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Nonadiabatic transitions in the dynamics of the NeICl van der Waals complex
Volume 159, number 5,6
CHEMICAL PHYSICS LETTERS
21 July 1989
NONADIABATIC TRANSITIONS IN THE DYNAMICS OF THE NeICl VAN DER WAALS COMPLEX Thomas A. STEPHENSON
‘, Yujian HONG and Marsha 1. LESTER 2
Department of Chemistry, UniversityofPennsylvania, Philadelphia. PA 191046323, USA Received 14 April 1989
Both electronic energy transfer and vibrational predissociation decay channels are observed following excitation of NefCl complexes to the 8 ion-pair state. Dispersed fluorescence is detected on the E-+X transition as well as the D’ *A’ and/or 8-A transitions. The vibrational quantum number of the emitting level is reflected in the shape of the Franck-Condon envelope, while its electronic character is determined from the emission wavelength and profile. The relaxation pathways are found to depend on the initial vibrational excitation of the NeICl complex.
1. Introduction The electronically excited van der Waals complexes of halogen and interhalogen diatomic molecules with rare gas atoms have proven to be important and convenient systems for the examination of the structure and vibrational predissociation dynamics of weakly bound molecular complexes [ l-8 1. To date, however, these studies have focused on the behavior of the van der Waals complexes confined to single adiabatic potential energy curves. The influence of additional electronic states and nonadiabatic couplings has been inferred from the behavior of certain vibrational levels in electronically excited ArIz [9], HeCl, [4] and HeICl [lo] but direct observation of electronic energy transfer in polyatomic van der Waals complexes has been limited to fine structure relaxation in HgNz [ 111. In the latter case, excitation of the complex to a molecular electronic state correlating with a 3P, mercury atom leads to dissociation and formation of a ‘PO mercury atom. In this Letter, we report the first direct observation of such electronic state changing processes in the dissociation of a diatom-rare gas complex, namely NeICl in the B (a= 1) ion-pair state. I On sabbatical leave (1988-89)
from the Department of Chemistry, Swarthmore College, Swarthmore. PA 1908 I, USA. * Alfred P. Sloan Research Fellow and Camille and Henry Dreyfus Foundation Teacher-Scholar.
0 009-2614/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
Three strongly bound electronic states correlating with the ionic atoms I+ ( 3P2) + Cl- (‘So) form the lowest energy tier of ICI ion-pair states. Conventionally labeled (using Hund’s case c notation) as E(O+), D’(2) and b(l), these states are closely spaced (T,=39059.5, 39061.8, and 39103.7 cm-‘, respectively) and have similar, though slightly displaced, potential energy curves [ 12 1. The ion-pair states can bc excited from the lower lying valence states of ICl with a strong propensity for dR= 0 electronic transitions. Both the D’ (2) and E (O+ ) states are heterogeneously mixed with the B( 1) state, however, resulting in absorption to alI three ion-pair states from the A( 1) valence state [ 121. Previous reports from this laboratory have described in great detail the vibrational predissociation dynamics of the HeICl [ 7] and NeICl [ 61 complexes excited to the A( 1) and B (O+ ) valence electronic states. In these investigations a pump laser promotes complexes from me ground electronic state to either the A or B state; a probe laser interrogates the resulting photofragments by excitation to an ICI ion-pair state. Emission from the ion-pair state is monitored as a function of probe laser wavelength, thus providing a fluorescence excitation spectrum of the nascent A or B state ICl products. For several A [ 13j and B [ 141 state vibrational levels in NeICl, the predissociation lifetime of the van der Waals complex is comparable to the pulse duration of the B.V.
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pump and probe lasers ( 10 ns). By temporally overlapping the lasers, the NeICl complex can be promoted to the ion-pair states. An example of the resulting optical-optical double resonance spectrum of NeICl has been presented elsewhere (see fig. 1 of ref. [ 131). The large geometry and binding energy changes that accompany excitation of the complex to the ion-pair state result in a progression of features due to excitation of the van der Waals vibrations. These features formed the basis for a rudimentary vibrational analysis of the van der Waals modes of NeICl in the ion-pair state [ 8 1. In our most recent experiments, we have obtained wavelength-resolved emission spectra following excitation of NeICl to the ion-pair states that demonstrate the presence of clear electronic energy transfer following excitation of the p state in the van der Waals complex.
