The A-X system of Br2+ radical cations

JOURNALOFMOLECULARSPECTROSCOPY~~,

269-281(1983)

The A-X System of Bri Radical Cations T.

HARRIS AND J. H. D. ELAND

Physical Chemistry Laboratory, South Parks Road, Oxford, England AND

R. P.

TUCKETT

Department of Physical Chemistry, Lensfield Road. Cambridge, England The 2II.-X2II, transitionin Bri was reexamined using dispersed laser-induced fluorescence, and emission spectroscopy in a seeded molecular beam. New constants are derived, confirming the large difference between A2HX,2and A211l,z, and reconciling the emission spectrum with photoelectron data.

1. INTRODUCTION

The emission spectrum of the radical cations of diatomic bromine has been known since 1928, when Uchida and Ota (I) observed it in a discharge kindled for the purpose of investigating the spectrum of neutral Brz. The spectrum was attributed to Br: and analyzed by Haranath and Rao (2), and their analysis has stood until the present day. The spectrum is extremely congested because of the overlapping of the 3/2-3/2 and l/2-1/2 systems, and also because of the tripling of every line and head in 1:2:1 proportions by the isotopes in their natural abundance. According to Haranath and Rao’s analysis the vibration frequencies in the A2113,2and A211112states are very different, and this unusual feature led Huber and Herzberg to question the vibrational analysis in their compilation (3). An even more serious objection to the old analysis is provided, however, by the high-resolution photoelectron spectrum of Br: , obtained by several groups (46), which can hardly be reconciled with the quoted origins of the A-X bands. This discrepancy between the optical and photoelectron work has led us to reexamine the spectrum of BrZ using modem techniques. It was also hoped that a detailed examination of the Br$ spectrum might shed some light on our persistent failure to detect the predicted (7) 1: emission, and on the peculiar spectrum of the Cl: ion (8). We began this investigation by remeasuring the emission spectrum of Br$ , excited in a flowing afterglow of helium, and also in a magnetically concentrated discharge of the form developed by Cossart (9). Unambiguous information on the Br2f ground state was then obtained from dispersed fluorescence of the Br2f ions in an argon afterglow, excited by lines from an Ar+ laser. Finally, a rotationally cold emission spectrum was recorded by electron impact excitation of a seeded supersonic jet of Br2. The enormous simplification afforded by this last technique allowed the whole 269

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1983by Academic Press, Inc.

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270

HARRIS, ELAND, AND TUCKETT

spectrum to be vibrationally analyzed leading to an assignment which reconciles the optical and photoelectron spectra. 2. EXPERIMENTAL DETAILS

