Analysis of the 2770-Å emission system in I2

JOURNAL OFMOLECULAR SPECTROSCOPY 86,393-405 (1981)

Analysis

of the 2770-A

Emission

System

in I,

K. S. VISWANATHAN, ABHA SUR, AND JOEL TELLINGHUISEN Department

of Chemistry.

Vanderbilt

University, Nashville.

Tennessee

37235

A weak emission spectrum of I, near 2770 A is reanalyzed and found to terminate on the A(luW) state. The assigned bands span u” levels 5- 19 and v’ levels O-8. The new assignment is corroborated by isotope shifts, band profile simulations, and Franck-Condon calculations. The excited state is an ion-pair state, probably the lg state which tends toward I-(‘S) + I+(V,). In combination with other results for the A state, the analysis yields the following spectroscopic constants: T; = 10 907 cm-‘, 2; = 1640 cm-‘. 0:: = 95 cm-‘, R; = 3.06 A: T: = 47 559.1 cm-‘, w: = 106.60 cm-‘, R; = 3.53 A. INTRODUCTION

Despite considerable spectroscopic effort spread over some six decades (I ), only a few of the electronic states of molecular iodine are presently known in any detail. The X(O+g’Z) and B(O+u3H) states have now been characterized with scrutinizing precision (2-4). Of the other 21 valence and 20 ion-pair states (I), only a handful have been analyzed rotationally, and those just recently and in some cases, sparsely (3 -8). Included among the latter is the A( 1u311) state, which was first reported in infrared absorption by Brown in 1931 (9) and first analyzed rotationally by Ashby in 1979 (8). In further work,Ashby and Johnson have revised the vibrational numbering of the A state upward, obtaining T, and Ba, values of 10 906 and 1641 cm-‘, respectively (10). These results corroborate an earlier suggestion (11) that the A state, which dissociates to ground-state I atoms, may be much more deeply bound than Brown concluded. In the present work we have analyzed a weak band system near 2770 A in the emission spectrum of I, in Ar. This spectrum, which had previously been thought to involve the X or B state (12, Z3), is now found to terminate on the A state. Vibrational isotope shifts confirm Ashby’s revised numbering. By combining our data with Ashby’s and Brown’s, we obtain moderately reliable spectroscopic constants for levels v = O-35 of the A state, which encompass 95% of the binding energy. Franck-Condon and band profile calculations are then used to deduce the rotational constants for the ion-pair excited state. The latter has the largest vibrational frequency and one of the smallest internuclear distances of any IZ ion-pair state identified to date. EXPERIMENTAL

DETAILS

The 2770-A emission band was recorded using procedures like those outlined previously (14,15). Spectra were photographed on Kodak Ia- plates, using the 393

0022-2852/81/040393-13$02.00/O Copyright 8 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.

394

VISWANATHAN,

SUR, AND TELLINGHUISEN

first order of a JY HR 1500 1.5-m spectrometer equipped with a 3600-groove/mm holographic grating. The reciprocal dispersion was 1.55 A/mm. The sources contained lz712or lzg12in equilibrium with excess crystalline iodine at ambient temperature, together with -l/2 atm Ar. The emission was excited with a Tesla coil. For typical slit widths of 10 pm, exposures ranged from 15 min to an hour. Iron calibration spectra were obtained from a microwave discharge Fe/I, lamp using exposures of l-3 sec. The plates were developed 4 min in Kodak D-19 developer. The spectra were measured on a Grant Comparator. The Fe calibration lines were fitted to low-order polynomials, with typical standard deviations of 0.0020.003 A. Most of the I, bandheads were determined with a precision estimated at 0.2-0.3 cm-l. This figure is comparable to that obtained in our least-squares fits, described below. RESULTS

