Profound Isotope Effect in Dissociation of Triatomic Hydrogen

Article pubs.acs.org/JPCA

Profound Isotope Effect in Dissociation of Triatomic Hydrogen P. C. Fechner, K. Mozer, and H. Helm* Department of Molecular and Optical Physics, Albert-Ludwigs-Universität, Hermann-Herder-Strasse 3, D-79104 Freiburg, Germany

ABSTRACT: Three-particle dissociation of high-lying Rydberg states of D3 is induced by an external electric field. We observe that the momentum vector correlation map of the center-of-mass motion of the fragments converges near the ionization threshold to two distinct fragment configurations, the near linear geometry and the symmetric acute angle geometry. A comparison is made with the momentum vector correlation map recorded in dissociative recombination of D3+ with slow electrons and with the corresponding results for H3 where the acute angle geometry is conspicuously absent.

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INTRODUCTION The importance of dissociative recombination of H3+ with slow electrons1,2 in astrophysics and in hydrogen plasmas in general has prompted many studies regarding the fragmentation channels of the neutral fragments formed in dissociative recombination (DR). Experiments have been refined to trace the final state distribution, specifically the distribution of total energy among the translational degrees of freedom3−8 in threeparticle decay H3* → H(1s) + H(1s) + H(1s)

It is now generally accepted that the dissociative recombination of H3+ and D3+ ions with slow electrons involves capture into Rydberg states. Among these the npe′ states diabatically connect to the dissociative ground state through Jahn−Teller coupling between electronic and nuclear motion.15 The cold molecular ion in DR experiments is in equilateral triangular geometry, as are the cores of Rydberg states in the experiments involving neutral triatomic hydrogen. Thus it may be surprising that atomic fragments in reaction 1 appear in near-linear orientation. A plausible explanation is found in the potential energy surfaces of Jungen.8 These show avoided crossings that can steer three-atom wavepackets from D3h geometry into the linear configuration. These avoided crossings appear at energies ≈2.7 eV above the dissociation limit (eq 1), thus also lending a plausible argument for the different MVC maps observed in low- and high-lying Rydberg states. In this work we present measurements of MVC maps for Rydberg states of D3 and compare with data recently reported for high Rydberg states in H3 molecules.14 As we observed in H3, the MVC maps for D3 rapidly change with total energy. Equally, a preferred dissociation into near-linear geometries occurs near the ionization threshold, D3+ + e. A marked isotopic difference is, however, the near equal probability for fragmentation into acute angled geometries, the transition

(1)

and among the internal and translational degrees of freedom3,5,9 in the molecular channel H3* → H(1s) + H 2(v ,J )

(2)

Remarkable agreement between experiment9 and theory10 has been achieved for the vibrational distributions for low vibrational levels in reaction 2. The subtle dynamics involved in reaction 1 is not fully understood at this point. Attempts to clarify the dynamics and to predict the momentum vector correlation maps (MVC) for reaction 1 have been made for the lowest lying (n = 2) Rydberg states by Lehner and Jungen11,12 and Galster.13 Experiments revealed increasing complexity in the MVCs for (n = 3) Rydberg states.8 Yet for higher principal quantum numbers, for Rydberg states near the ionization threshold, experiments in H314 revealed a surprisingly simple fragmentation pattern: two atoms with antiparallel momentum vectors, the third atom being nearly at rest. We refer to this configuration as near-linear geometry. © 2013 American Chemical Society

Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 20, 2012 Revised: May 30, 2013 Published: July 18, 2013 9794

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calculated as well as the total kinetic energy W = ∑i Ei released in the three-body decay, i = 1−3. The spectra of total kinetic energy release (KER) serve to identify the energetic location of the parent Ryberg state involved.

region between reactions 2 and 1 being a feature that is completely absent in H3. Possible origins for this drastic difference are discussed.

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EXPERIMENTAL SETUP Our experiment is performed with the fast-beam apparatus discussed previously.14 A schematic of the experimental setup is shown in Figure 1. D3 molecules are formed in charge exchange

RESULTS AND DISCUSSION OF KER SPECTRA IN D3 The charge-exchange process generates a wide spectrum of neutral states, most of which dissociate within a few nanoseconds in the immediate vicinity of the charge transfer cell and remain undetected. Molecules in the rotationless 2p 2 A2″ (N = 0, K = 0) state are metastable16 with a lifetime of ≈1 μs.20 These form the dominant contribution to the beam of neutral molecules which survive the time-of-flight to the Starkfield section. In addition to these metastable molecules a smaller fraction of higher-lying Rydberg states that are also long-lived are present in the neutral beam.14 Some of these undergo unimolecular dissociation and thereby contribute to the KER-spectrum of D3 at zero electric field shown in Figure 2.

