J. Phys. Chem. 1992, 96, 4301-4306
4301
Electronic Spectroscopy and Fluorescence Decay Dynamics of Matrix Isolated IBr Michel Macler, Matthew Erickson, Hong-Sun Lin, and Michael C. Heaven* Department of Chemistry, Emory University, Atlanta, Georgia 30322 (Received: December 5, 1991; In Final Form: February 4, 1992)
The spectroscopy and relaxation processes of IBr isolated in a solid Ar matrix have been studied using laser excitation and resolved fluorescence techniques. Excitation wavelengths in the range of 420-630 nm yielded fluorescence from the B(O+), A(1), and A’(2) states. Vibrational structure was absent from both the B(O+)-X(O+) excitation and emission spectra. All levels of B(O+) were subject to rapid nonradiative decay. Emission spectra for the A( I)-X(O+) and A’(2)-X(O+) systems yielded ground-state vibrational constants which were virtually identical to the gas-phase values. Electronic term energies of TJA) = 12 130 f 30 and T,(A’) = 11 180 f 30 cm-I were determined. Radiative lifetimes of 7(A) = 140 f 10 ps and T(A’) = 25 f 3 ms were obtained from time-resolved fluorescence measurements. Excitation of IBr/Ar matrices at 193 nm produced an emission feature at 419 nm, which has been tentatively assigned to the D’-A’ transition.
Introduction The photodissociation and relaxation dynamics of diatomic halogens and interhalogens trapped in raregas matrices have been studied for many This family of molecules provides a set of convenient prototypes for investigation of condensed-phase recombination and relaxation processes. The characteristics which are exploited in such studies may be described by reference to Figure 1, which shows the relevant low-lying electronic states of IBr. Excitation using visible radiation induces transitions to the B311(O+),Y(O+), ‘II(l), and A%( 1) states. Typically, the B and In(1) states are most strongly excited. The A state may experience weak direct excitation, but in most of the reported work it was populated via nonradiative transfer from B, Y, and/or III(1). Population of A”II(2) has been observed in many instances. This state can only be accessed through nonradiative channels. Wavelength-resolved fluorescence,l-I9 multiphoton e~citation,~ and polarization r a t i ~ ’ ~ ,measurements ’~J~ have provided insights concerning specific relaxation channels. It has been shown that matrix induced transfer between the B and I l l ( 1) states does not occur for Br2” or IC1.16 Predissociations from Br2(B) and bound regions of ICl(B) do not lead to population of Br2(A)” or IC1(A).I6 The relaxation dynamics of IF in solid Ar are also somewhat sele~tive.’~ Excitation of the bound region of IF(B) resulted in B X emission, with no detectable transfer to A or A’. Direct excitation of IF(A) yielded emission from IF(A’). Transfer between the closely nested A and A’ states precluded detection of A-state emission. In contrast, the relaxation dynamics of I2 and
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( I ) Howard, W. F.; Andrews, L. J. Roman Spectrosc. 1974, 2, 447. (2) Grzybowski, J. M.; Andrews, L. J. Raman Spectrosc. 1975, 4, 99. (3) Ault, B. S.;Andrews, L. J. Mol. Spectrosc. 1978, 70, 68. (4) Beeken, P.B.; Hanson, E. A.; Flynn, G. W. J . Chem. Phys. 1983, 78, 5892. (5) BBhling, R.; Langen, J.; Schurath, U. Chem. Phys. 1989, 130, 419. (6) Macler, M.; Nicolai, J. P.; Heaven, M. C. J. Chem. Phys. 1989, 91, 674. (7) Macler, M.; Heaven, M. C. Chem. Phys. 1991, 151, 219. (8) A u k B. S.;Howard, W. F.; Andrews, L. J. Mol. Spectrosc. 1975,55, 217. (9) Mandich, M.; Beeken, P.; Flynn, G. W. J . Chem. Phys. 1982,77,702. (10) Beeken, P. B.; Mandich, M.; Flynn, G. W. J . Chem. Phys. 1982, 76, 5995. (1 1) Bondybey, V. E.; Bearder, S.S.; Fletcher, C. J . Chem. Phys. 1976, 64, 5243. (12) Nicolai, J. P.; van de Burgt, L. J.; Heaven, M. C. Chem. Phys. Leu. 1985, l l S , 496. (13) Nicolai, J. P.; Heaven, M. C. J. Chem. Phys. 1985,83, 6538. (14) Langen, J.; Lodeman, K.-P.; Schurath, U. Chem. Phys. 1987, 112, 393. (15) Bondybey, V. E.; Fletcher, C. J . Chem. Phys. 1976, 64, 3615. (16) Bondybey, V. E.; Brus, L. E. J . Chem. Phys. 1975,62,620; 1976,64, 3724. (17) Wight, C. A.; Auk, B. S.; Andrews, L. J . Mol. Spectrosc. 1975, 56, 239. (18) Miller, J. C.; Andrews, L. J . Mol. Spectrosc. 1980, 80, 178. (19) Nicolai, J. P.; Heaven, M. C. J . Chem. Phys. 1987, 87, 3304.
