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     

Reexamination of the phosphorescence of Ba2Pt2(H2P2O5)4 via thermally modulated emission spectroscopy

Reexamination of the phosphorescence of Ba₂Pt₂(H₂P₂O₅)₄ reveals that the ≈10 K spectrum is a superposition of two electronic transitions [³A₂u(E_u,A_1u → A_1g] separated by ≈40 cm^?1. Each band displays a prominent 110 cm^?1 vibrational progression. Franck-Condon analysis yields a ≈0.25 Å distortion of the PtPt bond in the excited states, interpreted as a contraction.     

Isotopic mixed crystal exciton spectra in the far infrared: Naphthalene

The far infrared vibrational exciton spectra of isotopic mixed crystals of naphthalene-h₈ and d₈ were studied. The two observed translational phonon modes were determined to fall into the amalgamated band limit while the lowest energy B_3u, A_u and B_1u vibrational exciton bands were found to be in the separated band limit. The lowest energy B_3u “butterfly” mode with its large (15 cm^?1) exciton splitting was found to agree well with CPA calculations of mixed crystal spectra. A peak at 185 cm^?1 was also assigned as a peak in the vibrational exciton density-of-states of the B_3u mode.     

Electronic structure of Re2Cl82−

The multiple scattering Xα method has been used to calculate the ordering of both occupied and unoccupied one-electron energy states of Re₃Cl₈^2?. Single crystal polarized electronic spectra of [(n-C₄H₉)₄N]₂[Re₂Cl₈] have been measured at 5 K. Principal band maxima are observed at 14 180 (z), 30870 (xy), and 39 215 (z) cm^?1. The calculation, observed polarizations, and a comparison of band positions in Re₂Cl₈^2? and Re₂Br₈^2? suggest the following transition assignments for the former complex: 14 180 cm^?1, b_2gδ → b_1uδ*; 30 870 cm^?1, e_g → b_1uδ*; 39 215 cm^?1, e_uπ → e_gπ*.     

Exciton dynamics in molecular crystals from line shape analysis. Spectral moment analysis of the origin region of the 4000 Å transition of crystal anthracene

A spectral moment analysis of the line shape function ω?₂(ω) in the region of the (0—0) band of the 4000 ŹB_2u ← ¹A_g transition in crystal anthracene at various temperatures is performed. The data are compared with the predictions of three coupling models, viz., weak exciton—photon with weak exciton—phonon coupling, strong exciton—photon with weak exciton—phonon coupling, weak exciton—photon with strong exciton—phonon coupling. The terms contributing to each spectral moment for each model are rendered explicit. The experimental data indicate that the exciton—intermolecular phonon coupling is primarily weak. The exciton interacts with optical phonons of about 90 cm^-1 frequency with a coupling strength of about 140 cm^-1 , a value near that predicted by a weak coupling model. The coupling strength is nearly the same irrespective of whether the exciton is created by b- or a-polarized light probably indicative of the importance of higher multipole contributions to the coupling although the existence of strong interband scattering could affect that suggestion. The coupling parameters g_op and g_ac are about 10^-1 and 10^-4 respectively.     

Dissociation of OCS in liquid argon excited by an electron beam. Evidence of S(1S → 3P) and S2(B 3Σ−u → X 3Σ−g) emissions

Optical emission from e-beam excited liquid argon doped with OCS consists of a prominent S₂(B ³Σ^?_u → X ³Σ^?_g) band progression (v′ = 0 to v″ = 5–18 and v′ = 1 to v″ = 4–8), similar to the observation made in an argon matrix, but with a lesser red shift. The time decay of these bands exhibits a fast component (<0.5μs) and a long non-exponential one, extending to 1 ms, that appears to be due to recombination of S(³P) atoms: S(³P) + S(³P) → S₂(B ³Σ^?_u). Spectral study of the slow component (r > 5 μs) shows a peak at 456 nm identified as the S(¹S → ³P) transition. A possible mechanism for this behavior 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.     

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.     

Fluorescence and recombination-emission spectra from S2 formed by ultraviolet laser photolysis of CS2 in an argon matrix at 15 k

Two series of emission bands were observed for the CS₂/Ar(1 : 100–500) system at 15 K with excitation at 257.3 nm. They are assigned to B³Σ^?_u → χ³Σ^?_g and B″³Π_u → X³Σ^?_g of S₂, which was formed by photodissociation of CS₂, CS₂ + hv → CS + S, followed by recombination of two S atoms. The B″³Π_u state has been found 524 cm^-1 lower in energy than B³Σ^?_u     

On the lower excited electronic states of glyoxal

The polarization of both nπ* absorption bands of glyoxal has been measured in a glass matrix of 2-methyltetrahydrofuran by the photoselection method. The second absorption band in the 30 000 cm^?1 region has been assigned to a ¹A_g → ¹B_g nπ* transition. Its intensity is mainly induced by interaction with the solvent. An absorption band at about 43 000 cm^?1 has been ascribed to a charge transfer transition in complexes of glyoxal and 2-MTHF.     