AND DISCUSSION

Assignments The 2770-A emission system is confined to a relatively narrow band in the region 2730-2785 A (15). By way of contrast the better known F + X (25002720 A), E + B (4000-4300 A), and D’ + A’ (3100-3450 A) systems all show long short-wavelength extensions of gradually weakening intensity (24-16). In the 2770 system the intensity drops off rapidly with decreasing wavelength, and the short-wavelength tail is completely lost in the F --j X system, which is particularly congested at its long-wavelength end (16). Correspondingly the V” progressions are short, making assignments difficult. However, with the aid of the vibrational isotope effect, we have obtained a unique assignment of the measured features, which include many violet-degraded edges and spike-like features in addition to the dominant red-degraded bands. The analysis is corroborated through FranckCondon and band profile calculations, discussed further below. At the outset we sought an assignment scheme in which the bulk of the emission could be attributed to low V’ levels of a nearly thermalized emitting state. This model has proven correct for other halogen and rare-gas halide emissions from tesla discharges at similar pressures (14-17). Initially we concentrated on the reddegraded features and were able to find several plausible assignments which could account for most of the measured band heads. Only one of these, shown in Fig. 1, proved compatible with the observed isotope shifts, which were quite small (<4 cm-‘) for all measured bands. On comparing the vibrational structure of our lower state with Ashby’s recent results for the A state, we found complete agreement with both his high-precision measurements (8) and his revised numbering (10). Using his data and Brown’s (9) together with our own, we were able to define a fairly reliable potential curve for the A state, which we then employed in trial-and-error Franck-Condon calculations to locate the excited state on the internuclear axis. With the aid of band profile simulations employing these provisional potentials, we were eventually able to assign every clearly discernible feature in the spectrum, in a manner which was consistent with the FranckCondon calculations. Incorporation of the additional assignments in the least-

395

I, EMISSION SPECTRUM Fe 2739.55

I

Fe 2770.22

I

127

FIG. 1. 2770-A emission spectra of 1*91,and 12712,with assignments (estimated band origins)

shown

for the latter. squares fits then yielded improved constants and potentials for both states. This type of iterative analysis was employed previously in the interpretation of XeF emission spectra (18, 19) and is described in more detail in those papers. The assigned bands for both isotopes are listed in Table I, along with the residuals from the least-squares fit described below. In keeping with the model of a thermalized excited state, most of the assignments (and all of the strong bands) involve V’ = O-3. Many of the tabulated bands display multiple features, which are fully accounted for by the band profile calculations discussed below. A few of these bands were omitted from the least-squares analysis, because the head -+ origin correction was so large (> 10 cm-l) as to render the origin very uncertain. We emphasize that we could find no plausible assignment in which the lower state was assumed to be X or B, as concluded in previous studies (12, 13). Even without the isotope shifts, Franck-Condon calculations, and band profile simulations, the increased dispersion and resolution of our spectra permitted us to rule out these earlier assignments. Band Structure

The vibrational intensity distributions and rotational structure within the individual V’-v” bands of this system are qualitatively similar to those encountered in other halogen emission spectra (14) and in XeF spectra (18,19). In every case the emission originates from a deeply bound excited state of ion-pair character and terminates in a weakly bound lower state. Because the lower state has a

396

VISWANATHAN,

SUR, AND TELLINGHUISEN TABLE I

Band Origins (cm-‘) in the 2770-A Emission System of I, “‘_$I

v( 1271*)

Weight

Ava

v( 12912)

