- Email: [email protected]
Photoionization, fragmentation and proton transfer in 1-naphthol(NH3)n clusters
8 October 1999
Chemical Physics Letters 311 Ž1999. 439–445 www.elsevier.nlrlocatercplett
Photoionization, fragmentation and proton transfer in 1-naphthol žNH 3 /n clusters R. Knochenmuss
)
LOC, UniÕersitatsstrasse 16, ETH Zentrum, CH-8092 Zurich, Switzerland ¨ ¨ Received 28 May 1999; in final form 8 July 1999
Abstract The fragmentation of photoionized 1-naphtholP ŽNH 3 . n clusters is examined. In one-color resonant two-photon ionization experiments, fragmentation in the ion state is observed for every cluster size above n s 1. For small clusters Ž n F 4., a single neutral ammonia is evaporated in the ion state. This ceases at energies 1500–2500 cmy1 above the ionization thresholds. Larger clusters, n G 5, are readily ionized at much lower energies than n F 4. They yield Ž1-naphtholP ŽNH 3 . n .q, 1-naphtholP ŽNH 3 . n Hq, and ŽNH 3 . n Hq ion fragments. Several neutral ammonia molecules can be lost. These observations support predictions that n G 5 clusters undergo proton transfer in the ground state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Clusters of 1-naphthol with ammonia have been particularly important in the study of solvent-controlled gas-phase excited state proton transfer ŽESPT. reactions. This system was used to show that such reactions can be induced in the gas phase, in a manner analogous to bulk solution w1,2x. Early studies used resonant two-photon ionization ŽR2PI. and mass spectrometry to measure the S 1 § S 0 spectra of individual clusters. This knowledge enabled selective excitation of 1-naphtholP ŽNH 3 . n clusters. For n s 1–3, narrow line emission was observed, very similar to that of free naphthol. For n s 4, broad, redshifted naphtholate ESPT emission was found. This result is consistent with the n s 4 ESPT threshold ) Fax: q41-1-632-1292; e-mail: [email protected]
found for the closely related phenolP ŽNH 3 . n system w3x, and 2-naphtholP ŽNH 3 . n w4x. In a subsequent study of ammonia clusters of 1-naphthol, Kim, Li and Bernstein ŽKLB. w5x used filtered fluorescence to suggest that one conformer of n s 3 is ESPT active. It was later shown that no n s 3 species exhibits naphtholate emission, and that the small clusters have a broader spectrum than earlier realized w6x. In two-color R2PI spectra of n s 3 ammonia clusters, one peak was more prominent at low ionization energy, corresponding to the special conformer from fluorescence spectroscopy. This was taken to support pump–probe 2-photon ionization experiments w7–10x in which picosecond transients were observed for n s 3 and 4. ESPT was the explanation given for the pump–probe decay in both clusters. Both the existence of a low ionization potential n s 3 conformer and the n s 3 ESPT interpretation
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 7 9 - 9
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of the pump–probe results have recently been called into question w6,11,12x. At the same time, arguments have been renewed for ESPT in a non-fluorescent conformer of n s 3 w13x. These rely largely on pump–probe data and require that cluster fragmentation be insignificant. These varying interpretations showed the need for a clear picture of the ionization and fragmentation behavior of 1-naphtholP ŽNH 3 . n clusters. This is particularly important for interpreting the pump–probe data, since this can be dramatically affected by ion state fragmentation. While isolated aspects of naphthol–ammonia cluster fragmentation have been or will be noted elsewhere w11,12x, the present work gives the first comprehensive view of this topic. In particular, the n ) 4 clusters, where proton transfer is believed to occur in the ground state, have received little attention to date, and are found to have remarkable properties.
2. Experimental The experimental method was similar to that reported in earlier studies w2,6x. 1-naphthol was entrained in a stream of neon carrier gas and 1% anhydrous NH 3 at a partial pressure of - 3 mbar, the total stagnation pressure was 1–2 bar. This mixture was expanded through a pulsed valve into a vacuum. After skimming, the cooled molecules were intercepted with unfocused nanosecond pulses from frequency doubled pulsed, Nd:YAG-pumped dye lasers ŽLambda Physik FL-2002 and FL-2001 with added amplifier stage.. The resulting ions were mass analyzed in a 1 m linear time-of-flight mass spectrometer. The acceleration voltage used was 5 kV, and the ions were detected with a micro-sphere plate ŽEl-Mul Technologies, Israel.. The laser was scanned while observing the relevant mass peak with a transient digitizer. Two-color two-photon ionization experiments were performed by attenuating the pump and ionization lasers until 1-color signals were absent or very nearly so. Nevertheless, to avoid potential systematic error, a 1-color spectrum was measured after every 2-color scan. The ionization laser was blocked, the scan repeated and subtracted from the 2-color result. For comparison of efficiencies at different ionization
energies, the laser pulse energies were measured with a pyroelectric detector, and adjusted to identical values for each set of wavelengths. Care was taken to separate the UV from residual visible or near-IR light by using a dispersing prism. Yields were measured only by analysis of 2-photon ionization spectra, not by scanning the ionization laser. This allowed the evaluation and control of fragmentation contributions, which can enter unnoticed into ionization laser scans.