Bright emission flames of Bri were obtained by injecting Brz vapor into a stream of helium excited by a microwave discharge. The pressure in the flowing afterglow was estimated as about 1 Tot-r, but was not measured directly. Purer but weaker spectra free from atomic Br lines, could be obtained by separating the discharge and afterglow regions by an opaque nozzle. Gases from the discharge flowed through a liquid-nitrogen-cooled trap, from which bromine was recovered, and were pumped by a 160 liter/m rotary pump. Dispersed laser-induced fluorescence spectra were recorded from Br2+produced in the afterglow and excited by the visible lines of an Ar+ laser, whose beam passed coaxially through the rare gas discharge and through the nozzle. When argon was substituted for helium as discharge gas the normal Br2+emission spectrum disappeared, and the LIF spectrum became about 10 times brighter; the fluorescence was readily visible in a darkened room. The enhancement is presumably due to increased population of the Br,f ground state, because Ar* metastable atoms have only just enough energy to populate the lower levels of Br: A2JT, by Penning ionization. More intense Br: emission, suitable for measurement at high resolution but contaminated by many atomic lines, was obtained from a magnetically concentrated discharge source of the type designed by Cossart (9). The pressure in this source was estimated as - 10m3Tot-r under the optimum conditions of a flat emitting region close to the molybdenum cathode. The emission spectra and laser-induced fluorescence spectra were recorded using a 2 m in-plane Ebert monochromator with an 1800 line/mm-’ grating, in first order (2.7 &mm), using slitwidths between 200 and 20 pm. Detection was by a cooled RCA 31034 photomultiplier, and wavelength calibration was made by comparison with a simultaneously recorded thorium hollow cathode lamp spectrum; the uncertainties of this calibration have been demonstrated before (10). The emission spectrum of rotationally cold Br: was obtained using an apparatus which has already been described (I I). Briefly, helium at a pressure of one atmosphere was bubbled through bromine held at 273 K, and the gas mixture was expanded adiabatically through a 45pm nozzle hole. The stainless-steel nozzle was heated to 373 K to prevent it blocking. The molecular beam is skimmed, then crossed orthogonally by an electron beam. The electron gun is home built, operated at 200 eV electron energy, and a current of 5 mA was collected. Light from the crossing region is focused on to the entrance slit of a 1.26-m f/9 scanning monochromator. The grating ( 1800 line/mm-‘) was used in first order, and slitwidths of 180~pm were used. The dispersed fluorescence was detected by an RCA 31034 photomultiplier tube (cooled to 253 K). The numerous atomic Br lines in the spectrum serve as an internal calibration, and we estimate that the line positions of the Br: bands are accurate to one fifth of a linewidth (i.e., +l cm-‘). 3. RESULTS AND ANALYSIS

Representative sections of the different forms of Brl spectra are shown in Figs. 1 and 2. Figure 1 shows a dispersed laser-induced fluorescence spectrum excited by the

271

THE A-X SYSTEM OF Br;

J\ 0,1,4

480

5

6 7

520

0

9

560

!I

13

15

n

“”

600 640 Wavelongthtnm

FIG. 1. Fluorescence spectrum of Br; excited by the 4727-A multimode Ar+ line in an Ar afterglow.

4727 line of ArII. As only one rotational component (J’) in each of two bands is strongly pumped, each emission band has four strong rotational components; two more weak lines arise from a third band, but are not well seen in the illustration. Figure 2a shows a part of the very extensive normal Br: emission spectrum produced by the flowing afterglow technique, while Fig. 2b shows the same region produced in the crossed molecular beam/electron beam apparatus. The spectral simplification afforded by rotational cooling is readily seen. These three different forms of spectra are now discussed in detail, and the final constants resulting from the analysis are given in Table I. 3.1. Laser-Induced Fluorescence The spin-orbit splitting in the X2& ground state of Br: is known from photoelectron spectroscopy to be 2820 f 100 cm-‘. Such a large splitting suggests that relaxation, likely to be rapid for ionic species, will leave only the lower 211s,zgcomponent to absorb laser light, and it is therefore no surprise that all the dozen or so resonance series excited by six ArII laser lines (see Table II) can be assigned to just

17800 18000 Frequency(cm?

FIG. 2. Emission spectra of Bri excited (a) in a helium afterglow at ambient temperature and (b) by electron impact on an isentropically cooled supersonic molecular beam. Strong peaks in the upper spectrum are atomic lines.

272

HARRIS,

ELAND,

AND TUCKETT

TABLE I Molecular ConstantsforBri (incm-')

79 81 Br2+

2% lu

7gBr2+

8lBr2+

15946(2.5)+2820

Te

154.1(0.4)

ee w xe # 10 WeYe

0.13(0.03) -8.1(0.4)

154.1(1.8) 0.09(0.06) -9(1.0)

152.8(0.7) 0.12(0.03) -8.2(0.4)

0.0642(0.0015) Bt ae 10

A2"3/2u

3.7(1.5

',/Z

2.56(0.02)

Te U e

16620.7(1.5) 204.0(0.2)

oeXe 103,,Y,

6.5 (0.2) 0.0654(0.001)

