Resonance enhanced (2+1) multiphoton ionization spectrum of Br2: The lower Rydberg states

Chemical Physics 148 ( 1990) 3 15-323 North-Holland

Resonance enhanced (2 + 1) multiphoton ionization spectrum of Br2: the lower Rydberg states Trevor Ridley, Kenneth P. Lawley, Robert J. Donovan Department of Chemistry, University ofEdinburgh, West Mains Road, Edinburgh EH9 3JJ, UK

and Andrew J. Yencha Department of Chemistry and Department of Physics, State University ofNew York at Albany, Albany, NY 12222, USA Received 26 June 1990

The resonance enhanced (2 + 1) multiphoton ionization spectrum of Br, has been recorded in the region 61000-85000 cm-’ and gerade Rydberg states up to n = 11 have been identified. Two series are seen, based on the Q= 3/2 g and l/2 g core states. All of the higher states are members of the ns series but four components of the [P]4d multiplet close in energy to [P]6s have been identified for the tirst time. The quantum defects are very similar to those of the Br atom and to those of the lower ,membe.rsof the d series in CHsBr. The s series is at least an order of magnitude more intense than the d series, but intensities within each [Q&s+-X and [L?&td+X multiplet present some unexpected features.

1. Introduction

The ungerade Rydberg states of Brz have been studied extensively by Venkateswarlu [ 1 ] using conventional high resolution vacuum-ultraviolet absorption spectroscopy, and this work has been extended using synchrotron radiation [ 21. To reach the corresponding gerade states, at least two photons are needed, and the first observation of the even Rydberg states based on the ‘II, core was from resonantly enhanced ( 2 + 1) MPI [ 3 1. Four band systems were observed in the one photon energy region 272-295 nm, total energy 67800-73500 cm-‘. The same energy region was investigated by Koenders et al. [4] who used the REMPI technique in conjunction with photoelectron spectroscopy in order to determine the molecular core state of five Rydberg states in this region. More recently the four components of the lowest gerade Rydberg state, [ sl,] 5s have been observed [ 51 in the energy range 55500-61000 cm-‘. In the present work the (2 + 1) REMPI spectrum of molecular bromine has been recorded in the onephoton energy range 325-235 nm (2hvs61539-

85106 cm-‘), thus passing beyond the first ionization limit at 84825 cm-’ (A,=236 nm) [6,7]. The work to be presented is of medium resolution (8Ex 1 cm-’ ), and our aim is to investigate and assign the lower, more penetrating, members of the ns Rydberg progressions up to n = 12. In addition, we have seen components of the first member ( n = 4 ) of the weaker d series which are reported here together with assignments which are relevant to the work of Koenders et al. [4].

2. Experimental Tunable UV radiation was generated by a Lambda Physik 3002E dye laser pumped by a Lambda Physik EMG 20 1MSC excimer laser. The frequency doubled output of the dyes coumarin 102, coumarin 307, coumarin 153, rhodamine 6G, rhodamine B and rhodamine 101 covered the range 235-325 nm. The radiation was focused by a 15 cm focal length lens into a conventional ionization cell fitted with nickel electrodes and containing 1 Torr of bromine vapour. The

0301-0104/90/S 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)

T. Rid& et ai. /Mdtiphoton ionization spectrum of3r2

316

ion current was amplified by an operational amplifier [ 8 1, processed by a Stanford Research Systems 250 boxcar integrator and stored on an IBM XT computer. All spectra were normalized to the square of the simultaneously recorded laser power. Wave length calibration of the dye fundamental was against the optogalva~c spectrum of a neon-filled hollow cathode lamp.

3. Results and discussions 3.1. Genera/features and assignments The (2 + 1) NPI spectrum is remarkably simple in the 560~84000 cm--I region (figs. l-4). Each series exhibits short vibrational progressions with spacings in the range 345-378 cm-‘. The latter is characteristic of Br$ in its lowest electronic state (o,(Brz+)=376cm”) [9] andservestodistinguish progressions in the upper electronic state from the quite extensive hot bands with characteristic ground state spacing 318-320 cm-’ (o,(Brt(X))=325 cm- ’ ) [ 9 1. The principal quantum number n and the Evalue of each Rydberg state can usually be unambiguously assigned from the quantum defect 6(I) that is

calculated from the observed molecular electronic term value T( [ Sa,f nl) (Br$ [ Ss,] nl) and the ionization potential (IP) of the molecule to which the series converges:

