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MOLECULAR RYDBERG SPECTROSCOPYMAGNETIC FIELD EFFECTS IN ALKYL HALIDES

S. Mcglynn, J. Scott, W. Felps

To cite this version:S. Mcglynn, J. Scott, W. Felps. MOLECULAR RYDBERG SPECTROSCOPY MAGNETIC FIELDEFFECTS IN ALKYL HALIDES. Journal de Physique Colloques, 1982, 43 (C2), pp.C2-305-C2-315.�10.1051/jphyscol:1982224�. �jpa-00221835�

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Colloque C2, supplément au n°ll, Tome 43, novembre 1982 page C2-305

MOLECULAR RYDBERG SPECTROSCOPY MAGNETIC FIELD EFFECTS IN ALKYL HALIDES

S.P. McGlynn, J.D. Scott * and W.S. yelps

Chemistry Department, The Louisiana State University, Baton Rouge, LA 70803, U.S.A.

Chemistry Department, The University of Montana, Missoula, MT 59812, U.S.A.

Résumé. - Le moment angulaire total de tous les niveaux du premier complexe s de Rydberg des molécules HI, CH3I, CD3I et CH3 Br se révèle être quantifié, et cela même dans les molécules de symétries 'C pour lesquelles la brisure de symétrie im­plique l'absence d'une telle quantification. Ceci suggère que la symétrie effective est celle d'un "bit" moléculaire dans lequel la transition est localisée tandis que le reste de la molécule se comporte de manière perturbatoire. Les moments angulaires des différents états vibroniques ont aussi été mesurés. On a trouvé que des vibra­tions non totallement symétriques couplées à la partie électronique produisent sou­vent des changements importants du moment magnétique. Une approche perturbationnelle complètement moléculaire permet d'expliquer toutes les observations. Elle comporte des termes décrivant (i) la déviation de la molécule de la non-linéarité .(ii) les effets Renner-Teller.(iii) les effets Jahn-Teller.

Abstract. - The total angular momentum of all states of HI, CELI, CD-I and CK.Br in the 1st s-Rydberg complex is found to be quantized, even in Cjy molecules where symmetry-breaking decrees the absence of such quantization. This observation sug­gests that the operative symmetry is that of a molecular "bit"" within which the transition is localized and that the remainder of the molecule is merely perturba-tory. The angular momenta of various vibronic states have also been measured. It is found that non-totally symmetric vibrations coupled to electronic origins often produce profound changes of magnetic moment. A perturbation approach of "whole molecule" nature is developed and is capable of explaining all observations. The perturbation expression includes terms descriptive of (i) deviation of the molecule from non-linearity; (ii) Renner-Teller effects; and (iii) Jahn-Teller effects.

1. Introduction. - There exists no reason why the phenomenology associated with the Rydberg states of molecules should be less formidable or less interesting than that for atoms. Indeed, the molecular realm should be considerably richer. It is worth emphasizing, therefore, that the diatomic molecule, while a good starting point for molecular excursions, is not a truly representative "molecule". Indeed, the dia­tomic molecule cannot exhibit (i), vibronic effects, such as Renner-Teller or Jahn-Teller [1] phenomena; (ii) , optical activity of molecular nature; (iii), stability, when the molecule is loaded with more vibrational energy than the minimum required for its dissociation; (iv), interactions of discrete-continuum nature (i.e., auto-dissociations) which branch into more than one dissociative channel; (v), anti-resonances caused by interactions of a discrete state with a bound-bound intravalence transition unless, for some reason, the density of states in the bound-bound continuum is abnormally high; and (vi), a total symmetry-breaking of angular momentum quantization. For these reasons, this work will emphasize non­linear polyatomic molecules.

Zeeman measurements on polyatomic molecules are not usually feasible: Molecular band widths are normally larger than the splitting produced by accessible cryogenic magnet field strengths. Consequently, resort to other experimental Zeeman variants is necessary. The variant used in this work is magnetic circular dichrolsm (MCD) [2,3]. The experimental technique, particularly in the VUV region, is a difficult one which requires the whole panoply of constant-intensity high-flux VUV sources; CaF. optics; MgF„ photoelastic modulators; high-field (~7T)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982224

C2-306 JOURNAL DE PHYSIQUE

end-corrected, long-path length (-lOcm) cyrogenic magnets; and detectlvity ade uate to the task of extracting information from a background which may be 3 to 5 or%ers more intense. Nonetheless, the MCD technique is the method of choice because of the ease of analysis of the data and the fact that all the detail inherent in conven- tional polarized Zeeman spectroscopy is retained.

