Showing papers on "Excited state published in 1977"

Hyperpolarizabilities of the nitroanilines and their relations to the excited state dipole moment

Abstract: We have measured the second and third order hyperpolarizabilities of the three nitroaniline isomers and of two related molecules. For some isomers the intramolecular charge transfer is found to cause a very large enhancement of the second order polarizabilities. We present a theory relating this contribution to the first excited state of energy, oscillator strength, and dipole moment of the molecules. Experimental results are accounted for with an excellent accuracy.

Optical nonlinearities of conjugated molecules. Stilbene derivatives and highly polar aromatic compounds

Abstract: We have investigated the influence of donor and acceptor substituents on the second and third order hyperpolarizabilities β and γ of large conjugated molecules such as stilbene and styrene derivatives. This was performed by two independent measurements of β and γ using the techniques of dc electric‐field induced second‐harmonic generation and tunable four‐wave mixing in liquids and solutions. For trans−stilbene derivatives, β and γ are typically 10 times larger than for the corresponding benzene compounds, and are strongly correlated with the mesomeric effect of the substituents. A series of disubstituted molecules with strong donor–acceptor intramolecular charge transfer exhibit very large β, and it is shown that this enhancement can be predicted from the basic properties of the first electronic excited state.

Photon Antibunching in Resonance Fluorescence

Abstract: The phenomenon of antibunching of photoelectric counts has been observed in resonance fluorescence experiments in which sodium atoms are continuously excited by a dye-laser beam. It is pointed out that, unlike photoelectric bunching, which can be given a semiclassical interpretation, antibunching is understandable only in terms of a quantized electromagnetic field. The measurement also provides rather direct evidence for an atom undergoing a quantum jump.

On the calculation of multiplet energies by the hartree-fock-slater method

Abstract: It is shown that a consistent application of the p 1/3 approximation of the Hartree-Fock-Slater method requires the use of one specific procedure, the sum method, for the calculation of the energy E 1 of singlet excited states of closed shell molecules. Further, E 1 is found to be in reasonable agreement with experiment for a number of molecules, contrary to the energy E 2 obtained according to another method discussed in the literature. The calculation of other multiplet splittings than singlet-triplet in the Hartree-Fock-Slater method is also considered.

Theory of thermal unimolecular reactions at low pressures. I. Solutions of the master equation

Abstract: The master equation for a thermal unimolecular reaction in gases at low pressures is formulated. Steady‐state solutions are derived in analytical form with an exponential model of collisional transition probabilities, (i) for vibrational energy transfer in molecules with variable densities of states, and (ii) for combined rotational and vibrational energy transfer in molecules with variable heights of the centrifugal barriers. Other models of transition probabilities are treated numerically. The diffusion limit of energy transfer is discussed. In all cases, the nonequilibrium populations of excited states and the weak collision efficiency factors βc are calculated.

Lasers and Excited States of Rare Earths

Abstract: 1. Analogies and Differences Between Monatomic Entities and Condensed Matter.- A. The Configuration 4fq as an Instance of Spherical Symmetry.- B. The "Ligand Field" as Minor Deviations from Spherical Symmetry.- C. Inter-shell Transitions to Empty 5d and 6s Orbitals.- D. Electron Transfer Bands, Including the Uranyl Ion.- References.- 2. Rare-Earth Lasers.- A. Introduction.- B. Spontaneous and Stimulated Emission.- C. Three-level Laser System.- D. Four-level Laser System.- E. Modes of Oscillation.- F. The Threshold of Optical Pumping.- G. Laser Output.- H. Examples of Rare Earth Lasers.- a) Liquid Rare Earth Lasers.- b) Generalities About Solid State Lasers.- c) Relaxation Processes in Solid State Lasers.- d) Crystal Rare Earth Lasers.- e) The Specific Case of Nd3+Crystal Lasers.- f) Laser Emission of Ho3+, Er3+ and Tm3+.- g) High-power Nd3+ Glass Lasers.- h) Nd-doped Glass Ceramic Laser.- i) Lasers from Vapours of Rare-Earth Compounds.- References.- 3. Chemical Bonding and Lanthanide Spectra.- A. The Nephelauxetic Effect and the Photo-electron Spectra.- B. The Intensities and the Hypersensitive Pseudoquadrupolar Transitions.- C. The Chemistry Behind the Judd-Ofelt Parametrization.- References.- 4. Energy Transfer.- A. Transfer Probabilities.- a) Resonance Energy Transfer.- b) Exchange Interaction.- c) Magnetic Interaction.- d) Electrostatic Interaction.- e) Statistical Aspects of Macroscopic Energy Transfer.- B. Migration of Excitation.- C. Inhomogeneous Broadening.- D. Phonon-assisted Energy Transfer.- E. Selected Examples and Suggestions.- a) Donors with Spin-forbidden 3d Transitions.- b) Post-transitional (Mercury-like) Donors with Laporte-allowed Transitions.- c) Electron Transfer Bands in Early Transition Elements as Donors.- d) Energy Transfer from the Uranyl Ion to Lanthanides.- e) Energy Transfer Between Lanthanides.- f) Processes of Infra-red to Visible Up-conversion.- g) Energy Transfer from Organic Ligands to Lanthanides.- h) Energy Transfer from Lanthanides to Other Species.- References.- 5. Applications and Suggestions.- A. The Standard Emission Spectrum of Opaque Objects (in Atomic Units).- B. Induced Thermonuclear Reactions.- C. Cathodoluminescence and Energy Dissipation from Rapid Elementary Particles and Fission Products.- D. Communications and Holography.- E. Geodesy and Trigonometry in the Solar System.- F. Antiferromagnetic Coupling with Adjacent Transition Group Ions.- G. Final Comments on Spectroscopy in Condensed Matter.- References.- 6. Subject Index.- 7. Author Index.

