Ab initio trajectory surface-hopping study on ultrafast deactivation process of thiophene

The ultrafast S(1)((1)ππ*) → S(0) deactivation process of thiophene in the gas phase has been simulated with the complete active space self-consistent field (CASSCF) based fewest switch surface hopping method. It was found that most of the calculated trajectories (～80%) decay to the ground state (S(0)) with an averaged time constant of 65 ± 5 fs. This is in good agreement with the experimental value of about 80 fs. Two conical intersections were determined to be responsible for the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. After thiophene is excited to the S(1)((1)ππ*) state in the Franck-Condon region, it quickly relaxes to the minimum of the S(1)((1)ππ*) state, then overcomes a small barrier near the conical intersection (CI((1)ππ*/(1)πσ*)), and eventually arrives at the minimum of one C-S bond fission (S(1)((1)πσ*)). In the vicinity of this minimum, the conical intersection (CI((1)πσ*/S(0))) funnels the electron population to the ground state (S(0)), completing the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. This decay mechanism matches well with previous experimental and theoretical studies.     

Ultrafast S1 to S0 internal conversion dynamics for dimethylnitramine through a conical intersection

Electronically nonadiabatic processes such as ultrafast internal conversion (IC) from an upper electronic state (S(1)) to the ground electronic state (S(0)) though a conical intersection (CI), can play an essential role in the initial steps of the decomposition of energetic materials. Such nonradiative processes following electronic excitation can quench emission and store the excitation energy in the vibrational degrees of freedom of the ground electronic state. This excess vibrational energy in the ground electronic state can dissociate most of the chemical bonds of the molecule and can generate stable, small molecule products. The present study determines ultrafast IC dynamics of a model nitramine energetic material, dimethylnitramine (DMNA). Femtosecond (fs) pump-probe spectroscopy, for which a pump pulse at 271 nm and a probe pulse at 405.6 nm are used, is employed to elucidate the IC dynamics of this molecule from its S(1) excited state. A very short lifetime of the S(1) excited state (～50 ± 16 fs) is determined for DMNA. Complete active space self-consistent field (CASSCF) calculations show that an (S(1)/S(0))(CI) CI is responsible for this ultrafast decay from S(1) to S(0). This decay occurs through a reaction coordinate involving an out-of-plane bending mode of the DMNA NO(2) moiety. The 271 nm excitation of DMNA is not sufficient to dissociate the molecule on the S(1) potential energy surface (PES) through an adiabatic NO(2) elimination pathway.     

1B2(1Sigma(u)+) excited state decay dynamics in CS2

The authors report time resolved photoelectron spectra of the (1)B(2)((1)Sigma(u) (+)) state of CS(2) at pump wavelengths in the region of 200 nm. In contrast to previous studies, the authors find that the predissociation dynamics is not well described by a single exponential decay. Biexponential modeling of the authors' data reveals a rapid decay pathway (tau<50 fs), in addition to a longer lived channel (tau approximately 350-650 fs) that displays a marked change in apparent lifetime when the polarization of the pump laser is rotated with respect to that of the probe. Since the initially populated (1)B(2)((1)Sigma(u) (+)) state may decay to form either S((1)D) or S((3)P) products (the latter produced via a spin-orbit induced crossing from a singlet to a triplet electronic surface), this lifetime observation may be rationalized in terms of changes in the relative ionization cross section of these singlet and triplet states of CS(2) as a function of laser polarization geometry. The experimentally observed lifetime of the longer lived channel is therefore a superposition of these two pathways, both of which decay on very similar time scales.     

Excited-state processes in the carotenoid zeaxanthin after excess energy excitation

Aiming for better understanding of the large complexity of excited-state processes in carotenoids, we have studied the excitation wavelength dependence of the relaxation dynamics in the carotenoid zeaxanthin. Excitation into the lowest vibrational band of the S2 state at 485 nm, into the 0-3 vibrational band of the S2 state at 400 nm, and into the 2B(u)+ state at 266 nm resulted in different relaxation patterns. While excitation at 485 nm produces the known four-state scheme (S2 --> hot S1 --> S1 --> S0), excess energy excitation led to additional dynamics occurring with a time constant of 2.8 ps (400 nm excitation) and 4.9 ps (266 nm excitation), respectively. This process is ascribed to a conformational relaxation of conformers generated by the excess energy excitation. The zeaxanthin S state was observed regardless of the excitation wavelength, but its population increased after 400 and 266 nm excitation, suggesting that conformers generated by the excess energy excitation are important for directing the population toward the S state. The S2-S1 internal conversion time was shortened from 135 to 70 fs when going from 485 to 400 nm excitation, as a result of competition between the S2-S1 internal conversion from the vibrationally hot S2 state and S2 vibrational relaxation. The S1 lifetime of zeaxanthin was within experimental error the same for all excitation wavelengths, yielding approximately 9 ps. No long-lived species have been observed after excitation by femtosecond pulses regardless of the excitation wavelength, but excitation by nanosecond pulses at 266 nm generated both zeaxanthin triplet state and cation radical.     