C12 appear to be less restricted. Visible and near-UV excitation of C12 results in emission from C12(A’) exclusively, indicating efficient transfer between the low-lying states.15 Visible excitation of 1, produced emissions from the B, A, and A’ state^."^ While it is obvious that the observed energy-transfer processes are caused by interactions with the matrix cage, details of how the transitions are induced and which physical properties govern the degree of selectivity, are still unclear. Beeken et a1.4 proposed a model which was based on the idea that transfer between electronic states occurred when right-branch curve crossings were traversed. Accessible crossings were those which were encountered before the maximum internuclear separation permitted by the matrix cage was reached. Unfortunately, this attractively simple model could not account for many of the observed processes. At present, the only clear trend is that transfer is facilitated when minimal amounts of energy are released to the lattice and there is good Franck-Condon overlap between the states involved. Population of the halogen and interhalogen A and A’ states via dissociation-recombination events in rare-gas matrices has provided a valuable means for characterizing these metastable states. Gas-phase fluorescence decay lifetimes are difficult to measure for the A states, and they have never been reported for the A’ states. In many instances, these lifetimes are easily determined in rare-gas matrices.4-79J0,14-16J9 The emissions originate from v’ = 0 levels which have poor Franck-Condon overlaps with nearby states, so the lifetimes are usually unaffected by nonradiative processes. Useful spectroscopic data may also be derived from matrix studies. In the gas phase, the A’ states are known mostly through observations of the D’-A’ transition^.^(!-^^ As these Cf = 2 states cannot be directly accessed from the ground state, problems have been encountered in determining their absolute energies. Despite the inherent low resolution of matrix spectra, characterizations of the A’ X systems in rare-gas solids have yielded reliable estimates for A’ state term v a l ~ e s . ~ J ~ J ~ J ~ J ~ Detailed information concerning the B(O+),25-27B’(O+),28-30 A(l),2s*31and X(0+)32,33 states of IBr has been obtained from
-
(20) See: Brand, J. C. D.; Hoy, A. R. Appl. Spectrosc. Rev.1987,23,285, and references cited therein. (21) Guo, B.; Tellinghuisen, J. J . Mol. Spectrosc. 1988, 127, 222. (22) Lipson, R. H.; Hoy, A. R.; McDonald, N. J. Mol. Spctrosc. 1990, 142, 24. (23) Tellinghuisen, J.; Fei, S.;Zheng, X.;Heaven, M. C. Chem. Phys. Lett. 1991, 176, 373. (24) Hoy,A. R.; Jordan, K. J.; Lipson, R. H. J. Phys. Chem. 1991, 95, 611. (25) Selin, L.-E. Ark. Fys. 1962, 21, 479. (26) Selin, L.-E.; Soderborg, B. Ark. Fys. 1962, 21, 515. (27) Clyne, M. A. A.; Heaven, M. C. J. Chem. Soc., Farday Trans. 2 1980, 76, 49. (28) Selin, L.-E. Ark. Fys. 1962, 21, 529. (29) Kniickel, H.; Tiemann, E.; Zoglowek, D. J . Mol. Spectrosc. 1981,85, **
4 ms), a single exponential decay was observed (Figure 5b). The temporal profile was analyzed by fitting to the equation I(r)IC(@r - e - f / r I ) + &-‘/Ti (1) SI),
lo00
750
1500
TABLE I: Vibronic Band Centers (cm-I) for the A-X and A‘-X Transitions of IBr in Solid Argon U’’0 u”0 -u” v(A-+X) u(A’+X) +u” v(A-X) u(A’+X)
9144 9492 9240 8993 8756 8521
500
Time / ~ s
Wavelength /nm Figure 4. Near-IR emission spectra for IBr in solid Ar. Both traces were recorded using 532-nm excitation and a boxcar gate width of 3 ps: (a) 180-c~~ gate delay; (b) IO-ms gate delay. The numbers above the peaks give the ground-state vibrational assignments.
