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Radiative and non-radiative decay of selected vibronic levels of the B state of alkoxy radicals
Chemical Physics Letters 380 (2003) 749–757 www.elsevier.com/locate/cplett
Radiative and non-radiative decay of selected vibronic levels e state of alkoxy radicals of the B Sandhya Gopalakrishnan, Lily Zu, Terry A. Miller
*
Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, OH 43210, USA Received 13 June 2003; in final form 20 August 2003 Published online: 10 October 2003
Abstract We have measured the lifetimes of some of the prominent bands observed in the moderate-resolution, jet-cooled e e transition of ethoxy, all the structural isomers of propoxy laser induced fluorescence excitation spectra of the B X and butoxy, and 1-pentoxy. Recent high-resolution, rotationally resolved studies on primary alkoxy radicals have given conformer specific assignments for the bands for which lifetimes have been measured. We report observed lifetime trends as a function of vibrational excitation for specific isomers and conformers. The implications of these observae state will be discussed. tions for the dynamics of the B Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction The competition between radiative and nonradiative decay paths for electronic states has for many years been of great interest. It has long been accepted that Ôother things being equalÕ, non-radiative decay paths grow increasingly important as the size of the molecule increases [1–4]. Most of the experimental investigations of such processes have involved closed shell molecules. The alkoxy radicals, Cn H2nþ1 O, offer a convenient openshell species where the size of the molecule can easily be varied. Moreover the recent work [5,6] correlate –X e laser-induced fluorescence ing lines in the B (LIF) spectra of the alkoxies with not only the empirical formula, but the structural isomer and con*
Corresponding author. Fax: +1-614-2921948. E-mail address: [email protected] (T.A. Miller).
former, offer an opportunity to investigate such processes in unprecedented detail. In this Letter we reported the natural lifetimes, and by implication the quantum yields for fluorescence, for the ethoxy, propoxy, butoxy, and pentoxy radicals, which can be combined with similar, previously reported data for methoxy. Our results show relatively small variations in lifetime among the alkoxies for the vibrationless level, but very dramatic changes upon vibrational excitation. These latter changes are clearly structural isomer and conformer specific.
2. Experimental The alkoxy radicals were generated in a supersonic free-jet expansion by UV laser photolysis of the corresponding alkyl nitrites at the base of
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a 0.5 mm circular nozzle. The alkyl nitrites were synthesized according to well-known procedures. A few torr of the alkyl nitrite vapor was seeded into a supersonic jet expansion by using a suitable carrier gas like helium at a backing pressure of 60 psi and expanding the mixture through the pulsed nozzle (general valve) into a vacuum chamber. The background pressure in the vacuum chamber, pumped by a mechanical booster pump, was 150 and 10 mTorr, respectively, with and without operation of the free jet. The alkoxy radicals were produced by photolysis of the alkyl nitrite precursor using the output of a tripled Nd:YAG (Quanta Ray DCR, 355 nm). The radicals generated in situ were then excited by a probe laser about 15 mm downstream from the photolysis laser. The probe laser was the frequency doubled output (Inrad Autotracker II) of a tunable dye laser (Questek PDL-1) pumped by a Nd:YAG (Quanta Ray DCR, 532 nm) laser. We have previously reported [5–7] the laser e –X e transition of all the excitation spectra of the B alkoxy radicals, except ethoxy, for which lifetime measurements were attempted. The jet-cooled LIF excitation spectrum of ethoxy has been reported by several groups [8–10] and selected vibrational bands in this spectrum were chosen to obtain the natural lifetimes. The lifetimes were measured using the following procedure. The probe laser was tuned to the frequency of the chosen vibrational band at which the maximum fluorescence was observed. The fluorescence was collected on a photomultiplier tube (EMI 9659Q). The signal from the PMT was sent to an oscilloscope and the resulting temporal decay was averaged over 512 shots and stored digitally. For weak bands, a soft focus f =3 lens was used to image the fluorescence onto the PMT. However, for bands with long lifetimes (>1 ls), no optics were used to focus the fluorescence on the PMT in order to prevent excited molecules from moving out of the field of view during emission. The experimental temporal resolution, limited by the PMT and electronics, was 40 ns. Several experiments were performed to minimize systematic errors in recording the lifetimes. The natural lifetimes of several vibronic bands of jet-cooled methoxy have been reported [11,12].
e –X e origin band of methoxy The lifetime of the A was recorded to serve as a test and the measured lifetime was found to be in agreement with the reported one [11] to within 10%. Multiple measurements of the lifetime were made for several alkoxy bands at slightly different frequencies within the line profile and no deviation was observed beyond experimental accuracy. Care was also taken to ensure that there was no collisional quenching by recording lifetimes at different backing pressures of the helium carrier gas. No deviation beyond experimental error were observed for pressures between 40 and 100 psi. In order to ensure that the temporal decay was single exponential, the natural log of the intensity was plotted against time to check for linearity in all measurements. The corresponding experimental data were fit to an exponential decay to obtain the lifetime. The standard error for the lifetime obtained from each of these fits was typically 1% whereas the variation between the lifetimes obtained from the different fits for any given band was typically on the order of 10–15%. We estimate that the absolute error in any of our reported lifetimes is less than 20%.