Figure 1. Experimental setup for studying dissociation of D3 molecules. Unimolecular decay is detected over the region from the aperture to the beam flag (length 115 mm). Electric-field-induced dissociation is localized to the 3 mm region of the Stark electrodes which is shown on an expanded scale.

of 5 keV D3+ ions with Cs atoms and are guided through a 5 mm long region of low-pressure cesium vapor (p ≈ 10−5 mbar), D3+ + Cs → D3(n , l) + Cs+ + ΔE

(3)

The energy defect ΔE varies according to the Rydberg state quantum numbers (n, l). Reaction 3 is weakly endothermic (ΔE = −0.197 eV) for the 2p A2″ (n = 2, l = 1) state. The defect energy rises with principal quantum number n and reaches −3.89 eV for n → ∞. Residual ions are removed from the beam by a weak electric field after the charge-exchange cell. Downstream from the charge transfer cell (300 mm, corresponding to a time-of-flight of ∼750 ns), the undissociated neutral molecules pass a 1 mm diameter aperture. Molecules that pass this aperture are termed long-lived in the following. Among these are molecules in the metastable 2p 2A2″ state in a variety of vibrational levels, their rotational state being N = 0 (as only these levels are metastable16), and in addition longlived molecules in Rydberg states with principal quantum numbers n ≥ 6 in H3.14 The experiments described below show that in D3 long-lived molecules appear with n ≥ 4. These long-lived molecules enter a Stark-field section and after a distance of about 115 mm from the aperture a beam flag intercepts undissociated molecules. In the 3 mm long shielded Stark-field region an inhomogeneous dc electric field of up to E = 42 kV/cm can be generated by two razor blade electrodes spaced by 2 mm. The electric field induces dissociation of some of the neutral molecules by Stark-mixing with short-lived molecular states.14,17,18 Dissociation fragments with sufficiently high transverse momentum pass the beam flag and reach the multihit coincidence detector after a free flight of variable length, ranging from L = 1150 to 2450 mm. Atomic fragments from a dissociated molecule are detected in triple coincidence on a planar multihit detector with a spatial resolution of 60 μm and a temporal resolution of 25 ps. The coincidence technique permits us to distinguish reactions 1 and 2 from the two arrival time differences and the impact coordinates. For triple coincidences the three momentum vectors, p⃗i, of each atom in the center-of-mass frame can be determined19 and from these momenta the individual kinetic energies Ei can be

Figure 2. Unimolecular KER spectrum of D3. The ionization threshold, D3+ (N = 0, K = 0, v1 = 0, v2 = 0) + e, lies at 4.636 eV above D(1s) + D(1s) + D(1s). It is marked by the vertical arrow. Distinct peaks appear at the principal quantum numbers n = 4, 5 and from levels near the ionization threshold. The Rydberg series marked at the top converges to the first bending mode excited state of D3+. At low energies distinct peaks appear from the molecular 2s and 2p states.

When an electric field is applied to the Stark electrodes, longlived molecules can be forced to dissociate in the Stark-field region because the electric field E couples long-lived Rydberg molecules D3(nl) to rapidly predissociating states D3*, E

D3(nl) → D3* → D(1s) + D(1s) + D(1s)

(4)

The electric field may couple to ns-intermediate states, which (with the exception of 3s) are known to dissociate rapidly21,22 or else directly to the ground state continuum. In contrast to unimolecular decay, reaction 4 occurs well localized in space. Figure 3 compares the high-energy portion of the KER spectrum at two field strengths with the spectrum from unimolecular decay in Figure 2. The electric field leads to a substantial increase in the coincidence rate from Rydberg states with n ≥ 5, the signal from n = 4 being only weakly affected by the field. Origin of High-Lying Rydberg States in D3. The low count rates in Figure 3 signify that only a tiny fraction of charge transfer processes leads to long-lived molecules studied here. A total of about 3 molecules s−1 are detected in the spectrum at zero field, rising to about 15 molecules s−1 at 36 kV. This is to 9795

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Figure 3. High-energy KER spectra obtained at an electric field of 18 and 36 kV/cm in comparison to the zero field spectrum (unimolecular decay) of Figure 2. Labels as in Figure 2.

