J. Phys. Chem. 1989, 93, 6625-6628
excited-state conversion, we would predict a time constant of over 90 ns for the reverse process. This is considerably longer than the normal fluorescence lifetime of ACMA-H+ (18 ns) and explains why the excited-state trans to cis process can be neglected at room temperature. The observed dependence of the fluorescence emission spectrum on the excitation wavelength a t low temperatures can also be accounted for in terms of the theoretical results depicted in Figure 2. At low temperatures, cis-trans isomerization is "frozen out", and consequently, excitation at the red edge of the ACMA-H+ absorption spectrum will lead preferentially to excitation and emission from the trans isomer. Excitation within the main absorption band will, on the other hand, lead to emission from both the cis and trans isomers. In this way, we can account for the observed wavelength dependence of the low-temperature spectra. It also follows from the proposed role of cis-trans isomerization that the nature of the solvent and the solvent viscosity are not expected to have much effect on the ACMA-H+ emission behavior. The intrinsic barrier to cis-trans isomerization in the excited state arises from the generation of partial double bond character in the Cz-0 bond between the ring and the methoxy group and does not depend upon some special solvent effect. Moreover, rotation of the methoxy group is expected to have only a small solvent viscosity dependence, judging from studies of the excited-state cis-trans isomerization of analogous vinyl-substituted anthracene.% We should note, however, that since the calculations are for ACMA in a vacuum, we have not taken into account the interactions with solvent molecules. Since the changes in electronic charge distribution are not large, this may not be too serious. According to our interpretation, we expect no effect of salt, no counterion effect, no deuterium isotope effect, and no effect of pH in the range where the ring nitrogen is protonated. All of these predictions are in agreement with experimental observations. Finally, our theoretical calculations provide a clear prediction regarding the effects of deprotonation on cis-trans isomerization. As shown in Table I, the excited-state cis-trans barrier is reduced from 5.4 to 0.8 kcal/mol when the ring nitrogen proton is removed. Because of the very low barrier, cis-trans isomerization is predicted to be approximately 2000 times faster in ACMA relative to ACMA-H+ at room temperature. This is clearly too fast to be followed with nanosecond resolution and explains why neutral (26) Flom, R.;Nagarajan, V.; Barbara, P. F. J . Phys. Chem. 1986, 90, 2085-2092.
6625
ACMA exhibits single-exponential decay in our experiments. The key role of protonation in the complex emission behavior is thus accounted for. In some of our other theoretical studies, we looked for other possible explanations for the unusual fluorescence emission behavior of ACMA-H+. In the ground state, the molecule is predicted to be planar, whereas in the excited state it is predicted to be bent by about 20'. The possibility that the conformational rearrangement from a planar to bent geometry might be responsible for the slow 2-11s process is eliminated since no barrier is predicted to exist. We also explored the possibility that the orientation of the ring nitrogen proton might generate a metastable state and that interconversion between excited states with different N-H orientations might be involved. Again, this possibility was eliminated by the calculations, which show that the conformation with axial hydrogen is energetically unstable. Moreover, if reorientations of the N-H bond from axial to equatorial were involved, a deuterium isotope effect might have been expected, but none was observed. Calculations on the protonated 9AA show that in the ground state the molecule is planar, but bent by 18' in the SI state. Because of the absence of a methoxy group, however, no unusual emission behavior is expected or observed." Thus 9AA-H+ exhibits single-exponential decay. Summary The theoretical calculations that we have presented here account for all of our experimental spectroscopic observations on ACMA-H+, including (i) emission from two different excited-state species, (ii) the barrier to cis-trans interconversion, (iii) the effect of deprotonation in removal of the excited-state interconversion barrier, (iv) the lack of solvent, salt, and buffer effects, and (v) the effect of temperature on the emission properties. Since similar experimental results are also obtained with quinacrine, we may presume that cis-trans isomerization contributes to the complex emission behavior of this molecule as well. It is known that the AM1 program gives a good account of ground-state conformational properties of organic molecules. Our results suggest that it is successful in accounting for the conformational properties of the excited-state molecules as well. Acknowledgment. This work was supported by grants from the National Science Foundation and the American Cancer Society (to D.R.K.). We especially thank Professors Jay Siege1 and John Simon for many helpful discussions.
