R of the thermal CO molecules is ignored because it averages out due to random orientation) one calculates b = 4.6,1.6,0.7 A in the above order. These are all impact parameters of a plausiile order of magnitude, although, as b becomes smaller, the model of a “glancing” collision would be expected to become less and less appropriate. This is-m complete accord with our observations, which show that the portion of the rotational d&iiution due to glancing collisions is dominant only at 1000 eV, (fig_ 10). At 45 eVm, both mechanisms contribute to a similar extent (fig. 1 l), while at 7.5 eVu the “glancing”

325

collisions are hardly recognizeable as a separate mechanism, because they require, from (7), an impact parameter so small as to lead inevitably to stronger interaction. It is interesting to apply eq. (7) to the excitation of u’= 5, where AE = 3.0 eV_ At each energy one now finds a smaller b value than before (using the slightly A, respectidifferent Jm+J, namely b = 29,1.2,0.5 vely. A smaller impact parameter would facilitate vibrational excitation and at the same time enhance the “tail” of the rotational distribution relative to the “thermal” part. This, too, agrees with the observation, except in the 1000 eV, case where the lower . u’have a more prominent rotational ‘w The u’B 20 levels in the 30 eV,,_ spectrum canno? be discussed in these terms. They are probably populated via different potential hypersurfaces, as is discussed below. The anomalously broad rotational distribution observed for these particular product levels also points to a separate mechanism. This distribution is almost as broad as that measured for CO+(A) produced by Cc + Oa ion-molecule reaction [21] _ An important difference is that in the case of the exchange reaction all vibrational Ievels have a very broad roational distribution, while in the present charge transfer experiment this is only found for the very high levels populated at intermediate collision energies. An ideal limiting case to test eq. (7) would be a reaction with AE = 0, i.e. resonant. F’ ions, with a recombination energy of 17.43 eV, would populate the CO+(A, u’= 5) level with an energy deficit of only 0.03 eV [30]. At the same time the FCF for the ionizing transition CO@, u = 0) + CO*(A, u’= 5) is fairly large (OLl9,compared to the maximum value of 021 for u’= 2 formation [30]). These conditions, if simultaneously !XfrLled, favour long-range charge tram&et with large cross section [3]. The “‘glancing’ model leading to eq. (7) should then apply, and eq. (7) predicts zero angular momentum transfer at exact resonance_ In preliminary luminescent charge transfer experiments on the system F‘ + CO 1461 we have indeed found a strongly selective population of the uc= 5 level (exceeding v’= 4 and 6 by a factor of 2.7), with umneasurably small rotational excitation. The rotational structure of the (5,0) a.IIsn’ E+ subband was resolved, using a resolution of 1 A fwhm,intheJ=15-26range.Asimpleanalysis.

D- Neuschiifer et aL /Luminescent

326

charge transfer rmctio~

Table 4 Relative viirationd

level population

a) of CO*(A) produced

&,b = 75 ev

by charge transfer reactions NC. Ar*. Ne+ + CO b, Ebb = 1000 eV

Et& = 45 ev

UC

N+

Ar+

Net

N’

AI*

Ne*

N+

Ar+

Ne+

0 I 2

0.53 0.24

05.5

0.15

0.38 0.14

050 033

0.46 0.16

0.40 0.17

0.11 0.07 0.03 0.02

0.13 0.08 0.05 0.04

0.17 0.10 0.10 0.11

0.12 0.10 0.03 0.02

0.12 0.09 0.08 0.09

0.16 0.10 0.09 0.08

0.35 0.27 O-20

0.38 0.20 0.15

0.35 0.21 0.18

0.11 0.04 0.03

0.11 0.09 0.07

0.12 0.07 0.01

3 4 5

a) Normalized to unity for the sum U’= O-5. b, N+ data from spectrum simulation of this work. Ar*. Ne+ from band head intensities measured

yielded a rotational temperature of 360 -C50 K, which agrees with the gas temperature in the reaction chamber. A more precise analysis employing spectrum simulation is in progress. 4.2. CO*(A j uibmtional exitdon The relative vibrational level population as given in fig. 14 and in table 4 can be compared wit& the distriiutions reported with Ar’ and Net impact on CO in ref. [4] _This has been done in table 4 and shows a very surprising degree of agreement_ In all three cases the dist&utions peak 2t v' = 0 and then fall off monotonically in a very similar manner * although with Ne’ and Are not quite as fast zs with ;u’. Furthermore th2 f&-off is much the same at each energy for alI three projectiles. Both the similarity of P($ for different projectiles and the weak energy dependence are in sharp contrast to the case of s(B) as produced by charge transfer. Here the vibrational excitation was found to be markedly energy-dependent, and different for different primary ions [47] _Neither ffie s(B) nor th2 CO’(A) vibrational distriiutions can be explained by Fran&-Condon transitions from the parent moIecules. For example. the FCFs for CO@, u = 0) -+ CO’(A, v’) transitionsareintherati08.8: 19.7: 24.1:215: 15.7: 10.0 (in % of their sum) for u’= O-5 [30] -On. this basis peaking of the CO+(A) vibrational distribution at u’= 2 might have been expected, while the. + Corresponding. 3000-6OOO

roughiy.to

a therm& f&off

K over the lowest

few 6.