Figure 4. Model for unimolecular and electric-field-induced fragmentation of Ryberg series members formed in charge transfer of a single rovibrational level of D3+.

The probability for electric-field-induced dissociation should scale as the square of this matrix element24 and be weighted by the expressions 5 and 6. This signal and the sum of unimolecular and field-induced signal are given on relative scales in Figure 4 by the dashed and full lines, respectively. Our model does not assume competition between unimolecular and field-induced decay, yet it represents the main features of the experiment in Figure 3, provided the observed contributions peak at the position of the first bending mode excited state, as marked at the top of Figures 2 and 3. This is in contrast to the situation observed in H3 where similar agreement is obtained for the ground vibrational state.14 The experimental spectra show broader features due to participation of a range of rovibrational parent states of the ion. Low-Lying Rydberg States in D3. At low kinetic energies discrete peaks appear in the spectrum in Figure 2, which are shown on an expanded scale in Figure 5. These peaks are also enhanced by the presence of an electric field. This signal arises from field-induced mixing of the long-lived 2p 2A2″ state (in various forms of vibrational excitation) with the rapidly predissociated 2s 2A′1 state.17,18 In addition, weaker contributions appear at energies of the 2s levels. These are due to

be compared with the total number of neutral molecules formed in charge transfer, which is on the order of 2 × 108 s−1. A model helps to understand the KER spectra of the longlived molecular states. We assume that the probability of electron capture into D3(nl)-orbitals scales with the inverse third power of the principal quantum number

P1 = c /n3

(5)

with c being a constant, neglecting effects of endothermicity in reaction 3. This argument is derived from the volume occupied by the Rydberg orbital. The orbital volume increases with n3 and hence finds increasingly weaker overlap with that of the 5s electron of the cesium atom. Over the distance from the charge transfer cell to the dissociation region, Rydberg states will decay by spontaneous emission, the rate of spontaneous emission decreasing with increasing n and l. We set for the probability of survival P2 = exp[−t /τ(n ,l)]

(6)

with the empirical scaling τ(n,l=n−1) = n /350 μs. This gives for the maximal l-values, τ(7,6) = 0.98 μs and τ(6,5) = 0.62 μs, in good agreement with the lifetime of the corresponding hydrogenic states.23 In the region between the aperture and the beam flag, our long-lived molecules may undergo unimolecular dissociation, which we assume to also occur with the decay rate (eq 6). The product function P1P2 when integrated over the length of the beam path detected gives the spectrum labeled no f ield in Figure 4. This signal describes a single Rydberg series (zero quantum defect) converging to a single rovibrational parent ion D3+. The data were folded with the apparatus broadening function with a fwhm of 100 meV. This spectrum is smeared out in energy primarily because dissociation occurs over the 115 mm long distance between the aperture and the beam flag. On the other hand, the electric-field-induced dissociation signal is localized at the position of the Stark electrodes and hence the flight length of fragments is better defined. To quantify the field-induced dissociation rate, we consider the matrix element for the electric-field-induced overlap between molecular state and rapidly predissociated state23 3 ⟨nl| r |n , l + 1⟩ = − n n2 − l 2 (7) 2 3

Figure 5. Vibrational states of the 2s and 2p appear following electricfield-induced dissociation of D3* and H3* at high Stark-field voltages. The D3 spectrum has been offset by +5 for clarity. The vibrational labeling is ν1ν2. The H3 peak at 1.2 eV cannot be attributed to any known state of the molecule, its origin is currently not understood. 9796

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Figure 6. Connection between hyperspherical coordinates and corresponding spatial and momentum configurations. The spatial configuration sphere spanned by the three hyperspherical parameters {ϑ, φ, ρ} is shown at the left. If ρ is fixed and the sphere surface is projected into the equatorial plane, the spatial counterpart of Dalitz plot appears, which describes the spatial arrangement of the fragments (center). Correspondingly, the momentum vectors are given by drawing arrows from the center-of-mass of any given triangle to its corners and one obtains the Dalitz plot, which images the momentum configurations of the fragments in three-body decay (right).

