第9回日韓合同シンポジウム「気相，凝縮相および生体系中の光化学過程：実験と理論の協力的展開」, 2001年1月, 分子科学研究所(愛知県岡崎市

Inokuchi, Yoshiya

Nishi, Nobuyuki

Hiroshima University Institutional Repository

INTRODUCTIONIn the last decade, we have studied the photodissociation spectroscopy and dynamics of benzene cluster ions, (C6H6)n+.     Charge Distribution of (C6H6)n+The positive charge is delocalized in the dimer.  (C6H6)n+ has a dimer ion core structure.(C6H6)2+(C6H6)n+  (n = 3–5)Charge Hopping in the Excited StatesIntermolecular vibrations of the ejected neutral monomers are highlyexcited in the energy disposal occurring in the photodissociation of (C6H6)n+.The ejected molecules should be vibrationally excited along the directions of the change in the equilibrium geometries between the neutral and the ion,  if the charge continually hops in the cluster after the photoexcitation.1[1] Ohashi and Nishi,  J. Chem. Phys.  95, 4002 (1991).  [2] Ohashi, Inokuchi, and Nishi, Chem. Phys. Lett.  263, 167 (1996).[3] Nakai, Ohashi, and Nishi,  J. Phys. Chem.  101, 472 (1997).E n e r g yI ntermolecular DistanceC R(C6H6)n+ n = 2345ψ1+ψ2ψ1ψ2+ψ+ψ-The charge resonance (CR) bandis due to the transition fromthe ground state to the repulsivestate of the wave functions,ψi+: electronic wave function of C6H6+(X)ψi  : electronic wave function of C6H6(X)ψ+ = ψ1+ψ2 + ψ1ψ2+ψ- = ψ1+ψ2 - ψ1ψ2+The presence of the CR bandimplies that the two moieties should be equivalent.The positive charge is delocalized in the dimer.CRPhotodissociation spectra of (C6H6)n+  (n = 2–5)The absorption bands exhibit essentially the same features withthat of (C6H6)2+.  These cluster ions havesimilar dimer ion core structures.2 3IN THIS STUDY,we want to make clear whether the charge hopping really occurs or not in the vibrational excited states of the benzene cluster ions.IR photodissociation spectroscopy is applied to the benzene cluster ions.  We present vibrational spectra and the assignment of    (C6H6)n+      (n=2–5),   (C6H6•C6D6)+,       [(C6H6)m•(C6D6)l]+   (m+l=3).  According to the relationship between the vibrational spectra and the photofragment species, we demonstrate that the frequent charge hopping really occurs in the vibrational excited states prior to the predissociation.  4We used a tandem mass spectrometer with a quadrupole ion guide.Photodissociation spectra of the parent ions are obtained by plotting the yields of the fragment ions against the wavenumbers of the IR laser.  0 5 0 cme-Q uadrupoleIon GuideI on BenderQ -MassFilterQ -MassFilterP ulsedNozzleI on BenderE lectron-impactIonizerIRfor selecting parent ionsIon Detector for analyzing fragment ionsEXPERIMENTAL6Cross Section3150310030503000Wavenumber / cm -1n = 2n =3n = 4n = 5 Experimental Points Fitting Curves      Component 1      Component 2      Component 33066304530933066306630453093VIBRATIONAL SPECTRAThe spectra of n = 3–5 exhibit sharp and well-resolved bands at 3066 cm-1.  In the spectra of n = 4 and 5, we can find two maxima at 3045 and 3093 cm-1.  The intensity ratios of these bands relative to the central band at 3066 cm-1increase more and more with increasing cluster size from n = 3 to 5.  dimer ion coresolvent molecules7Because ...n Integrated Intensity of component 1a     the number of solvent benzene moleculesb10.3320.6430.95vibrational structures of (C6H6)n+  (n = 3–5) are composed of the C–H stretching vibration bands of the dimer ion core and the solvent benzene molecules.  We conclude that the electronic spectra of (C6H6)n+ show that there is a dimer ion core in the cluster.  neutral benzene has three C–H stretching bands in this region (we call them "triad") .we can reproduce the band shapes by decomposing into three components:   component 1     triad (solvent benzene molecules)   component 2     Lorentzian curve (dimer ion core)   component 3     plateau the ratios of the integrated intensity of the triad component to that of the Lorentzian component increase proportional to the number of the solvent benzene molecules.  aRelative intensity to that of component 2.  bThis is based on the fact that these have dimer ion core structures.ASSIGNMENT10Cross Section31403120310030803060304030203000Wavenumber / cm -1[(C6H6)2•C6D6]+  Æ   (C6H6•C6D6)+[(C6H6)2•C6D6]+Æ  (C6H6)2+[C6H6•(C6D6)2]+Æ  (C6H6•C6D6)+[C6H6•(C6D6)2]+Æ  (C6D6)2+      Experimental      (C6H6)3+VIBRATIONAL SPECTRA OFMIXED BENZENE TRIMER IONSAll the spectra of the mixed benzene trimer ions show almost the same spectral feature as that of (C6H6)3+.  