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Photodissociation of 1,4-dioxane ions. Competitive formation of two fragment ions
International Journal of Mass Spectrometry and Ion Processes, 76 (1987) l-10 EIsevier science Publishers B.V.. Amsterdam - Printed in The Netherlands
1
PHOTODISSOCIATION OF 1,4-DIOXANJZIONS. COMPETITIVE FORMATION OF TWO FRAGMENT IONS
J.H. CHEN and R.C. DUNBAR * Deport
of ~hern~t~, Case Western Reserve ~niversi~, ~level~~
OH 44104 (U.S.A.)
(First received 28 March 1986; in final form 8 December 1986)
ABSTRACT Photodissociation of the dioxane parent ion, m/z 88, in the ICR ion trap gives predominantly m/z 57 fragments at wavelengths between 515 and 457 nm, while at wavelengths increasing from 595 to 615 nm, an increasing amount of m/z 58 fragment is observed, reaching a 58/57 ratio of 5/l at 615 nm. This is in contrast to the observed fragmentation of m/z 88 ions energies produced by photoionization, where m/z 58 is the predominant fragment for photon energies in this energy range. However, comparing monoenergetic fragmenting ions, as approximated by the first derivative of the photoionization efficiency plots, the photodissociation pattern is in qualitative agreement with photoionization; the higher 57/58 ratio in photodissociation is attributed to the electron-impact-produced ions in the ICR ion trap containing 0.3 eV of excitation in excess of thermaL The observations and ~te~re~tion are consistent with prehminary ion-beam phot~ss~iation results from Bowers’ laboratory and a consistent picture of the competitive production of these two fragment ions as a function of parent ion internal energy is obtained.
INTRODUCTION
An interesting question in the study of gas-phase ion phot~he~st~ is whether, and when, the photochemical decomposition of ions will follow processes specific to particular excited electronic states, as opposed to proceeding by a statistical break-up of a vibrationally excited ground electronic state. The latter type of fra~entation, known as quasi-equilibrium theory or RRKM fragmentation, has been invoked with widespread success to rationalize mass-spectral fragmentation patterns in electron-impact ionization, chemical ionization, and photoio~tion. Fra~entations of polyatomic ions which do not occur in this way are still sufficientlyunusual to be worth studying. * To whom correspondence should be addressed. 0168-1176/87/$03.50
0 1987 Ekevier Science Publishers B.V.
2
In analogy with neutral-molecule photochemistry, it might be anticipated that photodissociation could show frequent instances of specifically excitedstate-governed phot~he~st~, but clear instances of such behavior have been limited to small ions and to the direct photocleavage of carbon-halogen bonds in alkyl halides [l]. Several ionic systems showing multiple fragmentation chmels have been studied without conclusive evidence for factors beyond QET playing a role in the branching ratios [2-41. It was recently observed that photodiss~iation of dioxane parent ions in the ICR ion trap gives fragmentation patterns substantially different from photoionization efficiency plots and it seemed of interest to consider carefully whether this might be an instance of state-specific photochemistry. This possibility is attractive because, in the wavelength range of interest, there are known to be two excited electronic states of the ion [S] and state-specific fragmentation from either or both of them could give interesting effects. However, the intriguing possibility of competitive photoexcitation to both of these ion states, followed by state-specific fragmentation, is made somewhat unlikely by the fact that, for the undistorted dioxane structure, one of them, the lb, state, is optically allowed from the ground state ion (la,), while the other, the 2a, state, is optically forbidden by parity. Nevertheless, the possibility that photodissociation of the parent ion involves state-specific behavior not seen in dissociative photoionization was interesting enough to lead us to a full set of experiments and a careful assessment of the results. To clarify these questions, the photodissociation at several wavelengths was studied in our ICR ion-trap instrument. Following our communication of the results, the Bowers laboratory at Santa Barbara investigated the photodissociation in the ion-beam instrument; their experiments will be described in detail later, but the results are of such importance for clarifying the situation that Bowers’ group has kindly allowed presentation of preliminary results here. EXPERIMENTAL
The ion-trap photodissociation experiments were carried out in a manner similar to other recent competitive photofragmentation studies in this laboratory [3,6]. The Fourier-transform ICR spectrometer [7], run in a single-ion bridge detector mode, was used to trap and irradiate ions for a period ranging from 1 to 4 s in different experiments, followed by detection of parent or daughter ion abundance. Low ionizing electron beam energies were used, typically 10.5 eV nominal, to blaze fragmentation in the absence of light, except in control experiments at 70 eV designed to examine the photodissociation or ion/molecule reactions of electron-impact-pro-
3
duced fragment ions. The indicated neutral-dioxane pressure at the ionization gauge was 2-3 X lo-* Torr. This was sufficiently low to suppress the ion/molecule reactions leading to the (M + 1) ion at m/z 89 although, as noted below, there was sufficient pressure of either sample or background gas to give significant reactive depletion of the highly reactive m/z 28 fragment. Photodissociation in the blue-green used the Coherent Radiation CR12 argon-ion laser and in the red the argon-ion-pumped dye laser. At the entrance window to the vacuum chamber, the laser-beam diameter was typically 2 cm. An argon-ion laser power of 1 W was sufficient to dissociate perhaps 75% of the parent ions in 4 s, while 300 mW from the dye laser dissociated perhaps 25%. RESULTS
The only observed photodissociation products under ion-trap conditions were m/z 57 and 58. At the optical wavelengths used, searches for product
I
10
z .