Figure 7. Signal and error bars for the segments in a Dalitz map shown together with the absolute location of the numbered segment in each 60° slice of the phase space. The signal gives the relative intensity normalized to the maximal count rate, the total counts being ≈1 × 104.

explained in detail in the appendices of refs 8 and 13. In spatial representation the hyperradius ρ controls the overall size of the triangular molecular geometry, ϑ and φ describe its shape (Figure 6). Analogously, in momentum-space ϑp and φp determine the relative orientation of the three momentumvectors, and ρp scales their length and is thus a measure of the total kinetic energy release, W ∝ ρp2. Each molecule that contributes to a peak in the data shown in Figures 2−5 also delivers a data point to the Dalitz map for the respective Rydberg state. For the Cartesian coordinates of the Dalitz map we choose

radiative decay of higher lying long-lived Rydberg states. The low-energy KER spectrum of H3 is included in Figure 5 for comparison. The difference in zero-point energies in the two molecules is clearly seen in the shift of vibrational states. A profound difference appears in the population of vibrational levels of the 2p-metastable states in H3 and D3. The ratio of populations in (ν1,ν2) is for H3 (0,0):(0,1):(1,0) = 20:1:3 whereas it is 5:8:1 for D3. In other words, the bending mode excited contribution in D3 amounts to ≈60% whereas it is only ≈4% in H3. The 15 times larger contribution of bending mode excited levels in D3 is very likely a consequence of the nuclear spin statistics discussed by Gellene and Porter.16 These authors showed that the statistical population of the parent ion rotational level leading to metastable neutral molecules is reduced in D3 by an order of magnitude for the vibrational ground state whereas no such reduction should appear for the bending mode excited state.

x = 3(ε2 − ε1)/ 3

y = 3(ε3 − 1/3)

(9)

where the reduced kinetic energy of the ith atom in the centerof-mass frame, εi = p⃗i2/(2mW) is normalized to the total kinetic energy release

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W=

DALITZ PLOTS The high dimensionality of experimental information obtained in three-particle decay of a triatomic molecule D3(nl) → D(1s) + D(1s) + D(1s)

and

1 2m

∑ pi ⃗ 2 i

(10)

Our definition of Dalitz coordinates (eq 9) includes a factor of 3 for easier labeling of the figures. Due to momentum conservation, all experimental data fall inside a circle of radius x2 + y2 = 1. The event position inside this circular area reflects the relative orientation of the momentum vectors of the three hydrogen atoms in the center-of-mass frame, as apparent from

(8)

can be reduced by mapping the center-of-mass momentum or spatial arrangement configuration space in terms of three hyperspherical parameters, {ρ, ϑ, φ}. These coordinates are 9797

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Figure 8. Comparison of Dalitz plots for high Rydberg states in D3 and H3 at 36 and 21 kV/cm, respectively. The two plots labeled Strasser et al. are taken from ref 6. These are Dalitz maps obtained in an indirect way from three-particle coincidences detected in dissociative recombination of slow electrons at the TSR in Heidelberg.

Figure 9. Dalitz maps for Rydberg states of H3 and D3 above the lowest ionization limit. H3 fragments appear barred from access into the acuteangled fragmentation geometry. This geometry is prominent in D3 and is likely due to the preferred population of bending mode excited states in the D3+ precursor.

Using slices of the energy interval shown in Figure 3, Dalitz maps as a function of Rydberg state energy can be obtained. Examples are given in Figure 8, which compares maps obtained for H3 and D3 Rydberg states in electric-field-induced dissociation. Comparing the results for D3 and H3 in Figure 8, we see that the Dalitz maps are similar in many aspects. More complex maps appear for lower principal quantum numbers (n ≤ 7), but they simplify at higher values of n. In the case of H3 the simplicity is extreme. For Rydberg states near and above the ionization threshold practically all fragments appear in linear geometry. This trend is also apparent in D3. However, in D3 an equal portion of fragments chooses the acute-angled configuration, a correlation that is practically absent in H3. Our finding for states above the ionization threshold is consistent with the indirect determination of a Dalitz map in DR experiments of H3+ and D3+ with slow electrons.6 We include these maps in the last row in Figure 8. Strasser’s Dalitz maps were inferred by a Monte Carlo reconstruction method from spatial fragmentation patterns in the absence of timing information of the correlated fragments. This rather different experiment and method of analysis also points to a dominance of fragment orientations in near-linear geometries.

