ORGANIC MASS SPECTROMETRY, VOL. 28, 327-334 (1993)

Ion-pair (X' + Y -) Formation from Photodissociation of the Interhalogen Molecules BrCl, ICl and IBrt

Devinder Kaurt and Andrew J. Yen&*$ Department of Chemistry, State University of New York at Albany, Albany, New York 12222, USA

Robert J. Donovan Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK

Agust Kvaran Science Institute, University of Iceland, Dunhaga, 107 Reykjavik, Iceland

Andrew Hopkirk SERC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK

Excitation functions for both positive and negative ions (X+ + Y-), formed by photodissociation in the vicinity of the onset of molecular ionization, for the jetcooled interhalogen systems BrCl, ICl and IBr are reported. The overall shape of the excitation functions indicates that a number of overlapping Rydberg systems are accessed leading to dissociation. From analyses of the ion-pair excitation functions of the three molecules, by means of simulation calculations, the dominant Rydberg systems involved are found to be X('II,,,),nsa and X(zlI, l ,) ,nsa: other weaker Rydberg systems, with configurations X (21T312)c npz, A ('I1312)c npc, A ('IIl12)npz, A (zl13/z)c nsa and A (2111,2),nsa are also found to be present. It is concluded that the formation of ion pairs occurs by the same process in all three interhalogens, i.e. via predissociation of bound Rydberg states by ion-pair states.

INTRODUCTION

The formation of ion pairs (X' + X-) in the homo- nuclear diatomic halogens has been demonstrated by photoabsorption mass spectrometry in F, , l C1, ,, Br,3p4 and I, .' Berkowitz et al., studied the photoexci- tation of C1, in three ion channels, C12+, C1+ and C1-. The mechanism for ion-pair formation proposed involved the initial excitation into bound Rydberg states, which are predissociated by ion-pair states. The observed structure was assigned to at least five Rydberg states with the associate cores X 'He, A 'nu and ,Ze+.

Similarly, in the cases of Br,3.4 and I, ,5 ion-pair for- mation has been detected mass spectrometrically, and the mechanism for ion-pair formation was described in terms of homogeneous coupling between Rydberg states and ion-pair states correlating with X- (IS,,) + X + (3PJ). The states involved in Br, were assigned to

the 8pn(O,+) Rydberg state, which converges to the X 2H1,2e Br, + ion, and the D (0,') ion-pair state.3 In I,, the main structure in the excitation functions was

t Dedicated to the memory of Professor Einar Lindholm. Present address: Laporte Industries Singapore Pte Ltd, 14 Tuas

8 Also, Department of Physics, State University of New York at Avenue 20, Singapore 2263.

Albany, Albany, New York 12222, USA.

0030-493X/93/040327-08 $09.00 0 1993 by John Wiley & Sons, Ltd.

assigned to coupling between three Rydberg states, 9pn, lOpn and llpn, which converge to the X2113/2e12f limit, and the ion-pair continuum.

In this paper, we report the excitation spectra for both X + and Y- formation in the diatomic inter- halogens BrCl, IC1 and IBr in the region from threshold to above the first ionization limit of these molecules. As was found for the homonuclear halogens, the excitation spectra for the interhalogens are remarkably simple in structure considering the high density of electronic states present in the vicinity of the first ionization limit.

EXPERIMENTAL

The experiments were carried out on beam line U11 of the National Synchrotron Light Source (NSLS) facility of the Brookhaven National Laboratory. The molecular beam apparatus and its general operating conditions for producing photoabsorption ion yield spectra have been described in detail elsewhere6 and only a brief descrip- tion will be given here.

