4864
J. Phys. Chem. A 2010, 114, 4864–4869
† Ultraviolet and Visible Light Photodissociation of H+ 3 in an Ion Storage Ring
A. Petrignani,*,‡ D. Bing,‡ O. Novotny´,§,‡ M. H. Berg,‡ H. Buhr,|,‡ M. Grieser,‡ B. Jordon-Thaden,‡ C. Krantz,‡ M. B. Mendes,‡ S. Menk,‡ S. Novotny,‡,⊥ D. A. Orlov,‡ R. Repnow,‡ J. Stu¨tzel,‡ X. Urbain,# and A. Wolf‡ Max-Planck-Institut fu¨r Kernphysik, 69117 Heidelberg, Germany, Columbia Astrophysics Laboratory, MC5247, 550 West 120th Street, New York, New York 10027, Department of Particle Physics, Weizmann Institute of Science, 76100 RehoVot, Israel, and PAMO, UniVersite´ Catholique de LouVain, 1348 LouVain-la-NeuVe, Belgium ReceiVed: October 31, 2009; ReVised Manuscript ReceiVed: December 15, 2009
Ultraviolet and visible photodissociation of a vibrationally excited H+ 3 ion beam, as produced by standard ion sources, was successfully implemented in an ion storage ring with the aim of investigating the decay of the excited molecular levels. A collinear beams configuration was used to measure the photodissociation of H+ 3 into H+ 2 + H fragments by transitions into the first excited singlet state with 266 and 532 nm laser beams. A clear signal could be observed up to 5 ms of storage, indicating that enough highly excited rovibrational states survive on the millisecond time scale of the experiment. The decay into H+ 2 + H shows an effective time constant between about 1 and 1.5 ms. The initial photodissociating states are estimated to lie roughly 1 eV below the dissociation limit of 4.4 eV. The expected low population of these levels gives rise to an effective cross section of several 10-20 cm2 for ultraviolet and some 10-21 cm2 for visible light. For using multistep resonant dissociation schemes to monitor rotational populations of cold H+ 3 in low-density environments, these measurements open promising perspectives. Introduction The triatomic hydrogen ion H3+ has one single bound electronic state 11A′ with H2 + H+ as the dissociation limit and with a large binding energy of 4.4 eV. (See Figure 1.)1-3 The first excited singlet state 21A′ is repulsive and at the equilibrium geometry of the ground state lies almost 20 eV higher in energy. Its dissociation limit is H2+ + H, ∼1.8 eV above the H2 + H+ limit. The first photodissociation experiments of electronic ground-state H+ 3 were performed on a microsecond time scale inducing infrared radiative transitions around 10 µm,4-7 which yielded a complex line spectrum of the H+ fragment rate. These fragments were proven to arise from optical transitions between levels close to the dissociation limit, with the upper and often also the lower states being predissociating. No H2+ signal was observed, which is not surprising because direct transitions into the 21A′ state require much higher photon energies. A later experiment using near-infrared (NIR) light at ∼1 µm revealed supporting data for the predissociation mechanism.8 In this experiment, the initial states were determined to lie ∼1 eV below the dissociation limit. Again, only H+ and no H+ 2 signal was observed. The first photodissociation experiment probing the excited electronic state was reported by Bae and Cosby in 1990.9 Until then, the excited state was calculated only theoretically.3 Photon energies between 2 and 5 eV, that is, visible (vis) to ultraviolet (UV), were used to investigate the dissociation. Instead of H+ fragments, H2+ fragments now were †
Part of the special section “30th Free Radical Symposium”. * To whom correspondence should be addressed. E-mail: A.Petrignani@ mpi-hd.mpg.de. ‡ Max-Planck-Institut fu¨r Kernphysik. § Columbia Astrophysics Laboratory. | Weizmann Institute and Max-Planck-Institut fu¨r Kernphysik. ⊥ Universite´ Catholique de Louvain. # Current address: Wihuri Physical Laboratory, University of Turku, 20014 Turku, Finland.