3. Results and discussion 3.1. 1-naphtholP (NH3 )n , n s 1 The 1-naphthol P ŽNH 3 . n n s 1 cluster has a ground state binding energy of 2680 " 5 cmy1 w14x. The S 1 binding energy was estimated to be ; 9% higher: 2916 " 5 cmy1 . It is stable in a 1-color R2PI experiment, as seen in Fig. 1a. No features are found in the n s 0 mass channel that correspond to the n s 1 spectrum. Assuming an ionization threshold of 58 000 cmy1 as reported by KLB, ; 4000 cmy1 of excess energy are deposited in the ion state. As will be shown below, this is enough to extensively fragment larger cluster ions. This ion is probably stabilized by ion–dipole interactions, which raise the binding energy at least 1000 cmy1 above that of the S 1 , and so above the energy deposited by the laser w15x. 3.2. n s 2 Two strong peaks in the n s 2 R2PI spectrum have been identified as electronic origins of different conformers w1x. Of these, only the one at 31 184 cmy1 fragments to n s 1 in a 1-color experiment, the other is stable. The process is evaporation of a neutral ammonia molecule after ionization: 2 hn
naphtholP Ž NH 3 . 2 ™ Ž naphtholP Ž NH 3 . 2 .
q
q
™ Ž naphtholP Ž NH 3 . . q NH 3 . That the evaporation occurs in the ion state is proven by its dependence on the ionization energy. At sufficiently low excess energies in the ion state, the process is stopped, as seen in Fig. 1b.
R. Knochenmussr Chemical Physics Letters 311 (1999) 439–445
Linear and cyclic hydrogen bonding topologies are the most likely structural possibilities for the two conformers. With two hydrogen bonds per ammonia, the cyclic structure should be more stable, corresponding to the stronger, non-fragmenting peak at 31 103 cmy1 . The analogous cyclic 1-naphtholP ŽH 2 O. 2 cluster has been identified by rotational co-
441
herence w16x and IR dip w17x spectroscopies, and is the dominant conformer in that case. Once again, the ion state excess energy in a 1-color experiment is around 4000 cmy1 , yet only the non-cyclic conformer fragments substantially Žapproximate yield s 50%, yield of the cyclic conformer - 2%.. This appears consistent with the relatively low binding energy of the ammonia dimer, which has been measured to be - 970 cmy1 w18x. Without the second hydrogen bond, the second ammonia can be evaporated at lower excess energy. 3.3. n s 3 At least two n s 3 conformers exist. The conformer with an origin at 31 096 cmy1 is the most stable and is observed under all conditions where n s 3 appears, see Fig. 1 of Ref. w6x. The group of several bands extending slightly to higher energy than the n s 2 origin at 31 103 cmy1 are all probably due to this conformer, as shown by preliminary fluorescence studies. Again, by analogy with water clusters of the same size w17x, this species may have a cyclic structure. Under slightly warmer expansion conditions, as in Fig. 1a, a group of three new lines is observed at 31 132, 31 139 and 31 155 cmy1 , belonging to at least one other conformer. The low-energy conformer has an ionization threshold near 58 000 cmy1 w11x. The spectra of Fig. 1 show that all n s 3 species undergo efficient 1color ion state fragmentation by loss of one ammonia. The lower-energy conformers fragment in a 1-color experiment with a yield of ; 50%. The higher-energy conformer fragments more efficiently,
Fig. 1. Mass-selective resonant two-photon ionization spectra of 1-naphtholPŽNH 3 . n clusters. The spectra are plotted on the same vertical scale. Ža. The same laser was used for excitation and ionization. The total ionization energy is therefore twice the values on the horizontal axis. The ns 2, 3 and 4 clusters all fragment to the next smaller size, as is seen from the corresponding spectral structure. Žb. The excitation laser was scanned over the indicated range, and was attenuated sufficiently that it did not generate any signal by itself. A second laser was used to ionize the clusters, at 28 100 cmy1 . The total ionization energy in the range of the sharp line structure was therefore near 59 200 cmy1 . At this total energy, the fragmentation processes of Ža. are either absent or very weak.
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the yield is up to 75%. Fragmentation is found to decrease steadily as the total ionization energy is reduced. At 59 400 cmy1 total 2-photon energy, the n s 3 peaks are only barely detectable in the n s 2 mass channel w11x. This represents ; 1500 cmy1 of excess energy in the ion state. 3.4. n s 4 A very low ionization threshold for n s 4 of ; 56 000 cmy1 has been reported w5x. This is 2000 cmy1 below that of n s 1, 2 or 3, and appears to be correlated with the appearance of ESPT. This is as expected, since the ionization energy of the naphtholate anion should be lower than that of the neutral. The ionization energy of free phenoxide anion is only 2.25 eV or 18 150 cmy1 w19x, for example. Photoionization of the proton-transferred cluster zwitterion leaves behind a positive Žproton-centered. ion rather than a neutral. Because of the increased Coulomb energy, the ionization energy of the zwitterion will be higher than that of phenoxide, but still less than before proton transfer. While it certainly lies below that of n s 3, the n s 4 threshold is difficult to measure reliably due to fragmentation from n ) 4. An improved evaluation of the n s 4 ionization threshold may be obtained by analyzing the pulse energy corrected 2-color R2PI spectra. Complete spectra enable more accurate measurements since 1-color contributions can be subtracted, and a direct estimation of n ) 4 fragmentation is possible on the basis of spectral structure. A generally linear drop in n s 4 ionization efficiency was found from above 62 000 cmy1 to below 58 000 cmy1 . This extrapolates to a true threshold of ; 56 900 cmy1 . Notably, the earlier data show a distinct step at 57 000 cmy1 , below this point that curve appears to reflect n s 5 fragmentation. As seen in Fig. 1, n s 4 fragments to n s 3. In Fig. 1a, the n s 3 channel shows a broad structure that correlates very well with the n s 4 spectrum. This broad component of the n s 3 spectrum can be varied vs. the sharp lines by changing either the excess energy in the ion state, as in Fig. 1b, or by changing the relative amounts of n s 3 and 4 in the expansion as in Ref. w11x. The 4 ™ 3 yield is, in a 1-color experiment, 30–40%. As in the smaller clusters, the yield drops steadily at lower ion state en-
