CuBr chemiluminescence from the Cu* (2Dsol32 ,52 ) + Br2 reaction

Chemical Physics North-Holland

160 (1992)

73-83

CuBr chemiluminescence P. Kowalczyk

from the Cu* ( 2D3,2,5,2)+ Br2 reaction

‘, I. Hikmet and N. Sadeghi 2

Laboratoire de Spectromt%rie Physique (CNRS URA 08). UwersltP Joseph-Fourier/Grenoble B. P. 8 7, 38402 Sam-Martm d’H&res CPdex, France Received

I,

30 July 199 1

The chemiluminescence spectrum of CuBr formed in the reaction CU*(~D 3,2,5,2) +BrZ in a flow reactor using a Cu sputtering source was recorded in the 400-600 nm range. Between 465 and 535 nm, besides the known A ‘TI, +X ‘E+ band system, a new system was observed, attributed to the transrtion from a triplet A’ state to the ground X ‘Z+ state. The molecular constants of the A’ state were determined and the constants of the A state were improved. The spectra resulting from the reaction of either Cu*( 2D9,2) or CU’(‘D,,~) metastable atoms with Br, were simulated to deduce the branching ratios for the population of different electronic states and the vibrational population within each state. From the highest vibrational level in the B ‘II state populated m the reaction involving Cu*( *D,,,) atoms, we estimated the lower limit of the bmding energy of the CuBr(X) molecule. The total reaction rate constants for the ground ‘S and metastable *D states of Cu reacting with Brz were measured.

1. Introduction The spectroscopy and electronic properties of copper halide molecules have been the topics of many experimental and theoretical investigations (see e.g. refs. [ l-91 and references therein). This is a particularly challenging subject, since these molecules show complex optical spectra, which become more complicated the heavier the halide. In particular, large spinorbit effects allow otherwise forbidden triplet-singlet band systems to be observable, thus increasing the number of transitions. Nevertheless, studies of copper monohalides are worthwile for both practical and theoretical reasons. Chemiluminescence spectroscopy of copper halides has been used to study reactions of the type cu/cu*+x,+cux*+x,

(1)

where X is a halogen atom and an asterisk denotes electronic excitation. Reaction ( 1) is of possible interest as a pumping process in chemical lasers [ lo], although the common ionic structure, which leads to nearly the same B,, values in most of the known states, ’ On leave from the Institute of Experimental ’

Umversrty, Poland. To whom correspondence

0301-0104/92/$05.00

Physics, Warsaw

should be addressed.

0 1992 Elsevier Science Pubhshers

appears unfavourable. These reactions show also fundamental questions in reaction dynamics due to the presence of numerous interacting potential energy surfaces [ 111 of varying ionic and covalent nature, associated with the ground state, 3di04s, and excited state, 3d94s2, configurations of Cu. Electronic spectra of copper bromide (CuBr) have been studied since the 1920s but the information available is still fragmentary. Four band systems are known in the visible region: the A system (460-5 10 nm), the B system (420-460 nm), the C system (390-460 nm) and the D system (370-400 nm) [ 12,131. All these systems involve the ground state (X ‘C+ ) as the lower state. The molecular constants of this state are known precisely from microwave measurements [ 141. On the other hand, the excited states have been subject to merely a rough vibrational analysis [ 12,13 1. The presence of four isotopic species ( 63Cu79Br, 63Cu81Br, 65Cu79Br and 65Cu81Br with an abundance ratio of about 7 : 7 : 3 : 3 ) and the consequent complexity of the rotational spectrum for a long time prevented any rotational analysis of the A, B, C and D states. Until now the B, rotational constants have been measured only for the v= 0, 1 and 2 vibrational levels of these states [ 15-l 7 1. Nevertheless, the observation of rotational structure together with lifetime measurements [2] has clarified the

B.V. All ngbts reserved.

