Electronic to vibrational energy transfer and relaxation in matrices. I. Hg in N2 matrix

Under KrF laser (249 nm) excitation of Hg atoms in a N₂ matrix in the temperature range 12–25 K we observed (i) a strong, broad-band fluorescence in the near-UV region assigned to a (Hg-N₂)^* exciplex (decay time from 800 to 100 μs) and (ii) infrared Δυ = −1 emission from high vibrational levels (υ = 6−12) of N₂ molecules in the electronic ground state, populated by the electronic-to-vibrational energy transfer (decay time 7 s on υ = 10−12 at 14 K). From time-resolved UV and IR spectra, one can conclude that the predominant part of the EV transfer takes place from thermally non-equilibrated levels, on a short time scale. This is unexpected in view of gas phase data and preliminary ab initio potential energy surface calculations. Temperature effects are discussed. Excited N₂ levels decay along a radiative path (predominant for the highest levels) and by VV transfer to N₂(υ=0) molecules with a rate decreasing rapidly with υ but strongly increasing with the sample temperature.     

Pulsed,Crossed, Supersonic Molecular Beam Studies of Refractory Atom Reactions

Dynamics of refractory atom reactions have been studied with a crossed beam apparatus combining two pulsed, supersonic molecular beam sources, a pulsed UV laser for creating the refractory atoms in the gas phase by laser ablation, and a pulsed dye laser to probe the reaction products by laser-induced fluorescence. Examples of the A1(²P_j) + O₂(X³∑_g)→ A10(X²∑⁺) + O(³P_j), Mg(¹S_o) + N₂O(X¹∑⁺) → MgO(X¹∑⁺,a³Π) + N₂(X¹∑_g⁺) andC(³P_j) + NO(X²Π_r) → CN(X²∑⁺) + 0(³P_j) systems are given. Comparisons with the studies performed using the conventional steady-state beam approach are made.     

Spin—orbit relaxation of Pb(3P01) by atomic hydrogen

Lead atoms in 7 ³P⁰₁ state were generated by 405.8 nm laser irradiation of metastable Pb(³P₂) in a series of premixed H₂/N₂/O₂ flames. Fluorescence was observed from both the ³P⁰₁ and ³P⁰₀ states. The ratio of the intensities of fluorescence from the two states varied with flame parameters in a manner which indicated that spin—orbit relaxation of Pb(³P⁰₁) occurred rapidly in collisions with atomic hydrogen. The rate constant for this process is estimated as 3 × 10^?10 cm³ molecule^?1 s^?1 at 1800 K.     

A violet emission from optically excited mercury vapour. Kinetics of the formation and decay of excited dimers and trimers

A new electronic systems has been observed from excited Hg vapour, which is assigned to collisionally induced emission from the Hg₂ O^±_g first excited states of the dimer: Hg₂O^±_g + M → 2Hg(6 ¹S₀) + M + hv(γ_max 3950 A). For M = N₂, the rate coefficient is 5.3(±0.7) × 10^?19 cm³ molecule^?1 at 298 K. From time resolved measurements of the luminescence in the afterglow following pulsed excitation, the decay rate of the green emission, in an excess of N₂, is shown to be a linear function of [Hg][N₂]. It is concluded that the reaction which controls the decay of the excitation is formation of an excited trimer in a termolecular reaction; the trimer is the carrier of the green emission: Hg₂ O^±_g + Hg(6 ¹S₀ + Hg(6¹S₀ + N₂ → Hg₃³Π_u + N₂. The rate coefficient is 1.10(±0.07) × 10^?30 cm⁶ molecule^?2 s^?1 at 298 K.     

