Transient Absorption Features Research Articles

Dynamics of photochromism in salicylideneaniline: A femtosecond spectroscopic study

Excited state intramolecular proton transfer (ESIPT) of salicylideneaniline enol and subsequent isomerization of the proton transferred keto structure to the final long-lived photochromic product has been studied in different solvents using femtosecond transient absorption and picosecond fluorescence spectroscopy. Three different types of transient absorption have been identified, viz. (a) from initially excited enol form, (b) proton transferred keto form and (c) corresponding to the final photochromic product. The ESIPT rate was determined from the rise time of the characteristic transient absorption originated from the proton transferred keto form and was found to be in the order of (200–300 fs)−1 in most of the solvents. Photochromic product formation is completed within a few tens of picoseconds. The time dependent spectral change observed at the early time delay after excitation was considered to be due to cooling of the vibrationally “hot” proton transferred keto form towards the formation of the final photochromic product. The cooling process leads to the formation of some “metastable” state within a few hundred of femtoseconds in the ground state potential energy surface of the final trans product.

Formation of trithiocyanate in the oxidation of aqueous thiocyanate.

Oxidation of SCN(-) in acidic media by a deficiency of aqueous chlorine generates a transient UV absorption feature in less than 3 ms. This intermediate has the same spectral and kinetic features as were found in prior studies of the oxidation of SCN(-) by H(2)O(2) or ClO(2). The UV absorbance increases strongly with increasing concentration of SCN(-) but not with increasing [H(+)]. These observations are evidence that (SCN)(2) and (SCN)(3)(-) both contribute to the absorbance and are in equilibrium with each other. Values of the corresponding molar absorptivities and the equilibrium constant are derived. Hydrolysis of (SCN)(2) to HOSCN is shown to have a small equilibrium constant.

Transient Absorption Features in Gamma‐Ray Bursts and Their Implications for Gamma‐Ray Burst Progenitors

The recent detection of a transient absorption feature in the prompt emission of GRB 990705 has sparked multiple attempts to fit this feature in terms of photoelectric absorption or resonance scattering out of the line of sight to the observer. However, the physical conditions required to reproduce the observed absorption feature turn out to be rather extreme compared to the predictions of current gamma-ray burst (GRB) progenitor models. In particular, strong clumping of ejecta from the GRB progenitor seems to be required. Using detailed three-dimensional hydrodynamic simulations of supernova explosions as a guideline, we have investigated the dynamics and structure of pre-GRB ejecta predicted in various GRB progenitor models. Based on our results, combined with population synthesis studies relevant to the He-merger model, we estimate the probability of observing X-ray absorption features as seen in GRB 990705 to 1%. Alternatively, if the supranova model is capable of producing highly collimated, long-duration GRBs, it may be a more promising candidate to produce observable, transient X-ray absorption features.

Dynamic infrared band–band spectroscopy of peripheral light-harvesting complexes from R. acidophila

Infrared (IR) kinetic spectroscopy measurements have been employed to investigate and compare the electronic structure of B800 and B850 bacteriochlorophyll (BChl) pigment aggregates within peripheral (LH2) antenna complexes of photosynthetic purple bacteria. Following initial excitation of the B800 chromophores at 800 nm, a strong absorption band centred at ∼1.3 μm was observed in the transient spectrum which is characteristic of monomeric BChls. Evolution of a transient spectrum in the near-IR and IR regions was then tracked while the excitation energy migrated to the B850 chromophores on a ∼0.9 ps time scale. The B850 excited-state IR spectrum was found to differ substantially from that of B800, exhibiting a broader absorption band in the near-IR (centred at 1.1 μm) and new transient absorption features at 2.9 and 3.7 μm which are absent in B800. These results support a localised description of excitations for B800 but indicate the presence of considerably stronger inter-chromophore interactions between the B850 pigments. Differences between the excited-state spectra of B800 and B850 are interpreted as reflecting the formation of delocalized exciton and charge-resonance bands in B850.

Time-dependent photoionization opacities in dense gamma-ray burst environments

The recent detection of a transient absorption feature in the X–ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma–ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.