2. Experimental For the investigations discussed in this report, our existing apparatus [8] has been modified to incorporate a scanning monochromator for dispersed emission studies. Briefly, a continuous expansion of ICI (Sigma) seeded in 75 psig of first-run grade Ne (A&o) is excited by two independently tunable extimer-pumped dye lasers. The pump laser ( 6 16-622 nm) selectively promotes either ground state NeICi or uncomplexed ICI to the v= 14 or 15 levels of the A( 1) electronic state. The probe laser (430-437 nm) excites NeICl (or ICI) to the ion-pair state. The resulting laser-induced fluorescence is collected and collimated by a 2 in. diameter, f/l lens and is focused onto the entrance slit of a 0.25 m f/4 scanning monochromator with a 2 in. diameterff4 lens. A blue sensitive phototube (Thorn/EM1 9535QA) is mounted directly on the exit slit body to detect emission from the ion-pair state. A red sensitive phototube (Thorn/EM1 9658B) detects the A-+X emission collected by a second set off/ 1 collection optics. The resulting emission signals are processed by gated integrators (PAR) and transferred to a laboratory computer for signal averaging and graphics output.
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3. Results and discussion In figs. la-If, we present the dispersed emission spectra from the v=O and 1 levels of the E, B and D’ states of uncomplexed ICI. As noted earlier the lowest energy ion-pair states perturb one another, particularly at the low vibrational levels discussed here [12].ExcitationtotheE(O’)andD’(2)statesfi-om the A(1) state (ALL-1 and +1, respectively) is exclusively due to “intensity borrowing” from the intense p( 1) tA( 1) transition. We find that the EcA and D’ +A excitation features are at least a factor of 200 weaker than the 8+A features. Several aspects of the emission spectra shown in fig, 1 are worthy of note: First, emission from the E state lies in a clearly distinguishable wavelength region from the D’ and 8 state emissions, which are significantly overlapped. The strong ALL0 propensity rule for ion-pair to valence state electronic transitions dictates that the E(O+ ) emission spectrum is dominated by transitions to the X (O+ ) ground state, while B( 1) +A( 1) and D’(Z)+A’ (2) transitions are prominent in emission from the B and D’ states, respectively. There are no manifestations (E+A emission, for example) of the relatively weak E-B and D’-B interactions in the dispersed emission spectra of uncomplexed ICI ( see below). Second, the vibrational quantum number of the initially excited ion-pair state is reflected in the shape of the envelope (no nodes for u= 0, one node for Y= 1, etc.) of the dispersed emission spectrum. The equilibrium internuclear separations of the ion-pair states are shifted significantly to larger values relative to the A(l), A’(2) and X(0+) states. Since the lower state vibrational wavefunctions are highly oscillatory, the only internuclear separations that contribute to the Franck-Condon overlap with the ion-pair states are those near the lower states’ outer turning points. Emission is observed, roughly, to lower state levels whose outer turning points fall within the range of internuclear separations which have significant ion-pair state wavefunction amplitude. The intensity of each emission transition is largely determined by the amplitude of the ion-pair state wavefunction in the vicinity of the lower state (A, A’ or X, for J$ D’ or E state emission, respectively) outer turning point. The similarity of the A and A’ potential energy curves
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Fib 1. Dispersed emission spectra of uncomplexed ICl from ion-pair states. ICI is prepared in tbe ion-pair state by double resonance excitation on ion-pairtA+X transitions. For spectra (a)-(e) the pump laser is fixed on the A(v= 14)tX(u=O) transition (16083 cm-‘);forspectnun(f),thepumplaserisf~~ontheA(v=15)cX(v~O)transition(16212cm~‘). (a)EmissionfiumIClE(u=O); theprobelasetiafixedontheE(u~O)+A(~=14)transition(2287Scm-’).(b)EmissionfromICIE(u=1);thep~belaserisfixedon the E(v= l)cA(~= 14) transition (23038 cm-‘). (c) Emission from ICI p(u=O); the probe laser is fucd on the p(u=O)tA(v= 14) transition(22921cm-‘).(d)EmissionfromICI~(u=l);theprobelaserisfuedonthe~(v=1)~A(u=14)vansition(23089~-‘). (e)EmissionfromIClD’(u=O);theprobelaserisfuedontheD’(u~O)+A(v=14)transition(22882cm-1).(f)EmissionfromICI D’(u= 1);theprobe laser is fixed on the D’ (u= I )cA(v= 15) transition (22924 cm-‘). X and A statevibrational assignments derived from ref. [ IS] and ref. [ 161, respectively.