Figure 1. Ground and first excited singlet states of H+ 3 with the two lowest ionic dissociation channels H2 + H+ and H+2 + H. The potentials are shown in the geometry depicted, as obtained in ref 3, in Cs geometry as a function of the HH-H distance R, with r corresponding for any R to the minimum energy on the ground-state surface. Transitions used in the recent photoexcitation studies10,11 from the H+ 3 ground state (see text), and the present photodissociation studies are schematically indicated.
observed, with a steep rise around 2.5 eV. High rovibrational levels of the ground electronic state, reaching internuclear distances as large as 3 Å, narrow the energy difference between the ground and first excited states to approximately this photon energy, allowing for direct transitions to the 21A′ state. The above-mentioned photodissociation experiments were all performed with a high-current vibrationally excited H3+ beam of kiloelectronvolt energies in a collinear configuration, except for ref 8, where a high-power pulsed laser was used in a crossedbeam setup. The time scale on which photodissociation occurred was microsecond after ion-source extraction.
10.1021/jp9104163 2010 American Chemical Society Published on Web 01/25/2010
Ultraviolet and Visible Light Photodissociation of H+ 3
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4865
+ Figure 2. Experimental scheme for observing photodissociation of H+ 3 into H2 + H in the heavy ion-storage ring TSR.
The aim of the present investigation is to probe cold rotational distributions of H+3 in its vibrational ground state on an ion beam of megaelectronvolt energy inside a storage ring utilizing photodissociation. Populations produced by ion sources, which are characterized in separate experiments, are often modified during the ion storage,12 and in situ probing will allow a better determination of the population inside the ring. Whereas H3+ single-photon dissociation from the vibrational ground state requires very high photon energies, that is, ∼20 eV at the equilibrium configuration as mentioned, we want to apply a different scheme: multistep photodissociation. In this scheme (Figure 1), the first step entails photoexcitation of the respective low rotational states of the vibrational ground state to excited rovibrational levels, with either one or more photons in the NIR to vis; the second step then photodissociates the excited ions into fragments by UV dissociation and, with a fast ion beam, these fragments can be detected with near-unity efficiency. The multistep photodissociation provides state selectivity and allows for larger internuclear distances to be sampled, thereby decreasing the energy gap between the electronic states in the photodissociation step. The first results on near-visible photoexcitation of cold H+ 3 detected by chemical probing in a 22-pole trap have been recently published,10 and we have currently extended our investigations to the visible excitation of the cold H3+, populating bound vibrational states up to 2 eV above the vibrational ground state. Calibration procedures are under development to determine the transition probabilities needed to derive the initial H+ 3 population in the multistep dissociation scheme. Here we report on the second component of the scheme, the UV photodissociation of high-lying rovibrational H3+ states in the ion storage ring TSR, Heidelberg. The ring allows for storage of the ions and thereby the investigation of the time dependence of the photodissociation signal, that is, the H2+ fragments. As a consequence, for the first time, our experiments are able to demonstrate that the photodissociation signals in previous experiments9 originate from a decaying fraction of the H+3 beam with lifetimes above the microsecond range typical for single-pass fast beam experiments. The size of the expected photodissociation signal is determined by the product of the relative populations of the various excited levels in the beam and their individual photodissociation cross sections, which we denote as the effective cross section referring to the total H3+ current. In the previous fast beam experiment,9 the effective cross section for producing H+2 fragments was as low as 10-22 cm2. Because
the 11A′-21A′ photodissociation of H+3 involves a high optical dipole moment,3 its maximum cross section, at a favorable photon energy EL depending on the vibrational level, can be expected to be high (up to some 10-17 cm2, taking results13 for H2+ as an orientation); it will significantly depend on EL and, to our knowledge, no calculated photodissociation cross sections are available for the excited H+ 3 states. The effective cross section is even harder to predict because it is strongly influenced by the state populations in the ion source. Because the photon energies reachable by UV laser sources are still much lower than the direct transition energy from the H3+ ground state to the dissociating potential (Figure 1), only high vibrational levels with correspondingly low relative populations are expected to contribute to the observed signal. In the present experiment, we realized the photodissociation on a megaelectronvolt beam stored under ultrahigh vacuum conditions. The typical storage lifetimes of the ions amounted to several seconds and hence the time dependence in the decay of the photodissociating excited H3+ levels should be clearly observable. Experimental Section Figure 2 shows the experimental setup of the storage-ring photodissociation experiment. The H3+ ions were produced in a Penning source, accelerated to 5.07 MeV within a few microseconds before injection into the ring, and stored in the ring as a dc (unbunched) beam for up to 200 ms. The corresponding beam velocity is V0 ) 1.8 × 107 m/s. The ion storage ring TSR was previously applied to investigate the dissociative recom14,15 bination of H+ 3 as well as the internal excitation of these ions and the ion source conditions correspond to those applied in the work of ref 15. Significant vibrational relaxation with time constants down to ∼5 ms was shown to occur using foil-induced Coulomb-explosion measurements on the extracted H3+ ion beam, starting several milliseconds after injection.15 Whereas most other experiments12 at the TSR are performed after an initial phase-space cooling of the ion beam, yielding typically a 1 mm diameter beam after a few seconds of cooling time, the present photodissociation measurements have to use the uncooled, high emittance beam freshly injected into the ring with root-mean-square diameters of 1.5 cm in the horizontal and 0.5 cm in the vertical direction. The size and angular divergence of the ion beam limit the efficiency of photodissociation experiments through both the large extension and the diffuse borders of the beam overlap volume as well as through the relatively large forward angle of the products.