with Tyib =-

in ref. 141.

observed maximum is at u’= 0. Jipeles [37] attempted to explain this by bondstretching of the parent molecule just prior to the charge transfer. Specifically, he attriiuted the stretching to the’target polarisability. While in our earlier paper [4] we advocated this qualitative model, later quantitative studies by Simonis [5] showed that it cannot describe the process fully. The dependence of the polarisabiities (parallel and perpendicular to the m0IecuIa.r axis) as weU as of the quadrupole moment on the target bond length is not strong enough to give a sufficient bond stretching, except for unreasonably small distances of the approaching ion from the target (< 1 A, a distance too small for the electrostatic potential model to apply). The problem is aggravated in a dynamical picture. Simonis [S] showed by trajectory calculations that an even closer approach of the projectile, up to 0.5 A at-1000 eV, would be necessary to stretch the bond sufficiently during the co&ion: However, his caIcuIation also showed considerable vibrational excitation ofthe target molecule. Furthermore, he demonstrated that Fra&kCondon transitions frdm a v&rating CO molecule to the CO*(A) state would givl a CO+(A) @K&MI distriiution not too diffirent from what-is observed: Speci&Uy, as long ask0 is viir&onall$ eic&d at all, whether into u = 1,2-or 3. a s&sequ&nt FranckCondo? &n&on will always_ @opul&c t& CO*(A) u’= 0 level most. TXrefoF, even th&glLthe vi&ational excititioti of the CO.target &I vii& -&depending on the energy and th&.@4+ile ion; ti will have. comparatiyely. little effect 0 the f&l C@+),~b&~ tioml dist&Uti~~ -+li th&_ &ulatioIis,-wgL+-

D. Neuschzifer et al.

/Luminescent

will be presented in detail elsewhere, can explain the observed maximum of p(u’) at IJ’= 0 as well as its insensitivity against collision energy and projectile ion species. The same model of vibrational target excitation followed by a Franck-Condon type electronic transition have also been used by Keliey et al. [48] to calculate, by means of Perturbation theory, vibrational distributions of ionic reaction products. The arromalously high population of the u’= 3 level shown in fig. 14, especially at 30 eV,,_, does not appear to be present with Ar* and Ne+ (table 4), although the analysis was in that case done without spectrum simulation and therefore less accurately. The effect in the N’ case is certainly genuine, and not due to e.g., an erroneous detection sensitivity calibration at the (3,0) band wavelength. We have checked the (3,0] band intensity relative to the neighbouring bands using a different spectrometer and again found an enhancement_ Also the unusually high rotational excitation of the 0’= 3 level points to some special effect in this case. The vibrational enhancement could in Siionis’ model be due to that fraction of CO molecules which are excited into u = 2 before charge transfer, as the FCFs from ref. [S] show. The absence of a u’= 3 enhancement with Ar* and Net would then indicate a slightly different CO vibrational excitation distribution with those ions. Finally, let us discuss the high-u’ portion of P(u’) measured at 30 eV,,_. This component gives rise to the peculiar Tmmp” in the mediumenergy spectra, as was explained above. Fig. S shows that this hump is not without any structure. The undulations on its short-wavelength flank are, however, clearly not individual bands, hecause in contrast to the band structure visible near the peak they do not show a sharp head and shading to the red, but are much rounder. The simulation revealed that they are in fact band sequences of a special type. For example, the inset in fg 5 shows between 2800 and 2900 A the heads of the bands (lO,O), (12, l), (14,2), (16,3), which form a head of heads. The lengths of the lines in the inset indicate the relative FCFs. It is very striking that in charge transfer of CO with AI+, Ne+ [4] as well as He+ [40,49] and H’ 2461 no “hump” appears in the CO+ spectra_ Because N+CP\ has a different spin from the ram gas ions, the +e of spin conservation would allow formation of CO* quartet states with N’, but not with the rare

charge transfff renctions

327

gas ions: N+cP) + CO(’ Z’) -+ N(4 S) -I-CO+

(quartet or doublet)