in the Franck-Condon accessible region means that there are only small differences in the emission profiles for the D’ and p states. Third, weak emission is observed from E state levels in the wavelength range expected for E+A emission, but with intensity that is larger than that predicted by a simple perturbation treatment of the known E-/3 [ 12 ] state interactions. This emission does not follow the Franck-Condon pattern expected for emission to the A state from the ion-pair
levels. In particular, the overall envelope is not sensitive to the vibrational quantum number of the initially excited level. These findings suggest that E state emission in this wavelength region is due to perturbations in the valence electronic states, perhaps similar to those known to couple higher vibrational levels of the A and X states [ 151. This unexpected emission pattern along with bound-free emission observed at lower emission frequencies wilI be a focus of continuing investigations. For the purposes of 551
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our discussion of the NeICl complex, it suffices to note that the dispersed fluorescence spectra are an unambiguous signature of the vibrational character of the emitting level and allow us to distinguish E state emission from that originating in the p and D’ states. In fig. 2, the dispersed emission spectrum of the NeICl complex excited to a U= 1 ion-pair state level is shown. The excitation feature pumped to obtain this spectrum was assigned in a previous report to theE (~=l)tA (~~14) transitioninNeIC1 [13]. Consideration of the relative intensities of the ICl Ec-A and /LA excitation features demonstrates, however, that a more appropriate description of the initially prepared NeICl level is /3( v= 1) #I. This revised assignment will have an impact on the NeICl binding energies previously reported [ 13 ] for the ionpair, A and X electronic states; this issue will be discussed in more detail in a future publication. Fig. 2 clearly shows emission at wavelengths corresponding to E+X transitions following excitation of the g( u= 1) level in NeICl along with emission that can be assigned to the @+A and/or D’+A’ transitions. The presence of E state emission demonstrates that the Ne atom induces an enhanced interaction between the E and p states. This is the first direct observation of electronic energy transfer in the I’ In earlier experiments, the use of cut-off filters that transmit wavelengths < 400 nm restricted the detected emission to E+ X
fluorescence.
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dynamics of halogenjinterhalogen-rare-gas van der Waals complexes. At the spectral resolution used to record the dispersed spectra following excitation of the van der Waals complex (75 cm-l), we cannot unambiguously distinguish between emission from the p and D’ states. We note, however, that the Franck-Condon profile for this emission region (fig. 2) is more symmetric than that observed (fig. 1) from ICI prepared exclusively in either the p or D’ states. This observation suggests that emission in this region arises from both the fl and D’ states after exciting NeICl to the p ( U= I ) level. In any event, the Franck-Condon profiles of the p/D’ and E state fluorescence are characteristic of emission from the v=O level of an ion-pair state. The loss of a quantum of ICI vibrational excitation can be understood in the traditional framework of the vibrational predissociation dynamics of van der Waals complexes. When NeICl is excited to the p(v=2) level, emission is observed from U= 1 but not u=O of the ion-pair states. Thus a Au= - 1 propensity rule for vibrational predissociation appears to be obeyed in ion-pair state excited NeICl, as found in the valence states of numerous rare-gas-halogen systems [ 1,2,4-71, For the u= 1 and 2 levels no emission is detected from the initially excited vibrational level, suggesting that vibrational predissociation must occur on a time scale that is short relative to the fluorescence lifetime (r, G 10 ns [ 17 ] ) #*. We have also recorded the dispersed emission spectrum resulting from excitation of another van der Waals feature associated with the p( u= 1) state, located 28 cm-’ lower in energy than that discussed above. This spectrum (not shown) is identical to fig. 2, suggesting that for p ( V= 1) the electronic relaxation process is largely insensitive to the degree of excitation of certain intramolecular motions of the Ne atom relative to ICl. Dispersed emission spectra resulting from excitation of NeICl to the g ( U= 0) level have also been recorded. In this case, the vibrational predissociation channel is closed, yet we do observe electronic relaxation as indicated by emission from
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Fig. 2. Dispersed emission spectrum following double resonance excitation of NeICl to the v= I level of the 3 ion-pair state. NeICl issequentiaUyexcitedontheA(v=I4)+X(u=O) (16087~m-‘) and~(t)=l)tA(~=l4)transitions(23062cm-L)ofthevander Waals complex. 552
21 July 1989
** Our discussion assumes that the observed emission spectra result exclusively from E, D’ and g state ICI photofragments. We have insufftcient spectral resolution to exclude the possibility that the emitting species are NeICl molecules with predissociation lifetimes that exceed the ion-pair state fluorescence lifetime.