Synchrotron radiation emanating from the 750 MeV electron strorage ring at the NSLS was dispersed using a normal incidence 4 m monochromator (1200 lines mm-' grating). The light was then passed through a 2 mm thick LiF filter to eliminate second- and higher-

Received 16 September 1992 Revised manuscript received 5 January 1993

Accepted 6 January 1993

328 D. KAUR ET AL.

order radiation. A wavelength bandpass of 0.09 nm was used throughout. Radiation intersecting the molecular beam was monitored using a sodium salicylate quantum converter plus photomultiplier. This arrangement per- mitted the normalization of the measured ion intensities from the quadrupole mass spectrometer, thereby cor- recting for the decay with time of the synchrotron radi- ation and also the wavelength dependence of the monochromator.

To facilitate the measurement of negative ions without interference from background electrons, two permanent magnets, with an effective field of about 10 G, were placed outside the vacuum system on either side of and half way along the quadrupole mass spec- trometer. This was done to inhibit the collection of elec- trons by the channeltron multiplier at wavelengths below the ionization limit. With the magnets in the optimum position, a background count rate of less than 20 cps of electrons was obtained.

The samples of bromine monochloride, BrCl, that were used in the experiments were a mixture of 1% BrCl in helium. BrCl was prepared from a stoichiomet- ric mixture of Br, (J. T. Baker, 99.8% purity) and C1, (Matheson Gas Products, 99.9% purity). This mixture was refluxed about six or seven times. After degassing at 77 K, it was transferred in to a glass bulb for storage. The samples of ICl (Eastman Kodak, 98% purity) and IBr (Aldrich Chemicals, 98% purity) were degassed with liquid nitrogen before use. Neat samples of ICl and IBr were connected to the gas inlet line of the vacuum chamber and helium was passed over the samples to obtain a 1-2% mixture of ICl or IBr in helium. For all three samples, the pressure in the nozzle chamber during the scans was in the Torr range and the pressure in the interaction chamber was -6 x lo-' Torr (1 Torr = 133.3 Pa).

The molecular beam was formed by expanding the mixture of sample plus helium through a heated (100 "C) glass nozzle of 200 pm diameter. The beam was collimated by a skimmer 1 mm in diameter, located 5-10 mm downstream of the nozzle. The pressure of the gas mixture passing through the nozzle was adjusted to give the maximum intensity of ion counts. It was found that as the nozzle pressure was increased an optimum pressure was reached, beyond which a decrease in all the mass-selected ion signal intensities was observed. This was presumed to be caused by the formation of clusters.

The wavelength scale of the synchrotron monochro- mator was calibrated by setting the zero at the maximum of the zero-order radiation. The photoioniza- tion yield spectrum of argon was also recorded to check the accuracy of this calibration as well as to determine the resolution. The wavelength scale was found to be accurate to within k0.02 nm. Photoabsorption ion yield curves were obtained using standard pulse counting procedures, a CAMAC interface and a PDP 11/23 minicomputer.

Condon factors for transitions from the ground states of the molecules to various accessible Rydberg states.' Individual Rydberg systems were thus obtained employ- ing Gaussian line-shape functions for each vibrational band, in which the identical width of all bands in a system was an adjustable parameter. The total simu- lated spectrum was derived from the sum of the individ- ual Rydberg systems with appropriate weighting factors to give the best overall fit to the experimental spectrum. Contributions from hot bands were incorporated into the simulation calculations assuming a vibrational Bolt- zmann temperature of 250 K.3*5*7

RESULTS AND DISCUSSION

Ion-pair excitation functions for BrCl

Figure 1 shows the excitation functions for the forma- tion of the ion pairs, Br' and C1-, together with the ionization function for the parent molecule, BrC1. The spectrum of Br+ is a result of a total of six scans, while the spectrum of the negative ion C1- contains half as many scans. This accounts for the lower signal-to-noise ratio for C1-. Comparing the overall contours of the two excitation functions, it is evident that the two are very similar to one another. This implies that both Br'

Analysis procedure

Analysis of the ion-pair excitation functions were per- formed utilizing simulation calculations of the Franck-

103 106 109 112 115 118 121

Wavelength / nm Figure 1. Excitation functions for Br+ and CI- formation, together with the ionization function for the parent molecule, BrCI. The thresholds for formation of the ion pairs Br+ (3P2,1,0) + CI- ( 'So) and the parent ion, BrCI+ (X2n,/ , , l i 2 ) are indicated.

