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The A′ 3Π(2u-X 1Σg+ emission spectrum of Br2 in an argon matrix
Volume 115, number 6
CHEMICAL
THE A’ 3n(2,)-X Jean-Philippe
‘2;
NICOLAI,
EMISSION Lambertus
PHYSICS
19 April 1985
LETTERS
SPECIRUFvI OF Br, IN AN ARGON J. VAN
DE BURGT
and Michael
MATRIX
C. HEAVEN
Depxzrtment of Chcmistty, IIlinoir Institute of Tiichtnology,Chicago, IL 60616, USA Received 29 November 1984
The emission spectrum of the A' 311(2,)-X ‘2: system of Br, in an argon matrix is reported. The vibrationally resolved spectra have been analysed and two possible vibrational numberings have been determined. These provide estimates for T, of 12966&S and 12679&S cm-‘, respectively. Excitation spectra have been examined, and these provide insight into the excitation and relaxation mechanisms present in the matrix.
1_ Introduction
During the past decade the diatomic halogens have been used to investigate the effects of rare gas matrix cages on molecular dissociation, vibrational relaxation, and transfer between electronic states. Optical excitation of the well known halogen B 3 lI(O:)-X 1 Zz systems, and the continuum transitions, has been used to this end. The most extensive studies have been conducted on Br, [l-3] and the picture that emerges is one of a delicate balance between competing relaxation processes_ Ault et al. [4] and Bondybey et al. [l] have examined the B-X transition of Br2 isolated in Ar. They found that re-emission from the B state occurred for excitation into both the bound and continuum regions. Complete vibrational relaxation occurred prior to emission, and the fluorescence decay lifetime indicated that the u’ = 0 level did not relax non-radiatively to other electronic states. Flynn and co-workers [2,3] extended these studies by investigating the far-red and IR regions, where the A 311(1,) and A’ 311(2,) states emit. In Ar and Kr matrices they observed fluorescence corresponding to two different lifetimes following excitation at 500 nm 121. The short-lived emission was assigned to the A-X transition, on the basis of a spectral analysis. The long lifetime emission was too weak for spectral resolution, but it was tentatively assigned to the A’-X system_ FIYM and co-workers [2,3] made a careful study of the dynamics of the Br, emissions 496
in Ar, Kr, and Xe matrices. These data; and similar dynamical information for matrix isolated I2 [SJ and Cl2 161, were used to develop a model for the excitation and relaxation processes that occur in the matrix environment [3,5]. In the description of the mechanisms initially populating the various electronic states, emphasis was placed on the role of the cage effect on photodissociation IS]_ In the present work we have reexamined the farred and IR emissions of Br2 in an Ar matrix. Resolved fluorescence spectra have been recorded for both the A-X system and the long-lived emission. Analysis of the latter confirms the previously tentative A’-X system assignment. The spectrum provides au approximate determination of the electronic term value, Y-c, for the A’ state. The mechanisms by which the A and A’ states are populated following absorption of light in the 500540 nm region has been considered_ In contrast to previous interpretations [S], arguments are presented which suggest that the photodissociation mechanisms are unimportant when wavelengths greater than 500 run are used to excite Br, and Cl,.
2. Experimental Samples of Br, in Ar (1 : 1000) were deposited on an Al mirror cooled to -15 K by a closed cycle helium refrigerator (CT1 model 20). The excitation source was 0 009-2614/85/S (North-Holland
03.30 0 Elsevier Science Publishers B.V. Physics Publishing Division)
19 April 1985
CHEMICAL PHYSICS LETPERS
Volume 115, number 6
a pulsed tunable dye laser (Quanta-Bay PDLl) pumped by the third harmonic of a YAG laser (Quanta-Ray
DCR2). Lasing over the range of 490-550 nm was obtained using the dye coumarin 500. The fluorescence from the sample was focused onto the slits of a 0.25 m monpchromator (Bausch and Lomb)_Visible fluorescence (700-950 nm) was detected by a GaAs photo-
B ‘2
in ARGON A-X
multiplier (RCA 4832, response time 10 ns), while near-IR radiation (0.951.6 run) was detected by a
germanium semiconductor device (ADC model 403, response time 2 ms). A boxcar integrator @‘AR model 162) was used to record the emission spectra.
3. Results
I
Over the range of excitation wavelengths investigated, the resultant fluorescence spectrum of Br, contained three different components. These corresponded
in
Br2 A’-
ARGON X
to emission from u’ = 0 levels of the B, A, and A’ states.