AV

O-10

40

2-13

40

o-9

40

968.5

-0.2

35972.3

0.1

l-10

40

36015.2

-0.1

36018.6

0.0

-0.6

024.5

35908.8 963.3

0.1 -0.4

3-14

10

022.4

O-8

40

031.3

0.0

4-16

10

042.1

0.2

Z-11

40

065.0

l-9

40

074.6

4-15

10

083.4

-0.2

o-7

40

097.7

5-17

10

107.8

3-12

40

117.6

2- 10

40

120.7

0.1

4-14

10

126.9

-0.2

6-19

10

141.7

0.1

O-6

40

167.7

4-13

10

172.9

5-15

10

4-12

-0.5

035.0

0.1

-0.1

067.8

-0.1

-0.1

077.8

0.0

0.1

101.4

0.0

0.6

108.3

0.7

119.8

-0.1

0.0

171.1

-0.1

0.1

174.2

0.3

187.5

0.2

187.6

0.7

40

222.3

0.1

-0.1

5- 14

10

230.6

6-16

10

250.9

l-6

40

273.8

275.7

0.6

l-5

40

347.5

-0.2

349.1

0.2

2-6

40

379.8

-0.2

380.7

0.5

2-5

40

453.5

-0.5

7-13

10

485.8

-0.6

484.1

0.0

8-14

10

542.3

-0.4

540.2

0.0

a

vcalc

-

vobs

from

least-squares

0.6 -0.1 0.1

fit.

smaller internuclear distance, the strong bands involving low a’ levels are predominantly red degraded. However, with increasing U’ the intensity maximum shifts to higher a”, and the individual bands begin to display violet-degraded edges far to the red of their origins. Finally, at sufficiently large U’ and u”, the bands are entirely violet degraded. The reason for this transformation is the rapid decrease of the lower-state rotational constant B':with increasing v”, together with strong lower-state centrifugal distortion. In the excited state, on the other hand, the B, value is nearly constant and the centrifugal distortion is weak. In the present case, unlike that for XeF, we are unable to resolve individual rotational lines. Consequently we must rely totally on the Franck-Condon intensity pattern and the v’-V” band shapes to locate the potential curves and determine the rotational constants. Fortunately the A state is fairly well determined by Ashby’s vibrational and rotational constants for o = lo-21 (8). We find that

I, EMISSION SPECTRUM

397

FIG. 2. Synthetic band profiles for selected bands involving u’ = 5. The calculations assumed a temperature of 360 K and a resolution of 0.05 A, and employed the constants B; = 0.02078 cm-l, 0; = 3.18 x 10e9 cm-i, and the A-state rotational and distortional constants given in Table III. The absolute wavelength and intensity scales are artibrary, but a constant relative intensity scale is used throughout. Band origins are marked with arrows.

the Franck-Condon calculations permit us to determine Rd,within about 0.01 A. For v’-v” bands where B: = BE, the synthetic band profiles are even more sensitive to slight changes in Ri, through their effect on the Bh values. From these bands we are thus able to extract fairly reliable Bh values, from which we can determine B: and CX:.The band profile calculations are also used to estimate the band origins from the measured features, which in some cases are several cm-l removed from the origins. It is of course the origins, rather than the band edges, which are required in the least-squares determination of the vibrational constants. In calculating the band profiles we have assumed a simple two-branch (P and R) structure, as would be expected if the excited state is a lg state and if(as expected) the fI doubling in both states is small. If the excited state were Og or 2g, a strong Q branch would occur. The agreement between observed and calculated band profiles supports the lg assignment, because a strong Q branch would significantly alter the band shapes. In Figs. 2 and 3 we illustrate the dependence of the band profiles on zl” for V’ = 5. Note that for small v”, weak violet-degraded edges occur in both branches far to the red of the origin and at large J. This doubling back of the lines at large J is directly attributable to the large centrifugal distortion in the lower state. With increasing u”, BI: approaches B:; and the violet-degraded edges move toward the origin and intensify, finally producing a strong spike near the origin. Further decreases in BI: yield a single, normal violet-degraded edge near the origin. Because of weak Franck-Condon factors (FCFs), not all of the illustrated bands are present in the spectrum; however we do observe bands of similar appearance for

VISWANATHAN,

SUR, AND TELLINGHUISEN 3

17 A

1

18 1

4

, L

3

a0H

A-7

>

FIG. 3. Band profiles for u’ = 5, calculated as in Fig. 2.

other a’ levels. In some cases the observed bands are distorted from their calculated shapes due to a strong rotational dependence in the FCFs, which is neglected in these figures. These effects are similar to those observed for XeF and can be accounted for quantitatively. Least-Squares