4866
J. Phys. Chem. A, Vol. 114, No. 14, 2010
In one of the straight sections of the ring, a UV laser beam was aligned antiparallel to the ion beam. The ultraviolet beam used was the fourth harmonic of a pulsed Nd/YAG laser producing 266 nm pulses of ∼10 ns duration and 0.5 mJ on average. The antiparallel configuration with the megavolt ion beam leads to a photon energy of 4.9 eV (∼6% Doppler shift included). The laser beam had a diameter of 2.5 cm to overlap most of the ion beam. The laser beam was centered on the entrance and exit windows of the storage ring to coincide with the nominal ion-beam axis over an overlap length of ∼10 m; regarding the ion beam, it is expected that possible deviations of its axis from the nominal storage-ring orbit amounted to ∼1 cm at a maximum, although this could not be verified directly and is only estimated from the possible deviations considered to be acceptable for stable storage. The laser pulse energy corresponded to a peak power density of ∼10 kW/cm2. Further measurements were performed with the second harmonic of the Nd/YAG laser at 532 nm, where the Doppler shifted photon energy amounted to 2.45 eV, and the peak power density was adjusted in a similar range. The laser pulses were shot at random with respect to the time of ion injection into the ring, leaving the laser run unsynchronized at its repetition rate of 10 s-1, and H2+ fragments from photodissociation were detected in coincidence with the laser pulse. The neutral fragments could not be measured as the laser blocked access to the available neutral detectors. The main beam and any charged fragments are deflected around the storage ring corner at the first downstream dipole magnet (D) with the lighter fragments being bent more strongly because of their lower mass, so that they hit a 2 × 2 cm2 Ce/YAG scintillator detector. (See Figure 2.) The limiting count rate of the detection system was 4 MHz, and the time resolution was 0.2 ms. Counting of H2+ fragments started with the H+3 injection into the ring. All possible H+ fragments are too strongly bent and cannot be observed; however, the contribution of this channel is expected to be small at the current photon energy.9 The background signal from H3+ collisions with rest gas was determined through the H2+ signal noncoincident with the laser, as will be explained later. This signal was corrected for influences of time-dependent vibrational excitation in the stored ion beam and provided a measure of the ion-beam current. The contribution from photodissociating HD+ ions in the H3+ beam has been significant. Although the contamination is low, HD+ ions have a much smaller vertical energy difference between the minimum of the ground-state potential and the dissociating potential, and thus at the current photon energies, they already can be efficiently photodissociated for much lower vibrational excitation than H3+. To investigate the HD+ contamination, we have performed measurements with ion beams of mass 3 produced using H2 and HD gas in the Penning source, respectively. Before the mass selection, the ion beam from H2 gas consisted of ∼30% H3+, the remainder being mainly H2+. The overwhelming fraction of the beam after selecting the mass-3 ions will be H3+ with an expected contamination of ∼0.05% through the natural abundance of D of 0.015%, and we will denote the beam of this type, created from H2 gas, as the H3+ beam. With HD gas, mass 3 dominated the source output at >80%, whereas only a small fraction of triatomic hydrogen was produced, as revealed by some of the weak remaining mass components. Therefore, the 3-amu mass component of this beam, created from HD gas, will be denoted as the HD+ beam; it is estimated to be contaminated with at most a few percent of H3+.