Be 104ae

3.6 (0.5) 2.54(0.02)

r,/l

E2"tg

1.31(0.02)

0 + 2820a

Te

361.2(0.2)

we

l.lO(O.03)

oeXe

363.0(0.8)

358.9(0.4)

l.Ol(O.15)

1.07(0.08)

368.0(0.5)

362.8

0.0887(0.0007) 1:4,

3.3b

r,J X2

3/2g

2.18(0.01) 0

Te

364.9(0.3)

e

1.13(0.03)

exe

1.12(0.04)

1.13(0.04)

0.0885(01007) e4 lo e r,/ll

3.1(0.3) 2.18(0.01)

aFrom the photoelectron

spectrum

b Estimated using the Pekeris equation Uncertainties

in parentheses are two standard deviations.

three (isotopic) forms of Br2+on the basis of the observed vibrational intervals. The resonance progressions cover u” = 0 to tJ’ = 17, and the variation in splitting of the R,P doublets as a function of V”allows & to be determined directly, though rather imprecisely. The vibrational parameters of X2II3,2g can then be obtained from the line positions, and these parameters are quoted in Table I. Because the fluorescence spectra cover a much wider range of u”, and are much less congested, the parameters derived from them should be more reliable than the slightly different ones obtained from the emission spectra. The S values can be obtained from the resonance doublet splittings if B” is known. Estimates of B& were obtained by Franck-Condon analysis of the photoelectron

THE A-X SYSTEM OF Br2+

273

TABLE II Brt Laser-Induced Fluorescence A21131zu-X2113~~g Ar II line

Transition

Isotope

v” values observed

v'

v"

4545 S

33

0

P20.5

81-81

4579 s

(37

2

P30.5

79-79)

32

0

R48.5

79-81

10

27

0

P42.5

79-79

13

28

0

P71.5

79-81

13

(32

2

R43.5

79-79)

7

4765

(27

0

R79.5

79-81)

8

4880

(27

2

J"1.5

79-79)

6 6

II 4727 1 !! II

7 8

II

25

1

P39.5

79-81

(1

25

1

R46.5

79-79

5

4889

22

0

R17.5

79-79

3

5017

19

0

R27.5

81-81

7

(21

1

R31.5

79-79)

7

22

1

P61.5

81-81

5

II !!

Parentheses mark assignments which are slightly uncertain

spectrum, together with the precisely known Bz of neutral Br2 ground state (12), initially by approximate methods (13) and finally by trial and error calculations of the photoelectron intensities using RKR potentials for Br, and Br2+derived from the vibrational constants. Vibrational hot bands are prominent in the photoelectron spectrum, and were included on the assumption that these spectra were measured at 300 K. The B, values obtained in this way for Br: X2113,2and X211,,2 are quoted in Table I among the other final parameters, as no emission bands have been sudciently resolved to yield better values by rotational analysis. In the similar case of S: , Franck-Condon calculations of this sort (14) were shown to be very accurate by later rotational analysis (10). The uncertainty in the B”,values leads to an uncertainty in S of at most &2. 3.2. The Rotationally Cold Emission Spectrum The clarity of the rotationally cold spectrum compared with the normal emission spectrum is demonstrated in Fig. 2. In particular, it is easy to recognize many isotopic 1:2: 1 triplets, and to see that in any spectral region there are triplets with two distinct splittings. When these splittings are plotted against the position of each triplet two parallel lines are found, Fig. 3, cutting the axis at 15 900 f 100 and 16 550 -+ IO0 cm-‘. The slopes of the lines are equal to the expected isotope shifts in Br2+;the two systems. The identification intercepts are the origins of the ‘II 1,2-211rj2and 2113,2-2113,2

274

HARRIS, ELAND, AND TUCKETT 44-

/’

3b -

1’

32 -

4 /'

~ 28-

“p’ ,’

1: ,"'O

5 24.gzog f~ Ib.Y erz2 84-

/ 1'