(1) where IP(Br~[3/2~)=84825 cm-’ [6,7] and IP(Br2+ [ l/21)=87645 cm-’ [IO]. The spin-orbit splitting between the &= 3/2 and l/2 states of the core, 2820 cm-’ [ lo], is sufficiently large for L&as well as n to be assigned from eq. ( 1). Furthermore, in Br; , S for Rydbexg s orbitals is found to fall in the narrow range 2.87-3.04, exhibiting a slight but regular increase between n= 6 and n= 12. For the four members of the [f2,]4d multiplet of Brz assigned below, we find 6 between 1.19 and 1.38. These are very similar to the range of defects for the two series found in the free atom. Thus, for n = 5 to n = 10,6( s ) ranges between 3.03 and 3.14 and the atomic quantum defect for the J components of the 3p44d configuration ranges between 1.14 and 1.40. It is thus often possible to assign n, I and &&unambiguously from the molecular term value alone, though one case of an alternative assignment that is permitted on these grounds is given below. The situation is rather similar in

70500 2-Photon Energy

/cm*’

Fig, 1.The REMPI (2 + 1) spectrumof Brs for A,= 278-292 nm normalized to the square of the laser power.

317

T. Ridky et al. /Multiphoton ionization spectrum of Br,

74500

P-Photon

Energy

/cm“

- 260-278 mn normalized to the square of the laser power.

F~2.~eREMPX(2+l)~mofBr~fora~-

1

f

-

--

/i

79500 2-Photon Energy

Fig. 3. The REMPI (2+ 1) spectrum of Br, for k-=243-259

CHSBr, where the quantum defects for the [ 3/2] 5s E and A, Rydberg states are 3.07 and 3.05, respectively [ 111, though the d series has not been observed. The assignment of the origin bands that gives the maximum similarity between molecular and atomic quantum defects is given in table 1, where the effect of the two alternative core states is i~us~ted. A complete assignment of all the band positions is given in table 2 where the term values refer to the band maximum in each case. The accuracy of the calibra-

AL

/cm-’

nm r~ormalizedto the square of the laser p_gwer.

tion technique can be judged from the positions of the Br atomic lines which are also observed. These are the very sharp features in figs. f-4 and originate from either 2P9,2or 2P,,z states, terminating in known upper levels [ 12.1. The wavenumbers of the transitions and their assignments are given in table 3. The accuracy of our ~b~tion technique ( + 1 cm-‘) can be deduced from the errors in these observations. To summarise, six progressions have been observed in the range of 68000 to 75000 cm-’ and a

318

T. Ridley et al. /Multiphoton ionization spectrumof Brz

I

82500 P-Photon

Energy

1

80500

/cm-’

Fig. 4. The REMPI (2 + I ) spectrum of Br2for 1,~ 234-248 nm normalized to the square of the laser power. The first ionization limit is arrowed.

further 15 between 75000 and 85000 cm-‘. In addition, previous work [ 51 has revealed three or possibly four gerade stat& between 56000 and 68000 cm-’ which are all Rydberg 5s states. Of the six states between 68000 and 75000 cm-‘, three have previously been identified by Koenders et al. [ 41 as being components of the 4d level #’ and we have found a fourth component at 73727 cm-‘. In this particular portion of the spectrum the quantum defect argument is ambiguous. The vibrational progressions with origins at 72408 and 72908 cm-’ could either be assigned to two components of the [ 1/2]4d level or to the [ 3/ 2 ] 6s level (or even one to each). Support for the latter assignment comes from the work of Koenders et al. [ 41, who found the core state to be SE=3/2. With the-new evidence of thl pattern of the extended sRydberg progression, we now believe that these two progressions arise from the [3/2]6s configuration. Our arguinant is as follows. The two systems are sim1)1These authors assign the Rydberg orbital as 5d, following Venkatekvarlu [ 1 ] who was apparently misled by earlier tabulations of atomic term values in which the 4d levels where missing..See ref. [ 12 ] for a fuller compilation of the spectrum.

