Femtosecond Absorption Spectroscopy Research Articles

The dynamics of photodissociation reactions in solution

Elementary photoreactions were examined in solution by femtosecond absorption spectroscopy. The transition state properties, energy and coherence transfer into product molecules, various solvent-mediated relaxation mechanisms and influence of solvent diffusion in governing reaction rates are discussed. Two types of reaction are presented. The first involves the impulsive photodissociation of HgI 2 in a solution of ethanol and the second demonstrates how condensed phase hydrogen abstraction processes differ from those in the gas phase.

Short-Time Electron Transfer Processes in Ionic Aqueous Solution: Counterion and H/D Isotope Effects on Electron−Atom Pairs Relaxation

Counterion and H/D isotope substitution effects on early electron transfer steps in aqueous sodium chloride solution (X2O/NaCl = 55, X = H,D) have been investigated by femtosecond absorption spectroscopy of short-lived electron−chlorine atom pairs in the 18500−8000 cm-1 spectral range. The frequency dependence of the overall signal rise time and early signal decay (geminate recombination process) from 1.77 to 2.29 eV has been analyzed in the framework of inhomogeneous populations of hydrated electron for which the absorption bands overlap in the visible spectral region. By use of a kinetic model, two well-defined electron hydration channels have been discriminated. The spectral signature of a delayed hydration channel (e-hyd‘) is fully developed 4.5 ps after the initial energy deposition in the sample against 1.4 ps for the fastest electron hydration channel (e-hyd). Compared to the absorption band of the hydrated electron ground state (e-hyd), which peaks around 1.7 eV, the e-Hyd‘ population exhibits a spectral blue shift of about 0.15 eV. This electronic ground state is assigned to a polaron-like state, i.e., an electron localized in the vicinity of sodium ion: {e-···Na+}hyd. The time-resolved spectroscopic experiments demonstrate that this electron localization process can be triggered by short-range counterion effects on transient electron−chlorine atom pairs ({Cl:e-}pair:Na+). The existence of selective H/D isotope substitution effects on short-lived electronic configurations of aqueous Cl- (electron−Cl atom)pairs provides direct evidence that solvent cage dynamics around Cl- can differently interfere with multiple electron transfer channels. Contrary to infrared presolvated electron relaxation (p-like excited hydrated electron), early electron−chlorine atom pair relaxation (Cl:e- → Cl-) or 1D geminate recombination of fully hydrated electron with chlorine atom (Cl + e-hyd → Cl-) for which no significant isotope effect is observed, the electron transfer process involving the deactivation of {Cl:e-}pair:(Na+) population is drastically influenced by a change of intramolecular OH/OD vibrational mode frequency. In aqueous NaCl solution, the electron photodetachment from these transient electronic configurations [{Cl:e-}pair:(Na+)] → {Cl}, {e-···Na+}hyd] takes place with a characteristic time of 750 fs in H2O and 980 fs in D2O. The complete buildup of a polaron-like state (second electron hydration channel) is delayed by a factor of 1.6 (4.5 ps in H2O and 7.4 ps in D2O) when the energetic vibrational modes of water molecules (OH vs OD) are decreased by √2. The conjugate effects of very fast responses and slower cooperative motions of water molecules during short-range (electron−atom)pairs−Na+ couplings and a second electron hydration channel are discussed.

Dynamics of the formation of an electron bubble in liquid helium.

The dynamics of excess electron localization in a bubble (equilibrium radius 17 \AA{}) in liquid He is described in terms of the initial formation of an incipient bubble of radius 3.5\AA{}, followed by an adiabatic bubble expansion in the ground electronic state, which is described by a hydrodynamic model for the expansion of a spherical cavity in an incompressible liquid. The bubble expansion time at pressure $P=0$ in liquid $^{4}\mathrm{He}$ is ${\ensuremath{\tau}}_{b}=3.9$ ps; the expansion time exhibits a marked pressure dependence (e.g., ${\ensuremath{\tau}}_{b}=1.5$ ps at $P=16$ atm) and a small (3%) isotope effect in liquid $^{3}\mathrm{He}$. The interrogation of the electron bubble dynamics by femtosecond absorption spectroscopy is analyzed.