11(C6H6)3+ [(C6H6)2•C6D6]+ [C6H6•(C6D6)2]+(C6H6•C6D6)+•••C6H6(C6H6)2+•••C6D6 (C6H6•C6D6)+•••C6D6 (C6D6)2+•••C6H6(C6H6)2+•••C6H6+++= =+=1     :     2 2     :     1SpeciesIsomersExpectedbandfeature Statistical abundanceratioObserved spectral featureIon core (C6H6)2+ (C6H6)2+ (C6H6•C6D6)+ (C6H6•C6D6)+ (C6D6)2+StatisticallymixedspectraINTERPRETATION OF VIBRATIONAL SPECTRAHowever, photodissociation spectra detected with the respective fragment ions are nearly the same with each other.  If we construct the two spectra of these isomers with the statistical abundance ratio, we obtain overall photoabsorption spectra of each species with the same feature as that of (C6H6)3+.  The dimer ion core switching, in other words, the charge hopping occurs in the photoexcited vibrational states of benzene trimer ions prior to the dissociation.  The mixed benzene trimer ions may also have the dimer ion core structure because of small difference in ionization potentials of C6H6 and C6D6 (0.003 eV).  If the photoexcitation induces the cleavage of the weakest intermolecular bond after the intracluster vibrational energy redistribution without changing the position of the charge, expected band feature is as follows,  There are two possible isomers for each trimer species.(C6H6•C6H6)+•••C6H60.27 eV (the weakest bond)0.67 eVbinding energy+(C6H6)2+•••C6D6=(C6H6•C6D6)+•••C6H6+=expected band feature1 : 2Example:[(C6H6)2•C6D6]+Overall absorption spectrum of [(C6H6)2•C6D6]+ is+ 2 X 2 X =( )This is the same with the band shape of (C6H6)3+.  AbundanceRatio12isomer13Photodissociation spectra of benzene cluster ions are measured in the IR region.In particular we pay our attention to the trimer ions.We can reasonably explain the IR spectrum of (C6H6)3+ by the dimer ion core model;  the C–H stretching band of (C6H6)3+ is composed of the C–H stretching bands of the dimer ion core and the solvent benzene.The IR spectra of the isotopically mixed benzene trimer ionsshow that the positive charge frequently hops from one dimer pair to another pair in the photoexcited vibrational states in the course of the dissociation.SUMMARY51.00.80.60.40.20.0Relative Cross Section1250010000750050000.0140.0120.0100.0080.0060.0040.0020.00070006000500040003000Wavenumber / cm -1CRCRThe cross section is not zero up to 2500 cm-1.  (C6H6)2+ELECTRONIC SPECTRUMCR : Charge Resonance BandThe CR band has essentially a broad feature because this transition is from the bound state to the repulsive state.X 514Vibrational temperature of (C6H6)n+ becomes lower and lower with increasing the cluster size from n = 3 to 5.  [4] Ohashi, Adachi, and Nishi, Bull. Chem. Soc. Jpn.  69, 915 (1996). The intensity of the plateau (component 3) becomes lower and lower with increasing the cluster size. These results also support the assignment of the plateau to the CR transitions from the vibrational excited states.8What is the origin of the plateau (component 3) in the IR spectra?E n e r g yI ntermolecular DistanceC RConsequently, the hot bands of the CR transition may mainly contribute to the plateau component of the IRspectra.  In the case of of (�C6H6)2+, the tail of the charge resonance (CR) band extends down to 2500 cm-1 over the threshold of the binding energy of 5400 cm-1 (0.67 eV). (C6H6)2+ dissociates through one photon process even though the photon energy is lower than the binding energy. 67890.12Intensity of C6H6+ (arb. units)4 5 6 7 8 910Laser Power (arb. units)Slope = 1.0(C6H6)2+ C6H6+ +hν@ 2900 cm-1 (0.36 eV)C6H6(C6H6)2+9Cross Section3120310030803060304030203000Wavenumber / cm -1(C6H6 • C6D6)+ hν C6H6+  +  C6D6C6D6+  +  C6H6(C6H6 • C6D6)+Difference in ionization potential between C6H6 and C6D6 is only 0.003 eV.It is probable that the positive charge is delocalized in (C6H6•C6D6)+because of the charge resonance interaction.VIBRATIONAL SPECTRA OF(C6H6•C6D6)+The C–H stretching frequency of C6H6 in the dimer ion core is independent of the partner (C6H6 or C6D6).  The spectra show the same feature as that of (C6H6)2+.  Photodissociation Spectroscopy of Benzene Cluster Ions(IMS, Japan) Yoshiya INOKUCHI and Nobuyuki NISHI  

Photodissociation Spectroscopy of Benzene Cluster Ions

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Photodissociation Spectroscopy of Benzene Cluster Ions

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