1' / Or'*""'-' /l;ooo,' /' ( #***'***.'.*A _ 17000 18000 19000 -4‘ ,' Frrqutncy/cm-' /

FIG. 3. Isotopic splittings between 79Brf and **Bri peaks in the cold beam emission spectrum, against frequencies of the corresponding ‘Ig*s’Brfpeaks.

of the lower origin with the l/2-1/2 system and the higher frequency one with 3/23/2 is completely unambiguous, on two grounds: (a) The LIF spectra are all found to arise from bands which can be fitted to the upper (16 550 cm-‘) origin. Because of relaxation, this upper origin must belong to the lower ionic ground-state component. (b) The present assignment gives a spin-orbit splitting in &I, of 0.27 eV, compared with 0.35 eV in X2& The contrary assignment would correspond to a splitting of 0.42 eV in A211,, which would be inconsistent with the photoelectron spectrum, where a splitting of 0.30 + 0.05 eV is discernible. The vibrational analysis of the cold emission spectrum of the 2113,2-2113j2 transitions is straightfonvard, since both the ground-state vibrational intervals and the approximate origin are known. The observed bands form a normal parabola in the Deslandres table (Table III) and account for many of the most intense peaks in the emission spectra. Because w’ - w“/2 there is considerable congestion, however, and some peaks are heavily overlapped despite the rotational cooling. For this reason, no separate vibrational analysis of the 3/2-3/2 system has been possible for the less abundant isotopic forms of Bri. For the l/2-1/2 system, it is evident from the photoelectron spectrum that the ground-state parameters must be similar to those of X211J,2g.The analysis is further aided by the easily recognizable isotopic splittings, and by knowledge of the approximate origin; there is no serious congestion, and separate vibrational analyses of all three isotopes were possible. In comparing the cold emission spectrum with the room-temperature spectra we have to allow for a difference between the positions of the observed peak maxima and the true band origins. According to measurements on neutral Br2 in a very similar

19199.0

19918.0

211a8.9

21335.4

2147v.5

21621

27

za

29

30

of

congestion,

35

Because

27312

3L

asterisks.

21317

22176

33

*

21179

2lLo4

22040

32

31

zoavi .a

21043.1

26

some strong

19660.3

20378.4

20743.6

24

25

19508.4

20228.3

20589.9

23

19710.7

20L35.a

peaks

fit

19152.6

18525.6 186E4.5

18881.4 19040.6

22

21

19241.8

19&x.2

19759.8

19963.3

20122.4

19

20

18362.6

18720.2

more

than

18796.5

18200.1

19079.1

19439.1

lVao2.0

18

18916.4

19275.7

19637.9

17

17682.0

18036.0

18748.9

19110.2

19473.1

17512.9

16

17347.6

17700.7 17867.8

18225.

18584.5

18944.3

19307.2

15

I

lao57.5

18414.7

18775.0

19138.4

14

17177.1

17531.9

one place

xan6.1

in

17979.1

17820.2

17493.2

16823.9 *

the

table,

17783.5

11668.2

16979.9

16643.5 *

16299.0 *

18244.0

18605.2

13

16126.5

16650.7

17357.8 *

17712.7

18070.6

18432.3

12

16650.7

15950.0

17361.1

16299.0 *

17719.6 16L75.7

la255.3

II

15587.0 15770.4

17181.2 *

180m.4

10

16288.9

15212.8

7

17538.6 *

16823.9 *

17181.2 *

17538.6 *

15566.1

15375.1

14’390.6

6

17896.3

16643.5 *

1

17mO.

