The influence of cluster formation on the photodissociation of sulfur dioxide: excitation to the E state

A femtosecond pump-probe technique was employed to study the dissociation dynamics of sulfur dioxide and sulfur dioxide clusters in real time. Dissociation is initiated by a multiphoton scheme that populates the E state. The SO(2) (+) transient is fit to a biexponential decay comprising a fast and a slow component of 230 fs and 8 ps, respectively. The SO(+) transient consists of a growth component of 225 fs as well as a subsequent decay of 373 fs. The pump-probe response obtained from the monomer clearly shows the predissociative cleavage of a S-O bond. Upon cluster formation, a sequential increase in the fast decay component is observed for increasing cluster size, extending to 435 fs for (SO(2))(4) (+). The transient response of cluster dissociation products SO(SO(2))(n) (+), where n=1-3, reflects no growth component indicating that formation proceeds through the ion state. Therefore, cluster formation results in a caging effect, which impedes the dissociation process. Further direct evidence for our proposed mechanism is obtained by a technique that employs a comparison of the amplitude coefficients of each respective component of the fit. This method makes possible the determination of branching ratios of competing relaxation processes and thereby the influence of cluster formation on each can be resolved. The caging effect is attributed to a steric hindrance placed on the SO(2) chromophore, preventing it from attaining a linear geometry necessary for dissociation.     

Single photon ionization of van der Waals clusters with a soft x-ray laser: (SO2)n and (SO2)n(H2O)m

van der Waals cluster (SO2)n is investigated by using single photon ionization of a 26.5 eV soft x-ray laser. During the ionization process, neutral clusters suffer a small fragmentation because almost all energy is taken away by the photoelectron and a small part of the photon energy is deposited into the (SO2)n cluster. The distribution of (SO2)n clusters decreases roughly exponentially with increasing cluster size. The photoionization dissociation fraction of I[(SO2)(n-1)SO+] / I[(SO2)n+] decreases with increasing cluster size due to the formation of cluster. The metastable dissociation rate constants of (SO2)n+ are measured in the range of (0.6-1.5) x 10(4) s(-1) for cluster sizes 5< or =n< or =16. Mixed SO2-H2O clusters are studied at different experimental conditions. At the condition of high SO2 concentration (20% SO2 partial pressure), (SO2)n+ cluster ions dominate the mass spectrum, and the unprotonated mixed cluster ions (SO2)nH2O+ (1< or =n< or =5) are observed. At the condition of low SO2 concentration (5% SO2 partial pressure) (H2O)nH+ cluster ions are the dominant signals, and protonated cluster ions (SO2)(H2O)nH+ are observed. The mixed clusters, containing only one SO2 or H2O molecule, SO2(H2O)nH+ and (SO2)nH2O+ are observed, respectively.     

Time-resolved study of solvent-induced recombination in photodissociated IBr-(CO2)n clusters

We report the time-resolved recombination of photodissociated IBr-(CO2)n (n = 5-10) clusters following excitation to the dissociative IBr-A' 2Pi12 state of the chromophore via a 180 fs, 795 nm laser pulse. Dissociation from the A' state of the bare anion results in I- and Br products. Upon solvation with CO2, the IBr- chromophore regains near-IR absorption only after recombination and vibrational relaxation on the ground electronic state. The recombination time was determined by using a delayed femtosecond probe laser, at the same wavelength as the pump, and detecting ionic photoproducts of the recombined IBr- cluster ions. In sharp contrast to previous studies involving solvated I2-, the observed recombination times for IBr-(CO2)n increase dramatically with increasing cluster size, from 12 ps for n = 5 to 900 ps for n = 8,10. The nanosecond recombination times are especially surprising in that the overall recombination probability for these cluster ions is unity. Over the range of 5-10 solvent molecules, calculations show that the solvent is very asymmetrically distributed, localized around the Br end of the IBr- chromophore. It is proposed that this asymmetric solvation delays the recombination of the dissociating IBr-, in part through a solvent-induced well in the A' state that (for n = 8,10) traps the evolving complex. Extensive electronic structure calculations and nonadiabatic molecular dynamics simulations provide a framework to understand this unexpected behavior.     

