Dissociation of methanol in intense femtosecond laser field

NOTESReferencesPart 1, 1992, 397(1): 337.(Received August 6, 2002)1. Rieger, E, Share, G. H, Forrest, D. J. et al., A 154-day peri-odicity in the occurrence of hard solar flares? Nature, 1984, 312:Dissociation of methanol in623.intense femtosecond laser field2. Bai, T, Sturrock, P. A.. Evidence for a fundamental period of thesun and its relation to the 154 day complex of periodicities, As-TANG Xiaoping, GAO Lirong, WANG Yonglin,trophysical Journal, Part 1, 1993, 409(1): 476.WANG Cao, WANG Sufan & KONG Fan' ao3. Massi, M, Neidhofer, J., Torielli-Ciamponi, G. et al., ActivityInstitute of Chemistry, Chinese Academy of Sciences, Beijing 100080,cycles in UX ARIETIS, Astronomy and Astrophysics, 1998, 332:ChinaCorrespondence should be addressed to Kong Fan' ao (e-mail: kong@ mrdlab.icas.ac.cn)Bogart, R. S., Bai, T. Confirmation of a 152 day peridicity in theoccurrence of solar flares inferred from microwave data, Astro-Abstract Methanol was irradiated by 80 fs laserphysical Journal, Part 2, Letters to the Editor, 1985, 299: L51.intensity range of 1013 一4014 W/cm2. A TOF-mass spectrome-ter was coupled to the laser system and used to detect theIchimoto, K., Kubota, J.. Suzuki, M. et al., Periodic behaviour ofons produced. The parent ions CH,OH+ appeared firstly atsolar flare activity, Nature, 1985, 316: 422.the laser intensity of 1.4 - X013 W/cmr. While the laser inten-6. Bai, T, Sturrock, P. A., The 152-day periodicity of the solar flaresity was gradually increased, the parent ions were dissociatedand the primary ions CH,OH+ were given as verified fromoccurrence rate, Nature, 1987, 327: 601.7. Bai, T, Cliver, E. W.. A 154-day periodicity in the ocurrence ratethe irradiation of deuterated methanol (CH,OD) showing theC - H bond cracking firstly. While the laser intensity was .of proton flares, Astrophysical Jourmal, Part 1, 1990, 363(1): 299.further increased to 2.0 . X013 W/cmr, the C-0 bonds ofGabriel, S., Evans, R., Feynman, J.. Periodicities in the occurrencethe parent ions also broke to give CH. When the laser in-rate of solar proton events, Solar Physics, 1990, 128: 415.tensity was higher, smaller fragment ions like CH+, C, OH+Bouwer, S. D.. Periodicities of solar iradiance and solar activityand O+ also appeared. Among the fragment ions, only H+ ionyield had anisotropic angular distribution dependence on theindices, Part - I, Solar Physics, 1992, 142(2): 365.laser polarization vector in the dissociation of methanol. All10. Ballester, J. L, Oliver, R., Baudin, F, Discovery of the near 158the experimental observations show that the dissociation ofday periodicity in group sunspot numbers during the eighteenthmethanol proceeds through stepwise mechanism but notCoulomb explosion.century, Astrophysical J., 199, 522: L153.11. Lou Yuqing, Rossby-type wave-induced periodicties in flare ac-Keywords: intense laser field, femtosecond laser, dissociation,methanol, TOF-mass spectra.tities and sunspot areas or groups during solar maxima, The As-trophysical Joumal, 2000, 540(2): 1102.The intensity of the coulomb field, which binds the12. Carbonel, M., The near 160 day periodicity in the emergence ofelectrons and nuclei together in molecules, is about 1Cphotospheric magnetic flux, Proceedings of SOLSPA 2001 Euro-V/cm. The electric field of an ultrashort intense laser pulconference: Solar Cycle and Space Weather, Vico Equense 2001,se can be comparable to this magnitude. Such strong laserfield interaction with molecules will result in changing ofESA SP Series, SP-477.he potential energy surface and the configuration of13. Meyer, Y., Wavelets and Operators, Cambridge: Cambridge Uni-molecules. Many new phenomena occur, such as above-versity Press 1992.threshold ionization!