飞秒泵浦-探测质谱方法研究苯内转换动力学

以飞秒400及266 nm激光脉冲结合泵浦-探测飞行时间质谱方法研究了苯S2态内转换动力学. 400 nm双光子过程将苯分子激发到S2电子态,布居在S2电子态的分子由于能级耦合无辐射弛豫到S1电子态. 通过测定C6H6+讯号强度随泵浦-探测延迟时间的改变,获得苯S2及S1电子态的衰减寿命分别为(48±1)fs及(6.5±0.2)ps, S2态及S1电子态的消激发机理被认为是相应势能面间的锥形交叉引起的内转换.     

Dibenzo-p-dioxin. An ab initio CASSCF/CASPT2 study of the pi-pi* and n-pi* valence excited states

The pi-pi* and n-pi* valence excited states of dibenzo-p-dioxin (DD) were studied via the complete active space SCF and multiconfigurational second-order perturbation theory employing the cc-pVDZ basis set and the full pi-electron active spaces of 16 electrons in 14 active orbitals. The geometry and harmonic vibrational wavenumbers of the ground state correlate well with the experimental and other theoretical data. In particular, significant improvements over previously reported theoretical results are observed for the excitation energies. All of the pi-pi* excited states exhibit planar D(2h)minima. Thus no evidence was found for a C(2v) butterfly-like relaxation, although the wavenumbers of the b(3u) butterfly flapping mode proved exceedingly low in both the ground S(0)((1)A(g)) and the lowest dipole allowed excited S(1)((1)B(2u)) state. The calculations of oscillator strengths established the 2(1)B(2u) <-- 1(1)A(g) and 2(1)B(1u) <-- 1(1)A(g) transitions as by far the most intense, whereas the only allowed of the n-pi* transitions ((1)B(3u)) should possess only a modest intensity. Studies into dependence of the oscillator strengths on the extent of the butterfly-like folding showed that the electronic spectrum is more consistent with a folded equilibrium geometry assumed by DD in solution.     

Ultrafast photodynamics of furan

Ultrafast photodynamics of furan has been studied by time-resolved photoelectron imaging (TRPEI) spectroscopy with an unprecedented time resolution of 22 fs. The simulation of the time-dependent photoelectron kinetic energy distribution (PKED) has been performed with ab initio nonadiabatic dynamics "on the fly" in the frame of time-dependent density functional theory. Based on the agreement between experimental and theoretical time-dependent photoelectron signal intensity as well as on PKED, precise time scales of ultrafast internal conversion from S(2) over S(1) to the ground state S(0) of furan have been revealed for the first time. Upon initial excitation of the S(2) state which has π-π* character, a nonadiabatic transition to the S(1) state occurs within 10 fs. Subsequent dynamics invokes the excitation of the C-O stretching and C-O-C out of plane vibrations which lead to the internal conversion to the ground state after 60 fs. Thus, we demonstrate that the TRPEI combined with high level nonadiabatic dynamics calculations provide fundamental insight into ultrafast photodynamics of chemically and biologically relevant chromophores.     

Femtosecond Time-Resolved Stimulated Raman Spectroscopy of the S(2) (1B(u)) Excited State of beta-Carotene

The electronic and vibrational structure of beta-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived ( approximately 160 fs) second excited singlet state (S(2),1B(u) (+))of beta-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at approximately 1100, 1300, and 1650 cm(-1) that decay with a time constant corresponding to the electronic lifetime of S(2). The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1B(u) (+) and 2A(g) (-)) in the ultrafast relaxation pathway of beta-carotene.     

