7338
J . Phys. Chem. 1989, 93, 7338-7342
centers along with large overall absorption for the larger peaks (206 and 208 isotopic transitions) becomes apparent at relatively low nl values while the smaller peaks (204 isotopic and 207b hyperfine transitions) remain reldtively unchanged up to rather high nl values. This effect is due to the relative magnitudes of the (nl)i values in each case. The calculated values of ZJZ0 vs nl are shown by the solid curve in Figure 3. The agreement between this calculated curve and the data is excellent and lends strong support to the procedure used here. It is emphasized that using the actual measured spectrum of the lamp as the source function in the calculations leaves the 283.3-nm absorption cross section as the only adjustable parameter, and hence, the tabulated value' used here is supported by these results. It is noted that there are two effects contributing to the curvature of the plot in Figure 4. One effect is the preferential absorption at the line center for each peak, and the other effect is the relatively strong absorption of the 208 and 206 isotope peaks due to the large abundances of these isotopes in the naturally occurring lead absorber. Example calculations indicate that the second effect is the major contributor to the curvature in the present results. An earlier experimental and theoretical study of the absorption by lead atoms of the 283.3-nm radiation from a lead hollow cathode lamp incorrectly concluded that a calculational model very similar to that used here, but for reasonable assumed source functions, could not explain their absorption calibration curve.4 It is our conclusion that it is not the model that is inadequate but that the experimental calibration data used for comparison were in error.
Atomic resonance line absorption studies for other atoms, using a theoretical model similar to that used here, indicate that this approach can indeed give accurate and quantitative absorbances as a function of atom number density.k-8-'0 Conclusion An important conclusion to be drawn from this work is that accurate atomic absorptions as a function of absolute atom number density can be calculated by the methods described here if one has a Doppler-limited spectrum of the light source and the value of the atomic absorption cross section. This conclusion is in substantial agreement with that in a recent study of atomic absorption by H atoms.'O Acknowledgment. J.W.S. gratefully acknowledges the financial assistance and kind hospitality of the Photochemistry Group (Chem-4) while on sabbatical leave at Los Alamos National Laboratory. Support from SDIO/IST is gratefully acknowledged. We are particularly indebted to Douglas Hof and Byron Palmer for their perseverance in obtaining the FTS spectrum of our lamp. Registry No. Pb, 7439-92-1; 204Pb, 13966-26-2; 2MPb, 13966-27-3; *"Pb, 141 19-29-0; "'Pb,
13966-28-4.
(8) Bemand, P. P.; Clyne, M. A. A. J. Chem. Soc.,Faraday Tram. 2 1973, 69, 1643, and references cited therein. Phillips, L. F. Chem. Phys. Lett. 1976, 37, 421, and references cited therein. (9) Braun, W.; Carrington, T. J. Quant. Spectrosc. Radiat. Transfer 1969, 9, 1133. (10) Maki, R. G.; Micheal, J. V.; Sutherland, J. W. J. Phys. Chem. 1985, 89, 48 15, and references cited therein.
Vacuum-Ultraviolet Absorption, Fluorescence Excitation, and Dispersed Fluorescence Spectra of BrCl Andrew Hopkirk, David Shaw, SERC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K.
Robert J. Donovan, Kenneth P. Lawley, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ. U.K.
and Andrew J. Yencha* Department of Chemistry and Department of Physics, State University of New York at Albany, Albany, New York 12222 (Received: February 8, 1989; In Final Form: May 9, 1989) Absorption and fluorescenceexcitation spectra of BrCl have been recorded between 137 and 180 nm with the use of synchrotron radiation. A pronounced resonance structure has been observed in the fluorescence excitation spectrum and is attributed to coupling of ion-pair and Rydberg states. Dispersed fluorescence spectra have been obtained through vacuum-ultraviolet (vacuum-UV) excitation that display several oscillatory continua. These are assigned to emission predominantly from the lowest R = O+ ion-pair state (the E state) with probably some contribution from theflO+) state of the second cluster. states is postulated. Fluorescence to four of the five 0' states correlating with the separated atomic J = 3 / 2 and
Introduction Diatomic halogen and interhalogen molecules display rich absorption spectra in the far-ultraviolet and vacuum-ultraviolet regions. This structure arises through transitions to Rydberg, ion-pair, and repulsive electronically excited states that are accessible from the ground state. We have investigated several such molecular systems and found that ion-pair states are vital to an understanding of their photochemistry and photo physic^.'-^ (1) Yencha, A. J.; Donovan, R. J.; Hopkirk, A.; Shaw, D.; J. Phys. Chem. 1988, 92, 5523.