17357.8 *

17719.6

17901.0

9

16458.9

16814.3

17173.5

7

8

15917.0 16104.2

15726.3

5

16270.9

4

16080.0

3

16436.2

2

16627.5

1

16985.7

0

5

v”

6

4

3

2

1

0

V’

Partial Deslandres Table for A211~,2u-XZIIg,~ of 79*8’Br;

TABLE IIIa

and are

16465.2

16126.5

15950.0

6

markedby

16117.4

15950.0

15779.9

15603.5

9

17146.5

11

3L

20L73.5

33

lY860.2

20354.3

19732.5

27

28

20233.2

19600.1

26

32

39472.3

25

31

19341.3

2L

19981.9

19206.6

23

20109.5

19071.2

22

30

18818.2

18937.2

29

18720.2

18796.5

20

21

19373.7

18982.2

18577.5

18637.5 18220.

1808o.L I

17910.5

18657.2

19

17802.2

18519.1

ltl

181%.7

18375.4

17

17726.5

17586.8

17917.7

17783.5

176L9.9

17512.9

17375.3

17236.8

17700.7

16746.1

16604.3

15970.4

17520.8

18159.2

17876.0

18234.0

16

15825.3

16317.6

17020.4

17375.3 17660.7

17732.1

Nn89.5

15

16875.6

leoO4.8

10354.5

18123.6

17888.2 18234.0

17766.2

17520.8

17267.3 17613.4 17867.8

17133.5

16875.6

16741.1

16191.3

16053.5

15912s

15770.4

15035.7

7

17482.2

17353.6

17221.8

16255.5

15680.4

16172.4

16376.9

16731.0

17229.8

15533.1

15381.3

15232.7

17085.5

16029.3

15731.2

15083.8

16231.2

16062.2

15582.1

15434.6

15286.3

15136.3

6

16583.9

16436.1

15936.3

15787.5

15639.2

16983.9

S

of 79.s’Br2+

16938.5

16791.1

16287.5

16139.5

16495.3 16643.5

15991.4

16346.4

18017.2

17586.8

17946.0

14

17442.0

17802.2

13

17295.3

17Oco.l

10

12

16852.3

9

8

15489.0

15334.6

4

15861.9

3

7

2

15180.9

I

15691.2

17506.8

0

5

V”

6

L

3

2

1

0

V’

Partial Deslandres Table for A211,z-X211,,,

TABLE IIIb

17544.2

17177.1

1666L. 1

16531.8

16126.5

15985.4

15709.8

15566.1

15122.6

8

15920.9

15647.5

153680

9

THE A-X SYSTEM OF Br;

277

system (15) the rotational temperature of Brz in the beam is likely to be about 22 K. By assuming that this temperature is not altered in the electron-impact ionization process, and using our final rotational parameters we have calculated the expected rotational band contours; the instrumental resolution was represented by a Gaussian profile of half-height width equal to the width observed for atomic lines. This simulation indicates that peak maxima in the dold spectrum shift by - 1.75 cm-’ from the origins, which essentially coincide with the heads (Av < 0.2 cm-‘). The shift is determined almost entirely by the instrumental resolution, and should not vary significantly over the spectrum, or between one subband and another. The T, values in Table I include this correction, which is comparable with the uncertainty of the long extrapolations to the origins of the upper states. The Deslandres tables have not been so corrected. The band intensities in the cold emission spectrum were subjected to FranckCondon analysis in order to confirm the vibrational assignments, and to determine B values in the upper states. As the photoelectron spectrum is wholly unresolved in the A bands it allows only the crudest estimates of B’, using the semiclassical ap proximation (13). These estimates nevertheless served as starting points for the more precise calculation of relative intensities of bands from particular upper levels in the emission spectrum. The parameters of the ground ionic states were fixed (B, from photoelectron intensities, cy, from laser-induced fluorescence), and B’, and c& were varied in search of the best overall fits. Because the w,y, values are large in the A states, it seems unlikely that (Y,alone will suffice to represent the variation of B with V’correctly. Nevertheless, we judged it unreasonable, as well as difficult, to determine a third parameter in this indirect way from intensity data. The B’ values obtained in these ways from relative band intensities have been checked further using the laser-induced fluorescence data. Since the J’ have already been estimated, the B’ and B” values enable us to estimate the positions of the origins of bands into which laser absorption occurs, and so to identify the upper states of the fluorescence. Calculated and observed bandheads agree within a few cm-’ (*6 cm-’ mean deviation), confirming the reasonableness of the parameters adopted. Because of the uncertainty in J’, we have not used the wavenumber intervals between pumping lines and observed heads to constrain the rotational parameters. On the other hand, when the upper levels are identified, the LIF spectra provide a better test of the assignment and parameters through their relative line intensities compared with the Franck-Condon calculations, since they extend to high u”, and are accurately measured. An example of the intensity fit achieved is shown in Fig. 4. 3.3. The Normal Emission Spectrum At normal temperature the emission spectrum of Br2+consists of a great number of strongly overlapped bands, some showing clear heads degraded to the red. In the violet the normal spectrum continues beyond the end of the cold spectrum, presumably because ionization of vibrationally hot Brz can populate higher levels of Brz A*&. The bandheads in this region augment the cold spectrum, particularly for high vibrational levels of A*I&, and have been incorporated into the vibrational analysis. The heads in the normal emission spectra are found to be shifted on average by +4

