Function Of Excitation Wavelength Research Articles

Time dependent luminescence spectroscopy of highly doped Zn 1−xMn xS and Cd 1−xMn xTe

The time dependence of low energy emission bands as a function of excitation wavelength and temperature is reported and related to energy transport processes.

Photoconduction in Single Crystals of the Thianthrene-Tetracyanobenzene 1:1 Adduct

Abstract The photoconduction of single crystal thianthrane—tetracyanobenzene has been measured as a function of excitation wavelength, 200–600 mm, excitation intensity, temperature and applied field. In the 200–325 nm region only electron photocurrents are observed which show a symbatic behaviour; in the 325–340 nm region antibatic photocurrent behavior occurs for holes and electrons; for longer wavelengths both photocurrents are weak with no wavelength dependent structure. The electron mobility was ∼ 3.6 cm2/Vs at 300°K. These results are discussed in terms of the dominate photocarrier generation processes.

Stepwise two-photon excitation of Cl 2 to the E(0 g+) ion-pair state

The E(0 g + ) ion-pair state of Cl 2 has been investigated by stepwise two-photon excitation through the B 3 Π(0 u + ) state. The method yields rovibronically resolved E-B emission spectra, which are analyzed as a function of excitation wavelength. The E(0 g + ) state is identified as lying at T e = 57820 cm −1 and is characterized by ω e = 250 cm −1 and r e = 2.89 A.

Photoinjection of Charge Carriers into 1,3,5–Trintrobenzene Single Crystal

Abstract The DC photoresponse has been measured for 1,3,5 trinitrobenzene crystals as a function of excitation wavelength and applied field. Photoelectrodes used included a-Se, CdTe, Te, ZnO and gold. Current-voltage characteristics of all systems barring Se-TNB were similar. The orgin and nature of the photocurrent peaks at 480-450 nm and the following increase (decrease) of the photocurrent, common to all systems independent of electrode material is discussed and analysed in terms of a two component process, injection and optical detrapping. From the analysis an energy diagram of the systems together with probable photoinjection mechanisms are obtained.

Absolute yields I( 2P 1/2 as a function of wavelength in the photolysis of n-C 3F 7I

The absolute branching ratio I( 2P 1/2)/I( 2P 3/2) in the photolysis of n-C 3F 7I is examined optoacoustically as a functionof excitation wavelength. The yield of I( 2P 1/2) is much less than the normally assumed value of unity. Comparison in terms of the sub-bands present is made with CH 3I, CD 3I and CF 3I.

A Microprocessor Controlled System For Photoelectrochemical Measurements

ABSTRACT An automatic system for measuring photocurrent of electrodes as a function of excitation wavelength and electrode potential is described. Applications to a chemically modified transparent Sn02 electrode and an oxide-covered tantalum electrode are shown.

Photoconduction of phthalocyanines in the near infrared

The photovoltaic action spectra of x-phase metal-free phthalocyanine (x-H/sub 2/Pc) with zinc oxide, cadmium sulfide, and electrolytic Schottky junctions were investigated in the near-infrared. Photocurrent measurements as a function of excitation wavelength, temperature, oxygen content, and light intensity are reported. A new photovoltaic response in the near-infrared (0.98 ..mu..m) was observed which is attributed to carrier generation following excitation of a surface charge-transfer state. A charge-transfer energy of 1.25 eV and activation energy for dissociation of the electron-hole pairs of 0.08 eV were determined. These results strongly indicate that exciton dissociation at surface states play an important role in mediating charge carrier generation in phthalocyanines.

Observation of Raman band shifting with excitation wavelength for carbons and graphites

The Raman spectra of crystalline graphite, graphite damaged by ion-etching, and structurally disordered pyrolytic and glassy carbons were examined as a function of excitation wavelength λ over the range 488.0 to 647.1 nm. The bands located at 1360, 2720 and 2950 cm 1 with λ = 488.0 nm undergo a progressive red-shift as λ increases, while the positions of the other major bands (1580 and 1620 cm 1) are invariant. The significance of these observations is briefly discussed.