Multiphonon relaxation of rare-earth ions in oxide glasses

Abstract: Nonradiative decay of excited rare-earth ions by multiphonon emission has been investigated in a series of oxide glasses. Various rare-earth electronic levels were selectively excited by short-duration laser pulses and multiphonon relaxation rates were determined from measurements of fluorescence rise and decay times. Time resolution for fluorescence measurements was 3 nsec, so excited states were probed for which the decay was predominantly nonradiative. Excited states of ${\mathrm{Nd}}^{3+}$, ${\mathrm{Er}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ with energy gaps to the next-lower $J$ state ranging from 1300 to 4700 ${\mathrm{cm}}^{\ensuremath{-}1}$ were studied. The multiphonon relaxation rates for each glass investigated exhibited an approximately exponential dependence on energy gap. Evidence of breakdown of this dependence was observed in the region of small energy gaps. The measured temperature dependences of the decay rates establish that the relaxation occurs predominantly by excitation of the highest-frequency vibrations associated with stretching modes of the glass network former. Borate, silicate, phosphate, germanate, and tellurite glasses were studied. From Raman spectra, the highest-frequency vibrations for these glasses ranged from 700 to 1400 ${\mathrm{cm}}^{\ensuremath{-}1}$. The corresponding multiphonon relaxation rates for a given energy gap differed by three orders of magnitude. The strength of the ion-phonon coupling was found to be approximately equal for all glasses.

An investigation of the structure near the L2,3 edges of argon, the M4,5 edges of krypton and the N4,5 edges of xenon, using electron impact with high resolution

Abstract: The authors have investigated structure near the L2,3 edges of argon, the M4,5 edges of krypton and the N4,5 edges of xenon, using electron impact at an incident energy of 1.5 keV and with an energy resolution of typically 70 meV. The structure observed corresponds to the promotion, via an optically allowed or optically forbidden transition, of an electron from an inner shell of the atom to an unoccupied Rydberg orbital. The energies of these excited states have been obtained, enabling the accurate determination of the Rydberg series limits. The effective quantum numbers of the excited states have been found to be closely related to those of the 'equivalent-core' atoms K I, Rb I and Cs I.

Resonant Photoemission in Nickel Metal

Abstract: A strong resonance effect has been observed for the first time in photoemission: This happens on a metallic nickel (100) surface when the photon energy is close to that of the $3p$ energy level. The effect is interpreted as the autionization of an excited quasidiscrete atomic configuration into a continuum of excitations with two holes in the $3d$ shell.

Conjugated polyene fatty acids on fluorescent probes: spectroscopic characterization.

