Volume 113, number 1 CHEMICAL PHYSICS LETTERS 4 January 1985

TRANSLATIONAL ENERGY SPECTROSCOPY OF NO+ IONS

FORMEDBYCHARGETRANSFERFROMAr+

Anthony O’KEEFE I, Renee DERAI * and Michael T. BOWERS Departmenr of Ckmutry, Univenrty of Gdifomio. Sanra Barbara, Glifcnmia 93306, USA

Rcccrvcd 6 Aprd 1984;in final form 5 Septcmbcr 1984

Transhrtional energy speclroscopy (TES) of NO+ ions formed by Ar* charge exchange has been studred. The two fealurcs observed in the spectrum are assigned to transitions from the U” = 0 and possibly u” = 1 and 2 levels of the a3Z+ state to the

low vibrational lcvcls of the w 3n and b’ 3C- states. Comparison wilh prcv~ous TLS spectra of NO’ formed by electron tm-

pact is reported and dcmonstratcs the high sclcctivrty of the charge transfer reactson rn popuIating the first excited stale of NO+_

I _ Introduction

Translational energy spectroscopy (TES) of NO+ rons formed by electron impact has recently been studied in this laboratory [l] and the observed peaks have been asstgned on the basis of the potential energy curves constructed by Albritton et al. [2] assuming that the ion ?oFGation is similar to that obtained III photoelectron spectroscopy [3,4] _ The main features observed are the A Ill + X ‘C+ electronic system

around 9 eV, and several weaker peaks wmch were assigned to transrtlons into the triplet manifold. Little is known about the spectroscopy of NO+. only the A t X system has been rotationally analysed [5-71. Maier and Holland [8] observed emissions oflong-lived states of NO* which could be due to b 311 + X rZ+ and b’ 3x_ +X lZ+ transitions while in a laser photo- fragment spectroscopy experiment, Cosby and Helm [9] studred the predissociated products of the 2 %l + b’ 3Z- transition.

The number of states of NO+ formed by electron impact LS quite large and, since TES has been shown to be very sensitive to the initial state of the ion [ 101,

t Prcscnt address: Chemistry Divrsion. Naval Rcscarch L&o- ratory, Washinpton. D.C.. USA.

2 Permanent address- Laboratolrc dc kkonsnw Jkctromquc ct lonlquc. UruversitC de Pans&d. Orsay, France_

a way to sunphfy the spectroscopy would be to start from ions produced in a more specific way. Charge transfer from thermal Ar+ ions is helieved to form NO+ ions in the low vibrational levels of the a 3C* state [11-l 31. The only radtative transition available to this state, a ?Z+ + X ‘I?, is dipole forbidden. Hence, it has a long lifetime, estimated to be 313 s [S], making it relatively easy to study. TES of NO+ ions formed by the reaction

Ar++NO+NO++Ar

is examined in the present paper. The TES spectrum of ions; is obtairzd by measunng

the kinetic energy changes induced by collision with a structureless neutral gas, helium. For high collision energy and small scattering angles, these changes are directly related to the changes in internal energy cor- responding to transitions between drscrete levels of the ion [ 14]_ These transitions are due either to inelas- tic collisions in wluch the kmetic energy of the ion is used to drive a vibronic tranution or to superelastic collisions where the gam in ion kinetrc energy corre- sponds to a transition from an mrtially excited state to a lower state.

The selection rules for such transrtions have already been discussed [I]. The electric-drpole optically allowed transitrons are usually the more intense, spin forbidden ones are severai orders of magnitude weaker. Smce

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Volume 113, number 1 CHEhltCAt PHYStCS LEl7ERS 4 Junumy 1985

the excitation process Is a collision between two heavy pxticles, the change in orbital angular momentum, AA, as well as the change in rotational levels, AJ, is not limited to 0, *I and some transitions such as A + E0rE:- + E+ may bc observed.

The arnngcmcnt for the expcrunent has already been dcsxibcd [ 141. It consists of a conuncrcial dou- blc kcusing rcversc-gcometry mass spcctmmetcr, ZAB2F (VG Analytical Ltd.) with a source specially design& [IS] to nllow long rcvdcncc times (lo-20 11s) of the ions. At I~C cxlt of the source. the ions nre accclcratcd at 8 kV, mass analyscd by the magnetic sector and then focused into a collision cell containing helium at a prcssurc of 1 X 10S3 Torr. The main beam and its scattered ~OIII~OIWIIIS m-c ~ACII cncrgy onalyscd through the clcctrostatic nualyscr of the iustrumcnt.