(2) Austin, D. I.; Donovan, R. J.; Hopkirk, A,; Lawley, K. P.; Shaw, D.; Yencha, A. J. Chem. Phys. 1987, 118, 91. (3) Donovan, R. J.; MacDonald, M. A.; Lawley, K. P.;Yencha, A. J.; Hopkirk, A. Chem. Phys. Lett. 1987, 138, 571.
0022-3654/89/2093-7338$01.50/0
Indeed, the molecular fluorescence detected following excitation in the ultraviolet and vacuum-ultraviolet is dominated by ionpair-state fluorescence bands. There has been little work on the vacuum-ultraviolet absorption spectroscopy of BrCl save the assignment of the b, X and a,
-
(4) Kerr, E.; MacDonald, M.;Donovan, R. J.; Wilkinson, J. P. T.; Shaw, D.; Munro, I. J. Photochem. 1985, 31, 149. (5) Donovan, R. J.; Gilbert, G.;MacDonald, M.; Munro, I.; Shaw, D.; Mant, G.R. Chem. Phys. Lett. 1984, 109, 379. (6) O'Grady, B. V.; Donovan, R. J. Chem. Phys. Lett. 1985, 122, 503. (7) Wilkinson, J. P. T.; Kerr, E. A.; Lawley, K.P.; Donovan, R. J.; Shaw, D.; Hopkirk, A.; Munro, I. Chem. Phys. Lett. 1986, 130, 213. ( 8 ) Austin, D. I. Ph.D. Thesis, University of Edinburgh, 1987. (9) Hiraya, A,; Shobatake, K.; Donovan, R. J.; Hopkirk, A. J. Chem. Phys. 1988, 88, 52.
0 1989 American Chemical Society
Absorption and Fluorescence Excitation Spectra of BrCl
-
X bands that are the first members (n = 5) of the nsa Rydberg series.1° Additional absorption has been reported below 154 nm but was not assigned." BrCl continuous emission bands have been reported in the UV wavelength range 255-360 nm,I2 and the D' A' and E B systems have been identified.13J4 The He I photoelectron spectrum of BrCl has been recorded, and the spin-orbit splitting for the ground state of BrCP was found to be 2070 f 30 cm-I.I5 In this work, we concentrate upon the vacuum-ultraviolet-induced UV fluorescence excitation spectrum and its relationship to the absorption spectrum in the 137-180-nm region and upon dispersed fluorescence spectra over the 130400-nm range. It will be seen that BrCl is in many respects similar in its spectroscopic behavior to other interhalogen molecules, but the various fluorescence systems tend to overlap more.
