Peter Vöhringer

Optical dephasing on femtosecond time scales: Direct measurement and calculation from solvent spectral densities

The connection between dephasing of optical coherence and the measured spectral density of the pure solvent is made through measurements and calculations of photon echo signals. 2‐pulse photon echo measurements of a cyanine dye in polar solvents are presented. Signals are recorded for both phase matched directions enabling accurate determination of the echo signal time shift. Echo signals are calculated by two approaches that employ the response function description of nonlinear spectroscopy; (i) a single Brownian oscillator line shape model, and (ii) the line shape obtained using the solvent spectral density. The strongly overdamped Brownian oscillator model incorporates only a single adjustable parameter while the experimental data present two fitting constraints. The second model incorporates the measured solvent spectral density. Both give very good agreement with the experimental results. The significance of the second method lies in this being a new approach to calculate nonlinear spectroscopic sign...

Time-gated photon echo spectroscopy in liquids

Abstract Optical dephasing of a cyanine dye chromophore in solution is investigated by time-gated photon echo spectroscopy in a three-pulse scattering geometry. The third-order polarization response is gated by up-conversion with a reference pulse in a nonlinear optical crystal. Time gating establishes two controllable periods of system evolution in an optical coherence. The measurements are compared with numerical simulations employing the measured OKE spectrum of solvent (butanol, DMSO) fluctuations. The time-shift of the time-gated echo signals for several gating forming pulse time delays agrees with simulation while the narrower simulated widths suggest a frequency-dependent solute-solvent interaction and the consideration of chromophore vibronic transitions.

Vibrational relaxation and geminate recombination in the femtosecond-photodissociation of triiodide in solution

The dynamics of product vibrational deactivation and subsequent geminate recombination of diiodide ions with atomic iodine following 400‐nm photolysis of triiodide in ethanol solution has been studied using femtosecond transient absorption spectroscopy. The excess vibrational energy of the diatomic product was found to decay on two distinct time scales. An ultrafast subpicosecond component, which accounts for the dissipation of most of the energy that is initially deposited into fragment vibrations, is followed by thermalization near the bottom of the I−2 potential on a time scale of several picoseconds. The former process is associated with recoil of the fragments in the exit channel of the potential energy surface relevant to bond breakage whereas the latter process represents relaxation in the asymptotic limit where interaction between the atom–diatom fragments becomes negligible. Transient product vibrational distributions are determined for delay times larger than the dephasing time of nuclear cohere...

The solvent spectral density and vibrational multimode approach to optical dephasing: Two-pulse photon echo response

A rigorous theoretical connection between the polarizability spectral density obtained from optical Kerr effect (OKE) measurement with the correlation function describing solvent‐induced optical dephasing detected in photon echo measurements is given. The experimentally obtained spectral density has a more reasonable physical basis than model correlation function descriptions of solvent fluctuations. The experimental OKE spectrum is demonstrated to provide a natural description of solvent motions that modulate the chromophore electronic states in the case of weak induced‐dipolar interactions. The chromophore optically active vibrational modes are obtained from pump‐probe spectra and are employed in the calculation of echo signals. It is found that the fast decays of the two‐pulse echo signals result from both solvent and solute intramolecular motions while the echo peak shifts are dominated by the solvent intermolecular modes.

Pressure dependence of solvent-induced barrier shifts in the photoisomerization of trans-stilbene

Abstract The photoisomerization of trans-stilbene is liquid n -hexane solution was studied over wide pressure and temperature ranges. Measurements of the pressure and temperature dependence of the rate, as in earlier studies from our laboratory, allow for the separation of specific solute—solvent interactions from purely frictional effects in the liquid phase reaction dynamics. The viscosity dependences along isotherms of the photoisomerization rate reveal strong changes of the activation energy with solvent density. From the analysis of the isotherms we conclude that specific solute—solvent interactions lead to a substantial lowering of the reaction barrier in the S 1 state with increasing density. As observed earlier ethane as a solvent, also in n -hexane manifestations of the multidimensionality of the barrier crossing process are found. The viscosity as well as the temperature dependence of the isomerization rate coefficient over the entire range investigated can surprisingly well be represented by a combination of standard unimolecular rate theory with Kramers frictional model.

Femtosecond photodissociation of triiodide in solution: Excitation energy dependence and transition state dynamics.

Abstract Femtosecond transient absorption measurements are reported for the photodissociation of triiodide in ethanol solution using 266 nm excitation light. The quantum yield for diiodide formation at this pump wavelength has found to be 0.8, significantly lower than for excitation with 400 nm. Furthermore, the data reveal distinct ultrafast dynamics on a time scale below 300 fs which are associated with the initial evolution of the wavepacket along the symmetric stretching coordinate. Real-time observation of the wavepacket motion at these very early stages of the reaction is made possible through wavelength selective probing, thereby defining detection windows along the nominal reaction coordinate. The results obtained with 266 nm excitation are supplemented by resonant impulsive stimulated scattering experiments. Detailed comparisons with quantum dynamics simulations are intended to provide insight into the time scale of product formation and the nature of the potential energy surface that facilitate the optical probing process.