Time-resolved photoelectron imaging of large anionic methanol clusters: (methanol)n-(n approximately 145-535)

The dynamics of an excess electron in size-selected methanol clusters is studied via pump-probe spectroscopy with resolution of approximately 120 fs. Following excitation, the excess electron undergoes internal conversion back to the ground state with lifetimes of 260-175 fs in (CH3OH)n- (n=145-535) and 280-230 fs in (CD3OD)n- (n=210-390), decreasing with increasing cluster size. The clusters then undergo vibrational relaxation on the ground state on a time scale of 760+/-250 fs. The excited state lifetimes for (CH3OH)n- clusters extrapolate to a value of 157+/-25 fs in the limit of infinite cluster size.     

Charge-transfer-to-solvent-driven dissolution dynamics of I- (H2O)2-5 upon excitation: excited-state ab initio molecular dynamics simulations

In contrast to the extensive theoretical investigation of the solvation phenomena, the dissolution phenomena have hardly been investigated theoretically. Upon the excitation of hydrated halides, which are important substances in atmospheric chemistry, an excess electron transfers from the anionic precursor (halide anion) to the solvent and is stabilized by the water cluster. This results in the dissociation of hydrated halides into halide radicals and electron-water clusters. Here we demonstrate the charge-transfer-to-solvent (CTTS)-driven femtosecond-scale dissolution dynamics for I-(H2O)n=2-5 clusters using excited state (ES) ab initio molecular dynamics (AIMD) simulations employing the complete-active-space self-consistent-field (CASSCF) method. This study shows that after the iodine radical is released from I-(H2O)n=2-5, a simple population decay is observed for small clusters (2 相似文献   

Association patterns in (HF)(m)(H2O)(n) (m + n = 2-8) clusters

In an attempt to understand the phase behavior of aqueous hydrogen fluoride, the clustering in the mixture is investigated at the molecular level. The study is performed at the mPW1B95/6-31+G(d,p) level of theory. Several previous studies attempted to describe the dissociation of HF in water, but in this investigation, the focus is only on the association patterns that are present in this binary mixture. A total of 214 optimized geometries of (HF)n(H2O)m clusters, with m + n as high as 8, were investigated. For each cluster combination, several different conformations are investigated, and the preferred conformations are presented. Using multiple linear regressions, the average strengths of the four possible H-bonding interactions are obtained. The strongest H-bond interaction is reported to be the H2O...H-F interaction. The most probable distributions of mixed clusters as a function of composition are also deduced. It is found that the larger (HF)n(H2O)m clusters are favored both energetically and entropically compared to the ones that are of size m + n < or = 3. Also, the clusters with equimolar contributions of HF and H2O are found to have the strongest interactions.     

Geometrical and electronic structures of Au(m)Ag(n) (2 < or = m + n < or = 8)

The structural and electronic properties of Au(m)Ag(n) binary clusters (2 < or = m + n < or = 8) have been investigated by density functional theory with relativistic effective core potentials. The results indicate that Au atoms tend to occupy the surface of Au(m)Ag(n) clusters (n > or = 2 and m > or = 2). As a result, segregation of small or big bimetallic clusters can be explained according to the atomic mass. The binding energies of the most stable Au(m)Ag(n) clusters increase with increasing m+n. The vertical ionization potentials of the most stable Au(m)Ag(n) clusters show odd-even oscillations with changing m+n. The possible dissociation channels of the clusters considered are also discussed.     

Photodetachment and photofragmentation pathways in the [(CO2)2(H2O)m]- cluster anions

The mass-selected [(CO(2))(2)(H(2)O)(m)](-) cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO(2) (-)(H(2)O)(m)CO(2) and (O(2)CCO(2))(-)(H(2)O)(m) parent cluster structures, particularly for m=2 and 3, only the CO(2) (-) based clusters are both required and sufficient to explain all fragmentation pathways for m>/=3. Three types of anionic photofragments are observed: CO(2) (-)(H(2)O)(k), O(-)(H(2)O)(k), and CO(3) (-)(H(2)O)(k), k6) is attributed to hindrance from the H(2)O molecules.     

