Femtosecond Pump-probe Experiments Research Articles

Evidence for thermal mechanisms in laser-induced femtosecond spin dynamics

Recent pump-probe experiments using powerful femtosecond lasers and x-ray magnetic circular dichroism have opened a debate on the origin of the magnetization modification on the femtosecond time scale. We show a quantitative agreement between femtosecond optical pump-probe experiments and thermal micromagnetic modeling in nickel, which reveals a predominant thermal demagnetization mechanism. Magnetic fluctuations are introduced in the system as spin-flip processes due to scattering mechanisms in the electron system. In our model the Landau-Lifshitz-Bloch equation for a macrospin (containing the statistically averaged magnetic fluctuations) is coupled to the electronic temperature of the two-temperature model whose parameters are extracted from the measured reflectivity. We show that the demagnetization and the magnetization recovery time slow down as the laser pump fluence is increased and identify the longitudinal relaxation as a key factor for the observed behavior.

Two-Photon Fluorescence Property and Ultrafast Dynamics of Two Dipolar Compounds with Dipicolinate as Electron Acceptor

Linear and nonlinear photophysical properties of two novel dipolar compounds named as trans-dimethyl-4-[4′-(N,N-dimethylamino)-styry1]-pyridin-2,6-dicarboxylate (Xiao-1) and trans-dimethyl-4-[4′-(N,N-diphenylamino)-styry1]-pyridin-2,6-dicarboxylate (Xiao-2) are investigated by steady-state absorption and fluorescence spectroscopy, Z-scan and two-photon excited fluorescence measurements. Strong two-photon fluorescence emission and the pronounced positive solvatochromism are observed from two compounds. The two-photon absorption cross section of Xiao-2 is about 1.5 times larger than that of Xiao-1. One-color and two-color femtosecond pump-probe experiments are employed to investigate the excited state dynamics of two compounds. The relaxation lifetime of the intra-molecular charge transfer state is determined to be in the hundreds of picosecond domain for both the compounds in THF, and several tens of picosecond in DMSO solutions.

Ultrafast spectroscopy of semiconductor saturable absorber mirror

Ultrafast spectroscopy of semiconductor saturable absorber mirror (SESAM) is measured using a femtosecond pump-probe experiment. This allows dynamic responses of SESAM in the cavity to be concluded by ultrafast spectroscopy. Change in reflection is measured as a function of pump-probe delay for different pump excitation fluences. Change of nonlinear reflection of SESAM is measured as a function of incident light energy density. When the excitation fluence increases, nonlinear change in ultrafast spectroscopy of SESAM becomes increasingly significant. When SESAM is pumped by an ultrahigh excitation fluence, the energy density of which is approximately 1400 \mu J/cm2, two-photon absorption can be observed visibly in its ultrafast spectroscopy.

Coherent excitation phenomena in time-resolved experiments

We report on the influence of coherent phenomena on femtosecond pump-probe experiments in molecular systems. The signature of Coherent Population Return (CPR) has been observed, and satisfactorily simulated, by following the dynamics of the on and off resonance excitation of the S(0)-S(1) transition of aniline. While for the on resonance experiments the system dynamics can be described in terms of the simple incoherent rate equations, for the off resonance experiments a more detailed analysis based on the time-dependent Schrödinger equation that takes into account coherent effects is needed. This fact points out the necessity of considering coherent effects during and after the interaction process, even in systems where ordinarily they have been neglected, for a rigorous description of laser mater interaction.

