Laser intensity determination using nonadiabatic tunneling ionization of atoms in close-to-circularly polarized laser fields.

We conceive an improved procedure to determine the laser intensity with the momentum distributions from nonadiabatic tunneling ionization of atoms in the close-to-circularly polarized laser fields. The measurements for several noble gas atoms are in accordance with the semiclassical calculations, where the nonadiabatic effect and the influence of Coulomb potential are included. Furthermore, the high-order above-threshold ionization spectrum in linearly polarized laser fields for Ar is measured and compared with the numerical calculation of the time-dependent Schrödinger equation in the single-active-electron approximation to test the accuracy of the calibrated laser intensity.

Angle-resolved conical emission spectra from filamentation in a solid with an Airy pattern and a Gaussian laser beam.

Filamentation dynamics in fused silica are investigated using an Airy pattern and a Gaussian laser beam. The angle-resolved conical emission spectra are measured and compared with the predictions of several models. Our experimental observations are consistent with the X-waves model in both cases. This indicates that both laser beams spontaneously evolve into nonlinear X-waves and suggests a universal evolution of filaments in fused silica, regardless of the initial laser beam profile.

The methyl- and aza-substituent effects on nonradiative decay mechanisms of uracil in water: a transient absorption study in the UV region.

The nonradiative decay dynamics of photo-excited uracil (Ura) and its derivatives, i.e., thymine (5-methyluracil, Thy), 6-methyluracil (6-MU) and 6-azauracil (6-AU) in water, has been studied using a femtosecond transient absorption method. The molecules are populated in the lowest (1)ππ* state by a pump pulse at 266 nm, and a broadband continuum in the deep UV region is then employed as the probe. The extension of the continuous UV probe down to 250 nm enables us to investigate comprehensively the population dynamics of the ground states for those molecules and to uncover the substituent effects on nonradiative decay dynamics of uracil. Vibrational cooling in the ground states of Ura, Thy and 6-MU has been directly observed for the first time, providing solid evidence of the ultrafast (1)ππ* → S0 decay. In combination with the ground state bleaching signals, it is consolidated that their lowest (1)ππ* state decays via two parallel pathways, i.e., (1)ππ* → S0 and (1)ππ* → (1)nπ*. Moreover, the contribution of the (1)ππ* → (1)nπ* channel is found to be much smaller for Thy or 6-MU than for Ura. Different from methyl-substitution, the initial (1)ππ* state of the aza-substituent 6-AU decays primarily to the (1)nπ* state, while the (1)ππ* → S0 channel can be negligible. Our study provides a comprehensive understanding of the substituent effects on the excited-state dynamics of uracil in water.

Ultrafast Excited-State Dynamics of 6-Azauracil Studied by Femtosecond Transient Absorption Spectroscopy.

The excited-state dynamics of 6-azauracil in different solvents have been studied using femtosecond transient absorption spectroscopy. The molecule is populated to the S2 state with a pump pulse at 264 nm. Broad-band white light continuum which covers from 320 to 600 nm is used as the probe. With a global fitting analysis of the measured transient spectra, three decay time constants, i.e., <0.3, 5.2 ± 0.1, and >1000 ps, are directly obtained in the solvent of acetonitrile. These newly observed lifetime constants are important in clarifying its decay dynamics as well as in providing a criterion for the ultrafast dynamics simulations in 6-azauracil using quantum chemical theories. In combination with previous theoretical works, the main decay channel is proposed: the initially populated S2 decays to S1 through internal conversion in <0.3 ps, followed by an intersystem crossing from S1 to T1 in 5.2 ± 0.1 ps. The >1000 ps component is due to the decay of the T1 state. A comparison of the excited-state dynamics in different solvents reveals that the decay from S1 to T1 shows a clear dependence on the polarity of the solvents. With higher polarity, the S1 excited state decays faster. This observation is in line with the prediction by Etinski et al. [ Phys. Chem. Chem. Phys. 2010 , 12 , 15665 - 15671 ], where a blue-shift of the T1 state potential energy surface leading to an increase of the intersystem crossing rate was proposed. With the new information obtained in the present measurement, a clearer picture of the decay dynamics of 6-azauracil on the S2 excited state is provided.