Gas phase synthesis, structure and unimolecular reactivity of silver iodide cluster cations, Ag(n)I(m)(+) (n = 2-5, 0 < m < n)

Multistage mass spectrometry (MS(n)) experiments reveal that gas phase silver iodide cluster cations, Ag(n)I(m)(+), are readily built up in a stepwise fashion via ion-molecule reactions between mass selected silver (Ag(3)(+) and Ag(5)(+)) or silver hydride (Ag(2)H(+) and Ag(4)H(+)) cluster cations and allyl iodide, in contrast to their reactions with methyl iodide, which solely result in ligation of the clusters. The stoichiometries of these clusters range from 1 < or = n < or = 5 and 1 < or = m < or = 4, indicating the formation of several new subvalent silver iodide clusters. Collision induced dissociation (CID) experiments were carried out on each of these clusters to shed some light on their possible structures. The products arising from CID of the Ag(n)I(m)(+) clusters are highly dependent on the stoichiometry of the cluster. Thus the odd-electron clusters Ag(4)I(2)(+) and Ag(5)I(+) fragment via loss of a silver atom. In contrast, the even-electron cluster ions all fragment via loss of AgI. In addition, Ag(2)I(2) loss is observed for the Ag(4)I(3)(+) and Ag(5)I(2)(+) clusters, while loss of Ag(3)I(3) occurs for the stoichiometric Ag(5)I(4)(+) cluster. DFT calculations were carried out on these Ag(n)I(m)(+) clusters as well as the neutrals associated with the ion-molecule and CID reactions. A range of different isomeric structures were calculated and their structures are described. A noteworthy aspect is that ligation of these silver clusters by I can have a profound effect on the geometry of the silver cluster. For example, D(3h) Ag(3)(+) becomes C(2v) Ag(3)I(+), which in turn becomes C(2h) Ag(3)I(2)(+). Finally, the DFT predicted thermochemistry supports the different types of reaction channels observed in the ion-molecule reactions and CID experiments.     

Microsolvation of Co2+ and Ni2+ by acetonitrile and water: photodissociation dynamics of M(2+)(CH3CN)n(H2O)m

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560-660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M(2+)(CH(3)CN)(n) (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H(+)(CH(3)CN), is observed as a minor channel for M(2+)(CH(3)CN)(2)(H(2)O)(2) and M(2+)(CH(3)CN)(2)(H(2)O) but is not seen in M(2+)(CH(3)CN)(3)(H(2)O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M(2+)(H(2)O)(4). Measurements of the photodissociation yield reveal that four-coordinate Co(2+) clusters dissociate more readily than Ni(2+) clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni(2+) clusters. For the two-coordinate clusters, dissociation is via electron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co(2+)(CH(3)CN)(2)(H(2)O) is calculated to occur via a (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3)) salt-bridge transition state, reducing kinetic energy release in the dissociation.     

Guided ion-beam studies of the reactions of Co(n)+ (n=2-20) with O2: cobalt cluster-oxide and -dioxide bond energies

The kinetic-energy dependence for the reactions of Co(n)+ (n=2-20) with O2 is measured as a function of kinetic energy over a range of 0 to 10 eV in a guided ion-beam tandem mass spectrometer. A variety of Co(m)+, Co(m)O+, and Co(m)O2+ (m < or = n) product ions is observed, with the dioxide cluster ions dominating the products for all larger clusters. Reaction efficiencies of Co(n)+ cations with O2 are near unity for all but the dimer. Bond dissociation energies for both cobalt cluster oxides and dioxides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different methods. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry and the stabilities of oxygenated cobalt clusters are discussed. The bond energies of Co(n)+-O for larger clusters are found to be very close to the value for desorption of atomic oxygen from bulk-phase cobalt. Rate constants for O2 chemisorption on the cationic clusters are compared with results from previous work on cationic, anionic, and neutral cobalt clusters.     