Our results for the B-X and A-X systems are in good agreement with the observations of Mandich, Beeken, and Flynn [2]. The IR detector used in this work provided a high sensitivity for detection of light in the 0.95-l .6 m range. Sufficient sensitivity was available for spectral resolution of these wavelengths, which had not been possible in previous studies [2,3]. Two emission lifetimes were observed in this spectral region, but the slow response time of the detector precluded accurate determination of the decay rates. The fluorescence signals were convoluted with the detector response characteristics, producing curves which could be represented by the expression I(t) = A(emkr - e-*/T)
,
where k is the fluorescence decay rate, and r is the time constant for the detector. Thus a maximum was seen in the output signal, after a period determined primarily by k. The difference in k between the two IR emissions waslarge enough to allow for the use of time resolved fluorescence detection (k(A) = 1.5 X lo4 s-l, k(_4’) = 91 s-l [a]). In this way the spectrum associated with a single lifetime could be recorded, free from overlapping emissions. With the boxcar gate set on the maximum of the short-lived emission, the spectrum shown in fig. la was recorded. This spectrum corresponds to a single vibrational progression. When corrected for the frequency response
1
7350
1
1300
1
I
I
1250
1200
1150
WAVELENGTH
1100
inm)
Fig. 1. Emission spectra for Bq In an argon matrix. Excitation wavelength 500 nm. (a) Spectrum recorded with 0.1 ms boxcar gate width and 0.2 ms delay- Monocbtomator slit width = 2.5 mm. (b) Spectrum recorded with 0.6 ms gate width and 4 ms delay. Monocbromator slit width = 1.5 mm. In both spectra the anomalous intensity of the peaks around 1200 run axe an instrumental artifact. Emission intensity versus laser fluence plots showed that both spectra result from single
photon absorption_
of the monochromator/detector combination, the intensity distribution has a single maximum, and the overall pattern indicates emission from U’ = 0. The vibrational intervals identify the lower state as X 1 Z$, and at long wavelengths the bands begin to show the expected resolution into isotopic components. The bands observed by Mandich et al. [2] at shorter wavelengths (0.92-i _1 m) frt well as an extension of 497
CHEMICAL
Volume 115. number 6
PHYSICS
this progression. Analysis of the complete spectrum gives a vibrational numbering which is in agreement with the previous study, and the A-X system assignment is confirmed. With the boxcar gate set at a delay of 4 1msthe spectrum shown in fig. 1 b was obtained_ Under these conditions the appearance of the A-X bands has been compIetely suppressed. The characteristic isotopic structure of Br, is evident in fig. 1 b, and the vibrational intervals are consistent with emission to the ground state. Once again, the corrected intensity distribution indicates emission from u’ = 0. The wavelengths and wavenumbers for the band centers are given in table 1, along with a tentative vibrational numbering_ Unfortunately, the uncertainties associated with the measurement of the band centers in this low-resolution spectrum make it difficult to detemine the absolute numbering_ Neither the intervals between bands, nor the isotopic splittings are determined well enough for unambiguous assignment_ In table 1 we compare the measured band separations with the separations calculated from the constants of Bondybey et al. Cl]_ The indicated numbering gives a reasonably good fit, but changing this numbering by as much as *2 units will also give fits which are within the experimental error (this is true for the isotopic splitting as well)_ Comparison of figs. la and lb shows that the two Table 1 Band oriis and vibrational intervals for the A’-X of 7gBr ** Br
system
u” a)
x (nm) b)
v (cm-r ) c)
Ap’=)
Au c&d)
12 13 14 15 16 17 18 19 20
1080 1116 1153 1194 1235 1297 1327 1377 1431
9259 8961 8673 8375 8097 7819 7536 7262 6988
298 288 298 278 278 283 274 274
290 288 286 284 282 280 278 276
a) Aii transitions originate from U’ = 0. The numbering given here may be one unit too high. See text. C) Error limits + 8 cm-’ _ b, Error limits 2 1 run. dl Calculated from the constants given by Bondybey, Bearder, md Fhtcher [l].