- -3, above- threshold dissociation'4, 5),14. Chao, B. F, Naito, L, Wavelet analysis provides a new tool forbond-softening|01_, Coulomb explosion!7 -121. , and align-studying earthi s rotatin, EOS, 1995, 76: 161.mentl3].15. Liao, D. C. Liao, X. H., New evidence for possible impact of so-Some reports on the dissociation of molecules inlar activity on long-term fluctuation of the earth rotation, Chineseintense, femtosecond laser field have already appeared.Science Bulletin, 2001, 4611): 905.Most of them were performed in the very high intensity16. Han Yanben, Han Yonggang, Wavelet analysis of sunspot relativeregion of 10I5- -1016 W/cm2 and the results were explainedin terms of Coulomb explosion. Cornaggia et a1.[14] studiedmumbers, Chinese Science Bulletin, 2002, 47(7): 609.17. Han Yanben, Zhao Juan, Li Zhian, Inter- annual variation of thethe dissnciationnf C0. sinσ 5 >10I5 W/cm2 laser pulse.earth rotation and El Nino, Chinese Science Bull. 2002, 47(2):The中国煤化工C2+(Z=1-4), or+(z-104.=1-YHCNMHG1-2)wereproducedvi18. Sturrock, P. A. Bai, T, Search for evidence of a clock related toCoulomb explosion of CO2 Ledingham et al.!15] havethe solar 154 day complex of periodicities, Astrophysical Journal,studied the dissociation processes of cyclic aromaticmolecules (furan, pyrrole, pyridine and pyrazine) using 75Chinese St亦数据/etin Vol. 47 No. 23 December 20021973NOTES .fs, intensity of 4. >106 W/cm? laser pulse. They .conclud-1 Experimental setuped that the Coulomb explosion takes place in multi-The laser system used in this experiment was acharged parent ions with a deformed molecular structure.home-built mode-locked femtosecond Ti: sapphire oscil-All of these reports were conducted at laser intensitylator which was pumped by a diode-pumped, frequency-higher than 105 Wcm7. At this intensity, multichargeddoubled laser (532 nm, Verdi, Coherent). As a seed pulse,ions appear and Coulomb explosion model can be used tothe 800 nm, 50 fs laser pulse from the oscillator wasexplain the results. In the Coulomb explosion, moleculesstretched and then led to a multipass Ti sapphire laserirradiated by intense laser will be multiply ionized to giveamplifier Quantronix, Odin) which was pumped by therise to multicharged molecular ions at very short time,second harmonic of a Nd: YLF laser. The amplified laserwhere the strong repulsion in the molecular ion leads tobeam was compressed to 80 fs and the energy output wasdissociation into small multicharged fragment ions orabout 300 μJ per pulse.multicharged atomic ions.The amplifed fs pulse was focused into the chamberHowever, at lower intensity (1013 - 104 W/cm2),of the linear TOF mass spectrometer by a 15 cm focal lens.molecules can acquire single charge only forming singlyThe gaseous molecules were continuously effused into thecharged ions, where the Coulomb explosion model cannotchamber through an orifice with 500 um, with a back-be used to explain the dissociation process of singlyground pressure of 1.6 X10+ Pa and methanol gas pres-charged ion. Haradae et al.101 studied the dissociation ofsure of 5..>10+ Pa. The gas molecules were crossed withtwo pairs of molecules (2,3-dimethyI- 1,3-butadiene, 2,5-the focused laser beam. The fragment ions produced in thedimethyl-2,4-hexadiene; 1,4-cyclohexadiene, 1,3- cyclo-laser beam were extracted by 200 V and then acceleratedhexadiene) iradiated with 120 fs laser pulse in the inten-.by 900 V in a two-stage electric field and flew freely to asity region of 104 W/cm?. One molecule in each pair haddual micro-channel plate (MCP) through a 50 cm free-no allowed electronic transition in the cation at the excita-field flight tube. The ion signal was detected by the MCPtion wavelength, resulting in parent ion dominance. Inand acquired by a 100 MHz high-speed transient recordercontrast, the counter molecule, which had a similar struc-and then transferred to the computer for data acquisition.ture but with absorption in the cation at the excitationFor the laser polarization experiment, the extractedwavelength, showed heavy fragmentation with a negligi-voltage was set at zero and the ions produced in the laserbly small signal of the parent ion. Chin et al.