-
d
b
1’1
l.O0
5;
-
0
O.l-
2.0
I
I
2.5
3.0
Ion Energy
(eV)
Fig. 1. Values of the m/z 57/58 ratio from different studies. 0, Present results, assuming thermal ions; 0, present results assuming 0.3 eV of superthermal energy in the photodissociating ions; A, results from the ion-beam photodissociation experiment of Bowers et al. [8]; results from the photoionization efficiency curves of Fraser-Monteira et al. [9] derived from the first-derivative plots of Fig. 3, below.
ions at 45,44,43, 31, 30, 29 and 28 showed no substantial photoproduction and none of these ions constitutes as much as - 10% of the observed photoproducts. Photodissociation cross-sections were not measured accurately, but the cross-section for parent ion disappearance was modest, probably near lo-l9 cm2. There was no dramatic variation in cross-section with wavelength over the wavelengths used. The observed 57/58 ratio is shown at various photon energies in Fig. 1. Also shown at the argon-ion-laser wavelengths are the corresponding 57/S ratios from the ion-beam photodissociation results of Bowers et al. [8]. In addition to 57 and 58, they also observed fragments of the order of 12% of the total fra~entation at 44 and 45 (at 515 run) and at 28 and 30 (at 458 nm), although at all wavelengths 57 plus 58 constituted the bulk of the photoproducts. DISCUSSION
Possible ion rearrangements
Before interpreting the photodissociation observations in terms of fragmentation of excited dioxane ions, it is important to rule out the possibility of re~~gement of the ions in the ion trap prior to photon absorption_ While this is not easy, several arguments make it seem unlikely.
--.
‘-A. LL&
0.4
0.8 Photon
1.2
Flux
Fig. 2. Exhaustive photodissociation results for two wavelength regions. Shown is the fraction of parent ion, m/z 88, remaining after 4 s of irradiation as a function of irradiating power level. For simple phot~s~tion of a homogeneous ion population having a single photodissociation rate constant, the expected curve is a simple exponential. The solid and broken curves are exponential decay curves fitted to the two data sets. --A--, 515 + 488 nm; o-a, 615 nm.
5
Fast, partial rearrangement to give a mixture of ion populations with different structures and different photodissociation characteristics was tested by exhaustive photodissociation [lo] with the results shown in Fig. 2. It is seen that irradiation at blue-green wavelengths gives dissociation of more than 90% of the ions with a single rate constant, while similar irradiation at 615 nm gives more than 75% dissociation with a single rate constant. So the ion population shows good photochemical homogeneity at both green and red wavelengths. The possibility of a slow, irreversible rearrangement was ruled out by gated-laser photodissociation experiments. In these experiments, the ions were trapped for a constant period of about 4 s, while dissociation was brought about by a laser pulse of about 0.5 s duration. The laser pulse was placed at varying times during the trapping period. In the event that the chemical character of the ion population varied during the trapping period, the photodissociation behavior would vary with the time of the laser pulse [ll]. At both green and red wavelengths, photodissociation was found to be the same whether the laser pulse came early or late in the ion-trapping period. This is good evidence against a rearrangement changing the photochemical character of the ion population with a time constant of the order of 1 s. Intermolecular scrambling of hydrogen atoms during ion/molecule collisions was ruled out by photodissociation in a mixture of C,H,O, and C,D,O,. The expected photodissociation products of the C,H,02+ and CaDsO+ ions were observed, but no products attributable to photodissociation of mixed H/D parent ions, indicating negligible intermolecular hydrogen scrambling. No feasible way to test for intramolecular hydrogen scrambling was devised. There remains the remote possibility of a rearrangement following electron-impact ionization on a l-10 ms time scale. Such a rearrangement would be negligible in the photoionization and ion-beam photodissociation experiments, but would be complete on the ion-trap time scale. The pulsedlaser photodissociation experiments needed to test this may soon be feasible, but have not yet been done. So ion rearrangement in the ion trap cannot be ruled out entirely, although it seems unlikely. Possible sequential dissociation Sequential photodissociation, in which initial photoproducts are further dissociated by further photon absorption, is often a serious possibility in ion-trap experiments. The possibility that m/z 58 was further photodissociated to m/z 57 at green/blue wavelengths was dismissed because, under conditions where substantial amounts of m/z 58 were present from electron
6