ilar in strength, roughly in the ratio 1: 3, and both are comparable in intensity to the [ l/2] 6s pair, rather than to the [ 3/2] 4d pair, which are an order of magnitude weaker. Secondly, two components of the [3/2]7s and 8s Rydberg states are unambiguously observed, rather than just one, and there is some evidence that the [ 3/ I] 5s configuration also displays two components in two photon excitation [ 51. Thirdly, based on theoretical grounds (see section 3.2), one would expect an equal number of components to be accessible for both the [ l/2 ]4d and [ 3/2]4d levels as assigned here, whereas three components of the [ 1/ 2]4d level would be accessed but only one component of the [ 3/2]4d level if the assignment of Koenders et al. [ 41 is used in conjunction with our newly found system. Finally, it might be argued that the weaker member of each [ 3/2] ns doublet, assigned by us as the G?=2 component should beassignedtoanunusuallystrong [1/2](n-2)dtX transition that has borrowed intensity by considerable mixing with the adjacent {3/2]6s state. However, except in the case of n = 6 the quantum defect would not then lie in the range associated with the d states ( 1.62 and 1.08, respectively, for the states as-

T. Ridky et al. /Multiphoton ionization spectrum of Brt Table 1 Assignments of the origin bands of the gerade Rydberg systems observed in the J origin (cm-‘)

(n-6) with [ 3/2] core

56544”

1.97

58940” 59430.’ 68827 70571 71752 72408 72908 73727

2.06 2.08 2.62 2.78 2.90 2.98 3.04 3.15

75288 75662 78041 78252 80503 80607

3.39 3.46 4.03 4.09 5.04 5.11

80886 81045 81894 82649 83164

5.28 5.39 6.12 7.10 8.13

83327 83406 84654 84706

8.56 8.79 25.33 30.37

(2 + I ) REMPI specMm of Brz

Assignment

(n-6)

nW)

[l/21

5s(l)

4d 4d 6s(2) 6s(l)

319

with

cat

Assignment

W-J)

1.88 1.96 1.97 2.42 2.54 2.63 2.69 2.73 2.81

5s(O) 5s(l)

4d

4d 6s(O) 69(l)

7s(2) 7s(l) 8s(2) 8s(l)

2.98 3.03 3.38 3.42 3.92 3.95

9s(l) lOs(1) lls(1)

4.03 4.08 4.37 4.69 4.95

7s(O) 7s(l)

5.04 5.09 6.06 6.11

8s(O) 8s(l) 9HO) 9s(l)

‘1 Ref. [5].

signedhereto [3/2]7sand [3/2]8s)andwouldvary erratically with n. With our assignments S is almost independent of n. Rotational structure is not resolved and additional errors in the band positions due to the rotational contours arise. The bromine sample contained the three isotopomers of Br, in their natural abundance, 1: 2 : 1. In the (0,O) band the difference between the origins in 79Br2and ‘lBr2 is only ~~0.2cm-i but is =2 cm-’ in the ( 1,O) band. Because of this, contours become much broader, and we estimate the error in all measurements to be x + 3 cm-l. All the rotational contours are blue degraded as expected for upper states having a slightly smaller R,. The relative intensity of the [sl,]4dtX bands compared with the adjacent [LZJ 5-X transitions is roughly 1: 10, and possibly as a consequence, only the first member (n = 4) of the d series is seen.

3.2. Classifiation of electronic states The gerade Rydberg states that converge on the positive molecular ion in its lowest electronic contiguration, [of r# at] ( *IISi2, 2II,,2), belong to four series, [3/2]ns, [ 1/2]ns, [3/2]nd and [ 1/2]nd. In Hund’s case c, only Q=QC+%Y is a good quantum number, but in these Rydberg states coupling between the core and Rydberg electron through eleo tron exchange is fairly small compared with spin-orbit coupling within the core. Thus we may additionally take S2, and c+,. to be good quantum numbers. Furthermore, the separation of the valence 4po and 4prt orbitals is sufficiently large for the dominant configuration of the ground state of the Br,+ core, given above, to contribute over 90% with configurations such asok nl xi oi playing very little part. Each [ 3/2] ns term splits into two states yielding G&2, 1 component while each [ 1/2]ns has three components 8= O+, O-, 1, which in the QC%Ynotation may be written as:

T. Ridteyet al. /Mdtiphotm ionizationspectrumof Bra

320

Table 2 Detailed vibronic assignment of the gerade Rydberg systems of Brz

v’, VW

w01wa

[3/2]5s( 1) ‘) [1/2]%(O) a) [1/2]5s(l)*’ 13/214d [3/234d Il/2]4d t3/2]6sW [3/‘2]6s(l) f 1/2]4d

%2

0,1

0, 0

170

2,o

3,o

430

5,O

55902 58304 58796

56226 58620 59110 68512 70250 71424 72089 72589

56544 58940 59430 68827 70571 71752 72408 72908 73727 75288 75662 78041 78252 80503 80607 80886 81045 81894 82649 83164 83327 83406 84654 84706