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CHEMICALPHYSICSLEl-l’ERS
the E (u= 0) state. Finally, excitation of the complex to l3( v=2) results in negligible emission from E(v=l) or E(u=O), but only the u=l level of the p and/or D’ states. Electronic energy transfer in NeICl is likely to be sensitive to changes in the relevant bare molecule potential curves and the welldocumented nonadiabatic perturbations in ICI upon addition of a Ne atom. The matrix elements coupling the S, D’ and E states in uncomplexed ICI [ 12 ] are somewhat smaller for the u= 2 levels than for v= 1 but the reason for the significant change in the dynamics of NeICl remains unclear at this time. Freed has pointed out that even small perturbations between electronic states can have dramatic effects on the nature of collision-induced electronic energy transfer [ 181, our data suggest that an analogous phenomenon occurs in van der Waals complexes. We are presently extending our investigations to higher ICI vibrational levels and working towards a more complete mapping of the dependence of the dynamics of NeICl on the degree of van der Waals vibrational mode excitation. Our eventual goal is a comparison with experiments on collision-induced energy transfer in halogen ion-pair states [ 19 1, the extant theory of collision-induced electronic energy relaxation [ 18 ] and the emerging theory of nonadiabatic electronic effects on the dynamics of small van der Waals complexes [ 20,2 11.
Acknowledgement
Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work Partial equipment support for this research was provided by the National Science Foundation through the Physical Chemistry Program. MIL
thanks the Natural Science Association at the University of Pennsylvania for a Young Faculty Award. TAS thanks
Swarthmore
Lang Faculty Fellowship
College
for a Eugene
M.
( 1988-89).
References [l] D.H. Levy, Advan. Chem. Phys. 47 (1981) 323. [2] K.C. Janda, Advan. Chem. Phys. 60 (1985) 201. [ 33 D.D. Evard, C.R. Bieler, J.I. Cline, N. Sivakumar and KC. Janda, J. Chem. Phys. 89 (1988) 2829, and references therein. [4] J.I. Cline, B.P. Reid, D.D. Evard, N. Sivakumar, N. HalberstadtandKC. Janda,J. Chem. Phys. 89 (1988) 3535. [ 51 N. Sivakumar, J.I. Cline, CR. Bieler and KC. Janda, Chem. Phys. Letters 147 (1988) 561, and references therein. [6] J.C. Drobits and MI. Lester, J. Chem. Phys. 89 ( 1988) 47 16,and references therein. [ 71 R.L. Waterland, J.M. Skene andM.1. Lester, J. Chem. Phys.