74

P. Kowalczyk et al. / CuBr chemrlummescence

controversy concerning assignment of these four lowlying excited states. They are known now as A3TI,, B’II,C’Z+andD3A, [2].Tothebestofourknowledge, there are no theoretical calculations on the electronic structure, spectroscopic properties or potential energy curves of CuBr, which can be compared with the experimental data. In the present work we used a room temperature flow reactor, running with 0.1 Torr neon carrier gas, to study the chemiluminescence of the CuBr molecule in the spectral range 400-600 nm. The emission intensity of the B-X and C-X bands was very weak. Between 465 and 535 nm, in addition to the known A ‘II, +X ‘C+ system, a new band system was observed corresponding to the transition from a formerly unknown excited state (labelled thereafter as A’) to the ground X state. The molecular constants and nature of the A’ state were investigated and the vibrational constants of the A 311, state were determined with a precision superior to previous measurements. We deduced the vibrational and rotational distributions in the A and A’ states resulting from reaction (1) and studied the efficiency of the metastable copper ‘D3,* and ‘D,,, atoms in the production of CuBr* molecules in the A, A’ and B states. Simple considerations concerning the energy available to the product molecules allowed us to propose a more precise value for the dissociation energy of the ground state of CuBr than reported before. Finally, the total reaction rate constants for Cu ( 2S, ,2 ), CU*(‘D,,~) and CU*(~D~,~) with Br2 were measured at 300 K.

2. Experimental The copper bromide was produced in a flowing afterglow reactor described in detail elsewhere [ 111. In brief, copper atoms sputtered from a hollow cathode in a discharge were entrained in a neon flow, a fraction of them (around 1%) being in the metastable 2D 3/2,5/2 states. A few centimeters downstream from the cathode a 5% Br,-95% Ne mixture was introduced, yielding a bright, blue-green chemiluminescent flame. The chemiluminescence resulted from the reaction of metastable copper atoms with bromine molecules

C~*(~D3,2,5,2)+Br~+CuB?+Br,

(2)

CuBrQCuBr+hu.

(3)

For energetic reasons (see fig. 1) the reaction involving the ground state Cu (‘S) atoms can produce only ground state CuBr( X ‘Z+ ) molecules. The total pressure in the reaction zone was about 0.1 Torr, due to the neon carrier gas. The total Cu concentration was in the range of 10’ ’ cmp3. For the spectroscopic measurements the chemiluminescence signal was observed perpendicularly to the flow direction, about 1 cm after the Br2 inlet point. The spectrum of the emitted light was either viewed with a 1 m monochromator (Jobin-Yvon HRlOOO) or studied in detail with a 2 m grating monochromator (SOPRA), both equiped with 1200 grooves/mm gratings. The spectra were recorded with an intensified diode array multichannel detector (Spectroscopy Instruments) interfaced to a PC-AT computer for storage and further treatment of the data. For a width of 100 urn of the entrance slit used, the spectral resolution (fwhm ) was 0.15 nm for HR 1000 and 0.02 and 0.0 13 nm for SOPRA working in the 2nd and 3rd order of the grating, respectively. The wavelengths of the observed chemiluminescence were precisely calibrated using the emission spectra of titanium and neon [ 18 ] from a commercial Ti hollow cathode lamp tilled with Ne. In order to separate the chemiluminescence from the Cu*( ‘D3,2) and Cu*( ‘D,,l) reactions, the 2D3/2 or 2D 5,2 atoms were selectively eliminated from the reaction zone. This was achieved by “pumping out” the ‘D3,2 or/and 2D,,2 states with the 578.2 nm (2P,,2t2D3,2 ) or/and 5 10.6 nm ( *P3/2t2D5/2) lines of a copper vapour laser (Oxford Lasers), as described in more detail in ref. [ 111. The total reaction rate constants of CU(‘s,,~), CU*( ‘D3/2) and Cu*( ‘D5,2) were measured by monitoring the density of the copper atoms 20 cm downstream from the reaction zone as a function of the bromine flow rate. The relative density of CU(~S,,~) was obtained by the conventional optical absorption technique, using a copper hollow cathode lamp as the source of the 327.4 nm (2P,,2-+2S,,2) resonance line. The relative density of CU*( 2D3,2) and Cu*( ‘D,,,) atoms was determined from the intensity of the resonance fluorescence at 327.4 nm (2P,,2+2S,,2) and