Analysis

In our initial least-squares fits we included just the red-degraded bandheads, for which the separation between head and origin is negligible in comparison with our measurement precision. On simultaneously fitting the assigned bands for both lz712and lzgIz to double polynomials in p(v’ + l/2) and p(u)’ + l/2) as described previously (14),’ we obtained minimum variance for a 0” numbering identical to the A-state numbering suggested by Ashby and Johnson (10). Changing the Y” numbering by + 1 and - 1 increased the variance by factors of 3 and 1.2, respectively. The former figure we consider definitive, but the latter is not large enough to rule out the reduced numbering. On the other hand the A +X bandhead measurements of Ref. (10) appear to eliminate the reduced numbering but not the increased numbering. Thus in combination the two studies strongly support the numbering recommended by Ashby and Johnson (10). After the Y”numbering had been ascertained, we expanded the fits to include both Ashby’s high-precision band origins (8) and the less precise, low-v bandhead measurements (IO), as well as Brown’s bandhead measurements (9). The highprecision data serve to locate the excited state very precisely, but the less precise low-v and high-v data are required to determine the A state outside the region of our observations (V = 5-19) and Ashby’s precise band origins (v = 10-21). The different data sets were assigned weights proportional to their estimated reciprocal variances: The actual weights were 2000 for the data from (8), 10 for those from * Here p is the isotopic ratio, (~1z7//.#‘*, which is 1.000 for ‘*‘I, and 0.99221 for **‘%.

399

I2 EMISSION SPECTRUM TABLE II Spectroscopic

Parameters (cm-‘) for States Involved in 277t?-w Band System of lz712 A (lu

‘II)

IkT (3Pl)

10906.81(94)a

Te

47559.08(12)a

CvI (Qb

94.954(825)

fv2(-wa

-2.4290 1.7471

c”3

-2.0286

c”4

1.2038

c”5

-3.6837

=vs

5.6717

cv7

-3.4898

=vtJ v

106.598(74) (11%)

-0.21S1(98)

x 10-I

(27%)

x lo-*

(22%)

x m-3

(20%)

x lo-’

(21%)

x IO-’

(24%)

x 10“

(27%) 31710c

1640.2(g) 2.845

C:l (Bd

x 10-2

2.134

CT2 (-a&

-4.202

x 10-4

c

-5.096

x 10-6

r3

f

1.407

r4, Re (A)

in parentheses fits.

b

cri

and

represent

x 1O-2 x 10-4

x m-7

3.0~i6~

aFigures squares

cvi

-1.028

3.52Sd

represent

standard

coefficients

in

enors

customary

(10)

from

polynomials

leastin

(v

+

l/2). ‘Calculated dUncertain

for by

dissociation about

0.01

to

I’(3Pl)

+ I-&).

i.

(lo), 1 for those from (9) (with the A-state numbe~ng increased by 14), 40 for our red-degraded bandheads, and 10 for our less precisely estimated band origins (obtained from the violet-degraded edges and spikes interpreted through the band profile calculations). All of the A + X data were converted to A-state term values using Luc’s very precise X-state energies (2). To compensate for possible systematic errors in the A c X bandhead measurements from Refs. (9) and (lo}, we incorporated in the fits two extra degrees of freedom, in the form of “throwaway“ TeA values for these two data sets. The least-squares equations were set up using formalism like that reviewed in (3) and were solved for various upperand lower-state polynomial orders. Minimum variance was obtained for a 14parameter fit containing 2 vibrational parameters for the excited state and 8 for the A state. Results are summarized in Table II. Our values for TeA and &, agree with those of Ashby and Johnson (IO), as well they should, since they are almost entirely determined by their data. The 5$,, value is calculated from the very precise value, $&, = 12 547.02 cm-’ for the X state (201, which shares the same dissociation asymptote. Both TeA and BeA are rather imprecise, in accord with the reduced precision of the A +- X bandhead data. However, the TJ value is quite precise, as is the A-state energy ToA (=TeA -t- GI,A) in the region u = 5-21 of the high-quality data. The error in I’,,, , as calculated from the relevant part of the least-squares variance-covariance matrix (3), is illustrated