Laser Ionization Studies of Organophosphonates and PO Radicals S. Randolph Long* and Steven D. Christesen US.Army Chemical Research, Development and Engineering Center, Aberdeen Proving Ground, Maryland 21 01 0-5423 (Received: January 23, 1989; In Final Form: April 26, 1989) Mass spectra of dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP) have been generated in a quadrupole mass spectrometer by ionization with the KrF (248 nm) and ArF (193 nm) excimer lasers. High mass ions, including the molecular ion, are prominent. PO radicals, produced by multiphoton dissociation of parent DMMP, are ionized in resonant two-photon ionization via the AZZ+-XZntransition with (0-0) bandheads near 247 nm.
Introduction Laser ionization of molecules via multiphoton processes holds considerable potential advantage for mass spectrometric analysis of chemical materials. Certainly this is true for molecules such as aromatic hvdrocarbons. whose ionization Dotentials are relativelv low and which have excited electronic statks which couple to thk ground state via strong transitions' The ionization Of such I'ilol@Xle permits the sensitive and Selectivedetection Of these species, while also making possible soft ionization (low degree of
fragmentation) by application of low laser intensities.' In order to ascertain the breadth of potential application of laser ~onizationas a of generating mass spectra, it is important to apply the technique to t n 0 h ~ l e not s apparently easily i o n i z d k (1) The following reviews provide ample background and references on multiphoton ionization mass spectrometry: Grotemeyer, J.; Schlag, E,W, Anpew. Chem.. Int. Ed. E n d . 1988. 27. 447. Lubman, D. M. Anal. Chem. 19g7, 59, 31A:
This article not subject to US. Copyright. Published 1989 by the American Chemical Society
Long and Christesen
6626 The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 A
One such class of molecules of interest to us is that of the organophosphonates. These molecules have ionization potentials around 10 eV and higher and are characterized generally by quite low absorption at wavelengths longer than 200 nm. In this report we discuss mass spectra of two members of this class, dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP), generated by excimer laser ionization. (The most recent measurement of the ionization potential2 of DMMP is 10.7 eV. The ionization potential of DIMP has apparently not been measured but is likely very similar to that of DMMP.) In addition to ions, neutral radical fragments are typically produced by focused ultraviolet laser irradiation of molecules. The most important of these fragments for organophosphonates is the PO radical, whose appearance as an ion in mass spectra is of significance for structural identification. We also present in this report results on the ionization of PO radicals generated by focused UV radiation.
