Red Excitation Research Articles

1 P 1 level decay in high pressure krypton

The 1P1 decay in high pressure krypton (densities 1×1020–5×1020 cm−3 ) has been investigated using selective photoexcitation. At densities above 4×1020 cm−3, the decay occurs fully through the 3P1 level. 1P1 blue or red excitation lead to identical results with evidence of a long lived reservoir effect with a rate constant associated with 1P1→3P1 transfer of ≊0.2×10−32 cm6 s−1. The kinetic scheme includes formation of short distance g symmetry excimers followed by 1g dissociation into the 3P1 state. At low density the results strongly support the hypothesis of a competing O+g(1P1) infrared excimer decay into the relaxed 1u(3P2) state.

Comparative study of resonance Raman and surface-enhanced resonance Raman chlorophyll a spectra using soret and red excitation

Surface-enhanced resonance Raman scattering (SERRS) spectra are reported for chlorophyll a adsorbed on a silver electrode at 298 and 77 K with 406.7-, 457.9-, 514.5-, and 647.1-nm excitation. Submerging the electrode in degassed water at 298 K was found to improve the spectral quality by minimizing sample heating and photooxidation. Spectral intensities and peak resolutions were greater at all excitation wavelengths at liquid nitrogen temperature. Most significantly, roughened silver at the low temperature quenched the fluorescence accompanying red excitation and minimized sample photooxidation, resulting in richly detailed SERRS spectra of chlorophyll a. The close correspondence between chlorophyll a resonance Raman (RR) and SERRS spectra suggests that an electromagnetic mechanism is the major source of the surface enhancement, rather than a chemical mechanism (e.g. a charge-transfer complex between chlorophyll a and the metal). The spectral similarities, together with the presence of the MgN{sub 4} vibration band in the SERRS spectra, also provide evidence that structural alterations (e.g. cleavage of ring V or loss of Mg) do not occur in chlorophyll a after adsorption at the electrode surface. A distinctive SERRS spectrum was obtained for each excitation wavelength. Selective excitation within the various electronic transitions can thus be utilized to verify assignmentsmore » of the vibrational modes of chlorophyll a and to monitor its interactions and photochemical behavior in biomimetic systems.« less

Ultralow Detection Limit of a Near-Infrared Dye by Diode-Laser-Induced Fluorescence in a Flowing Stream

Diode lasers are compact, highly efficient, temperature-tunable, long-lived, low-noise, simple-to-operate, and relatively inexpensive solid-state lasers that are rapidly becoming more useful in spectrochemical studies. Recently, Imasaka and Ishibashi reviewed the uses of diode lasers in trace chemical analyses. Due to the virtual elimination of background fluorescence from sample impurities, and the large wavelength shift of the Raman scatter from the solvent at red and near-infrared excitation wavelengths, the rather powerful diode lasers are very useful in laser-induced fluorescence (LIF) detection of very small amounts of red and near-infrared fluorophors. To our knowledge, the lowest detection limit obtained of a fluorescent species with the use of diode laser excitation is 46,000 molecules in a 56-nL volume of the laser dye IR-140 flowing in a liquid jet emanating from a capillary. With some simple improvements upon that system, the detection limit has been lowered to 3000 molecules of IR-140 in a 250-pL volume.

The red and near-infrared resonance Raman spectroscopy of photo-induced defects in the mixed-valence linear chain complex [Pt II(en) 2][Pt IV(en) 2Cl 2][ClO 4] 4

Resonance Raman (RR) spectra of the quasi-one-dimensional solid [Pt II(en) 2][Pt IV(en) 2Cl 2][ClO 4] 4 were obtained wth red excitations (1.65 and 1.41 eV) as a function of photolysis of the solid with energy within the intervalence charge transfer band (2.41 eV). The 1.65 eV excitation corresponds to a polaronic local gap absorption which is known to increase in intensity upon photolysis [S. Kurita & M. Haruki, Synthetic Metals 29, F129 (1989)]. However, the RR spectrum which best reveals the photo-induced defects is obtained with 1.41 eV excitation and indicates the presence of both hole and electron polaron defects.

