FORMATION AND CHEMILUMINESCENT DECOMPOSITION OF DIOXETANES IN THE GAS PHASE

Abstract— High resolution chemiluminescence spectra have been obtained of the singlet electronically excited products of O₂(¹Δ) plus alkene, dioxetane forming, reactions. The experiments were conducted in a flow apparatus at pressures of 1–5 torr. The spectra are a measure of the unrelaxed initial distribution of energy in the excited product. Results are reported for ethylene, 1, 1-difluoroethylene. methyl vinyl ether, ethyl vinyl ether, n -butyl vinyl ether, ketene, ketene-d₂, allene, unsymdimethyl allene, dimethyl ketene, 2-methoxy propene, 1-ethoxy propene, 2-bromo propene, and N, N- dimethyl isobutenyl amine. Chemiluminescence activation energies, representing the cycloaddition process, and absolute quantum yields for singlet excited product, ranging from 10^--⁴ to 2.5 × 10^--². are reported for 10 alkenes. Several of the reactions, 1,1-difluoroethylene, ketene, ethylene and allene give formaldehyde ¹ nπ* product with excess vibrational-rotational energy and a higher quantum yield than reactions not displaying this phenomenon. This is an indication of at least partially statistical partitioning of the energy in excess of that needed to electronically excite the formaldehyde. The experiments with ketene and ketene-d₂ provide the first evidence for the existence of unsubstituted 1,2-dioxetanone. The results from several of the experiments, particularly those with 2-methoxy propene and I-ethoxy propene are consistent with the mechanism of Goddard, which predicts regioselective and stereoselective attack of O₂(¹Δ) upon alkoxy substituted alkenes having allylic hydrogen.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—IX. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRUM OF THE BACTERIOCHLOROPHYLLIDE a ANION RADICAL

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for 19 doublet states of the anion radical of ethyl bacteriochlorophyllide a (Et-BChl a˙), and are employed in a resolution of the electronic absorption spectrum, as well as in a comparison with a previously reported study of the electronic states and spectrum of the anion radical of ethyl bacteriopheophorbide a (Et-BPheo a˙). The lowest two excited doublet states, D₁ and D₂, of Et-BChl a˙ are found to be approximately degenerate and are predicted to contribute to the experimentally observed absorption at 10000 cm^?1. In contrast, the D₂←D₀ transition in Et-BPheo a˙ is predicted to contribute to the 11000 cm^?1 absorption, while the D₁←D₀ transition appears at approximately 8600 cm^?1 with a low oscillator strength (f= 0.002). The prominent visible absorption at ～15700 cm^?1 in both molecules is found to be due to the D₄← D₀ transition. Another difference between the predicted spectra of the two molecules appears in the low-energy shoulder of the Soret band. Here, two intense transitions, D₁₀←D₀ and D₁₁←D₀, are predicted for Et-BChl a˙, as opposed to three fairly intense transitions, D₇←D₀, D₈←D₀ and D₉←D₀, for Et-BPheo a˙, differences which may provide a means of distinguishing between the two molecules using resonance Raman spectroscopy. The remainder of the Soret band of Et-BChl a˙ above 26000 cm^?1 consists of a number of closely-spaced transitions to states D₁₂←D₁₈. The intense transitions D₁₂←D₀, D₁₃←D₀, D₁₄←D₀ are predicted to contribute to the Soret maximum near 30000 cm^?1. The ground state spin densities of the two molecules are similar, with the minor difference of somewhat less spin density located on the methine carbon atoms of Et-BChl a˙ compared with Et-BPheo a˙.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—VII. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRA OF CHLOROPHYLLIDE a AND BACTERIOCHLOROPHYLLIDE a CATION RADICALS