9 10 11 12 13 14
250
L
400 425 450 Wavelength /nm Figure 6. Visible and near-UV emission spectra obtained by exciting an IBr/Ar matrix with 193-nm radiation.
350
315
ference being that the maximum appeared at a slightly shorter wavelength (a504 nm). Near-IR emissions from more concentrated matrices (Ar:(IBr Br2) a 700:l) were examined. At this ratio the band positions, lifetimes, and excitation spectra were unchanged from those descrikd above. The only difference was the appearance of a strong I(2P1/2) I(2P3/2)line at 1322 nm. An excitation spectrum for the atomic line was the same as those obtained by monitoring the IBr near-IR emissions. Power dependence measurements indicated that the atomic fluorescence was induced by single photon absorption. Excitation of a dilute IBr/Ar matrix at 193 nm yielded the emission spectrum shown in Figure 6. The most intense features, at 380 and 396 nm, were readily identified as ion-pair transitions of 12? However, the band centered at 419 nm was seen only from IBr containing samples. Decay of the 419-nm emission was too fast to be measured using a photomultiplier with a 10-ns response time. (ii) IBr/Xe Matrices. Spectra for the A X and A’ X systems of I2 have been reported by several groups.e7 In general, the spectra resemble those of Figure 4 in that they show matrix
+
-
-
-
Macler et al.
4304 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
broadend vibronic bands. An exception was noted for Iz in Xe. In addition to the broad features, Bohling et ale5reported sharp zero-phonon lines in this host. Observation of zero-phonon lines in the IBr spectra would permit determination of the isotopic splittings, from which absolute vibrational numberings could be established. Consequently, IBr/Xe matrices were briefly examined, in the hopes that sharp features could be detected. Unfortunately, zero-phonon lines were not evident. Apart from a red-shift of about 250 cm-l, the near-IR spectra from IBr/Xe matrices were the same as those shown in Figure 4.
TABLE II: Spectroscopic Constantso (cm-I) for IBr in Solid Argon and the Gas Phase constant Ar matrix gas phase Ab
Discussion (i) Far-Red Emission. Assignment of the far-red emission system is complicated by the lack of structure in the spectrum. That IBr is the carrier is almost certain, as this feature does not correspond to any of the spectra reported for Iz or Brz. Figure 3 shows a close match between the excitation spectrum and the gas-phase continuum absorption spectrum of IBr.37 Within the 450-600-nm range, the latter is dominated by the B-X absorption system. Figure 2 is consistent with re-emission from B, u’ = 0, under conditions where the vibronic band widths exceed the vibrational spacings. Gas-phase potential energy curves predict that B(v’=O) X emission will be most intense for the 0-16 band, which occurs a t 830 nm (isotopic average).27 When a matrix induced red shift of about 250 cm-l (see below) is taken into account, the 0-16 band is expected at 847 nm. This is in good agreement with the maximum of the far-red emission. On the basis of these considerations, we assign the far-red system to the B-X transition. The red shift of the B-state excitation spectrum, relative to the gas-phase continuum absorption spectrum, slightly exceeds the =250-cm-l matrix shift. The B-state excitation spectrum is also red-shifted relative to the A and A’ excitation spectra. As the In(1)-X absorption makes its greatest contribution at short wavelengthsj7 (=480 nm), these differences may indicate that In(1)-B transfer is not efficient. As noted in the Introduction, this channel is not open for IC116 or Brz.1z-14 Comparisons of the properties of matrix isolated ICl(B) and IBr(B) reveal some interesting similarities and contrasts. For ICl, Bondybey and Brus16 found that transitions to levels lying below the B/Y curve crossing exhibited sharp zero-phonon lines. Above the crossing, all vibrational structure was lost. Bondybey and BrusI6 had speculated that the quasi-bound B’(O+) state, formed by the B/Y interaction, may be stabilized by the matrix cage. The lack of structure above the crossing showed that the degree of stabilization achieved was not sufficient to create sharp vibrational levels. Excitation of