56922 59317 59805 69152 70947 72114 72764 73269 74092 75645 76017 78397 78615 80864 80970 81243 81405 82256 83025 83514 83687 83776 85012 85066

57295

57663

58030

58398

60185 69554 71313

60542 69894 71679

60917

61270

73122 73635

73479 74003

74370

74735

75996 76378 78758 78973 81219 81329 81598 81762 82620 83376 83877

76345 76725 79114 79332

77072 79482 79688

71768 72272

[1/2lWO) 11/216s(l) W2l'W) IWlWf)

74650

74967 75337 77722 77933 80182 80287 80566 80724

77404 77613

[3/218s(21

t3/2lW

1)

[1/217s(o)

tv217s(l) 13/219s(l) t312lWl) t3/211wl) t 1/218s(O) tu218s(l) 11/21%0) tlf2Fwl)

80407

81684 81950 82112 82979 84243

84136

‘) Ref. (51.

Table 3 Atomic bromine tmnsitions observed in the (2 f 1) REMPI spectrum of Br2

jj .>

Ohs.-lit.

(cm-‘)

(cm-*)

72012 7499 1 75698

0 0 +1 +1

75814 76183 76743

0 0 0

75494

0

78076 78511 78677 79177

*) Two-photon energy.

-1

Upper state term (4s24p’Sp)

0 -1 -1 +1

83366 =) 83815

+1

[ 121. e) Unidentified.

L5wer state term (4sZ4P5)

w&2

2R,2

‘P&2

lfl,*

%,2

2e[2

‘%2

2~,2

“R,2

%/2

“$2

2P?,2

‘9%~

“Fh

“$2

22

24,2

79695 79869

b, Ref.

b,

V2

‘4,2

24,*

55x,2

2N,2

2%2

W,2

2p1112

“@,2

2@,*

2P(ijt

84596

1: Ridleyet al. /Multiphoton ionizationspectrumof Br2

~1[-1/21,)1+1/2)), 1=][+1/2],)]+1/2).

(2) (3)

The [ 3/2 ] nd series is split into 12 components havingQ=4 (l), 3 (2), 2(2), 1 (3),0+(2) andO- (2). Only states with Q< 2 are accessible in a two-photon transition from an r(2=0 state, and the O- component is forbidden. The [ l/2] nd term is similarly split into 12 states (51=3 (l), 2 (3), l(4), O+ (2) and O( 2 ) ) . The actual spectrum is thus one of greater simplicity than the density of electronic states and formal selection rules would indicate. The relative intensities of two-photon transitions is not easy to predict because of the contribution of distant virtual states and the continuum. Furthermore, in REMPI studies the final ion signal may not be proportional to the two-photon cross section at the resonant stage because of possibly a wide variation in lifetimes of the upper Rydberg states. Nevertheless, some guidance comes from a consideration of the dominant virtual states involved in the absorption of the first photon. Although there is no intense absorption feature in the 235-325 mm single-photon range in Br, [ 13 1, any state that is fairly well matched in energy with the first photon will make a major contribution to the two-photon cross section. In fact, weak continuous absorption has been detected in Br, throughout the entire wavelength ranges studied here [ 13 1. The number of dipole allowed transitions is very limited. The ground state electronic configurationis[2440]andonlythe[2431] (‘lI,)and[1441] ( ‘EC1 ) states can be reached without invoking spinorbit coupling, Coriolis coupling or configuration interaction. Only the first of these intermediate states can leave the core in the right configuration for further promotion to Rydberg states converging on the lowest configuration on the ion, [ 24301. Absorption to this state has been identified to be partly due to the continuous absorption feature with a maximum at 24000 cm-’ (4 17 mm) [ 141. The second photon can then induce either the po,+ns transitions or the much weaker po,+nd transitions. In the d series, spin-orbit coupling in the Rydberg orbital is much less than the exchange interaction of the Rydberg electron with the core, and an appropriate basis is In the L&=3/2 core state the un[@I (m%h,