Laser-flash photolysis of 124 kDa oat phytochrome: Studies concerning the late steps of P fr formation

Phototransformation kinetics of 124 kDa oat phytochrome at 298 K after red (660 nm) laser-flash excitation were recorded at different wavelengths under various conditions. Only the last steps of the phototransformation were investigated. On changing the environment of the apoprotein, especially by increasing the microviscosity, the formation of meta-R c and P fr, which presumably involve rearrangement of the apoprotein, was retarded. In contrast, the decay of the intermediate meta-R a was not retarded. Using phytochrome specifically labelled by covalent attachment of fluorescein mercuric acetate acting as a reporter group, it was possible to trace changes in the apoprotein conformation. In this way, it was demonstrated that major changes on exposed sites of the protein take place at the very last step of phototransformation, i.e. during formation of P fr from the preceding meta-R c. The data are discussed in terms of molecular aspects, such as conformational changes of the protein.

Stimulated switch-off and repetitive switching of a bistable optically controlled semiconductor switch

The response of CdS:Cu bistable optically controlled semiconductor switches to 10 ns laser pulses had been measured with green (532 nm), red (650 nm) and IR (1064 nm) excitation and pulse mixtures. In addition to light-stimulated conductivity the authors show a new mechanism: the stimulated switch-off with an infrared light pulse. With the present low-doped samples it was possible to realise turn-on times as fast as 15 ns (10-90%) and turn-off times shorter than 250 ns; the minimum switched-on time was 150 ns for 60 ns on-off pulse separation. Furthermore it was possible to demonstrate repetitive switching with a minimum switching period of 1 mu s.

Line narrowing and site selectivity in tryptophan fluorescence from proteins and glasses: Cryogenic studies of conformational disorder and dynamics

Fluorescence line narrowing and site selectivity are observed for red edge excitations (≈300 nm) of tryptophan in alcohol glasses and proteins at cryogenic temperature (2–200 K). Structured and broad fluorescence bands are observed. Both components are excitation wavelength dependent. These two spectral features are assigned to emission from 1L b and an exciplex version of 1L a (designated 1L a exc) respectively. The latter can be populated either through barrier crossing from 1L b or directly via highly excited vibrational levels of 1L a exc. The barrier separating 1L b and 1L a exc appears to be on the order of a few hundred cal/mole suggesting that in solution at ambient temperatures the 1L a exc state should dominate the emission process. Influence of protein relaxation upon the 1L a exc emission is not apparent until T>100 K. A model incorporating the conformational substate description of proteins and the rotamer model for tryptophan is developed and discussed in terms of the presented results.

Laser flash photolysis of purpurins: Novel potential photosensitizers of interest in photodynamic therapy

Purpurins, a new class of photosensitizers of potential use in the photodynamic therapy of cancer, have been studied by laser spectroscopy to determine the properties of their excited states. For the two molecules studied, generation of triplet states and singlet oxygen ( 1O 2) is very efficient. Singlet oxygen yields are presented for red and blue excitation though no significant wavelength effect is apparent. The promise these sensitizers have shown merits further investigation of other derivatives.

Use of Allophycocyanin Allows Quantitative Description by Flow Cytometry of Alveolar Macrophage Surface Antigens Present in Low Numbers of Cells

Autofluorescence has limited quantitative description by flow cytometry of certain alveolar macrophage surface antigens. Autofluorescence is most significant when macrophages are excited by blue light and emission is monitored in the green-yellow light range. Because of recent advances in flow cytometry and in the development of fluorescent labeling compounds, such as allophycocyanin, autofluorescence can be minimized by the use of red excitation and emission wavelengths. We have developed a three-step labeling technique that discretely identifies the minor subpopulation of normal rat alveolar macrophages that express Class II major histocompatibility (Ia) antigens. Using this technique, we observed that alveolar macrophage Ia antigen expression increases at least fourfold after intravenous administration of bacillus Calmette-Guerin to previously primed rats. Additionally, we observed that in vitro treatment of alveolar macrophages by exposure to gamma interferon results in an increase in Ia antigen expression. This new method is significant because flow cytometry is a powerful tool with which to analyze quickly, accurately, and reproducibly alveolar macrophage surface antigens.

Transport of light in tissue in photodynamic therapy.

Abstract The dose rate in photodynamic therapy is proportional to the energy fluence rate and the concentration of the photosensitizer. Calculations of the energy fluence rate have been performed in slab, cylindrical and spherical geometries with the discrete ordinates transport method and diffusion theory. The attentuation of the energy fluence rate is least in slab geometry and greatest in spherical geometry. Violet (405 nm) light is attenuated much more rapidly than red (630 nm) light. Small tissue dimensions or narrow beam irradiation further decrease the energy fluence rate with radius and depth. Anisotropic scattering increases the energy fluence rate at large depths, but decreases it near the source. Measurements of the absolute energy fluence rate vs depth in a mouse tumor model exhibit an order of magnitude attenuation through the skin and a 3 mm thick tumor. Calculations of the energy fluence rate of the DHE fluorescence have been carried out to guide measurement of the concentration. Violet light excitation is much more efficient than red light excitation.