Abstract: This paper is the first in a series which extends introductory studies of parinaric acid and its phospholipid derivatives as membrane probes (Sklar, L.A., Hudson, B., and Simoni, R.D. (1975), Proc. Natl. Acad. Sci. after U.S.A. 72, 1649; (1976), J. Supramol. Struct. 4, 449). Parinaric acid has a conjugated tetraene chromophore and exhibits many spectroscopic properties common to linear polyenes. Its absorption spectrum is characterized by a strong near-ultraviolet transition with vibronic structure, which is strongly affected by solvent polarizability. The fluorescence emission occurs at considerably lower energy than the absorption and the wavelength of the emission is nearly independent of the solvent. The fluorescence quantum yield and lifetime are strongly affected by temperature and solvent. These spectral features are interpreted in terms of an excited electronic-state order such that a weak transition occurs at longer wavelengths than the strongly allowed transition which dominates the absorption. The sensitivity of the fluorescence quantum yield an lifetime to environment is shown to be due primarily to variations in the nonradiative rate, although changes in the radiative rate constant are also observed and interpreted. The absorption spectrum (epsilon max greater than 65 000) is in the 300-320-nm range, a region relatively free of absorption due to intrinsic biological chromophores. Shifts of several nanometers are characteristic of different environments. These shifts are compared to similar effects observed for a series of diphenylpolyenes for which new data are given and are correlated using a simple but adequate theory of solvent shifts. The intrinsic (or radiative) fluorescence lifetime is near 100 ns in a wide variety of environments. This is much longer than the intrinsic lifetime calculated from the absorption spectrum and strongly supports the proposed excited-state order.

Rovibronic interactions in the photoabsorption spectrum of molecular hydrogen and deuterium: An application of multichannel quantum defect methods

Abstract: Multichannel quantum defect theory (MQDT) is applied to the treatment of electron motion in molecular Rydberg states and its coupling to the vibrational and rotational motions of the nuclei. The full rovibronic Hamiltonian is taken into account and the theory formulated in terms of parameters which relate to the standard Born–Oppenheimer (BO) theory. The development is an extension of Seaton’s quantum defect theory and relies on Fano’s frame transformation method. MQDT is not restricted to the BO approximation, but includes the so‐called adiabatic corrections and nonadiabatic effects (i.e., vibronic coupling and rotational l uncoupling), however strong, without requiring the explicit evaluation of the interactions between individual levels. The general formalism can be simplified for the H2 and D2 molecules in the range below or near the ionization threshold and for vibrational excitation well below the dissociation energy. The only input data required are the potential energy curve of the ion core in its...

Beam-foil spectroscopy

Abstract: The measurement techniques of beam-foil spectroscopy are reviewed. These comprise the study of wavelengths of heavy ions with low and high charge states at visible, UV and X-ray wavelengths from normal excited states, multiply excited states and high-n Rydberg levels; the measurements of mean lifetimes above 10-12 s and metastable states up to 100 ns; high-resolution Auger electron measurements; and the application of atomic alignment and orientation by both perpendicular and tilted foils to measurements of atomic structures and analysis of the beam-foil excitation process. A theoretical description of the emission of light from anisotropically excited systems is included. Associated techniques of time-resolved spectroscopy are compared.

Geometries and energies of the excited states of O3 from ab initio potential energy surfaces

Abstract: The geometries and relative energies of the nine lowest states of the ozone molecule have been determined in C2v symmetry from ab initio configuration interaction calculations in a [3s2p1d] contracted Gaussian basis Calculations were carried out over a two‐dimensional grid of points in C2v symmetry to locate the optimum geometrical parameters R and ϑ for each state For the ground 1A1 state the calculated properties (with experimental values in parentheses) are as follows: Re=1299 A (1271 A), ϑe=1160° (1168°), ω1=1235 cm−1 (1110 cm−1) and ω2=707 cm−1 (705 cm−1) Of the excited states only the lowest 3B2 state is found to have an adiabatic excitation energy (092 eV) less than the dissociation energy (De=113 eV) and hence to be a likely bound species The 1B2 state responsible for the strong absorption in the Hartley band (47–58 eV) is stabilized by asymmetric distortions away from its equilibrium C2v geometry (Re=1405 A, ϑe=108°) suggesting unequal bond lengths for this state or else purely disso

Photoelectron spectra of polynuclear aromatics. V. Correlations with ultraviolet absorption spectra in the catacondensed series

Abstract: IP’s derived from the high‐resolution PE spectra of 61 catacondensed hydrocarbons are presented, most of them for the first time, and correlated with the energies of the four main singlet–singlet transitions. For the p and β′ bands (1La and 1Ba) a linear correlation with IP1 and IP2, respectively, is established. The α and β band energies (1Lb and 1Bb) correlate linearly with the mean of IP1 and IP2. The regressions encompass planar as well as nonplanar hydrocarbons of the helicene type; standard deviations range from 0.070 to 0.126 eV. Comparison with Huckel and PPP theory shows the empirical correlations to be superior; their potential in disentangling diffuse uv spectra is demonstrated. Many β′ bands are assigned for the first time, and estimates are given for the position of the α band in anthracene, tetracene, and pentacene. From the slopes and intercepts of the regressions, important conclusions regarding the wavefunctions of the excited states and the role of electron repulsion and CI can be drawn....