278

HARRIS, ELAND, AND TUCKETT

FIG. 4. Measured vibrational band intensities in LIF spectra of Brf (solid lines) compared with FCF calculations (dotted). No calibration of the monochromator and detector sensitivity has been attempted, but response is expected to be smooth in the region involved.

cm-’ from the corresponding peaks in the cold spectrum. This shift arises in part from the peak-origin gap of - 1.75 cm mentioned before, and in part from the effect of finite instrumental resolution on the apparent head position. Simulation of the profile indicates that the observed heads are shifted from the true heads by +2.2 cm-‘, making a total shift of 3.95 cm-’ between cold peaks and hot heads. Because the data from the cold spectrum are much more numerous the vibrational analysis was made by incorporating normal heads shifted down by 4 cm-‘, to put them onto a common scale. The band origins may therefore be estimated from figures in the Deslandres tables by adding a correction of 2 cm-‘. Br: emission from the flowing afterglow and Cossart sources was sufficiently intense that higher-resolution recordings could be made. At the highest resolution used (0.15 cm-‘) the bands are still too congested for complete rotational analysis, but some single branches can be followed over a wide range of J and the corresponding AB values derived. For the band at 5880 A, which exhibits the clearest branches, the partial rotational analysis yields (Bb - B;)3,2 = -0.0253$:$$, while the parameters in Table I predict a value of -0.025 1 for this quantity. Two bands of the l/2-1/2 system were also partially analyzed in the same way, and indicated AB values which agree with the parameters of Table I within the error limits. 4. LIFETIME MEASUREMENTS

Collision-free radiative lifetimes of a large number of vibronic bands of Brf were measured with the crossed molecular beam/electron beam apparatus. In these experiments, the electron energy was reduced from 200 to about 20 eV where the gun could be pulsed with a rapid on time. It is usual to apply the turn-on pulse to the grid in front of the filament of the electron gun. In our apparatus, however, we find it most effective to apply the pulse simultaneously to the final anode of the gun and its Faraday cage, the two components which encompass the molecular beam. A 25V pulse of variable width was applied to these two components, pulsing the electron energy rapidly from 20 eV (approximately threshold for production of Br; A211,) to 45 eV. Lifetime distributions were recorded by the single-photon counting method,