ergy, and is near zero at 58 000 cmy1 , or ; 1000 cmy1 above the n s 4 ionization threshold w11x. 3.5. n G 5 As shown in Ref. w1x for n s 5, clusters larger than n s 4 have broad absorptions that are further red-shifted vs. the smaller clusters. On the basis of thermodynamic estimates, it has been predicted that these clusters undergo naphthol-to-ammonia proton transfer in the electronic ground state w1,2,9x, because the basicity of the ammonia aggregate increases with size. In the related phenol–ammonia clusters, ground state proton transfer appears to occur at n s 6 w20x. The increased red-shift of the absorption spectra would be consistent with such a difference in cluster behavior above n s 4. The ionization and fragmentation behavior is also qualitatively different for these clusters. As seen in Fig. 2, they are readily ionized in a 1-color experiment at very long wavelengths. Difficulties in reaching the necessary combinations of wavelengths made accurate 2-color ionization threshold measurements impractical. These are in preparation, currently it can only be said that either the thresholds for n G 5 are below 54 000 cmy1 or 3-photon processes are far more efficient than in smaller clusters. For comparison, free trans 1-naphthol has a sharp ionization threshold of 62 610 cmy1 . The vertical 2-photon ionization threshold of 1-naphtholate in the large clusters has apparently been depressed by ) 1 eV vs. free 1-naphthol. Fragmentation of these clusters is far more extensive than for n - 5. The clusters n s 0, 1, 2, 3 and 4 are all formed by ion state loss of neutral ŽNH 3 . m from n G 5, as seen in Fig. 2. The fragment distribution of the smaller clusters is unusual; n s 4 and n s 1 are preferred products, while n s 3 is the least efficiently formed. Most of the mass spectra in Fig. 2 were taken at photon energies below the absorption band of n s 4, which ends at ; 30 500 cmy1 . The strong n s 4 signal is then due to fragmentation of n G 5, as also shown by the distinct tailing of the mass peak in the expanded part of the 29 520 cmy1 spectrum of Fig. 2a. The 2-photon energy where 1-color n G 5 fragmentation becomes weak is near 56 000 cmy1 .
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Further important evidence for ground state proton transfer in n G 5, is the observation of abundant protonated fragments. For all n G 5 in Fig. 2, 1naphtholP ŽNH 3 . m Hq clusters are observed in large quantities, as seen in the insets of Fig. 2. Sometimes these are as abundant than the non-protonated radical
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cations. From the tailing of the mass peaks, and the protonated fragments it is clear that these clusters fragment extensively, and it is difficult to ascertain parent–daughter relationships or yields. In some cases the ŽM q 2H.q signals appear to be stronger than expected from the natural 13 C abundance. The mechanism leading to such ions may involve hydrogen abstraction by the naphthol radical cation prior to fragmentation. The charge and the unpaired electron are then on separate fragments, as is relatively common in electron impact mass spectrometry. Also very revealing is the observation of ŽNH 3 . m Hq fragments of n ) 5 clusters, as shown in Fig. 3. These are not formed efficiently, and are only observed when exciting large quantities of n G 5, but they also suggest ground state proton transfer. This behavior is in contrast to extensive ŽNH 3 . m Hq production from phenol–ammonia clusters w21,22x. This difference is likely due to the lower ionization energy of the naphthol clusters Ž; 1 eV., which makes the asymmetric fragmentation channel energetically uncompetetive for all but the larger clusters. 3.6. Implications of 1-naphtholP (NH3 )n cluster fragmentation The possibility of ion state fragmentation was not significantly considered in the interpretation of picoand femtosecond pump–probe experiments on 1naphtholP ŽNH 3 . n clusters w7–10x. The data presented here demonstrate that the pump–probe data must be critically reexamined in the light of fragmentation processes.
Fig. 2. One-color two-photon ionization mass spectra of 1-naphtholPŽNH 3 . n clusters. The mass spectra are scaled to have about the same maximum intensity. The photon energies are indicated to the right of each spectrum in wavenumbers, the total ionization energy is twice this value. Ža. The top insert shows the strong 1-naphtholPŽNH 3 . n Hq ions which result from fragmentation in the ion state of larger clusters. The lower insert shows the ns 4 peak which results from excitation and ionization with a photon energy below the lowest ns 4 absorption band. The tailing to longer flight times is an indication that this ion is a fragment of larger clusters, as is the case for ns 0, 1, 2 and small amounts of ns 3. Žb. The series of Ža. is continued to lower photon energies. The fragmentation of n) 4 into n- 4 mass channels becomes less important at the lowest energies. The insert shows the decrease in 1-naphtholPŽNH 3 . n Hq fragment ion yield.
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this does not show that the decay is determined by proton transfer. Any process in the excited state which alters the Franck–Condon factors to the ion state will lead to such a signal. In addition, the present results show that dynamics in larger clusters will be projected into the lower mass channels by fragmentation, substantially complicating the interpretation of the observed time scales. Furthermore, the pump–probe signal in any given mass channel can be strongly modified or even negated by the superposition of ionic and neutral evaporation events, as found in Ref. w23x for ammonia clusters of NaI. 4. Conclusions
Fig. 3. Partial one-color two-photon ionization mass spectra of 1-naphtholPŽNH 3 . n clusters, at a photon energy of 29 100 cmy1 . The higher mass part of the spectrum is as in the lower spectrum of Fig. 2a. Small amounts of ŽNH 3 . n Hq ions are produced as fragments of ionized n) 4 clusters. Note also the appearance of metastable fragments above the ms 4 cluster, leading to a broad ion signal extending to an apparent m r z of 140.