Ion-pair formation dynamics of F(2) at 18.385 eV studied by velocity map imaging method

We studied the ion-pair formation dynamics of F2 at 18.385 eV (67.439 nm) using the velocity map imaging method. It was found that there are two dissociation channels corresponding to production of F(+)((1)D(2)) + F(-)((1)S(0)) and F(+)((3)P(j)) + F(-)((1)S(0)). The measured center-of-mass translational energy distribution shows that about 98% of the dissociation occurs via the F(+)((1)D(2)) channel. The measured angular distributions of the photofragments indicate that dissociation for the F(+)((3)P(j)) channel occurs via predissociation of Rydberg states converging to F(2)(+)(A(2)Pi(u)) and dissociation for the F(+)((1)D(2)) channel involves mainly a direct perpendicular transition into the ion-pair state, or X(1)Sigma(g)(+) --> 2(1)Pi(u), which is also supported by the transition dipole moment calculations .     

Investigating excited state dynamics of salinixanthin and xanthorhodopsin in the near-infrared

Excited state dynamics of native Xanthorhodopsin (XR), of an XR sample with a reduced prosthetic group, and of the associated Carotenoid (CAR) salinixanthin (SX) in ethanol were investigated by hyperspectral Near Infrared (NIR) probing. Global kinetic analysis shows that: (1) unlike the transient spectra recorded in the visible, fitting of the NIR data requires only two phases of exponential spectral evolution, assigned to internal conversion from S(2) → S(1) and from S(1) → S(0) of the carotene. (2) The rate of the internal conversion from S(2) → S(1) in the reduced sample is well fit with a decay time of 130 fs, significantly longer than in XR and in SX, both of which are well fit with τ ≈ 100 fs. This increased lifetime is consistent with a ～30% efficiency of ET from SX to retinal in XR. (3) S(1) of salinixanthin is verified to lie ～12,700 cm(-1) above the ground electronic surface, excluding its involvement in the retinal sensitization in XR. (4) The oscillator strength of the S(1) → S(2) transition is determined to be no more than 0.16, despite its symmetry allowedness. (5) No long lived NIR absorbance decay assignable to the carotenoid S* state was detected in any of the samples. Inconsistencies concerning previously determined S(2) lifetimes and kinetic schemes used to model these data are discussed.     

Femtosecond Time-Resolved Stimulated Raman Spectroscopy: Application to the Ultrafast Internal Conversion in beta-Carotene

We have developed the technique of femtosecond stimulated Raman spectroscopy (FSRS), which allows the rapid collection of high-resolution vibrational spectra on the femtosecond time scale. FSRS combines a sub-50 fs actinic pump pulse with a two-pulse stimulated Raman probe to obtain vibrational spectra whose frequency resolution limits are uncoupled from the time resolution. This allows the acquisition of spectra with <100 fs time resolution and <30 cm(-1) frequency resolution. Additionally, FSRS is unaffected by background fluorescence, provides rapid (100 ms) acquisition times, and exhibits traditional spontaneous Raman line shapes. FSRS is used here to study the relaxation dynamics of beta-carotene. Following optical excitation to S(2) (1B(u) (+)) the molecule relaxes in 160 fs to S(1) (2A(g) (-)) and then undergoes two distinct stages of intramolecular vibrational energy redistribution (IVR) with 200 and 450 fs time constants. These processes are attributed to rapid (200 fs) distribution of the internal conversion energy from the S(1) C=C modes into a restricted bath of anharmonically coupled modes followed by complete IVR in 450 fs. FSRS is a valuable new technique for studying the vibrational structure of chemical reaction intermediates and transition states.     

State preparation and excited electronic and vibrational behavior in hemes

The temporally overlapping, ultrafast electronic and vibrational dynamics of a model five-coordinate, high-spin heme in a nominally isotropic solvent environment has been studied for the first time with three complementary ultrafast techniques: transient absorption, time-resolved resonance Raman Stokes, and time-resolved resonance Raman anti-Stokes spectroscopies. Vibrational dynamics associated with an evolving ground-state species dominate the observations. Excitation into the blue side of the Soret band led to very rapid S2 --> S1 decay (sub-100 fs), followed by somewhat slower (800 fs) S1 --> S0 nonradiative decay. The initial vibrationally excited, non-Boltzmann S0 state was modeled as shifted to lower energy by 300 cm(-1) and broadened by 20%. On a approximately 10 ps time scale, the S0 state evolved into its room-temperature, thermal distribution S0 profile largely through VER. Anti-Stokes signals disappear very rapidly, indicating that the vibrational energy redistributes internally in about 1-3 ps from the initial accepting modes associated with S1 --> S0 internal conversion to the rest of the macrocycle. Comparisons of anti-Stokes mode intensities and lifetimes from TRARRS studies in which the initial excited state was prepared by ligand photolysis [Mizutani, T.; Kitagawa, T. Science 1997, 278, 443, and Chem. Rec. 2001, 1, 258] suggest that, while transient absorption studies appear to be relatively insensitive to initial preparation of the electronic excited state, the subsequent vibrational dynamics are not. Direct, time-resolved evaluation of vibrational lifetimes provides insight into fast internal conversion in hemes and the pathways of subsequent vibrational energy flow in the ground state. The overall similarity of the model heme electronic dynamics to those of biological systems may be a sign that the protein's influence upon the dynamics of the heme active site is rather subtle.     