ICI(B) above or below the curve crossing resulted in emission from ICl(B), u’ = 0. Lifetime measurements indicated purely radiative relaxation. However, the quantum yield for transfer to B, u’ = 0, changed dramatically as the B/Y curve crossing was traversed. Below the crossing the quantum yield was close to unity; above it, dropped by roughly 2 orders of magnitude. The B(u’=O) X emission line shapes depended on the excitation wavelength in a complex fashion. Excitation spectra for IBr(B) in Ar did not show vibrational structure. Below the B/Y curve crossing neither zero-phonon lines or steplike excitation thresholds were seen. Despite the fact that the B’(O+) state of IBr is more stable than that of ICl, evidence of bound B’ levels above the crossing was not found. There were no intensity anomalies in the IBr(B) excitation spectrum at energies corresponding to the curve crossing. It appeared that the quantum yield for transfer to B, u’ = 0, was roughly constant throughout the visible absorption region. The intensity of the B X fluorescence, relative to that of A X or A’ X (see below), was very low. Hence, there was rapid nonradiative loss from the B state. Furthermore, agreement between the low-energy regions of the absorption37and excitation spectra showed that B, u’ = 0, was also subject to efficient nonradiative decay. (The inability of Stephenson et al.38to detect fluorescence from the
IBr(B)-Ne binary complex probably stems from a related rare-gas atom induced electronic predissociation.) With regard to the B X emission spectrum (Figure 2). It should be noted that fluorescence quantum yield considerations rule out the possibility that lifetime broadening, resulting from transfer of population out of B, u’ = 0, is responsible for the lack of structure. The radiative decay rate for the B state is predictedz7 A lifetime broadened line width to be on the order of 2.5 X lo5 in excess of 300 cm-’ would be needed to mask the vibrational structure. This corresponds to nonradiative decay rates greater than 5 X lOI3 s-I, and fluorescence quantum yields of less than 5 X lo4. At such low quantum yields the B X emission would be orders of magnitude below our detection limit. Among the halogens and interhalogens, IBr and Iz are the only molecules with structureless B X systems in rare-gas matrices. As these are the largest and most polarizable members of the family, they are expected to interact strongly with the host lattice. Phonon side bands and inhomogeneous site effects resulting from these interactions may be responsible for the lack of resolvable vibrational structure. Rapid loss of population from the B, u’ = 0, levels is another common property of IBr and 12. This may be related to the circumstance that these are the only interhalogens/halogens where B, u’ = 0, lies above the ground-state disswiation limit, allowing predissociation via repulsive or shallow-bound states. Loss of population from Clz B, u’ = 0, which is also known to be relatively rapid,lS is a distinctly different process. Here, the level lies well below the ground-state dissociation limit, and transfer must occur to nearby bound levels of A, A’, and/or X. As noted in the Introduction, transfer is facilitated by the close proximity of the B, A, and A’ states. A similar situation exists for ClF. It is of interest to note that Wight et alai7were unable to observe visible fluorescence from ClF/Ar matrices, and J a ~ ~ could d a ~not ~ detect B-state laser induced fluorescence from ClF-Rg complexes. (ii) Near-IR Emission Systems. By analogy with the results for other halogen^^-^*^-'^ and interhalogens,16J9 the near-IR emission systems can be assigned to the A X (Figure 4a) and A‘ X (Figure 4b) transitions. As the 179Br/I*1Brisotopic splitting could not be resolved, absolute vibrational numberings could not be determined from the fluorescence spectra alone. There is a substantial body of data which show that, relative to the gas-phase, T, values for halogen and interhalogen B,A, and A‘ states are red-shifted by 200-250 cm-l in Ar matrices.’ Suzuki and Fujiwara3’ recently reported a gas-phase value for TJA) of 12 365.84 (64) cm-l. Therefore, the expression
-
-
-
-
-
(37) Seery, D. J.; Britton, D. J . Phys. Chem. 1964,68, 2263.