(7 studied thebeam moved just through a 3 mm diameter hole in theionization and fragmentation of NO molecules using 102grid to the acceleration region. The distance between the一1015 W/em2 , 200 fs laser pulse. Their results showedhole and the interaction region was 8 mm. Only those ionsthat the fragmentation process was principally due to dis-whose angle between the ion flight direction and the FOFsociative recombination. This latter process meansaxis is less than 11° can reach the MCP. A入2 wave-platestretching the molecule which will lead to enhanced ioni-was inserted into the path of the laser beam before thezation at larger internuclear distance. Talebpour et al.8l1window of the chamber. A polarizer was also used tostudied the multiphoton ionization and fragmentation ofdetermine the intial polarization direction of the laserethylene using linearly polarized 200 fs laser pulses withbeam and the initial angle of the入2 wave-plate. The massthe intensity of 1013-1015 W/cm2. Their results showed .spectrum can be obtained at different laser polarization bythat fragmentation of the molecule occurred as a result ofrotating the λ2 wave -plate.multiphoton ionization of inner-valence electrons fol-2 Experimental results and discussionlowed by radiationless transitions to various dissociationchannels of the molecular ion. Until now, a systemic,(. i) Mass spectra and threshold of dissociation:Fig. 1(a) -(d) shows the mass spectra of methanoldetailed research on the dissociation process of polya-irradiated at different laser intensities.. lonization of mo-tomice molecules in the laser intensity of 10B3- -104 W/cm2lecules is very fast (of the order of 0.1 fs) in the intenseis lacking. At this laser intensity region, chemical reac-laser field19]. Neutral molecules ionize first to form parenttions take place. The molecule-laser field interaction re-ions, fllowed by dissociation into fragment ions. Fig. 1 (a)search needs more inrestigations in the laser intensity ofshows the formation of the parent ions CH,OH+ (32 amu)at laser intensity of 1.4- )1013 W/cm?. When the laser1013- -1014 W/cm2.intensity increases to 1. >1013 W/cm?, a peak with massThis note concerms about the dissociation of metha-number of 31 amu appears (fig. 1(b)). This peak may bnol rradiated by 80 fs (1013 - -104 Wcm) laser pulse. Theeither CHO+ qr CH.OH+ To verify which ions werestepwise dissociation of the parent ion and anisotropicform中国煤化工H:OD) was studied usingangular distribution of H+ have been observed. Thesethelndition as CHjOH. WhenYHC N M H GW/cm?, only two peaks atphenomena suggest that we must propose a new model tomass number of 33 and 32 appear (ig. l(e)). Obviously,explain the interesting results.these peaks are parent ions CH,OD+ and fragment ionsCH2OD*. No CH,O+ with mass number of 31 appears, soChinese Science Bulletin Vol. 47 No. 23 December 20021974NOTES1.4x 10! W. cmr2CH,OH*(肉)2.8X 1013 W.cm~2CH2d)x5 10 1202530 354045303540453.0X 101 W.cm2(e)1.7X 102 W.cm3(bHX65 1013 2025303540 4510 15 202530”340~45(C)2.0X 10' W.cm°207X 108 W.cm2CH,OD* | CH,OD* (f)CH;5 10152025303540 - d$2Mass numbcrMass numberFig. 1. Mass spectra of methanol in different laser intensity.Table 1 The appearance laser intensity of the ions during thewe deduce that the first bond-broken in methanol isC- -Hdissociation of methanol in intense laser fieldsbond rather than O- -H bond. When the laser intensity is .Appearance intensityIonincreased to 2.0.