ION-PAIR FORMATION FROM PHOTODISSOCIATION OF INTERHALOGEN MOLECULES 329

J I

and C1- are formed via the same process (i.e. via a common intermediate). The thermodynamic thresholds for the formation of ion pairs in the Br+(3P,) + C1- ('So) states are indicated in the figure, along with

the ionization potentials for the formation of the spin- orbit components of the molecular ground state ions, X 21-13/2 and X 2111/2. From the ionization function of BrCl, the ionization energy of the X21-13/2 state was determined to be 11.02 & 0.01 eV (112.5 nm). The second ionization energy, 11.29 eV (109.8 nm), was obtained by adding the spin-orbit splitting energy' to the first ionization energy.

Table 1 lists the peak positions and energy separa- tions for the excitation functions for the formation of Br+ and C1-. From the photoelectron spectrum of BrCl, Dunlavey et a[.' found average values of the vibrational constant o, in BrCl+ to range from 486 f 20 cm-' for the X2J13/2 state to 510 f 20 cm-' for the X 21-11,2 state. Hopkirk et u1.' observed average vibrational separations of 507 f 3 and 508 f 4 cm-' in the u5 and b, Rydberg series of BrC1, respectively, which was found to agree reasonably well with the work of Cordes and Sponer." The energy separations listed in Table 1 range from 110 to 510 cm-'. From these data it can be inferred that sections of the spec- trum contain vibrational peaks from two overlapping Rydberg series, i.e. alternate peaks, rather than adjacent peaks, belong to one particular Rydberg series. The vibrational energy separations for the Br+/Cl- excita- tion functions are then determined to be in the range 470-507 cm-', in good agreement with the values in Refs. 8-10.

As in the case of 12,, the overall shape of the excita- tion functions for Br+/Cl- suggests that more than one Rydberg state is involved. The oscillator strength for a transition to a Rydberg state decreases rapidly as the principal quantum number n increases (I cc K 3 ) , imply- ing that the formation of Rydberg states with low prin- cipal quantum numbers will be favored.

10 Q 8 0 I I

l b 9 '- 1 0 2 1 0 I I 1

Table 1. Peak positions in the excitation functions of Br' and CI -

nso[x.1/2],

nso[X. 3/21,

Wavelength' (nm)

1 1 8.5 1 1 8.0 1 1 7.8 117.4 117.1 1 1 6.7 1 1 6.6 1 1 5.9 1 15.4 114.7 114.1 1 1 3.8 1 1 3.4 1 13.1 1 1 2.6 1 1 2.0

a Uncertainty = *0.02 nm. "uncertainty = i15 cm-'.

Energyb (cm-')

84 400 84 720 84 900 85 200 85 400 85 670 85 780 86 250 86 640 87 150 87 660 87 91 0 88 200 88 400 88 800 89 300

Separationb (cm-')

320 180 300 200 270 110 470 390 51 0 51 0 250 290 200 400 500

BrCl spectral analysis

The assignment of the bands below 115.9 nm will be discussed first. The vibrational structure between 115.4 and 114.1 nm is assigned as [X, 1/2],8sa (see Fig. 2). The (0, 0) band of this system is assigned to the peak at 115.4 nm, which is in good agreement with the calcu- lated value of 115.5 nm, based on extrapolation of the b, series. The (0,O) band of the [X, 1/2Ic 9sa system was calculated to be at 113.7 nm. Therefore, the peaks in the spectrum at 113.7, 113.1 and 112.6 nm are assigned as the (0, 0), (1, 0) and (2,O) bands, respectively, of the [ X , 1/2],9sa system. According to the n - 3 intensity dis- tribution, the intensity ratio for the 8sa to the 9sa peaks should be 1 : 0.7. However, the (0, 0) bands in the 8sa and 9s0 series are of almost equal peak intensity, and so are the (1, 0) bands. This can be accounted for by the occurrence of the threshold for formation of the ion pairs Br' (3P0) + C1- ( 'So) at 113.7 nm. The opening up of this new channel increases the probability for ion- pair formation, thereby enhancing the intensity of the bands from the 9so Rydberg state. The [ X , 1/2],10sa state is expected to occur at 112.6 nm. However, the onset of the first ionization limit (BrCl' + e-) occurs at 112.5 nm, and this may be expected to cause the inten-