498
LETTERS
19 April 1985
excited states involved are radiating to a similar series of ground state levels, and must have a relatively small separation between Their T, valr.%s. In addition,-thecorrected intensity distributions are quite similar, implying littIe change in the equilibrium internuclear distance, R,, between the two states. Within the accessible energy range, only the A’ or 3II(O;) states could give rise to the emission spectrum shown in fig. lb (the other bound states have been accounted for) [7]. The latter assignment may be discounted, as the 311(O;) state is expected to have a T, value greater than T,(A), and the equilibrium internuclear separation is expected to be significantly greater than the A state R, value [6]. Conversely, in the gas phase, the A and A’ states are known to be energetically close with very similar R, values [9,10]. Thus we conclude that the long-lived IR fl,r?orescence belongs to the A’ -X system. Sur and Tellinghuisen [lo] have estimated the A’ state T, value in the gas phase. They observed high vibrational levels of the A’ state in the D’-A’ emission spectrum and, by extrapolation, determined that Te(A> = 13220 + 100 cm-l. A value for Te in the matrix can be estimated by using the numbering of table 1, the known ground state constants [ 11, and the assumption that the A’ state gas phase vibrational constants are valid for the matrix. This gives T, = 12966 2 8 cm-l and a matrix red-shift of approximately 250 cm-l, which is in good agreement with the 255 cm- 1 red-shift seen for the A state [2]. Increasing the numbering by one unit results in a blue-shift which is contrary to the general observation of red-shifts caused by guest-host interactions. Decreasing the numbering by one unit gives T, = 12679 2 8 cm-l and an acceptable, though large, red-shift of a.540 cm-l _ Consequently it is possible that the given numbering is one unit too high. However, further reduction of the numbering gives an unreasonably large matrix shift. Excitation spectra for both A and A’ were briefly investigated_ In each case the monochromator was used to isolate a single vibrational band, and the resolved fluorescence intensity was monitored as a function of the laserwavelength. Essentially identical spectra were recorded for both states, and our results are in agreement with the excitation spectra published by Mandich et al. [2]. (See fig. 4 of ref.‘ [2].)_
Volume 115, number 6
CHEMICAL. PHYSICS LETTERS
4. Discussion The principal result of the present study is the assignment and characterization of the BrZ A’-X transition in an Ar matrix. This transition has been tentatively identified in the studies of Flynn and co-workers [2,3], and these investigators proposed a model for the excitation and relaxation processes occurring in the matrii environment [S] . The A’ assignment was assumed to be correct in the development of the relaxation model. Although we confirm this assignment, we find that close inspection of the proposed model raises questions concerning its validity. For matrix isolated Br,, Fly~ and co-workers [2,3] conclude that absorption of wavelengths around 500 nm leads primarily to population of the B state with a small component of the excitation going to the 111(1,) repulsive state. The majority of the population in the A and A’ states then arises due to non-radiative transfer from the B state. This occurs by direct transfer from the higher vibrational levels, and by an indirect route involving crossing from the B state into lII( 1”) [ 111. Beeken et al. proposed that the latter mechanism dominates [S] . Complete dissociation is prevented by the matrix cage, and the atoms recombine into the A and A’ states. The physical size of the cage is thought to influence the branching ratio for population of the lower energy states [5]. The above excitation scheme is brought into question by consideration of the B state excitation spectrum reported by Bondybey et al. El]. It was found that absorption into the B state continuum effectively populated the bound levels, whereas excitation into the lII( I,) continuum did not produce 3 state emission. In the gas phase, rotational motion couples the I3 and 1 D( 1 u) states, resulting in a rotationally dependent predissociation [ 11,12]. The reverse process, crossing from the repulsive potential into the bound state, has also been observed [ 111. This channel allows for population of the B state by recombination of ground state Br atoms. The results of Bondybey et al. [l] show that the B and lII(l,) states are not signifrcantly coupled in an Ar matrix. Consequently the B state predissociation will be very inefficient, and it cannot account for the reported B : A state population ratio of 1 : 25 121. Another important feature of the B state excitation spectrum is the maximum, which occurs at 488 nm