Convenient Method of Measuring the Chirp Structure of Femtosecond White-Light Continuum Pulses

We have developed a convenient method of measuring the temporal chirp structure of white-light continuum pulses used in femtosecond time-resolved absorption spectroscopy. This method, based on the nonresonant optical Kerr effect, is superior in many respects to the existing one that uses the sum or difference frequency generation by a nonlinear crystal.

Femtosecond absorption and emission spectroscopy of the DCM laser dye

The results of a femtosecond laser investigation of the photophysical and photochemical properties of the DCM styryl laser dye molecule are reported. Femtosecond pump-probe experiments (absorption/gain spectroscopy using a white light continuum and up-conversion fluorescence spectroscopy) show a red shift of the gain and emission spectra and a blue shift of the S 1 → S N absorption band only due to a fast reorganization of the solvent cage around the highly polar fluorescent first singlet excited state of DCM. This first complete study (spontaneous emission, gain and absorption) of DCM with a pump-probe intercorrelation function around 300 fs (FWHM) demonstrates that the hypothesis of a transition from a locally excited (LE) to a twisted intramolecular charge transfer (TICT) state is not valid for DCM.

Ultrafast energy transfer dynamics resolved in isolated spinach light-harvesting complex I and the LHC I-730 subpopulation

The energy migration in isolated bulk LHC I and its subpopulations LHC I-680 and LHC I-730 have been studied by fluorescence spectroscopy and femtosecond absorption spectroscopy. LHC I-680 and LHC I-730 display significant differences in both their steady-state and time-resolved fluorescence properties. The steady-state emission maximum is located at 680 nm for LHC I-680 and at 730 nm for LHC I-730. It was also found that different kinetic models were required to fit the time-resolved fluorescence data of the two subpopulations, two components of 374 ps and 3.9 ns were required for LHC I-680 and three components of 157 ps, 510 ps and 2.8 ns for LHC I-730, respectively. Using femtosecond absorption recovery spectroscopy we were able to resolve a fast 200–400 fs depolarization in bulk LHC I at 655, 665 and 670 nm. The results indicate a fastest hopping time between individual chlorophylls of 200–400 fs in bulk LHC I. In the subpopulation LHC I-730, on the other hand, both the isotropic absorption recovery decay and the depolarization process occurred with a 15 ps lifetime, while in the subpopulation LHC I-680 we could not resolve any ultrafast relaxation process. The 15 ps phase is assigned to a transfer of excitation energy to the pigment giving rise to far-red emission component F735 of PS I.

Femtosecond phase spectroscopy by use of frequency-domain interference

We present the principles, experimental procedures, applications, and theoretical analyses of femtosecond phase spectroscopy, which is complementary to femtosecond absorption spectroscopy. In femtosecond phase spectroscopy difference spectra of both phase and transmission are simultaneously measured with a frequency-domain interferometer, which is only slightly modified from the conventional pump–probe method. Femtosecond time-resolved dispersion relations for CdSxSe1−x-doped glass and CS2 are obtained with transform-limited pulses of 60-fs duration and 620-nm center wavelength. The results are theoretically analyzed and are well reproduced by numerical simulations. Although time-resolved data are not expected to satisfy the Kramers–Kronig (K–K) relations, the degree of discrepancy from the K–K relations is more substantial for CS2 than for CdSxSe1−x-doped glass. These results arise from the difference in the linear susceptibility and in the excited-population dynamics. The conditions for which the K–K relations are applicable to time-resolved spectra are obtained theoretically and verified experimentally. It is shown that induced amplitude and phase modulations of the probe pulses cause a deviation from the K–K relations.

Ultrafast spectroscopic studies of photoinduced electron transfer from semiconducting polymers to C60.