Temperature dependence of radiationless processes. Isoquinoline in solution

The temperature dependence of the fluorescence quantum yield φ_f, the fluorescence lifetime τ_f, and the oscillator strength f(S₀→S₁) of isoquinoline in solution has been measured between room temperature and 77 K. Following an Arrhenius type expression, φ_f in ethanol increases from 0.012±0.002 at 295 K to 0.61±0.03 at 77 K paralleled by an increase of τ_f from 0.25±0.10 ns to 9.0±0.2 ns. Over the same temperature range f(S₀→S₁) and the radiative fluorescence lifetime remain constant. By analyzing the temperature dependent data, it is shown that a spin-allowed internal conversion process with an activation energy of ～1500 cm^?1 is responsible for the observed temperature effect. A mechanism is proposed based upon a thermally activated depopulation of the S₁(ππ^*) state of isoquinoline via a slightly higher state, presumably the S₂(ππ^*) singlet state. An extremenly fast process involving the dissociation of the hydrogen bond deactivates this latter state, by possing S₁.     

Sensitization of the OH(A 2Σ+ → X 2Πi) emission by Hg(6 3P0) metastables

The electronic energy transfer process Hg(6 ³P₀) + OH(X²Π_i, υ = 0,K) → Hg(6 ¹S₀) + OH(A ²Σ⁺, υ,K) has been studied by the sensitized fluorescence method. A rather broad spectrum of rotational population, N_υ′K′, was obtained under conditions of minimum relaxation, which illustrates the non-resonant and non-optical nature of this energy transfer process. The fractions of the exoergicity, above electronic excitation of OH(A ²Σ⁺, υ = 0, K = 0), going into vibrational, rotational and translational excitation are 0.11, 0.31, and 0.58, respectively. A statistical mode of energy partitioning, such as would result from long-lived complex formation, seems to account well for these observations.     

Combination of metastable and ground-state mercury atoms into ungerade dimer states

The time evolution of the 2537 Å sensitised luminescence of mercury vapour has been examined at 304 K. With a trace of N₂ to generate Hg(6³P_o) and an excess of Ar to chaperon dimer formation, full time profiles of the ultraviolet and green bands have been recorded. The ultraviolet emission is of much shorter duration than the green with these gas mixtures. The reservoir of the ultraviolet band is the O^?_u state of the mercury dimer which correlates with Hg(6³P_o), Hg(6¹S_o) and is generated in the combination of metastable and ground-state mercury atoms. The Hg₂ O^?_u is efficiently relaxed to the O^±_g firstexcited states of the dimer in collision with Hg atoms, but rather inefficiently in collisions with Ar atoms. The rate coefficient for formation of excited trimers in the reaction Hg₂ O^±_g+ Hg(6¹S_o) + N₂ → Hg^*₃ + N₂ has been recorded as 1.95(±0.1) × 10^?30 cm⁶ molecule^?2s^?1 at 304 K.     

EXCIPLEX STUDIES–V. ELECTRON EJECTION FROM INDOLE AND METHYL-INDOLE DERIVATIVES*

Abstract— Electron-scavenging experiments with N₂O as scavenger demonstrate at least two electron-producing reactions of the excited singlet states of the exciplex species formed by indole or 1 -methyl-indole with water. Most electrons reacting with N₂O result from collision of the scavenger with a metastable state formed from the initial exciplex state but finite electron yields from indole and 1-methyl-indole at limiting scavenger concentrations suggest that the intermediate states also eject electrons directly into the solvent. The formation of the first metastable state from the fluorescent exciplex state has an activation energy, E_M, estimated to be about 13 kcal/mole for both indole and 1 -methyl-indole water exciplexes. The E_M values for 1-methyl-indole from fluorescence and electron yields are the same, Indicating that at neutral and alkaline pH fluorescence quenching and electron extraction are both being controlled by the formation of the first metastable intermediate. Observed electron yields from indole-water and indole-methanol exciplexes are less than predicted using fluorescence data, although E_M values of 1 kcal/mole are obtained for the indole-methanol exciplex by both methods. At pH 12·0 and 28°C the total electron yields for indole-water and 1 -methyl-indole-water exciplexes are 0·30 and 0·25, respectively. The residual yields attributed to outright formation of hydrated electrons from the initial exciplex excited stateare 0·11 and 0·05, respectively. Electron yields from the indole-water exciplex are strongly pH dependent only near pH 1 where the fluorescence yields as well as the electron yields decrease rapidly with increasing acidity. The 1-methyl-indole-water exciplex shows an additional pH dependence which is first-order in hydrogen-ion activity and has an effective pK_a of about 11·5. Comparable yields for indole and 1-methyl-indole are found only above pH 12. High electron yields are found with indole in the exciplex-forming solvent dioxane and in the non-exciplex forming solvent cyclohexane. For the latter system electrons are probably derived only from the lowest excited state of indole on collision with N₂O.     