Most importantly, n s 4 fragmentation in the ion state will project any n s 4 excited state dynamics into the n s 3 channel. The 2-photon ionization energies reported in Refs. w7–9x were in the range of 60 100–60 600 cmy1 , where n s 4 to n s 3 fragmentation is 15–20%. If the n s 4 ion yield decreases with pump–probe delay, the corresponding 15–20% ion state fragmentation contribution to the n s 3 signal will also decrease with the same time dependence. Due to the wide n s 4 absorption spectrum vs. the few sharp n s 3 lines, n s 4 fragments can dominate the intrinsic n s 3 signal under broadband excitation conditions w11x. This effect has recently been confirmed by pump–probe data showing the absence of fast decaying n s 3 pump–probe signal at low ionization energy w12x. This result is satisfying in that pump–probe data are then no longer in conflict with the spectroscopy, they indicate ESPT only at n s 4, not n s 3. A conclusion of Refs. w7–9x was that the pump– probe data revealed the time scale of ESPT. Although a fast decay component first appears at n s 4, where spectroscopy has found the ESPT threshold,
Fragmentation in the ion state is observed for every 1-naphtholP ŽNH 3 . n cluster size above n s 1. For the cluster sizes n s 2, 3 and 4, the only observed fragmentation pathway is loss of a single neutral ammonia in the ion state. This process ceases at energies 1500–2500 cmy1 above the ionization thresholds. The one-color fragment yield is typically around 50%. Larger clusters, n ) 4, are readily ionized at remarkably long wavelengths. They fragment more extensively than the smaller clusters, yielding three types of products: Ž1-naphtholP ŽNH 3 . n .q, 1-naphthol P ŽNH 3 . m Hq, and ŽNH 3 . m Hq clusters. In the case of the first group, more than one neutral ammonia can be lost, and products range to free naphthol. These observations support predictions that these clusters undergo proton transfer in the ground state. Earlier pump–probe studies of 1-naphthol P ŽNH 3 . n clusters must be reinterpreted in light of these data. Consistent with spectroscopic studies, there is no evidence for ESPT in n s 3. The pump– probe dynamics are probably only indirectly relevant to the ESPT time scale in the n s 4 cluster. References w1x O. Cheshnovsky, S. Leutwyler, J. Chem. Phys. 88 Ž1988. 4127. w2x R. Knochenmuss, O. Cheshnovsky, S. Leutwyler, Chem. Phys. Lett. 144 Ž1988. 317. w3x C. Jouvet, C. Lardeux-Dedonder, M. Richard-Viard, D. Solgadi, A. Tramer, J. Phys. Chem. 94 Ž1990. 5041. w4x T. Droz, R. Knochenmuss, S. Leutwyler, J. Chem. Phys. 93 Ž1990. 4520.
R. Knochenmussr Chemical Physics Letters 311 (1999) 439–445 w5x S.K. Kim, S. Li, E.R. Bernstein, J. Chem. Phys. 95 Ž1991. 3119. w6x R. Knochenmuss, Chem. Phys. Lett. 293 Ž1998. 191. w7x J.J. Breen, L.W. Peng, D.M. Willberg, A. Heikal, P. Cong, A.H. Zewail, J. Chem. Phys. 92 Ž1990. 805. w8x M.F. Hineman, G.A. Bruker, D.F. Kelley, E.R. Bernstein, J. Chem. Phys. 97 Ž1992. 3341. w9x S.K. Kim, J.J. Breen, D.M. Willberg, L.W. Peng, A. Heikal, J.A. Syage, A.H. Zewail, J. Phys. Chem. 99 Ž1995. 7421. w10x J.A. Syage, J. Phys. Chem. 99 Ž1995. 5772. w11x R. Knochenmuss, Chem. Phys. Lett. 305 Ž1999. 233. w12x R. Knochenmuss, I. Fischer, D. Luhrs, Israel. J. Chem. ¨ Ž1999., accepted. w13x D.F. Kelley, E.R. Bernstein, Chem. Phys. Lett. 305 Ž1999. 230. w14x T. Burgi, T. Droz, S. Leutwyler, Chem. Phys. Lett. 246 ¨ Ž1995. 291.
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w15x J. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1991. w16x L.L. Connel, S. Ohline, P.W. Joireman, T.C. Corcoran, P.M. Felker, J. Chem. Phys. 94 Ž1991. 4668. w17x R. Yoshino, K. Hashimoto, T. Omi, S. Ishiuchi, M. Fujii, J. Phys. Chem. A 102 Ž1998. 6227. w18x F. Huisken, T. Pertsch, Chem. Phys. 126 Ž1988. 213. w19x J.E. Bartmess, in: M. W.G., S.E. Stein ŽEds.., Negative Ion Energetics Data, NIST ŽNatl. Inst. Stand. Technol.., Gaithersburg, MD, 1998. w20x C. Jouvet, personal communication. w21x J.A. Syage, J. Steadman, J. Chem. Phys. 95 Ž1991. 2497. w22x J. Steadman, J.A. Syage, J. Chem. Phys. 92 Ž1990. 4630. w23x G. Gregoire, M. Mons, I. Dimicoli, C. Dedonder-Lardeux, C. ´ Jouvet, S. Martrenchard, D. Solgadi, J. Chem. Phys. 110 Ž1999. 1521.