279

THE A-X SYSTEM OF Brz

using a time-to-amplitude converter and a multichannel analyser. Lifetimes were determined by least-squares fitting of the decay curves to single exponentials after background subtraction. The technique was tested on excited electronic states of some well-characterized ions (N: B*Z:, CO: A%, and B*2:), and gave results in agreement with the literature. Some of the Br: results are shown in Table IV; all these bands are clean and unblended. Since the only known higher state of Br:, B2Zi, is almost certainly predissociated, and indeed, its first dissociation limit lies just above A*&, these lifetimes are probably cascade-free. Although we measure the lifetime of particular ~1’- u” bands, we stress that the lifetime is a property only of the initial u’Q’state, and Table IV indicates that they show no distinct correlation with either 2)’or Q’. This is not surprising as several different factors contribute to the radiative lifetimes. The lifetimes can be expressed in terms of the Einstein A coefficients for dipoleallowed vibronic transitions,

7d= C

’

d

where C A,,& 0”

2 v:,~(v’~v”); Y”

l?, is the electronic transition moment which may vary over a large range of v’, while the second term represents the sum of the u3 factors weighted by their appropriate Franck-Condon factors. It is not at all obvious which term will dominate as v’ changes, so the lack of dramatic change of 7 with either v’ or fl should not be worrying. The error limits quoted in Table IV are one standard deviation, but are almost certainly over optimistic. In order to reduce the accumulation time for each band, relatively long pulse widths of 1 psec were used. In theory, for a pure square wave pulse this should not affect the decay curves; in practice, however, the turn off of the pulse is not instantaneous, and this may distort the initial part of the decay curves.

TABLE IV Collision-Free Radiative Lifetimes of Some Vibrational Levels of Br; A2n, air

a

v’

Band

1

10

10,3

613.8

513 (12)

1

14

14,l

568.5

524 (15)

h

/nm

r/ns

1

17

17,l

554.9

506 (13)

1

24

24,l

526.7

482 (15)

312

6

6,6

642.3

505 (15)

312

14

14,l

532.5

486 (11)

312

19

19,l

510.2

581 (15)

312

23

23,0

485.7

562 (21)

280

HARRIS, ELAND, AND TUCKETT

All curves were analyzed in the same way, so any error, if present, should be systematically constant, but the error limits on the lifetimes should be treated with caution. The general conclusion that 7 is independent of both v’ and Q is definitely valid. 5. DISCUSSION

In the present analysis 329 of the 525 bands observed in the cold emission spectrum are directly assigned, and the majority of the unassigned bands can probably be attributed to isotopic variants of the 3/2-3/2 system, or to bands involving very high values of u” and u’. The emission spectra therefore seems to be satisfactorily understood. The laser-induced fluorescence results confirm the analysis of the 3/2-3/2 system, and both systems fit well with the photoelectron data. It therefore seems certain that the analysis is now basically correct, despite the lack of rotational resolution of any band. The major conclusions, that in Bri the two components of the A%, state are indeed different and that the spin-orbit splitting is less in the A state (2040 + 100 cm-‘) than in the X state (2820 f 100 cm) are enshrined in the potential energy curves derived for the four states, shown in Fig. 5. Part of the difference between the

24000 22000 20000 2 18000-5? s ki 5

<*

6000 40002000 O-

11 2.0 8

11

11

18

1

”

1

2.4 1%1uc:e2ar d&k

(6’

FIG. 5. Potential energy curves for the states of Bri, derived as described in the text. The absolute spacings between the multiplet components rest on the determination of the spacing in X*II, by photoelectron spectroscopy.

THE A-X SYSTEM OF Br;

281

A2113,2and A211,,2states can be attributed to the closeness of the common dissociation limit (Br 2P3,2 + Br+ 3P2), to which all the low-lying states of Br: correlate adiabatically. The difference is in any case no greater than that between other well-established multiplet components in heavy diatomics, so there is no need to invoke any perturbation. The smaller spin-orbit splitting in A than in X is in line with common experience in photoelectron spectroscopy of other ions with two 211 states. Only in 1: is the splitting apparently greater in A than in X, in view of the present analysis for Br: there may be some doubt of this interpretation of the photoelectron spectrum, since the larger apparent splitting of A might alternatively stem from a large difference in the 1: A’II, state curves. RECEIVED: September

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