Kinetically Controlled Photoinduced Electron Transfer Switching in Cu(I)-Responsive Fluorescent Probes

Copper(I)-responsive fluorescent probes based on photoinduced electron transfer (PET) switching consistently display incomplete recovery of emission upon Cu(I) binding compared to the corresponding isolated fluorophores, raising the question of whether Cu(I) might engage in adverse quenching pathways. To address this question, we performed detailed photophysical studies on a series of Cu(I)-responsive fluorescent probes that are based on a 16-membered thiazacrown receptor ([16]aneNS(3)) tethered to 1,3,5-triarylpyrazoline-fluorophores. The fluorescence enhancement upon Cu(I) binding, which is mainly governed by changes in the photoinduced electron transfer (PET) driving force between the ligand and fluorophore, was systematically optimized by increasing the electron withdrawing character of the 1-aryl-ring, yielding a maximum 29-fold fluorescence enhancement upon saturation with Cu(I) in methanol and a greater than 500-fold enhancement upon protonation with trifluoroacetic acid. Time-resolved fluorescence decay data for the Cu(I)-saturated probe indicated the presence of three distinct emissive species in methanol. Contrary to the notion that Cu(I) might engage in reductive electron transfer quenching, femtosecond time-resolved pump-probe experiments provided no evidence for formation of a transient Cu(II) species upon photoexcitation. Variable temperature (1)H NMR experiments revealed a dynamic equilibrium between the tetradentate NS(3)-coordinated Cu(I) complex and a ternary complex involving coordination of a solvent molecule, an observation that was further supported by quantum chemical calculations. The combined photophysical, electrochemical, and solution chemistry experiments demonstrate that electron transfer from Cu(I) does not compete with radiative deactivation of the excited fluorophore, and, hence, that the Cu(I)-induced fluorescence switching is kinetically controlled.

Optoacoustic response of a single submicronic gold particle revealed by the picosecond ultrasonics technique

The optoacoustic response of a single submicron (430 nm) gold particle embedded in a silica thin film is experimentally revealed by femtosecond pump-probe experiments. A semianalytical model is developed to calculate the transient reflectivity accounting for optical index changes in both media and for particle and film surface displacements. The displacement of the particle-film interface turns out to be the major contribution to the measured signal. The amplitude of the acoustical component of the transient reflectivity is modulated by the depth at which the particle is buried in the film.

Ultrafast singlet and triplet dynamics in microcrystalline pentacene films

The exciton dynamics of microcrystalline pentacene films is investigated by femtosecond pump-probe experiments. Measurements are performed with ultrashort laser pulses applied at normal incidence and at an angle of incidence of $65\ifmmode^\circ\else\textdegree\fi{}$ to disentangle singlet and triplet contributions by exploiting the different orientations of the molecular transition dipoles. The results indicate that the initial 70 fs fast relaxation step transforms the optically excited excitons in a reasonable mobile species with strongly reduced radiative transition strength. Fission into triplet excitons takes place on the picosecond time scale as a secondary, thermally activated process and with a small total yield of approximately 2%. Evidence is provided that the dominant species are singlet excitons with excimer character. To the subsequent dynamics contribute diffusion driven exciton-exciton annihilation and trapping of singlet and triplet excitons. Values for diffusion constants and trap densities are extracted by modeling the measurements with rate equations which include singlet and triplet dynamics.

Photofragment Spectroscopy and Predissociation Dynamics of Weakly Bound Molecules

Photofragment spectroscopy is combined with imaging techniques and time-resolved measurements of photoions and photoelectrons to explore the predissociation dynamics of weakly bound molecules. Recent experimental advances include measurements of pair-correlated distributions, in which energy disposal in one cofragment is correlated with a state-selected level of the other fragment, and femtosecond pump-probe experiments, in some cases with coincidence detection. An application in which coincident measurements are carried out in the molecular frame is also described. To illustrate these state-selective and time-resolved techniques, we review two recent applications: (a) the photoinitiated dissociation of the covalently bound NO dimer on the ground and excited electronic states and the role of state couplings and (b) the state-selected vibrational predissociation of hydrogen-bonded acetylene dimers with HCl (acid) and ammonia (base) and the importance of angular momentum constraints. We highlight the crucial role of theoretical models in interpreting results.

