V. M. Farztdinov

Femtosecond relaxation of photoexcited para-nitroaniline: solvation, charge transfer, internal conversion and cooling

Abstract The ultrafast relaxation of p -nitroaniline (PNA) in water and acetonitrile is studied experimentally and theoretically. Transient absorption spectra are measured by the pump–supercontinuum probe technique (PSCP) after 50 fs excitation at 400 nm. The relaxation includes several stages with distinct time scales: solvation, intramolecular charge transfer (CT), internal conversion and cooling. The spectral evolution before 100 fs reflects mainly solvation with dynamic Stokes shift of 3500 cm −1 in acetonitrile and 4000 cm −1 in water. CT and internal conversion are governed by twisting of the –NO 2 group and proceed in water with 120 and 250 fs, respectively. A hot ground state upon internal conversion is characterized by an initial temperature of 1400 K. The subsequent solute–solvent energy transfer is characterized by exponential behavior between 1 and 3 ps and by a nonexponential decay at longer delays, the solute cooling time lies in the range 0.85–1.3 ps.

Femtosecond fluorescence spectroscopy by upconversion with tilted gate pulses

Time-resolved emission bands can be observed by simultaneous sum-frequency mixing with femtosecond near-infrared gate pulses over a wide frequency range. Using tilted gate pulses in a noncollinear geometry we achieve 80 fs time resolution and background-free measurement of upconverted spectra. All components were optimized resulting in a spectrometer for routine applications. As example we report the initial fluorescence from Coumarin 153 in acetonitrile after excitation at 400 nm. The fluorescence is modulated by nuclear coherence in the excited state which affects band width and asymmetry more than the energy gap itself.

Coumarin 153 in the gas phase: optical spectra and quantum chemical calculations

We report the absorption spectrum of Coumarin 153 in the gas phase at 383 K and its fluorescence spectra in a supersonic jet. At excess vibrational energy above 200 cm-1 in the first excited state S1, intramolecular vibrational redistribution leads to emission from interconverting conformers. For the hot isolated molecules, the emission spectrum is dominated by active modes of 350, 810, and 1150 cm-1 and the distributions of oscillator strength for absorption and emission cross at 25420±40 cm-1. This value should be used in solvation studies for the electronic S1 energy of the bare molecule. The oscillator distributions change with temperature indicating a dependence of Franck-Condon factors for high-frequency modes on a low-frequency coordinate. Calculations of electronic structure with density-functional theory show that the second excited state is well separated from the first. It is proposed that spectral changes observed during nonequilibrium polar solvation are caused by intramolecular reorganisation of the julolidine group in the S1 state on the 200 fs time scale.

Frequency and time-domain analysis of excited-state intramolecular proton transfer. Double-proton transfer in 2,5-bis(2-benzoxazolyl)-hydroquinone?

Abstract 2,5-bis(2-benzoxazolyl)-hydroquinone has two reactive sites for excited-state intramolecular proton transfer (ESIPT) AH…B→A…HB. To carry out a frequency and time-domain analysis of the PT the reactive system is modeled in terms of diabatic educt and product potentials for each proton plus a single heavy atom coordinate per proton. The latter are assumed to modulate the proton motion which is treated in the adiabatic approximation. By dissipative propagation of this four-mode system, the structured absorption spectrum of the isolated molecule is simulated. Comparison with the observed vibronic spectra suggests the existence of a double-proton transferred state in the isolated molecule. This is confirmed by semiempirical quantum-chemical calculations.

Solvent dependence of structure and electronic properties in the ground and first excited singlet state of 4-dimethylamino-4′-nitrostilbene (DANS) – semiempirical calculations

Abstract Ground and excited state properties of trans -4-dimethylamino-4 ′ -nitrostilbene (DANS) in different solvents are calculated with the semiempirical SAM1 Hamiltonian. The solvent is treated as a continuum with the conductor-like screening model. Singlet excited-state conformations at various stages of relaxation, their optical transitions, and triplet states for these geometries are reported for the first time. Relaxation pathways are suggested which may be distinguished by characteristic optical spectra and which explain why fluorescence, intersystem crossing, and internal conversion strongly depend on the solvent.

Femtosecond dynamics of semiconductor microcavity polaritons

Dynamics of semiconductor microcavity modes in ZnS-Ni structure was investigated by femtosecond pump-supercontinuum probe spectroscopy in wide spectral region 1.6 - 3.2 eV. The change of reflectivity of the ZnS thin films (of 0.29 micrometer, 1.08 micrometer and 1.17 micrometer) on thick Ni film on quartz substrate was monitored. Two different pumping photon energies

Ultrafast optical response of IR‐treated polyacrylonitrile films

HBAR(omega) pul equals 2.75 eV and

Femtosecond spectroscopy and new possibilities for a correlated electron system study

HBAR(omega) pu2 equals 5.5 eV were used for the excitation of the microcavity and produced different photoresponse spectra. The first pumping energy (2.75 eV) is lower than the energy gap of ZnS (3.7 eV) and the powerful laser pulse excites mainly electrons of metal (i.e. boundary of the microcavity) and of ZnS layer (by two-photon absorption). Nonequilibrium carriers of metal penetrate through Schottky electron barrier into the semiconductor. For the second pumping energy (5.5 eV) the pumping pulse is practically totally absorbed in thin surface ZnS layer of 40 nm in thickness and creates nonhomogeneous hot carriers distribution semiconductor. The differences in the processes of carrier excitation are responsible for the differences in the changes of dielectric function of ZnS and Ni and for the differences in the change of cavity modes.

Random electric fields and coherent phonon excitation in C60 films by femtosecond laser pulses

An ultrafast photoinduced optical response of infrared (heat) treated polyacrylonitrile films (PAN-IR) was studied by a femtosecond pump–supercontinuum probe technique in the energy region of 1.6 – 3.2u2009eV. Two types of films were used corresponding to 600°C (1st) and to 700°C (2nd) heating. The samples were excited by optical pulses with a duration of 50u2009fsec and energy of ħωpumpu2005=u20052.34u2009eV. The temporal evolution of the photoinduced response is characterized by two relaxation times τ1 and τ2. For the 1st film type, the spectral dependence of the ultrafast relaxation time shows nonmonotonous behavior, it decreases from τ1u2005≈u2005170u2009fsec to 70u2009fsec at 1.6u2005<u2005ħωprobeu2005<u20051.9u2009eV and increases from τ1u2005≈u200580u2009fsec to 140u2009fsec at 1.9u2005<u2005ħωprobeu2005<u20053.2u2009eV. The spectral regionxad1.6–1.95u2009eV corresponds to photoinduced darkening while the region 1.95–3.2u2009eV corresponds to photoinduced bleaching. For the 2nd film type, only photoinduced bleaching is observed, and the ultrafast relaxation time decreases monotonously from τ1u2005≈u2005200u2009fsec to τ1u2005≈u200570u2009fsec. The second relaxation time τ2 is of ∼1u2009psec for both types of films.

Direct study of nonequilibrium carriers near Fermi level of Au film by optical reflection and transmission in femtosecond scale

The new method of investigation of Fermi surface and Fermi liquid (FL) versus non-Fermi liquid (NFL) behavior in strongly correlated electron systems by femtosecond laser spectroscopy is discussed. The method consists in the study of spectral dependence of nonequilibrium charge carriers relaxation time by femtosecond pump-supercontinuum probe technique. The usefulness of this method is demonstrated on weakly correlated electron systems in Au and Cu and strongly correlated electron system in high Tc oxide-superconductor YBa2Cu3O7-(delta ) by studying the temporary changes of optical density of thin metal films in wide spectral region of probing