Papers by Alexander Dobryakov
Photon Echo and Coherent Spectroscopy, 1997
ABSTRACT The excitation of semiconductor microcavity modes (ZnSe films on Cr and Cu surfaces) wer... more ABSTRACT The excitation of semiconductor microcavity modes (ZnSe films on Cr and Cu surfaces) were investigated by femtosecond pump- supercontinuum probe spectroscopy in wide spectral region 300 - 800 nm. Photoinduced red shift of cavity modes due to parametric pump of ZnSe film and film-metal interface and the generation of coherent phonon oscillations in ZnSe were observed.
The Journal of Chemical Physics, 2008
Molecular vibrational coherence from impulsive stimulated Raman (SR) scattering, as observed by b... more Molecular vibrational coherence from impulsive stimulated Raman (SR) scattering, as observed by broadband transient absorption spectroscopy, is treated within the well-known third-order perturbation formalism. Shaped femtosecond optical pulses are used for the pump and supercontinuum probe fields. Dephasing is assumed to be homogeneous in the Bloch approximation. A key step requires threefold time integration over response functions and electric fields. For well-separated pulses the triple integral can be solved analytically, resulting in lineshape functions. These allow to describe the SR signal through absorption/emission/dispersion profiles which are associated with the inherent contributions. A clear physical interpretation of the amplitude and phase of the oscillatory signal is thereby obtained, and a direct connection with the vibronic structure of the molecular system is provided. Calculations for model molecular systems illustrate the spectral dependence of the vibrational coherence seen, for example, with perylene in cyclohexane. The nonoscillatory and oscillatory parts of the transient absorption spectra are compared to each other. Observed mode beatings are explained.
The Journal of Chemical Physics, 2022
Resonance stimulated Raman signal and line shape are evaluated analytically under common electron... more Resonance stimulated Raman signal and line shape are evaluated analytically under common electronic/vibrational dephasing and exponential Raman/probe pulse, exp(−|t|/τ). Generally, the signal from a particular state includes contributions from higher and lower electronic states. Thus, with S0 → S1 actinic excitation, the Raman signal consists of 15 Feynman diagrams entering with different signs. The negative sign indicates vibrational coherences in S1 or higher Sn, whereas the positive sign reveals coherences in S0 or Sn via S1 → Sn → Sm (n < m) coupling. The signal complexity is in contrast to spontaneous Raman with its single diagram only. The results are applied to femtosecond stimulated Raman spectra of trans–trans, cis–trans (ct), and cis–cis (cc) 1,4-diphenyl-1,3-butadiene, the ct and cc being reported for the first time. Upon actinic excitation, the Stokes spectra show negative bands from S1 or Sn. When approaching higher resonances Sn → Sm, some Raman bands switch their s...
By studying the temporal behavior of the optical density of C60 thin film with the help of femtos... more By studying the temporal behavior of the optical density of C60 thin film with the help of femtosecond laser spectroscopy the coherent phonon oscillations in the frequency region 10 - 400 cm-1 (including with odd symmetry) in the wide spectral region of probing h(omega) pr equals 1.78 divided by 2.34 eV was detected. Spectral dependence of photoinduced response permits to observe selectively the relaxation of electrons in different bands. The possibility of reversible polymerization in excited state is discussed.
Journal of the American Chemical Society, Oct 25, 2017
Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization eff... more Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes is observed and calculated for planar ring-fluorinated stilbenes, trans-2,3,5,6,2',3',5',6'-octofluorostilbene (tF2356) and trans-2,3,4,5,6,2',3',4',5',6'-decafluorostilbene (tF23456). Due to the fluorination, Franck-Condon states S and S are dominated by the quasi-degenerate HOMO-1 → LUMO and HOMO-2 → LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S state. After optical excitation of tF2356, one observes an ultrafast (∼0.06 ps) evolution that reflects relaxation from initial nonpolar S to long-lived (1.3 ns in n-hexane and 3.4 ns in acetonitrile) polar S. The polarity of S is evidenced by a solvatochromic shift of its fluorescence band. The experimental results provide a sensitive test for quantum-chemical calculations. In particular, our calculations agree with the...