89 ( 1988) 7277, and references therein. [8] J.C. Drobits and M.I. Lester, J. Chem. Phys. 86 (1987) 1662. [ 91 G. Kubiak, P.S.H. Fitch, L. Wharton and D.H. Levy, J. Chem. Phys 68 (1978) 4477. [ IO] J.M. Skeneand M.I. Laster, Chern. Phys. Letters 116 ( 1985)
[ II] ?Jonvet and B. Seep, J. Chem. Phys. 80 ( 1984) 2229. [ 121 D. Bussieresand A.R. Hoy, Can. J. Phys. 62 (1984) 1941. [ 131J.C. Drobits, J.M. SkeneandM.1.
Lester, J. Chem. Phys. 84 (1986) 2896. [ 141 J.M. Skene, Ph.D. Thesis, University of Pennsylvania (1988). [ 151J.C.D. Brand and A.R. Hoy, J. Mol. Spectry. 114 (1985) 197. [ 161 J.A. Coxon and MA. Wickrarnaaratchi, J. Mol. Spectry.79 (1980) 380. [ 171 J.G. Eden, M.L. Dlabal and S.B. Hutchison, IEEE J. Quantum Electron. QE-17 (1981) 1085. [ 18 ] K-F. Freed, Advan. Chem. Phys. 42 ( 1980) 207, [ 191 J.P.T. Wilkinson, M. MacDonald and R.J. Donovan, J. Photochem. 35 (1986) 123. [ 201 J.A. Beswick, R. Mono& J.-M. Philippoz and H. van den Bergh, J. Chem. Phys. 86 (1987) 3965. [21] C. Jouvet and J.A. Bcswick, J. Chem. Phys. 86 (1987) 5500.
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NONADIABATIC TRANSITIONS IN THE DYNAMICS OF THE NeICl VAN DER WAALS COMPLEX Thomas A. STEPHENSON
‘, Yujian HONG and Marsha 1. LESTER 2
Department of Chemistry, UniversityofPennsylvania, Philadelphia. PA 191046323, USA Received 14 April 1989
Both electronic energy transfer and vibrational predissociation decay channels are observed following excitation of NefCl complexes to the 8 ion-pair state. Dispersed fluorescence is detected on the E-+X transition as well as the D’ *A’ and/or 8-A transitions. The vibrational quantum number of the emitting level is reflected in the shape of the Franck-Condon envelope, while its electronic character is determined from the emission wavelength and profile. The relaxation pathways are found to depend on the initial vibrational excitation of the NeICl complex.
1. Introduction The electronically excited van der Waals complexes of halogen and interhalogen diatomic molecules with rare gas atoms have proven to be important and convenient systems for the examination of the structure and vibrational predissociation dynamics of weakly bound molecular complexes [ l-8 1. To date, however, these studies have focused on the behavior of the van der Waals complexes confined to single adiabatic potential energy curves. The influence of additional electronic states and nonadiabatic couplings has been inferred from the behavior of certain vibrational levels in electronically excited ArIz [9], HeCl, [4] and HeICl [lo] but direct observation of electronic energy transfer in polyatomic van der Waals complexes has been limited to fine structure relaxation in HgNz [ 111. In the latter case, excitation of the complex to a molecular electronic state correlating with a 3P, mercury atom leads to dissociation and formation of a ‘PO mercury atom. In this Letter, we report the first direct observation of such electronic state changing processes in the dissociation of a diatom-rare gas complex, namely NeICl in the B (a= 1) ion-pair state. I On sabbatical leave (1988-89)
from the Department of Chemistry, Swarthmore College, Swarthmore. PA 1908 I, USA. * Alfred P. Sloan Research Fellow and Camille and Henry Dreyfus Foundation Teacher-Scholar.