Using femtosecond time-resolved photoinduced absorption spectroscopy, we have studied the dynamics of photoinduced electron transfer from semiconducting polymers to ${\mathrm{C}}_{60}$. We find that with ${\mathrm{C}}_{60}$ in poly[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene)], poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), and poly(3-octylthiophene) (P3OT), ultrafast photoinduced electron transfer occurs from the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band of the semiconducting polymer to the lowest unoccupied molecular-orbital level of ${\mathrm{C}}_{60}$. Time-resolved measurements of the dichroic ratio place a 300-fs upper limit on the electron transfer time. The charge-separated state is rendered metastable due to the spatial separation of the electron and hole and to polaron formation associated with the electron on the ${\mathrm{C}}_{60}$ and the hole on the semiconducting polymer. Increasing the ${\mathrm{C}}_{60}$ concentration in P3OT increases the lifetime of the charge-separated state. The two p-phenylene vinylene derivatives also displayed metastable charge separation.

Ultrafast Optical Response in Quasi-One Dimensional Halogen-Bridged Mixed-Valence Complexes [Pt(en)2][PtBr2(en)2](ClO4)4and [Pd(en)2][PdCl2(en)2](ClO4)4

Abstract Ultrafast optical response in two quasi-one-dimensional halogen-bridged mixed-valence complexs [Pt(en)2][PtBr2(en)2](ClO4)4 (en=ethylenediamine) (abbreviated as PtBr) and [Pt(en)2][PtBr2(en)2](ClO4)4 (as PdCl) has been investigated by femtosecond absorption spectroscopy at room temperature by pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed in PtBr. Both consist of a fast-decay component due to STEs and a slow-decay component due probably to polaron pairs. The former decays exponentially with the time constant of 1.4 ± 0.2 ps. The latter decays as erf(t β) with β = -0.22 ± 0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model (β = -0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination. A pump-probe absorption spectrum of PdCl is obtained from the ...

Femtosecond Studies on Third-Order Optical Nonlinearities of Polyanilines

The ultrafast dynamics of third order nonlinearities of polyaniline (PAn) and polyaniline doped in polymethyl mathacrylate (PAn-PMMA) films are investigated using femtosecond absorption spectroscopy. An ultrafast initial exciton deformation time of about 100 fs is observed in PAn film. In PAn-PMMA film, the third-order nonlinearity results in an efficient two-photon absorption at about 2 eV.

Ultrafast Excitation Relaxation in Colloidal Particles of Chloroaluminum Phthalocyanine: One-Dimensional Exciton-Exciton Annihilation

The excited-state dynamics of crystalline ClAl phthalocyanine colloidal particles have been studied by means of picosecond and femtosecond absorption spectroscopy. In the measured relaxation dynamics, the initial excited-state evolution, taking place during the first picosecond after excitation, was distinguished from the overall exciton decay. Excitation transfer between different molecular species is suggested to be the most probable reason to the initial (I1 ps) excited-state dynamics. Exciton decay was found to be dominated by exciton-exciton annihilation with a t112 time dependence of the excited-state population, suggesting a one- dimensional exciton diffusion. A 0.2-1 ps intermolecular exciton hopping time was determined from the annihilation dynamics.

Electron Photodetachment from Halide Ions in Solution: Excited-State Dynamics in the Polarization Well

The electronic relaxation and photodetachment dynamics of an electron originating from an aqueous iodide ion have been studied using femtosecond absorption spectroscopy. The results indicate the importance of an upper excited charge transfer to solvent (CTTS) state, of odd parity, in the potential well of the solvated ion. The upper excited CTTS state, which is accessed by two-photon excitation, relaxes in <50 fs to the lowest excited CTTS, which in turn relaxes to the ground state of the I-in 80 fs. It is found that most of the solvated electrons are produced by a three-photon excitation to a photoionized state, rather than from the CTTS excitation. 25 refs., 4 figs.

Wave packet dynamics of the HgI2 photodissociation reaction in solution

Using femtosecond absorption spectroscopy, the photodissociation dynamics of HgI2 in solution were investigated. Upon photolysis, the HgI fragment is formed in a coherent superposition of its vibrational states. Solvent dependence of the wave packet motion is evidenced by the different results for ethanol and DMSO and between solution and gas phase.

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2][PtBr 2(en) 2](ClO 4) 4

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2][PtBr 2(en) 2](ClO 4) 4 (en=ethylenediamine) has been investigated by femtosecond absorption spectroscopy at room temperature using pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed. Both consist of a fast-decay component due to STEs and a slow-decay component due to polaron pairs. The former decays exponentially with the time constant of 1.4±0.1 ps. The latter decays as erf( t β) with β =−0.22±0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model ( β= −0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination.