3. Results 3.1. Methoxy and ethoxy The assignment and natural lifetimes of the bands investigated in ethoxy are shown in Table 1. If we assume that the quantum yield for fluorese state is cence from the vibrationless level of the B unity, then the quantum yield for any vibrationally excited band, m0 , can be given approximately by sm0 =s0 where s is the natural lifetime of the band whose vibrational quantum is given by the subscript and the subscript 0 refers to the vibrationless level of the radical. (This is a reasonable assumption because the natural lifetimes of the origins of the smallest alkoxy radicals, methoxy and that of ethoxy are to within 1%, and the other alkoxies are all within approximately a factor of 2 of this value.) This approximation clearly neglects any small variations in radiative lifetime with vibrational level, such as might be caused by a R-dependent
S. Gopalakrishnan et al. / Chemical Physics Letters 380 (2003) 749–757 Table 1 Measured natural lifetimes (s in ls) of selected vibrational e state ethoxy bands (approximate frequency in cm1 ) of B Frequency 29 181 29 784 30 382 30 974 31 559 32 139
(00 ) (m1 ) (m2 ) (m3 ) (m4 ) (m5 )
s 2.58 2.34 2.15 1.53 0.58 0.13
e state. mn denotes the CO stretch vibration in the B
transition moment, m3 factors, etc. A plot of the ratio sm0 =s0 against the vibrational energy in the excited state is shown in Fig. 1 for methoxy, from previously published data [11,12], and for ethoxy from our present measurements. Since the CO stretch progression dominates the excitation spectrum of all the alkoxy radicals our measurements are almost exclusively for levels with the excess vibrational energy deposited in the CO stretch mode. Referring to Fig. 1, it is clear that for CH3 O the lifetime and hence the approximate quantum yield decreases slightly, but less than 20%, as 5
Fig. 1. Plot of lifetime (sv0 ) of various CO stretch levels divided by that of the vibrationless level (s0 ) vs excess vibrational energy for the methoxy, ethoxy, and propoxy radicals. The solid straight line indicates a value of ðsv0 =s0 Þ ¼ 0:05, which is the approximate quantum yield below which the LIF excitation spectrum could not be detected. Dashed lines for the various radicals indicate no LIF signal could be seen for the next CO stretch excitation, although on the basis of the lower-level observations and expected Franck–Condon factors, one would expect an observable signal.
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quanta (3500 cm1 in energy) of the CO stretch are excited. For 6 quanta there is an order of magnitude drop and our earlier work [11,13] showed the quantum yield is K104 for levels v P 7. The very sharp drop in quantum yield in methoxy has been attributed to an interaction with repulsive states arising from the CH3 + O dissociation limit, similar to the well-known predissociation in the excited vibrational levels of the OH radical. By contrast Fig. 1 shows that C2 H5 O shows a somewhat more pronounced decrease in lifetime between v ¼ 0 and 5 (3000 cm1 of energy) of the CO stretch, with the approximate quantum yield decreasing by over an order of magnitude. However this decrease appears gradual with no sharp discontinuity as is observed with CH3 O. (A continued gradual decrease in the lifetime of C2 H5 O above v ¼ 5 would place those levels below detectability, as was experimentally observed.) 3.2. Propoxy There are two structural isomers for propoxy, 1- and 2-propoxy. While 2-propoxy has a single structure, 1-propoxy can exist in two conformations, gauche (G) and trans (T), with spectral transitions of both of them having been observed and assigned in the jet [5]. (Fig. 2 illustrates schematically the structure of the conformations of all the alkoxy whose lifetimes were measured.) Lifetime measurements were performed on both structural as well as conformational isomers. The assignments and lifetimes of the bands studied are given in Table 2. The results are shown in Fig. 1 for propoxy where sv0 =s0 vs vibrational energy is plotted. The results for 2-propoxy are in fact similar to ethoxy and even to some extent, methoxy. As CO stretch quanta from v ¼ 0–6 (3500 cm1 in energy) are excited there is a gradual decrease in lifetime of greater than a factor of 10, with somewhat surprisingly the largest per quanta drop being a factor of 2 between the v ¼ 0 and 1 states. However the results for the structural isomer 1-propoxy are very different from any of the other radicals Cn H2nþ1 O with n 6 3. Neither conformer has a detectable quantum yield, i.e.,
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Fig. 2. Structural formulae for the conformers of 1-propoxy (a), 1-butoxy (b), and 1-pentoxy (c), for which lifetime measurements were conducted. The corresponding Newman projections are also shown as well as the designation in terms of the dihedral angles at their local minima (G for gauche, T for trans).
K5%, for vCO >1, while all the others fluoresced for vCO up to at least 5 and CH3 O up to vCO ¼ 6. As Fig. 1 shows the quantum yields for both 1-propoxy conformers drop for vCO ¼ 1, but much more precipitously for the G conformer.
3.3. Butoxy There are three structural isomers for butoxy, 1-, 2-, and t-butoxy. t-Butoxy has a single conformation while 1-butoxy has five conformers and 2-butoxy has three. Our high-resolution,
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Table 2 e state propoxy isomers and Measured natural lifetimes (s in ls) of selected vibrational bands (approximate frequency in cm1 ) of the B conformers 2-Propoxy
1-Propoxy G
Frequency 27 171 27 740 28 304 28 863 29 412 29 961 30 504
0
(0 ) (m1 ) (m2 ) (m3 ) (m4 ) (m5 ) (m6 )
s 1.97 1.09 0.830 0.713 0.348 0.167 0.078
T
Frequency 0
28 634 (0 ) 29 230 (m1 )
s 1.57 0.238
Frequency 0
29 219 (0 ) 29 895 (m1 )
s 1.78 1.40
e state. mn denotes the CO stretch vibration in the B
rotationally resolved studies [11] on 1-butoxy revealed the presence of three conformers of 1butoxy in the jet environment and established their identity. We have also obtained similar rotationally resolved spectra for 2-butoxy, although we do not yet have full rotational analyses (and hence the conformational identity) for these bands. However, two distinctly different sets of rotational profiles can be identified that indicate the presence of two different conformers e of 2-butoxy which we excite and measure their B state lifetimes. The plots of sv0 =s0 vs vibrational energy for the three isomers and the various conformers are shown in Fig. 3. The results are again striking. While t-butoxy shows a long progression in the CO stretch (up to v ¼ 6), 1- and 2-butoxy exhibit fluorescence from at most one quantum of excitation in this mode. Whereas in the case of G and T 1-propoxy the lifetimes of the CO stretch vibrational band differed by an order of magnitude, in the case of 1-butoxy, the CO stretch is not even observed for conformers G1 T2 and T1 G2 of 1-butoxy. A single CO stretch excitation band is observed for conformer T1 T2 and has a lifetime comparable to its vibrationless level. These results are all summarized in Table 3. In the case of 2-butoxy, a single CO stretch excitation is observed in the LIF spectrum for both conformers. However there is a substantial difference in lifetime and presumably quantum yield for the two conformers.
Fig. 3. Plot of lifetime (sv0 ) of various CO stretch levels divided by that of the vibrationless level (s0 ) vs excess vibrational energy for the 1-, 2-, and t-isomers and several conformers of the butoxy radical. The solid straight line indicates a value of ðsv0 =s0 Þ ¼ 0:05, which is the approximate quantum yield below which the LIF excitation spectrum could not be detected. Dashed lines for the various radicals indicate no LIF signal could be seen for the next CO stretch excitation, although on the basis of the lower level observations and expected Franck–Condon factors, one would expect an observable signal.