Electronic energy transfer from the S2 state of rhodamine 6G

In this paper we present the results of an experimental study of intermolecular electronic energy transfer (EET) from the short-lived Second excited singlet state of rhodamine 6G (R6G) to the ground state of 2,5-bis [5′-tert-butyl-2-benzoxazolyl] thiophene (BBOT). The S₂ state of the donor was excited by sequential, time-delayed, two-photon excitation (STDTPE) utilizing the second harmonic and the first harmonic of a mode-locked Nd³⁺: glass laser, while the EET process was interrogated by monitoring the enhancement of the S₁ → S₀ fluorescence of BBOT. The enhancement of the fluorescence intensity of BBOT was found to be linear in the energies of the two exciting pulses, and linear in the concentration of the energy acceptor (over the BBOT concentration range of (0.3–7) × 10^?5 M), which is in accord with the predictions of the Forster—Dexter mechanism for resonant EET from an ultrashort-lived donor state at low acceptor concentrations. Quantitative measurements of the S₂ → S₀ fluorescence yield in R6G solution directly excited by STDTPE and of the S₁ → S₀ fluorescence of BBOT from R6G + BBOT solutions resulting from EET led to the values of Y_D(S₂ → S₀) = (2.1 ± 0.5) × 10^?6 for the emission quantum yield of the S₂ state of R6G and τr_D(S₂) ≈ 3 × 10^?14 s for the lifetime of the metastable S₂ state of this molecule.     

Quenching cross sections for the deactivation of metastable mercury 6 (3P0)

By monitoring the relative concentrations of mercury ³P₀ atoms using absorption photometry, absolute rate constants have been measured for the excimer-forming reaction Hg (³P₀) + Hg + N₂ → Hg₂* + N₂ and for the quenching of Hg (³P₀) atoms by ethane, propane, deuterated propanes, and nitrous oxide.     

Sensitization of the CN(B2Σ+ → X2Σ+) emission by Hg(63P0) metastables

The vibrational analysis of the CN(B²Σ⁺ → X²Σ⁺) emission sensitized by Hg(6³P₀) metastables has shown that the energy transfer process, Hg(6³P₀) + CN(X²Σ⁺) → Hg(6¹S₀) + CN(B²Σ⁺), populates the CN(B²Σ⁺) state in a non-Franck-Condon fashion. The relative vibrational populations for the ν = 0 to 4 states are 1.00, 0.56 ± 0.06, 0.26 ± 0.03, 0.11 ± 0.03 and 0.04 ± 0.01, respectively. Long-range attractive interaction between the Hg(6³P₀) atom and the CN(X²Σ⁺) radical is evidenced by the observed high rotational excitation of the CN(B²Σ⁺) radical following the energy transfer process.     

Short-chain model studies on emission properties of poly(9-vinylphenanthrene-co-p-N,N-dimethylaminostyrene): Exciplex quenching and subsequent formation of another fluorescent exciplex

The emission properties of 1-(4-dimethylaminophenyl)-3-(9-phenanthryl)propane, Ph(CH₂)₃ DMA, and 1,3-di-(9-phenanthyrl)propane, Ph(CH₂)₃Ph, were studied in comparison with those of poly(9-vinylphenanthrene-co-p-N,N-dimethylaminostyrene). Ph(CH₂)₃DMA showed an intense intramolecular exciplex fluorescence in dioxane. Ph(CH₂)₃Ph did not exhibit a clear intramolecular excimer fluorescence. The quenching of the intramolecular exciplex by several electron acceptors was studied. As a result moderate electron acceptors, such as cyanobenzene, methyl benzoate, and acrylonitrile, selectively quenched the intramolecular exciplex, and in the case of cyanobenzene the subsequent formation of another fluorescent exciplex was observed. The results were discussed in terms of the reduction potentials of electron accepting quenchers.     