Coherent anti-Stokes Raman spectroscopy study of the energy partitioning in the sodium(3P)-hydrogen collision pair with red wing excitation

Coherent anti-Stokes Raman spectroscopy (CARS) has been used to analyze the internal state distribution of H/sub 2/ after energy transfer with Na(3P) dependent on excitation wavelength. The detuning range was 600 cm/sup -1/ to the red starting with excitation at 589 nm. They observed a marked change in the H/sub 2/ vibrational distribution. To interpret this behavior one needs detailed knowledge about potential energy surface crossings with scattering dynamical calculations.

INTERPRETATION OF THE ‘DICHROIC ORIENTATION’ OF PHYTOCHROME

The dichroic orientation of phytochrome observed both in the phytochrome‐mediated phototropism in Adiantum protonemata and in the phytochrome‐mediated chloroplast movement in Mougeotia were analyzed in terms of the orientation of the transition moment associated with the long‐wavelength absorption band, assuming that phytochrome, associated with the plasma membrane, rotates around the normal to the membrane. The orientation of the long‐wavelength transition moment of the phytochrome chromophore was calculated using the zero‐differential overlap approximation of the molecular orbital theory for ir‐electrons. The results indicate that the orientation of the long‐wavelength transition moment mainly changes later than 2 ms after red light excitation of Pr, and that the different dichroic orientations of Pr and Pfr can be attributed to the change in the angle of the long‐wavelength transition moment of phytochrome with the plasma membrane from 18o to 72o during phototransformation.

Optical spectroscopy and resonance Raman scattering of polyaniline during electrochemical oxidation and reduction

In situ optical absorption and resonance Raman measurements are reported for the salt form of polyaniline. The Raman spectra show a strong dependence of the intensities on the electrochemical potential for blue laser excitation. Line shifts and new peaks were observed for green laser excitation. The behaviour observed for the blue laser excitation is consistent with a decrease in preresonance scattering due to an upshift of the fundamental absorption during oxidation. The spectra for green and red laser excitations are characteristic of a change in benzoid-quinoid ring concentrations. Hysteresis behaviour observed for the transmission of light at constant photon energy during cycling is interpreted as being due to diffusion processes of heavy counter-ions.

EVIDENCE FOR A SEQUENTIAL PATHWAY FROM Pr TO Pfr OF THE PHOTOTRANSFORMATION OF 124‐kDa OAT PHYTOCHROME

Abstract. Phototransformation kinetics of 124‐kDa oat phytochrome at 298 K after a red (660‐nm) laser flash excitation were recorded at different wavelengths. The kinetics of the dark relaxation processes for lumi‐R to Pfr can be satisfactorily described by only 3 rate constants: k= 28000 s‐1 370 s‐1 and 20 s‐1. The first rate constant is due to the decay of lumi‐R to meta‐Ra. The latter two rate constants correspond to processes establishing the far‐red (>700 nm) absorption band. No meta ‐Rb could be detected. From the wavelength dependency of the amplitudes of these two rates, parallel pathways in the formation of Pfr could be excluded. A unique sequential pathway for the dark relaxation leading to Pfr seems to be an intrinsic property of 124‐kDa phytochrome, however. Assuming a sequential pathway, molar extinction coefficients for intermediates have been calculated. These values agree with molar extinction coefficients obtained from low‐temperature spectra. The process with a rate constant of 370 s‐1 corresponds to absorbance changes for the formation of meta‐Rc from meta‐Ra and the rate constant of 20 s‐1 describes the absorbance changes due to the transformation of meta‐Rc to Pfr.

Resonance Raman study of the v1 band structure of [Pt(en) 2][Pt(en) 2Br 2](ClO 4) 4

The resonance Raman spectrum of [Pt(en) 2][Pt(en) 2Br 2](ClO 4) 4 (en = H 2NCH 2CH 2NH 2) has been studied as a function of temperature and excitation wavelength. Six features are observed for the symmetric Pt-Br stretch v 1, at low temperature and each shows a distinctly different excitation profile. The 181 cm −1 component dominates the spectrum with blue excitation while the 168 cm −1 component dominates with red excitation. The results help to explain the origin of the previously described dispersion of v 1 with excitation wavelength and the unusual excitation profiles that the v 1 bands of these mixed-valence complexes exhibit.