19 April 1985
and coincides with the maximum of the gas phase B-X continuum absorption [ 1,131. The A state excitation spectra recorded in the present work reached a maximum at about 520 nm, indicating direct absorption into the A-X continuum region [14]. (Extrapolation of the data from ref. [ 141 gives absorptivities of 5.5 and 8.6 mol-1 cm-l at 500 and 520 run respectively.) The photofragmentation spectrum of Br2 measured by Oldman, Sander, and Wilson [ 151 offers support for this conclusion. For 532 nm excitation they found that absorption into the A-X continuum dominated the spectrum. In the matrix, transfer from B(v’ > 0) to the -4 state may well be present, but it must represent a minor channel for population of the A state. If this were not true, the B-X continuum would be more strongly represented in the A state excitation spectrum. Experimental confirmation that the A state is populated by direct excitation can be obtained from a photoselection study 16,161, and this will be undertaken in the near future. It is most unlikely that the A’ state is populated by direct excitation, as the A’-X transition moment is much too small for this to be efficient. Ener,T gap considerations favor transfer from the A state and, because of the similarity of the A and A’ potential energy curves, this processwill have large Franck-Condon factors for levels near the ground state dissociation limit. However, transfer will only occur if there is electronic coupling between the states [17]. Mandich et al. [2] noted a shortening of the A state u’ = 0 lifetime in an Ar matrix, which was attributed to non-radiative transfer into nearby vibrational Ievel of the A’ state (leakage from A into the ground state is forbidden by u/g symmetry restrictions). The transition has a low Franck-Condon overlap, but this will be offset if the energy gap is small. The important point about this interpretation is that it does indicate a significant degree of electronic coupling. We conclude that transfer between the A and A’ states is the most probable mechanism for population of the latter. Primarily the transfer occurs at the inner turning points of these states, and it does not involve hindered predissociation. Beeken et al. [3,5] have used state mixing and hindered predissociation arguments to explain the absence of emission from the A state in a Xe matrix. As the predissociation channel is not open, it would appear that state mixing in a Xe matrix results in an A -+ A’ transfer rate which is much greater than 499
Volume 115, number 6
CHEMICAL
PHYSICS
the A state radiative decay rate. Extensive state mixing is known to occur for Cl2 in an Ar matrix [6] _ Here, excitation of the B state produces emission from the A’ state exclusively, even when levels below the ground state dissociation hmit are excited [6]. Hindered predissociation and cage size arguments are clearly not applicable in this cast.
Acknowledgement We thank Professor J.S. Shirk for the loan of the matrix isolation equipment_ Professor Shirk and Mr. W. Hoffmann also assisted with the operation of this unit. We are grateful to Professor G.W. Flynn for helpful discussions. The germanium detector used in this work was provided, on loan, by Dr_ S.J. Davis of AFWL, Albuquerque. This work was supported by grants from the Air Force Office of Scientific Research (AFOSR-83-0173), The Petroleum Research Fund (PRF 14409-G6), and the Research Corporation (#9899).
References [ 11 V-E. Bondybe-y, S.S. Bearder and C. Fletcher, J. Chem. Phys 64 (1976) 5243.
500
LETTERS
19 April 1985
M. Mandich, P. Beeken and G. Flynn. J. Chem. Phys 77 (1982) 702. f31 P-B. Beeken, M. Mandich and G. Flynn, J. Chem. Phys 76 11982) 5995. E41 B.S. Ault, W-F. Howard 3r. and L. Andrews, J. MoL Spectry. 55 (197.5) 217. iSI P-B. Beeken, E.A. Hanson and G.W. Fiynn, J. Chem. Pbys. 78 (1983) 5892. WI V.E. Bondybey and C. Fletcher, J. Chem. Phys 64 (1976) 3615. 171 R.S. MuIIiken, Phys. Rev. 46 (1934) 549; 57 (1940) 500; J. Chem. Phys. 55 (1971) 288. 181 J-A. Coxon, In: Molecular spectroscopy, VoL 1 (The Chemical Society. London, 1973)). PI J.A. Coxon, J. MoL Spectry. 41(1972) 548,566. IW A. Sur and J. TeBIngImisen, J. Mol. Spectry. 88 (1981) 323. r111 M.A.A. Clyne, MC; Heaven and 3. Telhnghuisen, J. Chem. Phys. 76 (1982) 5431. [ 123 M.AA. Clyne and MC. Heaven, J. Chem. Sot. Faraday II 74 (1978) 1992. 1131 R-1. Personov, E.I. AI’Shits and L-A. Bykovskaya, Opt. Commun. 6 (1972) 169. [141 C.P. Hemenway. T-G. LIndeman and J-R. WlesenfeId. J. Chem. Phys. 70 (1979) 3560. 1151 R.J. Oldman, R.K. Sander and K-R. Wilson, J. Chem. Phys 63 (1975) 4252. 1161 AC. AIbrecht, J_ MoL Spectry. 6 (1961) 84. 1171 R. Kubo and Y. Toyozawa, Progr. Th:oret_ Phys 13 (1955) 160.