Physical Review B, 1997
The ultrafast relaxation of excitations in a C 60 thin film was probed in the energy range 1.6-3 ... more The ultrafast relaxation of excitations in a C 60 thin film was probed in the energy range 1.6-3 eV with femtosecond laser spectroscopy. The relaxation time has minima at 2 eV in the region of photoinduced darkening and 2.4 eV in the region of photoinduced bleaching. The shape of the optical density variation at zero time delay in the region of interband transitions at 2.3-3 eV is similar to the shape of the second derivative of the stationary absorption spectrum. We propose that this may indicate the creation of a random electric field in the sample during the absorption of the pump pulse. Coherent excitation of phonons in the 60-300-cm Ϫ1 frequency range were detected over a wide spectral probe range. An oscillation with a frequency of 118 cm Ϫ1 indicates that the nonequilibrium dimerization of C 60 molecules takes place following optical excitation. The full splitting of the H g ͑1͒ intramolecular oscillation mode is observed clearly, which demonstrates that a strong deformation of the molecules upon photon absorption takes place.
The Journal of Physical Chemistry Letters, 2016
Transient absorption (TA) measurements. Our setup with applications has been described elsewhere ... more Transient absorption (TA) measurements. Our setup with applications has been described elsewhere 1,2. It provides spectral coverage 275-690 nm with 0.1 ps instrumental response (fwhm) over the full probe range and timing precision of 0.02 ps. A solution of 1,1'dicyanostilbene in n-hexane (~ 0.1 mM) was flown through a sample cell of 0.4 mm internal thickness. Transient absorption spectra ΔA(λ, t) were recorded upon 351 nm excitation. 16 pump-probe scans were averaged to improve the signal-to-noise ratio. All TA spectra were recorded at the magic angle between pump-and probe polarizations. Femtosecond-stimulated Raman (FSR) measurements. The transient Raman setup was similar to that for TA 3,4. The narrow picosecond Raman-pump (0.1 µJ, 920 Hz) was tuned to λ R = 470 nm or λ R = 510 nm. The polychromator dispersion was adjusted to cover a 1000 cm-1 probe range. Stokes Raman signal was recorded by chopping the Raman-pump beam, with actinic excitation at λ ac = 340 nm. In this registration scheme, signals at negative pump-probe delays correspond to ground-state Raman contributions from both solute and solvent. For positive delays these contributions are eliminated by subtracting the (averaged) signal from negative delays. 64 pump-probe scans were averaged to improve the signal-to-noise ratio. All FSR spectra were recorded at the 0° angle between pump-and probe polarizations. The evolution of the transient absorption spectrum is discussed in the main text. In addition, the three kinetic traces shown in Fig. S1 help to understand the evolution of trans-1,1'dicyanostilbene in n-hexane upon excitation.
ChemPhysChem, 2015
Beta-Carotene in n-hexane was examined by femtosecond transient absorption and stimulated Raman s... more Beta-Carotene in n-hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals.Overlaid on the decay of S1 excited-state absorption, a picosecond process is found which is absent when the C9-methyl group is replaced by ethyl or isopropyl. It is attributed to reorganization on the S1 potential energy surface, involving dihedral angles between C6 and C9. In Raman studies, electronic states S2 or S1 were selected through resonance conditions. We observe a broad vibrational band at 1770 cm-1 in S2 already. With 200 fs it decays and transforms into the well-known S1 Raman line for an asymmetric C=C stretching mode. Low-frequency activity (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;800 cm-1) in S2 and S1 is also seen. A dependence of solvent lines on solute dynamics implies intermolecular coupling between beta-carotene and nearby n-hexane molecules.
SPIE Proceedings, 1998
The new method of investigation of Fermi surface and Fermi liquid behavior by femtosecond laser s... more The new method of investigation of Fermi surface and Fermi liquid behavior by femtosecond laser spectroscopy is considered. The method consists in study of spectral dependence of the nonequilibrium charge carriers relaxation time by pump-supercontinuum probe technique. Photoinduced optical properties and relaxation of nonequilibrium charge carriers in Au film in a femtosecond time scale are analyzed. It is shown that
The Journal of Physical Chemistry Letters, 2015
Raman scattering with stimulating femtosecond probe pulses (FSR) was used to observe vibrational ... more Raman scattering with stimulating femtosecond probe pulses (FSR) was used to observe vibrational activity of all-trans β-carotene in n-hexane. The short-lived excited electronic state S 2 was accessed in two ways: (i) by transient FSR after an actinic pulse to populate the S 2 state, exploiting resonance from an S x ← S 2 transition, and (ii) by FSR without actinic excitation, using S 2 ↔ S 0 resonance exclusively and narrow-band Raman/broad-band femtosecond probe pulses only. The two approaches have nonlinear optical susceptibilities χ (5) and χ (3) , respectively. Both methods show low-frequency bands of the S 2 state at 200, 400, and ∼600 cm −1 , which are reported for the first time. With (ii) the intensities of lowfrequency vibrational resonances in S 2 are larger compared to those in S 0 , implying strong anharmonicities/ mode mixing in the excited state. In principle, for short-lived electronic states, the χ (3) method should allow the best characterization of low-frequency modes.