Experimental Section Laser ionization mass spectra of DMMP and DIMP were generated by focusing (with a 10- or 20-cm focal length Suprasil lens) the output of a KrF (248 nm) or ArF (193 nm) excimer laser (Quanta-Ray EXC-1) into an effusive beam of the target molecules. The sample was leaked into the vacuum chamber through a 0.5-mm-diameter orifice along the axis of an Extranuclear Model 275 quadrupole mass spectrometer. Pressure in the vacuum chamber during mass spectral recording was typically 3 X 10" Torr. The laser was operated typically at 20- or 50-Hz repetition rate, and the mass spectrometer scanned at 0.2 amu/s. The electron multiplier output of the spectrometer was amplified and broadened in a Keithley Model 427 current amplifier. The amplified current pulse was averaged in a Stanford Research Systems boxcar integrator, whose output was recorded by computer. For power dependence measurements, dielectric filters (Acton Research) were interposed in the laser beam for attenuation. Mass spectra generated by the standard electron impact (EI) ionization technique were recorded with the same mass spectrometer operating under the same conditions as for laser ionization. The wavelength dependence of mass 47 PO' ions was recorded with the same system except that a Quanta-Ray Nd:YAG-pumped dye laser system was used to produce the ionizing laser radiation. Results and Discussion The purpose of the study discussed here is twofold: (1) to ascertain whether the application of excimer laser ionization could lead to a bias toward high mass ions (particularly, molecular ion) in the mass spectra of organophosphonates; (2) to determine whether the structurally significant PO' ion can be generated by ionization of neutral PO radicals created by the focused UV laser. Results on these aspects will be discussed in turn. Mass Spectra. The diisopropyl methylphosphonate (DIMP) molecule provides a striking contrast between electron impact and excimer laser ionization mass spectra. In Figure 1 are reproduced for comparison the 70-eV E1 spectrum (panel A) and ArF and KrF laser ionization mass spectra (panels B and C, respectively). The laser ionization mass spectra were recorded under the same mass spectrometer conditions as the E1 spectrum. These spectra represent near-threshold ionization at the relatively low laser intensities indicated in the captions, obtained by translating the focusing lens so that the beam was not tightly focused at the region sampled by the mass spectrometer. The laser ionization mass spectra have most of the ion intensity residing in the high mass fragment peaks (at m / e = 97, 123, and 139) and particularly in the molecular ion at m / e = 180. By contrast, the molecular ion (M') is barely visible in the E1 spectrum and a greater degree of fragmentation is observed. Interestingly, the same high mass fragment ions are observed in all three cases, with qualitatively similar relative abundances. It appears probable that, following ionization of the neutral parent, (2) Chattopadhyay, S.; Findley, G. L.; McGlynn, S. P. J . Electron Specrrosc. Relar. Phenom. 1981, 24, 27.
I8
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Figure 1. Mass spectra of DIMP: (A) 70-eV electron impact; (B) 0.04-mJ ArF laser (estd intensity 15 kW/cm2); (C) 0.6" KrF laser (estd intensity 200 kW/cm2).
the DIMP ion absorbs an additional photon(s) and fragments to produce the high mass fragment ions. The relative abundances reflect the relative stabilities of these ions. The laser intensity required for 193-nm ionization is so small that a resonant twophoton process must dominate. Recent vacuum-UV absorption spectra recorded by Syage et al. do show that DIMP absorbs at this ~ a v e l e n g t h . ~ For the dimethyl methylphosphonate (DMMP) molecule, the difference between the laser ionization and E1 mass spectra (displayed in Figure 2) is not as marked as for DIMP, since the molecular ion is prominent for both means of ionization. For KrF (248 nm) laser ionization under the near-threshold ionization conditions applied here, the principal ions observed are M' and the high mass fragments at m / e = 109, 94, and 79. Smaller fragments at m / e = 63 (PO2'), 47 (PO'), 31 (CH30+ and/or P'), 15 (CH,+), and 12 (C') are more prominent in the ArF (193 nm) laser ionization mass spectra and become much more prominent with increase of laser energy at this wavelength. These smaller fragments appear as the result of greater photochemical activity at 193 nm. For both KrF and ArF laser ionization of DMMP, the dependence of ion current signal for the principal high mass ions (79,94, 109, 124) on laser energy is near linear and nearly the same for all (n = 0.95 for KrF and n = 0.85 for ArF, where n is defined by S a I",with I = laser energy, and S = signal). This result is interesting, because, since DMMP absorbs only very weakly at 193 nm and immeasurably at 248 nm? the initial event might be expected to be a simultaneous, two-photon absorption. One would anticipate accordingly at least a quadratic power dependence for parent ionization. Two possible mechanisms may be postulated. The observed reduced power dependence may result from laser power dependent fragmentation of the parent (molecular) ion. Alternatively, saturation of some step in the overall excitation/ionization process may occur. This possibility requires that the laser radiation populates an electronic state at the onephoton energy level via a very weak transition (recalling the transparency of DMMP in the >190-nm region3). The subsequent excitation/ionization step would then be the more likely saturated process. Further planned experimentation on the UV laser photodissociation of the molecular ions and high mass fragment ions should provide insight on the laser energy dependence of the mass spectrum in addition to understanding the formation of the high mass fragment ions. With increase in laser intensity comes increased probability for ionization and fragmentation processes, in general. As demon(3) Syage, J. A.; Pollard, J. E.; Cohen, R. B. Appl. Opt. 1987, 26, 3516; Air Force Systems Command (Space Division) Technical Report SD-TR88-13, February 1988.