3.4. Pentoxy We have obtained [7] the LIF spectra of 1-, 2-, 3-, and t-pentoxy. Of these four isomers, we have reported the rotationally resolved LIF spectra for just 1-pentoxy [6]. Thus the conformational and vibrational assignments are available only for this isomer and we focus our lifetime measurements on it. Our rotationally resolved spectra established the
Frequency
27 069 27 680
s
2.50 1.78
e state. m denotes the CO stretch vibration in the B
Frequency
26 762 27 321 1.39
s Frequency
29 164 (00 ) 1.10 28 650 (00 )
s
1.38 1.13
Frequency s Frequency
1.97 1.06 1.00 0.994 0.700 0.560 0.384 (00 ) (m1 ) (m2 ) (m3 ) (m4 ) (m5 ) (m6 ) 26 415 26 945 27 480 28 000 28 512 29 020 29 525
s
T1 T2
Frequency
T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 G3 T1 G2 T3 T1 G2 T3 G1 T2 T3
29 096 (00 ) 29 768 (m1 )
Conformer 1 T1 G2
Table 4 Measured natural lifetimes (s in ls) of vibrational levels (approximate frequency in cm1 ) of 1-pentoxy; mCO and mCCO denote CO stretch and CCO torsional vibrations, respectively Conformer
G1 T2
1-Butoxy t-Butoxy
Table 3 e state butoxy Measured natural lifetimes (s in ls) of vibrational bands (approximate frequency in cm1 ) of B
2-Butoxy
Conformer 2
s
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2.43 1.04
754
Frequency 29 013 29 141 29 684 28 987 29 122 29 232 28 644
Assignment 0
0 m1CCO m1CO 00 00 m1CCO 00
s 1.20 1.20 1.10 1.40 1.37 1.07 0.99
presence of 6 of the 14 possible conformers in the jet environment of which five have been assigned in the observed spectra. The lifetimes for selected vibrational bands of these five conformers of 1pentoxy are listed in Table 4. The LIF spectra of all the observed isomers of pentoxy are terminated within K1000 cm1 of the electronic origin, much like the spectra of 1-butoxy, 2-butoxy, and 1-propoxy. An exposition of the natural lifetimes of the various bands of 1-pentoxy is given in Table 4 and shows that the origin bands of all observed conformers have comparable lifetimes. However, only three conformers, namely T1 T2 T3 and T1 T2 G3 and G1 T2 T3 exhibit a single CO stretch excitation band strong enough to rotationally resolve and assign. Of these three conformers, the lifetime of the CO stretch band of conformer G1 T2 T3 is considerably shorter (though not precisely measured) than those of conformer T1 T2 T3 . This indicates dynamical behavior in 1-pentoxy that is conformer dependent just as was observed in 1-propoxy and 1-butoxy.
4. Discussion 4.1. Observed trends With the results for the lifetimes of individual species documented, it is useful to summarize some general trends among the species. 1. In the absence of vibrational excitation the measured lifetimes of all the alkoxy radicals differed by less than a factor of 2.5. We expect that this is a result of the similarity of the transition moment in all the species, with the small varia-
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tions depending upon a number of small details of the electronic structure. 2. With the exception of the abrupt decrease at v P 6 in methoxy, molecules that fluorescence from CO stretch levels v > 1 show only a gradual decrease in lifetime as the vibrational energy is increased. This gradual decrease seems slightly more pronounced for the longer alkyl chains. 3. Structural isomers with only a single conformer always have observable fluorescence with roughly comparable lifetimes for all vibrational levels with 65 quanta of CO stretch energy. Structural isomers that have more than one conformer in no case had detectable fluorescence from levels with v > 1 in the CO stretch. 4. For the species with multiple conformers, lifetimes decreased most rapidly for the species without a plane of symmetry. The Cs conformers (T, T1 T2 , and T1 T2 T3 ) show only slightly shorter lifetimes (compared to the vibrationless level) with 1 quanta of CO stretch excitation,
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while lower symmetry species showed dramatically shorter vCO ¼ 1 lifetimes or no observable fluorescence at all for vCO ¼ 1 or higher. Indeed for multiple conformer species no fluorescence was observed above vCO ¼ 1 for any conformers even those with Cs symmetry. 4.2. Possible mechanisms It is possible to explain several, but not all, of the general trends of the previous section by reference to Fig. 4. This figure schematically represents the energetics of the various alkoxy radicals e state dissociation limit to R þ Oð3 PÞ. near the X e state and the reIt illustrates both the bound X pulsive states going to this limit. It also shows the e state is CH3 O since its X e state (formally the A e B state is degenerate) which has a corresponding R þ Oð1 DÞ dissociation limit. Long ago it was established that the first excited state of the OH radical was pre-dissociated above
e and X e potential energy surfaces of the alkoxy radical. The Fig. 4. Schematic cut along the C–O bond coordinate, through the B e state curve for CH3 O is shown but only the position of the B e state vibrationless level for the higher alkoxies. (The state complete B e for methoxy because of the degeneracy of its X e for the higher alkoxies but usually is denoted A e state.) The depicted is always labelled B e state vibrationless level is determined in the following manner with respect to the R þ Oð3 PÞ dissociation limit. The position of the B e state vibrationless level with respect to this limit is determined by B3LYP DFT quantum chemistry claculations. The B e position of the X e state level the experimental value of T00 from the B e –X e spectra. state vibrationless level is then determined by adding to X
