Investigation of multiple electronic excited state relaxation pathways following 200 nm photolysis of gas-phase imidazole

Imidazole acts as a subunit in the DNA base adenine and the amino acid histidine-both important biomolecules which display low fluorescence quantum yields following UV excitation. The low fluorescence quantum yields are attributed to competing non-radiative excited state relaxation pathways that operate on ultrafast timescales. Imidazole is investigated here as a model compound due to its accessibility to high level ab initio calculations and time-resolved gas-phase spectroscopic techniques. Recent non-adiabatic dynamics simulations have identified three non-radiative relaxation mechanisms which are active following 6.0-6.2 eV excitation. Presented herein is a comprehensive investigation of each mechanism using a combination of femtosecond time-resolved ion yield and total kinetic energy release spectroscopies to monitor the formation of associated photoproducts. Relaxation along the (1)πσ state constitutes the predominant deactivation pathway. Timescales for NH-dissociation are extracted and distinguished from alternative H-atom sources based on their kinetic energy distributions. Larger photoproducts are observed to a lesser extent and attributed to ring fragmentation following NH-puckering and CN-stretching relaxation paths.     

Nonradiative deexcitation dynamics of 9H-adenine: an OM2 surface hopping study

The nonradiative relaxation of 9H-adenine was studied at the semiempirical OM2/MR-CI level using the surface-hopping approach. Geometry optimizations of energy minima and conical intersections as well as single-point calculations of excitation energies at critical points were performed to characterize the relevant potential energy surfaces of 9H-adenine and to assess the accuracy of the OM2 results. Surface-hopping calculations were performed to describe the nonradiative dynamics of 9H-adenine after vertical excitation into the optically active state. They showed that the deexcitation process is mainly governed by a two-step relaxation consisting of an ultrashort component and a longer component. These findings compare well with experimental results from time-resolved photoelectron spectroscopy.     

Substituent effects on dynamics at conical intersections: alpha,beta-enones

Femtosecond time-resolved photoelectron spectroscopy and high-level theoretical calculations were used to study the effects of methyl substitution on the electronic dynamics of the alpha,beta-enones acrolein (2-propenal), crotonaldehyde (2-butenal), methylvinylketone (3-buten-2-one), and methacrolein (2-methyl-2-propenal) following excitation to the S2(pipi*) state at 209 and 200 nm. We determine that following excitation the molecules move rapidly away from the Franck-Condon region, reaching a conical intersection promoting relaxation to the S1(npi*) state. Once on the S1 surface, the trajectories access another conical intersection, leading them to the ground state. Only small variations between molecules are seen in their S2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the S1 surface and the branching ratios to the products. Ab initio calculations used to compare the geometries, energies, and topographies of the S1/S0 conical intersections of the molecules are not able to satisfactorily explain the variations in relaxation behavior. We propose that the S1 lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S1/S0 conical intersection.     

Solvation dynamics of the electron produced by two-photon ionization of liquid polyols. 1. Ethylene glycol

Solvated electrons have been produced in ethylene glycol by two-photon ionization of the solvent with 263 nm femtosecond laser pulses. The two-photon absorption coefficient of ethylene glycol at 263 nm is determined to be beta = (2.1 +/- 0.2) x 10(-11) m W(-1). The dynamics of electron solvation in ethylene glycol has been studied by pump-probe transient absorption spectroscopy. So, time-resolved absorption spectra ranging from 430 to 710 nm have been measured. A blue shift of the spectra is observed for the first tens of picoseconds. Using the Bayesian data analysis method, the observed solvation dynamics are reconstructed with different models: stepwise mechanisms, continuous relaxation models, or combinations of stepwise and continuous relaxation. Comparison between models is in favor of continuous relaxation, which is mainly governed by solvent molecular motions.     

Excited State Intramolecular Proton Transfer of 1-Hydroxyanthraquinone

The excited state intra-molecular proton transfer dynamics of 1-hydroxyanthraquinone in solution are investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. Two characteristic bands of excited state absorption and stimu-lated emission are observed in transient absorption spectra with the excitation by the pump wavelength of 400 nm. From the delayed stimulated emission signal, the time scale of the intra-molecular proton transfer is determined to be about 32 fs. The quantum chemistry calculations show that the molecular orbits and the order of the S₂ and S₁ states are rever-sal and a conical intersection is demonstrated to exist along the proton transfer coordinate. After proton transfer, the second excited state of tautomer populated via the conical intersection undergoes the internal conversion with ～200 fs and the following intermolecular energy relaxation with ～16 ps. The longer component 300 ps can be explained in terms of the relaxation from excited-state tautomer to its ground state. From our observations, two proton transfer pathways via a conical intersection are proposed and the dominated one preserves the molecular orbits.     