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for 29 doublet states and three quartet states of the cation radicals of ethyl chlorophyllide a (Et-Chl a⁺) and ethyl bacteriochlorophyllide a (Et-BChl a⁺). In Et-Chl a.⁺ I the lowest excited doublet state D₁ is estimated to lie 5220 cm^-1 above the ground state D₀, with a negligibly small D₁← D₀ transition probability. The lowest quartet state, Q₁, is estimated to lie 7980 cm^-1 above D₁. The absorption spectrum up to 20,000cm^-1 is shown to consist primarily of numerous low-intensity ‘background’ transitions, with transitions to D₅ and D₁₁ accounting for the observed peaks at 12,200cm^-1 and 17,500cm^-1, respectively. The large intense band at 25,000cm^-1 is due primarily to transitions to D₂₂ and D₂₃, with numerous lower-intensity transitions to neighboring states. In Et-BChl a.⁺ D₁ is estimated to lie 7112 cm-¹ above D₀, and Q, is approximately 5725 cm^-1 above D. A pair of states, D₃ and D₅, account for the absorption at 11,000 cm^-1, while another pair of states, D₁₃ and D₁₄, are associated with the broad, weak absorption near 20,000 cm^-1. The two prominent intense peaks at 23,700 cm^-1 and 27,700 cm^-1 are assigned to D₂₃ and D₂₈, respectively, while the shoulder located at 25,500cm^-1 is attributed to transitions to D₂₄ and D₂₆. As in Et-Chl at, numerous background transitions are found throughout the spectrum. The π spin density distribution in D₀ of both molecules is similar, with spin density found predominantly on the α-carbon atoms. In both systems, approximately 65% of the π spin density in D₁ is found on the methine carbon atoms, with the remainder found largely on the nitrogen atoms.     

A THEORETICAL TREATMENT OF THE ELECTRONIC STATES OF THIONINE AND RELATED MOLECULES

Abstract— –Problems associated with the protolytic equilibria of thionine and related molecules in their lowest excited electronic states were investigated. The theoretical arguments are based on semi-empirical SCF MO (CI) calculations for the π-electronic system of these molecules; all singly excited configurations were included in the CI. The results indicate that the basic form of thionine in its ground, first excited singlet and lowest triplet state is protonated at the heterocyclic N atom. The difference of the p K values of these three states can be explained in terms of the calculated charge densities. The photochemical reactivity of the lowest triplet of the acidic form of thionine (³TH₂²⁺) differs greatly from that of the lowest triplet of the basic form (³TH⁺). Some arguments for the assignment of nπ* character to ³TH₂²⁺ and ππ* character to ³TH⁺ are advanced.     

SOLVENT EFFECTS ON THE ELECTRONIC ABSORPTION AND FLUORESCENCE SPECTRA OF INDOLE-4-CARBOXYLIC ACID: PROTOTROPIC EQUILIBRIA IN AQUEOUS SOLUTIONS

Abstract— The absorption and fluorescence spectra of indole-4-carboxylic acid in various solvents have indicated that the -COOH group is more planar with respect to the indole ring in the first excited singlet state (S₁) than in the ground (S₀) state. Relatively large Stokes' shifts indicate that polarisability and dipole moment of the molecule are increased predominantly upon excitation. Prototropic reactions in the S₀ and S₁ states are the same. The -COO^- and -COOH+₂ groups are not coplanar in the S₀, but coplanar in the S₁ state. pH-dependent fluorescence spectra have revealed that both protonation and deprotonation of the -COOH group increase the basicity of the molecule upon excitation.     

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 (φ_ISC) 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 (φ_ISC˜ 0.7) which, in conjunction with fluorescence, accounts for the fate of all absorbed photons. For alloxazine (φ_ISC˜ 0.45) and lumichrome (φ_ISC˜ 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 φ_ISC 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.     