P. Kowalczyk et al. /CuBr chemduminescence

75

X

CuBr + Br

Cu + Br2

Fig. I. Schematic energy diagram for the Cu+Br, reaction.

324.7 nm (2P3,2-+2S,,2) induced by the 578.2 and 510.6 nm lines of the copper vapour laser, respectively. The reaction constants were normalized to absolute values [ 19 ] by comparison with the rate constant for quenching of Af( 3P,) by Br2, measured in the same experimental geometry, which has a known value of 6.5 x 1O-lo cm3 s-i [ 201. The relative density of Af( 3P2) was obtained by optical absorption of the 8 11.5 nm argon line.

3. Results 3.1. Spectroscopy The energy considerations (fig. 1) allow for production of CuBr molecules by reaction (2) in the ground state X ‘E+ and excited states A 3111,B ‘II and c Ix+. Indeed, the observed chemiluminescence spectrum extended between 400 and 540 nm and apparently consisted of the A-X, B-X and C-X band systems. However, since the B-X and C-X transitions were very weak, we focused our spectroscopic investigation on the 465-535 nm spectral range, containing almost the total chemiluminescence inten-

sity. Fig. 2 shows a low-resolution (0.15 nm) spectrum recorded in this range. According to the previous studies [ 12,15,2], the A 311+X ‘Z+ band system is expected in this region, and our spectrum reveals the well resolved vibrational structure of this transition. Since the potential curves and internuclear distances of the A and X states are similar, the spectrum consists of characteristic groups (sequences) of bands [21 I. Our attempt to tit the positions of all the observed bands with the vibrational constants available for the A and X states [ 12,141 failed and only part of the bands fitted the A-X transition (note in particular the presence of strong bands at 490,497 and 504 nm in fig. 2). This convinced us that we observed two different molecular transitions with spectra located in the same range of wavelengths. By checking the separation of the appropriate bands, we verified that the new transition, appearing besides the A-X system, also terminated in the CuBr (X iC+ ) state. The goal of this part of the experiment was to determine the nature and molecular constants of the newly discovered excited state, A’, and to improve the rather unreliable vibrational constants of the A state [ 121. Figs. 3 and 4 show portions of the che-

P. Kowalczyk

Av=

Av=

et al. / CuBr chemilummescence

AI-X

G&r”-++ 3

A-X

a)

wave

length

[nm]

Av=

AI-X +

+

+-

Av=

A-X

b)

500

510

520 wave

length

530

[nm]

Fig. 2. Part of the chemiluminescence spectrum of the CuBr molecule (corrected for the wavelength response of the detection channel) recorded under low resolution (0.15 nm). The sequences in the A-X and A-X systems are pointed out. Note the presence of Cu atomic lines at 521.8, 515.3 and 498.4 nm (the 249.2 nm line observed in the 2nd order of the grating) due to the scattered hght from the discharge.