A versatile apparatus for time-resolved photoemission spectroscopy via femtosecond pump-probe experiments

A laser-based system for time-resolved photoemission spectroscopy using up to 6.2 eV photons is presented. The versatility of the laser source permits several combinations of pump and probe photon energies with pulse durations of 50-100 fs. The ultrahigh vacuum system, equipped with evaporators, a low energy electron diffraction system and an Auger spectrometer, grants the possibility to grow and characterize thin films in situ. The electron energy analyzer is a time-of-flight spectrometer with a multianode detector allowing high count rates. The performance of the whole experimental setup is investigated on Cu(100), Cu(111), and Ag(111) single crystals.

Optimized optical generation and detection of superlattice acoustic phonons

We present a device for the optimization of the folded acoustic phonon signal in femtosecond pump-probe experiments through the spatial decoupling of the generation and detection processes in two different GaAs/AlAs superlattices separated by a thick GaAs layer. A thickness gradient in one of them allows a fine tuning of the generation and detection spectral responses. In the optimal overlap condition, a clear enhancement of the signal is observed at a time delay that matches the travel time of acoustic phonons through the intermediate layer. This nanostructure provides a sensitive tool for studying terahertz phonon propagation over long distances.

Photoinduced electron-transfer within osmium(II) and ruthenium(II) bis-terpyridine donor acceptor dyads

The synthesis and photophysical characterization of two different series of electron donor-acceptor dyads containing Ru(II) and Os(II) bis-terpyridines (M(tpy)(2)(2+)) were prepared and studied in order to compare the oft-studied Ru(tpy)(2)(2+) chromophore with the less studied Os(tpy)(2)(2+) chromophore. The first series of dyads incorporates a benzoquinone (BQ) group as the electron acceptor, whereas the second contains a substituted pyromellitimide (PI) group as the electron acceptor. Steady-state emission experiments indicated efficient quenching of the 3MLCT emission of the electronically excited Os(II)-BQ complexes (7-8) compared to both model complexes (3-4) and the Os(II)-PI complex 10. Femtosecond pump-probe absorption experiments on 7-8 yielded ultrafast electron transfer rate constants (kET) of approximately 2.0 x 10(11) s(-1) (7) and 1.3 x 10(10) s(-1) (8) that were in good agreement with the low emission quantum yield results. Charge-recombination (kCR) in these complexes was also quite rapid, with rate constants of approximately 6.7 x 10(10) s(-1) (7) and 1.2 x 10(10) s(-1) (8). The analogous Ru(II) complexes underwent charge separation with rate constants of 7.6 x 10(10) s(-1) (5) and approximately 2.3 x 10(10) s(-1) (6), while charge recombination in these complexes occurred with rate constants of approximately 2.1 x 10(10) s(-1) (5) and approximately 5.3 x 10(10) s(-1) (6). Electron transfer in the pyromellitimide-containing complexes occurred only for Os(II)-PI (10), which exhibited significantly slower electron transfer (approximately 4.3 x 10(6) s(-1)) and charge recombination (approximately 7.7 x 10(6) s(-1)) rate constants. The nearly thermoneutral free energy of electron transfer and short excited state lifetime in the case of Ru(II)-PI (9) presumably prevents electron transfer in this compound.

Thymine relaxation after UV irradiation: the role of tautomerization and πσ* states

Ab initio calculations and femtosecond pump-probe ionization experiments were carried out to identify excited state relaxation processes in isolated thymine monomer and small thymine-water clusters. Three transient species with life times of < or =100 fs, 7 ps and >1 ns were observed in the experiments on gas phase thymine. The longer-lived transients were weak or absent in thymine-water clusters. Available theoretical results on thymine agree with the assignment of low-lying pi-pi* and n-pi* excited states to the femtosecond and picosecond transients but the assignment of the third transient remains opaque. Our theoretical results seem to exclude the possibility of ground or excited state tautomerization as well as the involvement of states with pi-sigma* character. Remaining explanations for the observed transients are: very fast intersystem crossing to the triplet manifold or the observation of transient signals from local minima on the potential energy surfaces.