ACTION SPECTRA OF CATION EFFECTS ON THE FLUORESCENCE POLARIZATION AND INTENSITY IN THYLAKOIDS AT ROOM TEMPERATURE

Abstract— The degree of polarization of chlorophyll‐a(Chl‐a) fluorescence is known to monitor the extent of excitation migration and/or the orientation of the photosynthetic pigment molecules. We report here the effects of cations, at room temperature, on the degree of polarization of Chl‐a fluorescence, and fluorescence intensity in thylakoids as a function of excitation wavelength. Observations of maxima at 650 and 675 nm in the cation‐induced changes in the excitation spectrum for fluorescence at 730 and 762 nm, and, in the action spectra for the depolarization of fluorescence lead us to suggest that the regulation of the initial distribution of excitation to photosystem II involves the better coupling of Chl‐b and‐a in the light harvesting complex with Chl‐a in the reaction center II complex.

Laser study of the photophysical and photochemical properties of gaseous UF6

Abstract In view of possible laser isotope separation schemes of uranium, the photophysical and photochemical properties of gaseous UF 6 have been the subject of experimental and theoretical investigations in the last few years. In particular the use of laser light to exite the fluorescence emission of the UF 6 molecules has provided a fruitful tool for the study of the dynamics and structure of the excited electronic states. The fluorescence emission from UF 6 excited in the visible-UV (A-X band: 340–410 nm) exhibits different time behaviour and quantum yield as a function of excitation wavelength. These experimental data can be explained in terms of a model involving two excited electronic states. In addition due to the very low quantum yield ( −4 ) a very efficient quenching mechanism must be present. This mechanism may arise from the following processes: (i) Deactivation of the excited molecule through the internal degrees of freedom of either the same molecule or through collisions with other molecules. (ii) Dissociation process. We may note, of course, that only process (ii), if followed by a chemical or physical removal of the photoproducts, would be useful for a laser isotope separation scheme. New experimental data are reported in order to elucidate the quenching process. Mixtures of UF 6 and H 2 in different ratios have been irradiated at 360 nm and 400 nm in a monel cell carefully passivated. The two excitation wavelengths were chosen in order to excite preferentially one or the other of the two electronic states, which contribute to the A-X absorption band. After irradiation, a definite pressure drop in the reaction cell due to photochemical reactions has been observed. The stoichiometry of the photochemical reactions that we expect to occur is: 2UF 6 + H 2 → 2UF 5 (solid) + HF (1) With a stoichiometric UF 6 :H 2 initial pressure ratio, a pressure drop very near to the expected value of 2/3 of the initial pressure has been found. This result enables us to rule out the possibility of UF 6 decomposition into UF 4 , which would lead to a final pressure higher than that expected. The experimental time behaviour of the overall pressure drop has been interpreted in terms of the following reaction mechanism: F + H 2 → HF + H (3) H + UF 6 → UF 5 + HF (4) H + H + M → H 2 + M (5) F + UF 5 → UF 6 (6) in which M represents all possible collisional partners. According to this reaction scheme, the following conclusions have been drawn: (i) UF 6 molecules excited at 360 nm dissociate with a yield that is almost 1; (ii) H atoms produced by reactions (3) undergo reaction (4) rather than (5). (iii) Under excitation at 400 nm a photodissociation quantum yield of 0.71 has been found. Thus the dissociation process seems to be the predominant quenching mechanism of UF 6 excited in the A-X band.

Saturation of intervalence-band transitions inp-type semiconductors

We present a theory of the saturation of heavy-hole to light-hole band absorption in $p$-type semiconductors with the diamond or zinc-blende crystal structure by high-intensity light with a wavelength near 10 \ensuremath{\mu}m. The absorption coefficient is found to decrease with intensity in a manner closely approximated by an inhomogeneously broadened two-level model. For temperatures and hole concentrations where hole-phonon scattering dominates hole-impurity and hole-hole scattering, the saturation intensity is independent of the hole concentration. We calculate the saturation intensity as a function of excitation wavelength and temperature for $p\ensuremath{-}\mathrm{Ge}$ and $p\ensuremath{-}\mathrm{GaAs}$. We find that the saturation intensity increases with photon energy and with temperature. The calculated results are compared with the available experimental data and good agreement is found.