The resolution of the cxperhncut is limited in this study to 0.15 CV by the cncrgy spread of the incident beam and the resolving power of the clectrostotic onn- lyscr. In the prcscnt study, this was insufficient to allow the rcsolutiou of any vibmtionnl structure.

In order to study NO* ions fomlcd by chcn~icnl ionization from Ar+, lhc condiliox in the source hnvc been optimized to lower the contribution of NO+ dirccrly produced by clcctron impnct: the pressure of NO is mniutoincd ns low ns po&blc, <1 X 1O-4 Torr, rclativc to thot of hr, 5 X lo-’ Torr. If, as cxpccted, both EU~~~OIWI~~S ?P tiz and zb,2 of Ar+ arc fonncd by electron impacl. lhcn most of the NO+ ions w!U bc ini!ially in the U” = 0, I and 2 1evcls of the n 3x.+ slate. Although the U” = 2 lcvcl lies nt 15.97 cV. Lc. just above the rccomblnution energy of 2Pt,z Ar’. 15.94 cV, it can also bc populotcd, considcrlng the hinctic cncrgy of llic lhcrmal Ar+ ions, =50 mcV at 475 K, the csthuotcd source tcnrpcmturc. Slncc the number of cohisions in the SOUL-CC is bctwccn 10 nnd 20, the NO+ ions can react back with argon. This rcnction gives mninly Ar+ ions mthcr than NO+ ions rcloxcd by collisions to the ground stntc [ 12,16]. Energclically thcsc Ar+ ions must bc formed In the ,,p 3,~ slalc from NO+ a 3C+ u” = 1 and 2 (the u” = 2 state reacting three times faster than the u” = 1) 1161. Conscqucntly, lhc 2P3,z Ar+ population will incrcnsc OS will tlrar of NO+ 3E+ u” = 0. since formation of

94

NO+ a %+ u = 1 from Ar+ 2P3,2 is endoergic by 0.06 eV (SC-~ f@. 2 for the relative energies of these levels). Thus the population of NO* ions cxlting rhe source will mainly be in the n 3x:+ Y” = 0 srate.

3. Results and discussion

The k.tr.ctic energy of NO+ formed by charge cx- chanpc: with At+ and scattered off helium is presented In ftg. 1. Only the energy loss side of the spectrum is rcportcd stnoc no superclastlc peak was observed. Two features labclcd A and B arc located nround 1.25 and 1.95 cV rcspectivcly, the fwhm cnn be estimntcd for both of thcnr to bc of the order of 0.5 cV. For com- pnrlson the TES SpCtiUnl of NO+ formed by electron impact is shown iu Bgs. 2a ond 2b. Both clnsttc nnd supcrelastic peaks arc seen in thcst spectra indicnting n significant dlffcrencc In states fomled by electron impact and churgc trnusfcr. lltc transition cnergles nnd suggested nssiguments arc pmscntcd In table I_

Slncc thcrc is good cvidcncc tlrnt the mnln bcnm in thcclr:lrge tmnsfcr study Iscocstltuted mostly of NO+ a 3z+, and since spinthanging trunsitions arc usually very weti- in TES, A and B of Bg. 1 must bc nsslgncd to tmnsltlons within the triplet manifold. The stntcs involved arc thus expcctcd lo be ones which nppcur in the photoclcctrou spectrum of NO. Whtlc TES should also rcvcal states which nrlsc from configumtlons la which clcctrons arc promoted to previously empty or- bltnls, such cxcitntlon ofNO+(n %5?) involves n spln change and so would bc very wcnk In TES. ‘llrc potcn- tlol cncrgy curves ofthc low lytng lrlplct slntcs of NO+ in the cncrgy mngc rclevnnt to the present study arc prcscntcd in l?g. 3, togctltor wltlt tltc TES spectrum.