The MCD measurement is defined as the difference of absorptivity of left-handed (lcp,-) and right-handed circularly polarized (rep,+) light of a particular frequency v in a field-on situation. That is

where H denotes magnetic field strength. The corresponding theoretical expression [ 2 ,4 ] when the ground state is non-degenerate and certain small approximations are imposed is

where the quantity a carries all the information.on the paramagnetism of the excited state and b, the diamagnetic component, describes field-induced state mixings. Furthermore, it is expected that b << a6 -The advanta e of Eq. 2 is best utilized by simultaneous generation of BOA;) aA (v)/as ?nd.bAfi (;) versus 7 curves. A "followingw of the AAH(;) and aA (vl/a; plots lndlcates that the excited state at J is paramagnetic and that it possesses a magnetic moment proportional to the ratio of curve ordinates. A following of AAH(;) and A'(;), on the other hand, indicates diamagnetism of the excited state at v.

EXPERIMENTAL RESULTS

The results for the 1st s-Rydberg Complex of CH I, methyl iodide, are shown in 3 Figure 1. This ccnfigurational excitation, 5p -t 6s of the iodine component, yields four exc'tpd states which;in order of increasing energy, are denoted [ 5 ] 3112, 3111, f - 3110 and II1. If the ground state, '.Y+be labeled 0; then the excited states may be labeled 1,2,3 and 4, again in order of increasing excitation energy. The corresponding transitions (O'+ 1, 0 + 2, etc.) are simply labeled 1,2,3 and 4 in Figure 1.

Fig. la: Absorption, derivative, and MCD spectra of methyl iodide.

Fig.. lb: Absorption, derivative, an&^ spectra of methyl iodide.

Origin 3, which is occluded by a hot-band 4;3 in CH I, becomes distinct and 1 3 .

unobscured in CD I because of a large decrease of the vibrational intensity of V3 3 in the per-deutero compound. The measured magnetic moments (in Bohr magnetons)

are given in the following tabulation, where they are compared to that expected on the basis of simple (A,C) - coupling.

EXPERIMENT PREDICTED

STATE HI CH3 I CD31 CH3Br ( A , C) - - -

4 1.3 1 1

3 0 0 0

2 1.34 1.6 1 1

1 3 2.7 2.9 3

Results for the 1st p- and d-complexes of CH31 are shown in Figure 2. Certain states may be identified immediately on the basis of absorptivity, MCD signal-type and MCD a-term signal intensity.

, , , . , , , '--I 61 60 59 58 57 56 55 54 - ENERGY IkK)

Fig. 2: Absorption and IiCD spectra of methyl iodide.

Fig. 3: Absorption, derivative, and MCD spectra of methyl iodide.

These states are labelled in Figure 2. Certain vibrational activities are also evident in Figure 2: A three-member progression of a totally symmetric mode built

1 on the Il origin is readily identified, as are two single-member progressions in 1 each of two different modes built on the I E + origin. The power of the MCD technique for WV studies of molecules is amply demonstrated in Figures 1 and 2.

We now emphasize the existence of some oddities. It is found that a non- totally symmetric vibration built on origins 2 or 4 may exhibit an inverted a signal, a normal a signal, a b signal or even a null signal. A clear example is given in Figure 3, where the 2;61 band exhibits a paramagnetic characteristic of equal magnitude but opposite sign to that of the origin band for state 2 (see Figure 1). Indeed, it is now known that coupling of non-totally symmetric vibrational modes can convert an a-type origin into a-, -a-, b- or 0-vibronic

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s t a t e s . It is with t h e meaning of these a l t e r a t i o n s t h a t the remainder of t h i s work i s concerned.

I. PRIMITIVE MODEL

The ob jec t ive i s a model of t h e various e l e c t r o s t a t i c fo rces t h a t a f f e c t e lec t - ron ic motion i n molecular systems, a model which, simultaneously, can account q u a l i t a t i v e l y f o r spin-orbit coupling i n t h e severa l l i m i t s . Since t h e ul t imate purpose is t o provide a context within which t o discuss MCD spec t ra , t h e model must f a c i l i t a t e p rec i se statements concerning magnetic phenomena.