Laser Ionization of Organophosphonates and PO Radicals
The Journal of Physical Chemistry, Vol. 93, No. 18, 1989 6627
DMMP
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Figure 3. Top panel: Spectrum of masses generated from D M M P at focal region by using KrF (248 nm) laser (energy 2 mJ; estd intensity
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14 MW/cm2). Bottom panel: Intensity of mass 47 PO' ion as a function of laser wavelength. Laser energies maintained constant at shown values for respective portions of plot. Ordinate range is same for both segments of plot.
l
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Figure 2. Mass spectra of DMMP: (A) 70-eV electron impact; (B) 1.5-mJ ArF laser (estd intensity 0.5 MW/cm2); (C) 8-mJ KrF laser (estd intensity 2.5 MW/cm2).
strated in the top panel of Figure 3 for KrF laser ionization of DMMP, the array of masses observed when the focal region of the laser radiation is sampled by the mass spectrometer is dominated by the structurally significant mass 47 PO+ ion. However, the relative intensities among the characteristic high masses are rather similar to those observed with lower laser intensity. Further observations on the mass 47 PO+ ion are discussed in the following section. Wavelength Dependence of the Mass 47 P o f Ion. Near the 248-nm wavelength of the KrF laser lies the A2Z+-X211electronic spectrum of the neutral PO radical, with the (0-0) bandheads of this transition located at 247.7 and 246.3 nm (doubled due to ground ~ t a t e ) . We ~ have the spin-orbit splitting of the 2111/2,s/z previously used laser-induced fluorescence in the A-X transition to demonstrate that PO radicals are produced in the focused KrF and ArF laser photofragmentation of organoph~sphonates.~*~ This transition was shown to be a sensitive spectroscopic vehicle for (4) (a) Dixit, M. N.; Narasimham, N. A. Proc. Indian Acad. Sci. 1968, ,468, 1. (b) Coquart, B.;Couet, C.; Tuan Arh, N.; Guenbaut, H. J. Chim. Phys. 1967, 64, 1197. ( 5 ) Long. S.R.;Sausa, R. C.; Miziolek, A. W. Chem. Phys. Lett. 1985, 117, 505.
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(6) Sausa, R. C.; Miziolek, A. W.; Long, S. R.J. Phys. Chem. 1986, 90, 3994. (7) Dyke, J. M.; Morris, A.; Ridha, A. J. Chem. SOC., Faraday Trans. 2 1982, 78, 2077. (8) Chou, J. S.; Sumida, D. S.; Wittig, C. Chem. Phys. Lett. 1983, 100, 397; J. Chem. Phys. 1985,82, 1376.