Cavity-enhanced field-free molecular alignment at a high repetition rate.

Extreme ultraviolet frequency combs are a versatile tool with applications including precision measurement, strong-field physics, and solid-state physics. Here we report on an application of extreme ultraviolet frequency combs and their driving lasers for studying strong-field effects in molecular systems. We perform field-free molecular alignment and high-order harmonic generation with aligned molecules in a gas jet at a repetition rate of 154 MHz using a high-powered optical frequency comb inside a femtosecond enhancement cavity. The cavity-enhanced system provides a means to reach suitable intensities to study field-free molecular alignment and enhance the observable effects of the molecule-field interaction. We observe modulations of the driving field, arising from the nature of impulsive stimulated Raman scattering responsible for coherent molecular rotations. We foresee the impact of this work on the study of molecule-based strong-field physics, with improved precision and a more fundamental understanding of the interaction effects on both the field and molecules.

The substituent effect on the MLCT excited state dynamics of Cu(I) complexes studied by femtosecond time-resolved absorption and observation of coherent nuclear wavepacket motion.

The substituent effect on the excited-state dynamics of bis-diimine Cu(I) complexes was investigated by femtosecond time-resolved absorption spectroscopy with the S1← S0 metal-to-ligand charge transfer (MLCT) photoexcitation. The time-resolved absorption of [Cu(phen)2](+) (phen = 1,10-phenanthroline) showed a slight intensity increase of the S1 absorption with a time-constant of 0.1-0.2 ps, reflecting the flattening distortion occurring in the S1 state. The transient absorption of the 'flattened' S1 state was clearly observed, although its fluorescence was not observed in the previous fluorescence up-conversion study in the visible region. The flattened S1 state decayed with a time constant of ∼2 ps, and the S0 bleaching recovered accordingly. This clarifies that the S1 state of [Cu(phen)2](+) is predominantly relaxed to the S0 state by internal conversion. The time-resolved absorption of [Cu(dpphen)2](+) (dpphen = 2,9-diphenyl-1,10-phenanthroline) showed a 0.9 ps intensity increase of the S1 absorption due to the flattening distortion, and then exhibited a 11 ps spectral change due to the intersystem crossing. This excited-state dynamics of [Cu(dpphen)2](+) is very similar to that of [Cu(dmphen)2](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline). In the ultrafast pump-probe measurements with 35 fs time resolution, [Cu(phen)2](+) and [Cu(dpphen)2](+) exhibited oscillation due to the nuclear wavepacket motions of the initial S1 state, and the oscillation was damped as the structural change took place. This indicates that the initial S1 states have well-defined vibrational structures and that the vibrational coherence is retained in their short lifetimes. The present time-resolved absorption study, together with the previous time-resolved fluorescence study, provides a unified view for the ultrafast dynamics of the MLCT excited state of the Cu(I) complexes.

Photodissociation dynamics of allyl chloride at 200 and 266 nm studied by time-resolved mass spectrometry and photoelectron imaging.

The photodissociation dynamics of allyl chloride at 200 and 266 nm has been studied by femtosecond time-resolved mass spectrometry coupled with photoelectron imaging. The molecule was prepared to different excited states by selectively pumping with 400 or 266 nm pulse. The dissociated products were then probed by multiphoton ionization with 800 nm pulse. After absorbing two photons at 400 nm, several dissociation channels were directly observed from the mass spectrum. The two important channels, C-Cl fission and HCl elimination, were found to decay with multiexponential functions. For C-Cl fission, two time constants, 48 ± 1 fs and 85 ± 40 ps, were observed. The first one was due to the fast predissociation process on the repulsive nσ*/πσ* state. The second one could be ascribed to dissociation on the vibrationally excited ground state which is generated after internal conversion from the initially prepared ππ* state. HCl elimination, which is a typical example of a molecular elimination reaction, was found to proceed with two time constants, 600 ± 135 fs and 14 ± 2 ps. We assigned the first one to dissociation on the excited state and the second one to the internal conversion from the ππ* state to the ground state and then dissociation on the ground state. As we excited the molecule with 266 nm light, the transient signals decayed exponentially with a time constant of ∼48 fs, which is coincident with the time scale of C-halogen direct dissociation. Photoelectron images, which provided translational and angular distributions of the generated electron, were also recorded. Detailed analysis of the kinetic energy distribution strongly suggested that C3H4(+) and C3H5(+) were generated from ionization of the neutral radical. The present study reveals the dissociation dynamics of allyl chloride in a time-resolved way.