impact ionization, there was no change in m/z 58 abundance under irradiation at any laser power while, under these same conditions, m/z 57 increased with laser power as expected, corresponding to the decrease in parent ion intensity, and photoproduced m/z 57 was shown by double resonance to originate entirely from the parent ion (m/z 88). These observations convinced us that the normal m/z 58 structure resulting from electron-impact ionization was entirely photo-inert. There is also a possibility that photodissociation produces a fraction of some rearranged 58 structure which itself photodissociates rapidly to 57. We ruled this out as a predominant process at 514 nm by the double-resonance experiment in which photoproduction of 57 was measured in the presence of rapid, continuous ejection of 58. Ejection of 58 did not give a major decrease in 57 production, which would have indicated the 88 + 58 + 57 sequence as the source of a major fraction of the observed 57. Ion molecule reactions of photoproducts The relative abundance of photoproducts might be distorted by ion/molecule reactions during the ion trapping period since, at the pressures and trapping times used, an ion has a significant probability of collision with a neutral molecule during the trapping period. In test experiments at higher electron energies (to increase primary fragment abundances), it was found that the m/z 57 and 58 ions produced by electron impact were entirely unreactive under the conditions used, while m/z 28 decayed with a time constant of the order of 1 s-l and the parent ion (m/z 88) increased slightly over several seconds (presumably corresponding to charge transfer from various minor primary ionization products). Assuming that electronimpact-produced ions and photodissociation-produced ions show similar ion/molecule reaction behavior, it seems that the photoproduction measurements of m/z 57 and 58 will not be disturbed by ion/molecule reactions, while m/z 28 photoproduction might be underestimated if reactive depletion effects are neglected. At the longer wavelengths of 615, 590, and 515 mn, the abundance of photoproduced fragments corresponded well to the observed decrease in parent ion signal, ruling out reactive depletion of the product ions into unobserved ionic products. However, at 488 and 458 nm, the observed photoproducts were only half or less of the parent ion disappearance, with the balance unaccounted for by any product ions we could find. This unsatisfactory mass balance suggests unidentified ion/molecule reaction complications and leads us to less confidence in the results at these wavelengths. At UV wavelengths, parent ion photodisappearance was observed, but no products at all were found.
7
Comparison with photoionization The most interesting comparison for illuminating the present results is with the dissociative photoionization results of Fraser-Monteiro et al. [9]. In m/z 58 is by far the most abundant fragment, with photoionization, substantial 44 and 45 production at the lower energies and a minor amount of 57 at higher energies. Monoenergetic PEPICO data would be the most useful for comparison, but they did not obtain monoenergetic data above 11.1 eV and, in any case, the mass-resolution limitations of their PEPICO capability precluded measuring the monoenergetic 57/58 ratio. In order to make a comparison of the photoionization efficiency (PIE) plots with photodissociation, the PIE plots must be reduced to monoenergetic form since the direct PIE measurement at a given photon energy represents a superposition of fragmentations of ions of various internal energies. As was pointed out by Morrison [12], this may be done approximately by taking the first derivative of the PIE curves with respect to photon energy. This is based on the assumption that the onset for photoionization to each molecular ion state varies with energy according to a step function. This should often be a reasonable approximation within a few eV of the threshold in the absence of serious distortion by autoionization [13]. First-derivative plots of the PIE data of Fraser-Monteiro et al. [9] are shown in Fig. 3. The 57/58 ratio derived from these derivative plots is also plotted in Fig. 1 along with ion-beam photodissociation results of Bowers et al. [8]. An overall picture of the m/z 88 fragmentation is clear. It yields 58 at low excitation energy, with 57 increasing gradually to predominance over the 2.0-2.8 eV excitation range; further fragmentations to 45, 44, 28, etc. rise in importance above 2.8 eV. This is a pattern easily fitted into quasiequilibrium theory fragmentation ideas. The ion-trap photodissociation results show a substantially higher 57/58 ratio than photoionization. This is most likely a consequence of residual excess internal energy in the ions. It has been believed that collisions plus infrared radiative cooling can be relied on to thermahze ions efficiently on the time scale of seconds in experiments like these, but this confident assumption has recently been found to be invalid. Study of several ions, including iodobenzene [14], phenol and thiophenol [15], and chlorobenzene [16] ions, has shown that, at low pressure, several tenths of an eV of excess energy are often deposited by electron impact ionization and this energy is dissipated only over many seconds. Theoretical understanding of infrared radiative emission rates has supported this very slow low-pressure thermalization [16]. The present results are brought into quantitative agreement with the other studies on the assumption that an average of 0.3 eV of energy above thermal is retained in the dioxane ions, corresponding to a vibrational
8 620
513
456
1
I""'""~~'""'
9
"In
10 Photon
12
11 Energy
(eV)
Fig. 3. Plots of the fist derivative of PIE versus photoionizing photon energy (from data of ref. 9). The units are arbitrary but consistent among the six ions. At the top are indicated the positions (vertical IP) of the two dioxane-ion excited electronic states known’ in this energy region from phot~l~tron spectroscopy [S]. Also shown at the top are the energies corresponding to three of the wavelengths used in the photodissociation experiments.