Tltrec upper stntcs, b 311, w 3A and b’ 3B’. arc cnn- dldntcs to cxplnin the observed A and B trandtlons seen in fig. 1. Only the b 3ll t-a 3E+ tmnsltion is op. tlcally nllowcd. nnd the FC factors for excitation of the a 3x*, u” = 0 co b %, u’ = 0.1 and 2. ~1-311 could bc rcsponsiblc for pcnk A, hove been calculntod [ 1 i: : nil nrc quite h&h. so thnt the resulting trnnsltion would cxtcnd from 0.9 eV (0 + 0) to 1.3 CV (2 t- 0), l.c. nt much lower cncrgy than A (see Bg. 3). Its contribution to A must couscqucnlly bc low.

ho strong nrgumcnts tend to favor the attribution for pcnks A and B of fig. 1 lo w 3A + a 3x+ and b’ 3x_ + n 3Z+ transitions respectively. First, although

NO+ from

Ar++ NO -Ar + NO+

L I I I I I I I I I I I I

100 90 6.0 70 60 50 4.0 30 2.0 IO 0.0 l V

Fb. 1. Spcclrunl oT wwlnlhul cncrgy loss of NO+ ions Cormcd by clrxjic cxclxmge Wilh At*. The peak nt 0.0 CV corresponds to I~IU matn beam. A resolution of 0.15 cV fwhm wns obtalncd In llils spectrum.

NO+

b

NO+

I I I I 1 I I

-3.0 -20 -I 0 0 IO 20 30

0V

Fig. 2. (IA) Spectrum of tnmslntlonal cncrgy loss oC NO’ ions formed by clcctrnn Impart of NO over m oncrg range covcrlng 215 CV nbout the mdn bonnI (strong pcok II center ofl3.g~~). (b) High-resolution trnnslntlonal cnorpy spectrum or NO* Ions formed

by clcctron impact oT NO. The pcahs lnbclcd A-F are Idcntlllcd In table 1. A rcsolulion of ~0.1 wns obtained In this spectrum.

Votume 1?3. number 1 CHEMICAL PHYSICS LEITERS 4 Janunry 1985

Table 1 Trunsilions observed in the tmnslalional energy spectrum of NO+

Transition Tnmsition Assignment cncrSy a) (CV) width (cv)

NO’ formed by cha-c transfer rcaclion from Ar+

+1.25 * 0.05b) -0.5 W3A+a32*

+1.95 f 0.05 c) =05 b’SX-c a3x +

NO+ Formed by c-impact (ref. [I])

~0.6 d) 410-2 W3A -b3n

21.2 4 CO2 bB3,-,,,3n

*1.90 >0.3 b’ %C- W3A

- a3C

a-9.0 Id 2.5-3 0 A1l-ItX1~+

3) The signs + and - rcfcr to intemnl energy gain and loss ic,qWcLtvcly.

“) Peak labclcd A in l-ii 1. =I Peak lobdcd B in Iii. 1.

d) Peaks labeled B and C m ii. 2b. =) Peaks lab&d A and D in J-II Zb. r) Peeks lnbeled E and F in fig. 2b. g) Pczk al - 9.0 cV in Jig. 2a_

FC factors for these trantiitions are not available, the relative position of the potential energy curves is such that good overlap for Au = 0 transitions is expected in both cases_ Moreover, the calculated energies for transitIons between the low vibrational levels of these states, presented III table 2, also indicate a very good agreement with cxpenmental values for Au = 0 for initial states U” = 0, 1 and 2. This agreement is also

illustrated in fig. 3 for a main beam of NO+ ions in the a %+, Y” = 0 state, where the posltions of the two peaks A and B match very well the u’ = 0 of the w 3A

and b’ 3X_ states, respectively. The low intensity of B relative to A in fig. 1 can be

explained in the following way: a transition C- + Z+ Occurring tn a coUision implies that the change in rota- tional levels is no more limited to 0, +l. The selection rule concemmg the symmetry of the rotational levels must still be satisfied, so that only AJ even are possible These transitlons are then expected to be weaker than the ones for which such restrictions do not apply, such as 3A +- ?Z+-

The results are compared in table 1 with the data previously obtained for NO* formed by electron im- pact [ 11. The two spectra are very different:

- The absence of superelastic peaks in fig. 1 at -0.6, -1.2 and -1.9 evmdicates that the excited elec- tronic states formed by electron impact are not pres-

iO-

0

I

i

.--- *

_______-

TES No+

Fig_ 3. Potential energy curves for the low lying tip:?t of NO+ (from ref [Z]). The energy of the two components >fAr+,

2p3n and 2P1 R, and the NO* TES spectrum from iiF_ 1 xc

also reported. The spccu-um has been positioned ~~ruming thnt the ions in the main beam IWE mainly in the a31: ‘, U” = 0

state. The symbols, fob. A and h represent the relative inten- nties or the FC factors for ban, U’ = 0, 1 and 2 c 3 3 2+, U” =

0 (from ref. [ 171).

ent m the mam beam of NO+ produced by charge transfer from AP. Those states were assigned to the de-excitation of the w 3a and b’ 3X_ states whose formation by APIN reaction is too endoergic to be observed.