Chemical Physics Letters 311 Ž1999. 439–445 www.elsevier.nlrlocatercplett
Photoionization, fragmentation and proton transfer in 1-naphthol žNH 3 /n clusters R. Knochenmuss
)
LOC, UniÕersitatsstrasse 16, ETH Zentrum, CH-8092 Zurich, Switzerland ¨ ¨ Received 28 May 1999; in final form 8 July 1999
Abstract The fragmentation of photoionized 1-naphtholP ŽNH 3 . n clusters is examined. In one-color resonant two-photon ionization experiments, fragmentation in the ion state is observed for every cluster size above n s 1. For small clusters Ž n F 4., a single neutral ammonia is evaporated in the ion state. This ceases at energies 1500–2500 cmy1 above the ionization thresholds. Larger clusters, n G 5, are readily ionized at much lower energies than n F 4. They yield Ž1-naphtholP ŽNH 3 . n .q, 1-naphtholP ŽNH 3 . n Hq, and ŽNH 3 . n Hq ion fragments. Several neutral ammonia molecules can be lost. These observations support predictions that n G 5 clusters undergo proton transfer in the ground state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Clusters of 1-naphthol with ammonia have been particularly important in the study of solvent-controlled gas-phase excited state proton transfer ŽESPT. reactions. This system was used to show that such reactions can be induced in the gas phase, in a manner analogous to bulk solution w1,2x. Early studies used resonant two-photon ionization ŽR2PI. and mass spectrometry to measure the S 1 § S 0 spectra of individual clusters. This knowledge enabled selective excitation of 1-naphtholP ŽNH 3 . n clusters. For n s 1–3, narrow line emission was observed, very similar to that of free naphthol. For n s 4, broad, redshifted naphtholate ESPT emission was found. This result is consistent with the n s 4 ESPT threshold ) Fax: q41-1-632-1292; e-mail: [email protected]
found for the closely related phenolP ŽNH 3 . n system w3x, and 2-naphtholP ŽNH 3 . n w4x. In a subsequent study of ammonia clusters of 1-naphthol, Kim, Li and Bernstein ŽKLB. w5x used filtered fluorescence to suggest that one conformer of n s 3 is ESPT active. It was later shown that no n s 3 species exhibits naphtholate emission, and that the small clusters have a broader spectrum than earlier realized w6x. In two-color R2PI spectra of n s 3 ammonia clusters, one peak was more prominent at low ionization energy, corresponding to the special conformer from fluorescence spectroscopy. This was taken to support pump–probe 2-photon ionization experiments w7–10x in which picosecond transients were observed for n s 3 and 4. ESPT was the explanation given for the pump–probe decay in both clusters. Both the existence of a low ionization potential n s 3 conformer and the n s 3 ESPT interpretation
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 8 7 9 - 9
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of the pump–probe results have recently been called into question w6,11,12x. At the same time, arguments have been renewed for ESPT in a non-fluorescent conformer of n s 3 w13x. These rely largely on pump–probe data and require that cluster fragmentation be insignificant. These varying interpretations showed the need for a clear picture of the ionization and fragmentation behavior of 1-naphtholP ŽNH 3 . n clusters. This is particularly important for interpreting the pump–probe data, since this can be dramatically affected by ion state fragmentation. While isolated aspects of naphthol–ammonia cluster fragmentation have been or will be noted elsewhere w11,12x, the present work gives the first comprehensive view of this topic. In particular, the n ) 4 clusters, where proton transfer is believed to occur in the ground state, have received little attention to date, and are found to have remarkable properties.
2. Experimental The experimental method was similar to that reported in earlier studies w2,6x. 1-naphthol was entrained in a stream of neon carrier gas and 1% anhydrous NH 3 at a partial pressure of - 3 mbar, the total stagnation pressure was 1–2 bar. This mixture was expanded through a pulsed valve into a vacuum. After skimming, the cooled molecules were intercepted with unfocused nanosecond pulses from frequency doubled pulsed, Nd:YAG-pumped dye lasers ŽLambda Physik FL-2002 and FL-2001 with added amplifier stage.. The resulting ions were mass analyzed in a 1 m linear time-of-flight mass spectrometer. The acceleration voltage used was 5 kV, and the ions were detected with a micro-sphere plate ŽEl-Mul Technologies, Israel.. The laser was scanned while observing the relevant mass peak with a transient digitizer. Two-color two-photon ionization experiments were performed by attenuating the pump and ionization lasers until 1-color signals were absent or very nearly so. Nevertheless, to avoid potential systematic error, a 1-color spectrum was measured after every 2-color scan. The ionization laser was blocked, the scan repeated and subtracted from the 2-color result. For comparison of efficiencies at different ionization
energies, the laser pulse energies were measured with a pyroelectric detector, and adjusted to identical values for each set of wavelengths. Care was taken to separate the UV from residual visible or near-IR light by using a dispersing prism. Yields were measured only by analysis of 2-photon ionization spectra, not by scanning the ionization laser. This allowed the evaluation and control of fragmentation contributions, which can enter unnoticed into ionization laser scans.
