Two-photon spectroscopy in the gas phase: 1B2u ← 1Ag transition of p-difluorobenzene

The two-photon excitation spectrum ¹B_2u ← ¹A_g of p-difluorobenzene in the gas phase is presented and analysed. The normal absorption is electric dipole allowed and shows no vibronic coupling, but the two-photon absorption is electric dipole forbidden and displays rich vibronic structure. Eight vibronic origins are assigned to their excited state fundamentals by analysis of the hot bands and by analogy with benzene. The only previously unassigned ground state frequency, an a_u mode, is active in the spectrum and is accordingly assigned. The sequences and the abundant Fermi resonances accompanying absorption are also partially assigned.     

1B2u1A1g fluorescence from benzene produced by electron impact (0–30 eV)

The ¹B_2u¹A_1g fluorescence of benzene resulting from the impact of low energy electrons (0–30 eV) has been studied in the pressure range 10^?4 ?2 × 10^?3 torr. It is found that the apparent emission cross section near threshold varies linearly with the pressure. A reaction scheme explaining this behaviour is given. From the absolute value of the apparent emission cross section it follows that excitation of the ³E_1u state is by far dominant over excitation of the ¹B_2u state at low electron impact energies.     

1B2u1A1g fluorescence from benzene produced by electron impact (30–1000 eV)

The ¹B_2u → ¹A_1g fluorescence resulting from electron impact (30–1000 eV) on benzene has been studied in the pressure range 10^?4 ?2 × 10^?3 torr. The fluorescence spectrum is compared with the spectrum obtained by other methods. The energy dependence of the absolute emission cross section indicates a small probability for internal conversion from higher singlet states to the ¹B_2u state.     

The 1B1u ← 1A1g 0—0 transition in crystal benzene

The transition linewidth ΔE in crystal C₆H₆, C₆D₆ and sym-C₆H₃D₃ has been measured as a function of temperature T from 4.2 to 135°K, and it extrapolates to a common value of ΔE_o = 50 cm^? at O°K. In C₆H₆ ΔE = (50 + 7T^{$\text{1}\text{2}$}) cm^?1, indicative of strong exciton—phonon coupling, and there is a line shift of +40 cm^?1 per substituent deuteron. Fluorescence excitation spectral data are used to separate the ¹B_1u(= S₂) decay rate k_H = 9.4 × 10¹² sec^?1, derived from ΔE₀, into S₂S₁ internal conversion (rate ≈ 6.6 × 10¹² sec^?1) and S₂S_x (channel 3) internal conversion (rate ≈ 2.8 × 10¹² sec^?1. A similar value of k_H = 9.9 × 10¹² sec^?1 is obtained from the S₂S_o fluorescence quantum yield of liquid benzene.     

The second-order stark effect on the 1B2u electronic origin of tetracene and pentacene in p-terphenyl

The electric field induced shift of the electronic origin of the lowest B_2u singlet state of tetracene and pentacene in p-terphenyl at 1.8°K has been measured. From this shift we calculate the difference in polarizability of this state and the ground state along the long molecular axis to be 4.5 ± 0.5 ų for tetracene and 19.8 ± 1.0 ų for pentacene (F_c′ = 0.505 F_eff). The ratio of these tensor elements (Δα)_zz for pentacene and tetracene is found to be 4.4 ± 0.6 and is independent of the local field approximation.     

Electronic absorption spectroscopy of cooled supersonic expansions: dynamics of the 1B1u state of trans-butadiene

An ultraviolet absorption technique is developed for studying molecules cooled in a supersonic expansion. In this experiment, the temperature-dependent contribution to the diffuseness of electronically excited states is reduced for CS₂ and butadiene. The broadness of the spectral features of the butadiene ¹B_1u state is independent of temperature     

Temperature dependence of the lifetime of naphthalene 1B3u under boltzmann conditions

The lifetime of naphthalene ¹B_3u was measured in the gas phase under Boltzmann conditions (argon pressures > 400 Torr) over the temperature range 368–460 K. The results were found to agree well with calculations based on an assumed energy-dependent decay constant derived from the analysis by Schlag et al. of the decay of single vibronic levels. The Boltzmann distribution over the vibronic levels of the excited state was calculated using densities of vibrational states obtained from a direct count procedure. The results support Schlag's contention that the energy dependence of the single vibronic level lifetimes in naphthalene is predominantly determined by the Franck-Condon factors; they also indicate that no additional mechanism is needed to explain the observed temperature effects.     