Transient spectroscopy of tetracene single crystals

Ultrafast dynamics of photoexcitations in ultrapure tetracene crystals were studied using transient photomodulation spectroscopy. We measured characteristic transient absorption and stimulated emission bands of Frenkel excitons and found that efficient internal conversion to the lowest exciton and subsequent vibronic relaxation occurs within 200 and 500 fs, respectively. Exciton relaxation to the ground state proceeds by direct radiative and nonradiative decays, as well as via intermediate long-lived excited states. We measure the transient absorption and dynamics of these states and discuss their possible origin.

Nitrogen dioxide as an oxidizing agent of 8-oxo-7,8-dihydro-2'-deoxyguanosine but not of 2'-deoxyguanosine.

The redox reactions of guanine and its widely studied oxidation product, the 8-oxo-7,8-dihydro derivative, are of significant importance for understanding the mechanisms of oxidative damage in DNA. Employing 2'-deoxyguanosine 5'-monophosphate (dGMP) and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in neutral aqueous solutions as model systems, we have used nanosecond laser flash photolysis to demonstrate that neutral radicals, dGMP(-H)(*), derived by the one-electron oxidation and deprotonation of dGMP, can oxidize nitrite anions (NO2(-)) to the nitrogen dioxide radical (*)NO2. In turn, we show that (*)NO2 can give rise to a one-electron oxidation of 8-oxo-G, but not of dGMP. The one-electron oxidation of dGMP was initiated by a radical cation generated by the laser pulse-induced photoionization of a pyrene derivative with enhanced water solubility, 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT). The dGMP(-H)(*) neutral radicals formed via deprotonation of the dGMP(*)(+) radical cations and identified by their characteristic transient absorption spectrum (lambda(max) approximately 310 nm) oxidize nitrite anions with a rate constant of (2.6 +/- 0.3) x 10(6) M(-1) s(-1). The 8-oxo-dG is oxidized by (*)NO2 with a rate constant of (5.3 +/- 0.5) x 10(6) M(-1) s(-1). The 8-oxo-dG(-H)(*) neutral radicals thus generated are clearly identified by their characteristic transient absorption spectra (lambda(max) approximately 320 nm). The rate constant of 8-oxo-dG oxidation (k(12)) by the (*)NO2 one-electron oxidant (the (*)NO2/NO2(-) redox potential, E degrees approximately 1.04 V vs NHE) is lower than k(12) for a series of oxidizing aromatic radical cations with known redox potentials. The k(12) values for 8-oxo-dG oxidation by different aromatic radical cations derived from the photoionization of their parent compounds depend on the redox potentials of the latter, which were in the range of 0.8-1.6 V versus NHE. The magnitude of k(12) gradually decreases from a value of 2.2 x 10(9) M(-1) s(-1) (E degrees = 1.62 V) to 5.8 x 10(8) M(-1) s(-1) (E degrees = 1.13 V) and eventually to 5 x 10(7) M(-1) s(-1) (E degrees = 0.91 V). The implications of these results, including the possibility that the redox cycling of the (*)NO2/NO2(-) species can be involved in the further oxidative damage of 8-oxo-dG in DNA in cellular environments, are discussed.

Transient absorption from the 1Bu+ state of all-trans-beta-carotene newly identified in the near-infrared region.

We have attempted subpicosecond time-resolved absorption spectroscopy of all-trans-beta-carotene in organic solvents in the 820-1060 nm region and found novel transient absorption features which lived in subpicosecond time scales. A first component that appeared immediately after excitation showed a lifetime of 190 +/- 10 fs in n-hexane in agreement with the 1Bu+ lifetime that had been determined by fluorescence upconversion spectroscopy (195 +/- 10 fs). (Kandori et al. [1994] J. Am. Chem. Soc. 116, 2671-2672.) Therefore, this component is assigned to a transient absorption from the 1Bu+ state.