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.     

Through-bond interactions and the localization of excited-state dynamics

The influence of through-bond interactions on nonadiabatic excited-state dynamics is investigated by time-resolved photoelectron spectroscopy (TRPES) and ab initio computation. We compare the dynamics of cyclohexa-1,4-diene, which exhibits a through-bond interaction known as homoconjugation (the electronic correlation between nonconjugated double bonds), with the nonconjugated cyclohexene. Each molecule was initially excited to a 3s Rydberg state using a 200 nm femtosecond pump pulse. The TRPES spectra of these molecules display similar structure and time constants on a subpicosecond time scale. Our ab initio calculations show that similar sets of conical intersections (a [1,2]- and [1,3]-hydrogen shift, as well as carbon-carbon bond cleavage) are energetically accessible to both molecules and that the geometry and orbital composition at the minimum energy crossing points to the ground state are directly analogous. These experimental and computational results suggest that the excited-state dynamics of cyclohexa-1,4-diene become localized at a single double bond and that the effects of through-bond interaction, dominant in the absorption spectrum, are absent in the excited-state dynamics. The notion of excited-state dynamics being localized at specific sites within the nuclear framework is analogous to the localization of light absorption by a subsystem within the molecule, designated a chromophore. We propose the utility of the analogous concept, denoted here as a dynamophore.     

Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy

A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2-60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump-probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core-shell model in which the properties of the core (interior of the nanopool) are taken to be those of bulk water and the properties of the shell (water associated with the headgroups) are taken to be those of w0 = 2. A weighted sum of the core and shell components reproduces the size dependent spectra and the nonexponential population relaxation dynamics. However, the same model does not reproduce the spectral diffusion and the orientational relaxation experiments. It is proposed that, when hydrogen bond structural rearrangement is involved (orientational relaxation and spectral diffusion), dynamical coupling between the shell and the core cause the water nanopool to display more homogeneous dynamics. Therefore, the absorption spectra and vibrational lifetime decays can discern different hydrogen bonding environments whereas orientational and spectral diffusion correlation functions predict that the dynamics are size dependent but not as strongly spatially dependent within a reverse micelle.     

Density matrix method and ultrafast processes

When femto-second (fs) time-resolved experiments are used to study ultrafast processes, quantum beat phenomena are often observed. In this paper, to analyze the fs time-resolved spectra, we will present the density matrix method, a powerful theoretical technique, which describes the dynamics of population and coherence of the system. How to employ it to study the pump-probe experiments and fs ultrafast processes is described. The ππ*→nπ* transition of pyrazine is used as an example to demonstrate the application of the density matrix method. Recently, Suzuki’s group have employed the 22 fs time resolution laser to study the dynamics of the ππ* state of pyrazine. In this case, conical intersection is commonly believed to play an important role in this non-adiabatic process. How to treat the effect of conical intersection on non-adiabatic processes and fs time-resolved spectra is presented. Another important ultrafast process, vibrational relaxation, which takes place in sub-ps and ps range and has never been carefully studied, is treated in this paper. The vibrational relaxation in water dimer is chosen to demonstrate the calculation. It should be noted that the vibrational relaxation of (H2O)2 has not been experimentally studied but it can be accomplished by the pump-probe experiments.     

Exploring ultrafast H-atom elimination versus photofragmentation pathways in pyrazole following 200 nm excitation

The role of ultraviolet photoresistance in many biomolecules (e.g., DNA bases and amino acids) has been extensively researched in recent years. This behavior has largely been attributed to the participation of dissociative (1)πσ* states localized along X-H (X ═ N, O) bonds, which facilitate an efficient means for rapid nonradiative relaxation back to the electronic ground state via conical intersections or ultrafast H-atom elimination. One such species known to exhibit this characteristic photochemistry is the UV chromophore imidazole, a subunit in the biomolecules adenine and histidine. However, the (1)πσ* driven photochemistry of its structural isomer pyrazole has received much less attention, both experimentally and theoretically. Here, we probe the ultrafast excited state dynamics occurring in pyrazole following photoexcitation at 200 nm (6.2 eV) using two experimental methodologies. The first uses time-resolved velocity map ion imaging to investigate the ultrafast H-atom elimination dynamics following direct excitation to the lowest energy (1)πσ* state (1(1)A" ← X(1)A'). These results yield a bimodal distribution of eliminated H-atoms, situated at low and high kinetic energies, the latter of which we attribute to (1)πσ* mediated N-H fission. The time constants extracted for the low and high energy features are ~120 and <50 fs, respectively. We also investigate the role of ring deformation relaxation pathways from the first optically bright (1)ππ* state (2(1)A' ← X(1)A'), by performing time-resolved ion yield measurements. These results are modeled using a (1)ππ* → ring deformation → photofragmentation mechanism (a model based on comparison with theoretical calculations on the structural isomer imidazole) and all photofragments possess appearance time constants of <160 fs. A comparison between time-resolved velocity map ion imaging and time-resolved ion yield measurements suggest that (1)πσ* driven N-H fission gives rise to the dominant kinetic photoproducts, re-enforcing the important role (1)πσ* states have in the excited state dynamics of biological chromophores and related aromatic heterocycles.     