3. Results and discussion 3.1. 1-naphtholP (NH3 )n , n s 1 The 1-naphthol P ŽNH 3 . n n s 1 cluster has a ground state binding energy of 2680 " 5 cmy1 w14x. The S 1 binding energy was estimated to be ; 9% higher: 2916 " 5 cmy1 . It is stable in a 1-color R2PI experiment, as seen in Fig. 1a. No features are found in the n s 0 mass channel that correspond to the n s 1 spectrum. Assuming an ionization threshold of 58 000 cmy1 as reported by KLB, ; 4000 cmy1 of excess energy are deposited in the ion state. As will be shown below, this is enough to extensively fragment larger cluster ions. This ion is probably stabilized by ion–dipole interactions, which raise the binding energy at least 1000 cmy1 above that of the S 1 , and so above the energy deposited by the laser w15x. 3.2. n s 2 Two strong peaks in the n s 2 R2PI spectrum have been identified as electronic origins of different conformers w1x. Of these, only the one at 31 184 cmy1 fragments to n s 1 in a 1-color experiment, the other is stable. The process is evaporation of a neutral ammonia molecule after ionization: 2 hn
naphtholP Ž NH 3 . 2 ™ Ž naphtholP Ž NH 3 . 2 .
q
q
™ Ž naphtholP Ž NH 3 . . q NH 3 . That the evaporation occurs in the ion state is proven by its dependence on the ionization energy. At sufficiently low excess energies in the ion state, the process is stopped, as seen in Fig. 1b.
R. Knochenmussr Chemical Physics Letters 311 (1999) 439–445
Linear and cyclic hydrogen bonding topologies are the most likely structural possibilities for the two conformers. With two hydrogen bonds per ammonia, the cyclic structure should be more stable, corresponding to the stronger, non-fragmenting peak at 31 103 cmy1 . The analogous cyclic 1-naphtholP ŽH 2 O. 2 cluster has been identified by rotational co-
441
herence w16x and IR dip w17x spectroscopies, and is the dominant conformer in that case. Once again, the ion state excess energy in a 1-color experiment is around 4000 cmy1 , yet only the non-cyclic conformer fragments substantially Žapproximate yield s 50%, yield of the cyclic conformer - 2%.. This appears consistent with the relatively low binding energy of the ammonia dimer, which has been measured to be - 970 cmy1 w18x. Without the second hydrogen bond, the second ammonia can be evaporated at lower excess energy. 3.3. n s 3 At least two n s 3 conformers exist. The conformer with an origin at 31 096 cmy1 is the most stable and is observed under all conditions where n s 3 appears, see Fig. 1 of Ref. w6x. The group of several bands extending slightly to higher energy than the n s 2 origin at 31 103 cmy1 are all probably due to this conformer, as shown by preliminary fluorescence studies. Again, by analogy with water clusters of the same size w17x, this species may have a cyclic structure. Under slightly warmer expansion conditions, as in Fig. 1a, a group of three new lines is observed at 31 132, 31 139 and 31 155 cmy1 , belonging to at least one other conformer. The low-energy conformer has an ionization threshold near 58 000 cmy1 w11x. The spectra of Fig. 1 show that all n s 3 species undergo efficient 1color ion state fragmentation by loss of one ammonia. The lower-energy conformers fragment in a 1-color experiment with a yield of ; 50%. The higher-energy conformer fragments more efficiently,
Fig. 1. Mass-selective resonant two-photon ionization spectra of 1-naphtholPŽNH 3 . n clusters. The spectra are plotted on the same vertical scale. Ža. The same laser was used for excitation and ionization. The total ionization energy is therefore twice the values on the horizontal axis. The ns 2, 3 and 4 clusters all fragment to the next smaller size, as is seen from the corresponding spectral structure. Žb. The excitation laser was scanned over the indicated range, and was attenuated sufficiently that it did not generate any signal by itself. A second laser was used to ionize the clusters, at 28 100 cmy1 . The total ionization energy in the range of the sharp line structure was therefore near 59 200 cmy1 . At this total energy, the fragmentation processes of Ža. are either absent or very weak.
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the yield is up to 75%. Fragmentation is found to decrease steadily as the total ionization energy is reduced. At 59 400 cmy1 total 2-photon energy, the n s 3 peaks are only barely detectable in the n s 2 mass channel w11x. This represents ; 1500 cmy1 of excess energy in the ion state. 3.4. n s 4 A very low ionization threshold for n s 4 of ; 56 000 cmy1 has been reported w5x. This is 2000 cmy1 below that of n s 1, 2 or 3, and appears to be correlated with the appearance of ESPT. This is as expected, since the ionization energy of the naphtholate anion should be lower than that of the neutral. The ionization energy of free phenoxide anion is only 2.25 eV or 18 150 cmy1 w19x, for example. Photoionization of the proton-transferred cluster zwitterion leaves behind a positive Žproton-centered. ion rather than a neutral. Because of the increased Coulomb energy, the ionization energy of the zwitterion will be higher than that of phenoxide, but still less than before proton transfer. While it certainly lies below that of n s 3, the n s 4 threshold is difficult to measure reliably due to fragmentation from n ) 4. An improved evaluation of the n s 4 ionization threshold may be obtained by analyzing the pulse energy corrected 2-color R2PI spectra. Complete spectra enable more accurate measurements since 1-color contributions can be subtracted, and a direct estimation of n ) 4 fragmentation is possible on the basis of spectral structure. A generally linear drop in n s 4 ionization efficiency was found from above 62 000 cmy1 to below 58 000 cmy1 . This extrapolates to a true threshold of ; 56 900 cmy1 . Notably, the earlier data show a distinct step at 57 000 cmy1 , below this point that curve appears to reflect n s 5 fragmentation. As seen in Fig. 1, n s 4 fragments to n s 3. In Fig. 1a, the n s 3 channel shows a broad structure that correlates very well with the n s 4 spectrum. This broad component of the n s 3 spectrum can be varied vs. the sharp lines by changing either the excess energy in the ion state, as in Fig. 1b, or by changing the relative amounts of n s 3 and 4 in the expansion as in Ref. w11x. The 4 ™ 3 yield is, in a 1-color experiment, 30–40%. As in the smaller clusters, the yield drops steadily at lower ion state en-
