Strong fields induce ultrafast rearrangement of H-atoms in H 2 O
aa r X i v : . [ phy s i c s . c h e m - ph ] D ec Strong fields induce ultrafast rearrangement of H-atoms in H O F. A. Rajgara, D. Mathur,
1, 2, ∗ A. K. Dharmadhikari, and C. P. Safvan Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400 005, India UM-DAE Centre for Excellence in Basic Sciences,University of Mumbai - Kalina Campus, Mumbai 400 098, India Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India
H-atoms in H O are rearranged by strong optical fields generated by intense, 10 fs laser pulsesto form H +2 , against prevailing wisdom that strong fields inevitably lead to multiple molecularionization and the subsequent Coulomb explosion into fragments. This atomic rearrangement isshown to occur within a single 10 fs pulse. Comparison with results obtained with ∼ ions helps establish thresholds for field strength and timerequired for such rearrangements. Quantum-chemical calculations reveal that H +2 originates in the A state of H O when the O-H bond elongates to 1.15 a.u. and the H-O-H angle becomes 120 o .Bond formation on the ultrafast timescale of molecular vibrations (10 fs for H +2 ) has hitherto notbeen reported. Ready availability of femtosecond lasers has fuelledwidespread contemporary interest in molecular dynamicsin intense optical fields (for a recent compilation of cogentreviews, see [1], and references therein). Intense fieldshave magnitudes that match intra-molecular Coulom-bic ones, typically ∼
50 V ˚ A − . Exposing molecules tothem ruptures one or more bonds, a consequence of field-induced electron ejection, leaving behind two or moreionic cores that experience strong Coulombic repulsion.The inevitable Coulomb explosion sets the time limit forthe ensuing fragmentation.Chemical reactions necessitate rearrangement ofmolecular constituents such that different moieties formnew bonds. A priori , this would be considered very un-likely in the strong-field regime as, by prevailing wis-dom, Coulomb explosion would occur on much shortertimescales. However, results we report in the followingchallenge this wisdom: we observe strong-field-inducedrearrangement of two H-atoms in H O that occursfaster than Coulomb explosion, within 10 fs.Techniques used to generate intense optical pulses areintricately coupled to those for generating short pulses.Several laboratories have recently succeeded in generat-ing pulses short enough that only a handful of opticalcycles constitute a single pulse [2, 3, 4, 5, 6]. Mightthe use of such “few-cycle” pulses yield new insights instrong-field molecular dynamics?Strong-field molecular dynamics is essentially drivenby three processes [1, 7, 8]: electron rescattering, spatialalignment and enhanced ionization. Rescattering per-tains to the action of the oscillating optical field on theionized electron wherein, after ejection, it is acceleratedback towards the molecular core half a cycle after its“birth”, causing further ionization. Spatial alignmentoccurs when the linearly polarized optical field acts onthe induced dipole moment. Enhanced ionization is me-diated by charge exchange effects that increase ioniza-tion propensity when the bond length becomes double ortriple its equilibrium value. The ultrashort domain al- lows considerable simplification in the scheme of thingsas there is sufficient time only for rescattering to effec-tively contribute to the overall dynamics [7, 8, 9, 10]: theother processes are effectively “switched off”.But what constitutes an ultrafast pulse? Until re-cently, 100 fs pulses would have been deemed “ultrafast”,but not so in the context of work reported here. In 100fs, the field amplitude changes little between successivecycles, implying an adiabaticity in molecular response:electrons in the outermost orbital tunnel ionize before thefield increases any further. Adiabaticity implies that theirradiated molecule ceases to be a molecule (it becomesan ion) well before the optical field has attained its peak.In the ultrafast domain, however, the electronic responseis not adiabatic if the pulse is short enough that the irra-diated molecule survives to higher fields before ionizing:the electron is thus exposed to much higher, rapidly in-creasing fields, gaining more energy before rescattering.Indeed, in experiments on high harmonic generation inAr, 25 fs pulses yield harmonics that are many ordershigher than with 100 fs pulses of the same intensity, in-dicating that Ar in the ultrashort field survives to higherintensities - a consequence of the non-adiabatic responseof the atomic dipole to the ultrafast rise-time [11, 12, 13].In our experiments 10 fs pulses were generated bythe recently-introduced filamentation technique [14]. Weadopted the variant of using two gas-filled tubes [15] intandem (Fig. 1). A typical mass spectrum obtained withwater vapour is shown in Fig. 2a. Molecular ionizationtotally dominates in the 10 fs regime, in contrast to thesituation with 40-100 fs pulses [7] wherein fragments likeO q + ( q =2-5) are obtained at 5-10% yield levels (normal-ized to H O + ) at similar intensities (2 × W cm − ).This is a signature of non-adiabaticity in the few-cycledomain [7]. We wish to focus here on data presented inFig. 2b which shows that at 1% yield, clear signature isobtained for formation of H +2 ions. We made measure-ments over the intensity range 2-8 × W cm − and atdifferent operating pressures to verify that the H +2 signaldoes, indeed, emanate from the laser-molecule interac-tion, and from a unimolecular process. 2 × W cm − appears to be the threshold intensity below which no H +2 signal is obtained.The most significant feature of the H +2 signal is itsdependence on the polarization of the ultrashort pulse.H +2 is obtained only when the polarization vector is par-allel to the TOFMS axis; the signal extinguishes whenthe polarization is orthogonal. This polarization depen-dence constitutes unambiguous signature of H +2 beingformed by an intramolecular rearrangement. If a bi-molecular reaction involving the interaction of a protonwith some other water molecule were involved, the pro-cess would not exhibit polarization dependence. More-over, a quadratic dependence on H O vapor pressurewould be expected, contrary to measurements. More sig-nificantly, the polarization dependence also indicates thatthe rearrangement occurs within a single pulse, and isdriven by the strong field. If H +2 formation were simplya consequence of rearrangement in structure of an ex-cited H O + state, no polarization dependence would beexpected. Taken together, these observations all point toformation of H +2 ions via motion of two H-atoms on anultrafast timescale, one that matches the period of O-Hvibration.Does quantum mechanics allow this? We carried out ab initio calculations of the potential energy surfaces ofneutral and ionized H O. We made unrestricted Hartree-Fock computations using a 6-311 basis set with two d -and one f -type of orbitals using the GAMESS suite ofprogrammes [16]. The C n ν symmetry of H O was con-served in all our calculations. An input file was auto-matically generated at each point of the potential energysurface (with specified O-H distance and H-O-H angle)and calculations were run individually. Part of the po-tential energy surface of the lowest A state of H O isshown in Fig. 3a as a contour plot and shows a distinctpotential minimum zone when the O-H bond elongatesto 1.15 a.u. and the H-O-H angle becomes 120 o . Twocuts through this region (shown in Fig 3b for two val-ues of H-O-H angle) indicate that the minimum is deepenough to support a long-lived H +2 state. We also com-puted various states of H O q + ( q =1-3) but obtained noevidence for a potential minimum capable of supportingH +2 in any other surface.While quantum chemistry appears to theoretically vin-dicate the formation of H +2 from doubly-charged watermolecules, no time-dependent information is forthcom-ing from such computations. How fast does a strong-fieldinteraction have to be before such H-H rearrangementsbecome impossible?We have attempted to answer this question by con-ducting experiments wherein a beam of fast, highly-charged ions generates a transient strong field of ap-proximately the same magnitude as our few-cycle laser.Specifically, we made large impact-parameter ( > A ) col- -50 -25 0 25 500.00.20.40.60.81.0 Time (fs) N o r m a li z ed i n t en s i t y