Observation of ultrafast NH3 (Ã) state relaxation dynamics using a combination of time-resolved photoelectron spectroscopy and photoproduct detection

The ultrafast excited state relaxation of ammonia is investigated by resonantly exciting specific vibrational modes of the electronically excited NH(3) (?) state using three complementary femtosecond (fs) pump-probe techniques: time-resolved photoelectron, ion-yield and photofragment translational spectroscopy. Ammonia can be seen as a prototypical system for studying non-adiabatic dynamics and therefore offers a benchmark species for demonstrating the advantages of combining the aforementioned techniques to probe excited state dynamics, whilst simultaneously illuminating new aspects of ammonia's photochemistry. Time-resolved photoelectron spectroscopy (TRPES) provides direct spectroscopic evidence of σ* mediated relaxation of the NH(3) (?) state which manifests itself as coupling of the umbrella (ν(2)) and symmetric N-H stretch (ν(1)) modes in the photoelectron spectra. Time-resolved ion yield (TRIY) and time-resolved photofragment translation spectroscopy (TRPTS) grant a measure of the dissociation dynamics through analysis of the H and NH(2) photodissociation co-fragments. Initial vibrational level dependent TRIY measurements reveal photoproduct formation times of between 190 and 230 fs. Measurement of H-atom photoproduct kinetic energies enables investigation into the competition between adiabatic and non-adiabatic dissociation channels at the NH(3) (?)/NH(3) (X?) conical intersection and has shown that upon non-adiabatic dissociation into NH(2) (X?) + H, the NH(2) (X[combining tilde]) fragment is predominantly generated with significant fractions of internal vibrational energy.     

Time-resolved relaxation dynamics of Hgn- (11 <or = n <or = 16,n = 18) clusters following intraband excitation at 1.5 eV

Electron-nuclear relaxation dynamics are studied in Hg(n) (-) (11 相似文献   

Monitoring the effect of a control pulse on a conical intersection by time-resolved photoelectron spectroscopy

We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two (2)A' states of NO(2) that is transformed to an avoided crossing. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.     

Time-resolved imaging of the reaction coordinate

Time-resolved photoelectron imaging of negative ions is employed to study the dynamics along the reaction coordinate in the photodissociation of IBr(-). The results are discussed in a side-by-side comparison with the dissociation of I(2) (-), examined under similar experimental conditions. The I(2) (-) anion, extensively studied in the past, is used as a reference system for interpreting the IBr(-) results. The data provide rigorous dynamical tests of the anion electronic potentials. The evolution of the energetics revealed in the time-resolved (780 nm pump, 390 nm probe) I(2) (-) and IBr(-) photoelectron images is compared to the predictions of classical trajectory calculations, with the time-resolved photoelectron spectra modeled assuming a variety of neutral states accessed in the photodetachment. In light of good overall agreement of the experimental data with the theoretical predictions, the results are used to construct an experimental image of the IBr(-) dissociation potential as a function of the reaction coordinate.     

飞秒时间分辨的光电子影像技术研究2-氯吡啶超快内转换动力学

利用时间分辨的飞秒光电子影像技术结合时间分辨的质谱技术, 研究了2-氯吡啶分子激发态的超快过程. 实时观察到了2-氯吡啶分子第二激发态(S₂)向第一激发态(S₁)高振动能级的的超快内转换过程,该内转换的时间常数为(162±5)fs. 实验结果表明, 通过S₂/S₀的锥形交叉衰减到基态的衰减通道也是退布居的重要通道, 其时间尺度为(5.5±0.3) ps.     

光电子成像方法研究Xe时间分辨多光子电离过程

利用自行研制的离子成像检测器研究了Xe的飞秒时间分辨双色多光子电离过程.Xe的408nm多光子电离对比实验结果表明,该离子成像检测器与相应的进口产品具有相近的光电子能量分辨率.在272nm飞秒激光作用下,3光子电离产生能量为1.57和0.26eV的光电子,分别对应于Xe+的两个自旋态;在408nm飞秒激光作用下,还观察到第一级阈上电离产生的光电子.在双色飞秒时间分辨实验中,随着两束光相对时间的改变,光电子能谱出现了一系列的变化;随着两束光时间重合程度的增加,由双色多光子电离(3+1'或4'+1)产生的光电子信号逐渐加强;在第二束光的作用下,由第一束单色光产生的光电子出现能量红移,第二束光同时也导致中间态布居数减少.这种光电子能谱的红移现象反映了原子体系中激光场诱导有质动力势的时间分辨动态调制过程.     