ICONO 2001: Ultrafast Phenomena and Strong Laser Fields, 2002
ABSTRACT A new method for determination of the mobility edge in disordered semiconductors by femt... more ABSTRACT A new method for determination of the mobility edge in disordered semiconductors by femtosecond pump-supercontinuum probe spectroscopy is presented. The method is based on the determination of the spectral dependence of a stretched exponential relaxation in a wide spectral range of probing, h(omega) probe equals 1.6 - 3.2 eV. The method is demonstrated for porous silicon. It is shown that the relaxation parameters for porous silicon have essential spectral dependence. The spectral dependence of stretched exponential index (beta) ((omega) ) give unique information about existence and position of the mobility edge in disordered materials, and thus may be used as effective tool in manifestation of the transition from localized to delocalized relaxation regime on the femtosecond time scale.
Optics Communications, 1994
ABSTRACT We study the superconducting gap DELTA which is manifested in optical spectra for freque... more ABSTRACT We study the superconducting gap DELTA which is manifested in optical spectra for frequencies omega much greater than DELTA/hBAR; these frequencies correspond to the interband transitions. The peak in the difference spectrum of the superconducting and normal phases is predicted, with the peak width being connected to DELTA. The effect is more pronounced for transitions from the heavy-effective-mass band to the light-effective-mass band. Experimental manifestations of the predicted effect are discussed.
SPIE Proceedings, 1999
The new method of investigation of Fermi surface and Fermi liquid (FL) versus non-Fermi liquid (N... more 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
Chemical Physics
Page 1. 8 Observing molecular structure changes and dynamics in polar solutionAlexander L. Dobrya... more Page 1. 8 Observing molecular structure changes and dynamics in polar solutionAlexander L. Dobryakov1, Nikolaus P. Ernsting1, Wojciech Gawelda2, Christian Bressler2, Majed Chergui2 1 Department of Chemistry, Humboldt ...
The Journal of Physical Chemistry B, 2014
Photoinduced isomerization of 1,1′-bis-indanyliden (stiffstilbene) in solution was studied with b... more Photoinduced isomerization of 1,1′-bis-indanyliden (stiffstilbene) in solution was studied with broadband transient absorption and femtosecond Raman spectroscopies, and by quantum-chemical calculations. Trans-to-cis S 1 isomerization proceeds over a 600 and 400 cm −1 barrier in n-hexane and acetonitrile, respectively. The reaction develops on multiple time scales with fast (0.3−0.4 ps) viscosity-independent and slower (2−26 ps) viscosity-dependent components. In the course of intramolecular torsion (which should be the main reaction coordinate) some excited molecules pass through the perpendicular conformation P and reach the cis geometry, to be temporarily trapped there. Subsequently they relax back to P and further to the ground state S 0. The cis-to-trans isomerization reveals ultrafast (0.06 ps) oscillatory relaxation followed by 13 ps decay in nhexane and 2 ps decay in acetonitrile, corresponding to barriers of 800 and 400 cm −1 , respectively. Raman S 0 and S 1 spectra are reported and discussed. The perpendicular conformation P was not detected, possibly due to its low oscillator strength and short lifetime, or because of strong overlap with hot product spectra. XMCQDPT2 calculations locate a stationary S 1 point on the cis side and two perpendicular-pyramidalized stationary points, to be reached from the former over 300 and 680 cm −1 barrier. Implications for parent stilbene are discussed; in this case we also see evidence for the trans-to-cis adiabatic path, as in stiffstilbene. Very similar viscosity dependence for the two compounds supports the common isomerization pathway: torsion about the central double bond.