PRIMARY PROCESSES IN THE LASER FLASH PHOTOLYSIS AND PULSE RADIOLYSIS OF l-EPHEDRINE

Abstract— Transient absorption spectra produced by laser flash-photolysis of an aqueous solution of ephedrine have been measured under a variety of conditions. Ephedrine was found to photoionise via a biphotonic process. The apparent yield of photoionisation increases with lowering of pH, a value of 8.8 being found for the p K _a associated with this change. The cation radical absorption spectrum has been determined using the techniques of both pulse radiolysis and laser flash photolysis. The extinction coefficient of the cation at 295 nm was determined to be 1.37 × 10⁴ dm³ mol^-1 cm^-1 and 1.2 × 10⁴ dm³ mol-¹cm-¹ by the two techniques, respectively, at pH 11. It is also shown that the rate constant for electron abstraction by the azide radical to form the ephedrine cation is controlled by protonation of the amine group in the side chain. The ephedrine anion radical spectrum and its extinction coefficient at 305 nm were also determined. The excited states responsible for photoionisation and photodegradation are discussed.     

THE IN VITRO PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES-III. ABSORPTION SPECTRA, LIFETIMES AND RATES OF OXYGEN QUENCHING OF THE TRIPLET STATES OF β-CAROTENE, RETINAL AND RELATED POLYENES

Abstract— The triplet-triplet absorption spectra of six polyenes have been characterised using flash photolysis, in the presence of anthracene as sensitizer, and pulse radiolysis, in the absence of a sensitizer. The polyenes include several which contain carbonyl groups whose triplet states, unlike retinal , could not be detected unsensitized by flash photolysis. The triplet lifetimes appear to be a function of the number of conjugated double bonds, and vary between 7 and 14 μ sec. In general, the longer the polyene, the shorter the lifetime. An empirical linear relation was found between the frequencies of the polyene triplet-triplet absorption maxima, and the frequencies of the corresponding ground singlet-singlet maxima. The rate constants for quenching by oxygen of nine polyene triplet states were determined to lie in the range 2–7 × 10⁹ M ^-1 sec^-1. The possible mechanisms for oxygen quenching of triplet states are discussed and analogies between the results for oxygen quenching of polyenes and of polyacenes are drawn. The rate constant for oxygen quenching of all- trans -β-carotene triplet was the same in benzene and hexane.     

ELECTRON TRANSFER FROM THE EXCITED STATE OF TYROSINE TO COMPOUNDS CONTAINING DISULFIDE LINKAGES

Abstract— The flash photolysis of aqueous solutions of tyrosine has been studied in the presence of various concentrations of the cyclic disulfide sodium lipoate (thioctic acid, Na⁺ salt). In addition to the formation of phenoxyl radicals and hydrated electrons (and possibly H atoms) from the photoionization of tyrosine, the characteristic spectrum of the radical anion RSSR^- of lipoate was also observed in neutral as well as in alkaline solutions. From the dependence of these yields upon the concentration of lipoate, it was found that a long–lived triplet excited state of tyrosine, rather than the singlet excited state, is involved in these reactions. The negative radical ions RSSR^- are formed by two distinct pathways: (a) Na⁺–lipoate reacts with the solvated electrons which are ejected from the tyrosine triplets ³Tyr → RO.+ e ^-_aq+ H⁺ followed by e ^-_aq+ RSSR → RSSR^-, and (b) by direct interaction of lipoate with triplet excited tyrosine, resulting in the transfer of a negative charge from tyrosine to the disulfide linkage. At high lipoate concentrations, the singlet excited state of lipoate is quenched, k ₄= 1.6 × 10¹⁰ M ^-1 sec^-1, but this reaction does not lead to the formation of RSSR^- radical ions.     

QUENCHING OF THE FLUORESCENCE OF 8-METHOXYPSORALEN BY PROTONS

Abstract— The quenching of 8-methoxypsoralen (8-MOP) fluorescence by protons was observed to occur at the diffusion controlled rates in aqueous solutions at room temperature. Enhanced basicity of 8-MOP in the excited state compared to the ground state is expected on theoretical grounds. The fluorescence yield. which we determined as 6.3 × 10^--⁴ at pH 1 is surprisingly low and indicative of extremely fast radiationless decay pathways. The fluorescence lifetime of 8-MOP in neutral aqueous solution is on the order of 1–2 ns.     