P. Kowalczyk et al. /CuBr chemdummescence

d

b

495.4

495.6 wave

length

[nm]

Fig. 3. Spectrum of the (0, 1) vibrational band in the A ‘II, + X ‘Z+ system recorded at a resolution of 0.013 nm. The four band heads wslble m the figure are formed in the R branches corresponding to the lsotoplc molecules (a) 65Cu8’Br, (b) 65Cu79Br, (c) ‘Wu8’Br and (d) 63Cu79Br.

miluminescence spectrum recorded under the highest resolution available (0.0 13 nm), which still did not resolve the individual rotational lines. However, we could observe the well-defined band heads in the vibrational bands (overlapped by the isotopic structure) and determine their positions with a precision of about 0.4 cm- ‘. In fig. 4a the (0, 0) vibrational band of the A-X system is shown. For this band the isotopic splitting is negligible and the spectrum reveals a single, prominent band head in the R branch and a much weaker band head in the Q branch. With the use of the rotational constants for the A 311, and X ‘C+ states known from the literature [ 14,15 ] and assuming a 300 K Boltzmann rotational distribution, we could simulate the shape of the observed band perfectly (fig. 4a). Considering the long lifetimes of the A and A’ states (about 8 and 12 us, respectively [ 221) and our working pressure (about 0.1 Torr) the rotationally inelastic collisions with neon were efficient enough to induce thermal equilibrium between the rotational levels. The relative intensities of the P, Q and R branches were taken as for an allowed ‘II‘C transition ( 1 : 2 : 1).

Fig. 4b shows the (0,O) band of the A-X system. Our first impression was that the Q branch was missing in this case. However, using a wide range of trial Bb values we were not able to simulate the observed spectrum under this assumption. Finally, we obtained a satisfactory fit with Bb =0.0985 &0.005 cm - ’ and all three rotational branches present, but with an unusual intensity ratio R: Q : P w 4 : 1: 1. One should mention that the quality of the simulation was not very sensitive to either the Bb value or the intensity ratio given above and did not allow for a more precise determination of them. This type of unusual intensity distribution has been predicted by the theoretical calculation of Schamps et al. [ 61 for the spinforbidden ‘II, ‘C-‘C transitions in copper fluoride. Since the rotational constants for the X ix+ and A ‘II, states are known [ 14,15 1, the positions of the band origins of the A-X system could be easily deduced from the positions of the R-branch band heads observed in the experiment. A similar procedure was applied to the A-X system, by using the rotational constant estimated above and the CY,and D, values of

P. Kowalczyk et al. I C&r

78

chem~iumlnescetzce

wave

488.4

488.2

488.0

487.8

length

[nm]

b)

504.2

504.4

504.6

wave

length

504. I3

[nm]

Rg. 4. Spectra of the (0,O) vibrational bands in the (a) A-X system and (b) A’-X system recorded at a resolution of 0.013 nm (solid lines) compared to the simulated spectra (dotted lines). The band marked by x is ( 1, 1f of A-X; y and z denote the isotopic bands (3,s) and (4.6) of A-X. respectively. We are presently not able to account for the unexpected intensity of the (3.5) band and the wavelength shift of the (4,6) band in the 6’Cu79Br isotopic mole&e.

P Kowalcqk

19

et al. / CuBr chemdummescence

the neighbouring A state. Using the positions of 137 band origins of the A-X system and 147 band origins of the A’-X system (including isotopic bands), we calculated the term energies and vibrational constants for the A and A’ states by a weighted linear leastsquares procedure. The constants of the ground X state were fixed at the values given by Manson et al. [ 141. The observed isotope shifts (fig. 3 ) established a unique vibrational numbering [ 2 1 ] in the A’ state and confirmed the existing numbering in the A state. The positions of the band origins were fitted to an rms deviation of 0.4 cm-‘, i.e. of the order of the experimental uncertainty. The molecular constants listed in table 1 describe the A 311, state up to v=9 and the A’ state up to v= 10. It must be remembered that the rotational constants assumed for the A’ state are not exact and changing them would slightly affect the vibrational constants assigned to this state. The A’ state is located 666.4 * 0.4 cm- ’ below the A ‘II, state and its vibrational frequency and anharmonicity are very similar to those of the A state. Additionally, the measured lifetime of the u= 0 level of the A’ state (T= 12 us [ 221) suggests a forbidden character for the A-X transition. Three candidates exist for the A’ state assignment among the molecular states correlated asymptotically with the ionic Cu+ ( 3d94s)Br- (4s24p6) configuration, namely the 3110,3112and 3C+ states. Unfortunately, there are no theoretical calculations on the electronic structure of the CuBr molecule, which might help to identify the A’ state. We can only use results for the similar molecules CuF and CuCl [ 1,3-8,231. The states 3112and 3Z+ are known to lie below the ‘II, state in these mol-