VI
CHEMICAL
THE A’ 3n(2,)-X Jean-Philippe
‘2;
NICOLAI,
EMISSION Lambertus
PHYSICS
19 April 1985
LETTERS
SPECIRUFvI OF Br, IN AN ARGON J. VAN
DE BURGT
and Michael
MATRIX
C. HEAVEN
Depxzrtment of Chcmistty, IIlinoir Institute of Tiichtnology,Chicago, IL 60616, USA Received 29 November 1984
The emission spectrum of the A' 311(2,)-X ‘2: system of Br, in an argon matrix is reported. The vibrationally resolved spectra have been analysed and two possible vibrational numberings have been determined. These provide estimates for T, of 12966&S and 12679&S cm-‘, respectively. Excitation spectra have been examined, and these provide insight into the excitation and relaxation mechanisms present in the matrix.
1_ Introduction
During the past decade the diatomic halogens have been used to investigate the effects of rare gas matrix cages on molecular dissociation, vibrational relaxation, and transfer between electronic states. Optical excitation of the well known halogen B 3 lI(O:)-X 1 Zz systems, and the continuum transitions, has been used to this end. The most extensive studies have been conducted on Br, [l-3] and the picture that emerges is one of a delicate balance between competing relaxation processes_ Ault et al. [4] and Bondybey et al. [l] have examined the B-X transition of Br2 isolated in Ar. They found that re-emission from the B state occurred for excitation into both the bound and continuum regions. Complete vibrational relaxation occurred prior to emission, and the fluorescence decay lifetime indicated that the u’ = 0 level did not relax non-radiatively to other electronic states. Flynn and co-workers [2,3] extended these studies by investigating the far-red and IR regions, where the A 311(1,) and A’ 311(2,) states emit. In Ar and Kr matrices they observed fluorescence corresponding to two different lifetimes following excitation at 500 nm 121. The short-lived emission was assigned to the A-X transition, on the basis of a spectral analysis. The long lifetime emission was too weak for spectral resolution, but it was tentatively assigned to the A’-X system_ FIYM and co-workers [2,3] made a careful study of the dynamics of the Br, emissions 496
in Ar, Kr, and Xe matrices. These data; and similar dynamical information for matrix isolated I2 [SJ and Cl2 161, were used to develop a model for the excitation and relaxation processes that occur in the matrix environment [3,5]. In the description of the mechanisms initially populating the various electronic states, emphasis was placed on the role of the cage effect on photodissociation IS]_ In the present work we have reexamined the farred and IR emissions of Br2 in an Ar matrix. Resolved fluorescence spectra have been recorded for both the A-X system and the long-lived emission. Analysis of the latter confirms the previously tentative A’-X system assignment. The spectrum provides au approximate determination of the electronic term value, Y-c, for the A’ state. The mechanisms by which the A and A’ states are populated following absorption of light in the 500540 nm region has been considered_ In contrast to previous interpretations [S], arguments are presented which suggest that the photodissociation mechanisms are unimportant when wavelengths greater than 500 run are used to excite Br, and Cl,.
2. Experimental Samples of Br, in Ar (1 : 1000) were deposited on an Al mirror cooled to -15 K by a closed cycle helium refrigerator (CT1 model 20). The excitation source was 0 009-2614/85/S (North-Holland
03.30 0 Elsevier Science Publishers B.V. Physics Publishing Division)
19 April 1985
CHEMICAL PHYSICS LETPERS
Volume 115, number 6
a pulsed tunable dye laser (Quanta-Bay PDLl) pumped by the third harmonic of a YAG laser (Quanta-Ray
DCR2). Lasing over the range of 490-550 nm was obtained using the dye coumarin 500. The fluorescence from the sample was focused onto the slits of a 0.25 m monpchromator (Bausch and Lomb)_Visible fluorescence (700-950 nm) was detected by a GaAs photo-
B ‘2
in ARGON A-X
multiplier (RCA 4832, response time 10 ns), while near-IR radiation (0.951.6 run) was detected by a
germanium semiconductor device (ADC model 403, response time 2 ms). A boxcar integrator @‘AR model 162) was used to record the emission spectra.
3. Results
I
Over the range of excitation wavelengths investigated, the resultant fluorescence spectrum of Br, contained three different components. These corresponded
in
Br2 A’-
ARGON X
to emission from u’ = 0 levels of the B, A, and A’ states.