0 009-2614/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
Three strongly bound electronic states correlating with the ionic atoms I+ ( 3P2) + Cl- (‘So) form the lowest energy tier of ICI ion-pair states. Conventionally labeled (using Hund’s case c notation) as E(O+), D’(2) and b(l), these states are closely spaced (T,=39059.5, 39061.8, and 39103.7 cm-‘, respectively) and have similar, though slightly displaced, potential energy curves [ 12 1. The ion-pair states can bc excited from the lower lying valence states of ICl with a strong propensity for dR= 0 electronic transitions. Both the D’ (2) and E (O+ ) states are heterogeneously mixed with the B( 1) state, however, resulting in absorption to alI three ion-pair states from the A( 1) valence state [ 121. Previous reports from this laboratory have described in great detail the vibrational predissociation dynamics of the HeICl [ 7] and NeICl [ 61 complexes excited to the A( 1) and B (O+ ) valence electronic states. In these investigations a pump laser promotes complexes from me ground electronic state to either the A or B state; a probe laser interrogates the resulting photofragments by excitation to an ICI ion-pair state. Emission from the ion-pair state is monitored as a function of probe laser wavelength, thus providing a fluorescence excitation spectrum of the nascent A or B state ICl products. For several A [ 13j and B [ 141 state vibrational levels in NeICl, the predissociation lifetime of the van der Waals complex is comparable to the pulse duration of the B.V.
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pump and probe lasers ( 10 ns). By temporally overlapping the lasers, the NeICl complex can be promoted to the ion-pair states. An example of the resulting optical-optical double resonance spectrum of NeICl has been presented elsewhere (see fig. 1 of ref. [ 131). The large geometry and binding energy changes that accompany excitation of the complex to the ion-pair state result in a progression of features due to excitation of the van der Waals vibrations. These features formed the basis for a rudimentary vibrational analysis of the van der Waals modes of NeICl in the ion-pair state [ 8 1. In our most recent experiments, we have obtained wavelength-resolved emission spectra following excitation of NeICl to the ion-pair states that demonstrate the presence of clear electronic energy transfer following excitation of the p state in the van der Waals complex.
2. Experimental For the investigations discussed in this report, our existing apparatus [8] has been modified to incorporate a scanning monochromator for dispersed emission studies. Briefly, a continuous expansion of ICI (Sigma) seeded in 75 psig of first-run grade Ne (A&o) is excited by two independently tunable extimer-pumped dye lasers. The pump laser ( 6 16-622 nm) selectively promotes either ground state NeICi or uncomplexed ICI to the v= 14 or 15 levels of the A( 1) electronic state. The probe laser (430-437 nm) excites NeICl (or ICI) to the ion-pair state. The resulting laser-induced fluorescence is collected and collimated by a 2 in. diameter, f/l lens and is focused onto the entrance slit of a 0.25 m f/4 scanning monochromator with a 2 in. diameterff4 lens. A blue sensitive phototube (Thorn/EM1 9535QA) is mounted directly on the exit slit body to detect emission from the ion-pair state. A red sensitive phototube (Thorn/EM1 9658B) detects the A-+X emission collected by a second set off/ 1 collection optics. The resulting emission signals are processed by gated integrators (PAR) and transferred to a laboratory computer for signal averaging and graphics output.
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3. Results and discussion In figs. la-If, we present the dispersed emission spectra from the v=O and 1 levels of the E, B and D’ states of uncomplexed ICI. As noted earlier the lowest energy ion-pair states perturb one another, particularly at the low vibrational levels discussed here [12].ExcitationtotheE(O’)andD’(2)statesfi-om the A(1) state (ALL-1 and +1, respectively) is exclusively due to “intensity borrowing” from the intense p( 1) tA( 1) transition. We find that the EcA and D’ +A excitation features are at least a factor of 200 weaker than the 8+A features. Several aspects of the emission spectra shown in fig, 1 are worthy of note: First, emission from the E state lies in a clearly distinguishable wavelength region from the D’ and 8 state emissions, which are significantly overlapped. The strong ALL0 propensity rule for ion-pair to valence state electronic transitions dictates that the E(O+ ) emission spectrum is dominated by transitions to the X (O+ ) ground state, while B( 1) +A( 1) and D’(Z)+A’ (2) transitions are prominent in emission from the B and D’ states, respectively. There