321

paired electron must have m,= - l/2 and in the 51,=+1/2statem,=l/2 (in the dominant configuration ( 2430) ) . The components of the 4d level accessed without spin-flip by two photons will thus be [3/2]4d(O, -1/2)&l [3/214d.(l, -l/2),62=2; and [3/2]4d( - 1, - l/2), 52=0+. Similarly, for the Qc= l/2 core, the d states accessible are also 2, 1 and O+. There would thus appear to be six 4d states that are readily accessible with comparable cross sections in the linearly polarised rotationally unresolved case @. However, note that only two components ml= 1 and 0, of the d shell are accessible by this route (i.e. & and d,). Precisely the same conclusion is reached by considering the less nearly resonant sequence of transitions: [ 2440]-+ [ 23401 ns, nd+ [2430]ns, nd without invoking a spin-slip. An experimental observation relevant at this point is that the splitting of the two 4d levels based on each core state is very large, 1744 cm-’ for Q= 3/2 and 1975 cm- * for Q= l/2. The atomic 4d term values are also split by up to 2000 cm-’ [ 161, considerably more than the splitting in the 5s manifold. The major source of this additional splitting of the 4d level is the electrostatic interaction of the quadrupole moment of the core with that of the d electron (an s electron has, of course, no orbital quadrupole moment). The quadrupole moment of a d orbital depends upon Iml1; %(n&)

= -Wz(nd,

1 ami @zz(ndl) = WrA~do).

The two L&O+ components arising from [3/2] 4d(-1, -l/2) and [1/2]4d(-1, l/2) are also strongly mixed by the core quadrupole moment. We thus expect the 8=2, 1 and 0 components of both core states to be split by > 1000 cm-‘, but on the present assignment only two [ s2,] 4d components are seen. The suppression of valence+Rydberg ndOtransitions has been reported in NO [ 17 1, where it was attributed [ 18 ] to out-of-phase ml,,/ ( n + 1)s mixing and perhaps the same effect is operating in bromine. *’ The actual rotational matrix elements governing the strength of the 0, P, Q, R and S branches have been given by Bray and Hochstrasser [ 15 1.Summing over all the rotational branches in the limit Jz+ 1, it can be shown that the two-photon cross section for the sequence X+lLlI (G?=O+ 1) is one halfthat for the sequence LJI-3 (a=OdO), assuming the same magnitude of transition dipole moment at the second step as would be the case with different mrcomponents of the same atomic state. The sequence lE-+lLA (6)=0+2) leads to the same averageH&l-London factors as the L&O+0 transition.

322

T. Ridley et al. /Multiphoton ionization spectrum of Br2

It can also be seen from the assignments in table 1 that the relative splitting between the two 51components of each core state (a=+ I /2) consistently departs from that predicted by simple perturbation theory. In a four state basis in the Sl,m,m, coupling scheme with core spin-orbit interaction Cand the ns Rydberg-4px, exchange K included [ 5,111, the two 51~ 1 states must repel each other. Thus the splitting between the 822 and $2~ 1 states derived from the Sz,= 3/2 core (the lower doublet) must always be less than the splitting in the upper doublet derived from the s2,= l/2 core. This conclusion is independent of the magnitude of K and [, and was the pattern observed by Felps et al. [ 113 in the lowest Rydberg s states of a series of aliphatic bromides and HBr. In the present case of Br,, the splitting in the upper O= l/G?=0 doublet is consistantly less than in the lower 52=2/O= 1 doublet for n = 6, 7 and 8 (the relative doublet splitting for n= 5 cannot fully be assessed without the firm location of the [3/2]5s (ii?= 2) Rydberg member [ 5 ] ). Presumably, additional interactions are at work, almost certainly involving the (n- 2)d levels that are interleaved with the ns levels with which they strongly interact through the large core quadrupole moment. This perturbation seems to be more pronounced in BrZ than in the RBr series and introduces shifts of the order of a few hundred cm- ‘, large enough to reverse the splitting order yet still small enough for the quantum defect to remain characteristic of the dominant s- or d-Rydberg atomic orbital.

4. Conclusions By comparing molecular and atomic quantum defects we have identified 22 new low-lying gerade states of Br,. Four of the states are based on the configuration [Q]4d and the remainder belong to the ns series, with n = 6-l 1. The assignment of principal quantum numbers is that of the Br atom to which the state dissociates and is made possible by the close similarity of Rydberg molecular and atomic quantum defects, 6(nl) for a given nl state. In both the atom and molecule there is a wider range of 6 values associated with the d series than with the s series. This is mainly due to the electrostatic quadrupole interaction between the core and the d electron in its var-