Spectrum of acetylene: 1650–1950 Å

A discontinuity in ¹³C-isotope shifts is used to fix the location of a diffuse 00 of gaseous ¹²C₂H₂ at 54 116 cm^?1. The shifts and analogy with a known band system of HCN suggest the assignment B? ¹B_u - X? ¹ Σ⁺_g for the new electronic transition.     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

Electronic relaxation of biacetyl in a supersonic jet

Emission spectra have been recorded and decay times measured for biacetyl selectively excited in the 0⁰₀ band of the S₁S₀ (¹A_u¹A_g) transition. The spectrum of the “long emission” corresponds to a superposition of the fluorescence and of the “hot” phosphorescence. The results may be treated in terms of a uniform distribution of the singlet oscillator strength among the quasi-stationary levels, in the absence of vibrational redistribution on a microsecond scale.     

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.     

Optical and ODMR investigation of the low lying excited states of neat α-oxalic acid dihydrate

The lowest excited singlet and triplet states of neat α-oxalic acid dihydrate have been investigated by optical, optical Zeeman, and zero-field optically detected magnetic resonance (ODMR) spectroscopy at T ? 4 K. The observed electronic transitions in absorption are assigned as ¹A_u ← ¹A_g (ν₀ = 34131 cm^?1) from its normal polarizatio These correspond to the expected lowest lying ^1,3nπ* excitations in trans-α-dicarbonyls. The ³A_u → ¹A_g phosphorescence is also observed. Monitoring the phosphorescence intensity, the fine structure splittings and principal axes' orientation and the kinetic parameters of the ³A_u s The fine structure constants are X = 2510.0, Y = ?1800.3, and Z = ?709.7 MHZ where the x axis is in-plane and parallel to the carbo The absolute signs of the constants have been established by optical Zeeman measurements. The τ_x zero-field spin state has the largest total phosphorescence rate, radiative rate, and populating rate. The τ_x activity in the 0 - 0 band is polarized mainly along the x axis. However, considerable normal polarization associated with an in-p     

Fluorescence spectrum of the benzene radical cation in solid argon

Argon/benzene samples were condensed at 12 K with continuous argon resonance radiation. Laser excitation at 421 nm produced a weak emission with structure at 19770, 19140 and 18290 cm^?1, assigned to the ²A_2u → ²E_1g emission of C₆H₆⁺. Observation of 630 and 1480 cm^?1 intervals for the vibrations v₁₈ (e_2g) and v₆ (e_2g), respectively, supports this assignment.     

Assignment of the vibrational bands of the B1u ← A1g transition in solid benzene films

Low temperature absorption spectra of benzene films were observed in the ¹B_1u ← ¹A_1g transition region. The origins of the two progressions of the totally symmetric vibration v₂(a_1g) are assigned to the crystal-field-induced 0—0 transition and to the false origin 0 + v_1g(e_2g).     

Vibrational relaxation in neat crystals of naphthalene by picosecond cars

Picosecond delayed CARS experiments on totally symmetric modes in naphthalene at 1.5 K are reported. The Raman lineshape of the vibrational excitons is lorentzian and vibrational relaxation can be surprisingly slow. The Raman lineshape of the A_g exciton level of the 766 cm^?1 vibrational mode reveals that the low-temperature lorentzian lineshape occurs by motional narrowing At higher temperature the exciton is trapped by interaction with lattice phonons.     

The radiative lifetime of the A1II state of BH

Second-order polarization propagator calculations of the X¹Σ⁺ → A¹II transition moment as well as the radiative lifetime of the A¹II state of BH are reported. The calculated vibrational lifetimes are τ(v′ = 0) = 121 ns, τ(v′ = 1) = 129 ns, and τ(v′ = 2) = 137 ns. The τ(v′ = 0 lifetime agrees with the most recent experiment of τ(v′ = 0) = 125 ± 5 ns. We show that the electronic oscillator strength computed at the ground state equilibrium is rather different from the band absorption oscillator strength f₀₀, which demonstrates that theoretical electronic oscillator strengths should not be expected to agree with experimentally determined band oscillator strengths.     

The structure of the dithionite ion

The Raman spectra of aqueous and solid sodium dithionite have been recorded. Differences in the location, intensity, and number of observed bands are attributed to conformational changes in the dithionite ion. The structure of the aqueous ion is non-planar with a C_2h symmetry with an SS bond distance estimated to be 0.220–0.226 nm, as opposed to the dithionite structure in the Na₂S₂O₄·2H₂O salt which is known to have C_2ν structure with a bond distance of 0.2389 nm. The Raman spectra of aqueous dithionite are assigned to A_g (SO) = 997 cm^?1; B_g (SO) at 912 cm^?1, B_g SO₂ twist at 324 cm^?1. The remaining bands are a strong A_g, the SO₂ wag, the SO₂ scissor, and the SS stretch at 584, 461, and 232 cm^?1, respectively, but due to coupling all three motions are expected to exhibit substantial SS character. The variation of the spectra of the solid and aqueous sodium dithionite indicate strong environmental effect on the structure of the anion.