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v ¼ 1, and it has long been assumed that the sudden onset of a non-radiative decay pathway in CH3 O was attributable to the same mechanism. Recent photofragmentation experiments by Neumark and co-workers [14,15] certainly confirm this assumption. Based upon their photofragmentation and our lifetime experiments [11,13] the conical e and repulsive states of intersection involving the A CH3 O must occur between the energies of 5 and 6 quanta of CO stretch excitation. At vCO ¼ 6 and above the non-radiative channel predominates the decay. When we began correlating the data from the present lifetime measurements of the larger alkoxies, we first thought the quenching of the quantum yield might be due to a similar mechanism with the details of the onset of the nonradiative decay channel strongly dependent upon the exact location of the corresponding conical intersections with the repulsive states. However Fig. 4 clearly shows this not to be the case. As the alkoxy radicals become larger the vibrationless e state sinks well below the dissocialevel of the B e state and correspondingly the tion limit of the X repulsive curves from that limit are far above the energetic threshold of the observed non-radiative behavior. Physically this is because the larger (and more branched) alkyl groups tend to stabilize the e state, compared to highly polar C–O bond in the B e state. This the much less polar bond in the X finding is consistent with the recent observations by Choi et al. [16] that dissociation does not occur e state of ethoxy as it does from the from the B corresponding state in methoxy. While Fig. 4 rules out the explanation involving the conical intersection with the repulsive state, for e state of the the non-radiative behavior of the B larger alkoxies, it also suggests an alternative explanation. All the levels investigated in this study lie within the dense manifold of the vibrational e state near its R þ Oð3 PÞ dissocialevels of the X tion limit. Indeed other lower dissociation limits, e.g., to R@O þ H, create a true continuum of levels in this energy region. We therefore suggest that the key mechanism for non-radiative decay is internal conversion to the ground state surface, with possibly subsequent isomerization or dissociation thereupon. This
mechanism is consistent [1] with several of the general trends noted above. There is a large elece and B e states of tronic energy gap between the X the alkoxy radicals consistent with a slow (>1 ls) non-radiative decay of the vibrationless level of all the radicals. Such a mechanism is also consistent with the gradual increase in the non-radiative decay as CO stretch vibrational energy is added to e state of C2 H5 O, 2-C3 H7 O, and t-butoxy. It is the B generally accepted that so long as one has a dense manifold of accepting levels, then the internal conversion rate will increase as the electronic gap (E0 ) decreases. Furthermore it has been found [1] that vibrational excitation of the bright (in this e ) state, tends to decrease the effective eleccase B tronic gap, E00 , roughly according to E00 ¼ E0 nx where n is the number of excited quanta of a given mode with frequency x. There are however several aspects of the general trends which the internal conversion model, as presently formulated, does not fully account. One way of categorizing the behavior of the decay dye state is to note that the radicals, namics of the B CH3 O, C2 H5 O, 2-C3 H7 O, t-C4 H9 O, that seem relatively resistant to non-radiative decay have several characteristics in common. They all have only one conformer and possess some symmetry (at least a reflection plane). For the alkoxy species where multiple structural isomers exist the nonradiative-decay resistant species obviously have the same number of degrees of vibrational freedom as do the single conformer species, but substitute in the latter methyl torsions for the C–C–C or C–C–O torsions present in the other conformers. It is also worth noting that all the trans (or at least local to the oxygen atom trans) species, that have a plane of symmetry (or at least an approximate one) show little if any non-radiative decay in the vCO ¼ 1 levels, while non-radiative decay in the other species is the predominant decay mode for vCO > 0. Of course when vCO J 2 is excited in all the trans species, non-radiative decay again becomes predominant. It is extremely interesting to note that we have preliminary calculations for the e state barrier for conversion of trans to gauche B conformers. Its height is between the energy of 1 and 2 CO stretch quanta. Thus for vCO P 2, the molecule has sufficient energy to lie above the
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barrier where unique gauche or trans configurations no longer exist. It is fairly clear then that either the decrease in symmetry and/or the increase in flexibility of molecules with multiple conformers increases the rate of internal conversion, presumably by increasing the coupling matrix element between the e and X e states. Such an increase might be qualiB tatively expected as the coupling operator is generally thought to be the kinetic energy terms neglected in the Born–Oppenheimer approximation. The torsional degree of freedom inherent in multiple-conformer species may well lead to a large Coriolis contribution to this kinetic energy coupling. While qualitatively similar observations have been made with acetaldehyde where large differences in radiationless decay rates have been noted [17] between e and a torsional levels, the details of the alkoxy behavior are quite different. Clearly, a quantitative correlation between theory and experiment requires more work for the alkoxy radicals.
5. Conclusions Detailed measurement of the temporal decay of e state of the alkoxy radthe fluorescence of the B icals have been performed. These measurements have demonstrated a significant lifetime shortening due to the deposition of an excess of vibrational energy, presumably indicative of the opening of competitive non-radiative decay channels. These lifetime variations show a surprising dependence on the molecular geometry, i.e., structural isomer and conformer, for radicals of the same empirical formula and hence the same number of nuclear degrees of freedom. Alkoxy radical isomers with only a single conformer seem relatively resistant to non-radiative decay compared to isomers with multiple conformers. Of the latter group, all trans conformers seem most resistant to non-radiative decay. It is argued that the present observations of non-radiative decay in the larger alkoxy radicals are not consistent with the well-known predissociation known in higher vibrational levels of CH3 O. Rather the presently observed behavior is
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most consistent with internal conversion to the e state. Apparently low frequency torsional X motions in species without Cs symmetry are efficient promoters of the internal conversion, although a detailed mechanism is not yet apparent.
Acknowledgements The authors gratefully acknowledge the financial support by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy, via Grant DE-FG02-01ER15172.