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

The multiphoton multichannel photodynamics of NO(2) has been studied using femtosecond time-resolved coincidence imaging. A novel photoelectron-photoion coincidence imaging machine was developed at the laboratory in Amsterdam employing velocity map imaging and "slow" charged particle extraction using additional electron and ion optics. The NO(2) photodynamics was studied using a two color pump-probe scheme with femtosecond pulses at 400 and 266 nm. The multiphoton excitation produces both NO(2) (+) parent ions and NO(+) fragment ions. Here we mainly present the time dependent photoelectron images in coincidence with NO(2) (+) or NO(+) and the (NO(+),e) photoelectron versus fragment ion kinetic energy correlations. The coincidence photoelectron spectra and the correlated energy distributions make it possible to assign the different dissociation pathways involved. Nonadiabatic dynamics between the ground state and the A (2)B(2) state after absorption of a 400 nm photon is reflected in the transient photoelectron spectrum of the NO(2) (+) parent ion. Furthermore, Rydberg states are believed to be used as "stepping" states responsible for the rather narrow and well-separated photoelectron spectra in the NO(2) (+) parent ion. Slow statistical and fast direct fragmentation of NO(2) (+) after prompt photoelectron ejection is observed leading to formation of NO(+)+O. Fragmentation from both the ground state and the electronically excited a (3)B(2) and b (3)A(2) states of NO(2) (+) is observed. At short pump probe delay times, the dominant multiphoton pathway for NO(+) formation is a 3x400 nm+1x266 nm excitation. At long delay times (>500 fs) two multiphoton pathways are observed. The dominant pathway is a 1x400 nm+2x266 nm photon excitation giving rise to very slow electrons and ions. A second pathway is a 3x400 nm photon absorption to NO(2) Rydberg states followed by dissociation toward neutral electronically and vibrationally excited NO(A (2)Sigma,v=1) fragments, ionized by one 266 nm photon absorption. As is shown in the present study, even though the pump-probe transients are rather featureless the photoelectron-photoion coincidence images show a complex time varying dynamics in NO(2). We present the potential of our novel coincidence imaging machine to unravel in unprecedented detail the various competing pathways in femtosecond time-resolved multichannel multiphoton dynamics of molecules.     

2-甲基呋喃分子激发态超快非绝热动力学

Excited-state dynamics of 2-methyl furan has been studied by femtosecond time-resolved photoelectron imaging. The molecule 2-methyl furan was simultaneously excited to the n=3 Rydberg series of S₁[¹A"(π3s)], ¹A'(π3p_x), ¹A"(π3p_y) and ¹A"(π3p_z) and the valence state of ¹A'(ππ^*) by two 400 nm photons and subsequently probed by two 800 nm photons. The average lifetime of the Rydberg series and the valence state was measured to be on the time scale of 50 fs by the time-dependent ion yield of the parent ion. Ultrafast internal conversions among these excited states were observed and extracted from the time-dependences of the photoelectron kinetic energy components of these excited states in the photoelectron kinetic energy spectra. Furthermore, it is identified that the ¹A'(ππ^*) state might play an important role in internal conversions among these excited states. The Rydberg-valence mixings, which result in numerous conical intersections, act as the driving force to accomplish such ultrafast internal conversions.     

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.     

Determination of the electron affinity of the acetyloxyl radical (CH3COO) by low-temperature anion photoelectron spectroscopy and ab initio calculations

The electronic structure and electron affinity of the acetyloxyl radical (CH3COO) were investigated by low-temperature anion photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of the acetate anion (CH3COO-) were obtained at two photon energies (355 and 266 nm) and under three different temperatures (300, 70, and 20 K) with use of a new low-temperature ion-trap photoelectron spectroscopy apparatus. In contrast to a featureless spectrum at 300 K, a well-resolved vibrational progression corresponding to the OCO bending mode was observed at low temperatures in the 355 nm spectrum, yielding an accurate electron affinity for the acetyloxyl radical as 3.250 +/- 0.010 eV. This experimental result is supported by ab initio calculations, which also indicate three low-lying electronic states observed in the 266 nm spectrum. The calculations suggest a 19 degrees decrease of the OCO angle upon detaching an electron from acetate, consistent with the vibrational progression observed experimentally.