ergy, and is near zero at 58 000 cmy1 , or ; 1000 cmy1 above the n s 4 ionization threshold w11x. 3.5. n G 5 As shown in Ref. w1x for n s 5, clusters larger than n s 4 have broad absorptions that are further red-shifted vs. the smaller clusters. On the basis of thermodynamic estimates, it has been predicted that these clusters undergo naphthol-to-ammonia proton transfer in the electronic ground state w1,2,9x, because the basicity of the ammonia aggregate increases with size. In the related phenol–ammonia clusters, ground state proton transfer appears to occur at n s 6 w20x. The increased red-shift of the absorption spectra would be consistent with such a difference in cluster behavior above n s 4. The ionization and fragmentation behavior is also qualitatively different for these clusters. As seen in Fig. 2, they are readily ionized in a 1-color experiment at very long wavelengths. Difficulties in reaching the necessary combinations of wavelengths made accurate 2-color ionization threshold measurements impractical. These are in preparation, currently it can only be said that either the thresholds for n G 5 are below 54 000 cmy1 or 3-photon processes are far more efficient than in smaller clusters. For comparison, free trans 1-naphthol has a sharp ionization threshold of 62 610 cmy1 . The vertical 2-photon ionization threshold of 1-naphtholate in the large clusters has apparently been depressed by ) 1 eV vs. free 1-naphthol. Fragmentation of these clusters is far more extensive than for n - 5. The clusters n s 0, 1, 2, 3 and 4 are all formed by ion state loss of neutral ŽNH 3 . m from n G 5, as seen in Fig. 2. The fragment distribution of the smaller clusters is unusual; n s 4 and n s 1 are preferred products, while n s 3 is the least efficiently formed. Most of the mass spectra in Fig. 2 were taken at photon energies below the absorption band of n s 4, which ends at ; 30 500 cmy1 . The strong n s 4 signal is then due to fragmentation of n G 5, as also shown by the distinct tailing of the mass peak in the expanded part of the 29 520 cmy1 spectrum of Fig. 2a. The 2-photon energy where 1-color n G 5 fragmentation becomes weak is near 56 000 cmy1 .
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Further important evidence for ground state proton transfer in n G 5, is the observation of abundant protonated fragments. For all n G 5 in Fig. 2, 1naphtholP ŽNH 3 . m Hq clusters are observed in large quantities, as seen in the insets of Fig. 2. Sometimes these are as abundant than the non-protonated radical
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cations. From the tailing of the mass peaks, and the protonated fragments it is clear that these clusters fragment extensively, and it is difficult to ascertain parent–daughter relationships or yields. In some cases the ŽM q 2H.q signals appear to be stronger than expected from the natural 13 C abundance. The mechanism leading to such ions may involve hydrogen abstraction by the naphthol radical cation prior to fragmentation. The charge and the unpaired electron are then on separate fragments, as is relatively common in electron impact mass spectrometry. Also very revealing is the observation of ŽNH 3 . m Hq fragments of n ) 5 clusters, as shown in Fig. 3. These are not formed efficiently, and are only observed when exciting large quantities of n G 5, but they also suggest ground state proton transfer. This behavior is in contrast to extensive ŽNH 3 . m Hq production from phenol–ammonia clusters w21,22x. This difference is likely due to the lower ionization energy of the naphthol clusters Ž; 1 eV., which makes the asymmetric fragmentation channel energetically uncompetetive for all but the larger clusters. 3.6. Implications of 1-naphtholP (NH3 )n cluster fragmentation The possibility of ion state fragmentation was not significantly considered in the interpretation of picoand femtosecond pump–probe experiments on 1naphtholP ŽNH 3 . n clusters w7–10x. The data presented here demonstrate that the pump–probe data must be critically reexamined in the light of fragmentation processes.
Fig. 2. One-color two-photon ionization mass spectra of 1-naphtholPŽNH 3 . n clusters. The mass spectra are scaled to have about the same maximum intensity. The photon energies are indicated to the right of each spectrum in wavenumbers, the total ionization energy is twice this value. Ža. The top insert shows the strong 1-naphtholPŽNH 3 . n Hq ions which result from fragmentation in the ion state of larger clusters. The lower insert shows the ns 4 peak which results from excitation and ionization with a photon energy below the lowest ns 4 absorption band. The tailing to longer flight times is an indication that this ion is a fragment of larger clusters, as is the case for ns 0, 1, 2 and small amounts of ns 3. Žb. The series of Ža. is continued to lower photon energies. The fragmentation of n) 4 into n- 4 mass channels becomes less important at the lowest energies. The insert shows the decrease in 1-naphtholPŽNH 3 . n Hq fragment ion yield.
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this does not show that the decay is determined by proton transfer. Any process in the excited state which alters the Franck–Condon factors to the ion state will lead to such a signal. In addition, the present results show that dynamics in larger clusters will be projected into the lower mass channels by fragmentation, substantially complicating the interpretation of the observed time scales. Furthermore, the pump–probe signal in any given mass channel can be strongly modified or even negated by the superposition of ionic and neutral evaporation events, as found in Ref. w23x for ammonia clusters of NaI. 4. Conclusions
Fig. 3. Partial one-color two-photon ionization mass spectra of 1-naphtholPŽNH 3 . n clusters, at a photon energy of 29 100 cmy1 . The higher mass part of the spectrum is as in the lower spectrum of Fig. 2a. Small amounts of ŽNH 3 . n Hq ions are produced as fragments of ionized n) 4 clusters. Note also the appearance of metastable fragments above the ms 4 cluster, leading to a broad ion signal extending to an apparent m r z of 140.