References [1] P. Avouris, W.M. Gelbart, M.A. El-Sayed, Chem. Rev. 77 (1977) 793. [2] A. Tramer, R. Voltz, in: Time-Resolved Studies of Excited Molecules, vol. 4, Academic Press, New York, NY, 1979, p. 281. [3] J. Kommandeur, W.A. Majewski, W.L. Meerts, D.W. Pratt, Annu. Rev. Phys. Chem. 38 (1987) 433. [4] D.J. Nesbitt, R.W. Field, J. Phys. Chem. 100 (1996) 12735. [5] S. Gopalakrishnan, C.C. Carter, L. Zu, V. Stakhursky, G. Tarczay, T.A. Miller, J. Chem. Phys. 118 (2003) 4954. [6] S. Gopalakrishnan, L. Zu, T.A. Miller, J. Phys. Chem. A 107 (2003) 5189. [7] C.C. Carter, J. Atwell, S. Gopalakrishnan, T.A. Miller, J. Phys. Chem. A 104 (2000) 9165. [8] X. Zhu, M. Kamal, P. Misra, Pure Appl. Opt. 5 (1996) 1021. [9] G. Inoue, M. Okuda, H. Akimoto, J. Chem. Phys. 75 (1981) 2060. [10] X.Q. Tan, J.M. Williamson, S.C. Foster, T.A. Miller, J. Phys. Chem. 97 (1993) 9311. [11] D.E. Powers, M. Pushkarsky, T.A. Miller, J. Chem. Phys. 106 (1997) 6878. [12] S.-R. Lin, Y.-P. Lee, J.B. Nee, J. Chem. Phys. 88 (1988) 171. [13] B.E. Applegate, M.B. Pushkarsky, T.A. Miller, J. Phys. Chem. 103 (1999) 1538. [14] D.L. Osborn, D.J. Leahy, E.M. Ross, D.M. Neumark, Chem. Phys. Lett. 235 (1995) 1484. [15] D.L. Osborn, D.J. Leahy, D.M. Neumark, J. Phys. Chem. 101 (1997) 6583. [16] H. Choi, R.T. Bise, D.M. Neumark, J. Phys. Chem. A 104 (2000) 10112. [17] D.C. Moule, R.H. Judge, H. Liu, E.C. Lim, Int. J. Quantum Chem. 71 (1999) 167.
Radiative and non-radiative decay of selected vibronic levels e state of alkoxy radicals of the B Sandhya Gopalakrishnan, Lily Zu, Terry A. Miller
*
Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, OH 43210, USA Received 13 June 2003; in final form 20 August 2003 Published online: 10 October 2003
Abstract We have measured the lifetimes of some of the prominent bands observed in the moderate-resolution, jet-cooled e e transition of ethoxy, all the structural isomers of propoxy laser induced fluorescence excitation spectra of the B X and butoxy, and 1-pentoxy. Recent high-resolution, rotationally resolved studies on primary alkoxy radicals have given conformer specific assignments for the bands for which lifetimes have been measured. We report observed lifetime trends as a function of vibrational excitation for specific isomers and conformers. The implications of these observae state will be discussed. tions for the dynamics of the B Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction The competition between radiative and nonradiative decay paths for electronic states has for many years been of great interest. It has long been accepted that Ôother things being equalÕ, non-radiative decay paths grow increasingly important as the size of the molecule increases [1–4]. Most of the experimental investigations of such processes have involved closed shell molecules. The alkoxy radicals, Cn H2nþ1 O, offer a convenient openshell species where the size of the molecule can easily be varied. Moreover the recent work [5,6] correlate –X e laser-induced fluorescence ing lines in the B (LIF) spectra of the alkoxies with not only the empirical formula, but the structural isomer and con*
Corresponding author. Fax: +1-614-2921948. E-mail address: [email protected] (T.A. Miller).
former, offer an opportunity to investigate such processes in unprecedented detail. In this Letter we reported the natural lifetimes, and by implication the quantum yields for fluorescence, for the ethoxy, propoxy, butoxy, and pentoxy radicals, which can be combined with similar, previously reported data for methoxy. Our results show relatively small variations in lifetime among the alkoxies for the vibrationless level, but very dramatic changes upon vibrational excitation. These latter changes are clearly structural isomer and conformer specific.
2. Experimental The alkoxy radicals were generated in a supersonic free-jet expansion by UV laser photolysis of the corresponding alkyl nitrites at the base of
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.08.124
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a 0.5 mm circular nozzle. The alkyl nitrites were synthesized according to well-known procedures. A few torr of the alkyl nitrite vapor was seeded into a supersonic jet expansion by using a suitable carrier gas like helium at a backing pressure of 60 psi and expanding the mixture through the pulsed nozzle (general valve) into a vacuum chamber. The background pressure in the vacuum chamber, pumped by a mechanical booster pump, was 150 and 10 mTorr, respectively, with and without operation of the free jet. The alkoxy radicals were produced by photolysis of the alkyl nitrite precursor using the output of a tripled Nd:YAG (Quanta Ray DCR, 355 nm). The radicals generated in situ were then excited by a probe laser about 15 mm downstream from the photolysis laser. The probe laser was the frequency doubled output (Inrad Autotracker II) of a tunable dye laser (Questek PDL-1) pumped by a Nd:YAG (Quanta Ray DCR, 532 nm) laser. We have previously reported [5–7] the laser e –X e transition of all the excitation spectra of the B alkoxy radicals, except ethoxy, for which lifetime measurements were attempted. The jet-cooled LIF excitation spectrum of ethoxy has been reported by several groups [8–10] and selected vibrational bands in this spectrum were chosen to obtain the natural lifetimes. The lifetimes were measured using the following procedure. The probe laser was tuned to the frequency of the chosen vibrational band at which the maximum fluorescence was observed. The fluorescence was collected on a photomultiplier tube (EMI 9659Q). The signal from the PMT was sent to an oscilloscope and the resulting temporal decay was averaged over 512 shots and stored digitally. For weak bands, a soft focus f =3 lens was used to image the fluorescence onto the PMT. However, for bands with long lifetimes (>1 ls), no optics were used to focus the fluorescence on the PMT in order to prevent excited molecules from moving out of the field of view during emission. The experimental temporal resolution, limited by the PMT and electronics, was 40 ns. Several experiments were performed to minimize systematic errors in recording the lifetimes. The natural lifetimes of several vibronic bands of jet-cooled methoxy have been reported [11,12].
e –X e origin band of methoxy The lifetime of the A was recorded to serve as a test and the measured lifetime was found to be in agreement with the reported one [11] to within 10%. Multiple measurements of the lifetime were made for several alkoxy bands at slightly different frequencies within the line profile and no deviation was observed beyond experimental accuracy. Care was also taken to ensure that there was no collisional quenching by recording lifetimes at different backing pressures of the helium carrier gas. No deviation beyond experimental error were observed for pressures between 40 and 100 psi. In order to ensure that the temporal decay was single exponential, the natural log of the intensity was plotted against time to check for linearity in all measurements. The corresponding experimental data were fit to an exponential decay to obtain the lifetime. The standard error for the lifetime obtained from each of these fits was typically 1% whereas the variation between the lifetimes obtained from the different fits for any given band was typically on the order of 10–15%. We estimate that the absolute error in any of our reported lifetimes is less than 20%.