Figure 6. As we detect indistinguishable atoms, the Dalitz maps show 6-fold symmetry. Our representation of Dalitz data is tailored to quantify the statistical error in each segment of the phase space of the Dalitz map. An example is given in Figure 7. The segmented presentation permits consideration of statistical and absolute measurement uncertainties. Altogether ten observables are used to calculate the position of a three-body event inside the Dalitz map. These are six impact positions, two time differences, the neutral beam speed and the distance between dissociation and detector. The absolute error in these observables limits the resolution in the Dalitz phase space; however, at the segment size used in Figure 7 these errors are much smaller than the statistical error due to the limited count rate. The area of circular segment fractions are chosen such that the statistical error of events represented by each segment remains below ≈10%. An exception are the segments 37−39, for which our coincidence detector geometry is virtually blind, the areas marked in pink color in Figure 7. The Dalitz data for Figure 7 are taken from the energy interval 4.81−4.92 eV of the spectrum at 36 kV/cm in Figure 3. They are representative for the signal of Rydberg states with energies around the ionization limit of D3+ + e. 9798

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(5) Müller, U.; Cosby, P. C. Product State Distributions in the Dissociation of H3 (n=2,3) Rydberg States. J. Chem. Phys. 1996, 105, 3532−3550. (6) Strasser, D.; Lammich, L.; Kreckel, H.; Krohn, S.; Lange, M.; Naaman, A.; Schwalm, D.; Wolf, A.; Zayfman, D. Breakup Dynamics and the Isotope Effect in H3 and D3 Dissociative Recombination. Phys. Rev. A 2002, 66, 032719-1−13. (7) Laperle, C. M.; Mann, J. E.; Clements, T. G.; Continetti, R. E. Three-body Dissociation Dynamics of the Low-lying Ryberg States of H3 and D3. Phys. Rev. Lett. 2004, 93, 153202-1−4. (8) Galster, U.; Baumgartner, F.; Müller, U.; Helm, H.; Jungen, M. Experimental and Quantum Chemical Studies on the Three-particle Fragmentation of Neutral Triatomic Hydrogen. Phys. Rev. A 2005, 72, 062506-1−14. (9) Galster, U.; Kaminski, P.; Beckert, M.; Helm, H.; Müller, U. Kinematically Complete Final State Investigations of Molecular Photodissociation: Two- and Three-body Decay of Laser-prepared H3 3s 2A1′. Eur. Phys. J. D 2001, 17, 307−318. (10) Schneider, I. F.; Orel, A. Non-adiabatic Couplings for H3. J. Chem. Phys. 1999, 111, 5873−5881. (11) Lehner, M.; Jungen, M. Three-particle Dissociation of D3 and H3 (2sa′1): Energies and Momentum Correlation Maps for Selected Rovibrational Levels. J. Phys. B: At., Mol., Opt. Phys. 2009, 42, 0651011−11. (12) Lehner, M.; Jungen, M. Three-particle Dissociation of D3 and H3 (2sa′1): Wave Packet Dynamics and Momentum Correlation. J. Phys. B: At., Mol., Opt. Phys. 2012, 45, 175101-1−8. (13) Galster, U. Formation of Fragment Momentum Correlations in Three-body Predissociation of Triatomic Hydrogen: The Interplay of Geometric Phase Effects and Ground State Dynamics. Phys. Rev. A 2010, 81, 032517-1−10. (14) Fechner, P. C.; Helm, H. Momentum Vector Correlation Maps for Three-particle Fragmentation of Neutral Triatomic Hydrogen Near Its Ionization Threshold. Phys. Rev. A 2010, 82, 052523-1−7. (15) Kokoouline, V.; Greene, C. H. Photofragmentation of the H3 Molecule, Including Jahn-Teller Coupling Effects. Phys. Rev. A 2004, 69, 032711-1−16. (16) Gellene, G.; Porter, R. Experimental Observations of Excited Dissociative and Metastable States of H3 in Neutralized Ion Beams. J. Chem. Phys. 1983, 79, 5975−5981. (17) Höffler, H.; Fechner, P. C.; Gisi, M.; Baumgartner, F.; Helm, H. Momentum Vector Correlations in Three-particle Fragmentation of Triatomic Hydrogen. Phys. Rev. A 2011, 83, 042519-1−8. (18) Baumgartner, F.; Helm, H. Stark Field Control of Nonadiabatic Dynamics in Triatomic Hydrogen. Phys. Rev. Lett. 2010, 104, 1030021−4. (19) Müller, U.; Eckert, T.; Braun, M.; Helm, H. Fragment Correlation in the Three-Body Breakup of Triatomic Hydrogen. Phys. Rev. Lett. 1999, 83, 2718−2721. (20) Reichle, R. Identifizierung Hochangeregter Zustände von H3 und D3 mit Zweiphotonenanregung. Diploma Thesis, Univ. Freiburg, 1997. (21) Tashiro, M.; Kato, S. Quantum Dynamics Study on Predissociation of H3 Rydberg States. J. Chem. Phys. 2002, 117, 2053−2062. (22) Tashiro, M.; Kato, S. Predissociation of H3 2s Rydberg State: Quantum Dynamics Study. Chem. Phys. Lett. 2002, 354, 14−19. (23) Bethe, H.; Salpeter, E. Quantum Mechanics of One- and TwoElectron Atoms, 3rd ed.; Plenum Publishing Corp.: New York, 1977. (24) Bordas, C.; Lembo, L.; Helm, H. Spectroscopy and Multichannel Quantum-defect Theory Analysis of the np Rydberg Series of H3. Phys. Rev. A 1991, 44, 1817−1827. (25) Fechner, P. C.; Helm, H. Manuscript in preparation. (26) Oka, T. Observation of the Infrared Spectrum of H. Phys. Rev. Lett. 1980, 45, 531−534.