POLARIZED UV-ABSORPTION SPECTRA OF RETINAL ISOMERS—II. ON THE ASSIGNMENT OF THE LOW AND HIGH ENERGY ABSORPTION BANDS

Abstract The polarized UV-absorption spectra of all- trans retinal and both crystal forms of 11- cis , 12-s- cis retinal (presented in the previous paper, Part I) are analyzed using Lowry-Hudson functions to describe the band profiles. The polarization ratios of the polarized bands is used to determine the direction of the corresponding transition moments. For all- trans retinal the polarization spectra show that the absorption between 23 and 36 X 10³ cm⁻¹ is caused by three overlapping bands labeled S, A and B. For 11- cis retinal the B-band is also clearly resolved whereas the S and A bands are separated with much less certainty than for all- trans retinal.
Comparing these bands with the excited state manifold resulting from semiempirical CI-calculations including double excitations, the S-band could be assigned to the ¹A_g⁻→¹A_g-* and the A-band to the ¹A_g⁻→¹B_u+* transition. However, no transition is found in this manifold which could positively be assigned to the B-band because the transitions predicted in this spectral region have negligible oscillator strengths. In all the crystal spectra a further band C is observed around 39 X 10³ cm⁻¹ which is particularly pronounced in the case of 11- cis retinal. For this band an assignment to the ¹A_g⁻→¹A_g+*-transition is proposed.     

RELATIONSHIP BETWEEN SURVIVAL AND THYMINE-DIMER EXCISION IN ULTRAVIOLET-IRRADIATED ESCHERICHIA COLI

Abstract— The influence of amino acid prestarvation on both the resistance to u.v. light and excision of thymine dimers of bacterial strains E. coli B/r hcr ⁺ thy- trp ^-, E. coli B/r hcr ^-thy^- trp ^-, and E, coli 15 T^- 555–7 thy ^- meth ^- trp ^- arg ^- has been studied.
The prestarvation increased the resistance of all the strains but reasonably inhibited excision of thymine dimers. Thus the enhancement of u.v. resistance after amino acid prestarvation was not due to more complete excision of thymine dimers.     

ABSORPTION SPECTRAL SHIFTS OF CAROTENOIDS RELATED TO MEDIUM POLARIZABILITY

Abstract–Solvent induced absorption spectral shifts of the electronic transition from ground 1 A_g state to the excited 1B_u state in carotenoids have been studied. It is shown that the shift depends only on dispersion interactions in non-polar solvents. In polar media there is just a small extra contribution to the red-shift, due to other forms of interactions. The spectral shifts are well described by the theory, which expresses the shift relative to the gas phase value, as a function of solvent polarizability. The main conclusion is that the dominating mechanism behind the large red-shifted absorbance of carotenoids in the proteinacous environment, in vivo, is the mutual polarizability interactions between the carotenoids and the surrounding medium. The solution-phase values of the dipole moments of the lA_g to 1B_u transitions and the differences of isotropic polarizability between 1B_u and lA_g states of carotenoids in non-polar solvents are calculated and found to be around 13 D and 360 ų respectively. From the great overlap of absorption spectra between carotenoids in quinoline and carotenoids in vivo in purple bacterial antenna complexes, it can be expected that the carotenoids are surrounded by several aromatic amino acids in vivo. Comparisons have been done between the exicted states in carotenoids and in linear conjugated polyenes.     