3. Results 3.1. Methoxy and ethoxy The assignment and natural lifetimes of the bands investigated in ethoxy are shown in Table 1. If we assume that the quantum yield for fluorese state is cence from the vibrationless level of the B unity, then the quantum yield for any vibrationally excited band, m0 , can be given approximately by sm0 =s0 where s is the natural lifetime of the band whose vibrational quantum is given by the subscript and the subscript 0 refers to the vibrationless level of the radical. (This is a reasonable assumption because the natural lifetimes of the origins of the smallest alkoxy radicals, methoxy and that of ethoxy are to within 1%, and the other alkoxies are all within approximately a factor of 2 of this value.) This approximation clearly neglects any small variations in radiative lifetime with vibrational level, such as might be caused by a R-dependent
S. Gopalakrishnan et al. / Chemical Physics Letters 380 (2003) 749–757 Table 1 Measured natural lifetimes (s in ls) of selected vibrational e state ethoxy bands (approximate frequency in cm1 ) of B Frequency 29 181 29 784 30 382 30 974 31 559 32 139
(00 ) (m1 ) (m2 ) (m3 ) (m4 ) (m5 )
s 2.58 2.34 2.15 1.53 0.58 0.13
e state. mn denotes the CO stretch vibration in the B
transition moment, m3 factors, etc. A plot of the ratio sm0 =s0 against the vibrational energy in the excited state is shown in Fig. 1 for methoxy, from previously published data [11,12], and for ethoxy from our present measurements. Since the CO stretch progression dominates the excitation spectrum of all the alkoxy radicals our measurements are almost exclusively for levels with the excess vibrational energy deposited in the CO stretch mode. Referring to Fig. 1, it is clear that for CH3 O the lifetime and hence the approximate quantum yield decreases slightly, but less than 20%, as 5
Fig. 1. Plot of lifetime (sv0 ) of various CO stretch levels divided by that of the vibrationless level (s0 ) vs excess vibrational energy for the methoxy, ethoxy, and propoxy radicals. The solid straight line indicates a value of ðsv0 =s0 Þ ¼ 0:05, which is the approximate quantum yield below which the LIF excitation spectrum could not be detected. Dashed lines for the various radicals indicate no LIF signal could be seen for the next CO stretch excitation, although on the basis of the lower-level observations and expected Franck–Condon factors, one would expect an observable signal.
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quanta (3500 cm1 in energy) of the CO stretch are excited. For 6 quanta there is an order of magnitude drop and our earlier work [11,13] showed the quantum yield is K104 for levels v P 7. The very sharp drop in quantum yield in methoxy has been attributed to an interaction with repulsive states arising from the CH3 + O dissociation limit, similar to the well-known predissociation in the excited vibrational levels of the OH radical. By contrast Fig. 1 shows that C2 H5 O shows a somewhat more pronounced decrease in lifetime between v ¼ 0 and 5 (3000 cm1 of energy) of the CO stretch, with the approximate quantum yield decreasing by over an order of magnitude. However this decrease appears gradual with no sharp discontinuity as is observed with CH3 O. (A continued gradual decrease in the lifetime of C2 H5 O above v ¼ 5 would place those levels below detectability, as was experimentally observed.) 3.2. Propoxy There are two structural isomers for propoxy, 1- and 2-propoxy. While 2-propoxy has a single structure, 1-propoxy can exist in two conformations, gauche (G) and trans (T), with spectral transitions of both of them having been observed and assigned in the jet [5]. (Fig. 2 illustrates schematically the structure of the conformations of all the alkoxy whose lifetimes were measured.) Lifetime measurements were performed on both structural as well as conformational isomers. The assignments and lifetimes of the bands studied are given in Table 2. The results are shown in Fig. 1 for propoxy where sv0 =s0 vs vibrational energy is plotted. The results for 2-propoxy are in fact similar to ethoxy and even to some extent, methoxy. As CO stretch quanta from v ¼ 0–6 (3500 cm1 in energy) are excited there is a gradual decrease in lifetime of greater than a factor of 10, with somewhat surprisingly the largest per quanta drop being a factor of 2 between the v ¼ 0 and 1 states. However the results for the structural isomer 1-propoxy are very different from any of the other radicals Cn H2nþ1 O with n 6 3. Neither conformer has a detectable quantum yield, i.e.,
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Fig. 2. Structural formulae for the conformers of 1-propoxy (a), 1-butoxy (b), and 1-pentoxy (c), for which lifetime measurements were conducted. The corresponding Newman projections are also shown as well as the designation in terms of the dihedral angles at their local minima (G for gauche, T for trans).
K5%, for vCO >1, while all the others fluoresced for vCO up to at least 5 and CH3 O up to vCO ¼ 6. As Fig. 1 shows the quantum yields for both 1-propoxy conformers drop for vCO ¼ 1, but much more precipitously for the G conformer.
3.3. Butoxy There are three structural isomers for butoxy, 1-, 2-, and t-butoxy. t-Butoxy has a single conformation while 1-butoxy has five conformers and 2-butoxy has three. Our high-resolution,
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Table 2 e state propoxy isomers and Measured natural lifetimes (s in ls) of selected vibrational bands (approximate frequency in cm1 ) of the B conformers 2-Propoxy
1-Propoxy G
Frequency 27 171 27 740 28 304 28 863 29 412 29 961 30 504
0
(0 ) (m1 ) (m2 ) (m3 ) (m4 ) (m5 ) (m6 )
s 1.97 1.09 0.830 0.713 0.348 0.167 0.078
T
Frequency 0
28 634 (0 ) 29 230 (m1 )
s 1.57 0.238
Frequency 0
29 219 (0 ) 29 895 (m1 )
s 1.78 1.40
e state. mn denotes the CO stretch vibration in the B
rotationally resolved studies [11] on 1-butoxy revealed the presence of three conformers of 1butoxy in the jet environment and established their identity. We have also obtained similar rotationally resolved spectra for 2-butoxy, although we do not yet have full rotational analyses (and hence the conformational identity) for these bands. However, two distinctly different sets of rotational profiles can be identified that indicate the presence of two different conformers e of 2-butoxy which we excite and measure their B state lifetimes. The plots of sv0 =s0 vs vibrational energy for the three isomers and the various conformers are shown in Fig. 3. The results are again striking. While t-butoxy shows a long progression in the CO stretch (up to v ¼ 6), 1- and 2-butoxy exhibit fluorescence from at most one quantum of excitation in this mode. Whereas in the case of G and T 1-propoxy the lifetimes of the CO stretch vibrational band differed by an order of magnitude, in the case of 1-butoxy, the CO stretch is not even observed for conformers G1 T2 and T1 G2 of 1-butoxy. A single CO stretch excitation band is observed for conformer T1 T2 and has a lifetime comparable to its vibrationless level. These results are all summarized in Table 3. In the case of 2-butoxy, a single CO stretch excitation is observed in the LIF spectrum for both conformers. However there is a substantial difference in lifetime and presumably quantum yield for the two conformers.