400

VISWANATHAN, I

SUR, AND TELLINGHUISEN /

I

I

%

_/-(X10)

0

b

0.5-

1.0 - LI;:0 0

IO

"

20

30

FIG. 4. Standard error in TVAas a function of v.

as a function of u in Fig. 4. The increased uncertainty at low and high u reflects the decreased weighting of the data in those regions. The modest error in TVAis to be contrasted with the seemingly large errors in the high-order polynomial coefficients themselves, which, however, are highly correlated so as to give a welldetermined term value function. The “throwaway” TeA values for the data from Refs. (9) and (10) agreed with the value in Table II within about 1 cm-l, indicating that the bandheads measured in those studies are indeed good estimates of the origins. To determine rotational parameters for the two states we relied heavily on the B, values of Ashby for v = IO-21 of the A state (8). These values were extrapolated smoothly to u = 0 and were fitted to polynomials in (V + 1/2).2 Minimum variance was obtained for a four-parameter fit. The derived parameters are included in Table II, along with the B, and (Y,values deduced for the excited state from the Franck-Condon and band profile calculations. Potentials and Derived Properties

The spectroscopic parameters in Table II were used to generate RKR curves (21) for the upper and lower states. When we subsequently used the A curve to calculate centrifugal distortion constants (22), we observed rather poor quantum mechanical consistency with the starting spectroscopic parameters. Although such inconsistency is always present to some extent due to the semiclassical approximation inherent in the first-order RKR method, large discrepancies are often indicative of anomalies in the potential. On examining the left branch of the A curve, we noted a slight decrease in the absolute slope in the v = IO-15 region. This behavior was essentially independent of the assumed low-v extrapolation of the rotational constants,2 hence appeared to be inherent in the data. Whether it represents a real effect or an artifact we cannot say. However, for the purpose of the Franck-Condon and centrifugal distortion calculations, we chose to replace the RKR left branch with a smooth R-” curve constrained to give reasonable agreement with the raw B, values and with the earlier analysis of the A + X diffuse absorption to the blue of 8000 A (see below) (II). The right branch was then adz Although the low-v region of the B, curve remains uncertain, the potential curve and derived therefrom for v > 10 are relatively insensitive to the low-v uncertainties.

properties

401

1%EMISSION SPECTRUM TABLE III Adjusted Potential for the A State of 12, and Computed Spectroscopic v

TV

By x 100

0 1 2 3

10953.79 11044.50 11131.37 11214.73

2.8297 2.7863 2.7422 2.6976

4 S

11294.74 11371.40

6 7 8 9 10 11 12 13 14

11444.67 11514.51 11580.86 11643.72 Il703.10 11759.06 11811.70 11861.14 11907.54 11951.11 11992.04 12030.56 12066.89 12101.22

2.6525 2.6068 2.5606 2.5138 2.4664 2.4183 2.3697 2.3206 2.2710 2.2211 2.1714 2.1222 2.0738 2.0264 1.9802 1.9353

12133.72 12164.54

1.8915 1.8487

12193.80 12221.59 12247.97 12272.99 12296.67 12319.05 12340.14 12360.00 12378.67 12396.24 12412.82 12428.54 12443.54 12457.97

1.8065 1.7648 1.7231 1.6812 1.6388 1.5958 1.552s 1.5091 1.4662 1.4247 1.3854 1.3493 1.3170 1.2882

1s 16 17 18 19 20 7.1 22 23 24 2s 26 27 28 29 30 31 32 33 34 35

3”

x IO8

1.07 1.13 1.18 1.23 1.30 1.38 1.49 1.62 1.77 1.94 2.13 2.34 2.56 2.78 2.98 3.17 3.34 3.49 3.63 3.75 3.86 3.98 4.13 4.29 4.50 4.73 5.00 5.29 5.57 5.81 5.98 6.02 5.90 5.62 5.22 4.83