ACID-BASE EQUILIBRIA OF THE EXCITED SINGLET AND TRIPLET STATES AND THE SEMI-REDUCED FORM OF ACRIDINE ORANGE

Abstract— The acid-base equilibria of the excited singlet and triplet states of acridine orange (AO) were studied by flash-photolysis and fluorometric methods. The dye is a stronger base in the first excited singlet state (pK_s= 13.3) than in the triplet and ground states (pK_r= 10.3: pK_c, = 10.2); acridine orange follows the trend observed with some other heterocyclic compounds, viz. pK_s > pK_r= pK,_c. At room temperature, an anomalous fluorescence occurs from the dye in basic media: the assignment of this emission is discussed.
The semi-reduced dye was studied as a function of pH. In a large pH range (3–14), only the protolytic equilibrium between the cationic (AOH₂⁺) and the neutral (AOH) radicals was observed; the pK value corresponding to this equilibrium was found to be in the range of pH 5–6.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS — V. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL CHLOROPHYLLIDE a AND ETHYL PHEOPHORBIDE a

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying excited singlet and triplet states of ethyl pheophorbide a (Et-Pheo a) and ethyl chlorophyllide a (Et-Chl a), and are employed in an analysis of the electronic absorption spectra of these systems. In both molecules the visible spectrum is found to consist of transitions to the two lowest-lying ¹(π, π*) states, S₁ and S₂. The configurational compositions of S₁ and S₂ in both molecules are similar, and are described qualitatively in terms of a four-orbital model. The S₁← S₀ transition in each case is predicted to be intense, and is largely in-plane y-polarized, while the S₂← S₀ transition is predicted to be extremely weak and in-plane polarized. The orientation of the S₂← S₀ transition dipole is not conclusively established in the present calculations. The Soret band in both molecules is composed of transitions to no less than ten states (S_3-S₁₂ in Et-Chl a and S₃-S₇S₉-S₁₂. and S₁₄ in Et-Pheo a), which exhibit primarily (π, π*) character. The configurational compositions of these states are generally a complex mixture of excitations from both occupied macrocyclic π molecular orbitals and occupied orbitals with electron density in the cyclopen-tanone ring and the carbomethoxy chain. No clear correspondences are evident between respective Soret states of the two systems. Transitions to these states are generally intense and display a variety of in-plane polarizations. Two additional Soret states of Et-Pheo a, S₈ and S₁₃, exhibit primarily (n. π*) character. S₈ is characterized by excitations from u and non-bonding regions of the carbomethoxy chain, while S₁₃ is described by n →π* excitations involving the nitrogen atom of ring II. No corresponding (n, π*) states were found for Et-Chl a. In both molecules the lowest two triplet states, T₁ and T₂, are found to lie lower in energy than S₁. while T, and S₁ are approximately degenerate. The configurational compositions of T₁-T₄ of both molecules are nearly identical, and may be described by a four-orbital model. However, the compositions of T₁-T₄ differ sharply from those of S₁ and S₂. A number of higher-lying ³(π, π*) states of both molecules (T₅-T₁₃ in Et-Chi a and T₈-T₉, T₁₁-T₁₃ in Et-Pheo a) are found to have energies similar to the singlet Soret states, relative to S₀. They are characterized by a complex mixture of configurations which do not include significant contributions involving the four-orbital model. In addition, two ³(n, π*) states of Et-Pheo a, T₁₀ and T₁₄, are found, which are somewhat analogous to S₈ and S₁₃. Additional data presented include the charge distributions and molecular dipole moments of the S₀. S₁, and T₁ states of both molecules, as well as energies and oscillator strengths of computed S_n←S₁ and T_n←₁ transitions.     

EXCITED STATES OF SIX-MEMBERED N-HETEROCYCLES. FLUORESCENCE, PHOSPHORESCENCE AND ACID-BASE EQUILIBRIA OF FIVE MONO- AND DIAZA-PHENANTHRENES IN THE LOWEST EXCITED (τ,τ) STATES

Abstract— Acid-base equilibria of five azaphenanthrenes (1-and 4-aza; 1,10-, 1,7-, and 4,7-diaza) in the two lowest excited (τ,τ) states have been determined by Förster's cycle. Pure electronic levels of the free bases and their conjugate acids have been located from the absorption, fluorescence and phosphorescence spectra. In diazaphenanthrenes the increase of the first pK_a and the decrease of the second pK_a under excitation has been observed, and an explanation is offered. In many cases the reactivity of the molecule excited to the lowest triplet state differs considerably from that in the ground state. The Pariser-Pam-Pople (PPP) calculations of the free bases have been performed. From the theoretical results, the changes of the net charge on nitrogen atoms under excitation and the magnitude of the S-T splitting in the L_a and L_b states have been discussed and compared with experiment.     