w/
wryc”
Tc(A)
T,(A’)
267.3 f 4.2 0.92 f 0.15 12130 f 30 11 180 f 30
267.63‘ 0.81‘ 12365.84 f 6431 114802’
0.3 f 4.2 -0.11 f 0.15 236 f 30 300
Isotopically averaged values. Difference between gas-phase and matrix contants. Isotopic average calculated from the constants of ref 33.
-
-
-
u = T,(A)
-
-
+ C’(0) - w,”(u”+!/~) + U , X , ” ( U ” + ! / ~ ) ~
(2)
was fit to the band centers from Table I, with the constraint 12 116 < TJA) < 12 166 cm-l. It was assumed that G’(0) was unchanged from the gas-phase value of 67 cm-’. This procedure established the vibrational numbering given in Table I. A similar approach was used to obtain the numbering for the A’-X bands. The gas-phase values of Guo and Tellinghuisen2I of G’(0) = 67 cm-’ and T,(A‘) = 11480 cm-’, plus the 200-25O-cm-’ gas-to-matrix shift, yielded the A’-X numbering of Table I. Finally, a simultaneous fit of the A-X and A’-X data was performed, resulting (38) Stephenson, T. A.; Simpson, W. R.; Wright, J. R.; Schnider, H. P.; Miller, J. W.; Schultz, K. E. J . Phys. Chem. 1989, 93, 2310. (39) Janda, K. C. Private communication.
Matrix Isolated IBr
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4305
TABLE III: Fluorescence Decay Lifetimes for the Halogem and Interhalogens
(i) A % ( i ) Lifetimes molecule I2 IBr Br2 IC1
7dPs
143 i 198 f 140 f 61 f 107 f 260‘
lob 1SC 10 Id 1
7dIILS
7WlPS
263 i 20 364 f 30 258 f 20 112 f 2 197 f 2 480
350 f
460 f 509
I2
(ii) A’Q(2) Lifetimes rJms molecule 6 f lb Br2
IF IBr
1I* 25 f 3
molecule
5W
rJms
~
c12
44 f 3’ 76/
Gas-phase lifetime estimated by scaling the matrix result. bReference 7. CReference 5. dSite dependent lifetime from ref 14. CClyne,M. A . A.; Heaven, M. C.; Martinez, E.J. Chem. Soc., Faraday Trans. 2 1980, 76, 177. /Reference 16. ZHarris, S. J.; Natzle, W. C.; More, C. B. J . Chem. Phys. 1979, 70, 4215. hReference 19. Reference 14. j Reference 15.