Space Telescope Imaging Spectrograph Echelle Observations of NGC 4151: Variable Ionization of the Intrinsic UV Absorbers

We present echelle observations of the intrinsic UV absorption lines in the Seyfert galaxy NGC 4151; these observations were obtained with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope on 1999 July 19. The UV-continuum flux at 1450 A decreased by factor of about 4 over the previous 2 years, and there was a corresponding dramatic increase in the column densities of the low-ionization absorption lines (e.g., Si II, Fe II, and Al II), presumably as a result of a decrease in the ionizing continuum. In addition to the absorption lines seen in previous low states, we identify a large number of Fe II absorption lines that arise from metastable levels as high as 4.1 eV above the ground state, indicating high densities (>106 cm-3). We find that the transient absorption feature in the blue wing of the broad C IV emission, seen in a Goddard High Resolution Spectrograph spectrum and thought to be a high-velocity C IV component, is actually a Si II fine-structure absorption line at a radial velocity of -560 km s-1 (relative to systemic). We also demonstrate that the satellite emission lines of C IV found in IUE spectra are actually regions of unabsorbed continuum plus broad emission that become prominent when the UV continuum of NGC 4151 is in a low state.

Absolute rate constants for the quenching of reactive excited states by melanin and related 5,6-dihydroxyindole metabolites: implications for their antioxidant activity.

The triplet-excited state of benzophenone and the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and its biogenetic precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The excited states were generated by the laser-flash photolysis technique and their reaction kinetics was examined by time-resolved transient absorption or fluorescence spectroscopy, respectively. The reaction between triplet benzophenone and DHI produced with unit efficiency the corresponding 6O-centered semiquinone radical, which was characterized by its characteristic transient absorption. The quenching rate constants for DHI (3.1-8.4 x 10(9) M-1 s-1) and DHICA (3.3-5.5 x 10(9) M-1 s-1) were near the diffusion-controlled limit, indicating excellent antioxidant properties. Kinetic solvent effects were observed. The reactivity of synthetic melanin, assessed through the quenching rate constant of Fluorazophore-P and normalized to the number of monomer units, was more than one order of magnitude lower (2.7 x 10(8) M-1 s-1) than that of its precursors. The trend of the quenching rate constants, i.e. DHI > DHICA approximately alpha-tocopherol > melanin, along with the preferential solubility of DHICA in aqueous environments, serves to account for several experimental results from biochemical studies on the inhibition of lipid peroxidation by these natural antioxidants.

Specific Fluorescence Properties and Picosecond Transient Absorption of 8-Azasteroids

The specific fluorescence properties as well as picosecond transient absorption features have been studied for two 8-azasteroids. It is shown that at various excitation wavelengths the essentially different final excited electronic states are realized. Because of the multicenter character of 8-azasteroids the spectroscopic data obtained may be analyzed on a basis of the “mesomeric tautomerism” model taking into account the dynamic combination of cis- and trans-configurations. The dependence of fluorescence spectral characteristics on the solvent nature is a manifestation of intermolecular H-bond interactions.

On the nature of SW Sex

We present spectrophotometry of the eclipsing nova-like variable SW Sex. The continuum is deeply eclipsed and shows asymmetries due to the presence of a bright spot. We derive a new ephemeris and, by measuring the eclipse width, we are able to constrain the inclination to i > 75^o and the disc radius to R_D > 0.6 L_1. In common with other members of its class (of which it is the proto-type), SW Sex shows single-peaked emission lines which show transient absorption features and large phase shifts in their radial velocity curves. In addition, the light curves of the emission lines show a reduction in flux around phase 0.5 and asymmetric eclipse profiles which are not as deep as the continuum eclipse. Using Doppler tomography, we find that most of the line emission in SW Sex appears to originate from three sources: the secondary star, the accretion disc and an extended bright spot. The detection of the red star allows us to constrain the radial-velocity semi-amplitude of the secondary to K_R > 180 km/s and hence the component masses to M_1 = 0.3-0.7 Msun and M_2 < 0.3 Msun.

Intermolecular interaction of photoexcited Cu(TMpy-P4) with water studied by transient resonance Raman and picosecond absorption spectroscopies

A photoinduced complex between Cu(TMpy-P4) and water molecules, reversibly axially coordinated to the central metal, was observed in picosecond transient absorption and nanosecond resonance Raman experiments. This complex is rapidly created ( τ 1 = 15 ± 5 ps) in the excited triplet (π, π ∗ ) state of Cu-porphyrin, and the subsequent relaxation is proposed to proceed via two parallel pathways. One is fast and efficient (⩾ 90% of molecules), and presumably involves a (π, d) charge-transfer state. The second pathway is slow ( τ 2 ⪢ 1 ns), has a low quantum yield (⩽ 10%) and involves the excited (d, d) state which is responsible for transient Raman features at ≈ 1553 cm −1 (ν 2 ∗) and ≈ 1347 cm −1 (ν 4 ∗) , and for low-intensity long-lived transient absorption features.