I I

110 112 114 116 118 120 Wavelength / n m

Figure 2. Comparison of the experimental and calculated excita- tion functions for Br+(CI-) formation. The positions of the first few vibrational bands in each Rydberg state are indicated at the top. The calculated spectrum is a sum of the transitions to seven Rydberg states, each of which is shown. The FWHM for the peaks in the calculated spectrum range from 220 to 240 cm-'.

330 D. KAUR ET AL.

sity of the lOsa state to decrease beyond that expected from the n V 3 relationship. The [X, 1/2],10sa Rydberg state bands do appear to be present in the spectrum, albeit with low intensity, and are obscured by more intense bands from other Rydberg series. The presence of the lOsa state is further supported by the observation of the l lso state in the molecular ion spectrum, which will be discussed in greater detail in a separate paper."

In the longer wavelength region, the bands at 116.6 and 115.9 nm are assigned as the (0, 0) and (1,O) bands, respectively, of the [X, 3/2],9sa Rydberg state, which is calculated to be at 116.6 nm (see Fig. 2). Since the 9sa state is present, it follows that the 8sa state, which is expected to lie near the threshold for ion-pair forma- tion, should also be present. The (0, 0) band of the [X, 3/2],8so state was calculated to lie at 118.5 nm. Except for a slight shoulder in the spectrum, there is no peak at 118.5 nm that could be assigned as the (0, 0) band. However, the peaks at 117.8 and 117.1 nm could be assigned as the (1, 0) and (2, 0) bands, respectively, of the 8sa state. Since the intensity of a band in the ion- yield profile is formally proportional to the product of two overlap functions, e.g. the Franck-Condon factor for a transition from the ground state to the Rydberg state and the overlap of the Rydberg state and ion-pair state wave functions, the absence or weak intensity of a particular band in the ion-pair excitation function can result from poor or zero overlap in either or both of these functions. In this particular case, the essential absence of the (0, 0) band in the 8so Rydberg system suggests a poor overlap of the u' = 0 vibrational state wave function with the ion-pair continuum wave func- tion.

Thus far, Rydberg states based on the Xzl13/2 and Xzlll l , core states account for the majority of the bands in the Br+/Cl- excitation functions, except for a series of bands at 118.0, 117.4 and 116.7 nm, which appear to be the (0, 0), (1,O) and (2, 0) bands of another Rydberg system, and a series of bands in the short wavelength region originating at 113.4 nm. The posi- tions of the Rydberg states converging on the AZI13, , and A 2111/z ion states were calculated using the 'light- atom' approximation (i.e. with the view that the lowest ionization potential in mixed diatomic halogen systems is associated with the removal of an electron that is pri- marily localized on the heavier atom, while the second ionization potential is associated with the removal of an electron primarily localized on the lighter atom).'2J3 Based on this assumption, calculations were performed using the quantum defects for atomic chlorine which suggest that the bands in the long wavelength region (116-119 nm) are due to the [A, 3/2],4so state and in the shorter wavelength region (111-114 nm) to the [A, 1/2],4sa state.