ious mlstates. The united atom approach used by Wu and Johnson [ 19 ] for I2 is not really relevant because at no point in space does the Rydberg electron experience the combined nuclear charges (whether partially shielded or not) of both atoms. In fact, from the close agreement of molecular and atomic quantum defects, it is clear that as the Rydberg electron approaches either of the atomic cores in the molecule, electron correlation ensures that the local environment is that of Br+. There are seven 52 components of the [ 3/2 ]4d configuration and nine [ l/2 ] 4d states that are, in principle, accessible by a two-photon transition from the ground state using the selection rule AQ=O, 1 or 2. We have shown that, if spin-flip is not involved, a consideration of the probable near resonant virtual intermediate states reduces the accessible final D states to three based on each of the two core states. We can only find two such states associated with each core state; the & component may be suppressed by a small out-of-phase mixing with the 6s orbital. In contrast, the lower members (n=6-9) of the [L&lns+X series exhibit all four possible electronic branches, although beyond n= 9 only one total Q component of each of the two core (A&) states is observed, presumably sd= 1. To reach the $2~0 and 2 components of each multiplet effectively requires a spin-flip during the two-photon transition. This may be mediated by Coriolis coupling which effectively decouples the spin of the promoted electron in high virtual Rydberg levels from that of the core electrons (Hund’s case b ) . It is particularly difficult to find a conventional route to the [ 3/2]ns, a= 2 states that could give a two-photon cross section comparable to that for reaching the 9= 1 state. For this reason, the possibility does remain that the weaker partner in eachofthe [L&]nsdoubletsisinfacta [L&l (n-2&, state that is strongly mixed with the [ 3/2]ns(S6= 1) configuration with which it is accidentally nearly degenerate. The close similarity of atomic and molecular quantum defects in the gerade series reported here suggest a re-evaluation of some of the assignments of the ungerade levels in Br, and I2 where 6 values quite different from atomic values have been put forward. We note, in particular, that all the members of the Rydberg f series in Brz assigned by Venkateswarlu [ I] seem misplaced. Thus, the k system starting at 709 13

T. Ridky et al. /Multiphoton ionization spectrumof Brz

cm- * is assigned by Venkateswarlu as [ 3/2 ] Sf which, even after amending 5f to 4f, which is the lowest vacant f shell, gives a quantum defect of 1.3, whereas the atomic values range between 0.023 and 0.001 for the [‘P,] 4f configuration.

Acknowledgement

We gratefully acknowledge support from the UGC for an equipment grant and the partial support from NATO for a travel grant (No. 870878 ) . We thank Mr. J. Luque Sanchez and Mr. Robert V. Flood for their assistance with part of this work.

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323

[ 51 K.P. Lawley, R.J. Donovan, T. Ridley, A.J. Yen&a and T. Ichimura, Chem. Phys. Letters 168 (1990) 168. [ 61 A.J. Yencha, D.K. Kela, R.J. Donovan, A. Hopkirk and A. Kvaran, Chem. Phys. Letters 165 (1990) 283. [7] V.H. Dibeler, J.A. Walker and K.E. McCulloh, J. Chem. Phys. 53 ( 1970) 47 15. [ 81 T.E. Adams, RJ.S. Morrison and E.R. Grant, Rev. Sci. Instr. 51 (1980) 141. [P] KP. Huber and G. Her&erg, Constants of Diatomic Molecules (Van Nostrand-Reinhold, New York, 1979). [lo] A.B. Comford, D.C. Frost, C.A. McDowell, J.L. Ragle and I.A. Stenhouse, J. Chem. Phys. 54 (1971) 2651. [ 111 W.S. Felps, G.L. Findley and S.P. McGlynn, Chem. Phys. Letters 8 1 ( 198 1) 490. [ 121 J.L. Tech, J. Res. Natl. Bur. Std. A 67 (1963) 505. [ 131 J.C. Calvert and J.N. Pitts, Photochemistry (Wiley, New York, 1967) p. 184. [ 141 J.A. Coxon, Molecular Spectroscopy, Vol. 1 (The Chemical Society, London, 1973) p. 177. [ 151 R.G. Bray and R.M. Hochstrasser, Mol. Phys. 31 ( 1976) 1199. [ 161 C.E. Moore, Atomic Energy Levels, Vol. 2, Natl. Bur. Std. Circular467 (1971). [ 171 C. Jungen, J. Chem. Phys. 53 (1970) 4168. [ 18 ] M.B. Robin, Higher Excited States of Polyatomic Molecules, Vol. 1 (Academic Press, New York, 1974). [ 191 M. Wu and P.M. Johnson, J. Chem. Phys. PO ( 1989) 74.