ecules. The CuF( ‘112) state was observed about 450 cm-’ below the 311, state [23]. However, since the 3112state radiates to the ground state mainly due to intensity borrowing from the ‘II,-X ‘Cf transition [ 5,6], the assignment of A’ as 3112cannot easily account for the low intensity of the corresponding Q lines observed in our experiment. Additionally, the lifetime of the 3112state is expected to be of the order of several milliseconds [ 6 1, contrary to the measured value. The ‘C+ state has been recently identified in CuCl as the molecular A state [ 4,6 1, originally expected to be the 52~ 2 component of the ‘II manifold [ 11. This state is placed around 1500 cm- ’ below the B ‘II, state. But also for a 3C+ assignment to CuBr(A’), the Q branches in the A’-X transition should be the strongest ones. Additionally, as for 3112, the lifetime of the ‘C+ state is expected to be much longer than that of the A 3111state. The third assignment to be taken into account, namely A’ 3110. provides the easiest way to understand the observed rotational intensity pattern. The intensity anomalies between the P and R branches for the 3II,,-X ‘C+ transition have been theoretically predicted for the similar CuF molecule [6] and were attributed to quantum interference effects between the ‘II,, 311, and 3C+ states. The observed weak Q branches in the A-X system may be an experimental artifact due mainly to the lack of precise values for the rotational constants of this state. The similarity of its lifetime (12us)withtheoneforA311, (sums) [22] support this assignment. However, there is both experimental and theoretical evidence that in CuF and CuCl the ‘II, state is located above the ‘II, state [l-8]. Pres-

Table 1 Molecular constants for the A ‘TI, and A’ states of the 63Cu79Brmolecule. All quantities are in cm-‘. Errors quoted m parentheses are one standard devtatton A ‘Tl, state

T, 0, W-G o&r. B, 01, 0. a) Taken from ref. [ 15 1.

A’ state

This work

Ref. [ 121

This work

20496.06( 16) 297.69( 13) 1.332(32) -0.0125(23) 0.96509x 10-l ‘) 0.65x IO-‘“’ 0.389x 10m7a’

20498.5 296.13 1.01

19828.21(16) 294.49( 12) 1.525(26) 0.0239( 16) 0.985(5)x10-’ 0.65~ lo-‘“’ 0.389x lo-‘“’

80

P. Kowalczyk et al. / CuBr chemdummescence

ently we cannot find any mechanism responsible for changing the order of the fine structure components of the ‘II manifold in CuBr. Another possibility is suggested by the very recent theoretical calculation for the CuI molecule done by Ramirez-Solis et al. [ 91. Contrary to the light copper monohalides (CuF and CuCl), where the lowest neutral asymptote does not form bound molecular states, in the CuI molecule the first excited states are the ‘II and %I states, correlated with the neutral atomic configuration Cu( 3d”4s)I (5~~5~‘). It is possible that the corresponding bound covalent states are also present in the CuBr molecule and located in the energy range we observed. This would open another possibility for the assignment of the new A’ state as a component of the covalent 311 multiplet. Theoretical calculations could help to clear up this controversy and the A-X system calls for a high-resolution spectroscopic study both for a direct assignment of the upper state symmetry and for a more precise determination of the rotational constants. 3.2. Reaction dynamics The total reaction rate constants for Cu (‘S, ,* ) and Cu*(‘D,,,,,,,) atoms reacting with Brz, based on comparison with the quenching rate constant of Ar*( 3Pz) by Br, [ 201, were determined from the decay of the density of these atoms, measured 20 cm downstream of the BrZ injection point, versus the reagent flow rate [ 19 1. The rate constants deduced from the slopes of the decay curves shown in fig. 5 are k=(4.1kO.l)x lo-” cm3/s for the ground C~(*S,,~)stateandk=(1.5+0.1)~10-’~cm~/sfor both the CU*(‘D,,~) and CU*(~D,,~) metastable states. Similarly to other halogen compounds, the total reaction rate constant for the metastable states is a few times smaller than for the ground state copper atoms [ 241. It must be noted, however, that whereas the total rate constant for reaction (1) with Br, is about four times larger than for the corresponding reaction with F2 [ 111, the chemiluminescence of the CuBr product molecule is around one order of magnitude weaker than that of CuF. As already mentioned, the chemiluminescence spectrum consisted of the A-X, A-X, B-X and C-X molecular systems. The relative contributions of the Cu*( ‘D3,z) and Cu* (*D,J states to the population

