The singlet nπ* states of para-benzoquinone

The vibronic nπ* singlet spectra of p-benzoquinone-h₄ and p-benzoquinone-d₄ have been observed in a supersonic jet and some as yet unknown excited state fundamentals in the vapor phase have been assigned. The electric dipole forbidden, magnetic dipole allowed origin of the ¹B_1g ← ¹A_g transition is observed at 20045 cm^?1. The origin of the¹A_u ← ¹A_g, transition has been indirectly determined at 19991 cm^?1 from the vibronic excitation spectra. Neither shows a deuterium shift.     

Absorption,fluorescence and phosphorescence spectra of the singlet and triplet states of s-tetrazine in the crystal and in mixed crystals at low temperatures

The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The ³B_3u-¹A_g (nπ^*) origin is at 18414 ± 5 cm^?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm^?1 for tetrazine in pyrazine; 17 701 cm^?1 in pyrimidine; and 17 676 cm^?1 in pyridazine. The ^eB_3u-¹A_g (nπ^*) origin is at 14 096 ± 2 cm^?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν_6a) built upon site origins and vibrational fundamentals. The ν_6a interval is: 743 (¹A_g), 715 (³B_3u), and 709 cm^?1 (¹B_3u) in the neat tetrazine crystal; 732 (¹A_g) and 705 cm^?1 (¹B_3u in pyrazine host, 737 (¹A_g) and 701 cm^?1 (¹B_3u) in pyrimidine host, and 732 (¹A_g) and 703 cm^?1 (¹B_3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″_6a)^o_n (i), i indicating the observed s     

Two-photon excitation into low-energy singlet states of anthracene in mixed crystals

The two-photon excitation spectrum of the first excited state of anthracene in fluorene and biphenyl at 4.2 K has been measured. Intensity is induced into the origin by the static dipole moment of fluorene, and into b_1u vibrons through coupling to an A_g state near 29400 cm⁻¹; the nature of this A_g state is discussed.     

Vibronic coupling of pyrene in the first ππ* excited state

The two-photon fluorescence excitation spectrum of pyrene in n-hexane and n-heptane matrices has been measured at 10 K in the region of the first electronic transition (26800–30200 cm^?1). The spectrum consists of a rich number of sharp bands, being in general better resolved in n-hexane than in n-heptane matrix. Shpol'skii multiplets have been observed for the most intense bands. A strong two-photon band dominates the spectrum = 1495 cm^?1 from the 0—0 line and was assigned to B_1u × b_1u = A_g symmetry. Other weaker vibronic origins occur in the spectrum which were correlated to vibrational modes of b_1u, b_2u, b_3u and a_u symmetry. Intense vibronic bands are observed close to the origin of the second electronic transition and were interpreted as combination bands of B_1u × b_1u × b_3g symmetry. A two-photon vibronic theory to account for their intensity is proposed where the electronic moment is linearly expanded in powers of the nuclear displacements.     

Molecular spectroscopy of 2,5-distyrylpyrazine

The molecular absorption and fluorescence spectra of 2,5-distyrylpyrazine (DSP) in a Shpol'skii matrix at 4 K together with the Raman spectrum of crystalline samples at 298 K have been investigated. The transition to the lowest excited singlet state of B_u symmetry is of π—π^* character and two sites with origins at 23621 cm^?1 and 23713 cm^?1 are observed in n-tetradecane at 4 K. The DSP molecule has the same conformation in the two sites and, furthermore, the geometry of the molecule in the first excited singlet states is unchanged as compared to the ground state. The vibrational progression observed for DSP is compared with the literature data for distyrylbenzene (DSB) and slilbene.     

Factor group splitting in the lowest triplet state of p-benzoquinone-d4 crystals

Polarized Stark-modulated Zeeman absorption experiments on p-benzoquinone-d₄ single crystals at 2 K show the factor group splitting in the origin of the lowest B_1g (nπ^*) triplet state at 18649 cm^?1 to be 0.62±0.06 cm^?1. The ordering of the crystal states is such that the orbital plus state lies at higher energy. The absence of a measurable factor group solitting in the ³A_u (nπ^*) state at 12.1 cm^?1 from the origin is taken as a further confirmation of the vibronic nature of this state. The ZFS parameter D of this level is found to be ?10±3 GHz.     

The polarised two-photon excitation spectrum of benzene monocrystals at 6 eV

The two-photon excitation spectrum of a benzene single crystal at 4.2 K has been recorded in the region of the second absorption system. The onset of two-photon absorption occurs near 46 500 cm^?1 (quoted as a two-photon frequency). The spectrum has the appearance of a forbidden transition in that the system begins with weak lines which become dominated by an intense continuum at higher energies. The two-photon cross section at 55 000 cm^?1 (the limit reached in this study) is about 200 times greater than at 47 490 cm^?1 although the peak of this strongly allowed system has not yet been reached. The fwhm of the bands near 47 000 cm^?1 is 280 cm^?1, the same as in the one-photon spectrum at these energies. The polarisation ratio is much the same over the entire energy range, and is consistent with the two-photon operators (xz, yz) or (zz). An analysis of all the data available from the one- and two-photon spectra suggests that the transition is ¹B_1u ← ¹A_1g in which the vibronic intensity is derived from the ¹E_1u state in the one-photon and ¹E_1g in the two-photon spectrum.     