Petrignani et al. Data Analysis The photodissociation signal was derived from the measured detector counts, Ni, in each time window, i, of length ∆t ) 0.84 µs, triggered by the laser pulse and comprising the times of arrival of all fragments created by the photodissociation along the ∼10 m long overlap region (time spread ∼0.55 µs). Background contributions, Nb, i, were also recorded, although over a much longer time interval (duration ∆T ) 0.2 to 1 ms) around the window i (cutting out the signal window). These counts are due to the production of mass-2 positive fragment ions from the collisions of the stored ions with the residual gas. The portion of the stored beam from which the detector can observe dissociation fragments is the straight section between the bending magnets directly upstream of the scintillation detector, filled by N0 ions. The counts measured in the signal window are given by
Ni ) N0εL
ei σ˜ (t ) + N0εbk(ti)∆t hνLQL γ i
(1)
and in the background window by
Nb,i ) N0εbk(ti)∆T
(2)
where ei denotes the laser pulse energy, hνL the photon energy, σ˜ γ(ti) the effective photodissociation cross section as defined above, and ti the time since the ion injection (large compared with ∆T, ∆t). In the case of full efficiencies εL and εb, eq 1 assumes complete transverse overlap of the laser with the ion beam inside a laser beam cross-section QL over the total interaction length and full detection efficiency for the mass-2 fragments. Deviations from this assumption in the real setup are described by the parameter εL for the laser process, whereas a similar parameter εb, expressing the counting efficiency only, is used for the background. The residual-gas signal is described by the rate constant
k(t) ) ngσ˜ g(t)V0
(3)
applying the effective collisional cross section σ˜ g(t) for producing mass-2 ionized products by collisions with residual gas molecules, which may vary during the storage time, t, through the changing internal excitation of the ion beam, and the residual gas density, ng. The overwhelming residual-gas fraction is H2,and ng is on the order of 106 cm-3. For both the H3+ and the HD+ beam, the background count rates dNb(t)/dt are found to decrease exponentially in very good approximation for t > 0.08 s. Directly ( 10 not tabulated in ref 19 were extrapolated from V e 10. The highest level
4868
J. Phys. Chem. A, Vol. 114, No. 14, 2010
Petrignani et al.
+ + Figure 4. Mass-2 (H+ 2 and D ) signal in coincidence with 532 nm laser pulses (Doppler-shifted photon energy 2.45 eV) for H3 ions in the same representation as that in Figure 3. The dashed lines show the HD+ photodissociation model at 2.45 eV photon energy, scaled to lie close to the experimental data, whereas the black line shows an exponential fit with a resulting time constant of 1.4(5) ms.
included was V ) 15. Scaled by constant factors, this model indeed represents the observed time dependences for the slow components on both ion beams (Figure 3). The modeled effective photodissociation cross section has a value of 1.5 × 10-18 cm2 at t ) 0. We also performed a short measurement on photodissociation with the second Nd/YAG harmonic (photon energy 2.45 eV). As shown in Figure 4, the photoinduced signal of the H3+ beam is found to have a similar time structure at a lower absolute level. The slow time dependence is in reasonable agreement with the model for the effective HD+ photodissociation cross section at the longer wavelength; on the basis of less populated, higher lying vibrational levels, its value is calculated to be 5 × 10-19 cm2 at t ) 0. The fast component is attributed to excited H3+ levels and has a slightly longer lifetime than that observed with UV light, as could be expected for the radiative lifetimes if even higher vibrational levels are considered than in the latter case. Discussion The observation of fast decay components on only the H3+ beam, together with the absence of a corresponding signal with an HD+ beam, is taken as a clear signature for the observation of a photodissociation signal from the stored H3+ beam, caused by initially populated high-lying H3+ states decaying with time constants on the order of 1 ms. The occurrence of vibrational excitation from the H3+ source and its fast relaxation during the first few milliseconds of storage are consistent with the previous measurement on this beam using foil-induced Coulomb explosion,15 performed under similar conditions including the absence of electron cooling. The present measurement demonstrates the feasibility of the photodissociation scheme for detecting vibrationally excited H3+ in a stored ion beam. Conversely, the sizes of the photoinduced signals show that the efficiency for observing them was seriously limited. Both the laser overlap and fragment detection efficiency can be affected by the use of large, uncooled ion beams. The ratio of ∼3 between the effective HD+ photodissociation cross sections at 4.9 and 2.45 eV as well as the expected natural HD+ contamination in the H3+ beam of ∼5 × 10-4 can be roughly reconciled with the data assuming efficiency ratios εL/ εb much below unity in eq 7. For two of the measurements, the contamination signal in the H3+ data of 4 and the pure HD+ signal in Figure 3, agreement within a factor of 3 is found in the signals S regarding their relative size, which should be ∼3