10 fs -6-4-20246 P ha s e (r ad ) Fig. 1
Ap 1 BS 50 fs, 1 kHz Laser Tube 1 Tube 2 SPIDER CDM 2 CDM 1 CM3 CM1 CM2 CM4 M 1 M 3 M 2 Ap 2 Ap 3 M4 TOFMS Channeltron SM f =5cm FIG. 1: Laser light from a Ti-sapphire based amplified system(800 nm wavelength, 0.4 mJ pulses of 50 fs duration at 1 kHzrepetition rate) were focused with a metal-coated sphericalmirror (CM1, f = 1 m) onto a 1.5 m long tube filled with Argas at 1.2 atm where filamentation occurred. The resultingwhite light output was compressed using a set of chirped di-electric mirrors (CDM1) to produce 16 fs pulses with 0.3 mJenergy which were, in turn, passed through an aperture andfocused (using CM4) on to a second 1 m long tube filled withAr gas at a pressure of 0.9 atm. The resulting broadbandlight was compressed once again (using CDM2) to yield 10fs pulses, with typical energy of 0.2 mJ. Temporal and phasefeatures of the pulses were characterized by spectral phase in-terferometry for direct electric field reconstruction (SPIDER)and steered by sets of high reflectivity mirrors into an ultrahigh vacuum (UHV) chamber through a 300 µ m thick fusedsilica window. The laser light was focused within the UHVchamber by a spherical mirror (SM, f=5 cm) to typical peakintensities in the range 10 -10 W cm − . Ions formed inthe laser-molecule interaction were electrostatically extractedwith unit efficiency into a linear (20 cm) time-of-flight spec-trometer (TOFMS). A typical SPIDER trace is shown for a9.7 fs pulse; note the constant, zero phase within the pulseduration. lisions of H O molecules with 100 MeV Si ions froman ion accelerator [17]. The 8+ charge on the Si-ion en-sures a strong field and the energy ensures that the ion-molecule interaction time is only ∼
300 attoseconds. Theresulting ion spectrum (Fig. 4) shows multiply chargedfragments, including hydrogen-like O at 0.2% yield lev-els, but there is no evidence for H +2 ions. It appears that300 attoseconds is far too short a time for the two H-atoms to reach the potential minimum indicated in Fig.3. Does this ultrafast phenomenon in water in the strong-field regime have relevance outside the intense laser lab-oratory? The answer is in the affirmative. Watermolecules also experience strong fields in natural, ter-restrial environments. Upon freezing water to -150 Cice is formed with hexagonal structure with disorderedH-bonds in which each O-atom is located at the centreof a tetrahedron formed by four nearest-neighbour oxy-gens [18]. Following the four-decade-old prescription ofCoulson and Eisenberg [19], water ice with I h structurecan be shown to possess an internal electrostatic field of ∼ V m − . This field corresponds to an effective laserintensity of I eff ∼ × W cm − .By invoking adiabaticity, it becomes mandatory to ac-count for how optical energy transfers to H O, resultingin formation of H O q + ( q ¿1) which, in turn, leads to thebreaking of O-H bonds, leading ultimately to the exper-imental observable of H + fragments rearranging to formH +2 . We estimate the energy transfer efficiency using theadiabatic parameter, λ M = τ laser / τ vib , which comparesthe duration of the laser-molecule interaction, τ laser , withthe period of O-H vibration (10 fs) in the water molecule, τ vib . One expects the adiabatic domain when λ M >> λ M be-comes more meaningful when we account for the dura-tion of the field as well as its peak magnitude