Most importantly, n s 4 fragmentation in the ion state will project any n s 4 excited state dynamics into the n s 3 channel. The 2-photon ionization energies reported in Refs. w7–9x were in the range of 60 100–60 600 cmy1 , where n s 4 to n s 3 fragmentation is 15–20%. If the n s 4 ion yield decreases with pump–probe delay, the corresponding 15–20% ion state fragmentation contribution to the n s 3 signal will also decrease with the same time dependence. Due to the wide n s 4 absorption spectrum vs. the few sharp n s 3 lines, n s 4 fragments can dominate the intrinsic n s 3 signal under broadband excitation conditions w11x. This effect has recently been confirmed by pump–probe data showing the absence of fast decaying n s 3 pump–probe signal at low ionization energy w12x. This result is satisfying in that pump–probe data are then no longer in conflict with the spectroscopy, they indicate ESPT only at n s 4, not n s 3. A conclusion of Refs. w7–9x was that the pump– probe data revealed the time scale of ESPT. Although a fast decay component first appears at n s 4, where spectroscopy has found the ESPT threshold,
Fragmentation in the ion state is observed for every 1-naphtholP ŽNH 3 . n cluster size above n s 1. For the cluster sizes n s 2, 3 and 4, the only observed fragmentation pathway is loss of a single neutral ammonia in the ion state. This process ceases at energies 1500–2500 cmy1 above the ionization thresholds. The one-color fragment yield is typically around 50%. Larger clusters, n ) 4, are readily ionized at remarkably long wavelengths. They fragment more extensively than the smaller clusters, yielding three types of products: Ž1-naphtholP ŽNH 3 . n .q, 1-naphthol P ŽNH 3 . m Hq, and ŽNH 3 . m Hq clusters. In the case of the first group, more than one neutral ammonia can be lost, and products range to free naphthol. These observations support predictions that these clusters undergo proton transfer in the ground state. Earlier pump–probe studies of 1-naphthol P ŽNH 3 . n clusters must be reinterpreted in light of these data. Consistent with spectroscopic studies, there is no evidence for ESPT in n s 3. The pump– probe dynamics are probably only indirectly relevant to the ESPT time scale in the n s 4 cluster. References w1x O. Cheshnovsky, S. Leutwyler, J. Chem. Phys. 88 Ž1988. 4127. w2x R. Knochenmuss, O. Cheshnovsky, S. Leutwyler, Chem. Phys. Lett. 144 Ž1988. 317. w3x C. Jouvet, C. Lardeux-Dedonder, M. Richard-Viard, D. Solgadi, A. Tramer, J. Phys. Chem. 94 Ž1990. 5041. w4x T. Droz, R. Knochenmuss, S. Leutwyler, J. Chem. Phys. 93 Ž1990. 4520.
R. Knochenmussr Chemical Physics Letters 311 (1999) 439–445 w5x S.K. Kim, S. Li, E.R. Bernstein, J. Chem. Phys. 95 Ž1991. 3119. w6x R. Knochenmuss, Chem. Phys. Lett. 293 Ž1998. 191. w7x J.J. Breen, L.W. Peng, D.M. Willberg, A. Heikal, P. Cong, A.H. Zewail, J. Chem. Phys. 92 Ž1990. 805. w8x M.F. Hineman, G.A. Bruker, D.F. Kelley, E.R. Bernstein, J. Chem. Phys. 97 Ž1992. 3341. w9x S.K. Kim, J.J. Breen, D.M. Willberg, L.W. Peng, A. Heikal, J.A. Syage, A.H. Zewail, J. Phys. Chem. 99 Ž1995. 7421. w10x J.A. Syage, J. Phys. Chem. 99 Ž1995. 5772. w11x R. Knochenmuss, Chem. Phys. Lett. 305 Ž1999. 233. w12x R. Knochenmuss, I. Fischer, D. Luhrs, Israel. J. Chem. ¨ Ž1999., accepted. w13x D.F. Kelley, E.R. Bernstein, Chem. Phys. Lett. 305 Ž1999. 230. w14x T. Burgi, T. Droz, S. Leutwyler, Chem. Phys. Lett. 246 ¨ Ž1995. 291.
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w15x J. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1991. w16x L.L. Connel, S. Ohline, P.W. Joireman, T.C. Corcoran, P.M. Felker, J. Chem. Phys. 94 Ž1991. 4668. w17x R. Yoshino, K. Hashimoto, T. Omi, S. Ishiuchi, M. Fujii, J. Phys. Chem. A 102 Ž1998. 6227. w18x F. Huisken, T. Pertsch, Chem. Phys. 126 Ž1988. 213. w19x J.E. Bartmess, in: M. W.G., S.E. Stein ŽEds.., Negative Ion Energetics Data, NIST ŽNatl. Inst. Stand. Technol.., Gaithersburg, MD, 1998. w20x C. Jouvet, personal communication. w21x J.A. Syage, J. Steadman, J. Chem. Phys. 95 Ž1991. 2497. w22x J. Steadman, J.A. Syage, J. Chem. Phys. 92 Ž1990. 4630. w23x G. Gregoire, M. Mons, I. Dimicoli, C. Dedonder-Lardeux, C. ´ Jouvet, S. Martrenchard, D. Solgadi, J. Chem. Phys. 110 Ž1999. 1521.