temperature of about 600 K. The energy-shifted points are replotted in Fig. 1, showing the resulting excellent agreement of the 57/58 ratio from these three different studies. CONCLUSION
Photodissociation of 1,3dioxane parent ions in the ion trap at visible wavelengths shows the same predo~nant m/z 57 and 58 products as monoenergetic photoionization at corresponding energies and ion-beam photodissociation at the same wavelengths, although with a higher 57/58 ratio. Based on the fact that similar dissociation products are observed and supported by the control experiments described, we consider it likely that structural rearrangement is not involved and that the same parent ion structure is involved in all of these experiments. It is suggested that the higher 57/58 ratio in the present results reflects inefficient therma~tion of the electron-impact-produced ions in the low-pressure ICR experiment. With a correction of the present results for excess internal energy, a consistent pattern for the 57/58 ratio is seen for photoionization, ion-beam
9
photodissociation, and ion-trap photodissociation. It appears that the fragmentation pattern of dioxane ions can be considered as reflecting the internal energy of the ion in QET terms and no special effects arising from the initial mode of excitation need be invoked. ACKNOWLEDGEMENTS
Appreciation is expressed to the National science Foundation and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research and to SOHIO, whose generosity provided the instrument. We are especially grateful to Professor MT. Bowers and Dr. N. Kimhner for providing us with preliminary ion-beam photodissociationresults.
1 See, for instance, R.C. Dunbar, in T.A. MiIIer and V.E. Bondybey (Eds.), Molecular Ions. Spectroscopy, Structure and Chemistry, North-Holland, Amsterdam, 1983, p. 231. R.C. Dunbar in M.T. Bowers (Ed.), Gas-Phase Ion Chemistry, Vol. 3, Academic Press, New York, 1984, Chap. 20. 2 Some examples have been reported in which photodissociation yields products differing in intriguing ways from the products of other dissociation methods [e.g. V. Franchetti, B.S. Freiser and R.G. Cooks, Org. Mass Spectrom., 13 (1978) 106. R.L. Hettich and B.S. Freiser, J. Am. Chem. Sot., 108 (1986) 25371, but the factors accounting for these differences have not yet been clarified. 3 There continues to be a possibility of very interesting state-specific photochemistry in some aromatic ketone ions [R. Gooden and J.I. Brauman, J. Am. Chem. Sot., 99 (1977) 19771, but the situation in these experimentally difficuh systems is stih uncertain. 4 J.H. Chen, J.D. Hays and R.C. Dunbar, J. Phys. Chem., 88 (1984) 4759. 5 D.A. Sweigart and D.W. Turner, J. Am. Chem. Sot., 94 (1972) 5599. T. Kobayashi and S. Nagakura, BuII. Chem. Sot. Jpn., 46 (1973) 1558. D. Gonbeau, M. Loudet and P.G. Guillouzo, Tetrahedron, 36 (1980) 381. F.S. Jorgenson, N.L. Lauritsen, R.B. Jensen and G. SchroIl, Tetrahedron, 37 (1981) 3671. 6 J.H. Chen and R.C. Dunbar, Int. J. Mass Spectrom. Ion Processes, 72 (1986) 115. 7 J.D. Hays and RC. Dunbar, Rev. Sci. Instrum., 55 (1984) 1116. 8 M.T. Bowers and N. Kichner. We are grateful to these workers for communicating their prehminary results to us. 9 M.L. Fraser-Monteiro, L. Fraser-Monteiro, J.J. Butler, T. Baer and J.R. Hass, J. Phys. Chem., 86 (1982) 739. 10 E.W. Fu, P.P. Dymerski and R.C. Dunbar, J. Am. Chem. Sot., 98 (1976) 337, 11 J.P. Honovich and RC. Dunbar, Int. J. Mass Spectrom. Ion Processes, 42 (1982) 33. 12 J.D. Morrison, J. Chem. Phys., 21 (1953) 1767; Rev. Pure Appl. Chem., 5 (1955) 22. 13 The quantitative unreliability of derivative plots can be illustrated by looking at the 58/45 ratio at the photoio~tion energy of, for instance, 10.9 eV. The PEPICO result (presumably the correct branching ratio) from Fig. 7 of ref. 9 gives a ratio of 4, while the
10 ratio derived from the PIE derivative plots of our Fig. 3 gives a ratio of 1, a discrepancy of a factor of 4. While such large discrepancies are not seen at most energies, this suggests the degree of ci.rcumspection appropriate in basing conclusions on derivative plots. 14 B. Asamoto and R.C. Dunbar, J. Phys. Chem., in press. 15 B. Asamoto, Ph. D. Thesis, Case Western Reserve University, 1986. 16 R.C. Dunbar, J. Phys. Chem., in press.