× 10-4 considering the effective HD+ photodissociation cross sections at both photon energies and the natural HD+ contamination. For these two measurements, which were performed with only a small temporal gap between them and with little intervening retuning of the laser and ion beams, we hence assume similar values of the efficiency ratio εL/εb, amounting to roughly 3 × 10-3. The slow (contamination) component in Figure 4, assuming the HD+ model result and the natural HD+ fraction, then roughly corresponds to an effective cross section of 3 × 10-22 cm2, whereas the H3+ photodissociation signal at t ) 0 represents an effective cross section of roughly 3 × 10-21 cm2. Next, assuming again the natural contamination and the HD+ model results at 4.9 eV, the HD+ contamination signal in the H+3 data of Figure 3 corresponds to an effective cross section of roughly 7 × 10-22 cm2, whereas the initial H3+ photodissociation signal represents an effective cross section up to two orders of magnitude higher, ∼7 × 10-20 cm2. The efficiency ratio for the UV H3+ measurement (red open circles in 3) then had a higher efficiency ratio εL/εb than the other measurements, amounting to roughly 3 × 10-2. In summary, comparing the H3+ and HD+ measurements with the help of the HD+ photodissociation model, we arrive at an effective H3+ photodissociation cross section of several 10-20 cm2 for hνL ) 4.9 eV and of some 10-21 cm2 for hνL ) 2.45 eV. Assuming a high optical dipole moment for the photodissociation,3 as discussed at the end of the Introduction, level populations of the order of 10-4 to 10-3 are estimated for the relevant excited states from these effective cross sections. Photodissociation of H3+ ions was also studied in a separate experiment20 at PAMO, Louvain-la-Neuve, where the kinetic energy release (KER) upon UV photodissociation of a hot H3+ ion beam at kiloelectronvolt energies was measured. A photon energy of 4.2 eV was used, and, in the applied single-pass fastbeam setup, photodissociation occurred on a microsecond scale after the H3+ production in a duoplasmatron ion source yielding similar conditions as those in the Penning ion source used at TSR. The acceleration process, taking place in a low-pressure environment, has been found to preserve largely vibrational level populations from ion sources;21 hence, similar H3+ initial excitation conditions are expected. The measurement yielded fragment kinetic energies (EKER in Figure 1) of ∼1.7 eV, whereas the effective cross section was determined to be on the order of 10-20 cm-2, in rough agreement with the storage ring result. Moreover, dissociative charge transfer with a potassium target and coincident detection of secondary H
Ultraviolet and Visible Light Photodissociation of H+ 3 fragments22 was applied to determine the vibrational population in the H2+ fragments, which were observed to be predominantly in their vibrational ground state (hence supporting only small values of Evib in Figure 1). Therefore, from energy conservation, the H3+ states populated in the Louvain experiment lie 0.7 eV below the first H+ 3 dissociation threshold and, consequently, 3.7 eV above the H3+ ground state. Finally, the measured KER of 1.7 eV on the H3+ (21A′) curve, as represented in Figure 1, corresponds to a dissociation coordinate of R ≈ 4a0, where the vertical distance between this and the 11A′ ground state agrees well with the photon energy (4.2 eV). Therefore, the observation supports a Condon-type vertical transition in the molecular geometry underlying Figure 1. Assuming a similar photodissociation mechanism for the TSR signal at 4.9 eV photon energy, implying a Condon-type vertical transition within Figure 1 and a small vibrational excitation of the H2+ product, the expected KER amounts to ∼2.1 eV. Therefore, the initial H+ 3 states lie ∼1 eV below the dissociation threshold and at an excitation energy of ∼3.4 eV. This supports the expectation that laser monitoring of rotational populations in cold stored H+ 3 ion beams should become feasible after some further development. As mentioned, suitable photoexcitation schemes and calibration procedures are under study. In our related H+ 3 spectroscopy work recently achieving visible excitation up to 2 eV above the ground state, additional increase in the excitation energy appears possible, thus further reducing the gap of roughly 1.4 eV presently remaining between the excitation and the dissociation step of the multiphoton dissociation scheme. Similarly, additional photodissociation experiments are planned to investigate further the dissociation process. Acknowledgment. X.U. is Senior Research Associate of the F.R.S.-FNRS. H.B. acknowledges partial support from the German Israeli Foundation for Scientific Research and Development (G.I.F.) under grant I-900-231.7/2005 and by the European Project ITS LEIF (HRPI-CT-2005-026015). We thank the TSR accelerator group for their support during the experiments.