PO detection, as evidenced by the short radiative lifetime of 9 ns for the A ~ t a t e ,indicative ~,~ of a very strong transition. Since the A state lies over halfway to the ionization potential of 8.4 eV,' ionization via this state should be efficient. In order to ascertain whether resonant two-photon ionization (R2PI) via the A-X transition could provide a means of ensuring the appearance of the structurally significant PO+ ion in mass spectra, we examined the wavelength dependence of mass 47 in the 243-253-nm region. The Nd:YAG-pumped dye laser system (DCM dye laser output frequency-doubled and mixed with the 1.06-rm YAG fundamental) was used to generate the tunable radiation. The mass spectrometer was set to pass mass 47. Due to shot-to-shot fluctuations in the signal which generated a relatively noisy spectrum in continuous wavelength scan mode, we employed a point-by-point approach to produce the mass 47 excitation spectrum; Le., the laser wavelength was changed in ca. 0.4-nm increments and the mass 47 signal averaged for about 600 laser shots. The resulting spectrum of the wavelength dependence of the PO+ ion is displayed in the bottom panel of Figure 3. In the 243-249-nm region of this spectrum, where a constant laser energy of 0.5 mJ was applied, several peaks are found. The known (0-0)bandhead positions of the PO neutral A-X transition are denoted as vertical lines. An excellent correspondence is noted between the observed peaks and the known A-X (0-0)bandheads. A third, weaker observed peak corresponds as well to an A-X (1-1) bandhead. The spectroscopy therefore suggests that the PO' generated at wavelengths within the structured region of this spectrum derives from ionization of PO neutral radicals via the A28+ state. Other observations lend support to assignment of this structured portion of the spectrum to the PO radical. In our previous studies of generation of PO radicals by excimer laser photolysis of DMMP, with LIF applied as the probe, we found that at least 95% of the PO radicals are produced in the urr= 0 ground state.6 (9) Wong, K. N.; Anderson, W. R.; Kotler, A. J. J. Chem. Phys. 1986,86, 2406.
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The Journal of Physical Chemistry, Vol. 93, No. 18, 1989
The spectrum of Figure 3 is qualitatively in agreement, since the (0-0) bandheads are by far the most prominent features in the spectrum. (Quantitation is not possible in the present experiment, due to insufficient spectral resolution and the fact that the (1-1) bands overlay the higher energy wings of the (0-0) bands.) The composite evidence therefore suggests that the structured spectrum in the 244-248-nm region of Figure 3 is due to the PO A-X neutral radical generated by photodissociation of the parent DMMP at these wavelengths. Since the pulse width of the beam from the dye laser system is approximately 5 ns, all the processes involved in decomposition of parent into neutral PO radicals as well as the resonant two-photon ionization of these radicals to produce PO+ occur within that time frame. We have also measured the laser energy dependence of PO+ at one of the (0-0)A-X bandheads (246.3 nm) and find n = 2.5 (f0.3). Within experimental error, this measurement is the same as the n = 2.2 (A0.2) we measured previously via LIF for the production of PO radicals using the KrF (248 nm) laser as photolysis s o ~ r c e .The ~ composite process for which we measure the laser energy dependence at 246.3 nm in the present work includes ionization of PO radicals subsequent to their production. The observation that the overall process including ionization to PO+ is characterized by nominally the same laser energy dependence as that for the production of PO neutral radicals suggests that the ionization of PO radicals is an essentially saturated process, over the laser intensity range applied here. This result is not surprising, given that the ionization at 246.3 nm occurs as a resonant two-photon process via the strong A-X transition. Naturally, in the case of a very complex overall process such as the photofragmentation of organophosphonates to produce PO radicals, the physical significance of the measured laser energy dependence is difficult to identify. W e can, however, postulate. We have noted earlier that DMMP has no detectable absorption ~ exists, therefore, no intermediate in the 248-nm r e g i ~ n .There electronic state even moderately coupled optically to the ground state to facilitate absorption of a single -248-nm photon. The expectation that readily follows is that the initial excitation in the DMMP molecule is likely a simultaneous two-photon process, with a quadratic laser energy dependence anticipated for this step. Given that the observed laser energy dependence for production of both PO neutrals and ions is only somewhat greater than quadratic, it would appear that the sequential dissociation and ionization steps subsequent to an initial two-photon excitation are essentially saturated at the laser intensities applied. If this postulated simple mechanism is correct, reconciliation of the greater than quadratic laser