Excited state dynamics and fragmentation channels of the protonated dipeptide H2N-Leu-Trp-COOH

The excited state dynamics of the isolated and protonated peptide H(2)N-Leu-Trp-COOH are analyzed by fs pump-probe spectroscopy. The peptides are brought into the gas phase by electrospray ionization, and fs pump-probe excitation is detected by fragment ion formation. The pump laser addressed the excited pipi* state of the indole chromophore of the amino acid tryptophan. The subsequent excited state dynamics agreed with a biexponential decay with time constants of 500 fs and 10 ps. This is considerably shorter than the lifetime of neutral tryptophan in solution and in proteins, but similar to isolated, protonated tryptophan. Several models are discussed to explain the experimental results but the detailed quenching mechanism remains unresolved.     

Electrochemistry of niobium(V) in sulfuric and methanesulfonic acids: formation of the Nb3O2(SO4)6(H2O)(3)(5-) cluster and designed electrochemical generation of "Nb3O2" core clusters by double potential pulse electrolysis

The electrochemical and spectroelectrochemical properties of niobium(V) and the Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) cluster in sulfuric acid and methanesulfonic acid were investigated using cyclic voltammetry, constant potential electrolysis, and spectroelectrochemistry. These chemical systems were suitable to probe the formation of "Nb(3)O(2)" core trinuclear clusters. In 9 M H(2)SO(4) the cluster Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) exhibited a reversible 1-electron reduction peak at E(pc) = -1.30 V vs Hg/Hg(2)SO(4) electrode, as well as a 4-electron irreversible oxidation peak at E(pa) = -0.45 V. Controlled potential reduction at E = -1.40 V produced the green Nb(3.33+) cluster anion Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(6-). In 12 M H(2)SO(4) Nb(V) displayed two reduction peaks at E(pc) = -1.15 V and E(pc) = -1.30 V. It was determined that the first process involves a quasi-reversible 2-electron reduction. After reduction of Nb(V) to Nb(III) the following chemical step involves formation of [Nb(III)](2) dimer, which further reacts with Nb(V) to produce the Nb(3)O(2)(SO(4))(6(H(2)O)(3)(5-) cluster (ECC process). The second reduction peak at E(pc) = -1.30 V corresponds to further 2-electron reduction of Nb(III) to Nb(I). The electrogenerated Nb(I) species also chemically reacts with starting material Nb(V) to produce additional [Nb(III)](2). In 5 M H(2)SO(4), the rate of the second chemical step in the ECC process is relatively slower and reduction of Nb(V) at E = -1.45 V/-1.2 V produces a mixture of Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) and [Nb(III)](2) dimer. [Nb(III)](2) can be selectively oxidized by two 2-electron steps at E = -0.65 V to Nb(V). However, if the oxidation is performed at E = -0.86 V, the product is Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-). A double potential pulse electrolysis waveform was developed to direct the reduction of Nb(V) toward selective formation of the Nb(3)O(2)(SO(4))(6)(H(2)O)(3)(5-) cluster. Proper application of dc-voltage pulses alternating between E(1) = -1.45 V and E(2) = -0.86 V yields only the target trinuclear cluster. Analogous double potential pulse electrolysis of Nb(V) in methanesulfonic acid generates the "Nb(3)O(2)" core cluster Nb(3)O(2)(CH(3)SO(3))(6)(H(2)O)(3)(+).     

Ab initio studies on the reaction of O2 with Ba(n) (n=2,5) clusters

Ab initio theoretical calculations have been performed to study the reaction of O(2) with Ba(n) (n=2,5) clusters. Our results show that O(2) can easily chemisorb and dissociate on small Ba(n) clusters and there is no obvious energy barrier in the process of the dissociation. The local magnetic moment contributed by oxygen must vanish during the intermediate states before the O(2) dissociation. Correspondingly, local magnetic moment only decreases from 2 mu(B) to about 1 mu(B) if O(2) molecularly adsorbs onto Ba(5) cluster. The electronic structure analysis indicates that the charge transfer from Ba(n) cluster to O(2) as well as the orbital hybridization between the cluster and the oxygen molecule may play a key role in O(2) dissociation.     