1
PHOTODISSOCIATION OF 1,4-DIOXANJZIONS. COMPETITIVE FORMATION OF TWO FRAGMENT IONS
J.H. CHEN and R.C. DUNBAR * Deport
of ~hern~t~, Case Western Reserve ~niversi~, ~level~~
OH 44104 (U.S.A.)
(First received 28 March 1986; in final form 8 December 1986)
ABSTRACT Photodissociation of the dioxane parent ion, m/z 88, in the ICR ion trap gives predominantly m/z 57 fragments at wavelengths between 515 and 457 nm, while at wavelengths increasing from 595 to 615 nm, an increasing amount of m/z 58 fragment is observed, reaching a 58/57 ratio of 5/l at 615 nm. This is in contrast to the observed fragmentation of m/z 88 ions energies produced by photoionization, where m/z 58 is the predominant fragment for photon energies in this energy range. However, comparing monoenergetic fragmenting ions, as approximated by the first derivative of the photoionization efficiency plots, the photodissociation pattern is in qualitative agreement with photoionization; the higher 57/58 ratio in photodissociation is attributed to the electron-impact-produced ions in the ICR ion trap containing 0.3 eV of excitation in excess of thermaL The observations and ~te~re~tion are consistent with prehminary ion-beam phot~ss~iation results from Bowers’ laboratory and a consistent picture of the competitive production of these two fragment ions as a function of parent ion internal energy is obtained.
INTRODUCTION
An interesting question in the study of gas-phase ion phot~he~st~ is whether, and when, the photochemical decomposition of ions will follow processes specific to particular excited electronic states, as opposed to proceeding by a statistical break-up of a vibrationally excited ground electronic state. The latter type of fra~entation, known as quasi-equilibrium theory or RRKM fragmentation, has been invoked with widespread success to rationalize mass-spectral fragmentation patterns in electron-impact ionization, chemical ionization, and photoio~tion. Fra~entations of polyatomic ions which do not occur in this way are still sufficientlyunusual to be worth studying. * To whom correspondence should be addressed. 0168-1176/87/$03.50
0 1987 Ekevier Science Publishers B.V.
2
In analogy with neutral-molecule photochemistry, it might be anticipated that photodissociation could show frequent instances of specifically excitedstate-governed phot~he~st~, but clear instances of such behavior have been limited to small ions and to the direct photocleavage of carbon-halogen bonds in alkyl halides [l]. Several ionic systems showing multiple fragmentation chmels have been studied without conclusive evidence for factors beyond QET playing a role in the branching ratios [2-41. It was recently observed that photodiss~iation of dioxane parent ions in the ICR ion trap gives fragmentation patterns substantially different from photoionization efficiency plots and it seemed of interest to consider carefully whether this might be an instance of state-specific photochemistry. This possibility is attractive because, in the wavelength range of interest, there are known to be two excited electronic states of the ion [S] and state-specific fragmentation from either or both of them could give interesting effects. However, the intriguing possibility of competitive photoexcitation to both of these ion states, followed by state-specific fragmentation, is made somewhat unlikely by the fact that, for the undistorted dioxane structure, one of them, the lb, state, is optically allowed from the ground state ion (la,), while the other, the 2a, state, is optically forbidden by parity. Nevertheless, the possibility that photodissociation of the parent ion involves state-specific behavior not seen in dissociative photoionization was interesting enough to lead us to a full set of experiments and a careful assessment of the results. To clarify these questions, the photodissociation at several wavelengths was studied in our ICR ion-trap instrument. Following our communication of the results, the Bowers laboratory at Santa Barbara investigated the photodissociation in the ion-beam instrument; their experiments will be described in detail later, but the results are of such importance for clarifying the situation that Bowers’ group has kindly allowed presentation of preliminary results here. EXPERIMENTAL
The ion-trap photodissociation experiments were carried out in a manner similar to other recent competitive photofragmentation studies in this laboratory [3,6]. The Fourier-transform ICR spectrometer [7], run in a single-ion bridge detector mode, was used to trap and irradiate ions for a period ranging from 1 to 4 s in different experiments, followed by detection of parent or daughter ion abundance. Low ionizing electron beam energies were used, typically 10.5 eV nominal, to blaze fragmentation in the absence of light, except in control experiments at 70 eV designed to examine the photodissociation or ion/molecule reactions of electron-impact-pro-
3
duced fragment ions. The indicated neutral-dioxane pressure at the ionization gauge was 2-3 X lo-* Torr. This was sufficiently low to suppress the ion/molecule reactions leading to the (M + 1) ion at m/z 89 although, as noted below, there was sufficient pressure of either sample or background gas to give significant reactive depletion of the highly reactive m/z 28 fragment. Photodissociation in the blue-green used the Coherent Radiation CR12 argon-ion laser and in the red the argon-ion-pumped dye laser. At the entrance window to the vacuum chamber, the laser-beam diameter was typically 2 cm. An argon-ion laser power of 1 W was sufficient to dissociate perhaps 75% of the parent ions in 4 s, while 300 mW from the dye laser dissociated perhaps 25%. RESULTS
The only observed photodissociation products under ion-trap conditions were m/z 57 and 58. At the optical wavelengths used, searches for product
I
10
z .