Structure of the Excited States in 12C. II

Abstract: Structure of both positive- and nCc:gative-parily slates in 12C is studied with a microscopic :l u-parlicle model. The c.m. motions of 3 a-clusters are treated by the generator coordinate method. All the levels with T=O below 15 MeV (except the 1:0.7 MeV r+ level) are suc­ cessfully reproclucecl, including the famous o,+ level and the next positive-parity level which is regarded as :o,+. Another 2', :l1+ and two 4+ stales which have a structure quite similar to that of the O, ,_ and :0, + stales are preclictecl with large K-rnixings. Furthermore the pre­ sent investigation predicts an existence of O, + state. The structure of all the above excited positive-parity slates is quite diiferenl from that expected in the shell model, hut rather should he considered that· o£ a finite a-boson ga,. The negative-parity leveis are quite well de":ribecl '" memhcrs of the K"=:l- and I bands in the present model. § 1. Introduetion The nucleus "C has been offering a testing field to vanous nuclear models. It is a stable and tight binding system though it is one of the lightest nuclei. On the assumption of a stable average nuclear Jield, shell model \Vas applied in se\·eral \·ersions to explain low energy properties of 12C.'' Unfortunately, all the efforts have resulted in obtaining only partial success. Above all the second 0 t level at 7.7 MeV and the next positive-parity level at 10.3 MeV have been difficult to be reproduced at such low excitation energies. They are also known to have anomalously large a-decay widths.'' Morinaga suggested that they form an excited rotational band with a linear chain structure of 3 a-particles. 11 According to Ikeda's cliagram5l' 61 one can expect appearance of nuclear states with some cluster structure in the neighbourhood o£ decay-threshold energies of a-particles. In 12C, the 0,' le\·el and the next positive-parity level noted aboYe are expected to be such states, that is, to ha\·e distinct cluster structure relevant to the 'Be -1--CY channel al 7.4 MeV. From the \'iewpoint of a-particle mudel this nucleus has been investigated since the early ages of nuclear study. The classical a-particle model was applied to it on the analogy o£ homonuclear tri-atomic mole­ cule." The modeL howeYer, predicts a :3 state at too low an excitation energy compared with experiment. This dr;cm·back is known to disi!ppear in the micro­ scopic model.RJ Recently sc1·eral dynamical calculations"' of :3 a-particle system haYe been made by the use of 1·arious a-ct interactions. Fuji\\'ara and Tamagaki 9b 1 11 Preliminary results were

Molecular Rydberg states. VII. Water

Abstract: The optical absorption spectra of H2O and D2O are reported at various pressures for 2000?λ?950 A. The 1670 A band is vibrationally structured (ν2 bend) and the excited 1B1 state contains a considerable intravalence 4a1 component. The 1280 A state 3sa1; 1A1 is linear and dominantly Rydberg. A Renner–Teller analysis of the 1280 A band leads to detection of a perturbing state at 1365 A which may be assigned as a bent, heavily Rydberg 3sa1; 1B1 state. A reanalysis of K‐shell excitation spectra bolsters this 3sa1; 1B1 assignment. Isotope shift studies in the 1130–980 A region lead to some vibronic reassignments. The terminal state of the 968 A absorption band is a linear 5sa1; 1A1 state. In sum, a total of 21 electronic states, comprising fragments of six Rydberg series, have been assigned.