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4869 References and Notes (1) Tennyson, J. Rep. Prog. Phys. 1995, 57, 421–467. (2) Cosby, P. C.; Helm, H. Chem. Phys. Lett. 1988, 152, 71–74. (3) Talbi, D.; Saxon, R. P. J. Chem. Phys. 1988, 88, 2235–2241. (4) Carrington, A.; Buttenshaw, J.; Kennedy, R. Mol. Phys. 1982, 45, 753–758. (5) Carrington, A.; Kennedy, R. A. J. Chem. Phys. 1984, 81, 91–112. (6) Carrington, A.; McNab, I. R. Acc. Chem. Res. 1989, 22, 218–222. (7) Carrington, A.; McNab, I. R.; West, Y. D. J. Chem. Phys. 1993, 98, 1073–1092. (8) Alvarez, I.; Yousif, F. B.; Urquijo, J. d.; Cisneros, C. J. Phys. B: At., Mol. Opt. Phys. 2000, 33, L317–L323. (9) Bae, Y. K.; Cosby, P. C. Phys. ReV. A 1990, 41, 1741–1743. (10) Kreckel, H.; Bing, D.; Reinhardt, S.; Petrignani, A.; Berg, M. H.; Wolf, A. J. Chem. Phys. 2008, 129, 164312. (11) Berg, M. H.; Bing, D.; Petrignani, A., Wolf, A., in preparation. (12) Wolf, A.; Buhr, H.; Grieser, M.; von Hahn, R.; Lestinsky, M.; Lindroth, E.; Orlov, D. A.; Schippers, S.; Schneider, I. F. Hyperfine Interact. 2006, 172, 111–24. (13) Dunn, G. H. Phys. ReV. 1968, 172, 1–7. (14) Kreckel, H.; Motsch, M.; Mikosch, J.; Glosik, J.; Plasˇil, R.; Altevogt, S.; Andrianarijaona, V.; Buhr, H.; Hoffman, J.; Lammich, L.; Lestinsky, M.; Nevo, I.; Novotny, S.; Terekhov, D. A. O.; Toker, J.; Wester, R.; Gerlich, D.; Schwalm, D.; Wolf, A.; Zajfman, D. Phys. ReV. Lett. 2005, 95, 263201. (15) Kreckel, H.; Krohn, S.; Lammich, L.; Lange, M.; Levin, J.; Scheffel, M.; Schwalm, D.; Tennyson, J.; Vager, Z.; Wester, R.; Wolf, A.; Zajfman, D. Phys. ReV. A 2002, 66, 052509. (16) Barnett, C. F. Collisions of H, H2, He and Li Atoms and Ions with Atoms and Molecules; Atomic Data for Fusion 1; Controlled Fusion Atomic Data Center: Oak Ridge, TN, 1990; ORNL-6086. (17) Dinelli, B. M.; Miller, S.; Tennyson, J. J. Mol. Spectrosc. 1992, 153, 718; see erratum J. Mol. Spectrosc., 1992, 156, 243 (18) von Busch, F.; Dunn, G. H. Phys. ReV. A 1968, 5, 1726. (19) Amitay, Z.; Forck, P.; Zajfman, D. Phys. ReV. A 1994, 50, 2304– 2308. (20) Urbain, X.; et al., to be published. (21) Amitay, Z.; Baer, A.; Dahan, M.; Knoll, L.; Lange, M.; Levin, J.; Schneider, I. F.; Schwalm, D.; Suzor-Weiner, A.; Vager, Z.; Wester, R.; Wolf, A.; Zajfman, D. Science 1998, 281, 75–78. (22) Urbain, X.; Fabre, B.; Staicu-Casagrande, E. M.; de Ruette, N.; Andrianarijaona, V. M.; Jureta, J.; Posthumus, J. H.; Saenz, A.; Baldit, E.; Cornaggia, C. Phys. ReV. Lett. 2004, 92, 163004.
JP9104163