-
d
b
1’1
l.O0
5;
-
0
O.l-
2.0
I
I
2.5
3.0
Ion Energy
(eV)
Fig. 1. Values of the m/z 57/58 ratio from different studies. 0, Present results, assuming thermal ions; 0, present results assuming 0.3 eV of superthermal energy in the photodissociating ions; A, results from the ion-beam photodissociation experiment of Bowers et al. [8]; results from the photoionization efficiency curves of Fraser-Monteira et al. [9] derived from the first-derivative plots of Fig. 3, below.
ions at 45,44,43, 31, 30, 29 and 28 showed no substantial photoproduction and none of these ions constitutes as much as - 10% of the observed photoproducts. Photodissociation cross-sections were not measured accurately, but the cross-section for parent ion disappearance was modest, probably near lo-l9 cm2. There was no dramatic variation in cross-section with wavelength over the wavelengths used. The observed 57/58 ratio is shown at various photon energies in Fig. 1. Also shown at the argon-ion-laser wavelengths are the corresponding 57/S ratios from the ion-beam photodissociation results of Bowers et al. [8]. In addition to 57 and 58, they also observed fragments of the order of 12% of the total fra~entation at 44 and 45 (at 515 run) and at 28 and 30 (at 458 nm), although at all wavelengths 57 plus 58 constituted the bulk of the photoproducts. DISCUSSION
Possible ion rearrangements
Before interpreting the photodissociation observations in terms of fragmentation of excited dioxane ions, it is important to rule out the possibility of re~~gement of the ions in the ion trap prior to photon absorption_ While this is not easy, several arguments make it seem unlikely.
--.
‘-A. LL&
0.4
0.8 Photon
1.2
Flux
Fig. 2. Exhaustive photodissociation results for two wavelength regions. Shown is the fraction of parent ion, m/z 88, remaining after 4 s of irradiation as a function of irradiating power level. For simple phot~s~tion of a homogeneous ion population having a single photodissociation rate constant, the expected curve is a simple exponential. The solid and broken curves are exponential decay curves fitted to the two data sets. --A--, 515 + 488 nm; o-a, 615 nm.
5
Fast, partial rearrangement to give a mixture of ion populations with different structures and different photodissociation characteristics was tested by exhaustive photodissociation [lo] with the results shown in Fig. 2. It is seen that irradiation at blue-green wavelengths gives dissociation of more than 90% of the ions with a single rate constant, while similar irradiation at 615 nm gives more than 75% dissociation with a single rate constant. So the ion population shows good photochemical homogeneity at both green and red wavelengths. The possibility of a slow, irreversible rearrangement was ruled out by gated-laser photodissociation experiments. In these experiments, the ions were trapped for a constant period of about 4 s, while dissociation was brought about by a laser pulse of about 0.5 s duration. The laser pulse was placed at varying times during the trapping period. In the event that the chemical character of the ion population varied during the trapping period, the photodissociation behavior would vary with the time of the laser pulse [ll]. At both green and red wavelengths, photodissociation was found to be the same whether the laser pulse came early or late in the ion-trapping period. This is good evidence against a rearrangement changing the photochemical character of the ion population with a time constant of the order of 1 s. Intermolecular scrambling of hydrogen atoms during ion/molecule collisions was ruled out by photodissociation in a mixture of C,H,O, and C,D,O,. The expected photodissociation products of the C,H,02+ and CaDsO+ ions were observed, but no products attributable to photodissociation of mixed H/D parent ions, indicating negligible intermolecular hydrogen scrambling. No feasible way to test for intramolecular hydrogen scrambling was devised. There remains the remote possibility of a rearrangement following electron-impact ionization on a l-10 ms time scale. Such a rearrangement would be negligible in the photoionization and ion-beam photodissociation experiments, but would be complete on the ion-trap time scale. The pulsedlaser photodissociation experiments needed to test this may soon be feasible, but have not yet been done. So ion rearrangement in the ion trap cannot be ruled out entirely, although it seems unlikely. Possible sequential dissociation Sequential photodissociation, in which initial photoproducts are further dissociated by further photon absorption, is often a serious possibility in ion-trap experiments. The possibility that m/z 58 was further photodissociated to m/z 57 at green/blue wavelengths was dismissed because, under conditions where substantial amounts of m/z 58 were present from electron
6