Effects of Chromatic Adaptation on the Photochemical Apparatus of Photosynthesis in Porphyridium cruentum

Cells of Porphyridium cruentum were grown in different colors of light which would be absorbed primarily by chlorophyll (Chl) (red and blue light) or by the phycobilisomes (green or two intensities of cool-white fluorescent light), and samples of these cells were frozen to -196 C for measurements of absorption and fluorescence emission spectra. Cells grown in the high intensity white light had least of all of the photosynthetic pigments, a higher ratio of carotenoid/Chl, but essentially the same ratio of phycobilin to Chl as cells grown in the low intensity white light. The ratio of photosystem II (PSII) to photosystem I (PSI) pigments was affected by light quality; the ratios of phycobilin to Chl and of short wavelength (PSII) Chl to long wavelength (PSI) Chl were both greater in the cells grown in red or blue light.Light quality also exerted a strong influence on the structural and functional organization of the photochemical apparatus. Data on the relative optical cross-sections of PSI and PSII as a function of excitation wavelength indicate that cells grown in light absorbed primarily by the phycobilisomes package a large fraction of their Chl into PSI (PSI Chl/PSII Chl approximately 20), whereas cells grown in light absorbed by Chl distribute their Chl much more equitably (PSI Chl/PSII Chl approximately 1.5). In both types of cells the phycobilisomes transfer their excitation energy predominantly to PSII Chl with little or no direct energy transfer to PSI, but the yield of energy transfer from PSII to PSI is approximately twice as large for cells grown in the phycobilin wavelengths of light. These differences in functional organization and energy distribution account for the physiological expressions of chromatic adaptation. The effects of chromatic adaptation on O(2) evolution can be predicted from our calculations of energy distribution between PSI and PSII for cells grown in the different colors of light.

Pre-resonance Raman spectra and conformations of nystatin in powder, solution and phospholipid-cholesterol multilayers

A Raman scattering study of the channel-forming polyene antibiotic nystatin, is reported in the solid state, in organic and aqueous solutions as well as in phospholipid and phospholipid-cholesterol multilayers. Measurements of the solid and solution spectra as a function of excitation wavelengths in the 459.7–514.5 nm range, and the phospholipid spectra as a function of temperature in the 10–60°C range, have also been made. The spectral features indicate a pre-resonance-enhanced Raman spectrum in which the CC and CC stretching modes of the polyene segment of nystatin are dominant. However, in contrast to previously published results on the nearly isostructural polyene antibiotic amphotericin B, a line at 1610 cm −1 assignable to the CO stretching mode is also observed to be strongly resonance enhanced. This is explained by a postulated ground-state conformation model in which a twisting of the molecule is facilitated by the break in the polyene chain. This allows the CO group at one end of the molecule to be aligned along the polyene unit at the other end, and the CC stretching vibration, which is strongly modulated by the polyene π → π∗ excited state, to mix with the CO stretching vibration. The peak frequencies and intensities of the CC and CC stretching modes in nystatin, however, remain essentially unchanged compared with amphotericin B, indicating that the polyene units in nystatin remain planar and trans both in the ground and excited states. The intensity of the CO mode with respect to the CC stretching mode was observed to vary appreciably with nystatin environment, indicating a

Theory of Nonlinear Infrared Absorption inp-Type Germanium

We present a theory of the saturation of inter-valence-band absorption in $p$-type Ge by high-intensity light with a wavelength near 10 \ensuremath{\mu}m. The absorption coefficient decreases with intensity in a manner closely approximated by an inhomogeneously broadened two-level model. The saturation intensity is calculated as a function of excitation wavelength and temperature and found to be in good agreement with measured values.

Thermal rotations and relaxation of vibrational energy of excited dye molecules in solution

Measurements of polarization and decay time of fluorescence of dyes in solution as a function of excitation wavelength were performed. The rate of thermal rotational depolarization was found to be independent of vibrational energy excess. To explain the observed influence of excitation wavelength on the emission anisotropy, a change in the magnitude of torsional vibrations, in addition to possible mixing of states, is proposed.