Simulations of the individual contributing Rydberg states are shown in Fig. 2, together with their summa- tion. It can be seen from the final simulated spectrum that a reasonable fit to the experimental data is obtained with seven Rydberg states. Residual differences between the calculated spectrum (i.e. the Rydberg exci- tation function) and the experimental spectrum (i.e. the ion-pair excitation function) can be attributed to the effect of the overlap function between the Rydberg states and the ion-pair continuum states.

Ion-pair excitation functions for IBr and ICI

The excitation functions for I+ and Br- formation in IBr, together with the ionization function for the parent molecule, IBr, are shown in Fig. 3. Figure 4 shows the corresponding ion-pair excitation functions and ioniza- tion function for ICl. Within experimental uncertainty, the excitation function of I+ is identical with that of Br-, and it can be concluded that both fragment ions are formed by the same process in IBr. This is also true for I+ and C1- formation from ICl. The calculated ther- modynamic thresholds for the formation of the ion pairs, I+ (3P,) + Br- ('So) in IBr and I+ (3PJ) + C1- ('So) in ICl, are shown in Figs. 3 and 4, respec-

tively. A listing of the peak positions and separations for the

vibrational structure in the ion-pair excitation functions of IBr is given in Table 2. Mason and Tuckett14 gave a value of 310 cm-' for the vibrational constant o, of the ground-state ion. From an analysis of the a6 and b, Rydberg series observed in the vacuum UV (VUV) absorption spectrum of IBr," the energy spacing between the main peaks in both systems was deter- mined to fall in the range 273-314 cm-'. The vibra- tional energy separations for the Rydberg series found to be present in the I+/Br- excitation functions shown in Fig. 3 are in the range 250-322 cm-', in good agree- ment with the previous work noted above.

137 142 112 117 122 127 132

Wavelength / nm Figure 3. Excitation functions for I+ and Br- formation, together with the ionization function for the parent molecule, IBr. The thresholds for the formation of the ion pairs I+ (3P2,0,1) + Br- ('So) and the parent molecular ion, IBr+ (Xzl13,2.1,,), are indicated.

ION-PAIR FORMATION FROM PHOTODISSOCIATION OF INTERHALOGEN MOLECULES 331

I

105 115 125 135

Wavelength / nm

Figure 4. Excitation functions for I + and CI- formation, together with the ionization function for the parent molecule, ICI. The thresholds for the formation of the ion pairs I+ (3P2,0,1) + CI- ('So) and the parent molecular ion, ICI+ (Xzn3,2,1,2), are indicated.

Table 3 gives the peak positions and separations for the vibrational structure in the I+ and C1- excitation functions. In this case, the vibrational separations found to be appropriate in the analysis procedure for the Rydberg series ranged from 400 to 463 cm-l. This com- pares fairly well with the separations determined for the b, Rydberg series from the VUV absorption spectrum of IC1,16 which ranged from 429 to 447 cm-'.

IBr spectral analysis

In the excitation functions for I+/Br- formation, the vibrational structure between 136.9 and 138.6 nm is assigned to the [X, 3/2],8su Rydberg state, with the (0, 0) band at 72 150 cm-' (138.6 nm). The (1, 0), (2, 0) and (3, 0) bands are also clearly observed for this system, as indicated in Fig. 5. The next Rydberg state in the [X, 3/23, nso series, [X, 3/2],9sc, is assigned to the peaks at 133.9 and 133.5 nm, with the first peak being the (0, 0) band and the second peak the (1, 0) band. The next series of nso Rydberg states comes from [X, 1/2Ic spin- orbit core state. The series of bands from 128.3 to 129.8 nm are assigned to the [X, 1/2],8so state, with the (0,O) band at 129.8 nm. The peaks from 135.4 to 134.4 nm are assigned to the [X, 3/2],8pn state (see Fig. 5). The broad peak at 135.4 nm is assigned as the (0, 0) band, while the (1, 0) and (2, 0) bands are at 134.8 and 134.4 nm, respectively. The next state in this series, [X,

Table 2. Peak positions in the excitation functions of I' and Br -

Wavelength' (nm)