impact ionization, there was no change in m/z 58 abundance under irradiation at any laser power while, under these same conditions, m/z 57 increased with laser power as expected, corresponding to the decrease in parent ion intensity, and photoproduced m/z 57 was shown by double resonance to originate entirely from the parent ion (m/z 88). These observations convinced us that the normal m/z 58 structure resulting from electron-impact ionization was entirely photo-inert. There is also a possibility that photodissociation produces a fraction of some rearranged 58 structure which itself photodissociates rapidly to 57. We ruled this out as a predominant process at 514 nm by the double-resonance experiment in which photoproduction of 57 was measured in the presence of rapid, continuous ejection of 58. Ejection of 58 did not give a major decrease in 57 production, which would have indicated the 88 + 58 + 57 sequence as the source of a major fraction of the observed 57. Ion molecule reactions of photoproducts The relative abundance of photoproducts might be distorted by ion/molecule reactions during the ion trapping period since, at the pressures and trapping times used, an ion has a significant probability of collision with a neutral molecule during the trapping period. In test experiments at higher electron energies (to increase primary fragment abundances), it was found that the m/z 57 and 58 ions produced by electron impact were entirely unreactive under the conditions used, while m/z 28 decayed with a time constant of the order of 1 s-l and the parent ion (m/z 88) increased slightly over several seconds (presumably corresponding to charge transfer from various minor primary ionization products). Assuming that electronimpact-produced ions and photodissociation-produced ions show similar ion/molecule reaction behavior, it seems that the photoproduction measurements of m/z 57 and 58 will not be disturbed by ion/molecule reactions, while m/z 28 photoproduction might be underestimated if reactive depletion effects are neglected. At the longer wavelengths of 615, 590, and 515 mn, the abundance of photoproduced fragments corresponded well to the observed decrease in parent ion signal, ruling out reactive depletion of the product ions into unobserved ionic products. However, at 488 and 458 nm, the observed photoproducts were only half or less of the parent ion disappearance, with the balance unaccounted for by any product ions we could find. This unsatisfactory mass balance suggests unidentified ion/molecule reaction complications and leads us to less confidence in the results at these wavelengths. At UV wavelengths, parent ion photodisappearance was observed, but no products at all were found.
7
Comparison with photoionization The most interesting comparison for illuminating the present results is with the dissociative photoionization results of Fraser-Monteiro et al. [9]. In m/z 58 is by far the most abundant fragment, with photoionization, substantial 44 and 45 production at the lower energies and a minor amount of 57 at higher energies. Monoenergetic PEPICO data would be the most useful for comparison, but they did not obtain monoenergetic data above 11.1 eV and, in any case, the mass-resolution limitations of their PEPICO capability precluded measuring the monoenergetic 57/58 ratio. In order to make a comparison of the photoionization efficiency (PIE) plots with photodissociation, the PIE plots must be reduced to monoenergetic form since the direct PIE measurement at a given photon energy represents a superposition of fragmentations of ions of various internal energies. As was pointed out by Morrison [12], this may be done approximately by taking the first derivative of the PIE curves with respect to photon energy. This is based on the assumption that the onset for photoionization to each molecular ion state varies with energy according to a step function. This should often be a reasonable approximation within a few eV of the threshold in the absence of serious distortion by autoionization [13]. First-derivative plots of the PIE data of Fraser-Monteiro et al. [9] are shown in Fig. 3. The 57/58 ratio derived from these derivative plots is also plotted in Fig. 1 along with ion-beam photodissociation results of Bowers et al. [8]. An overall picture of the m/z 88 fragmentation is clear. It yields 58 at low excitation energy, with 57 increasing gradually to predominance over the 2.0-2.8 eV excitation range; further fragmentations to 45, 44, 28, etc. rise in importance above 2.8 eV. This is a pattern easily fitted into quasiequilibrium theory fragmentation ideas. The ion-trap photodissociation results show a substantially higher 57/58 ratio than photoionization. This is most likely a consequence of residual excess internal energy in the ions. It has been believed that collisions plus infrared radiative cooling can be relied on to thermahze ions efficiently on the time scale of seconds in experiments like these, but this confident assumption has recently been found to be invalid. Study of several ions, including iodobenzene [14], phenol and thiophenol [15], and chlorobenzene [16] ions, has shown that, at low pressure, several tenths of an eV of excess energy are often deposited by electron impact ionization and this energy is dissipated only over many seconds. Theoretical understanding of infrared radiative emission rates has supported this very slow low-pressure thermalization [16]. The present results are brought into quantitative agreement with the other studies on the assumption that an average of 0.3 eV of energy above thermal is retained in the dioxane ions, corresponding to a vibrational
8 620
513
456
1
I""'""~~'""'
9
"In
10 Photon
12
11 Energy
(eV)
Fig. 3. Plots of the fist derivative of PIE versus photoionizing photon energy (from data of ref. 9). The units are arbitrary but consistent among the six ions. At the top are indicated the positions (vertical IP) of the two dioxane-ion excited electronic states known’ in this energy region from phot~l~tron spectroscopy [S]. Also shown at the top are the energies corresponding to three of the wavelengths used in the photodissociation experiments.