Single photon ionization of van der Waals clusters with a soft x-ray laser: (CO2)n and (CO2)n(H2O)m

Pure neutral (CO2)n clusters and mixed (CO2)n(H2O)m clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV. The distribution of pure (CO2)n clusters decreases roughly exponentially with increasing cluster size. During the ionization process, neutral clusters suffer little fragmentation because almost all excess cluster energy above the vertical ionization energy is taken away by the photoelectron and only a small part of the photon energy is deposited into the (CO2)n cluster. Metastable dissociation rate constants of (CO2)n+ are measured in the range of (0.2-1.5) x 10(4) s(-1) for cluster sizes of 5< or =n< or =16. Mixed CO2-H2O clusters are studied under different generation conditions (5% and 20% CO2 partial pressures and high and low expansion pressures). At high CO2 concentration, predominant signals in the mass spectrum are the (CO2)n+ cluster ions. The unprotonated cluster ion series (CO2)nH2O+ and (CO2)n(H2O)2+ are also observed under these conditions. At low CO2 concentration, protonated cluster ions (H2O)nH+ are the dominant signals, and the protonated CO2(H2O)nH+ and unprotonated (H2O)n+ and (CO2)(H2O)n+ cluster ion series are also observed. The mechanisms and dynamics of the formation of these neutral and ionic clusters are discussed.     

On the statistical nature of collision and surface-induced dissociation: a theoretical investigation of aluminum clusters

The unimolecular dissociation dynamics of aluminum clusters following collision with either a rare gas atom or a surface is investigated by classical trajectory simulations with model potentials. Two conformers of Al(6) with very distinct shapes, i.e., the spherical O(h) and planar C(2)(h) clusters, are considered in this work. The initial vibrational energy and angular momentum distributions resulting from collision, as well as the energy and angular momentum resolved lifetime distributions, of excited clusters were determined for both collision-induced dissociation (CID) and surface-induced dissociation (SID) processes. The partitioning of excitation energy acquired upon collision was found to depend on the excitation mechanism (CID or SID), as well as on the cluster molecular shape, especially in the case of CID. For both types of processes, the energy and angular momentum resolved excited cluster lifetime distributions were found to decay exponentially, in agreement with statistical theories of chemical reactions, suggesting intrinsic Rice-Ramsperger-Kassel-Marcus (RRKM) behavior. Moreover, the simulated microcanonical rate constants determined from the cluster lifetime distributions are in good agreement with the predictions of the orbiting transition state model of phase space theory (OTS/PST), which further supports the statistical character of cluster CID and SID. Thus, in the CID and SID of highly fluxional systems such as aluminum clusters, the rate of intramolecular vibrational energy redistribution (IVR) is much faster than the dissociation rate, which validates one of the key assumptions, i.e., post-collision statistical behavior, underlying the models that are routinely used to determine cluster binding energies from experimental CID/SID cross sections.     

Interpreting activity in H(2)O-H(2)SO(4) binary nucleation

Sulfuric acid-water nucleation is thought to be a key atmospheric mechanism for forming new condensation nuclei. In earlier literature, measurements of sulfuric acid activity were interpreted as the total (monomer plus hydrate) concentration above solution. Due to recent reinterpretations, most literature values for H(2)SO(4) activity are thought to represent the number density of monomers. Based on this reinterpretation, the current work uses the most recent models of H(2)O-H(2)SO(4) binary nucleation along with perturbation analyses to predict a decrease in critical cluster mole fraction, increase in critical cluster diameter, and orders of magnitude decrease in nucleation rate. Nucleation rate parameterizations available in the literature, however, give opposite trends. To resolve these discrepancies, nucleation rates were calculated for both interpretations of H(2)SO(4) activity and directly compared to the available parameterizations as well as the perturbation analysis. Results were in excellent agreement with older parameterizations that assumed H(2)SO(4) activity represents the total concentration and duplicated the predicted trends from the perturbation analysis, but differed by orders of magnitude from more recent parameterizations that assume H(2)SO(4) activity represents only the monomer. Comparison with experimental measurements available in the literature revealed that the calculations of the current work assuming a(a) represents the total concentration are most frequently in agreement with observations.     