139.1 138.6 138.0 137.5 136.9 136.3 135.4 134.8 134.4 133.9 133.5 133.0 132.5 132.0 131.6 131.4 130.9 130.4 129.8 129.3 128.8 128.3 127.8 127.3 126.8 126.4 126.0 125.6 125.2 124.7

Energyb (cm-')

71 880 72 150 72 440 72 730 73 020 73 380 73 850 74 160 74 41 0 74 670 74 930 75 190 75 450 75 740 76 000 76 120 76 380 76 670 77 030 77 330 77 620 77 920 78 240 78 560 78 860 79 130 79 380 79 630 79 870 80 170

Separationb (cm-')

270 290 290 290 360 470 31 0 250 260 260 260 260 290 260 120 260 290 360 300 290 300 320 320 300 270 250 250 240 300

a Uncertainty = *0.02 nm. Uncertainty = *I2 cm-'.

Table 3. Peak positions in the excitation functions of I' and CI -

Wavelength. (nm) Energyb (cm-') Separationb (cm-')

390 21 0 190 250 460 450 440 370 670 1 90 290 160 220 280 31 0 650 380 450 440 370 560 41 0 440 430

137.3 72 830 136.6 73 220 136.2 73 430 135.8 73 620 135.4 73 870 134.5 74 330 133.7 74 780 132.9 75 220 132.3 75 590 131.1 76 260 130.8 76 450 130.3 76 740 130.0 76 900 129.7 77 120 129.2 77 400 128.7 77 71 0 127.6 78 360 127.0 78 740 126.3 79 190 125.6 79 630 125.0 80 000 124.1 80 560 123.5 80 970 122.8 81 410 122.2 81 840

a Uncertainty = a0.02 nm. buncertainty= f12cm-'.

332 D. KAUR ET AL.

1J 3210 9

1 0 3 2 1 0 I

122 127 132 137 142

Wavelength / nm

Figure 5. Comparison of the experimental and calculated excita- tion functions for I+(Br-) formation. The positions of the first new vibrational bands in each Rydberg state are indicated at the top. The calculated spectrum is a sum of transitions to the eight Rydberg states shown. The FWHM of the peaks in the calculated spectrum ranged from 220 to 240 cm-’.

3/2],9pn, is assigned to the bands from 132.5 to 131.6 nm. As in the case of the 8pn state, the (0, 0) band is a relatively broad peak at 132.5 nm, with the two well defined peaks at 132.0 and 131.6 nm being the (1,O) and (2, 0) bands, respectively. None of the peaks in the I+/Br- excitation functions are assigned to any state from the [X, 1/21, npn Rydberg series as no close match is found between the experimental structure and the cal- culated values for this series.

Although the nsa and npn Rydberg series with [ X , 3/23, and [X, 1/21, cores account for all the structure from 131.6 to 138.6 nm, in addition to some of the prominent features in the higher energy region, a number of bands in the shorter wavelength range are not yet assigned. The next step is then to include Rydberg series converging to the A21’1312 and A 2 l l l I 2 ion states as possible contributors to structure in the short wavelength region. Using the ‘light-atom’ approx- imation, as in the case of BrC1, the quantum defects for atomic bromine are used to calculate the positions of the Rydberg series converging on the two spin-orbit components of the A state of IBr+. The following assignments are proposed for the rest of the features in the I+/Br- excitation functions (see Fig. 5). The two prominent peaks at 130.9 and 130.4 nm are assigned to the (0, 0) and (1, 0) bands of the [A, 3/2],5p state,

respectively. The peak at 129.8 nm is due to the overlap of the (2,O) band of this state and the (0,O) band of the [X, 1/2],8sa state. The peaks at 127.8, 127.3 and 126.8 nm are assigned as the (0, 0), (1, 0) and (2, 0) bands of the [A, 1/21, 5p state, respectively. Finally, the highest energy series of bands originating at 126.4 nm are assigned to the [A, 3/2],6so state.