The bes t zero-order s t a r t i n g point i s t h e l i n e a r ( t r ia tomic) molecule i n t h e (A,S)-spin-orbit coupling l i m i t . I n t h i s l i m i t t he space and sp in coordinates remain separate and simple considerat ions of e l e c t r o s t a t i c ( s p a t i a l ) per turbat ions can be indulged. Concurrently, one may consider o ther spin-orbit coupling limits i n t h e (A,S)-basis, remembering t h a t , because of the azimuthal space isotropy, t h e angular momentum = fS2h (5 being the u n i t vector i n the in te rnuc lear a x i s ) , must be t h e same, i n a l i n e a r molecule, f o r a l l spin-orbi tal ly mixed s t a t e s .

The most convenient s p a t i a l coordinate system is t h e cy l indr ica l system: s , in te rnuc lear ax i s ; p , perpendicular d i s tance from z; +, azimuth from an a r b i t r a r y but f ixed plane containing z. The e lec t ron ic wavefunction, expressed i n these and t h e sp in coordinates , S and C , has the form

where p, B and i n d i c a t e t h e s p a t i a l coordinates of a l l e lec t rons . The space- dependent funct ion, $0, may be expressed a s

where ?i represents t h e s e t of phase angles between $ and t h e azimuth $ f o r each individual e lec t ron (where b. r e f e r s t o t h e k th e lec t ron) . I n o tker words, ak = $k-$ and a =;{ak}. ' k ~ o r an n e lec t ron system, n.may be expressed i n

n ~ e r m s ef the ind iv idua l A . s a s A = I A ( ; A . = 2 J j = l j J

j -'L/h A.

Although A i s r e s t r i c t e d t o p o s i t i v e values, A . f o r t h e j t h ind iv idua l e lec t ron may be e i t h e r p o s i t i v e o r negat ive according ta the contr ibut ion which t h a t e lec t ron makes t o the t o t a l momentum J. The phase funct ion, F,, is

where p o s i t i v e and negative s igns d i f f e r e n t i a t e t h e double-degenerate components with A # 0.

I n add i t ion t o t h e various spin-orbi t coupling cases , we w i l l now t r e a t a l l o ther systems a s perturbed (A,S)-coupled, l i n e a r t r ia tomics. Since these l a t t e r per turbat ions w i l l be e l e c t r o s t a t i c i n na ture , t h e i r primary e f f e c t occurs only on the s p a t i a l funct ions. These systems a r e

(i) Non-linear molecule (loss of spatial isotropy in $);

(ii) Vibronic coupling in a linear triatomic;

(iii) Vibronic coupling in a non-linear molecule.

However, before proceeding with the electrostatic perturbations, the general effects of spin-orbit coupling will be reviewed in the context of the zero-order model. In strong-coupling cases, where & and lose uniqueness and R remains as the only "good" electronic quantum number, excited states which are electronic-dipole forbidden in the (A,S)-limit may gain spin-allowedness by coupling with states having S = 0 (and C = 0). The original "parent state" then gains spin-allowedness by mixing with contaminant singlet states. This terminology, of course, is meaningful only inthe context of perturbation theory. The mixing occurs because the spin-orbit perturbation Hamiltonian contains terms that are tbtally symmetric in the double group ~ 2 ~ . Thus, R of the parent state and ~2 of the contaminant state(s) must be the same. Since the basis is formed in the (A,S)-limit, it follows that (IL + = (A + "contaminant. Furthermore, because those contaminants which confer spin-allowedness on the parent state must have C = 0, we find that

A contaminant = (A + "parent = 'parent.

Since the electrostatic perturbation (i) A (iii) are functions only of the spatial coordinates, they cannot induce spin-orbit coupling. The "parent states", then, in the electrostatic cases are the spin-orbit coupled resultants expressed in the (A ,S)-basis.

(i) Non-Linear Molecule : Consider the atom "C" of the linear molecule A-B-C to consist of m (finite)

identical atoms symmetrically disposed about the internuclear axis. The symmetry of this system is now C for m 2 2 (Cs, m = 1). The electrostatic potential

mu associated with chese m atoms is

where the phase angle between the nuclear (off-axis) potential and $ is set equal to zero -- a simplification that does not affect generality. The loss of isotropy in $ may affect the parent states in two ways:

---States with A different from that of the parent may "admix" and destroy the integer nature of A (parent).

---A degenerate parent state may split into two non-degenerate perturbed states.