>1013 W/cm?, CH$ ions also begin to(- >103/W- cm-)CH,OH.1.4appear (fig. l()). The appearance laser intensity of CH2CH,OHis 2.6. >1013 W/cm2. When the laser intensity gets higherCOH+ or CHO+(2.8. >1013 W/cm?), smaller fragment ions CH+, C, OH+CH;+2.0and O+ also appear one after another (fig. 1(d)). When theCH,O*or CHOH+H+2.4laser intensity is 3.0_ >1013 W/cm2, H2 is also seen. TableCH2-2.61 shows the appearing sequence of ions produced fromCH..dissociation of methanol in the intense laser field of 103OH+-1014 W/cm2. This sequential appearance of the fragment2.ions as laser intensity increasing supports the stepwise中国煤化工2.8dissociation mechanism.(-i) Ion yield. Fig. 2 shows the dependence of the1HCNMHGion yield on the laser intensity. When the laser intensity is(fig.' 2(a)). When the laser intensity is 2.8. >1013- -3.0..>lower than 2.8.. >1013 W/cm2. , the parent ion (CH3OH+)1013 W/cm2.，the yield of CH,OH+ reaches the maximum.yield is increasing as the laser intensity increasesWhen the laser intensity is higher than 3.0_ >1 013 W/cm2 ,Chinese S府亦数letin Vol. 47 No. 23 December 20021975NOTESOH+120卜CH.OHCH*80 t0t(i(a120卜CH:*卜O*40个(b)(f)(0]20 tCH,O*t CH*↑H;40 t(cg)120CHO"十C+tH+80<240(}(d152.02.53.0 3.540 4.515 2.0 2.5 3.0 3.5 4.0 4.5 1.5 2.0 2.5 3.0 3.5 4.0 4.5laser intensityXx l0-10/W .cm 2Fig. 2. Dependence of yield of the ion produced from methanol on the laser intensity.the yield of CHzOH+ decreases, which means that thepathway. There are two ways of dissociation: one involvesparent ion is dissociated. A similar relationship betweenthe pull of H (or H+) or H, (or H2 ) from the parent ions,yield and laser intensity for CHgO+ and CH (fig. 2 (b),the other involves the C- 0 bond cracking. According to(e)) also exists. When the laser intensity is between 2.8- xthe above experimental findings, we assume that disso-1013- 3.5. >x103 W/cm2 (for CH,O+) and 3.5. >1013.- -37.> ciation of methanol has two pathways. Pathway A in-1013 W/cm2 (for CH,+), the yields almost do not change asvolves the sequential pull of the three H gradually, givingthe laser intensity increases. When the laser intensity isCH2OH+, CH2O+ (or CHOH+), and finally forming CHO+higher than 3.5. >1013 W/cm2 (for CH,O+) and 3.7.. >1013(or COH+) ions. The other pathway, B, proceeds at lowerW/cm2 (for CH), the yields of both CH,O+ and CHlaser intensity. The C- O bond of the parent ions cracks,also begin to decrease. This is because that smaller frag-forming CH and OH+. These two products graduallyment is formed from the dissociation of CH,O* and CH.lose their H (or H+), and finally form C+ and O+ ions (fig.However, the yields of other smaller ions, like CH,O+ (or3). The products of reaction route A begin to appear atCHOH+)_, CHO+ (or COH+), CH2, CH+, C+, OH+, O*,lower laser intensity (1.7. >1013 W/cm2), but those of re-action route B begin to appear at the laser intensity of 2.0H2 and H) show a monotonous increase as indicated in.>1013 W/cm2.fig. 2(c), (d), (f)- -(1), which shows that they ome fromWe estimated the ratio between the total yield of allthe dissociation of parent ions CH,OH+, fragment ionsproducts in route A and that in route B. The ratio is not aCH,O+ and CH'.cons中国煤化Ilaser intensity increasing.(i) Sequence and pathway of the dissociation.ThisCNMH(ute B have a correlation.AlltfMHGuced in route A can breakAlthough it is difficult to decide the precise disso-their C- O bonds and give the products which also can beciative pathway because of many possible pathways dur-produced in route B. This makes the total yield in route Bing the dissociation of methanol, we can explore the main1976Chinese Science Bulletin Vol. 47 No. 23 December 2002NOTESincrease faster. In other words, when the laser intensity isCH,OH*35卜\0t含25|OH* + CH号20f土15oCH;CH2O* or CHOH"-100 -500 50100150 200 250 300Angle between laser polarization and the TOF axis/()CH+Fig. 4. Dependence of yield of fragment ions dissociated from metha-CHO* or COH"nol on laser polarization vector.Cions produced in Coulomb explosion have an anisotropicangular distributionI20, 211. Another explanation for theFig.3. Possible dissociative pathways of methanol in intense laser fieldangular distribution is that large dipole moment can beinduced by the intense, linearly polarized laser field. Such(1013 - -104 W/cm2).dipole moment exerts large torque on the molecule, whichlower, i.e. 2.0..