Investigation of the temperature dependence of ruthenocene photoluminescence

Computer analysis of the luminescence decay times of ruthenocene in the range of 4.2 to 77 K has yielded energies and decay parameters for the lowest excited states. The derived level pattern has been rationalized on the basis of a ³E₁ (dd) term, split by ≈ 500 cm^-1 by spin-orbit coupling into A₂ + A₁ + E₁ + E₂ components in order of increasing energy. From a preliminary Franck-Condon analysis of the well-structured luminescence observed at 4.2 K, the molecule is shown to be expanded in the excited A₂ state, primarily along the principal symmetry axis.     

Structured weak absorption spectrum of p-terphenyl microcrystals dispersed in methylcyclohexane glass matrix at 77K

P-terphenyl microcrystals dispersed in a methylcyclohexane glass matrix at 77 K showed a sharp structured weak absorption system at lower energy than the well known strong conjugation band. We assign it to the short-axis polarized transition, ¹B_3u←¹A_g. It is concluded that the lowest excited singlet state is a long-lived state, ¹B_3u, in a planar conformation, whereas it is a short-lived state, ¹A_u, in a twisted conformation.     

The fluorescence of all-trans diphenyl polyenes

The temperature dependence of the fluorescence and fluorescence excitation spectra of all-trans diphenyl hexathene (DPH) and octatetraene (DPO) in six solvents confirms the S₁(¹A_g*) and S₂(¹B_u*) state assignment, and determines their energy difference ΔE. The S₁ fluorescence rate parameter k_F depends on ΔE, the solvent refractive index n, the S₂ (n = 1) fluorescence rate parameter k_F20 (2.23 × 10⁸ s^?1 for DPH, 2.33 × 10⁸ s^?1 for DPO), and the S₂-S₁ coupling matrix element V (745 cm^?1 for DPH, 500 cm^?1 for DPO). The S₁ fluorescence is induced by ¹B_u*-¹A_g* potential interaction (PI), via a b_u vibrational mode (≈ 900 cm^?1), and not by vibronic coupling. The main S₁ radiationless transition, rate parameter k_R, is thermally-activated internal rotation through an angle θ about the central ethylenic bond(s). The PI distorts the S₁ (θ) potential surface and thus influences k_R.     

The two-photon phosphorescence excitation spectrum of triphenylene

The two-proton excited phosphorescence of triphenylene in PMMA matrix at 77 K was measured using a tunable flashlamp-pumped rhodamine dye laser in the effect spectral region 32000–36000 cm^?1. The band origin of the S₀S₂ transition, which is uncertain in the one-photon absorption spectrum, was observed at 33200 cm^?1. The vibronic features in the one-photon spectrum were reinterpreted.     

Two-photon absorption study of Zn(II)tetraphenylporphin: Role of a higher excited singlet state of gerade symmetry

The S₂ state fluorescence of Zn(II)tetraphenylporphin has been studied by using two-photon absorption and optical—optical double-resonance techniques. The main process to populate the S₂ state was found to be a stepwise two-photon absorption to the S_nstate through the S₁ state. The large absorption cross section of the S_n ← S₁ transition (6.8 × 10^?16 cm² molecule^?1) at 540 nm suggests that there exists a higher excited singlet state of gerade parity.     

Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0_b (A_g → A_u) transition lies 50–100 cm^?1 above the bottom of the exciton band and the 0-0_c′ (A_g → B_u) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm^?1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0–1 vibronic transition. As a result of the 0–1_c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm^?1 above the first system. It is tentatively assigned as |a(A_g → B_u) charge transfer exciton transition in agreement with earlier observations.     

Absorption and emission spectra of Ca2 in solid krypton

The calcium van der Waals molecule, Ca₂, has been formed by codepositing calcium and krypton atoms on a substrate at 12 K. Absorption spectra revealed a structured band at 666 nm with 117 ± 2 cm^? spacings. Calcium-44 isotopic spectra confirmed the assignment and located the band origin at 14 432 ± 4 cm^?1. Emission spectra, pumping the 11 ← 0 absorption, exhibited a very strong 14 000 cm^?1 band with 78 ± 2 cm^?1 spacings to the band origin and unrelaxed emission with 78 ± 2 and 117 ± 2 cm^?1 spacings above the band origin from v′ levels up to 6. The transition ¹∑⁺_u(¹S + ¹P) ← ¹∑⁺_g (¹S + ¹S) and the bonding in ground and excited Ca₂ states fit a simple molecular orbital model.     