energy dependence for production of PO neutrals and PO+ ions with the near-linear dependence for production of molecular ions (previous section) requires one of the following to be also true. (1) The initial excitation events in the two overall processes are the same, and the apparently linear laser energy dependence of M+ is due to laser energy dependent molecular ion fragmentation; or (2) the initial excitation events in the two overall processes are different, with production of M+ p r d i n g via an intermediate state, at the onephoton level. Such a state may be populated only via a very weak transition from the ground state, as noted earlier. We cannot presently distinguish between these processes nor other mechanisms which may be conceived. The PO+ laser energy dependence is significantly greater than the near-linear dependence for the high mass ions as discussed in the previous section. The high mass fragment ions most probably result from an ion fragmentation route starting at the DMMP' molecular ion since they have the same laser energy dependence as the molecular ion. It would appear probable that the PO neutrals, from which the PO+ ions are generated, are produced via a neutral fragmentation route. It is useful to note that the mass spectrum generated by using 0.5 mJ at 246.3 nm
Long and Christesen is virtually identical with that of the upper panel of Figure 3. Since nearly all the PO+ signal (using 246.3 nm) is due to resonant ionization of PO neutrals formed by photofragmentation of the parent DMMP, a substantial portion of the total ion intensity (viz. PO+) generated by the focused laser is due to the neutral photofragmentation/PO ionization process. Both the neutral photodissociation and ionization/fragmentation processes are important in describing the action of a focused UV laser on organophosphonates. The mass 47 wavelength dependence at wavelengths longer than 248 nm is characterized by sharply increasing intensity with increasing wavelength (denoted by circles in Figure 3, bottom). The laser energy dependence measured at 25 1.5 nm is approximately n = 2.6 or similar to that measured for PO+ produced via R2PI of PO neutrals. This result allows one to postulate that the species responsible for the longer wavelength portion of the Figure 3 spectrum is also produced via the neutral fragmentation route. The PO radical is an unlikely candidate since it has no absorptions at these wavelengths. The carrier of this portion of the spectrum may be either (a) an ion, formed by ionization of a neutral photofragment, which is subsequently photodissociated to generate PO+, or (b) a neutral photofragment which undergoes subsequent ionization/photofragmentation to yield PO+.
Conclusion As a general observation, electron impact ionization on most organophosphonates produces mass spectra with little or no molecular ion detectable.1° The laser ionization mass spectra of DMMP and DIMP presented here demonstrate a potential for enhanced molecular ion production in comparison with electron impact ionization for organophosphonates. Since the molecular ion is the single most important ion in a mass spectrum, this is a significant potential enhancement. In the only previous open literature account of laser ionization studies of these molecules, Syage et al. recount results of laser ionization mass spectral studies of DIMP in a supersonic beam, time-of-flight mass spectrometers3 Their effort employed longer wavelength photons than those used here, in order to access high-lying excited states of the parent compound to study their photochemistry. As a result, the largest ion observed was mass 123. This report is, to our knowledge, the first to demonstrate resonance-enhanced two-photon ionization (R2PI) of PO radicals via the A2Z+-X211transition. This approach should be expected to be an efficient probe for PO radicals, since the A-X transition is strong and the A state lies at more than half the ionization potential of PO (8.4 eV). Two-frequency R2PI of PO radicals via the lower lying B2Z+state has been used by Chou et al. as a probe in the infrared multiphoton dissociation of organophosphonates.* In the present study, the R2PI of PO radicals in laser mass spectral studies of DMMP demonstrates that both neutral photodissociation and ionization/fragmentation processes are important in describing the effect of focused UV laser radiation on organophosphonates. It appears probable that use of the appropriate wavelength and energy can ensure the appearance of the structurally significant PO+ ion in the mass spectra of organophosphonates. In a cursory study of ionization of DMMP a t one of the A-X bandheads in a supersonic beam, T O F mass spectrometer, Syage et al. also appear to have observed iotlization of PO radicals, though with considerably lower yield than observed here.3 This difference may be due to differences in laser intensity applied, different sample molecule temperatures (our 300 K effusive beam versus their much colder supersonic beam), or other differences in experimental approach. (10) Sass,
S.;Fisher, T.L.Org. Mass Spectrom. 1979, 14, 257.