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The Journal of Physical Chemistry, Vol. 93, No. 21. 1989 7339
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Experimental Section The work presented here was performed with the Synchrotron Radiation Source (SRS) at the SERC Daresbury Laboratory on station 13.2. Details of the experimental apparatus have been given elsewhere.' Briefly, fluorescence, both dispersed and undispersed, was viewed at right angles to the S R S vacuum-ultraviolet radiation axis in the following manner. The undispersed fluorescence excitation spectrum was obtained through a quartz window with a Meles Griot No. UG3 (310-450-nm band-pass) filter in combination with an EM1 Model 98834 photomultiplier. Dispersed vacuum-UV and far-UV fluorescence, up to 330 nm, was detected through a CaF2 window and focusing optics with a 0.20-m vacuum monochromator (Acton) and EM1 Model G26K3 14LF photomultiplier. Longer wavelength dispersed fluorescence, in the range 200-400 nm, was viewed through a quartz window, employing a 0.25-m monochromator (Spex) and an EM1 Model 98834 photomultiplier. Absorption spectra were recorded by the use of a sodium salicylate quantum converter screen placed under vacuum at the rear of the gas cell beyond the SRS-radiation exit window (of lithium fluoride). The ultraviolet fluorescence so produced was monitored by an EM1 Model 98834 photomultiplier through a Kodak Wratten No. 47B filter. The absorption and fluorescence excitation spectra were recorded simultaneously along with a signal proportional to the storage ring current. The vacuum-ultraviolet and ultraviolet dispersed fluorescence spectra were recorded likewise for several excitation wavelengths. Corrections for ring-current decay and for the S R S station monochromator photon flux function have been incorporated where appropriate. There has been no attempt to correct for analyzer monochromator transmission efficiencies. Cell-empty runs were performed for the absorption data. The gas sample was prepared in a glass bulb by premixing Br2 in an excess of C12 (typically, BrC1:C12 = 1:12). The resultant equilibrium between Brz, C12, and BrCl is shifted toward BrCl, but precise BrCl concentrations are not known due to differential adsorption of the halogens on the cell walls, and we have not quoted absorption cross-sections in absolute terms. Absorption by residual Br2 was found to be negligible, indicating that the sample was essentially BrCl and C12 a t all times. Results ( a ) Absorption and Fluorescence Excitation Spectra. Figure 1 shows the BrCl absorption spectrum from 137 to 180 nm along with the corresponding fluorescence excitation spectrum. The prominent features in the absorption spectrum in the range 155-170 nm are identified as the as X and b5 X Rydberg series by analogy with similar transitions reported by Venkates-
-
-
(IO) Cordes, H.; Sponer, H. Z . Phys. 1932, 79, 170. (11) Donovan, R. J.; Husain, D. Trans. Faraday Soc. 1968, 64, 2325. (12) Haranath, P. B. V.; Rao, P. T. Indian J . Phys. 1957, 31, 368. (13) Diegelmann, M.; Hohla, K.; Rebentrost, F.;Kompa, K. L. J. Chem. Phvs. 1982. 76. 1233. 114) Chakraborty, D. K.; Tellinghuisen, P. C.; Tellinghuisen, J. Chem. Phys. Lett. 1987, 141, 36. ( 1 5) Dunlavey, S.J.; Dyke, J. M.; Morris, A. J. Electron Spectrosc. Relat. Phenom. 1977, 12, 259.
Fluorescence Excito tion
Y
0. 0 140
I50
I70
160
I80
Wavelength / n m
Figure 1. Comparison of absorption and fluorescence excitation spectra of BrCl (BrCI:C12= 1:12) in the vacuum-ultraviolet spectral range. The relative absorption cross section was obtained with a total pressure of 70 mTorr and a resolution AA = 0.1 nm, while the fluorescence excitation spectrum (310-450-nm detection) was recorded with a total pressure of 235 mTorr and an excitation wavelength resolution AA = 0.35 nm.
150
200
250
300
350
400
Fluorescence wavelength / nm
Figure 2. Dispersed fluorescence spectra recorded with a vacuum-UV monochromator (left) and UV monochromator (right), following excitation of BrCl with 147-nm radiation (band-pass, 2.6-nm fwhm). The two regions of the figure, 140-225 nm (resolution, AA = 6 nm) and 225-400 nm (resolution, Ah = 5 nm), are normalized to 1.0 for the most intense peak. Total sample pressure was 1.08 Torr.