Mercury-rare gas van der Waals complexes: From the lightest HgHe to the heaviest HgXe

Mercury helium and mercury xenon van der Waals complexes have been observed in a supersonic free jet expansion by laser-induced fluorescence in the vicinity of the Hg(³P₁¹S₀) transition. The helium complex very weakly bonded in the ground state (D₀ = 8 ± 1 cm⁻¹) exhibited a simple rotational structure. The strongly bonded xenon complex has shown a clear example of isotopic broadening in its vibronic spectra.     

Formation of hot hexafluorobenzene in the 193 nm photolysis

Time-resolved absorption spectra of hexafluorobenzene vapor have been observed with ArF laser (193 nm) excitation. The initial intermediate is postulated to be due to HFB2(S₀) (hot hexafluorobenzene, with internal energy of 639 kj/mol) because the transient spectrum can be simulated as part of the S₃(¹E_1u) ← S₀ transition at 3050 K.     

Polysulfonylamine. XXXVII. Darstellung von Quecksilberdimesylamiden. Kristall- und Molekülstrukturen von Hg[N(SO2CH3)2]2, Hg[{N(SO2CH3)2}2(DMSO)2] und Hg[{N(SO2CH3)2}2(HMPA)]

Polysulfonyl Amines. XXXVII. Preparation of Mercury Dimesylamides. Crystal and Molecular Structures of Hg[N(SO₂CH₃)₂]₂, Hg[{N(SO₂CH₃)₂}₂(DMSO)₂], and Hg[{N(SO₂CH₃)₂}₂(HMPA)] Hg[N(SO₂CH₃)₂]₂ ( 1 ) and Hg₂[N(SO₂CH₃)₂]₂ ( 2 a ) are formed as colourless, sparingly soluble precipitates when solutions of Hg(NO₃)₂ or Hg₂(NO₃)₂ in dilute nitric acid are added to an aqueous HN(SO₂CH₃)₂ solution. By a similar reaction, Hg₂[N(SO₂C₆H₄ ? Cl? 4)₂]₂ is obtained. 1 forms isolable complexes of composition Hg[N(SO₂CH₃)₂]₂ · 2 L with L = dimethyl sulfoxide (complex 3 a ), acetonitrile, dimethyl formamide, pyridine or 1,10-phenanthroline and a (1/1) complex Hg[N(SO₂CH₃)₂]₂ · HMPA ( 4 ) with hexamethyl phosphoramide. Attempted complexation of 2 a with some of these ligands induced formation of Hg⁰ and the corresponding Hg^II complexes. Crystallographic data (at -95°C) are for 1: space group 14₁/a, a = 990.7(2), c = 2897.7(8) pm, V = 2.844 nm³, Z = 8, D_x = 2.545Mgm^?3; for 4a: space group P1 , a = 767.8(2), b = 859.2(2), c = 925.2(2)pm α = 68.44(2), β = 86.68(2), γ = 76.24(2)°, V = 0.551nm³, Z = 1, D_x = 2.113 Mgm^?3; for 4: space group P2₁/c, a = 1041.3(3), b = 1545.4(3), c = 1542.5(3) pm, β = 100.30(2)°, V = 2.474nm³, Z = 4, D_x = 1.944Mgm³. The three compounds form molecular crystals. The molecular structures contain a linear or approximately linear, covalent NHgN moiety; the Hg? N distances and N? Hg? N angles are 206.7(4) pm and 176.3(2)° for 1, 207.2(2) pm and 180.0° for 3a, 205.7(4)/206.7(4) pm and 170.5(1)° for 4. In the complexes 3a and 4, the 0-ligands are bonded to the Hg atoms perpendicularly to the N? Hg? N axes, leading in 3a to a square-planar trans-(N₂O₂) coordination with Hg? 0 261.2(2) pm and N? Hg? O 92.3(1)/87.7(1)°, in 4 to a slightly distorted T-shaped (N₂O) geometry with Hg? 0 246.2(4)pm and N? Hg? 0 96.7(1)/92.0(1)°. In all three structures, the primary coordination is extended to a severely distorted (N₂O₄) hexacoordination by the appropriate number of secondary, inter- and/or intramolecular Hg…?0 inter-actions (0 atoms from sulfonyl groups, Hg…?O distances in the range 280—300pm). The intramolecular Hg…?O interactions give rise to nearly planar four-membered [HgNSO] rings. The molecule of 1 has a two-fold axis through the bisector of the N? Hg? N angle, the molecule of 3a an inversion center at the Hg atom. The molecule of 4 has no symmetry.     