Ultrafast internal conversion pathway and mechanism in 2-(2′-hydroxyphenyl)benzothiazole: a case study for excited-state intramolecular proton transfer systems

We study the ultrafast electronic relaxation of the proton transfer compound 2-(2'-hydroxyphenyl)benzothiazole (HBT) in a joint approach of femtosecond pump-probe experiments and dynamics simulations. The measurements show a lifetime of 2.6 ps for the isolated molecule in the gas phase in contrast to approximately 100 ps for cyclohexane solution. This unexpected decrease by a factor of 40 for the gas phase is explained by ultrafast internal conversion to the ground state promoted by an inter-ring torsional mode. The quantum chemical calculations based on multireference configuration interaction clearly demonstrate that a S(0)/S(1) conical intersection at a 90 degrees twisted structure exists and is responsible for the ultrafast decay. The reaction path leading from the keto form of HBT to this intersection is practically barrierless on the S(1) surface. The on-the-fly dynamics simulations using time-dependent density functional theory show that after electronic excitation to the S(1) state and after fast excited-state proton transfer (30-50 fs), HBT reaches the region of the S(1)/S(0) crossing within about 500 fs, which will lead to the observed 2.6 ps deactivation to the ground state. After the internal conversion, HBT branches in two populations, one that rapidly closes the proton transfer cycle and another (trans-keto) that takes approximately 100 ps for that step.

Photodissociation Dynamics of Nitromethane at 226 and 271 nm at Both Nanosecond and Femtosecond Time Scales

Photodissociation of nitromethane has been investigated for decades both theoretically and experimentally; however, as a whole picture, the dissociation dynamics for nitromethane are still not clear, although many different mechanisms have been proposed. To make a complete interpretation of these different mechanisms, photolysis of nitromethane at 226 and 271 nm under both collisional and collisionless conditions is investigated at nanosecond and femtosecond time scales. These two laser wavelengths correspond to the pi* <-- pi and pi* <-- n excitations of nitromethane, respectively. In nanosecond 226 nm (pi* <-- pi) photolysis experiments, CH(3) and NO radicals are observed as major products employing resonance enhanced multiphoton ionization techniques and time-of-flight mass spectrometry. Additionally, OH and CH(3)O radicals are weakly observed as dissociation products employing laser induced fluorescence spectroscopy; the CH(3)O product is only observed under collisional conditions. In femtosecond 226 nm experiments, CH(3), NO(2), and NO products are observed. These results confirm that rupture of C-N bond should be the main primary process for the photolysis of nitromethane after the pi* <-- pi excitation at 226 nm, and the NO(2) molecule should be the precursor of the observed NO product. Formation of the CH(3)O radical after the recombination of CH(3) and NO(2) species under collisional conditions rules out a nitro-nitrite isomerization mechanism for the generation of CH(3)O and NO from pi pi* CH(3)NO(2). The OH radical formation for pi pi* CH(3)NO(2) should be a minor dissociation channel because of the weak OH signal in both nanosecond and femtosecond (nonobservable) experiments. Single color femtosecond pump-probe experiments at 226 nm are also employed to monitor the dynamics of the dissociation of nitromethane after the pi* <-- pi excitation. Because of the ultrafast dynamics of product formation at 226 nm, the pump-probe transients for the three dissociation products are measured as an autocorrelation of the laser pulse, indicating the dissociation of nitromethane in the pi pi* excited state is faster than the laser pulse duration (180 fs). In nanosecond 271 nm (pi* <-- n) photolysis experiments, pump-probe experiments are performed to detect potential dissociation products, such as CH(3), NO(2), CH(3)O, and OH; however, none of them is observed. In femtosecond 271 nm laser experiments, the nitromethane parent ion is observed with major intensity, together with CH(3), NO(2), and NO fragment ions with only minor intensities. Pump-probe transients for both nitromethane parent and fragment ions at 271 nm excitation and 406.5 nm ionization display a fast exponential decay with a constant time of 36 fs, which we suggest to be the lifetime of the excited n pi* state of nitromethane. Combined with the 271 nm nanosecond pump-probe experiments, in which none of the CH(3), NO(2), CH(3)O, or OH fragment is observed, we suggest that all the fragment ions generated in 271 nm femtosecond laser experiments are derived from the parent ion, and dissociation of nitromethane from the n pi* excited electronic state does not occur in a supersonic molecular beam under collisionless conditions.