Elimination mechanisms of Br(2)+ and Br+ in photodissociation of 1,1- and 1,2-dibromoethylenes using velocity imaging technique.

Elimination pathways of the Br(2)(+) and Br(+) ionic fragments in photodissociation of 1,2- and 1,1-dibromoethylenes (C(2)H(2)Br(2)) at 233 nm are investigated using time-of-flight mass spectrometer equipped with velocity ion imaging. The Br(2)(+) fragments are verified not to stem from ionization of neutral Br(2), that is a dissociation channel of dibromoethylenes reported previously. Instead, they are produced from dissociative ionization of dibromoethylene isomers. That is, C(2)H(2)Br(2) is first ionized by absorbing two photons, followed by the dissociation scheme, C(2)H(2)Br(2)(+) + hv→Br(2)(+) + C(2)H(2). 1,2-C(2)H(2)Br(2) gives rise to a bright Br(2)(+) image with anisotropy parameter of -0.5 ± 0.1; the fragment may recoil at an angle of ∼66° with respect to the C=C bond axis. However, this channel is relatively slow in 1,1-C(2)H(2)Br(2) such that a weak Br(2)(+) image is acquired with anisotropy parameter equal to zero, indicative of an isotropic recoil fragment distribution. It is more complicated to understand the formation mechanisms of Br(+). Three routes are proposed for dissociation of 1,2-C(2)H(2)Br(2), including (a) ionization of Br that is eliminated from C(2)H(2)Br(2) by absorbing one photon, (b) dissociation from C(2)H(2)Br(2)(+) by absorbing two more photons, and (c) dissociation of Br(2)(+). Each pathway requires four photons to release one Br(+), in contrast to the Br(2)(+) formation that involves a three-photon process. As for 1,1-C(2)H(2)Br(2), the first two pathways are the same, but the third one is too weak to be detected.

Photodissociation of cis-, trans-, and 1,1-dichloroethylene in the ultraviolet range: characterization of Cl((2)P(J)) elimination.

By using photofragment velocity imaging detection coupled with a (2 + 1) resonance-enhanced multiphoton ionization technique, the elimination channel of spin-orbit chlorine atoms in photodissociation of cis-, trans-, and 1,1-dichloroethylene at two photolysis wavelengths of 214.5 and 235 nm is investigated. Translational energy and angular distributions of Cl((2)P(J)) fragmentation are acquired. The Cl((2)P(J)) fragments are produced by two competing channels. The fast dissociation component with higher translational energy is characterized by a Gaussian distribution, resulting from a curve crossing of the initially excited (pi, pi*) state to nearby repulsive (pi, sigma*) and/or (n, sigma*). In contrast, the slow component with a lower translational energy is characterized by a Boltzmann distribution, which dissociates on the vibrationally hot ground state relaxed from the (pi, pi*) state via internal conversion. cis-C(2)H(2)Cl(2) is found to have a larger branching of Boltzmann component than the other two isomers. The fraction of available energy partitioning into translation increases along the trend of cis- < trans- < 1,1-C(2)H(2)Cl(2). This trend may be fitted by a rigid radical model and interpreted by means of a torque generated during the C-Cl bond cleavage. The anisotropy parameters are determined, and the transition dipole moments are expected to be essentially along the C horizontal lineC bond axis. The results are also predicted theoretically. The relative quantum yields of Cl((2)P(J)) have a similar value for the three isomers at the two photolysis wavelengths.