Our results for the B-X and A-X systems are in good agreement with the observations of Mandich, Beeken, and Flynn [2]. The IR detector used in this work provided a high sensitivity for detection of light in the 0.95-l .6 m range. Sufficient sensitivity was available for spectral resolution of these wavelengths, which had not been possible in previous studies [2,3]. Two emission lifetimes were observed in this spectral region, but the slow response time of the detector precluded accurate determination of the decay rates. The fluorescence signals were convoluted with the detector response characteristics, producing curves which could be represented by the expression I(t) = A(emkr - e-*/T)
,
where k is the fluorescence decay rate, and r is the time constant for the detector. Thus a maximum was seen in the output signal, after a period determined primarily by k. The difference in k between the two IR emissions waslarge enough to allow for the use of time resolved fluorescence detection (k(A) = 1.5 X lo4 s-l, k(_4’) = 91 s-l [a]). In this way the spectrum associated with a single lifetime could be recorded, free from overlapping emissions. With the boxcar gate set on the maximum of the short-lived emission, the spectrum shown in fig. la was recorded. This spectrum corresponds to a single vibrational progression. When corrected for the frequency response
1
7350
1
1300
1
I
I
1250
1200
1150
WAVELENGTH
1100
inm)
Fig. 1. Emission spectra for Bq In an argon matrix. Excitation wavelength 500 nm. (a) Spectrum recorded with 0.1 ms boxcar gate width and 0.2 ms delay- Monocbtomator slit width = 2.5 mm. (b) Spectrum recorded with 0.6 ms gate width and 4 ms delay. Monocbromator slit width = 1.5 mm. In both spectra the anomalous intensity of the peaks around 1200 run axe an instrumental artifact. Emission intensity versus laser fluence plots showed that both spectra result from single
photon absorption_
of the monochromator/detector combination, the intensity distribution has a single maximum, and the overall pattern indicates emission from U’ = 0. The vibrational intervals identify the lower state as X 1 Z$, and at long wavelengths the bands begin to show the expected resolution into isotopic components. The bands observed by Mandich et al. [2] at shorter wavelengths (0.92-i _1 m) frt well as an extension of 497
CHEMICAL
Volume 115. number 6
PHYSICS
this progression. Analysis of the complete spectrum gives a vibrational numbering which is in agreement with the previous study, and the A-X system assignment is confirmed. With the boxcar gate set at a delay of 4 1msthe spectrum shown in fig. 1 b was obtained_ Under these conditions the appearance of the A-X bands has been compIetely suppressed. The characteristic isotopic structure of Br, is evident in fig. 1 b, and the vibrational intervals are consistent with emission to the ground state. Once again, the corrected intensity distribution indicates emission from u’ = 0. The wavelengths and wavenumbers for the band centers are given in table 1, along with a tentative vibrational numbering_ Unfortunately, the uncertainties associated with the measurement of the band centers in this low-resolution spectrum make it difficult to detemine the absolute numbering_ Neither the intervals between bands, nor the isotopic splittings are determined well enough for unambiguous assignment_ In table 1 we compare the measured band separations with the separations calculated from the constants of Bondybey et al. Cl]_ The indicated numbering gives a reasonably good fit, but changing this numbering by as much as *2 units will also give fits which are within the experimental error (this is true for the isotopic splitting as well)_ Comparison of figs. la and lb shows that the two Table 1 Band oriis and vibrational intervals for the A’-X of 7gBr ** Br
system
u” a)
x (nm) b)
v (cm-r ) c)
Ap’=)
Au c&d)
12 13 14 15 16 17 18 19 20
1080 1116 1153 1194 1235 1297 1327 1377 1431
9259 8961 8673 8375 8097 7819 7536 7262 6988
298 288 298 278 278 283 274 274
290 288 286 284 282 280 278 276
a) Aii transitions originate from U’ = 0. The numbering given here may be one unit too high. See text. C) Error limits + 8 cm-’ _ b, Error limits 2 1 run. dl Calculated from the constants given by Bondybey, Bearder, md Fhtcher [l].