RESONANCE RAMAN SPECTRUM OF THE EXCITED 21Ag STATE OF ß-CAROTENE

Abstract— Resonance Raman (RR) bands assignable to the 2¹A_g excited state of ß-carotene are recorded using picosecond time-resolved resonance Raman (PTR³) spectroscopy. The RR spectrum contains bands in both the C-C (1204 cm^?1, 1243 cm^?1, and 1282 cm^?1) and C=C (1777 cm^?1) stretching regions. The time-dependent intensities of these RR features, measured with ? 30 ps. resolution, are found (i) to closely correlate with picosecond transient absorption (PTA) data recorded at 575 nm on the same sample and (ii) inversely correlate with the time-dependent intensities of RR bands assigned to the 1¹A_g ground state. Both of these observations support the assignment of these four RR features to the 2¹A_g excited state. These results remove uncertainties associated with earlier experiments in which excited-state RR scattering from (3-carotene was not observed in spite of predicted trends emanating from studies of shorter polyene compounds. The observed C=C band position also agrees with a recent report of this feature.     

Photon-echo relaxation measurements with two dye-lasers application to pentacene-h14 and -d14 in p-terphenyl-h crystals at 1.5 K

Electronically controlled photon-echo relaxation measurements, using two nitrogen pumped dye-lasers, are reported for mixed crystals of pentacene-h₁₄ and -d₁₄ in p-terphenyl-h₁₄ at 1.5 K. In dilute mixed crystals (ca. 10^?8 M) the photon-echo lifetime is found to be exclusively determined by the fluorescence lifetime. Homogeneous linewidths of 7.1 and 5.9 MHz at 1.5 K are then calculated for the electronic origin of the ¹B_2u ← ¹A_1g transition of pentacene-h₁₄ and -d₁₄ respectively. The decrease in photon-echo lifetime at higher guest concentrations (ca. 10^?7 M) is ascribed to energy transfer between excited and neutral guest molecules.     

Electronic absorption spectrum of Fe3+ doped in ammonium chloride single crystal

The electronic absorption spectrum of the Fe²⁺ ion doped in ammonium chloride has been studied at room and liquid air temperatures. The observed bands have been assigned transitions from the ground ⁶A_1g(S) state to the excited ⁴A_1g(⁴E_g), ⁴T_1g(G) and ⁴T_2g(G) states. The cubic field approximation with D_q = 675 cm^?1, B = 645 cm^?1 and C = 4.4 B is found to give a good fit to the observed band positions.It is further concluded that the site symmetry of the Fe³⁺ ion in the crystal is lowered from O_h to C_4v symmetry at liquid air temperature.     

Polarization and assignment of the two-photon excitation spectrum of benzene vapor

The two-photon excitation (TPE) of benzene fluorescence in the vapor phase at 60 torr is reported for the total-energy region from 38 086 cm^?1 to 42 441 cm^?1 using both circular and linear polarized light from a nitrogen-pumped dye-laser. The theory of the polarization dependence of the vibronic transitions in benzene is briefly reviewed, and it is seen how transitions involving vibrations of b_1u symmetry are expressly forbidden for this type of TPE experiment in which the two photons are identical. Five vibronic origins with distinctive rotational contours and polarization dependence are identified in the TPE spectrum. The υ₁₄(b_2u) vibronic origin at 1570 cm^?1 (above the electronic origin of the ^IB_2u state) stands out very prominently in the linear polarized spectrum, but nearly disappears in the circular polarized spectrum. This striking polarization dependence indicates a significant contribution of A_2u electronic states to the intermediate states of this TPE vibronic transition. The relatively great strength of the υ₁₄ band may be due to vibronic borrowing by the b_2u mode from the ground electronic state (A_1g).     

The state energy and the displacements of the potential minima of the 2Ag− state in all-trans-β-carotene as determined by fluorescence spectroscopy

The fluorescence spectrum of all-trans-β-carotene was recorded at 170 K. The 1B_u⁺ → 1A_g⁻ fluorescence exhibited clear vibrational structures constituting a mirror image with those of the 1B_u⁺ ← 1A_g⁻ absorption, and the deconvolution of the entire spectrum identified the 2A_g⁻(0) → 1A_g⁻(0) transition at 14 500 cm⁻¹. The displacements of the 1B_u⁺ and 2A_g⁻ potential minima along ν₁ and ν₂ (the CC stretching and C–C stretching normal coordinates, respectively) were determined to be 1.2 and 0.9, and 1.6 and 1.5, respectively. Thus, much larger potential displacements in the 2A_g⁻ state than in the 1Bu⁺ state have been shown.     

Zeeman experiments on the 3Eu α 1A1g transition of platinum porphein in an n-alkane single crystal at 4.2

We report absorption spectra from the ground state to the photoexcited triplet state of platinum porphin (PtP) in single crystals of n-octane (C₈) and n-decane (C₁₀) at 4.2 K, with and without a magnetic field. For PtP in C₁₀ the same transition was studied in emission. From the experiments, values are derived of the spin-orbit coupling parameter Z, the crystal field splitting δ and the orbital angular momentum A for PtP in the two hosts: Z = 76 ± 2 cm^?1 (C₈, C₁₀), δ = 71 ± 1 cm^?1 (C₈), 55 ± 1 cm^?1 (C₁₀) and A = 1.6 ± 0.1 (C₈, C₁₀). For the ratio of the in-plane and the z-polarized electric dipole transition moments we obtain ¦Mx,y¦/¦Mz¦=76± 0.3 (C₈).