We now examine these effects from the standpoint of symmetry. The effect of H' on a degenerate pair, JI+( G,a,$), of parent states is determined in first order by the secular determinant-

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In defining the matr ix elements, t h e s impl i f i ca t ion i s made t h a t t h e parent s t a t e i s a pure s t a t e i n the (A,S)-basis. This, of course, is not t r u e i n o ther coupling l i m i t s . However, given a complete (A,S)-basis, t h e wavefunction i n any o ther coupling l i m i t can be expressed a s a l i n e a r combination of t h e (A,S)-functions. Hence, the treatment i n any o ther l i m i t is not more complicated but merely longer. The e l e m p t s of t h e determinant a r e read i ly expanded. It is found t h a t the compon- e n t s of H' t h a t can remove degeneracy a r e those f o r which km/2 is in teger , and t h e p a r t i c u l a r s t a t e s f o r which s p l i t t i n g can occur a r e those f o r whichA is equal t o one of these integers . In general , %?hen m is even, t h e l e v e l s f o r which degeneracy is removed a r e those withA = (m/2)R where R = 1, 2, 3, 4.... When m is odd, those l e v e l s f o r which A = mR l o s e degeneracy. For l e v e l s which remain degenerate, ihe per tu rba t ion energy, E ' , is

and f o r those which l o s e degeneracy, i t i s

E ' = v o , v. ; = - = i nteger . J m

The f i r s t - o r d e r energy correct ions lead t o degeneracy removal and provide a context ( t h e values of 0 ) i n which t o discuss these s p l i t t i n g s . A knowledge of t h e k per tu rba t ion wavefunctions ( i . e . , t h e nature of t h e contaminant s t a t e s ) leads t o severa l fundamental statements: For example, t h e mechanism by which a forbidden s t a t e may gain allowedness. The perturbed s t a t e t o f i r s t - o r d e r is

c.,

where t h e subscript 1 represents t h e parent s t a t e . The values of C. a r e given by 3

Thus, the coef f ic ien t C. i s non-zero only i f J

I f Eq. 12 appl ies , s t a t e j mixes with i ts o r b i t a l angular momentum vector & . a n t i p a r a l l e l t o t h a t of the parent s t a t e . Equation 13 i s t h e condition forJ p a r a l l e l mixing.

( i i ) Vibronic Coupling i n a Linear Molecule (Renner-Teller)

Perturbat ions a r i s i n g from t h e coupling of v i b r a t i o n a l and e l e c t r o n i c motions a r e discussed i n t h e context of t h e Franck-Condon Pr inc ip le . The per tu rba t ion

Hamiltonian is w r i t t e n i n a form p e r t i n e n t t o t h e (A,S)-linear t r i a t o m i c b a s i s . The i n i t i a l d i scuss ion of v ib ron ic coupl ing r e f e r s only t o l i n e a r t r i a t o m i c s where t h e p e r t u r b a t i o n Hamiltonian c o n s i s t s , i n t o t o , of Renner-Teller terms. Discuss ion of t h e i n t e r a c t i o n s i n C systems is based on t h e l inear-molecule r e s u l t s , bu t wi th t h e r equ i red a d d i t & of Jahn-Teller terms.

The v i b r a t i o n a l motion n e c e s s i t a t e s i n c l u s i o n of normal mode coord ina tes . It i s because of s e p a r a b i l i t y of t h e s e coord ina tes t h a t t h e Franck-Condon P r i n c i p l e holds . The p o s s i b i l i t y of observing vibronical ly-forbidden t r a n s i t i o n s r e q u i r e s a mixing of Franck-Condon-allowed s t a t e s i n t o t h e forbidden pa ren t . This mixing must be i n v e s t i g a t e d on t h e b a s i s of t h e p e r t u r b a t i o n Hami l ton ian , fu l l u se being made of t h e v i b r a t i o n a l wavefunction. In a l i n e a r t r i a t o m i c one of t h e v i b r a t i o n s ( t h e degenerate IT v i b r a t i o n ) is non-tota l ly symmetric and i s assoc ia ted wi th Franck-Candon-forbidden s t a t e s . Appropr ia te coord ina tes f o r t h i s v i b r a t i o n a l mode a r e given i n Figure 4. The e l e c t r o n i c coordinate 4 se rves a s a r e fe rence f o r t h i s coordinate system. The v i b r a t i o n i t s e l f i s dep ic ted i n Figure 5 .

Figure 4 : Relat ionship between Figure 5 : Geometry of T v i b r a t i o n a l mode nuc lea r coord ina tes , x and y , and of a l i n e a r t r i a t o m i c molecule. e l e c t r o n i c coordinate $J.