warlu in IC1,16 I,,{' and Br2.'* The only previous report of these absorption bands was given briefly many years ago by Cordes and Sponer,Io who used photographic methods. The present study extends these observations, and data on the relative absorption cross section in the range 137-180 nm are reported for the first time. Below 140 nm, C12 absorption19 underlies some BrCl absorption, making analysis difficult. However, the strong Br2 absorption feature known to occur in the 151-nm region2 appears only very weakly due to the low Br, concentration in our gas sample. The fluorescence excitation spectrum has a long wavelength threshold at around 167 nm, and initially, fluorescence increases in intensity, moving to shorter wavelengths. There is no correlation between the intensity of fluorescence and absorption by the Rydberg states in the range 155-167 nm. From 155 down to 148 nm, the fluorescence excitation spectrum shows peaks and troughs with a spacing similar to those for the Rydberg absorption systems to longer wavelengths. Dips in fluorescence excitation that seem to form a smooth extension of a Rydberg vibrational progression, seen to the red in absorption, are common in the corresponding (16) (17) (18) (19)
Venkateswarlu, P. Can. J . Phys. 1975, 53, 812. Venkateswarlu, P. Can. J . Phys. 1970, 48, 1055. Venkateswarlu, P. Can. J . Phys. 1969, 47, 2525. Lee, L. C.;Suto, M.; Tang, K. V. J . Chem. Phys. 1986, 84, 5277.
1340 The Journal of Physical Chemistry, Vol. 93, No. 21, 1989
Hopkirk et al. TABLE I: Band Maxima of as
X and bs t X Rydberg Systems in
BrCl
-
syst a5 X
v:v"" 031 090 190 2,o 3,O 4,O
58 b5
+
X
0,1 030 130 290
3,O 4,o
5,o
X,.,blnm
I1cm-l
Allcm-'
169.63 168.34 166.90 165.51 164.15 162.79 161.45 163.48 162.30 160.95 159.65 158.39 157.14 155.88
58952 59403 59916 60419 60920 61429 61939 61 169 61614 62131 62637 63135 63637 64152
451 513 503 501 509 510 445 517 506 498 502 515
"Assignment of the main component. Many of these bands have unresolved sequence structure. *Estimated error in relative peak wavelength is fO.O1 nm. Absolute error in the wavelength scale is fO. 1 nm, calibrated as previously described.l vibronic levels of the two 0 = 1 components of the Rydberg states [3/2]nu and ['/z]nu (the same Q value ensures that the Rydberg/core exchange terms are essentially the same because the spin alignments are the same) should be very close to that of the two spin-orbit states of BrCI+. The similarity in the spectral characteristics of the three interhalogens continues to shorter wavelengths with the sharp decline in ion-pair absorption at or near the onset of a third Rydberg series. Thereafter, additional Rydberg-like structure appears in BrCl below 140 nm as it does in the respective wavelength regions for the other two interhalogens. The upper states of the first two Rydberg series in the 163-170- and 155-164-nm ranges in BrCl are identified as a5(II1), with the electron configuration (2&/2)5SU, and b5(nl),with the electron configuration (2n1/2)5sU,respectively. Table I lists the wavelengths of the absorption peak maxima as measured in this work. The average upper-state vibrational separations of 507 f 3 and 508 f 4 cm-' in the as and b5 series, respectively, agree reasonably well with earlier work.1° The next Rydberg systems should be [Q3 zlSpu X and [2111~215pu X. Their electronic origins, which will be close to the vibrational origins, can be calculated from the Rydberg atomic term values E,, = IP - R / ( n - 1 3 ) ~
-
+ -
where IP is the adiabatic ionization potential for formation of BrCP (X(2113/2),88 300 cm-', and X(zIIl/2),90 370 cm-', estimated from Figure 2 in ref 1 9 , R is the Rydberg constant, n is the principal quantum number of the excited electron, and 6 is the quantum defect. By comparison with the analogous series in IBrI, IC1,16,21Br2,18and 12,i7the quantum defect for BrCl can be estimated to be 2.6. The calculated positions for the two 5pu Rydberg bands are 69 248 cm-' (144.5 nm, 2113 core) and 71 318 cm-' (140.2 nm, 2111j2core). In each case, t i e estimate lies to slighty longer wavelength of a group of Rydberg-like bands in the absorption spectrum. We assign these features to the first members of the 5pu Rydberg series, which is consistent with the pattern seen in both IBrl and IC1.zl The absorption between 144 and 155 nm is predominantly due to ion-pair states. In BrCl, two ion-pair clusters are expected to lie in this region: the first member of the lowest cluster, E(O+), correlates with B I - + ( ~ P+ ~ )C1-, while the first member of the second cluster,flO+), which lies 3840 cm-' higher, correlates with Br+(3Po)+ CI-. The extended profile of uah(v) to the red almost certainly results from absorption from X (u" = 1 or 2), which are appreciably populated in the cell. The irregular structure of the observed dispersed fluorescence contrasts with the very regular structure that would result from a single electronic transition originating in a high u'IeveLZ2 In other halogens, synchrotron excitation at or to the red of the peak of uabs(v)excites predom~~
~~
~~
(22) Tellinghuisen, J. Aduances in Chemical Physics; Wiley: Chichester, U K., 1985; Vol. 60.