ICI spectral analysis

The longest wavelength bands observed in the I+/Cl- excitation functions are assigned to the [X, 3/2],8sa Rydberg state, with the (0,O) band at 134.5 nm (see Fig. 6). In his work on IC1, Venkateswarlu” assigned a peak of medium intensity at 134.3 nm to the (0,O) band of the [X, 3/2],8so Rydberg state. The difference of 0.2 nm can be attributed to the difference of 0.2 nm between the ionization potential obtained from photoelectron data’ (123.1 nm) and that used by Venkateswarlu” (122.9 nm). However, the anomalously high intensity of this peak suggests the presence of another series of bands with its origin at 135.4 nm. The overlap of the (1, 0) peak of this series with the (0, 0) peak of the [X, 3/2],8sa peak gives rise to the intense peak at 134.5 nm. The assignment of this other series will be discussed later in this section.

2 1 0 0 2 1 0

nsu[X, 1/21, Ll nso[X, 3/2Ic

2 1 0 2 1 0

I

120 125 130 135 140 Wavelength / nm

Figure 6. Comparison of the experimental and calculated excita- tion functions for I+(CI-) formation. The positions of the first few vibrational bands in each Rydberg state are indicated at the top. The calculated spectrum is a sum of transition to the eight Rydberg states shown. The FWHM of the peaks in the calculated spectrum range from 220 to 240 cm-’.

333 ION-PAIR FORMATION FROM PHOTODISSOCIATION OF INTERHALOGEN MOLECULES

A series of weak bands occur in the spectrum from 131.1 to 128.7 nm (see Fig. 6). A number of these peaks can be assigned to the [X, 3/2],9so Rydberg state, with the (0, 0) band at 130.3 nm. This matches fairly well with Venkateswarlu's a~signment'~ of a weak peak at 130.0 nm to the (0, 0) band of the [ X , 3/2],9sa state. The rest of this weak structure can be accounted for by members of the [ X , 3/2],npn series, which will be discussed later. Since both the [ X , 3/2],8sa and [ X , 1/2],8so states were observed in IBr, we might expect the [ X , 1/2],8sa state to be present in ICl, since the orbital involved in the ground state of the ion is centred on the iodine atom (e.g. the 'heavy-atom' appr~ximation).'~~'' The series of bands from 126.3 to 124.1 nm is assigned to the [ X , 1/2]8sa state, with the (0, 0) band at 126.3 nm. This assignment is also sup- ported by Venkateswarlu's work on IC1, in which he assigned the (0, 0) band of the be state to a peak of medium intensity at 126.4 nm.I7 This state accounts for the majority of prominent features in the high energy region of the ion-pair excitation function.