The v i b r a t i o n a l wavefunction f o r such a motion i s

+n' ,$,& = +v+ = fn ' , L ( ~ ) exp(*iLO) (14)

2 2 l I 2 where p = (x + y ) ; n ' i s t h e number of quanta of t h e exc i t ed mode; and

$ = n ' , n ' -2, ... , 1 o r 0. (15)

I n t h e fol lowing d i scuss ion , t h e 6-dependent p a r t of Eq. 14 is of primary concern.

The v i b r a t i o n a l p a r t of t h e Hamiltonian may b e w r i t t e n

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j , k even

j ,k odd

where, because of our concerns, t h e primary i n t e r e s t lies in t h e @- and @-dependent p a r t s of Eq. 16.

For a s t a t e t h a t i s double degenerate , bo th e l e c t r o n i c a l l y and v i b r a t i o n a l l y ( i . e . R#O), t h e product wavefunction is

Since a l l four combinations a r e p o s s i b l e , zero-order quadruply-degenerate v i b r o n i c s t a t e s a r i s e . For convenience, t h e s e f o u r s t a t e s a r e l abe led

The f i r s t - o r d e r energy c o r r e c t i o n i s obtained from

The only non-zero off-diagonal elements a r e F;g = H,;; and t h e s e a r e non-zero only i f R = A . I n t h i s case , t h e degeneracy of s t a t e s 2 and 3 i s removed, and t h e zero- o r d e r pa ren t s t a t e s become t h e appropr ia t e symmetric (c+) and ant isymmetr ic (c-) l i n e a r combinations of func t ions 2 and 3 . Thus, t h e f i r s t - o r d e r energy e f f e c t i s e s s e n t i a l l y t h e s p l i t t i n g of degeneracy of s t a t e s 2 and 3 . S t a t e s 1 and 4 cannot mix and remain degenerate . For cases where A # i , mixing cannot occur , s t a t e s 2 and 3 r e ~ a i n degenerate , and second-order energy cor rec t ions a r e r equ i red i n o r d e r t o remove t h e degeneracy of t h e p a i r JI 1 , Q4 o r t h e p a i r J12, J 1 3 .

Informat ion more germane t o t h e p resen t concern can a l s o be der ived from f i r s t - o r d e r e f f e c t s . The ques t ion devolves on whether o r no t HI w i l l permit a forbidden pa ren t s t a t e t o mix w i t h an allowed contaminant s t a t e and, i f it does , how, i n terms of t h e i r angular momenta, t h e s e s t a t e s admix. Since t h e term "allowed" i s def ined i n t h e con tex t of t h e s p i n , o r b i t a l and v i b r a t i o n a l components of t h e ground s t a t e and s i n c e t h e ground s t a t e i s always taken t o b e v i b r a t i o n a l l y unexci ted, t h e allowed e x c i t e d s t a t e s i n t h e (A,S)-, non-vibronically-interacting b a s i s must s a t i s f y

A = 0 , +1; S = 0; Z = 0; and R = 0 .

Considerat ion of Eq's. 16 and 17 l e a d s t o t h e fol lowing conclusions:

---The parent states 1 and 4 cannot mix in the first-order of H' with any state that is both orbitally and Franck-Condon allowed.

---For the insertion of first-order allowedness, it is required that the parent state have

---For the cases, A - R = +1, degeneracy remains. In the event thatA - k =

il, the parent and contaminant mix with parallel A vectors, while for A - R = -1 the mixing is antiparallel. This is an important'feature because of the sensitivity of MCD to such mutual angular momentum directionalities.

(iii) Non Linear, C m > 3, molecules (Jahn-Teller Effect). It is assumed that d e g e n e r a t e p i n C molecules can be discussed in terms of Eq. 14 with the

mu coordinate 8 now defined as

- 1 2 2 % 8 - COS [ca/(ca + cb) 1

-1 2 2 % - sin [5h/(Sa + Sb) I

where 5 and Sb are the normal coordinates of the degenerate pair of vibrations.

The perturbation Hamiltonian in the C system is now mu

p j [ (e i (kinru) $e-ik8 + -i (ki-nm) +eilc8

+ f 0 f 0 E l '1k.n )

Equation20contains all the elements of Eq. 16 (ice., the Renner-Teller terms) plus a set of other terms which evolve from the non-linear symmetry (the Jahn-Teller terms). Utilization of Eq. 20 permits expansion in terms of the linear-molecule basis. Those of the Jahn-Teller terms that contain m in the exponential account for the off-axis perturbations directly attributable to non-linearity.