Symmetry-switching molecular Fe(O2)n(+) clusters

Experimental and theoretical studies based on mass spectrometry, collision-induced dissociation, and ab initio calculations are performed on the formation and stability of FeO(n)(+) clusters, as well as on their structural, electronic, and magnetic properties. In the mass spectra, clusters with an even number of oxygen atoms show increased stability, most prominently for FeO(10)(+). The extra stability of this cluster is confirmed by measurements of fragmentation cross sections through crossed molecular beam experiments. In addition, the calculations indicate a structural phase transition at this size, and most importantly, the FeO(n)(+) clusters show unique magnetic features, exhibiting isoenergetic low-spin (LS) and high-spin (HS) ground states. In the LS state, the magnetic moments of the O atoms adopt an antiferromagnetic alignment with respect to the magnetic moment of Fe(+), whereas in the HS state, the alignment is ferromagnetic. FeO(10)(+) is the largest thermodynamically stable complex, with the highest magnetic moment among the FeO(n)(+) clusters (13 μ(B) in HS).     

Tuning cluster reactivity by charge state and composition: experimental and theoretical investigation of CO binding energies to Ag(n)Au(m)(+/-) (n + m = 3)

Temperature-dependent gas-phase reaction kinetics measurements and equilibrium thermodynamics under multicollision conditions in conjunction with ab initio DFT calculations were employed to determine the binding energies of carbon monoxide to triatomic silver-gold binary cluster cations and anions. The binding energies of the first CO molecule to the trimer clusters increase with increasing gold content and with changing charge from negative to positive. Thus, the reactivity of the binary clusters can be sensitively tuned by varying charge state and composition. Also, multiple CO adsorption on the clusters was investigated. The maximum number of adsorbed CO molecules was found to strongly depend on cluster charge and composition as well. Most interestingly, the cationic carbonyl complex Au(3)(CO)(4)(+) is formed at cryogenic temperature, whereas for the anion, only two CO molecules are adsorbed, leading to Au(3)(CO)(2)(-). All other trimer clusters adsorb three CO molecules in the case of the cations and are completely inert to CO in our experiment in the case of the anions.     

Kinetics and Dynamics of Photocatalyzed Dissociation of Ethanol on TiO2(110) (cited: 2)

The kinetics and dynamics of photocatalyzed dissociation of ethanol on TiO₂(110) sur-face have been studied using the time-dependent and time-resolved femtosecond two-photon photoemission spectroscopy respectively, in order to unravel the photochemical properties of ethanol on this prototypical metal oxide surface. By monitoring the time evolution of the photoinduced excited state which is associated with the photocatalyzed dissociation of ethanol on Ti_5c sites of TiO₂(110), the fractal-like kinetics of this surface photocatalytic reaction has been obtained. The measured photocatalytic dissociation rate on reduced TiO₂(110) is faster than that on the oxidized surface. This is attributed to the larger defect density on the reduced surface which lowers the reaction barrier of the photocatalytic reaction at least methodologically. Possible reasons associated with the defect electrons for the acceleration have been discussed. By performing the interferometric two-pulse corre-lation on ethanol/TiO₂(110) interface, the ultrafast electron dynamics of the excited state has been measured. The analyzed lifetime (24 fs) of the excited state is similar to that on methanol/TiO₂(110). The appearance of the excited state provides a channel to mediate the electron transfer between the TiO₂ substrate and its environment. Therefore studying its ultrafast electron dynamics may lead to the understanding of the microscopic mechanism of photocatalysis and photoelectrochemical energy conversion on TiO₂.