of these states have been investigated by state selection of the metastable atoms with the light of a copper vapour laser [ 111. The vibrational energy available to each CuBr product state can be calculated [ 241 provided that the dissociation energies Do of the ground states of Brz and CuBr are known precisely. Unfortunately, this is not the case: whereas the Do value for Br2 has been accurately determined from spectroscopicdata (Do=15895.6+0.4cm-’ [25]), for CuBr only the thermochemical value is available (Do = 27400 cm- ’ [ 261) with an uncertainty of 2000 cm-‘. The contribution of the Cu*(*D,,?) and Cu*( *D5,*) atoms differs dramatically in production of CUB? (B ‘II) : the emission from the vibrational levels up to v= 11 present in fig. 6a, disappears when the Cu*(*D,,,) atoms are pumped out (fig. 6b). Using the dissociation energies of Br, [25] and CuBr [ 261 and the B state molecular constants from Huber and Herzberg [ 27 1, one can expect that the B state levels up to v=8 should be energetically accesssible when the reacting copper atom is in the *D3,, state, but only the lowest V= 0 level can be populated with the copper atom in the ‘Dg,* state. This prediction is in qualitative agreement with the experimental chemiluminescence spectra, although we observed population of B state levels by ‘D3,* copper atoms at least up to v= 11 (fig. 6). This allows to set the lower limit for the value of the dissociation energy of the ground X’C+stateofCuBrasD,>28200cm-‘. The branching ratio of reaction products, observed with the presence of both ‘D3,;! and ‘Ds,z copper atoms in the reaction zone, was CuBr(A) : CuBr(A’) : CuBr(B) : CuBr(C)=30 : 30 : 1 : (
P. Kowalczvk et al. / CuBr chemdummescence

2 f low

4 rate

of the

8

6 Br2-Ne

mixture

81

10 [cm3/min]

Fig. 5. Density of copper atoms in: ‘S I,z (crosses), 2D5,z (squares) and *D 3,2 (cucles) states and of A?( ‘Pz) atoms (stars), measured 20 cm downstream of the Br, inJection point, as a function of the Br,-Ne mixture flow rate. The straight lines represent least-squares fits to the experimental pomts. Despite the 2043 cm-’ energy difference between the 2D,,2 and 2D,,2 metastable states, we did not observe a marked difference in the A-X and A’-X spectra recorded in both cases, showing that the vibrational distributions in the A and A’ states were (at least in the low vibrational levels) practically independent of the state of the metastable copper atom participating in the reaction. Reaction (2) is then highly exoergic for both the CU*(~D,,,) and the Cu*( ‘D,,,) atomic state, allowing the population of vibrational levels up to v= 12/20 in the A state and up to U= 14/22 in the A’ state. Since our chemiluminescence spectra were insufficiently resolved, they could not be inverted directly to obtain the rovibrational populations of the excited states. Consequently, we performed spectral simulations to get information about the vibrational distribution in each case. As already mentioned in the previous section, the rotational distribution was a 300 K Boltzmann one. The vibrational distribution in the A state was well represented by the prior distribution [ 281 with (f;,) = 0.29. In the A’ state the population of the vi-