The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 7341
Absorption and Fluorescence Excitation Spectra of BrCl inantly only one ion-pair (IP) state, and so the present structure is likely to be mainly the result of superposition of several electronic transitions (IP valence) from a single upper state. There are five O+states correlating with the various J,M combinations of C1(,PJM) Br(,PJM). At large internuclear separations, the electronic structure of both ion-pair and valence states are best described by the pure precession model (Le., as the product of the appropriate free-ion or atom wave functions). This, in turn, leads to quite different predictions of the relative oscillator strengths of the five IP valence transitions from those given by a single M O description of the electronic structure appropriate at small separations and hence to the vertical excitation process from u" = 0. Inspection of the orbital character of the ionic/atomic wave functions (Russell-Saunders coupling scheme) shows that all five transitions become allowed, though one of them (to the B'(O+) state correlating with two J = 3/2 atoms) involves P,'~') piBr) electron transfer and is thus much weaker at large R than the others. We will interpret most of the structure seen under low resolution ( A v = 60 cm-I) in the dispersed fluorescence as red extrema of the various IP valence transitions. The lower vibrational levels of the E state have been analyzed by Chakraborty et al.,I4 who give T, = 48 759.8 cm-*. The lower portions of the B and X states have been the subject of an RKR analysis by CoxonZ3and a reanalysis by Tellinghuisen, from which @(X) is accurately known24q25to be 18 245 cm-l, although the X state potential itself is only known up to 3000 cm-I. Using this information, we could normally predict the positions of the red extrema of the E B and E X fluorescence, subject to an uncertainty of perhaps *200 cm-I in the extrapolation of the B potential. However, the B state is crossed by a repulsive 0' state correlating with ground-state atoms a t around u' = 9, and the vibrational term values are perturbed. With fluorescence being discussed from very high vibrational levels of an ion-pair state, transitions around the red extremum occur when the relative kinetic energy, typically -20000 cm-' in our experiments, is at its maximum. This relative velocity is retained in the motion over the lower-state potential. Coxon's analysis of the B state indicates a coupling matrix element of 300-500 cm-I, and so the motion in our experiments is essentially diabatic, i.e., governed by the deperturbed potentials that cross. Taking Coxon's deperturbed B-state RKR points, the position of the maximum in the Mulliken difference potential with the E state is found to be 3.10-3.12 A. The perturbing B' state, being purely repulsive, does not exhibit a maximum in the difference potential, at least at large R, though transitions to this state will lead to weak background fluorescence. We then estimate the red extremum of the E B fluorescence to lie at 330-332 nm. If the outer limit of the ground-state potential is assumed to run parallel to the deperturbed B state, but 882 cm-' below it, the red extremum of E X fluorescence is predicted at 320-322 nm. However, in IC1 and IBr, the red extremum of this E X fluorescence occurs at a point where the X-state potential has dropped roughly 2000 cm-I below its asymptote. This would place the E X red extremum in BrCl at 305-307 nm. Turning to Figures 2 and 3, we find with A, = 147 nm there is only a shoulder (d) in the fluorescence at 334 nm near the predicted position of the E B red extremum, but this becomes a well-developed peak in Figure 3. There is a prominent feature at 3 15 nm in both spectra, peak f, and this we assign to the E X red extremum. Its position implies that the X potential has dropped by approximately 1200 cm-' from its asymptote, relative to the B-state potential, around 3.15-3.18 A. Alternatively, the peak g could be the E X red extremum though, in the light of the discussion to follow, this would make identification of peak f, 1600 cm-' to the red, difficult. If the Mulliken difference potential exhibits a maximum, the envelope of the bound free fluorescence shows a series of
-
+
-
-
-
-
-
-
-
-
-
-
-
-
(23) Coxon, J. A. J . Mol. Spectrosc. 1974, 50, 142. (24) Clyne, M. A. A.; McDermid, I. S. Faraday Discuss. Chem. Soc. 1979,
67,316. ( 2 5 ) Brown, S.W.; Dowd, C. J., Jr.; Tellinghuisen, J. J . Mol. Spectrosc. 1988, 132, 178.