Triplet splitting of the 1p state of mercury atoms in inert gas matrices

The effect of O₂ and N₂ impurities on the spectrum of Hg atoms in solid krypton matrices at 20°K has been investigated. The relative intensity of the components of Hg ¹P⁰₁ are shown to depend strongly on the amount of O₂ or N₂ in the matrix. Under certain conditions, the long wavelength triplet component can be completely eliminated. It is shown that the triplet arises from the superposition of spectra of atoms trapped at three distinct trapping sites.     

Author index to volume 66

Collisional quenching processes of Cd(¹P₁) have been studied directly, using dye-laser excitation and time-resolved fluorescence. Absolute cross sections for the quenching of Cd(¹P₁) by several gases have been determined, and the branching ratios for production of Cd(³P_J) for the quenching molecules N₂, CH₄, C₃Hg, and i-C₄H₁₀ have been shown to be close to unity.     

Photophysical Properties of Pyrazinopsoralen,A New Monofunctional Psoralen*

Abstract— Pyrazinopsoralen (PzPs), a new monofunctional psoralen, has a UV absorption spectrum similar to other psoralens except that it absorbs more strongly in the long-UVA than 8-methoxypsoralen. The solvent effects on the UV absorption and fluorescence emission spectra indicate that the lowest excited singlet state is the π,π* state like other psoralen derivatives. It shows a much lower fluorescence quantum yield (0.0008 in ethanol at room temperature) than the other psoralens as expected by the increased proximity effect (vibronic perturbation) due to close ¹(n,π*) to ¹(π,π*) states. The fluorescence lifetime was 1.05 ns in methylcyclohexane with a single exponential decay, while more than two components were observed in other solvents with the short-lived component being the major (>95%). The triplet state of PzPs could not be detected by phosphorescence, laser flash excitation (T-T absorption) and singlet oxygen formation probably due to very low φ_isc, or short lifetime of the triplet state (τ_T) caused by the fast T₁→ S₀ intersystem crossing.     

Laser excited S2 → S1 and S1 → S0 emission spectra and the S2 → Sn absorption spectrum of azulene in solution

We present the S₁ → S₀ fluorescence spectrum, between 740 and 940 nm, of azulene solutions (10^?3 M in methanol) excited with a Q-switched ruby laser. The nitrogen-laser excited S₂ → S₁ fluorescence spectrum, between 700 and 930 nm, is also reported. The transient S₁ → S_n spectrum between 500 and 650 nm was studied, using synchronous nitrogen laser and dye laser excitation. The S₅ (¹B₁(3)) state of azulene was found to be located at 45500 cm^?1 and the cross section σ₂₅ of the transient absorption S₂ → S₅ is estimated to be 3 × 10^?18 cm²/molecule.     

Diphosphine derivatives: Part 5. Ligand behaviour and vibrational spectra of coordinated tetraalkyl-1,2-dithioxodi-λ5-phosphanes (tetraalkyldiphosphine disulphides)

Vibrational data are reported for complexes of tetraethyl-,tetrapropyl-and tetrabutyl-1,2-dithioxodi-λ⁵-phosphanes (tetraalkyldiphosphine disulphides) P₂R₄S₂ with zinc, cadmium, mercury and cobalt halides. The P—P stretching frequencies in the coordinated ligands occur near 500 cm^?1, substantially higher than in the corresponding P₂Me₄S₂ complexes, most probably because of interactions with P—C—C deformation modes. In the F—C stretching region the spectra of the P₂Et₄S₂ complexes are consistent with (but do not necessarily prove) cis-chelate ligand geometry with Zn, Cd and Co halides, and gauche-chelate geometry with Hg halides. No M—S stretching vibrations can be identified above 250cm^?1.