498
LETTERS
19 April 1985
excited states involved are radiating to a similar series of ground state levels, and must have a relatively small separation between Their T, valr.%s. In addition,-thecorrected intensity distributions are quite similar, implying littIe change in the equilibrium internuclear distance, R,, between the two states. Within the accessible energy range, only the A’ or 3II(O;) states could give rise to the emission spectrum shown in fig. lb (the other bound states have been accounted for) [7]. The latter assignment may be discounted, as the 311(O;) state is expected to have a T, value greater than T,(A), and the equilibrium internuclear separation is expected to be significantly greater than the A state R, value [6]. Conversely, in the gas phase, the A and A’ states are known to be energetically close with very similar R, values [9,10]. Thus we conclude that the long-lived IR fl,r?orescence belongs to the A’ -X system. Sur and Tellinghuisen [lo] have estimated the A’ state T, value in the gas phase. They observed high vibrational levels of the A’ state in the D’-A’ emission spectrum and, by extrapolation, determined that Te(A> = 13220 + 100 cm-l. A value for Te in the matrix can be estimated by using the numbering of table 1, the known ground state constants [ 11, and the assumption that the A’ state gas phase vibrational constants are valid for the matrix. This gives T, = 12966 2 8 cm-l and a matrix red-shift of approximately 250 cm-l, which is in good agreement with the 255 cm- 1 red-shift seen for the A state [2]. Increasing the numbering by one unit results in a blue-shift which is contrary to the general observation of red-shifts caused by guest-host interactions. Decreasing the numbering by one unit gives T, = 12679 2 8 cm-l and an acceptable, though large, red-shift of a.540 cm-l _ Consequently it is possible that the given numbering is one unit too high. However, further reduction of the numbering gives an unreasonably large matrix shift. Excitation spectra for both A and A’ were briefly investigated_ In each case the monochromator was used to isolate a single vibrational band, and the resolved fluorescence intensity was monitored as a function of the laserwavelength. Essentially identical spectra were recorded for both states, and our results are in agreement with the excitation spectra published by Mandich et al. [2]. (See fig. 4 of ref.‘ [2].)_
Volume 115, number 6
CHEMICAL. PHYSICS LETTERS
4. Discussion The principal result of the present study is the assignment and characterization of the BrZ A’-X transition in an Ar matrix. This transition has been tentatively identified in the studies of Flynn and co-workers [2,3], and these investigators proposed a model for the excitation and relaxation processes occurring in the matrii environment [S] . The A’ assignment was assumed to be correct in the development of the relaxation model. Although we confirm this assignment, we find that close inspection of the proposed model raises questions concerning its validity. For matrix isolated Br,, Fly~ and co-workers [2,3] conclude that absorption of wavelengths around 500 nm leads primarily to population of the B state with a small component of the excitation going to the 111(1,) repulsive state. The majority of the population in the A and A’ states then arises due to non-radiative transfer from the B state. This occurs by direct transfer from the higher vibrational levels, and by an indirect route involving crossing from the B state into lII( 1”) [ 111. Beeken et al. proposed that the latter mechanism dominates [S] . Complete dissociation is prevented by the matrix cage, and the atoms recombine into the A and A’ states. The physical size of the cage is thought to influence the branching ratio for population of the lower energy states [5]. The above excitation scheme is brought into question by consideration of the B state excitation spectrum reported by Bondybey et al. El]. It was found that absorption into the B state continuum effectively populated the bound levels, whereas excitation into the lII( I,) continuum did not produce 3 state emission. In the gas phase, rotational motion couples the I3 and 1 D( 1 u) states, resulting in a rotationally dependent predissociation [ 11,12]. The reverse process, crossing from the repulsive potential into the bound state, has also been observed [ 111. This channel allows for population of the B state by recombination of ground state Br atoms. The results of Bondybey et al. [l] show that the B and lII(l,) states are not signifrcantly coupled in an Ar matrix. Consequently the B state predissociation will be very inefficient, and it cannot account for the reported B : A state population ratio of 1 : 25 121. Another important feature of the B state excitation spectrum is the maximum, which occurs at 488 nm