interference maxima to the blue of the red extremum (supernumerary oscillations and the rainbow, respectively, in scattering terminology, where the deflection function plays the role of th'e difference potential in fluorescence). Identifying the E X red extremum as peak f allows its first supernumerary oscillaton to the blue to be approximately located. If the difference potential AV(R) is parabolic, the envelope to the structured fluorescence around a red extremum at vo is the square of an Airy function Ai and a simple semiclassical analysis gives
-
where AV/(Ro) is the second derivative of A V a t the position of the extremum Ro, Eothe kinetic energy of relative motion at the extremum, I.L the reduced mass, and MI2(RO) the transition dipole. The spacing xI - xo between the first and second maxima of [Ai(x)12is given by xl- xo = 2.25. The curvature of all ion-pair states at their minima is quite similar (lying in the range (2.6-3) X 10" cm-I A-2), and if the lower state is almost flat at the classical point of transition, the relationship 1 takes the approximately universal form for halogen ion-pair states
where p is in amu and E and 'v are in cm-I. Applying this formula to experimental extrema from different upper states ranging from the f state of IC1 to the D state of Br, and I,, the constant on the right-hand side ranges between 1.82 to 2.02. For the transition under discussion, the first supernumerary of the E X red extremum is calculated to lie 1770 100 cm-' to the blue, placing it at 298 nm. The peak g is at 300 nm and so is assigned as a supernumerary to (f), belonging to the E X transition. The peak h at 273 nm lies 4980 cm-I from the E X red extremum. It is thus probably the red extremum of the f X fluorescence. If the E and f (Br+(3Po))states run parallel from their asymptotes to their minima (which by analogy with other Elf pairs are expected to have very similar Re values), then the f X red extremum will fall at 281 nm rather than the observed 273 nm, a discrepancy of 900 cm-I. The peak i at 227 nm, lying 12330 cm-I from the E X extremum, lies close to the predicted position for the red extremum of fluorescence from a third-tier O+ state probably collisionally populated. BrCl is unusual among the halogens and interhalogens in that the lowest Br- Cl+(3Pz)asymptote, which lies 11 450 cm-l above Br+(3P2) C1-, is almost isoenergetic with the Br+(lD) C1- asymptote at 11 409 cm-'. Each of these separated ion combinations correlates with an O+ ion-pair state (among other R components) with, presumably, similar Re and De values. Their fluorescence would thus strongly overlap, leading to a prominent extremum. The intense fluorescence peak j, close to the excitation wavelength, must come from transitions at the inner turning point of the E state back to low vibrational levels of the ground state. The prominent peak a at 370 nm lies 4600 cm-l to the red of the E X red extremum. The Br(2P3/2)-(2Pl/2)splitting is 3685 cm-I, and the two J states of C1 are separated by 882 cm-I. A peak near 4500 cm-l might thus be the red extremum of fluorescence from the E state to the lower state correlating with Br(,PII2) + Cl(2P1,) that runs roughly parallel to the X and B states at large R . jhis would be the highest of the five O+states correlating with two different halogen atoms in their lowest electronic configuration. The next peak to the blue (b) at 356 nm would then be a transition similar to the next lower O+state, 4(0+), a shallow-bound state correlating with Br(*PIl2) + C1(2P32 ) . The predicted energy separation (a) - (b) is then 882 cm- i (observed 900 cm-I). Continuing this interpretation to peak c would then be the first supernumerary of the rainbow a (predicted spacing (a) - (c) from formula 2 is 1500 cm-I, observed 1500 cm-I). The distinct shoulder (e) at -31 200 cm-' is then possibly the red extremum of thef- 5 (O+)transition (predicted