Fig. 3. Plot of lifetime (sv0 ) of various CO stretch levels divided by that of the vibrationless level (s0 ) vs excess vibrational energy for the 1-, 2-, and t-isomers and several conformers of the butoxy radical. The solid straight line indicates a value of ðsv0 =s0 Þ ¼ 0:05, which is the approximate quantum yield below which the LIF excitation spectrum could not be detected. Dashed lines for the various radicals indicate no LIF signal could be seen for the next CO stretch excitation, although on the basis of the lower level observations and expected Franck–Condon factors, one would expect an observable signal.
3.4. Pentoxy We have obtained [7] the LIF spectra of 1-, 2-, 3-, and t-pentoxy. Of these four isomers, we have reported the rotationally resolved LIF spectra for just 1-pentoxy [6]. Thus the conformational and vibrational assignments are available only for this isomer and we focus our lifetime measurements on it. Our rotationally resolved spectra established the
Frequency
27 069 27 680
s
2.50 1.78
e state. m denotes the CO stretch vibration in the B
Frequency
26 762 27 321 1.39
s Frequency
29 164 (00 ) 1.10 28 650 (00 )
s
1.38 1.13
Frequency s Frequency
1.97 1.06 1.00 0.994 0.700 0.560 0.384 (00 ) (m1 ) (m2 ) (m3 ) (m4 ) (m5 ) (m6 ) 26 415 26 945 27 480 28 000 28 512 29 020 29 525
s
T1 T2
Frequency
T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 G3 T1 G2 T3 T1 G2 T3 G1 T2 T3
29 096 (00 ) 29 768 (m1 )
Conformer 1 T1 G2
Table 4 Measured natural lifetimes (s in ls) of vibrational levels (approximate frequency in cm1 ) of 1-pentoxy; mCO and mCCO denote CO stretch and CCO torsional vibrations, respectively Conformer
G1 T2
1-Butoxy t-Butoxy
Table 3 e state butoxy Measured natural lifetimes (s in ls) of vibrational bands (approximate frequency in cm1 ) of B
2-Butoxy
Conformer 2
s
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2.43 1.04
754
Frequency 29 013 29 141 29 684 28 987 29 122 29 232 28 644
Assignment 0
0 m1CCO m1CO 00 00 m1CCO 00
s 1.20 1.20 1.10 1.40 1.37 1.07 0.99
presence of 6 of the 14 possible conformers in the jet environment of which five have been assigned in the observed spectra. The lifetimes for selected vibrational bands of these five conformers of 1pentoxy are listed in Table 4. The LIF spectra of all the observed isomers of pentoxy are terminated within K1000 cm1 of the electronic origin, much like the spectra of 1-butoxy, 2-butoxy, and 1-propoxy. An exposition of the natural lifetimes of the various bands of 1-pentoxy is given in Table 4 and shows that the origin bands of all observed conformers have comparable lifetimes. However, only three conformers, namely T1 T2 T3 and T1 T2 G3 and G1 T2 T3 exhibit a single CO stretch excitation band strong enough to rotationally resolve and assign. Of these three conformers, the lifetime of the CO stretch band of conformer G1 T2 T3 is considerably shorter (though not precisely measured) than those of conformer T1 T2 T3 . This indicates dynamical behavior in 1-pentoxy that is conformer dependent just as was observed in 1-propoxy and 1-butoxy.
4. Discussion 4.1. Observed trends With the results for the lifetimes of individual species documented, it is useful to summarize some general trends among the species. 1. In the absence of vibrational excitation the measured lifetimes of all the alkoxy radicals differed by less than a factor of 2.5. We expect that this is a result of the similarity of the transition moment in all the species, with the small varia-
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tions depending upon a number of small details of the electronic structure. 2. With the exception of the abrupt decrease at v P 6 in methoxy, molecules that fluorescence from CO stretch levels v > 1 show only a gradual decrease in lifetime as the vibrational energy is increased. This gradual decrease seems slightly more pronounced for the longer alkyl chains. 3. Structural isomers with only a single conformer always have observable fluorescence with roughly comparable lifetimes for all vibrational levels with 65 quanta of CO stretch energy. Structural isomers that have more than one conformer in no case had detectable fluorescence from levels with v > 1 in the CO stretch. 4. For the species with multiple conformers, lifetimes decreased most rapidly for the species without a plane of symmetry. The Cs conformers (T, T1 T2 , and T1 T2 T3 ) show only slightly shorter lifetimes (compared to the vibrationless level) with 1 quanta of CO stretch excitation,
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while lower symmetry species showed dramatically shorter vCO ¼ 1 lifetimes or no observable fluorescence at all for vCO ¼ 1 or higher. Indeed for multiple conformer species no fluorescence was observed above vCO ¼ 1 for any conformers even those with Cs symmetry. 4.2. Possible mechanisms It is possible to explain several, but not all, of the general trends of the previous section by reference to Fig. 4. This figure schematically represents the energetics of the various alkoxy radicals e state dissociation limit to R þ Oð3 PÞ. near the X e state and the reIt illustrates both the bound X pulsive states going to this limit. It also shows the e state is CH3 O since its X e state (formally the A e B state is degenerate) which has a corresponding R þ Oð1 DÞ dissociation limit. Long ago it was established that the first excited state of the OH radical was pre-dissociated above
e and X e potential energy surfaces of the alkoxy radical. The Fig. 4. Schematic cut along the C–O bond coordinate, through the B e state curve for CH3 O is shown but only the position of the B e state vibrationless level for the higher alkoxies. (The state complete B e for methoxy because of the degeneracy of its X e for the higher alkoxies but usually is denoted A e state.) The depicted is always labelled B e state vibrationless level is determined in the following manner with respect to the R þ Oð3 PÞ dissociation limit. The position of the B e state vibrationless level with respect to this limit is determined by B3LYP DFT quantum chemistry claculations. The B e position of the X e state level the experimental value of T00 from the B e –X e spectra. state vibrationless level is then determined by adding to X