19 April 1985
and coincides with the maximum of the gas phase B-X continuum absorption [ 1,131. The A state excitation spectra recorded in the present work reached a maximum at about 520 nm, indicating direct absorption into the A-X continuum region [14]. (Extrapolation of the data from ref. [ 141 gives absorptivities of 5.5 and 8.6 mol-1 cm-l at 500 and 520 run respectively.) The photofragmentation spectrum of Br2 measured by Oldman, Sander, and Wilson [ 151 offers support for this conclusion. For 532 nm excitation they found that absorption into the A-X continuum dominated the spectrum. In the matrix, transfer from B(v’ > 0) to the -4 state may well be present, but it must represent a minor channel for population of the A state. If this were not true, the B-X continuum would be more strongly represented in the A state excitation spectrum. Experimental confirmation that the A state is populated by direct excitation can be obtained from a photoselection study 16,161, and this will be undertaken in the near future. It is most unlikely that the A’ state is populated by direct excitation, as the A’-X transition moment is much too small for this to be efficient. Ener,T gap considerations favor transfer from the A state and, because of the similarity of the A and A’ potential energy curves, this processwill have large Franck-Condon factors for levels near the ground state dissociation limit. However, transfer will only occur if there is electronic coupling between the states [17]. Mandich et al. [2] noted a shortening of the A state u’ = 0 lifetime in an Ar matrix, which was attributed to non-radiative transfer into nearby vibrational Ievel of the A’ state (leakage from A into the ground state is forbidden by u/g symmetry restrictions). The transition has a low Franck-Condon overlap, but this will be offset if the energy gap is small. The important point about this interpretation is that it does indicate a significant degree of electronic coupling. We conclude that transfer between the A and A’ states is the most probable mechanism for population of the latter. Primarily the transfer occurs at the inner turning points of these states, and it does not involve hindered predissociation. Beeken et al. [3,5] have used state mixing and hindered predissociation arguments to explain the absence of emission from the A state in a Xe matrix. As the predissociation channel is not open, it would appear that state mixing in a Xe matrix results in an A -+ A’ transfer rate which is much greater than 499
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PHYSICS
the A state radiative decay rate. Extensive state mixing is known to occur for Cl2 in an Ar matrix [6] _ Here, excitation of the B state produces emission from the A’ state exclusively, even when levels below the ground state dissociation hmit are excited [6]. Hindered predissociation and cage size arguments are clearly not applicable in this cast.
Acknowledgement We thank Professor J.S. Shirk for the loan of the matrix isolation equipment_ Professor Shirk and Mr. W. Hoffmann also assisted with the operation of this unit. We are grateful to Professor G.W. Flynn for helpful discussions. The germanium detector used in this work was provided, on loan, by Dr_ S.J. Davis of AFWL, Albuquerque. This work was supported by grants from the Air Force Office of Scientific Research (AFOSR-83-0173), The Petroleum Research Fund (PRF 14409-G6), and the Research Corporation (#9899).
References [ 11 V-E. Bondybe-y, S.S. Bearder and C. Fletcher, J. Chem. Phys 64 (1976) 5243.
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M. Mandich, P. Beeken and G. Flynn. J. Chem. Phys 77 (1982) 702. f31 P-B. Beeken, M. Mandich and G. Flynn, J. Chem. Phys 76 11982) 5995. E41 B.S. Ault, W-F. Howard 3r. and L. Andrews, J. MoL Spectry. 55 (197.5) 217. iSI P-B. Beeken, E.A. Hanson and G.W. Fiynn, J. Chem. Pbys. 78 (1983) 5892. WI V.E. Bondybey and C. Fletcher, J. Chem. Phys 64 (1976) 3615. 171 R.S. MuIIiken, Phys. Rev. 46 (1934) 549; 57 (1940) 500; J. Chem. Phys. 55 (1971) 288. 181 J-A. Coxon, In: Molecular spectroscopy, VoL 1 (The Chemical Society. London, 1973)). PI J.A. Coxon, J. MoL Spectry. 41(1972) 548,566. IW A. Sur and J. TeBIngImisen, J. Mol. Spectry. 88 (1981) 323. r111 M.A.A. Clyne, MC; Heaven and 3. Telhnghuisen, J. Chem. Phys. 76 (1982) 5431. [ 123 M.AA. Clyne and MC. Heaven, J. Chem. Sot. Faraday II 74 (1978) 1992. 1131 R-1. Personov, E.I. AI’Shits and L-A. Bykovskaya, Opt. Commun. 6 (1972) 169. [141 C.P. Hemenway. T-G. LIndeman and J-R. WlesenfeId. J. Chem. Phys. 70 (1979) 3560. 1151 R.J. Oldman, R.K. Sander and K-R. Wilson, J. Chem. Phys 63 (1975) 4252. 1161 AC. AIbrecht, J_ MoL Spectry. 6 (1961) 84. 1171 R. Kubo and Y. Toyozawa, Progr. Th:oret_ Phys 13 (1955) 160.
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