Constants (cm-l)

-HV x 1014

Rmi,., (;I

Rmax (11

2.6 3.0 3.5 3.9 4.4

2.9855 2.9453 2.9217

3.1367 3.2121 3.2722 3.3247 3.3786 3.4293 3.4797 3.5304 3.5819 3.6345 3.6885 3.7440 3.8013 3.8605 3.9214 3.9841 4.0482 4.1137 4.1804 4.2480 4.3167 4.3862 4.4569 4.5290 4.6028 4.6788 4.7573 4.8389 4.9238 5.0121 5.1038 5.1981 5.2941 5.3904 5.4855 5.5780

5.0 5.7 6.5 7.4 8.4 9.5 10.8 12.4 13.4 14.5 15.3 16.0 16.6 17.5 18.4 20.3 22.8 26.0 30.7 36.0 42.0 47.4 52.3 54.2 52.7 44.9 30.2 10.9 -?.6 -13.4 7.4

2.9049 2.8922 2.8820 2.8737 2.8666 2.8604 2.8549 2.8498 2.8449 2.8405 2.8364 2.8325 2.8290 2.8257 2.8227 2.8199 2.8172 2.8147 2.8124 2.8102 2.8081 2.8062 2.8044 2.8027 2.8010 2.7995 2.7981 2.7968 2.7956 2.7944 2.7933 2.7923 2.7913

justed to preserve the RKR turning-point differences, R,,,&D) - Rmin(u), which are determined by the vibrational parameters alone. Below z, = 30 these changes in the potential amounted to at most 0.0033 A. The adjusted potential and its computed spectroscopic constants are given in Table III. The error in T, is shown in Fig. 4. The B, values reproduce Ashby’s values within 2 x lop5 cm-’ for levels 13-16 and 18-20, with discrepancies as large as 8 x lo-” cm-* for the other levels. The latter discrepancies are somewhat larger than the estimated experimental precision, raising questions about the validity of the “smooth potential” assumption employed in our calculations. Outside of the region o = 10-21, the calculated B, values are of course determined by our assumptions in extrapolating the experimental B, values and the potential; as a rough indicator of their probable reliability, we estimate an uncertainty of 3 x W4 cm-‘. The D, and II,, values are estimated reliable within about 5 and 20%, respectively, below u = 22. Above u = 30 the calculated values for both constants decrease. This behavior is unusual and is not thought to be real, rather, is an artifact resulting from uncertainties in the vibrational parameters and the potential at high D. Within the v = lo-21 region, most of the calculated D, values

402

VISWANATHAN,

SUR, AND TELLINGHUISEN

I

6000

7000

I

8000

itA,

FIG. 5. Comparison of present calculated A +- X continuum extinction cm-l) with experimental values from Ref. (II).

coefficient (liters mole-’

agree with Ashby’s estimate of (3.5 + 1) x lOPa cm-l; however, they show a significant dependence on u. The A t X continuum absorption measurements obtained in Ref. (II) are compared with the present calculated spectrum in Fig. 5. This spectrum was calculated using the quantum methods outlined in Ref. (23). The dipole strength [assumed constant and defined as in (1 I )] needed to bring the calculated spectrum into quantitative agreement with experiment was 1pe 12 = 0.041 D2,which is about 7% below the earlier estimate. Most of the decrease occurs in the short-wavelength wing of the calculated spectrum, which is the region of overlap with the much stronger B tX system.3 Relative to the energy at the dissociation limit, the present repulsive branch of the A curve is given by U(R) = -3781 + 2.9580 x 108/Z?11.This curve has the same power dependence as the earlier version but is slightly less steep near Rex = 2.667 A, where the A +X diffuse absorption occurs. The new curve is also shifted slightly (-0.007 A) to smaller R. The latter change represents the error in the reflection method used to simulate the spectra in (II). The FCFs calculated for the 2770 emission system are summarized in Table IV for J = 0 and J = 100. For reference, the average J in the upper state at the estimated discharge temperature of 360 K (25) is 95. For most of the red-degraded features, which occur at low J, the J = 0 FCFs are appropriate. However, many of the violet-degraded edges occur at larger J, so the J dependence of the FCFs must be taken into account in interpreting these features. The intensity distribution in this spectrum can be better understood by examining the potential diagram in Fig. 6, which includes the difference potential, V(R) = U'(R)- U"(R). The latter quantity is just the spectral frequency as a function of R, as prescribed by the classical Franck-Condon principle (26). The concentration of the emission in a narrow wavelength region is attributable to the sharp minimum in V(R)near R$.With increasing U’ the emission shifts to higher u” in a way that roughly maintains the emission frequency near the minimum in V(R).The high-v’ transitions are progressively more difficult to identify, not only because of the declining v’ population, but also because the individual v’-u” FCFs decrease, as the emission is distributed over more U” levels in accord with the 3 The strength of the continuum absorption in the 600&A region remains experimentally uncertain (24).