The series of weak peaks at the low-energy end of the spectrum (see Fig. 6) from 137.3 to 135.8 nm can be assigned to the [ X , 3/2],7pa state, with the (0, 0) band at 137.3 nm and the (1, 0) and (2, 0) bands at 136.6 and 135.8 nm, respectively. Venkate~warlu'~ assigned the (0, 0) band of this state to be at 137.2 nm, and he described the relative intensity of the peak to be weak. The next Rydberg state in this series, [ X , 3/21, Spx, should occur at 131.5 nm. No peak is observed at this wavelength in the ion-pair excitation function. However, a peak is observed at 130.8 nm, and another at 130.0 nm, which could possibly be the (1, 0) and (2, 0) band of the 8pn state, respectively. Venkateswarl~'~ observed a peak, which he described as very weak in intensity, at 131.4 nm, which was assigned as the (0, 0) band of the [ X , 3/2],8pn state. He also assigned a peak of medium rela- tive intensity at 130.6 nm as the (1, 0) band of the same state and a third peak of very weak intensity at 129.9 nm as the (2,O) band. The last two peaks correspond to the peaks observed in the ion-pair excitation functions at 130.8 and 130.0 nm, respectively, taking into con- sideration the difference of 0.2 nm in the convergence limits. Therefore, this series of bands is assigned as the [ X , 3/2],8pn state. The [ X , 3/2],9sa and [ X , 3/2],8pn states account for almost all of the weak structure from 131.1 to 128.7 nm, except for a single peak at 128.7 nm. The (0, 0) band for the [ X , 3/2],9pn Rydberg state is calculated to be at 128.7 nm. Venkate~warlu'~ observed a very weak band at 128.4 nm, which he assigned to the (0,O) band of the 9pn state. Therefore, the peak at 128.7 nm is asigned to the (0, 0) band of the [ X , 3/2Ic9pn state. Since only the (0, 0) band was observed in the I+(Cl-) experimental spectrum, this state was not included in the simulated spectrum. The peak at 127.0 nm corresponds to the calculated position of the [ X , 3/2],1Opn state, within experimental error. Venkateswarlu" observed this band to be at 126.9 nm, and the (1,O) band of the same state at 126.2 nm. From an inspection of the experimental spectrum (see Fig. 6), the peak at 127.0 nm can be seen clearly, but the rest of the bands of the [ X , 3/2],10pn state are buried under

the more intense bands of the [ X , 1/2],8sa state. The enhanced intensity of this state relative to the 8pn and 9pn states can be attributed to the opening of a second channel for ion-pair formation in the I' ('Po) state (see Fig. 4).

After assigning the [ X , 3/21, and [ X , 1/21, core Rydberg states, two series of bands originating at 135.4 and 123.5 nm remain unassigned. Using the 'light-atom' approximation, two Rydberg states are found to lie in the region of interest. The [ A , 3/2],4sa Rydberg state was calculated to occur at 135.4 nm. This state can be assigned to the series of bands beginning with the (0, 0) band at 135.4 nm and the (1,O) and (2,O) bands at 134.5 and 133.7 nm, respectively. The (1, 0) peak of this state coincides with the position of the (0, 0) band of the [ X , 3/2],8so state, thereby giving rise to the intense peak at 134.5 nm. The next contribution from the A state is cal- culated to be at 123.4 nm, and set of bands in this region are assigned to the [A , 3/2],4pn state. The (0, 0), (1, 0) and (2, 0) bands are at 123.5, 122.8 and 122.2 nm, respectively. Figure 6 shows a comparison of the experi- mental spectrum with a simulation based on the Rydberg state assignment discussed above.

CONCLUSIONS

From the work presented here and previous work on the homonuclear halogen^,'.^ we conclude that the mechanism for the formation of free ion pairs (X' + Y-) involves predissociation of quasi-bound Rydberg states by ion-pair continuum states. Despite the high density of available Rydberg states, it is clear that only a few are selectively coupled to the ion-pair continuum. The present work shows that the nsu Rydberg states are most efficiently coupled to the ion- pair continuum, for the interhalogens. Our simulations, which are based on a simple Gaussian-like Franck- Condon absorption profile, together with a constant coupling term between the Rydberg and ion-pair states, suggests that the potential curves cross on the inner limb of the Rydberg states. A crossing on the outer limb would produce a rapidly varying overlap (coupling) term, with possible multiple maxima.'e*'9 We cannot eliminate this possibility with the present data, but we note that the ion-pair excitation function for Br, has a simple Gaussian-like shape,' and we would expect the potential crossings to be similar for the interhalogens that we considered here. Further work at higher resolution is desirable to confirm this point.

Acknowledgements

We appreciated the assistance of Dr J. R. Grover and Dr M. G. White in operating the beam line at the National Synchrotron Light Source facility. We gratefully acknowledge financial support for this study from the State University of New York at Albany (Faculty Research Fellowship and an Office for Research grant), from a NATO travel grant (No. 870878) and from the National Synchrotron Light Source (Faculty/Student research grant).

334 D. KAUR ET AL.

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