Solvent-dependent activation of intermediate excited states in the energy relaxation pathways of spheroidene

In carotenoids internal conversion between the allowed (S(2)) and forbidden (S(1)) excited states occurs on a sub-picosecond timescale; the involvement of an intermediate excited state(s) (S(x)) mediating the process is controversial. Here we use high time resolution (sub-20 fs) broadband (1.2-2.5 eV) pump-probe spectroscopy to study the solvent dependence of excited state dynamics of spheroidene, a naturally-occurring carotenoid with ten conjugated double bonds. In the high polarizability solvent, CS(2), we find no evidence of an intermediate state, and the traditional three-level (S(0), S(1), S(2)) model fully accounts for the S(2)→ S(1) process. On the other hand, in the low polarizability solvent, cyclohexane, we find that rapid (~30 fs) relaxation to an intermediate state, S(x), lying between S(1) and S(2) is required to account for the data. We interpret these results as due to a shift of the S(2) energy, which positions the state above or below the energy of S(x) in response to changes in solvent polarizability.     

Photoisomerization mechanism of 4-methylpyridine explored by electronic structure calculations and nonadiabatic dynamics simulations

In the present paper, different electronic structure methods have been used to determine stationary and intersection structures on the ground (S(0)) and (1)ππ? (S(2)) states of 4-methylpyridine, which is followed by adiabatic and nonadiabatic dynamics simulations to explore the mechanistic photoisomerization of 4-methylpyridine. Photoisomerization starts from the S(2)((1)ππ?) state and overcomes a small barrier, leading to formation of the prefulvene isomer in the S(0) state via a S(2)∕S(0) conical intersection. The ultrafast S(2) → S(0) nonradiative decay and low quantum yield for the photoisomerization reaction were well reproduced by the combined electronic structure calculation and dynamics simulation. The prefulvene isomer was assigned as a long-lived intermediate and suggested to isomerize to 4-methylpyridine directly in the previous study, which is not supported by the present calculation. The nonadiabatic dynamics simulation and electronic structure calculation reveal that the prefulvene isomer is a short-lived intermediate and isomerizes to benzvalene form very easily. The benzvalene form was predicted as the stable isomer in the present study and is probably the long-lived intermediate observed experimentally. A consecutive light and thermal isomerization cycle via Dewar isomer was determined and this cycle mechanism is different from that reported in the previous study. It should be pointed out that formation of Dewar isomer from the S(2)((1)ππ?) state is not in competition with the isomerization to the prefulvene form. The Dewar structure observed experimentally may originate from other excited states.     

Ultrafast dynamics of aniline in the 294-234 nm excitation range: the role of the πσ∗ state

The ultrafast relaxation of jet-cooled aniline was followed by time-resolved ionization, after excitation in the 294-234 interval. The studied range of energy covers the absorption of the two bright ππ? excitations, S(1) and S(3), and the almost dark S(2) (πσ?) state. The employed probe wavelengths permit to identify different ultrafast time constants related with the coupling of the involved electronic surfaces. A τ(1) = 165 ± 30 fs lifetime is attributed to dynamics along the S(2) (πσ?) repulsive surface. Other relaxation channels as the S(1)→S(0) and S(3)→S(1) internal conversion are also identified and characterized. The work provides a general view of the photophysics of aniline, particularly regarding the role of the πσ? state. This state appears as minor dissipation process due to the ineffective coupling with the bright S(1) and S(3) states, being the S(1)→S(0) internal conversion the main non-radiative process in the full studied energy range. Additionally, the influence of the off-resonance adiabatic excitation of higher energy electronic states, particularly S(3), is also observed and discussed.     