403

I, EMISSION SPECTRUM TABLE IV Franck-Condon

Factors

(X 103)

for l&P1)-A

decreasing AG: value. The Franck-Condon less strongly dependent on u’. Upper-State

System of 12712a

density (=FCF/AGz)

near 2770 A is

Designation

Up to now we have said nothing about the electronic character of the upper state, except that it is an ion-pair state, probably of 1g symmetry. There is evidence (15) that the ion-pair states of I, group in accord with the energies of the I+ ion. In that case it is reasonable to assign the upper state in our spectrum as the lg state arising from I-(‘S) + If(3P1). This assignment yields the 9& value in Table II. Of the other five ion-pair states expected in the 47 000 cm-l region, only one has been analyzed, namely, F(O+u) at 47 218 cm-l (16). The F state has a significantly smaller o, (96 cm-‘) and a slightly larger R, (-3.55 A) than our 1 g state. On expanding these comparisons to include the better characterized ion-pair states at lower energy (15,27), we find none having a higher o, value and only one [the E state of Danyluk and King (27)] having a comparably small R,.

VISWANATHAN,

SUR, AND TELLINGHUISEN

PIG. 6. Potential curves for the A and lg(“P,) states, and the difference potential V(R) (broken curve). Note the different ordinate scales for the three curves. CONCLUSION

With the completion of this work, which builds on results in Refs. (8-IO), the A state of Iz is now known with a precision comparable to that for the A state of Br, (28,29). In both molecules the A state has a slightly larger R, and appreciably smaller w, and gd, than the well-known B(O+u) component of the low-lying VI?, manifold, The differences are mainly ascribable to the different dissociation limit of the B state (2F3,2 + V,& as compared with the other components. Of the latter only the 0-u remains unobserved, the 2u having been identi~ed as the terminus of the uv laser transitions in both molecules (24). The ion-pair states in most of the halogens remain poorly characterized, although about half a dozen of these states are now fairly well known in Iz (15,27). With such a large number of these states, notation is a problem. As one possible solution we suggest the notation used here (e.g., Table II), which gives the case c symmet~ and the atomic ionic parentage of the state. This scheme is fully descriptive in Ip, since no two states of the same case c symmetry correlate with a given J level of the I+ ion, and since the fine structure components of I+ are well separated in energy. However, it may not be suitable for Cl, and F,, in which the ion-pair states must be closer to Hund’s case a. Also, the ionic correlation may not be obvious in all cases. According to this scheme the well known D, I)‘, E, and F states of IS are denoted O+U(~P~),2g(V,), O+g(V,), and O+U(~P~),respectively, where the ionic ancestry is probable but not yet certain (15). ACKNOWLEDGMENT We want to thank R. A. Ashby for helpful correspondence, including updated estimates of the band origins and& values in Ref. (8), and for permission to use the results in Ref. (IQ) prior to their publication. This work was supported by the Vanderbilt University Research Council. RECEIVED:

August 22, 1980

I, EMISSION

SPECTRUM

405

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