The more complex Hamiltonian of Eq. 20 multiplies vibronic-coupling possibili- ties relative to the linear triatomic case. The following observations may be made:

---States 1 and 4 (cf. Eq. 17) lose degeneracy whenever R + A = nm, n = 1, 2, 3, .... Also, states 1 and 4, even while retaining degeneracy, can now mix, in first-order, with totally-allowed states.

---States 2 and 3 lose degeneracy when

---The rules for the mixing of allowed contaminants are

C2-3 14 JOURNAL DE PHYSIQUE

which arise from the Renner-Teller effects, and

where, in the latter case, all possible sigq combinations are permitted.

The perturbation Hamiltonian of Eq. 20 also accounts for the C perturbation in the zero-vibronic-interaction limit. Thus, an electronic state % a C molecule may be described by the quantum numbers of the "ancestor" state ?; the linear basis. The vibrational quantum numbers n' and 9, also retain their linear- molecule identities.

DISCUSSION

In using the various coupling conditions developed under items (i), (ii) & (iii), some presumptions must be borne in mind. The first of these is that motions which make a paramagnetic contribution some times smaller than the electronic contribution, do not, of themselves, contribute to the observed magni- tude of a-terms. The second is tiiat the magnetic moment of an excited state is predominantly that of the "parent" state whereas the intensity may well be totally attributable to that of a "contaminant" state. In situations where this latter is the case, the phasing of a-terms, positive or negative, is a direct consequence of the phasing of parentlcontaminant angular momenta. With these comments in mind, it is a relatively simple matter to conclude that an a-type parent can yield, as a result of mixing, an a-type, -a-type, or b-type resultant (where Ibl may be zero) whereas a b-type parent will normally yield only a b-type resultant.

We now illustrate the alteration of a-signatures using origin 2 and vibronic states built on 2 .

Transition 2 to the 3n1 state possesses a large oscillator strength. It is clearly electric-dipole allowed. It exhibits paramagnetism and the magnetic moment is approximately lvg. Mixing of 3 ~ ~ 1 with lnl is permitted by the spin-orbit coupling operator, but only in such a way that the two angular momenta remain parallel. Thus, the MCD signature of 2 should be in phase with that of state 4 (i.e., the '11 state) and both should exhibit comparable a-magnitudes. Such is, in fact, the cask.

1 1 The 2 ; 5 and 2 ; 6 transitions exhibit a-terms comparable to that of state 2 but

1 of opposite signature. The 2 ; 4 transition exhibits either a null term or a very weak b-term. All those absorption bands, while considerably less intense than transition 2 , are relatively strong. Since all bands should be Franck-Condon- forbidden, vibronic coupling must account for the observed intensity. lN$w, Reqner- Teller terms remove the degeneracy of state 2 and 3 (Eq. 17) to yield and lZ- scates neither of which will exhibit a magnetic moment and only one of which, namely tz+, is permitted to connect via dipole radiation to the ground state. Thus, the 2;4 transition must occur as a result of RenneriTeller coupling. Jahn-Teller terms, on the other hand, mix Franck-Condon-allowed nl states(states 1 & 4 of Eq. 17) but only in such a way that the angular momenta of parent and contaminant are antiparallel. Thus, a negative a-term s ould resnlt. Since these conclusions P are congruent with observations for the 2 ; 5 and 2;6' states, we must conclude that these states obtain intensity as a result of Jahn-Teller mixing.

It is assumed that these few examples illustrate the detailed information which can be extracted from Rydberg spectra using the MCD technique.

CONCLUSION

MCD techniques provide a nice means of measuring magnetic moments, assigning vibrational progressions in the totally symmetric modes, detecting vibronic bands which depnd on J-T or R-T effects for their intensity, and determining the operative route, J-T or R-T, which produces that intensity.

ACKNOWLEDGEMENT

This work was suppo r t ed by t h e U.S. Department of Energy. The a d v i c e of P r o f e s s o r Gary L. F ind l ey (New York U n i v e r s i t y ) is d e e p l y a p p r e c i a t e d .

REFERENCES

[ l ] IIERZBERG, G., Molecular S p e c t r a and Molecular S t r u c t u r e . 111. E l e c t r o n i c S p e c t r a and E l e c t r o n i c S t r u c t u r e of Polya tomic Molecules (Van Nost rand Reinhold Co.) 1966.

[2] SCOTT, J.D., FELPS, W.S., FINDLEY, G.L. and McGLYNN, S.P. , J. Chem. Phys. 68 (1978) 4678.

[ 3 ] SCOTT, J.D., FELPS, W.S. and McGLYNN, S.P. , Nuclear I n s t rumen t s and Methods 152 (1978) 231.

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