Table 2 Calculated energies for transillons between w 3A - a 3E+ and

b’3z-+ a3Z* mound 1.25 eV (peak A offii. 1) and 1.95 eV (peak I3 of Iii. 1)

“’ + U” Peak A W3P.-,3x*

O+O 1.21 1.94 1-o 1.38 2.10 O-1 1.05 1.78 I+1 1.22 1.94 2+-l 1.38 2.08 lt2 1.07 1.79 2t2 1.23 1.93 362 1.37 z-09

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Volume 113. number 1 CHEMICAL PHYSICS LETTERS 4 January 1985

- The same remark applies to the inelastic peak in fig. 2 at a.6 eV, the b 311 state lying at higher energy than both components of Ar+.

- The complete absence in the Ar+/NO system of the transition at 9 eV, attributed to the A +X system, is more interesting; It clearly shows that NO+ ions in the low vibrational ievels of the ground state X ‘Z? are almost completely absent in the NO+ main beam formed by charge transfer from Ar+. This confirms that, in the present experimental conditions, the elec- tron impact contribution to NO’ formation is negli- gable. Another consequence IS the NO+ X ‘Z+ Ions do not appear to be formed m the Ar*/NO ck.arge trans- fer reaction If some high vibrational levels of the X state around u” = 28, i.e. energy resonant with the _4r+ recombmation energies were formed, then the A ill +X lYZ+ transition would appear at much lower

energy, around 4.2 eV, where there is no feature in the TES spectrum. This confirms that these states are not formed, in agreeement with the very low FC fac- tors calculated for their production [ 121. The upper limit of 15% determined by Dotan et al. [ 121 for this process can be considerably lowered, and, if it occurs,

formation of NO+ X lZ-c by Ar+/NO reaction contnb- utes less than 5% to the total production of NO*. In addition, the relaxation of NO+ a 3X+ by colhsion with Ar,

NO+(a 3Zf) + Ar + NO+(X lx+) + Ar , (1)

appears also to be a minor process as previously claimed

[Ia. - The two peaks A and B in fig. 1 lie at the same

energies as two of the features seen m the TES spec-

trum of electron impact produced NO+ (fig. 2). The one at +1.2 eV in fig_ 2 was assigned in ref. [I] to a b’ ?Z- + b 3lI transition and could not consequently

be oSsrrved in this work where only the a SE+ is in- volved_ The peak at +1.9 eV in fig. 2 was attributed to a bf3Z- + a 3Z+ and/or w 3A +-a 32+ both transl- , tions being likely to explam the weak peak B. Assign- ment of 1.2 eV transition in electron impact formed YO+ was made considering its narrowness [l] ; elec- tror. impact, unlike A_r+ charge transfer, populates a very large number of vibrational levels of the a ?Z+ state and a very broad transition would be expected. The feature observed in ref. [l] has a fwhm of 0.2 eV, i.e. smaller than peak A of fig. 1, and is_consequently u&kely to be due to a transition origmating from the

a %S+ state. On the contrary, the u = 0 level of the b state is populated by electron impact with a high effi- clency, which makes it a better candidate to explain a narrow structure. The fact that the two transitions are both at 1.2 eV seems to be a coincidence but does

suggest caution in interpreting either assignment as unambiguous.

4. Conclusion

The TES spectrum of NO+ ions produced by Ar+ charge transfer suggests a very high selectivity of NO+ states formed by this process. Most of the ions are formed in only a very few vibrational levels of the frost excited state a ?Z+ and less than 5% in the ground state. The two peaks A and B observed in the spectrum

at 1.25 and 1.95 eV are assigned to Au = 0 transitions from the low vibrational levels of the a 3x+ state to w 3A and b’ ‘IZ- states. Comparison with the TES spectrum of NO+ formed by electron impact indicates that the techmque is very sensitive to the initial state distribution of the ions. The common feature observed at -1.2 eV in both spectra is probably due to different transtions.

Acknowledgement

The support of the National Science Foundation under Grant CHEBO-20464 is gratefully acknowledged. One of us (RD) also wishes to acknowledge the sup- port of the North Atlantic Treaty Organization for a travel and subsistence senior fellowship.

References

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[*I

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[41

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Vohmc 113, number 1 CHEMICALPHYSICS LETTERS 4 January 1985

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