Direct mapping of recoil in the ion-pair dissociation of molecular oxygen by a femtosecond depletion method

Time-resolved dynamics of the photodissociation of molecular oxygen, O(2), via the (3)Sigma(u) (-) ion-pair state have been studied with femtosecond time resolution using a pump-probe scheme in combination with velocity map imaging of the resulting O(+) and O(-) ions. The fourth harmonic of a femtosecond titanium-sapphire (Ti:sapphire) laser (lambda approximately 205 nm) was found to cause three-photon pumping of O(2) to a level at 18.1 eV. The parallel character of the observed O(+) and O(-) images allowed us to conclude that dissociation takes place on the (3)Sigma(u) (-) ion-pair state. The 815 nm fundamental of the Ti:sapphire laser used as probe was found to cause two-photon electron photodetachment starting from the O(2) ion-pair state, giving rise to (O((3)P)+O(+)((4)S)) products. This was revealed by the observed depletion of the yield of the O(-) anion and the appearance of a new O(+) cation signal with a kinetic energy E(transl)(O(+)) dependent on the time delay between the pump and probe lasers. This time-delay dependence of the dissociation dynamics on the ion-pair state has also been simulated, and the experimental and simulated results coincide very well over the experimental delay-time interval from about 130 fs to 20 ps where two- or one-photon photodetachment takes place, corresponding to a change in the R(O(+),O(-)) interatomic distance from 12 to about 900 A. This is one of the first implementations of a depletion scheme in femtosecond pump-probe experiments which could prove to be quite versatile and applicable to many femtosecond time-scale experiments.

Molecular dissociation and nascent product state distributions detected with atomic wavepacket interferometry and parametric four-wave mixing: Rb2 predissociation observed by quantum beating in Rb at 18.2 THz

Dissociation of a diatomic molecule and the excited-state distribution of the nascent atomic fragments can be detected and characterized by atomic wavepacket interferometry and a coherent nonlinear optical process, such as parametric four-wave mixing (PFWM), in ultrafast pump–probe experiments. Underlying these experiments is a reliance on atom–atom interaction to alter the properties of an atomic wavepacket which, in turn, impacts the phase and amplitude of a coherent optical signal. Specifically, quantum beating in the atomic species provides a sensitive, in situ probe of molecular dissociation by detecting approaching dissociation fragments through long-range dipole–dipole interaction. The resulting influence of this interaction on the amplitude and phase of the quantum beating is observed in temporal or Fourier domains by probing the wavepacket by interferometry and PFWM with 100–150 fs laser pulses. The wavepacket thus serves as a detector of molecular dissociation fragments and the dynamics of atom–atom interactions are converted into the macroscopic domain by the PFWM signal and idler waves. Femtosecond pump–probe experiments are described in which the predissociation of electronically excited Rb2 states in the ∼24 000–28 000 cm−1 interval, and the distribution of nascent atomic fragments into Rb excited states (7s, 5d, 6s, 4d and 5p) spanning an energy range >1.25 eV, have been observed in Rb vapour with atomic number densities of ∼6 × 1013–3 × 1017 cm−3. Quantum beating at 18.2 THz (corresponding to the Rb 7s–5dJ (J = 5/2) energy defect of ∼608 cm−1) is superimposed onto the axially phase matched PFWM signal wave generated at λS ∼ 420 nm (Rb 62PJ → 52S1/2 transitions) and recovered by Fourier analysis of the signal wave intensity as the pump–probe time delay (Δt) is scanned. The dominant exit channels for Rb2 predissociation are found to be sensitive to the interval of internuclear separation R in which the molecular wavepacket, produced by the pump pulse through two-photon association of Rb–Rb collision pairs, is localized. Generating Rb2 wavepackets on 3Λu (Λ = Σ, Δ) potential surfaces at large R (∼7–9 Å) favours the Rb 7s and 5d dissociation channels. In contrast, producing dimer wavepackets localized in the R ∼ 3–4 Å region suppresses Rb (7s, 5d) generation and favours the production of Rb atomic fragments (primarily 5p) with less internal energy, but maximum velocities of ∼15–20 Å ps−1. Laser excitation spectroscopy on the nanosecond time scale suggests that the (3)3Δu and (7)3Σ+u states of Rb2 (correlated with Rd (5d) + Rb (6s) in the separated atom limit), and possibly a 3Σ+u state derived from the Rb (7p) + Rb (5s) asymptote, are populated by two-photon absorption of Rb–Rb ground-state collision pairs and predissociation of these levels provides the excited atomic fragments subsequently detected by atomic wavepackets. The data presented here demonstrate the observation of the molecular dissociation transient and the determination of the nascent statistical distribution of atomic product states in a manner that is unencumbered by radiative lifetime or collisional effects. A wavepacket, in tandem with the dipole–dipole interaction and a coherent nonlinear optical process, provides a new avenue for pursuing atom–atom and atom–molecular interactions over a broad range in inter-particle separations.