*
-
-
-
- --. -
+
+
+
-
-
J . Phys. Chem. 1989, 93, 7342-7346 position, assuming the f- and E-state potentials to run parallel to Re is 30900 cm-I). Alternatvely, the peak a may not be a rainbow but a transition occurring at the inner turning point of the E state to high on the wall of a purely repulsive lower state. In 12, the strong fluorescence system 7300 cm-I to the red of the D X red extremum is clearly such a transition. However, the spacing between the corresponding E X red extremum and peak a in BrCl is only 4700 cm-', which seems rather too small for this explanation. Also, a peak in fluorescence arising from such an inner turning-point transition should show a pronounced blue shift as bXmoves to the red. The peak under discussion, in fact, shows a very slight shift to the red on changing A,, from 147 to 153 nm. When the longer wavelength portions of the dispersed ion-pair fluorescence spectra of the other halogens are reexamined, hitherto unexplained features can be assigned to ion-pair nO+ valence red extrema. Thus, in fluorescence from I2 (D(O,+)), the quite prominent system extending between 425 and 460 nm is in exactly the position expected for the D 3(0,+) transition, the 3(0,+) state correlating with I(2P3/2)+ I(2Plj2),26,27 if bound by -800 cm-1.28
The principal ion-pair states excited at 147-153 nm are the E(O+) and to a lesser extent t h e n o + ) (perhaps populated collisionally). Transitions to several lower states are seen. Various assignments of the peaks in fluorescence are possible, but our results are consistent with the simplest conclusion that the red extrema of the transitions from the E(O+) state to four of the five shallow-bound O+ states, correlating with Br(2PJt) + C1(2P,r,), are seen. Furthest to the blue of these is the E X system (X l(O+)). The E 2(0+) extremum is missing because the 2(0+) is diabatically a purely repulsive state (adiabatically forming the B' surface when it avoids a crossing with the B state). The remaining three systems can be identified as terminating in shallow-bound states similar to the B state. These give rise to red extrema separated from each other by very nearly the separation of the atomic J states to which they dissociate. The remaining weaker peaks can plausiby be identified as either supernumeraries of these extrema or, much further to the blue as two of the correspondingf+ n(O+)extrema ( n = 1 and 4 being the strongest). Alternately, some of the prominent features in fluorescence may be due to transitions to lower repulsive states occurring at the inner turning point of the ion-pair-state vibration. Compared with the heavier halogens and interhalogens the asymptotes of all the valence states in BrCl lie much closer together. Thus, the red extrema of the corresponding ion-pair valence transitions in BrCl (for those lower states that are slightly attractive at long range) also move closer together and to the blue, so that for the first time they may all be seen in the 300-400-nm range.
-
-
-
-
-
-
-
Conclusions In this study, we have examined the vacuum-ultraviolet absorption spectrum and corresponding fluorescence excitation spectrum of BrCl and assigned for the first time a region of ion-pair-state absorption. The first members of the npa ( n = 5) Rydberg series have also been identified.
Acknowledgment. We thank the SERC for the use of the synchrotron radiation facilities at the Daresbury Laboratory and NATO (Grant No. 870878) for support that enabled A.J.Y. to work in the U.K.
(26) Hemmati, H.; Collins, G. J. Chem. Phys. Lett. 1980, 75, 488. (27) Martin, M.; Fotakis, C.; Donovan, R. J.; Shaw, M. J. Nuouo Cimento 1981, 638, 300. (28) Ishiwata, T.; Ohtoshi, H.; Sakai, M.; Tanaka, I. J. Chem. Phys. 1984, 80, 1411.
Registry No. BrCI, 13863-41-7.