‘
in the constants given in Table 11. Within the experimental error limits,the ground-state vibrational constants were unchanged from their gas-phase values. The Ar matrix shift of Te(A’) borders on the anomalous, but this is probably not significant. Guo and Tellinghuisenzl obtained their estimate for T,(A’) by indirect means. Although they did not specify error limits,a range of f 2 0 cm-I would be reasonable. For the A-X and A’-X spectra, the vibrational numberings and intensity distributions were consistent with current knowledge of the gas-phase potential energy curves. As the Re values for B, A, and A’ are quite similar, the intensity maxima of the u’ = 0 emission progressions should all appear at u” = 16. When the spectra were corrected for the effects of the detection system, this was found to be the case. Our numberings and constants were also compatible with the lack of resolvable isotopic splitting in the spectra. Ignoring the slight effect on the excited-state zeropoint levels, the isotopic separation (Av) can be approximately represented by
AU = (1 - p ) w / ( ~ ” + ’ / z )
- (1 - p 2 ) ~ d / ( ~ ” + 1 / 2 ) 2
(3)
where p2 (0.985) is the ratio of the reduced masses. Equation 3 predicts a splitting of 36 cm-*for U” = 20, which was the highest energy level observed. This is less than the intrinsic line width, but sufficient to account for the slight additional broadening noted at longer wavelengths. Excitation spectra for the A and A’ states were very similar to the gas-phase continuum absorption spectrum?’ implying that transfer from the B, Y,and In, states was effective in populating A and A’. Fluorescence decay measurements yielded a lifetime of 140 f 10 ps for the A state. Correcting for the effects of the matrix environment,40 a gas-phase lifetime of 258 f 20 ps is obtained. This should be a reliable determination of the radiative lifetime, provided that IBr(A) is unaffected by nonradiative decay in solid Ar. There is good evidence that I2(A)’ and IC1(A)l6 lifetimes are not influenced by nonradiative relaxation in Ar. However, such processes do shorten the Br2(A)I4lifetime by a factor of about 3. It is anticipated that the radiative lifetime of IBr(A) should be shorter than those of ICI(A) and Br2(A), and somewhat longer than that for 12(A). The relevant lifetime data is collected in Table 111, where it can be seen that the former expectation is borne out. We conclude that relaxation of IBr(A) in solid Ar is primarily radiative. Table I11 also contains a summary of lifetime data for the halogen and interhalogen A’ states. In keeping with the above considerations, we find that our IBr(A’) lifetime is intermediate between those of I2 and Br,. In contrast, it exceeds that of IF. (40)
Fulton, R. L. J . Chem. Phys. 1974,61,4141.
Nonradiative decay of IF(A’) in Ar is a possible reason for this departure from the general trend. Although the dynamics of matrix isolated I2 and IBr are quite similar, the rates at which population is transferred to the A’ states show a measurable difference. With visible laser excitation, Macler and Heaven’ observed biexponential decay of 12(A’). The faster component was assigned to radiative decay of 12(A’)which had been populated by a rapid (compared to the instrumental response time) transfer process. The longer-lived component was a consequence of slow population transfer from a shallow-bound metastable state. IBr(A’) decayed with single exponential characteristics and a lifetime that was of the order expected for radiative decay. Thus, if population reached IBr(A’) by passing through an intermediate metastable state, the rate(s) for the transfer step(s) must have been greater than the A‘-state radiative decay rate. (S) Atomic I(?,,2)-(?3$ Emissioa In the more concentrated matrices, single photon excitation of the I(2Pl 2)-(2P3/2) line was observed at wavelengths which did not provide sufficient energy for the channel IBr
+ hu
-
I(zP1/2)+ Br(2P3/2); X C 450 nm