are no manifestations (E+A emission, for example) of the relatively weak E-B and D’-B interactions in the dispersed emission spectra of uncomplexed ICI ( see below). Second, the vibrational quantum number of the initially excited ion-pair state is reflected in the shape of the envelope (no nodes for u= 0, one node for Y= 1, etc.) of the dispersed emission spectrum. The equilibrium internuclear separations of the ion-pair states are shifted significantly to larger values relative to the A(l), A’(2) and X(0+) states. Since the lower state vibrational wavefunctions are highly oscillatory, the only internuclear separations that contribute to the Franck-Condon overlap with the ion-pair states are those near the lower states’ outer turning points. Emission is observed, roughly, to lower state levels whose outer turning points fall within the range of internuclear separations which have significant ion-pair state wavefunction amplitude. The intensity of each emission transition is largely determined by the amplitude of the ion-pair state wavefunction in the vicinity of the lower state (A, A’ or X, for J$ D’ or E state emission, respectively) outer turning point. The similarity of the A and A’ potential energy curves
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Fib 1. Dispersed emission spectra of uncomplexed ICl from ion-pair states. ICI is prepared in tbe ion-pair state by double resonance excitation on ion-pairtA+X transitions. For spectra (a)-(e) the pump laser is fixed on the A(v= 14)tX(u=O) transition (16083 cm-‘);forspectnun(f),thepumplaserisf~~ontheA(v=15)cX(v~O)transition(16212cm~‘). (a)EmissionfiumIClE(u=O); theprobelasetiafixedontheE(u~O)+A(~=14)transition(2287Scm-’).(b)EmissionfromICIE(u=1);thep~belaserisfixedon the E(v= l)cA(~= 14) transition (23038 cm-‘). (c) Emission from ICI p(u=O); the probe laser is fucd on the p(u=O)tA(v= 14) transition(22921cm-‘).(d)EmissionfromICI~(u=l);theprobelaserisfuedonthe~(v=1)~A(u=14)vansition(23089~-‘). (e)EmissionfromIClD’(u=O);theprobelaserisfuedontheD’(u~O)+A(v=14)transition(22882cm-1).(f)EmissionfromICI D’(u= 1);theprobe laser is fixed on the D’ (u= I )cA(v= 15) transition (22924 cm-‘). X and A statevibrational assignments derived from ref. [ IS] and ref. [ 161, respectively.
in the Franck-Condon accessible region means that there are only small differences in the emission profiles for the D’ and p states. Third, weak emission is observed from E state levels in the wavelength range expected for E+A emission, but with intensity that is larger than that predicted by a simple perturbation treatment of the known E-/3 [ 12 ] state interactions. This emission does not follow the Franck-Condon pattern expected for emission to the A state from the ion-pair
levels. In particular, the overall envelope is not sensitive to the vibrational quantum number of the initially excited level. These findings suggest that E state emission in this wavelength region is due to perturbations in the valence electronic states, perhaps similar to those known to couple higher vibrational levels of the A and X states [ 151. This unexpected emission pattern along with bound-free emission observed at lower emission frequencies wilI be a focus of continuing investigations. For the purposes of 551
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our discussion of the NeICl complex, it suffices to note that the dispersed fluorescence spectra are an unambiguous signature of the vibrational character of the emitting level and allow us to distinguish E state emission from that originating in the p and D’ states. In fig. 2, the dispersed emission spectrum of the NeICl complex excited to a U= 1 ion-pair state level is shown. The excitation feature pumped to obtain this spectrum was assigned in a previous report to theE (~=l)tA (~~14) transitioninNeIC1 [13]. Consideration of the relative intensities of the ICl Ec-A and /LA excitation features demonstrates, however, that a more appropriate description of the initially prepared NeICl level is /3( v= 1) #I. This revised assignment will have an impact on the NeICl binding energies previously reported [ 13 ] for the ionpair, A and X electronic states; this issue will be discussed in more detail in a future publication. Fig. 2 clearly shows emission at wavelengths corresponding to E+X transitions following excitation of the g( u= 1) level in NeICl along with emission that can be assigned to the @+A and/or D’+A’ transitions. The presence of E state emission demonstrates that the Ne atom induces an enhanced interaction between the E and p states. This is the first direct observation of electronic energy transfer in the I’ In earlier experiments, the use of cut-off filters that transmit wavelengths < 400 nm restricted the detected emission to E+ X
fluorescence.