Polarized resonance Raman spectroscopy of fully-oriented crystalline trans-(CH) x

Fully-oriented non-fibrous crystalline trans-(CH) x films prepared by stress-orientation and conversion of poly-7,8-bis-(trifluoromethyl)-tricyclo-{4.2.2.0.}-deca-3,7,9,-triene, poly-(BTFM-TCDT), are investigated by polarized resonance Raman spectroscopy. With an excitation wavelength of 647.1 nm and parallel polarization (parallel to the c-axis, i.e., the stretching direction), two strong Raman bands are observed at 1078 cm −1 and 1469 cm −1, whereas for perpendicular polarization, no Raman scattered light can be detected. This polarization behaviour proves the perfect alignment of long unperturbed conjugated sequences parallel to the c-axis. Upon excitation with blue laser light ( λ l = 457.9 nm ), for which preferentially short conjugated sequences come into resonance, an increase of the depolarization ratio compared to the red excitation is observed. These results are discussed in terms of conjugation length distributions and resonance conditions with respect to the polarized optical absorption spectra.

Photodynamics and electronic structures of metal complexes

The emitting excited states of [Ru (2,2′-bipyridine) (ethylenediamine) 2] 2+ as well as [Ru (2,2′-bipyridine) 3] 2+ are described as the metal-to-bipyridine charge-transfer triplet state to which a variety of the spin-allowed transitions grant their intensities as deduced by molecular orbital calculations taking into account spin-orbit coupling. The structure of the lowest excited states of metalloporphyrins are discussed with relevance to the excited state lifetime. A variety of tetraphenylporphyrin metal complexes generate blue light upon red light laser excitation. The pumping up to the second excited singlet is ascribed to a successive two-photon absorption ▪.

Flicker photometric study of chromatic adaption: selective suppression of cone inputs by colored backgrounds.

Flicker photometric equivalence is both additive and transitive when the test and standard are alternated upon a relatively more intense colored background. When the balance of red versus green cone excitation from the background is unequal, the contribution of one cone type to flicker photometric spectral sensitivity may be depressed in relation to that of the other by at least 1 order of magnitude more than Weber's law predicts. The resultant spectral sensitivity is determined predominantly by only one class of cone. The cone spectral sensitivities of normals are then seen to be the same as those of dichromats, although there is some individual variation. A model is developed to explain this surprising phenomenon.

Band-edge photoluminescence of ZnSe using far-below band-gap excitations

Band-edge photoluminescent spectra of ZnSe were measured at 2 K using blue, green, and red krypton laser excitations. These results indicate the presence of a deep level whose binding energy is stronger than those for previous reported acceptor states. The nonlinear photoluminescent intensity dependence for the red excitation suggests that a two-photon excitation process takes place.

Spin-dependent phonon Raman scattering and electronic structure of CdCr2X4 (X:S,Se)

Spin-dependent (SD) phonon Raman scattering in ferromagnetic CdCr2X4 is closely associated with the resonance effect. In order to clarify the correlation between the excitation energy dependence of the SD phenomenon and the electronic structure, detailed temperature dependences of absorption and Faraday rotation spectra were measured between 20 and 300 K for ’’red-shifting (RS)’’ transitions in CdCr2S4 ∼1.9 eV) and CdCr2Se4 (∼1.2 eV). We found that the RS absorption peaks consists of a pair of electronic transitions with the energy separation of about 0.07 eV at 20 K. It turned out that the SD scattering cross section for Raman active phonon modes T2g and A1g in CdCr2S4 observed with red excitation light are caused by the resonance with the pair transitions. Magnetic circular polarization properties of Raman spectra for CdCr2S4 were measured in the applied fields up to 45 kOe in the Faraday configuration. The polarization and field-induced effects observed with Kr 647.1 nm excitation light support the idea of SD resonance scattering. SD effects for phonon modes T2g and A1g are also found with higher excitation energies C1 (2.4 eV; CdCr2S4, 2.0 eV;CdCr2Se4) and C2 (2.3 eV;CdCr2Se4), respectively. The correlations between the SD phonon scattering and the electronic structures are discussed on the basis of proposed transition diagrams.