Exciton transition energy and temperature dependence of ultrafast relaxation of self-trapped excitons in polydiacetylenes

Exciton transition energies and temperature dependence of ultrafast relaxation processes have been investigated in polydiacetylene (PDA-(12,8) Langmuir-Blodgett films with several exciton transition energies using femtosecond absorption spectroscopy. The relaxation from self-trapped exciton to the ground state in PDA with 1.88 eV exciton energy is faster than that relaxation in PDA with larger exciton energy. The decay kinetics in PDAs can be explained in terms of tunneling in the configuration space from the self-trapped exciton to the ground state.

Nonlinear optical susceptibilities of epitaxially grown polydiacetylene measured by femtosecond time-resolved spectroscopy.

Ultrafast nonlinear optical responses in polydiacetylene-3-butoxycarbonyl methylurethane prepared by epitaxial vacuum deposition on a KCl substrate are investigated by femtosecond absorption spectroscopy. Absorbance changes due to ``instantaneous'' processes which decay within the pulse duration of 100 fs, self-trapped exciton, and triplet exciton are resolved in time. The spectra of the corresponding nonlinear optical susceptibilities are obtained considering the pulse duration and response times. The ``instantaneous'' nonlinear optical response consists of several nonlinear effects, i.e., hole burning, coherent coupling between pump polarization and probe field, Raman gain, inverse Raman scattering, and induced-phase modulation.

Ultrafast relaxation in the fluorescent state of the laser dye DCM

Ultrafast relaxation in the fluorescent state of 4-(dicyanomethylene)-2-methyl-6-( p-dimethylaminostyryl)-4H-pyran (DCM) has been interrogated by time-resolved femtosecond absorption spectroscopy both in methanol and in ethylene glycol. Following a subpicosecond rise, each frequency-specific trace in ethylene glycol exhibits either a slower increase or a slower decrease in transmission; an additional long-term decay is seen in methanol solution. The data are generally consistent with recent solvation dynamics studies that employed different probe molecules, but the solvation time we measure for ethylene glycol is an order of magnitude shorter than previously reported.

Primary electron transfer kinetics in bacterial reaction centers with modified bacteriochlorophylls at the monomeric sites BA,B.

The primary electron transfer has been investigated by femtosecond time-resolved absorption spectroscopy in two chemically modified reaction centers (RC) of Rhodobacter sphaeroides, in which the monomeric bacteriochlorophylls BA and BB have both been exchanged by 13(2)-hydroxybacteriochlorophyll a or [3-vinyl]-13(2)-hydroxybacteriochlorophyll a. The kinetics of the primary electron transfer are not influenced by the 13(2)-hydroxy modification. In RCs containing [3-vinyl]-13(2)-hydroxybacteriochlorophyll a the primary rate is reduced by a factor of 10.

Ultrafast Response of Polymers with Large Optical Nonlinearity

Ultrafast optical response in polydiacetylenes and polythiophense was investigated by femtosecond time-resolved absorption spectroscopy. Several nonlinear optical processes, i.e. hole burning, Raman gain, and induced-frequency shift, were observed and the third-order nonlinear susceptibilities were determined. The ultrafast relaxation dynamics following after the photoexcitation was investigated and the formation and relaxation of self-trapped exciton was observed

Theory of femtosecond time-resolved infrared absorption spectroscopy with application to ligand dissociation in haemoglobin

In this paper we present a theory of femtosecond time-resolved infrared absorption spectroscopy utilizing the theory of nonstationary time-dependent absorption. We treat the pump-probe absorption with nonoverlapping pulses using the stochastic Liouville equation. In this case, we deal only with the linear susceptibility and obtain the generalized linear response theory due to the fact that the system is initially nonstationary because of the femtosecond pulse duration. The adiabatic approximation is introduced to derive the theoretical expressions for the time-resolved IR absorption. Theoretical results are applied to interpret the experimental fs time-resolved IR spectra reported by Hochstrasser and co-workers for the direct observations of ligand dynamics in haemoglobin.