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dynamics of halogenjinterhalogen-rare-gas van der Waals complexes. At the spectral resolution used to record the dispersed spectra following excitation of the van der Waals complex (75 cm-l), we cannot unambiguously distinguish between emission from the p and D’ states. We note, however, that the Franck-Condon profile for this emission region (fig. 2) is more symmetric than that observed (fig. 1) from ICI prepared exclusively in either the p or D’ states. This observation suggests that emission in this region arises from both the fl and D’ states after exciting NeICl to the p ( U= I ) level. In any event, the Franck-Condon profiles of the p/D’ and E state fluorescence are characteristic of emission from the v=O level of an ion-pair state. The loss of a quantum of ICI vibrational excitation can be understood in the traditional framework of the vibrational predissociation dynamics of van der Waals complexes. When NeICl is excited to the p(v=2) level, emission is observed from U= 1 but not u=O of the ion-pair states. Thus a Au= - 1 propensity rule for vibrational predissociation appears to be obeyed in ion-pair state excited NeICl, as found in the valence states of numerous rare-gas-halogen systems [ 1,2,4-71, For the u= 1 and 2 levels no emission is detected from the initially excited vibrational level, suggesting that vibrational predissociation must occur on a time scale that is short relative to the fluorescence lifetime (r, G 10 ns [ 17 ] ) #*. We have also recorded the dispersed emission spectrum resulting from excitation of another van der Waals feature associated with the p( u= 1) state, located 28 cm-’ lower in energy than that discussed above. This spectrum (not shown) is identical to fig. 2, suggesting that for p ( V= 1) the electronic relaxation process is largely insensitive to the degree of excitation of certain intramolecular motions of the Ne atom relative to ICl. Dispersed emission spectra resulting from excitation of NeICl to the g ( U= 0) level have also been recorded. In this case, the vibrational predissociation channel is closed, yet we do observe electronic relaxation as indicated by emission from
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Fig. 2. Dispersed emission spectrum following double resonance excitation of NeICl to the v= I level of the 3 ion-pair state. NeICl issequentiaUyexcitedontheA(v=I4)+X(u=O) (16087~m-‘) and~(t)=l)tA(~=l4)transitions(23062cm-L)ofthevander Waals complex. 552
21 July 1989
** Our discussion assumes that the observed emission spectra result exclusively from E, D’ and g state ICI photofragments. We have insufftcient spectral resolution to exclude the possibility that the emitting species are NeICl molecules with predissociation lifetimes that exceed the ion-pair state fluorescence lifetime.
Volume 159,number 5,6
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CHEMICALPHYSICSLEl-l’ERS
the E (u= 0) state. Finally, excitation of the complex to l3( v=2) results in negligible emission from E(v=l) or E(u=O), but only the u=l level of the p and/or D’ states. Electronic energy transfer in NeICl is likely to be sensitive to changes in the relevant bare molecule potential curves and the welldocumented nonadiabatic perturbations in ICI upon addition of a Ne atom. The matrix elements coupling the S, D’ and E states in uncomplexed ICI [ 12 ] are somewhat smaller for the u= 2 levels than for v= 1 but the reason for the significant change in the dynamics of NeICl remains unclear at this time. Freed has pointed out that even small perturbations between electronic states can have dramatic effects on the nature of collision-induced electronic energy transfer [ 181, our data suggest that an analogous phenomenon occurs in van der Waals complexes. We are presently extending our investigations to higher ICI vibrational levels and working towards a more complete mapping of the dependence of the dynamics of NeICl on the degree of van der Waals vibrational mode excitation. Our eventual goal is a comparison with experiments on collision-induced energy transfer in halogen ion-pair states [ 19 1, the extant theory of collision-induced electronic energy relaxation [ 18 ] and the emerging theory of nonadiabatic electronic effects on the dynamics of small van der Waals complexes [ 20,2 11.
Acknowledgement
Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work Partial equipment support for this research was provided by the National Science Foundation through the Physical Chemistry Program. MIL
thanks the Natural Science Association at the University of Pennsylvania for a Young Faculty Award. TAS thanks
Swarthmore
Lang Faculty Fellowship
College
for a Eugene
M.
( 1988-89).
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