Analysis of transient interband light modulation by ultrashort intersubband resonant light pulses in semiconductor quantum wells

We explore the transient characteristics of an interband resonant light modulation process by ultrashort intersubband resonant light pulses in semiconductor quantum wells (QW's). The modulation characteristics in a three-level semiconductor QW system, including the effects due to in-plane momentum, have been investigated by a numerical analysis of the coupled Bloch equations using a density-matrix approach. We have studied the effect of the carrier density and the nature of the intersubband coupling light on the transient absorption features of the interband light. The modulation process has been compared in doped and undoped QW's. The switching behavior of the strong interband light field in presence of a train of intersubband light pulses has also been discussed.

Time-resolved Optical Spectroscopy of the Cataclysmic Variable PG 0859+415

view Abstract Citations (12) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Time-resolved Optical Spectroscopy of the Cataclysmic Variable PG 0859+415 Hoard, D. W. ; Szkody, Paula Abstract We present time-resolved, high-resolution (2 Å) spectra of the nova-like cataclysmic variable (CV) PG 0859+415. The average optical spectrum of this star displays a continuum slope that is steeper than expected for a steady state accretion disk this is attributed to the presence of a hot spot with T ∼ 12,000 K at the impact site of the accretion stream. As first observed by Grauer et al., the time- resolved spectra exhibit a transient central absorption feature in the Balmer lines that appears near φ = 0.6 and lasts into the (partial) eclipse, and the radial velocity solution of the Balmer lines shows a phase offset of Δφ = -0.1 from the photometric phasing. System parameters are estimated from the radial velocity solution to the He II lines (which is consistent with an origin close to the white dwarf primary star). Doppler tomograms constructed from the Balmer emission lines reveal the presence of a region of enhanced emission at a location consistent with the expected impact site of the accretion stream with the edge of the disk. Finally, the similarities between PG 0859+415 and the recently identified SW Sextantis subclass of CV is discussed, and a qualitative model for the system is proposed to explain the observational results. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177930 Bibcode: 1996ApJ...470.1052H Keywords: ACCRETION; ACCRETION DISKS; STARS: BINARIES: CLOSE; STARS: INDIVIDUAL ALPHANUMERIC: PG 0859+415; STARS: NOVAE; CATACLYSMIC VARIABLES full text sources ADS | data products SIMBAD (12)

Ultrafast Dynamics of Chlorine Dioxide Photochemistry in Water Studied by Femtosecond Transient Absorption Spectroscopy

The ultrafast dynamics of chlorine dioxide (OClO) photochemistry in aqueous solution was studied by femtosecond transient absorption spectroscopy. Following the photoexcitation of OClO at 395 nm, the transient absorption dynamics were probed at 12 different wavelengths ranging from 350 to 700 nm. The transient absorption features observed in the visible wavelengths are assigned to correspond to the vibrationally hot photoisomer ClOO*. The spectral dynamics reveal the vibrational relaxation of this molecule in its ground electronic state. The total vibrational energy relaxation occurs within ∼9 ps. The dynamics of the formation of chlorine atom was examined by measuring the absorption dynamics in the 350−390 nm range. The time constant for Cl formation is calculated to be ∼200 ps. The data show that the dominant pathway for Cl formation is via the isomerized ClOO molecule. No fast component for Cl is detected, indicating that the ClOO molecule reaches thermal equilibrium before dissociating into Cl and O2.