An excitation spectrum for the atomic fluorescence followed those of the IBr A and A’ states, implying excitation by energy transfer. A similar transfer from I2(A and/or A’) to I has been observed previously in concentrated 12/Rg matrices.6~~’In both cases, the concentration dependencies suggest medium to short-range transfer processes. Reasons for the Occurrence of relatively high atomic concentrations in 12/Rg and IBr/Ar matrices remain uncertain. As prolonged photoexcitation of these samples does not result in growth of the atomic fluorescence, it is clear that in situ photodissociation is not the source. (iv) Excitation of IBr/Ar Matrices at 193 nm. The fluorescence produced by exciting IBr/Ar matrices at 193 nm can be assigned by analogy to gas-phase r e s ~ l t s ~and 9 ~the ~ behavior of I2 in Ar.7s42 Lawley et al.34have shown that 193-nm excitation of gas-phase IBr populates high vibrational levels of the E(O+) ion-pair state. In the presence of a high pressure of an inert buffer gas, such as Ar, population from E(O+) may be efficiently transferred to the D’(2) state.21s22 Fast radiative relaxation via the D’(2)-A’(2) transition then follows. The most intense component of the emission is centered at 385.0 nm. We propose that a similar sequence takes place when IBr/Ar matrices are irradiated with 193-nm pulses. Note that the I2 DCA’ bands present in Figure 6 were excited by a corresponding sequence.’ GUtler et have also found that excitation of C1, ion-pair states in Ne matrices results in D’-A’ fluorescence. On the basis of these observations, the IBr feature at 419 nm was assigned to emission from D’, u’ = 0, to high vibrational levels of A’. The short lifetime ( < l o ns) was in accordance with a fully allowed charge-transfer transition. The absence of structure in the 419-nm band is not surprising. Spacings between the high vibrational levels of A’ will be much smaller than the line broadening caused by phonon interactions and site inhomogeneities. A gas-to-matrix red-shift of T,(D’) of about 2100 cm-’ is implied by the D’-A’ assignment. This is comparable to the 2900-cm-I shift of 12(D’) in Ar.’ These large matrix effects are most probably caused by solvation of the excited-state electric dipole moment.@
Summary Visible laser excitation of IBr/Ar matrices resulted in fluorescence from the B, A, and A’ states. The B-X emission spectrum and the B-state excitation spectrum were found to be structureless. Efficient predissociation of all B-state levels was inferred from relative quantum yield data. Matrix stabilization of the B’ state was not observed. (41) Mhling, R.; Becker, A. C.; Minaev, B. F.; Seranski, K.;Schurath, U. Chem. Phys. 1990, 142,445. (42) Tellinghuisen, J.; Phillips, L. F. J . Phys. Chem. 1986, 90, 5108. (43) Giirtler, P.; Kunz, H.; LeCalve, J. J . Chem. Phys. 1989, 91, 6020. (44) Fajardo, M. E.; Apkarian, V . A. J . Chem. Phys. 1986, 85, 5660.
J . Phys. Chem. 1992, 96,4306-4310
4306
Wavelength-resolved fluorescence spectra were recorded for the A-X and A’-X systems. Ground-state vibrational constants, electronic term values, and matrix shifts were obtained from the spectra. Fluorescence decay lifetimes were determined for the A and A’ states. Comparisons with the corresponding results for other halogens and interhalogens suggest that the IBr A and A’ states are unaffected by nonradiative decay processes in solid Ar. Excitation spectra were consistent with efficient transfer from B, Y, and Ill, to the A and A’ states.
Excitation of IBr/Ar matrices at 193 nm produced an emission feature at 419 nm. This has been tentatively assigned to the W-A’ transition, red-shifted about 2100 cm-I relative to the gas-phase. This large red shift is attributed to solvation of the excited-state electric dipole.
Acknowledgment. Support of this work by the Air Force Office of Scientific Research (Grant AFOSR-88-0249) and the Emory University Research Committee is gratefully acknowledged.
Electronic Spectrum of the SiCI3 Radical Karl K. Irikura; Russell D. Johnson 111, and Jeffrey W. Hudgens* Chemical Kinetics and Thermodynamics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: December 10, 1991; In Final Form: January 28, 1992) Between 330 and 400 nm the 2 + 1 REMPI spectrum of SiC13exhibits two overlapping vibrational progressions composed of >30 members and one vibrational progression composed of five members. These band systems, labeled F,G, and H, are tentatively assigned to three distinct Rydberg states that reside between 50 100 and 59600 cm-I. The vibrational progressions have an average spacing of w2 = 262 cm-l and are assigned to the v2 umbrella modes of the Rydberg states. Electronic origins are not observed. Ab initio calculations on SiC13and SiC13+yield geometries and harmonic vibrational frequencies consistent with the spectroscopic assignments and provide the adiabatic ionization potential, IP, = 7.93 f 0.05 eV. In combination with a recent experimental value for pHr(SiC13+),this leads to pHf(SiCI3) = -353 12 kJ/mol.