The Influence of Autoionization Accompanied by Excitation on Dielectronic Recombination and Ionization Equilibrium

Abstract: In the process of dielectronic recombination, the doubly excited state formed by radiationless capture may autoionize preferentially into an excited state of the recombining ion. This additional autoionization process has not been discussed in previous treatments of dielectronic recombination. The dielectronic recombination rates for certain nonhydrogenic Fe ions, although still larger than the direct radiative recombination rates, are found to be substantially reduced by the inclusion of the additional autoionization rate in the branching ratio for the stabilizing radiative transition. Consequently, the temperatures of maximum equilibrium abundance are significantly lower than those predicted by recent calculations. Finally, the radiative energy loss rate coefficients are calculated for radiation processes involving electron Fe-ion collisions in high-temperature plasmas. Electron impact excitation of resonance line radiation is the dominant radiative cooling mechanism in steady-state plasmas at temperatures where ions with bound electrons are abundant. However, it is found that the radiation emitted during dielectronic recombination can be more important than direct recombination radiation and bremsstrahlung.

The Photoenolization of 2-Methylacetophenone and Related Compounds

Abstract: A reinvestigation of 2-methylacetophenone (1) by ns flash photolysis has provided detailed evidence for the reaction sequence of photoenolization. The triplet reaction proceeds adiabatically from the lowest excited triplet state of the ketone, 3K (1), to the enol excited triplet state, 3E (1), which decays both to enol and ketone ground state. The Z- and E-isomers of the photoenol, Z-E (1) and E-E (1) are formed in about equal yield by the triplet pathway, while direct enolization from the lowest excited singlet state of 1 yields (predominantly) the Z-isomer. Intramolecular reketonization from Z-E (1) to 1 proceeds at a rate of ca. 108s−1 in cyclohexane, but can be retarded to ca. 104s−1 in hydrogen-bond-acceptor solvents. The proposed mechanism is summarized in Scheme 1 and rationalized on the basis of a state correlation diagram, Scheme 2. 3,3,6,8-Tetramethyl-1-tetralone (2) was used as a reference compound with fixed conformational position of the carbonyl group, and some results from a brief investigation of 2,4-dimethylbenzophenone (3) are also reported.

Theory of differential phase fluorometry: Detection of anisotropic molecular rotations

Abstract: A general description of transformations in the excited state is employed to derive the general equations for the differential delay and the modulation ratio of the fluorescence owing to different pairs of excited species present. These equations yield directly the differential delay and the modulation ratio of the polarized components of the fluorescence from a rotating spherical molecule. Similar equations for a rotating irregular molecule are then derived from the sine and cosine transforms of the impulse response of the polarized components of the fluorescence emission. It is shown that for excitation at a wavelength at which the limiting polarization is high, namely, 1/2 to 3/11, the maximum differential tangent observed for anisotropic rotations is uniformly lower than the value for the sphere. The magnitude of this tangent defect permits an estimate of the minimum standard deviation of the principal rotational rates of the ellipsoid of inertia that describes the rotations of the molecule.

Structure and energetics of simple carbenes methylene, fluoromethylene, chloromethylene, bromomethylene, difluoromethylene, and dichloromethylene

Abstract: There is very little experimental data concerning the singlet--triplet separations of carbenes. In many cases, it is not even possible to form estimates of this separation from experimental results. CH/sub 2/ is a distinct exception; here substantial, although contradictory, data are available. The equilibrium geometries of the lowest singlet states of several carbenes have been determined experimentally. However, the geometries of many states are not known and the triplet state geometries are also generally unknown. In order to obtain theoretical information about these quantities for a series of halogenated carbenes, we have studied CH/sub 2/, CHF, CHCl, CHBr, CF/sub 2/, and CCl/sub 2/ using a consistent level of ab initio electronic structure theory. One configuration (triplet state) and two configuration (singlet state) wave functions have been computed. Double zeta basis sets augmented by a d function on the carbon atom, optimized separately for both the singlet and triplet states of each molecule, were used. The theoretical equilibrium geometries agree well with the available experimental data. The trends of the singlet-triplet separations are discussed and correlated with the carbon atom Mulliken gross populations for the singlet and triplet states.

Cis - trans isomerization in the photochemistry of vision

Abstract: Various models proposed for the primary photoevent in vision are critically discussed. It is concluded that the classical picture of a single cis-trans isomerization step is the only one which satisfactorily accounts for all the available experimental data. Experiments are performed showing that this process is temperature independent over a range of 200°C. Photoisomerization yields for the free protonated Schiff base of 11-cis (and all-trans) retinal are measured as a function of the excitation wavelength. In contrast to the efficient and wavelength independent photobleaching of rhodopsin, the yields of the 11-cis→al-trans isomerization of the free chromophore are small, exhibiting a marked dependence on the excitation wavelength. Potential energy curves for both ground and excited states of rhodopsin are derived from the analysis of the accumulated experimental data. In variance with the behavior of model compounds, photoisomerization in the pigment proceeds via the quantitative population of a common, barrierless, thermally relaxed excited state along the 11-12 torsional coordinate separating the 11-cis (rhodopsin) and all-trans (bathorhodopsin) configurations. In the ground state, interactions with the protein destabilize the all-trans isomerization product, leading to storage of a significant fraction of the photon's energy in the primary step.