with re-spect to E a , the field magnitude required to suppressthe Coulomb barrier to the ionization energy, IE. Tak-ing E a ∼ and noting that our laser intensitieswere in the PW cm − range, it is clear that our operating conditions were well away from adiabatic.In summary, we have probed the strong field ionizationof water molecules in the non-adiabatic regime that isaccessed when laser pulses of only 10 fs duration are used.We observe a counterintuitive rearrangement of H-atomson such an ultrafast timescale, manifesting itself in theobservation in mass spectra of H2+, a molecular speciesthat is not expected when H O dissociates.We are grateful to the Department of Science and Tech-nology for partial but important financial support for ourfemtosecond laser system. ∗ Electronic address: [email protected][1] K. Yamanouchi, S. L. Chin, P. Agostini, and G. Fer-rante, Progress in Ultrafast Intense Laser Science, Vols.1-3 (Berlin: Springer) 2007.[2] A. Baltuska, et al., Nature , 611 (2003).[3] S. Baker, et al., Science , 424 (2006).[4] H. Niikura, et al., Nature , 826 (2003).[5] C. A. Haworth, et al., Nature Physics , 52 (2007).[6] I. A. Bocharova, et al., Phys. Rev. A , 053407-1 (2008).[7] D. Mathur, F. A. Rajgara, A. K. Dharmadhikari, J. A.Dharmadhikari, Phys. Rev. A , 023414-1 (2008).[8] D. Mathur, A. K. Dharmadhikari, F. A. Rajgara, J. A.Dharmadhikari, Phys. Rev. A , 013405-1 (2008).[9] X. M. Tong, et al., J. Phys. B —bf 38, 333 (2005).[10] A. S. Alnaser, et al., Phys. Rev. Lett. , 183202 (2004).[11] I. P. Christov, et al., Phys. Rev. Lett. , 1743 (1996).[12] S. C. Rae, K. Burnett, Phys. Rev. A , 2490 (1993).[13] T. Brabec, F. Krausz, Rev. Mod. Phys. , 545 (2000).[14] C. P. Hauri, et al., Appl. Phys. B , 673 (2004).[15] A. K. Dharmadhikari, J. A. Dharmadhikari, F. A. Raj-gara, D. Mathur, Opt. Express , 7083 (2008).[16] M. W. Schmidt, et al., J. Comput. Chem. , 1347(1993).[17] D. Mathur, Phys. Rev. A , 032502-1 (2001).[18] D. Eisenberg, W. Kauzmann, The structure and proper-ties of water (Oxford University Press, London, 1969).[19] C. A. Coulson, D. Eisenberg, Proc. Roy. Soc. London A , 445 (1966). b)a) I on y i e l d ( % ) Time-of-flight ( m s) H +2 I on y i e l d ( % ) Time-of-flight ( m s) Parallel polarization Perpendicular polarization H O + Fig. 2
FIG. 2: a) A typical mass spectrum obtained upon irradia-tion of water vapour by 10 fs pulses of peak intensity 2 × W cm − . Note the absence of large-scale fragmentation andthe almost total domination of molecular ionization. b) Sig-nal corresponding to H +2 at yield levels of ∼
1% with respectto the H O + yield at an intensity of 4 × W cm − . Notethe polarization dependence of this signal: no H +2 is obtainedwhen the laser polarization vector points in a direction per-pendicular to the axis of the time-of-flight spectrometer. b) O-H distance (a.u.) H - O - H ang l e ( deg r ee s ) -76.00-74.63-73.25-71.88-70.50-69.13-67.75-66.38-65.00 a) E ne r g y ( H a r t r ee s ) q = 120 o q = 104 o Fig. 3
FIG. 3: a) Contour plot of the potential energy surface ofH O in its lowest A electronic state. Note the zone in-dicating a potential minimum when the O-H bond length is1.15 a.u. and the H-O-H angle is 120 o . b) Cuts through thepotential surface at H-O-H angles of 120 o and 104 o (the equi-librium angle for neutral H O in the ground electronic state)clearly illustrate the potential minimum.
200 400 600 800010002000 H O + OH + H + H + O O O I on c oun t s Channel nos. (arb. units) missing H +2 Fig. 4
FIG. 4: Time-of-flight spectrum obtained in collisions of abeam of thermal H O molecules with 100 MeV Si ionsfrom a heavy-ion tandem accelerator. The ion-water inter-action time is ∼
300 attoseconds. Note the absence of a signalcorresponding to H +2 ions. Channel numbers are a measureof flight time (in arbitrary units), with individual ion peakscalibrated with respect to the H O + parent ion peak and apeak generated by dopant N +2+2