Predissociation dynamics of the B state of CH3I by femtosecond pump-probe technique.

The laser induced predissociation dynamics of the B Rydberg state of CH(3)I following two-photon absorption of a pump pulse was studied with femtosecond pump-probe photoelectron imaging coupled with time-resolved mass spectroscopy. The predissociation lifetime was measured to be 1.55 ps induced by the crossing between the B state and the repulsive A-band. Two possible predissociation channels were observed originating from (a) direct coupling between the B state and the repulsive (3)Q(0) state and (b) a second crossing between the (3)Q(0) and (1)Q(1) states after the coupling between the B and (3)Q(0) states, respectively.

The intersystem crossing process of p-bromofluorobenzene studied with time-resolved photoelectron imaging.

Ultrafast processes of p-bromofluorobenzene are studied with femtosecond time-resolved photoelectron imaging spectroscopy. The photoelectron image revealed four photoelectron rings centered at 0.39, 0.86, 1.13, and 1.61 eV, respectively. The inner rings are more anisotropic than the outer rings. The decay traces of the different rings were recorded separately. Sharp photoelectron energy distributions and different anisotropy parameters extracted from the images indicated resonances with Rydberg states at the (1+1(')) photon energy. The quantum defect values of the four Rydberg states were determined to be 0.75, 0.52, 0.36, and approximately 0, respectively, with principal quantum number of 3. The electron dephasing mechanism of the S(1)(B(2)) state corresponds to the intersystem crossing from the S(1)(B(2)) to T(1)(B(2)) state and the predissociation of the S(1)(B(2)) state via the T(1)(B(1)) state. The lifetimes of S(1)(B(2)) and T(1)(B(2)) are determined from the decay of the photoelectron signals to be 40 and 33 ps, respectively. The variety of time-dependent anisotropy parameters in the first 5 ps shows the rotational wave coherences of p-bromofluorobenzene at the S(1)(B(2)) state.

Ultrafast dynamics of o-bromofluorobenzene studied by time-resolved photoelectron imaging.

Photodissociation dynamics and rotational wave packet coherences of o-bromofluorobenzene are studied by femtosecond time-resolved photoelectron imaging [figure: see text]. The decay of different photoelectron rings shows the population decay of states from which the lifetimes of different states are determined. The variation of photoelectron angular distributions reflects the evolution of rotational coherences.Photodissociation dynamics and rotational wave packet coherences of o-bromofluorobenzene are studied by femtosecond time-resolved photoelectron imaging (TR-PEI) spectroscopy combined with the (1+2') resonance-enhanced multiphoton ionization (REMPI). Photoelectron kinetic energy and angular distributions indicate ionization dynamics from some Rydberg states at the (1+1') photon energy. The lifetimes of the S(1) (A') and T(2) (A') states are determined from the decay of the photoelectron signals to be 38 ps and 27 ps. The electron population decay of the two states is attributed to predissociation and tunneling dissociation. The variation of time-dependent anisotropy parameters in the first 5 ps shows the rotational wave coherences of molecule.

Photoelectron imaging of atomic chlorine and bromine following photolysis of CH2BrCl.

Photoionization of chlorine and bromine atoms following photodissociation of CH(2)BrCl was studied in the wavelength range of 231-238 nm by photoelectron imaging technique. Final state-specific speed and angular distributions of the photoelectron were recorded. Analysis of relative branching ratios to different levels of Cl(+) and Br(+) revealed that the final ion level distributions are generally dominated by the preservation of the ion-core configuration of the intermediate resonant state. Some J(c) numbers of the intermediate states were newly assigned according to this regulation. The configuration interaction between resonant states and the autoionization in the continuum were also believed to play an important role in the ionization process since some ions that deviate from the regulation mentioned ahead were observed. The angular distributions of the electrons were found to be well characterized by beta(2) and beta(4), although the ionization process of chlorine and bromine atoms involves three photons.