brational levels was found to decrease more rapidly than the prior with increasing U. However, lacking precise molecular constants for the A’ state and, consequently, reliable Franck-Condon factors for the A’X system, we were not able to study the A’ state population in more detail. It is interesting to compare the vibrational distribution in the A3rI, state with the distribution observed in the b 3H, state of CuF [ 11,241 formed in the analogous reaction Cu*+Fz-+CuF+F.

(4)

Although the energy available in both reactions is comparable and the lifetime of the b state in CuF (r= 7.1 ps [ 1 ] ) has almost the same value as for the A state in CuBr (7= 8.1 l.~ts[ 22]), the vibrational distribution in CuF (b 3rI, ) was found to be strongly inverted, with a maximum around v=25 and (J;,) N 0.75 [ 11,241. Additionally, the efficiency of forming the b state molecules in reaction (4) by copper atoms was much larger than for 2D3,2 *J&,2 atoms [24]. This shows that, despite the apparent

P. Kowalczyk et al / CuBr chemdummescence

82

12

I

I

3

I

1

4

5

I

I

A-X,

Av=-3

B-X, v’=O

6

I

7

I

6

I

9

,

10

I

11

1

Av=4

7

a)

t

+

452

454

456

456

wave length

460

462

[nm]

Fig. 6. Chemiluminescence spectrum (resolutron 0. I5 nm) m the regton of the B-X, AZ)= - 3 transitton: (a) Cu laser off (both Cu’ metastables present), (b) 578 nm laser line on (CU’(‘D~,~) depopulated). Note the presence of atomic lines of Cu (451.3 and 452.5 nm) and Ne (452.9,457.5 and 461.4 nm) due to the scattered light from the discharge.

Table 2 Relative branching ratios for formation in different electronic states resulting Cu*(‘D 3,2,5,2) +Br,. N means negligible

of CuBf molecules from the reaction

CuBf:

Cu*(‘Dw) C~*(‘DS,Z)

A

A

B

C

100 70

100 70

7 N

N N

similarity, the mechanisms differ considerably.

of reactions

(2) and (4)

4. Conclusion By examination of CuBr chemiluminescence resulting from the reaction of copper metastable atoms with bromine we found a new CuBr state (called A’) of triplet symmetry, located about 666 cm- ’ below the A 3fI, state. The molecular constants of the A’ state

were determined and those of the A ‘ff, state were improved. A full identification of the A’ state would require high-resolution spectroscopic study or/and ab initio calculations of the electronic structure of the CuBr molecule. Observation of the vibrational population in the product CuBr( B ‘l-I) state allowed us to estimate the limiting value of the dissociation energy of the ground state of CuBr as Do> 28200 cm-‘. The vibrational distribution in the CuBr (A 3KI,) state was found to be similar to the prior one, in contrast to the highly inverted distribution found in CuF (b ‘IT, ) formed in the analogous Cu*( ‘D) reaction with Fz. Although the total reaction rate constants for the copper metastable states reacting with BrZ are larger than when reacting with F2, the observed chemiluminescence of CuBr is actually an order of magnitude weaker than that of CuF. This suggests that the Cu+ ion core conservation model, valid for the Cu* + Fz reaction [ 111, fails for Cu* + Br, and a large amount of ground state CuBr is produced even in the reactions of Cu*( 1D3,2,5,2) atoms.

P Kowalczyk et al. / CuBr chedummescence

Acknowledgement We are grateful to Professor D.W. Setser, Professor J. Schamps and Dr. F. Hartmann for helpful discussions. PK. wishes to thank the French Minis&e de la Recherche et de la Technologie for a fellowship.

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