C6- electronic relaxation dynamics probed via time-resolved photoelectron imaging

Anion time-resolved photoelectron imaging has been used to investigate the electronic relaxation dynamics of C(6) (-) following excitation of the C (2)Pi(g)<--X (2)Pi(u) and 2 (2)Pi(g)<--X (2)Pi(u) 0(0) (0) transitions at 607 and 498 nm, respectively. Analysis of evolving photodetachment energy distributions reveals differing relaxation pathways from these prepared states. Specifically, the C (2)Pi(g) 0(0) level relaxes on a time scale of 620+/-30 fs to vibrationally hot ( approximately 2.0 eV) anion ground state both directly and indirectly through vibrationally excited levels of the intermediate-lying A (2)Sigma(g) (+) state that decay with a time scale of 2300+/-200 fs. In contrast, the 2 (2)Pi(g) 0(0) level relaxes much more quickly (<100 fs) to vibrationally hot ( approximately 2.5 eV) anion ground state directly and with transient population accumulation in the A (2)Sigma(g) (+), B (2)Sigma(u) (+), and C (2)Pi(g) electronic levels, as determined by spectral and time-scale analyses. This work also presents the experimental observation of the optically inaccessible B (2)Sigma(u) (+) state, which is found to have an electronic term value of 1.41+/-0.05 eV.     

An examination of structural characteristics of phenylacetylene by vibronic and rovibronic simulations of ab initio data

The structural properties of phenylacetylene have been investigated in the S(0)((1)A(1)) neutral ground and S(1)((1)B(2)) and S(2)((1)A(1)) singlet excited states and the D(0)((2)B(1)) cationic state using both rovibronic and multidimensional Franck-Condon simulations from data determined via correlated ab initio methods. Results are compared to experimental and ab initio data reported in the literature. (10,10)-CASSCF and a hybrid CASSCF/SACCI frequency analysis using the cc-pVDZ Dunning basis set have been employed to produce vibronic simulations of REMPI/FES, dispersed fluorescence, TPES and MATI spectra. Calculated rotational constants are used where appropriate to compare to rotationally resolved experimental studies. Whilst the simulations are of generally good quality, it is apparent that the distortion of the ring along the long axis upon electronic excitation is underestimated, resulting in smaller predicted changes in ipso and para CCC bond angles and weaker activities in the 6a and 9a modes compared with experiment. Simulations of one-photon MATI spectra on the other hand, which do not rely on excited state methodologies, compare very well with experiment, suggesting that the neutral and cationic ground state geometries are quite accurate, as are the predicted changes in geometry accompanying ionisation. Simulated rotational and vibrational profiles, as well as other calculated physical data, show good agreement with the numerous experimental and computational studies of phenylacetylene in the literature.     

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.     

Photodissociation dynamics of D2O via the B̃(1A1) electronic state

Photodissociation dynamics of D(2)O in the B?((1)A(1)) state at different photolysis wavelengths have been investigated using the D-atom Rydberg "tagging" time-of-flight (TOF) technique, in combination with a tunable vacuum ultraviolet photolysis light source. TOF spectra of the D-atom product from the D(2)O photodissociation in both parallel and perpendicular polarizations have been measured. Product kinetic energy distributions and angular distributions have been derived from these TOF spectra. From these distributions, internal state distributions of the OD product as well as the OD quantum state specific angular anisotropy parameters have been derived. Two product channels governed by distinct dissociation dynamics have been clearly observed in the B?((1)A(1)) state photodissociation: ground electronic state radical product OD(X (2)Π) + D and excited electronic state OD(A (2)Σ(+)) + D. The OD(A) + D channel proceeds via adiabatic pathway on the B?((1)A(1)) state surface, producing rovibrational excitation in the OD(A) product, while the OD(X) + D channel is generated through nonadiabatic pathway mainly via conical intersections between the B?((1)A(1)) and the X?((1)A(1)) state surfaces. Due to strong angular force induced by the conical intersections, the OD(X) product is extremely hot in the rotational excitation close to the energy limit (N ～ 50 for v = 0). However, the vibrational excitation is cold in the OD(X) product with dominant population in the ground vibrational state v = 0. Detailed experimental results at different photolysis wavelengths show that at higher energy the unstable periodic orbit, from which dissociation starts, on the B? state has stronger excitation degree of the OD internal state. The negative angular anisotropy parameters of the OD(A) products suggest that the angular forces in this adiabatic dissociation pathway from these periodic orbits have changed the original angular distribution of the D(2)O molecule excited by the B?((1)A(1))←X?((1)A(1)) parallel transition.