Raman scattering from boron-implanted laser-annealed silicon

Measurements of the line shape of the silicon optic mode in boron-implanted ruby-laser-annealed silicon were analyzed assuming a Fano-type interaction between the discrete optic mode and the continuous valence band states. The results yielded a utilization coefficient of 0.89±0.09 for the boron impurities. Measurements of the ratio of the intensity of the boron local mode to that of the silicon optic mode as a function of excitation wavelength are consistent with the conclusion of other workers that laser annealing is more effective than thermal annealing in reducing lattice damage.

Line-narrowed fluorescence spectra and site-dependent transition probabilities ofNd3+in oxide and fluoride glasses

The fluorescence spectra, branching ratios, and decay of the $^{4}F_{\frac{3}{2}}$ state of ${\mathrm{Nd}}^{3+}$ in glass were measured under excitation by tunable pulsed laser radiation. The following glass types were studied: silicate, phosphate, borate, fluoroberyllate, and fluorophosphate. Measurements were made at liquid-helium temperatures by exciting into the $^{2}P_{\frac{1}{2}}$ state, and line-narrowed fluorescence spectra were obtained as a function of excitation wavelength. Large variations in Stark splitting of the $^{4}I_{\frac{9}{2}}$ and $^{4}I_{\frac{11}{2}}$ states have been observed and attributed to site-to-site differences in the local crystal field. The probabilities for radiative decay and for nonradiative decay by multiphonon emission also exhibit variations with excitation wavelength. Although similarities exist, each different glass type shows its own distinctive patterns of variation in crystal-field splitting and relative quantum efficiency. In the fluorophosphate glass, which contains large numbers of both fluorine and oxygen anions, comparison of the laser-excited fluorescence spectra and lifetime of ${\mathrm{Nd}}^{3+}$ with the corresponding results from pure oxide and fluoride glasses demonstrates the presence of ${\mathrm{Nd}}^{3+}$ sites having both fluorine and oxygen nearest-neighbor coordination.

Phosphorescence Emissions and Nonradiative Properties of Vapors of Aromatic Carbonyl Compounds at Low Pressure

Abstract The phosphorescence quantum yields of benzaldehyde, acetophenone, p-fluorobenzaldehyde, p-chlorobenzal-dehyde, and benzophenone vapors, measured at low pressure down to the order of 10−3 Torr as a function of excitation wavelength, are essentially constant in the region of excitation energy corresponding to the S1(n, π*) state, whereas the yields decrease very sharply as the excitation energy is raised to the value corresponding to the S2(π, π*) state. This indicates that the nonradiative pathway in the carbonyl compound varies according to the type of the singlet state to which the molecule is initially excited. When excited to the S1 and S2 states, the molecule passes to the phosphorescent triplet state and a decomposing state, respectively. Two mechanisms are suggested and examined for understanding the nonradiative properties of the carbonyl compound vapors: (I) The internal conversion S2→S1 is slow compared with the conversion from S2 to the decomposing state; (II) the vibrational energy redistribution in S1 between the vibrational mode populated through S2→S1 and the mode to be populated optically is slow compared with the conversion from the former mode to the decomposing state.

Time-resolved fluorescence excitation spectroscopy of styrenes

The time-dependent photophysical processes occurring in low pressures of styrene and trans-1-phenylprop-1-ene vapours have been investigated as a function of excitation wavelength using synchrotron radiation from the Orsay Storage Ring (ACO). Earlier observations made with an Argon Ion laser have been confirmed and it has been found that the fluorescence lifetimes of the observed dual emissions show a strong dependence on excitation wavelength.

Vibrational relaxation of excited acetaldehyde vapor. A search for unquenchable species

Quantum yields of intersystem crossing for the system acetaldehyde-CO 2 were determined at room temperature as a function of pressure (5–2200 torr) and excitation wavelength (250–330 nm). Fluorescence decay times of 10 torr of acetaldehyde were measured as a function of excitation wavelength (275–325 nm). The results are consistent with the formation of an nπ * singlet state which can be totally relaxed by collision. No evidence was found a collision insensitive excited state. The lifetime of the nπ * triplet state of acetaldehyde is also reported.