SENSITIZED PHOTOREDUCTION OF CO(EDTA)- IN AERATED AQUEOUS SOLUTION. ITS PHOTO-ELECTROCHEMICAL AND PHOTOBIOLOGICAL APPLICATIONS

Abstract— The reduction of Co(edta)^- in aerated aqueous solution containing 1-hydroxy-5-oxo-5H-pyrido[3,2a]phenoxazine-3-calboxylic acid (HPPC) as sensitizer and ethylenediaminetetraacetate ion(EDTA) as electron donor was studied. Semireduced HPPC, formed in a reaction between triplet HPPC and EDTA, was shown to reduce Co(edta)^- to Co(edta)^2-. The ratio of the rate constant for oxidation of semireduced HPPC by oxygen to that by Co(edta)^- was determined to be 0.45± 0.01 from competition reaction kinetics. A (concentration)half-cell (oxidation-reduction electrode) based on the photoreduction of Co(edta)^- was found to give an electrode potential decrease of 100 mV. Co(edta)^- was also found to be reduced during illumination in aerated suspension of chloroplasts. The rate of oxygen evolution for the Co(edta)^- -chloroplast system was nearly the same as that for the Fe(CN)₆^3- -chloroplast system both in the absence and presence of ammonium ions.     

THE EFFECTS OF HIGH-INTENSITY UV-RADIATION ON NUCLEIC ACIDS AND THEIR COMPONENTS—I. THYMINE

Abstract— The set of final products of thymine conversion induced by high-intensity UV irradiation (λ= 266nm, intensity 10²⁴-5 × 10²⁹ photons·s⁻¹·m⁻², pulse duration 10ns) of the dilute aqueous solution to the first approximation is similar to that formed with ionizing irradiation (γ-irradiation of aqueous solution or autoradiolysis of a solid 2-[¹⁴C]-thymine). The data obtained suggest that high-intensity UV-induced photochemical conversion of thymine involves photoionization and/or photodissociation. These processes pass through the higher excited state(s) populated as a result of the second photon absorption by excited (most probably in the T₁ triplet state) thymine molecules.     

Simulation of X-ray absorption near edge spectra of organometallic compounds in the ground and optically excited states

The Mg K-edge and Zn K- and L3-edge X-ray absorption near edge spectra of Mg and Zn porphyrins in the ground state and low-lying optically excited states are calculated. Also computed are X-ray absorption near edge spectra of Fe(II) spin crossover compound in its ground and low-lying optically excited states, motivated by a recent experiment (J. Phys. Chem. A 2006, 110, 38). The calculated absorption spectra of optically excited states can be used to simulate ultrafast optical pump/X-ray probe experiments.     

FLASH PHOTOLYSIS STUDIES OF OROTIC ACID

Abstract— The triplet state of orotic acid has been studied by flash photolysis. The rate for dimerization has been observed to vary from 2 × 10⁹ M ^-1 sec^-1 at pH 1 where both the triplet and ground state molecules are neutral, to under 10⁸ M^-1 sec^-1 above pH 9 where both the triplet and ground state molecules are doubly ionized. The p K of the triplet state has been measured as 4.6. The rate of oxygen quenching for the triplet is 2–3 × 10⁹ M^-1 sec^-1 while the rate of radiationless decay in solution is 0.73 × 10⁴ sec^-1. The triplet absorption spectra have been measured for the two ionic forms of the triplet.