temperature of about 600 K. The energy-shifted points are replotted in Fig. 1, showing the resulting excellent agreement of the 57/58 ratio from these three different studies. CONCLUSION
Photodissociation of 1,3dioxane parent ions in the ion trap at visible wavelengths shows the same predo~nant m/z 57 and 58 products as monoenergetic photoionization at corresponding energies and ion-beam photodissociation at the same wavelengths, although with a higher 57/58 ratio. Based on the fact that similar dissociation products are observed and supported by the control experiments described, we consider it likely that structural rearrangement is not involved and that the same parent ion structure is involved in all of these experiments. It is suggested that the higher 57/58 ratio in the present results reflects inefficient therma~tion of the electron-impact-produced ions in the low-pressure ICR experiment. With a correction of the present results for excess internal energy, a consistent pattern for the 57/58 ratio is seen for photoionization, ion-beam
9
photodissociation, and ion-trap photodissociation. It appears that the fragmentation pattern of dioxane ions can be considered as reflecting the internal energy of the ion in QET terms and no special effects arising from the initial mode of excitation need be invoked. ACKNOWLEDGEMENTS
Appreciation is expressed to the National science Foundation and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research and to SOHIO, whose generosity provided the instrument. We are especially grateful to Professor MT. Bowers and Dr. N. Kimhner for providing us with preliminary ion-beam photodissociationresults.
1 See, for instance, R.C. Dunbar, in T.A. MiIIer and V.E. Bondybey (Eds.), Molecular Ions. Spectroscopy, Structure and Chemistry, North-Holland, Amsterdam, 1983, p. 231. R.C. Dunbar in M.T. Bowers (Ed.), Gas-Phase Ion Chemistry, Vol. 3, Academic Press, New York, 1984, Chap. 20. 2 Some examples have been reported in which photodissociation yields products differing in intriguing ways from the products of other dissociation methods [e.g. V. Franchetti, B.S. Freiser and R.G. Cooks, Org. Mass Spectrom., 13 (1978) 106. R.L. Hettich and B.S. Freiser, J. Am. Chem. Sot., 108 (1986) 25371, but the factors accounting for these differences have not yet been clarified. 3 There continues to be a possibility of very interesting state-specific photochemistry in some aromatic ketone ions [R. Gooden and J.I. Brauman, J. Am. Chem. Sot., 99 (1977) 19771, but the situation in these experimentally difficuh systems is stih uncertain. 4 J.H. Chen, J.D. Hays and R.C. Dunbar, J. Phys. Chem., 88 (1984) 4759. 5 D.A. Sweigart and D.W. Turner, J. Am. Chem. Sot., 94 (1972) 5599. T. Kobayashi and S. Nagakura, BuII. Chem. Sot. Jpn., 46 (1973) 1558. D. Gonbeau, M. Loudet and P.G. Guillouzo, Tetrahedron, 36 (1980) 381. F.S. Jorgenson, N.L. Lauritsen, R.B. Jensen and G. SchroIl, Tetrahedron, 37 (1981) 3671. 6 J.H. Chen and R.C. Dunbar, Int. J. Mass Spectrom. Ion Processes, 72 (1986) 115. 7 J.D. Hays and RC. Dunbar, Rev. Sci. Instrum., 55 (1984) 1116. 8 M.T. Bowers and N. Kichner. We are grateful to these workers for communicating their prehminary results to us. 9 M.L. Fraser-Monteiro, L. Fraser-Monteiro, J.J. Butler, T. Baer and J.R. Hass, J. Phys. Chem., 86 (1982) 739. 10 E.W. Fu, P.P. Dymerski and R.C. Dunbar, J. Am. Chem. Sot., 98 (1976) 337, 11 J.P. Honovich and RC. Dunbar, Int. J. Mass Spectrom. Ion Processes, 42 (1982) 33. 12 J.D. Morrison, J. Chem. Phys., 21 (1953) 1767; Rev. Pure Appl. Chem., 5 (1955) 22. 13 The quantitative unreliability of derivative plots can be illustrated by looking at the 58/45 ratio at the photoio~tion energy of, for instance, 10.9 eV. The PEPICO result (presumably the correct branching ratio) from Fig. 7 of ref. 9 gives a ratio of 4, while the
10 ratio derived from the PIE derivative plots of our Fig. 3 gives a ratio of 1, a discrepancy of a factor of 4. While such large discrepancies are not seen at most energies, this suggests the degree of ci.rcumspection appropriate in basing conclusions on derivative plots. 14 B. Asamoto and R.C. Dunbar, J. Phys. Chem., in press. 15 B. Asamoto, Ph. D. Thesis, Case Western Reserve University, 1986. 16 R.C. Dunbar, J. Phys. Chem., in press.