Femtosecond pump-probe study of molecular vibronic structures and dynamics of a cyanine dye in solution

Time-resolved pump–probe spectra of 1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′indotricarbocyanine (HDITC), a cyanine dye, in ethylene glycol are obtained using 11 fs and 90 fs duration pulses and analyzed in order to study its potential energy surfaces and vibrational dynamics. Ten oscillatory frequencies ranging from 30 cm−1 to 1400 cm−1 are observed in the 11 fs duration wavelength-resolved pump–probe measurements. They are assigned as fundamental vibrational frequencies of HDITC. The relative displacements of the equilibrium position between electronic excited and ground states along the resolved ten vibrational modes are determined through the wavelength dependence of the oscillatory amplitude. After considering the contributions of the ten vibrational modes, it is found that most of the Stokes shift and the early fast decays of the pump–probe signals are due to relaxation along the low frequency overdamped modes of the chromophore. The overdamped modes are characterized by the 90 fs pump–probe ...

FEMTOSECOND DYNAMICS OF INTRAMOLECULAR CHARGE TRANSFER IN 4-DIMETHYLAMINO-4'-CYANOSTILBENE IN POLAR SOLVENTS

Abstract The femtosecond dynamics of 4-(N,N-dimethylamino)-4′-cyanostilbene (DCS) in acetonitrile and methanol solvent was studied by fluorescence upconversion and pump-probe absorption spectroscopy. The spectral evolution observed provides strong evidence for a fast internal charge-transfer (CT) process dominating the dynamics in the first few picoseconds, while Stokes shift dynamics seem to play only a minor role. The initially excited state seems to have already a large amount of CT character, in line with observations in jet-cooled DCS-solvent clusters. The non-exponential CT-state formation contains components significantly faster than longitudinal dielectric relaxation rates in the solvents studied, indicating a possible contribution by high-frequency vibrational modes.

OD stretch vibrational relaxation of HOD in liquid to supercritical H2O

The population relaxation of the OD stretching vibration of HOD diluted in H(2)O is studied by time-resolved infrared pump-probe spectroscopy for temperatures between 278 and 663 K in the density range 0.28<or=rho<or=1.01 g/cm(3). Transient spectra recorded after exciting the v=0-->1 OD stretching transition at low temperatures show a delay between excited state decay and formation of the thermalized spectrum pointing to an intermediately populated state. Above 400 K, the rates of excited state decay and ground state recovery become equivalent and the intermediate state is not detectable anymore. Over the entire thermodynamic range, the derived OD stretch relaxation rate constant k(r) depends linearly on the static dielectric constant epsilon of water, indicating a correlation of k(r) with the average hydrogen bond connectivity of HOD within the H(2)O network. However, in contrast to the OH stretch relaxation rate constant of the complementary system of HOD in D(2)O, the low density data of k(r)(epsilon) extrapolate to a nonzero intercept for epsilon-->1. Our analysis suggests that at ambient conditions the OD excited state is mainly depopulated by a direct v=1-->0 transition, avoiding the excited v=1 HOD bending state. Therefore, at room temperature the detected intermediate is assigned to a nonthermalized state with respect to nuclear degrees of freedom of the solvent molecules, and subsequent formation of the final product spectrum is related to a rearrangement of the hydrogen bond network. Passing over to the gas phase the excited OD stretch state shifts into close resonance with the HOD bend overtone, thereby opening up an additional relaxation channel.

Relating linear vibrational spectroscopy to condensed-phase hydrogen-bonded structures: Liquid-to-supercritical water

The pressure and temperature-dependent linear absorption spectrum of partially deuterated water HOD dissolved in heavy water D(2)O was measured in the OH-stretching spectral region. The temperature was varied in the interval of 298 K</=T</=700 K while the density was changed within the range of 12 moll</=rho</=58 moll corresponding to the liquid and the supercritical phases of the fluid solution. The spectra were analyzed in terms of the temperature and density dependent frequency of maximal absorbance nu(max)(T,rho) and their full widths at half maximum Deltanu(T,rho). In parallel, molecular dynamics simulations of the fluid solution were carried out to obtain the average nearest neighbor O-O distance r(OO) ((1))(T,rho) and its dispersion Deltar(OO) ((1))(T,rho) at any state point (T,rho) for which an absorption spectrum was recorded. A correlation is presented between the experimental spectroscopic quantities nu(max)(T,rho) and Deltanu(T,rho) on the one hand and the local structural quantities r(OO) ((1))(T,rho) and Deltar(OO) ((1))(T,rho) on the other. This intuitive correlation can be used as a critical test for future perturbational simulations of the OH-stretching frequency shifts with hydrogen-bond geometry. Finally, a connection is made to the average hydrogen-bond connectivity in the fluid via the temperature and density dependent dielectric constant of water.