Geometric Structure of Chlorocyanide Cation Matthias Rosslein and John P. Maier* Institut fur Physikalische Chemie, Uniuersitat Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland (Received: February 8, 1989; In Final Form: May 16, 1989) The rotationally resolved origin band in the B 2113 % . 2113/2 laser excitation spectra of 3sC113CN+,37C113CN+, 3sC1C'5N+, and 37C1C1sN+ has been recorded and analyzed. drom the obtained rotationa! constants, as well as those known for 3sC1CN+ and 37C1CN+,the complete r, structure of the chlorocyanide cation in the X 211state is obtained: rs(C-Cl) = 1.559 (12) A; r,(C-N) = 1.215 (12) A. +-
Introduction The number of polyatomic molecular ions for which the whole r, structure has been determined is still small. These include the open-shell species COZ+,lCS2+,2H20+,3and HCCCCH+4and the closed-shell ones HN2+,5 HC0+,6 FH2+,7C1H2+,8NH2-,9 FHF,'O ClHCI-," HOC+,12HBF+,13 HCNH+,14 H30+,15CH3+,16 (1) Gauyacq, D.; Larcher, C.; Rostas, J. Can. J. Phys. 1979, 57, 1634. (2) Callomon, J. H. Proc. R. SOC.London, A 1958, 244, 220. Balfour, W. J. Can. J . Phys. 1976, 54, 1969. (3) Strahan, S. E.; Mueller, R. P.; Saykally, R. J. J. Chem. Phys. 1986, 85, 1252. Dinelli, B. M.; Crofton, M. W.; Oka, T. J. Mol. Spectrosc. 1988, 127, 1. Woods, R. C. Philos. Trans. R. SOC.London, A 1988, 324, 141. (4) Lecoultre, J.; Maier, J. P.; Rijsslein, M. J. Chem. Phys. 1988,89,6081. ( 5 ) Owrutsky, J. C.; Gudeman, C. S.; Martner, C. C.; Tack, L. M.; Rosenbaum, N. H.; Saykally, R. J. J . Chem. Phys. 1986,84, 605. (6) Woods,R. C.; Saykally, R. J.; Anderson, T. G.; Dixon, T. A,; Szanto, P. G. J . Chem. Phys. 1981, 75,4256. (7) Schafer, E.; Saykally, R. J. J. Chem. Phys. 1984, 81, 4189. (8) Lee, S. K.; Amano, T.; Kawaguchi, K.; Oldani, M. J. Mol. Spectrosc. 1988. 130. I . (9) Tack, L. M.; Rosenbaum, N. H.; Owrutsky, J. C.; Saykally, R. J. J. Chem. Phys. 1986, 85,4222. (10) Kawaguchi, K.; Hirota, E. J . Chem. Phys. 1987, 87, 6838.
0022-365418912093-7342$01.50/0
and NH4+.17 The aim of this study was the determination of the geometry of the chlorocyanide cation in its ground electronic state. The obtained structural parameters of this species should be useful as a bench-mark test for calculations of properties of ions and of open-shell specizs containing second-row atoms.I8 The B 211 X 211electronic transition of chlorocyanide cation was first identified in an emission spectrum following electron impact excitation on an effusive beam containing its molecular
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(11) Kawaguchi, K. J . Chem. Phys. 1988, 88,4186. (1 2) Bogey, M.; Demuynck, C.; Destomtes, J. L. J . Mol. Spectrosc. 1986, 115, 229. (13) Saito, S.; Yamamoto, S.; Kawaguchi, K. J. Chem. Phys. 1987, 86, 2597. Cazzoli, G.; Degli Esposti, C.; Dore, L.; Favero, P. G. J . Mol. Spectrosc. 1987, 121, 278. (14) Amano,T.;Tanaka,T.J. Mol.Spectrosc. 1986,116, 112. Liu,D.-J.; Lee, S.-T.; Oka, T. Ibid. 1988, 128, 236. (15) Begemann, M. H.; Saykally, R. J. J. Chem. Phys. 1985, 82, 3570. (16) Crofton, M. W.; Jagod, M.-F.; Rehfuss, B. D.; Kreiner, W. A.; Oka, T. J. Chem. Phys. 1988.88, 666. (17) Crofton, M. W.; Oka, T. J. Chem. Phys. 1987, 86, 5983. (1 8 ) Botschwina, P. In Ion and Cluster Ion Spectroscopy and Structure; Maier. J. P., Ed.; Elsevier: Amsterdam, 1989, p 59.
0 1989 American Chemical Society