Excited State Dynamics of Rh(II) Tetramesityl Porphyrin Monomer from Nanosecond Transient Absorption and Emission Spectroscopy

The excited states of [tetrakis(2,4,6-trimethylphenyl)porphyrinato]rhodium(II), Rh(II)TMP, have been studied using nanosecond transient absorption spectroscopy, emission spectroscopy, and electrochemistry. Rh(II)TMP has a long-lived excited state, with a 460 nm absorption band, that decays with a time constant of 180 ± 22 ns, in benzene and 205 ± 28 ns in 1,3-difluorobenzene. No transient absorption features are seen between 600 and 700 nm, but weak phosphorescence is detected at 743 nm at 77 K. Thus, the excited state has the characteristics of a triplet (π,π*) state, although a low lying, charge transfer state had been expected from iterative extended Huckel calculations. The phosphorescence is biphasic, and highly temperature dependent, consistent with a trip-quartet [4T] lowest excited state, lying not far below a trip-doublet [2T] state. Electrochemistry establishes that the Rh(II)/(I) reduction potential is more negative than −1.5 V versus SSCE while the porphyrin cation reduction potential is +0.55 V. These potentials place the lowest LMCT state at >2.05 V, well above the triplet states. In addition to elucidating the photophysics of a novel type of metalloporphyrin, these results have significant bearing on the feasibility of using Rh(II) porphyrins in photoinduced catalytic systems.

The Activity of Weak-Lined T Tauri Stars. I. V410 Tauri

view Abstract Citations (18) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Activity of Weak-Lined T Tauri Stars. I. V410 Tauri Welty, Alan D. ; Ramsey, Lawrence W. Abstract From two intensive series of spectra we have studied the phenomenology of magnetic activity and its distribution on and around V410 Tau. Large-amplitude, quasisinusoidal radial velocity variations at the stellar rotation period appear to be due to large-scale photospheric temperature inhomogeneity that has been present at least since 1988, and not to orbital motion. Other radial velocity variations imply more localized spot groups. The radial velocity variations highlight the point that radial velocity searches for brown dwarfs and planets orbiting cool stars must take stellar activity into account. The activity-related radial velocity variations seen in V410 Tau are thousands of times larger than expected radial velocity reflex motion of low-mass stars due to planets. Variations of absorption linewidths are consistent with surface temperature inhomogeneities implied by the radial velocity variations. The implied temperature distribution is in qualitative with Doppler images made from one of our series of spectra. We found that the cooler regions of the photosphere were associated with greater Balmer emission during the first observing run, but about 90° out of phase 13 months later. The level of Balmer emission increased from one season to the next, and flares were observed only during the later season, suggesting an activity cycle. The net Balmer emission apparently has two strong contributors: a global chromospheric component and a variable asymmetric "halo." Prominence-like structures appear not to be a large contributor to the nonphotospheric Balmer features. Transient absorption features probably are due to an infall of previously ejected material. Publication: The Astronomical Journal Pub Date: July 1995 DOI: 10.1086/117524 Bibcode: 1995AJ....110..336W Keywords: STARS: PRE-MAIN SEQUENCE; STARS: KINEMATICS full text sources ADS | data products SIMBAD (5)

A femtosecond-time-scale photolysis study of vapor-phase GaCl

Time-resolved absorption spectra are recorded for Ga atoms produced by the photolytic action of 300 fs, 248.5 nm laser pulses on gas-phase GaCl molecules. Photodissociation of GaCl at 248.5 nm produces both ground-state (2P1/2) and spin–orbit-excited (2P3/2) Ga atoms, resulting in strong transient-absorption features at 403.3 and 417.2 nm, respectively. The experimental spectra are analyzed to obtain the kinetic energy of the photofragments, which is estimated to be ≂0.08 eV for both channels. The analysis suggests that the dissociation limits are (1) Ga 2P1/2+Cl* 2P1/2 for the 403.3 nm channel; and (2) Ga* 2P3/2+Cl 2P3/2 for the 417.2 nm channel. From these results, a new estimate for the dissociation energy of GaCl is obtained: D00=4.80±0.03 eV.

Prominence activity in G dwarfs of the Persei cluster

We have found evidence for prominence-like circumstellar material in four rapidly rotating G-K dwarfs of the α Persei cluster, He 373, 520, 622 and 699. Our time-series Hα spectra of these stars show transient absorption features similar to those seen in the southern field rapid rotator AB Doradus. The absorption transients have durations of order 1 hr, during which they drift across the rotationally broadened Hα profiles of the stars concerned. This behaviour is consistent with the model developed for AB Dor, in which the absorption transients are produced via scattering of Hα photons in clouds of mainly neutral material trapped in the summits of closed magnetic field structures extending several stellar radii above the surface of the star