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v ¼ 1, and it has long been assumed that the sudden onset of a non-radiative decay pathway in CH3 O was attributable to the same mechanism. Recent photofragmentation experiments by Neumark and co-workers [14,15] certainly confirm this assumption. Based upon their photofragmentation and our lifetime experiments [11,13] the conical e and repulsive states of intersection involving the A CH3 O must occur between the energies of 5 and 6 quanta of CO stretch excitation. At vCO ¼ 6 and above the non-radiative channel predominates the decay. When we began correlating the data from the present lifetime measurements of the larger alkoxies, we first thought the quenching of the quantum yield might be due to a similar mechanism with the details of the onset of the nonradiative decay channel strongly dependent upon the exact location of the corresponding conical intersections with the repulsive states. However Fig. 4 clearly shows this not to be the case. As the alkoxy radicals become larger the vibrationless e state sinks well below the dissocialevel of the B e state and correspondingly the tion limit of the X repulsive curves from that limit are far above the energetic threshold of the observed non-radiative behavior. Physically this is because the larger (and more branched) alkyl groups tend to stabilize the e state, compared to highly polar C–O bond in the B e state. This the much less polar bond in the X finding is consistent with the recent observations by Choi et al. [16] that dissociation does not occur e state of ethoxy as it does from the from the B corresponding state in methoxy. While Fig. 4 rules out the explanation involving the conical intersection with the repulsive state, for e state of the the non-radiative behavior of the B larger alkoxies, it also suggests an alternative explanation. All the levels investigated in this study lie within the dense manifold of the vibrational e state near its R þ Oð3 PÞ dissocialevels of the X tion limit. Indeed other lower dissociation limits, e.g., to R@O þ H, create a true continuum of levels in this energy region. We therefore suggest that the key mechanism for non-radiative decay is internal conversion to the ground state surface, with possibly subsequent isomerization or dissociation thereupon. This
mechanism is consistent [1] with several of the general trends noted above. There is a large elece and B e states of tronic energy gap between the X the alkoxy radicals consistent with a slow (>1 ls) non-radiative decay of the vibrationless level of all the radicals. Such a mechanism is also consistent with the gradual increase in the non-radiative decay as CO stretch vibrational energy is added to e state of C2 H5 O, 2-C3 H7 O, and t-butoxy. It is the B generally accepted that so long as one has a dense manifold of accepting levels, then the internal conversion rate will increase as the electronic gap (E0 ) decreases. Furthermore it has been found [1] that vibrational excitation of the bright (in this e ) state, tends to decrease the effective eleccase B tronic gap, E00 , roughly according to E00 ¼ E0 nx where n is the number of excited quanta of a given mode with frequency x. There are however several aspects of the general trends which the internal conversion model, as presently formulated, does not fully account. One way of categorizing the behavior of the decay dye state is to note that the radicals, namics of the B CH3 O, C2 H5 O, 2-C3 H7 O, t-C4 H9 O, that seem relatively resistant to non-radiative decay have several characteristics in common. They all have only one conformer and possess some symmetry (at least a reflection plane). For the alkoxy species where multiple structural isomers exist the nonradiative-decay resistant species obviously have the same number of degrees of vibrational freedom as do the single conformer species, but substitute in the latter methyl torsions for the C–C–C or C–C–O torsions present in the other conformers. It is also worth noting that all the trans (or at least local to the oxygen atom trans) species, that have a plane of symmetry (or at least an approximate one) show little if any non-radiative decay in the vCO ¼ 1 levels, while non-radiative decay in the other species is the predominant decay mode for vCO > 0. Of course when vCO J 2 is excited in all the trans species, non-radiative decay again becomes predominant. It is extremely interesting to note that we have preliminary calculations for the e state barrier for conversion of trans to gauche B conformers. Its height is between the energy of 1 and 2 CO stretch quanta. Thus for vCO P 2, the molecule has sufficient energy to lie above the
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barrier where unique gauche or trans configurations no longer exist. It is fairly clear then that either the decrease in symmetry and/or the increase in flexibility of molecules with multiple conformers increases the rate of internal conversion, presumably by increasing the coupling matrix element between the e and X e states. Such an increase might be qualiB tatively expected as the coupling operator is generally thought to be the kinetic energy terms neglected in the Born–Oppenheimer approximation. The torsional degree of freedom inherent in multiple-conformer species may well lead to a large Coriolis contribution to this kinetic energy coupling. While qualitatively similar observations have been made with acetaldehyde where large differences in radiationless decay rates have been noted [17] between e and a torsional levels, the details of the alkoxy behavior are quite different. Clearly, a quantitative correlation between theory and experiment requires more work for the alkoxy radicals.
5. Conclusions Detailed measurement of the temporal decay of e state of the alkoxy radthe fluorescence of the B icals have been performed. These measurements have demonstrated a significant lifetime shortening due to the deposition of an excess of vibrational energy, presumably indicative of the opening of competitive non-radiative decay channels. These lifetime variations show a surprising dependence on the molecular geometry, i.e., structural isomer and conformer, for radicals of the same empirical formula and hence the same number of nuclear degrees of freedom. Alkoxy radical isomers with only a single conformer seem relatively resistant to non-radiative decay compared to isomers with multiple conformers. Of the latter group, all trans conformers seem most resistant to non-radiative decay. It is argued that the present observations of non-radiative decay in the larger alkoxy radicals are not consistent with the well-known predissociation known in higher vibrational levels of CH3 O. Rather the presently observed behavior is
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most consistent with internal conversion to the e state. Apparently low frequency torsional X motions in species without Cs symmetry are efficient promoters of the internal conversion, although a detailed mechanism is not yet apparent.
Acknowledgements The authors gratefully acknowledge the financial support by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy, via Grant DE-FG02-01ER15172.
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