CONCLUSIONS Our experimental data reveal a profound isotope effect in the momentum correlation of the three hydrogen atoms that are formed in dissociation of Rydberg states near and above the lowest ionization threshold. To illustrate this, we show the two highest energy Dalitz maps W > IP for H3 and D3 in 3Drepresentation in Figure 9. Practically all events in H3 lead to the near-linear configuration. In D3 a near equal sharing between linear and acute angled geometry is observed. To explain our observation of the near-linear configuration in H3, we had previously used an argument from molecular dynamics.14 The diabatic predissociation channel 12Σ+g , shown in Figure 1 of ref 8, opens Rydberg states at energies higher than 2.7 eV above H(1s) + H(1s) + H(1s) to dissociation into linear geometry. One may argue that due to the mass difference the vibrational coupling of the Rydberg molecule to the dissociative state is somewhat weaker in the heavier triatomic deuterium molecule; nevertheless, this path should equally be open in D3 and H3. An alternative explanation may be found in the nonadiabatic coupling term, which initiates predissociation. For vibronic coupling of ns-states into the ground state continuum, this coupling term is explicitly known.13 It favors acute angled fragmentation for bending mode excited vibrational states, as can be shown25 in an analysis of the ∂/∂φ term in eq 9 of ref 13. Regardless of the electronic state symmetry of the initially long-lived molecules, the Stark effect makes the short-lived sstates possible intermediates to predissociation. The observed preferential bending mode excitation in long-lived D3 Rydberg states may thus be at the heart of the observed isotope effect. Similar isotope effects appear in Dalitz maps of lower lying states; they are currently a topic of more detailed study.25 After thirty years of study, the triatomic hydrogen molecule is still ripe with surprises and at the same time a prototype for the complexity of molecular dynamics. It appears that triatomic hydrogen will entertain generations to come and, very clearly, Oka’s analysis of its ionic structure26 laid the foundation to an understanding of excited neutral triatomic hydrogen, as intermediate or in its long-lived forms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Deutsche Forschungsgemeinschaft under Grant No. HE 2525/5. We thank Dr. Strasser for providing us with his original data.

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REFERENCES

(1) Suzor-Weiner, A.; Schneider, I. F. Chemistry: Mystery of an Interstellar Ion. Nature 2001, 412, 871−872. (2) Kokoouline, V.; Greene, C. H.; Esry, B. D. Mechanism for the Destruction of H+3 Ions by Electron Impact. Nature 2001, 412, 891− 894. (3) Cosby, P. C.; Helm, H. Photodissociation of Triatomic Hydrogen. Phys. Rev. Lett. 1988, 61, 298−301. (4) Peterson, J.; Devnyck, P.; Herzler, C.; Graham, W. Predissociation of H3 n=2 Rydberg States: Product Branching and Isotope Effects. J. Chem. Phys. 1992, 96, 8128−8135. 9799

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