Light-induced collapse of metastable magnesium complexes formed in helium nanodroplets

Doping helium nanodroplets with more than a single Mg atom leads to a shift of the atomic absorption from $279\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}282\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. This behavior is stable against a change of the chosen droplet size. There is evidence that the interplay of the weak long-range potential between individual Mg atoms and the interaction with the surrounding quantum fluid leads to the formation of metastable very weakly bound complexes high above the global energy minimum. They resemble a bubble foam, where the individual Mg atoms are separated by a helium layer. The formation of compact Mg clusters is hindered by density variations within the superfluid solvent. The distance between the Mg atoms is substantially larger compared to the ground state. We estimate a value of roughly $10\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and the barrier, preventing the formation of compact Mg clusters to only a few kelvin. Upon laser excitation the metastable complexes collapse on a time scale of $20\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ as measured with a femtosecond pump-probe experiment.

Ultrafast Vibrational Dynamics of Adenine-Thymine Base Pairs in DNA Oligomers

N-H stretching excitations of DNA oligomers containing 23 alternating adenine-thymine base pairs are studied in femtosecond two-color pump-probe experiments. For a DNA film in a zero relative humidity atmosphere, transient vibrational spectra and their time evolution up to 10 ps demonstrate negligible spectral diffusion and allow for discerning different N-H stretching bands and the O-H stretching absorption of residual water molecules. Lifetimes on the order of 0.5 ps are found for both N-H and O-H stretching modes. The time-dependent pump-probe anisotropies of the different N-H excitations point to a pronounced coupling among them, whereas the O-H stretching anisotropy remains essentially constant.

Influence of self-diffraction effect on femtosecond pump-probe optical Kerr measurements

We investigated influence of the self-diffraction effect on optical Kerr signals of Bi2O3-B2O3-SiO2 glass (BI glass) and CS2 in femtosecond non-collinear pump-probe optical Kerr experiments. By measuring the dependence of the Kerr signals on the pump power and on the polarization angle between pump and probe beams, we found that the optical Kerr signals of CS2 consisted of two components: a fast response and a slow response,which were attributed to the self-diffraction effect and the photoinduced birefringence effect, respectively. The contribution of the self-diffraction effect to the Kerr signals increased with increase of the pump power. For the BI glass, no evident influence of self-diffraction on the Kerr signals was observed.

A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH3I

The real time photodissociation dynamics of CH(3)I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH(3) fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH(3)(nu=0) and I((2)P(32)) and I(*)((2)P(12)) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I(*) channels when the CH(3) fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.