>1013 W/cm', only C- H bond in methanolgives rise to spatial alignment of the molecule in the di-ion can crack. However, when the laser intensity is higher,rection of the laser polarization vectorl22- -241. Such align-C- 0 bond and C- -H bond can also break. According toment only exists in the laser pulse of picosecond order. Itthe data of the bond energy of chemical bond (C- -H: 410is impossibe that significant reorientation takes place onkJ/mol, 0-H: 436.8 kJ/mol, C- O: 1076.5 KJ/mol), itthe time scale of the current experiment (1 - 100 fs)l251.seems that the sequence of the dissociation is related toWe assume that the molecules in the intense laser fieldbond energy (bond strength). The bond with low bondwill dissociate in a specific direction. When the laser po-energy will break first. But the proof is needed to verifylarization direction is parallel to the dissociative bond inthis hypothesis.the molecules, such a bond will break easily. But if theOnce the CH; ion appears .. , the CH; and CH-laser field is vertical to the chemical bond in a molecule,also appear almost at once. This shows that CH; ansuch a bond will not break easily.CH; are unstable. These mass spectra consisting of mass3 Conclusionnumberof 15, 14 and 13 always appear in the mass spec-tra of molecules with methyl dissociation in the intenseMass spectra of methanol show that the dissociationlaser field. For example, the mass spectra of CH,CHO",of methanol is the stepwise mechanism under our experi-CH:COCH32) and CH4 also exhibit the similar dissociationmental condition. The experimental observations showpattern.that no multicharged ions appear at the laser intensity(°- i1) Angular distribution of H+. H+ ions have a range of 1013- -1014 W/cm2." The results cannot be ex-strong anisotropic angular distribution during the disso-plained by the Coulomb explosion model. It is noted thatciation of methanol ions. When the angle between theall bonds in methanol can be broken. This means that thelaser polarization direction and flight axis of the TOFdissociation of the chemical bond is not dependent on themass spectrometer is 0-or 180-, the signal of H+ is thewavelength of the laser used in the experiment, which isstrongest. But when the angle is 90. or 270-, the signalonly 800 nm. A new dissociative mechanism must beof H+ is the weakest (fig. 4). Under this experimental con-proposed to explain the experimental results.dition, parent ions and other fragment ion's have not suchIn the linearly polarized laser experiment, only H+an anisotropic angular distribution. This is different from presents the anisotropic angular distribution. This impliesthe angular distribution of fragment ions produced in that the laser field pulls out the H+ from the molecularCoulomb explosion. Except for parent ions, all fragmentions in the dissociative process. Because the mass of the中国煤化工? rest of the molecular ion,Hteular distribution. This fact1) Tang, X. P.. Wang. S. F. Gao, L. R. et al, The step-wise field-assisted dissTYHc N M H G laser feld, sbmitcet tJ.Phys.Chem.2) Tang, X. P., Wang, S. F, EIshakre, M. et al, The field-assisted step-wise dissociation process of acetone in intense femtosecond laser field,submitted to Phys. Chem. Chem. 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