*
Introduction This paper reports the first electronic spectrum of the trichlorosilyl radical (SiC13). The spectrum was observed between 330 and 400 nm by resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Although the REMPI spectrum is very congested with vibrational bands, the REMPI scheme presented here can serve as a sensitive and selective method for detecting gas-phase SiC13radicals in other applications. In this paper we also report theoretical studies of the structure and spectroscopy of the SiC1, radical and cation and of the ionization potential of SiCl,. These theoretical results account for the congestion in the SiC1, spectrum and also lead to thermochemical values for SiCl,. This report is part of our continuing study of the spectroscopy of transient compounds of silicon and germanium that are relevant to chemical vapor deposition (CVD) and reactive etching of semiconductor materials.l-* The SiC13 radical is the major chlorosilicon species involved in the CVD of silicon films from SiC14/Ar microwave plasmas? SiCl, radicals also play important roles during the CVD of silicon carbide films.1° New detection methods for SiC1, may lead to better understanding of these processes. Prior spectroscopic and structural information about the SiC13 radical and cation is sparse. ESR spectroscopy has established that the ground state of SiC1, is nonp1anar.l’ Limited infrared absorption data for the SiC13radical and cation are also availabie.lz~3 Experimental Apparatus and Methods The experimental apparatus is described in detail elsewhere.’ It consists of an excimer-pumped tunable dye laser, a flow reactor, a time-of-flight mass spectrometer, and a computerized data acquisition system. In the flow reactor, atomic fluorine was generated in a microwave discharge of 5% F2/He. Trichlorosilane vapor was added downstream from the discharge to produce Sic& radicals by the reaction
SiHCl,
+F
-
SiC13 + H F
NRC/NIST Postdoctoral Associate.
* To whom correspondence should be addressed.
(1)
-
The flow reactor was operated a t a total pressure of -200 Pa (1-2 Torr) and a flow rate of 1 m/s with pressure ratios of F,/SiHCl,/He of about 1/5/25. A small amount of the reactor effluent effused through a 0.9-mm hole into the ion optics of the mass spectrometer. The vacuum chamber was operated a t 1-10 mPa (10-5-104 Torr). Light from a tunable, excimer-pumped dye laser (line width = 0.2 cm-I, pulse energy = 10-25 mJ) was focused into the center of this ionizer by a 250-mm lens. Photoions produced by laser ionization were mass-analyzed and monitored with boxcar integrators. Spectra were obtained using the Exciton14 laser dyes PTP (330-350 nm), DMQ (350-370 nm), and QUI (370-400 nm). No resolved spectra were obtained in the neighboring regions covered by PBBO (390-410 nm), stilbene 420 (410-440 nm), coumarin 440 (425-460 nm), or DCM (YAG-pumped, frequency-doubled, 3 10-330 nm). Laser wavelength was calibrated by recording the neon optogalvanic spectrum using a hollow cathode lamp.I5 (1) Johnson, R. D. 111; Fang, E.; Hudgens, J. W. J . Phys. Chem. 1988, 92. 3880-3883. (2) Dulcey, C. S.;Hudgens, J. W. Chem. Phys. Lett. 1985, 118, 444. (3) Johnson, R. D. 111; Hudgens, J. W. J . Phys. Chem. 1989, 93,6268. (4) Johnson, R. D. 111; Hudgens, J. W. Chem. Phys. Letr. 1987,141, 163. ( 5 ) Johnson, R. D. 111; Tsai, B. P.; Hudgens, J. W. J . Chem. Phys. 1989, 91, 3340. (6) Johnson, R. D. 111; Hudgens, J. W. J . Chem. Phys. 1991, 94, 5331. (7) Johnson, R. D., 111; Tsai, B. P.; Hudgens, J. W. J. Chem. Phys. 1988, 89, 4558-4563. ( 8 ) Johnson, R. D. 111; Tsai, B. P.; Hudgens, J. W. J. Chem. Phys. 1988, 89. 6064. (9) Avni, R.;Carmi, U.; Inspektor, A.; Rosenthal, I. J . Vac. Sci. Techno/. A 1985, 3, 1813-1820. (10) Fischman, G . S.; Petuskey, W . T. J . Am. Ceram. Soc. 1985, 68, 185-1 90. (1 1) Lloyd, R. V.; Rogers, M. T. J. Am. Chem. SOC.1973,95,2459-2464. (12) Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1968,49, 3130-3135. (13) Miller, J. H.; Andrews, L. J . Mol. Struct. 1981, 77, 65-73. (14) Certain commercial materials and equipment are identified in this
paper in order to adequately specify the experimental procedure. In no case does such an identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified is necessarily the best available for the purpose. (15) Striganov, A. R.; Sventitskii, N. S. Tables of Spectral Lines of Neutral and Ionized Atoms; IFI/Plenum: New York, 1968; pp 199-216.
This article not subject to US.Copyright. Published 1992 by the American Chemical Society