Cross sections for ionization of metastable rare-gas atoms (Ne*, Ar*, Kr*, Xe*) and of metastable N/sub 2/*,CO* molecules by electron impact. [6 to 250 eV, Born and binary encounter approximations]

Abstract: Cross sections for the collisional ionization of the metastable atoms Ne*, Ar*, Kr*, and Xe* by electrons with impact energy E in the range 6 < E < 250 eV are determined in the Born and binary-encounter approximations. For low energies epsilon of ejection, the s-d and s-f bound-free transitions dominate the associated form factor, while transitions to continuum states with progressively higher angular momentum gain importance with increasing epsilon. While up to nine partial waves are normally sufficient for convergence of the bound-free form factor at a given energy epsilon and momentum change K of the ejected electron, as many as 30 are required for those K in the vicinity of the Bethe ridge at large epsilon. These properties are the origin of the overall closeness obtained between the Born and binary-encounter cross sections. Also inner-shell ionization, as described by the binary-encounter treatment, becomes increasingly important as the target atom becomes more complex. Cross sections for ionization of metastable N/sub 2/* and CO* are also determined. Good agreement with available measurements (for Ne* and Ar*) is obtained. (AIP)

Laser Excitation and Ionization in a Dense Li Vapor: Observation of the Even-Parity, Core-Excited Autoionizing States

Abstract: The first optical absorption from excited states to autoionizing core-excited levels has been observed in lithium. A 1-MW dye laser tuned to the $1{s}^{2}2s\ensuremath{\rightarrow}1{s}^{2}2p$ resonance produced \ensuremath{\sim}${10}^{16}$ excited atoms/${\mathrm{cm}}^{2}$ in a heat-pipe oven. Far-uv absorption spectra of ${\mathrm{Li}}^{*}(2p)$ and ${\mathrm{Li}}^{+}$ were obtained. A curve of the evolution from excited neutrals to \ensuremath{\sim}95% ions is presented, and the significance of the Li observations for understanding the ionization mechanism is discussed.

A demonstration of one‐atom detection

Abstract: Resonance ionization spectroscopy, a photoionization method in which all of a given quantum selected species are converted to ion pairs, has been used to develop a detector for a single atom. We have demonstrated the detection of one atom by using a pulsed dye laser to photoionize Cs to saturation and a proportional counter for the detection of single electrons. Some current applications, e.g., the slow transport and chemical reactions of atoms, are briefly discussed. Future applications may include the detection of rare events such as quarks, solar neutrinos, and superheavy elements.

Photochemistry of flavins. ii. photophysical properties of alloxazines and isoalloxazines

Abstract: . Pulsed laser photolysis at 347nm has been used to study the transient spectroscopy of alloxazine, lumichrome, lumiflavin, and riboflavin in acidic (pH 2.2) aqueous solution and in ethanol. Intersystem crossing quantum yields (φISC) were determined by a modification of the comparative laser excitation method which utilizes the variation of the triplet yield with intensity in conjunction with a kinetic model for the various photophysical and photochemical processes occurring during the pulse. Fluorescence quantum yields and lifetimes are also reported. Correction for quenching of the excited singlet state by H+ ions shows that, in neutral aqueous solution, intersystem crossing for flavins is an efficient process (φISC˜ 0.7) which, in conjunction with fluorescence, accounts for the fate of all absorbed photons. For alloxazine (φISC˜ 0.45) and lumichrome (φISC˜ 0.7) the results are more difficult to interpret owing to interconversion between alloxazine and isoalloxazine structures in the singlet excited state. For all four compounds, the quantum yield of products derived from the singlet excited state is estimated as ˜0.04. There is evidence of biphotonic product formation at high laser energies. In ethanol, where φISC for lumichrome is about twice that of lumiflavin, internal conversion between the excited singlet and ground states appears to be a significant process. Complete triplet-triplet absorption spectra in the region 260–750nm are reported. For lumichrome at pH 2.2 there is spectral evidence for isomeric triplet states which appear to be in equilibrium.