The Journal of Physical Chemistry A, 2008
Femtosecond dynamics of riboflavin, the parent chromophore of biological blue-light receptors, wa... more Femtosecond dynamics of riboflavin, the parent chromophore of biological blue-light receptors, was measured by broadband transient absorption and stationary optical spectroscopy in polar solution. Rich photochemistry is behind the small spectral changes observed: (i) loss of oscillator strength around time zero, (ii) sub-picosecond (ps) spectral relaxation of stimulated emission (SE), and (iii) coherent vibrational motion along a′ (in-) and a″ (out-of-plane) modes. Loss of oscillator strength is deduced from the differences in the time-zero spectra obtained in water and DMSO, with stationary spectroscopy and fluorescence decay measurements providing additional support. The spectral difference develops faster than the time resolution (20 fs) and is explained by formation of a superposition state between the optically active (1 ππ*) S 1 and closely lying dark (1 nπ*) states via vibronic coupling. Subsequent spectral relaxation involves decay of weak SE in the blue, 490 nm, together with rise and red shift of SE at 550 nm. The process is controlled by solvation (characteristic times 0.6 and 0.8 ps in water and DMSO, respectively). Coherent oscillations for a′ and a″ modes show up in different regions of the SE band. a″ modes emerge in the blue edge of the SE and dephase faster than solvation. In turn, a′ oscillations are found in the SE maximum and dephase on the solvation timescale. The spectral distribution of coherent oscillations according to mode symmetry is used to assign the blue edge of the SE band to a 1 nπ*-like state (A″), whereas the optically active 1 ππ* (A′) state emits around the SE maximum. The following model comes out: optical excitation occurs to the Franck-Condon ππ* state, a ππ*-nπ* superposition state is formed on an ultrafast timescale, vibrational coherence is transferred from a′ to a″ modes by ππ*-nπ* vibronic coupling, and subsequent solvation dynamics alters the ππ*/nπ* population ratio.
Polymers for Advanced Technologies, 1998
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Physical Chemistry Chemical Physics, 2011
The dual photochemistry of anthracene-9,10-endoperoxide (APO) was investigated in a fs UV pump-su... more The dual photochemistry of anthracene-9,10-endoperoxide (APO) was investigated in a fs UV pump-supercontinuum probe experiment, along with anthracene (AC) and anthraquinone (AQ) for comparison. Excitation of APO at 282 nm leads to 100% product formation by two competing photoreaction channels. Cycloreversion generates with a ∼25% quantum yield (QY) (1)O(2) and AC vibrationally excited in the singlet electronic ground state (hot AC). 1-2% of the AC is generated in the lowest triplet state, but no AC is generated in electronically excited singlet states. Generation and cooling of hot AC are modeled using solution phase and broadened gas-phase AC absorption spectra at various temperatures. Results indicate ultrafast generation of hot AC within 3 ps, much faster than reported before for derivatives of anthracene endoperoxide, and subsequent cooling with an 18 ps time constant. The homolytic O-O cleavage pathway generates a biradical, which converts into electronically excited diepoxide (DE). Our data indicate a 1.5 ps time constant that we tentatively assign to the biradical decay and DE formation. Cooling of DE in this electronically excited state takes place with a ∼21 ps time constant. Excitation of AQ at 266 nm is followed by an ultrafast population of the T(1)(nπ*) triplet state of AQ with a time constant of (160 ± 60) fs.
The Journal of Chemical Physics, 2003
Electronic and vibrational coherence effects in broadband transient absorption spectroscopy with ... more Electronic and vibrational coherence effects in broadband transient absorption spectroscopy with chirped supercontinuum probing. [The Journal of Chemical Physics 119, 988 (2003)]. Alexander L. Dobryakov, Sergey A. Kovalenko, Nikolaus P. Ernsting. Abstract. ...
The Journal of Chemical Physics, 2012
Femtosecond stimulated Raman spectra of trans-stilbene (D0), its isotopomers D2, D10, D12, (13)C2... more Femtosecond stimulated Raman spectra of trans-stilbene (D0), its isotopomers D2, D10, D12, (13)C2 and of cis-stilbene in hexane are measured in the ground (S(0)) and excited (S(1)) electronic states. The ground (13)C2 and excited D12 spectra are presented for the first time; the excited cis-spectra differ substantially from previously published ones. S(1) Raman bands of trans-stilbene are 20 cm(-1) wide corresponding to ~1 ps vibrational dephasing. For cis-stilbene the bands are broadened to 40 cm(-1) reflecting a short excited-state lifetime of 0.3 ps, in agreement with transient absorption data. From a dynamic shift of the 1569 cm(-1) band, pump-induced intramolecular cooling is estimated to be less than 20 K. Many S(1) Raman lines are detected for the first time. Vibrational spectra are calculated at MP2/cc-pVTZ (for S(0)) and XMCQDPT2/cc-pVTZ (for S(1)) levels of theory. Experimental and computational results can be used for a re-evalution of Rice-Ramsberger-Kassel-Marcus (RRKM) predictions for this famous photoisomeration reaction.
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Papers by Alexander Dobryakov