Ar+(zP) + CO(’ E’) -+ Ar(’ S) + CO+ (doublet only). CO+(A) in high u’levels might then somehow be formed via the quartet CO’ states. Although no quartet states of CO* appear to have been observed, they have been calculated ab initio [50]. A ‘C* state is well-known in the isoelectronic molecule CN. It has been identified by level crossing spectroscopy [Sl] and perturbation of a rotation level [52], and its potential curve minimum has been calculated ab initio to lie 4.02 eV above the CN ground state [53]. In I$, also isoelectronic with CO+, ‘Xi + X ’ Zg emission has been reported [54], although this was questioned later [55]. If correct, the observation would place the N;(a”ZZ;3 state at 3.2 eV. This state has also been calculated 1561 to he at 3.5 eV above the s groune state. This is probably about 0.5 eV too high, as a comparison with the A “&and B ‘Zz states as calculated in the same work against +he experimental value suggests. In any case both in CN and in gz the a 42’ states seem to be close to the corresponding B *I;+ states. Assuming this to be also to,be true for CO+, reactions involving CO* (a”Z> would be about 5 5 eV endoergic. This conclusion supports the hypothesis that the observed quasi~ontinuoua hump in our spectra originates via the 48+ states, because according to fig. 2 this feature seems to just be appearing at 5 eVc_,_, but is strongly present at 8 eV,,_. Further tests were carried out to support the hypothesis that the spin of N+(3P) is the determining factor. We observed [46] CO*(A) production by charge transfer using two other triplet projectiles, OH+(3 Z-) and F”(‘P). In the former case spectra very similar to the mediumenergy N+ spectra, i.e. exhi%iting an intense quasi continuous hump, were indeed observed atpOand75eV~~.~neF’cCOspectza(fram25 eV,, to 50 eV,), on the other hand, this feature was completely absent. This can perhaps be explained by the overwhehning competition of the near-resonant charge transfer (see above) which has a very iarge cross section. OH+, with a recombination energy of 13.2 eV; certainly mimics the N+ collisions much better. Spectra from C* + CO and 0’ + CO collisions

D. NeuschZferet ~1 /Luminescent chargetmnsfw rexactions

328

at 40 eV,

were also briefly studied, because both C’e P) and O’e S) could populate CO’C E+) with spin conservation. ‘ihe observed CO*(A) emission showed a hump in the C' , although not in the O+ case. The exact mechanism by which CO’(a 4 Z’) might act as a precursor to CO’(A, bigh 0’) formation is not clear. Co&on-induced processes can be excluded, because spectra taken at 2,s and IO mTorr show no discernible differences (see fig. 8). Rzdiative transitionsa4Zf-+A21ishotlldbeweak,bothasaresult of the-spin change and the small energy separation. Perhaps nonradiative transitions between the outer ?imbs of the two curves take place. Here the viiirational wavefunctions have large nii ,@vingahigh transition probability, especially, as fig. 10 shows, into high u’levels of the A state. This would be consistent with the observation of A + X emission from these same levels, which rakes place near the inner turning point. Perhaps even very high u levels of the COc(X2 ZZf) state are involved in the a + A transition, because it has been shown 157-l that X2 C‘A 2 KIcoupling is important _ An entirely different explanation of the high-v’ population, which does not involve the a 4 Z’ state, is suggested by ihe NCOf potential curves calculated by Wr; and Schlier [Z] _They show *tit the collinear approach of N+ f CO can occur on two electronic energy surfaces, 3 Z- and s II, of which the former is attractive and the latter steeply repulsive, for either orientation of the CO moIecu!e. Non-adiabatic transitions populating the product channel N + CO’(A) may then occur in two distinct regions, depending on whether the initial approach is along 32- or 311. Possibly it is the collisions following the 3 l-i surface which make the transitions somewhere high up in the repulsive region, and therefore i&o high-u’levels of the CO+(A) state.

Icvels u’f 8 could be immediately identified, emission from high vibrational 1eveLs;up to u’= 26, could only be recognized as belonging to the COf(A-X) transition by means of a careful computer simulation of the spectra. This technique revealed great detail of-the vibrational and, for the E-st time with CO+(A), rotational population distributions, particularly for the low u’levels. The rotational distriiutions consist of a ‘YhernS component, with rotatiodtemperatures between 650 and 5000 K depending on u’and the energy, and a “tail?‘, extending from J’ = 30 to =150. The former shows some Qends which are consistent with a glancing collision model, while the latter require harder collisions. The most interesting feature of ‘rhe viirational distributions is the population of U’> 10 levels at medium (e.g. 30 eV,,.) energy. Comparisons with other charge transfer spectra, using raregasions,HC,C+,O’,OH+,andF+ asprojectiles shed additional light on the reaction mechanisms, in particular on the consequences which energy resonance has on the vibrational and rotational distribution and on the importance of spin conservation. Under certain conditions, CO+@” C’) may be formed as a precursor state.

We are gratell to Professor Ch. Schlier for stimu. _ chcuaons. The spectrum simulations were done on the computer of the Geselkchaft fiirwissenschaftlithe Datenverarbeitung, Giittingen. The apparatus was in part financed by the Deutsche ForschungsgemeinscbafL : . latmg

References [ 11 W.

Frobin. Ck Schlier;Ki Strein and E. Thy,

phys. 67

5:Condnsion

This work has shown that in 1owAergy .p i CO. co~ons.~~..ody mjor @minesgent pgodu$t is CO+(A), w&l? re5nangeinent products like (2@ or CN aji: Fpt formed in elecgron@lly excited s+tes. At its maxim@n of 1 A*, the cross section for CO’(A) formation ~otm+ to~%%of &e_fdal Q* &oducthn~~W@ile;tI;e well-structured bands f&n CO*(A)

:_

(1977) 55il5.

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