Laser fluorescence spectroscopy and lifetime of the 2B1 (K′ ⪢ 0) electronic state of NO2

Fluorescence of NO₂ excited by HeCd 442 nm laser radiation is found to exhibit a spectrum characteristic of perpendicular transitions from several levels belonging to ²B₁ (K′ ? 0) vibronic states. The lifetime of a level K′ = 4, N′ = 16 ± 1 is 36 μs, substantially greater than lifetimes given previously for K′ = 0 levels of the ²B₁ state. This result supports the mechanism of lifetime lengthening by the Renner interaction of the ²B₁ and ²A₁ components of the linear ²Π_u state.     

Raman spectroscopy of the [TeX6]2− ions (X  Cl or Br) at resonance with their lowest 3T1u and 1T1u states: Evidence for tetragonal distortion in these excited states

The observation that the v₂(e_g) band is the most enchanced Raman band at resonance with the bands assigned to both the ³T_1u ← ¹A_1g and ¹T_1u ← ¹A_1g transitions of the [TeX₆]^2? ions indicates that the ions are tetragonally distorted in these excited states. The depolarisation ratio of 2v₂ band of [TeBr₆]^2? at resonance with the ¹T_1u ← ¹A_1g transition is found to be 0.18, in close agreement with that expected (3/14) for the first overtone of a doubly degenerate vibration coupled to a triply degenerate excite state.     

Two-photon spectroscopy in the gas phase: first excited state of naphthalene, 1B3u

Molecular two-photon spectroscopy is promising to be a new tool for finding otherwise forbidden spectroscopic transitions. Work in the gas phase is particularly important for a study of vibronic two-photon states. The proof of such a new method, at least in part, must be based on its ability to make new assignments. Benzene has up to now been the first, and only example for new assignments. We now present naphthalene as a second system for this new technique. Assignments of new states are presented and based on a spectrum at a pressure of only 70 mtorr. The analysis makes use of rotational structure, polarization behavior and hot band information. We assign not only six new fundamentals in the first excited singlet state ¹B_3u but also fix some ground state assignments, which were ambiguous in the IR spectrum.     

Excited state geometry of and radiationless processes in the lowest B3u(nπ*) singlet state of s-tetrazine

High resolution absorption and laser induced emission spectra of the lowest B_3u(nπ^*) singlet state of s-tetrazine-h₂ and -d₂ in a benzene crystal at 1.8 K are presented and discussed. The absorption spectrum with origin at 17231 cm^?1 (h₂) is dominated by a progression in ν_6a and a Herzberg-Teller origin which has been assigned as ν₁. The absence of ν₁ in the emission spectrum is explained as being due to a destructive vibronic interference effect. The Franck-Condon envelope of the unique ν_6a progression in emission is used for a determination of the excited state structure and the limitations of this procedure are examined. Direct lifetime measurements using a dye laser and single photon counting techniques show the fluorescence lifetime of s-tetrazine-h₂ and -d₂ to be shorter than 1.5 ns. From a deconvolution of the emission pulse of dimethyl s-tetrazine its fluorescence lifetime in the gas phase is found to be 6.0 ± 0.3 ns. Through a comparison of the fluorescence quantum yield of s-tetrazine-h₂ and dimethyl s-tetrazine we calculate for s-tetrazine-h₂ a fluorescence lifetime of 1.5 ± 0.2 ns and a fluoresence quantum yield of 1.8 × 10^?3. The ratio of the emissive lifetimes of s-tetrazine-d₂ and -h₂ was measured from relative fluorescence yields and found to be 1.18 ± 0.05. Photodissociation quantum yield studies on s-tetrazine-h₂, -d₂ and dimethyl for excitation into the origin of the ¹B_3u(nπ^*) state show this yield to be in the range of 1.3 ± 0.3, and this could explain the low fluorescence yields of the s-tetrazines. The fluorescence quantum yields in the gas phase are found to vary among the vibronic levels of the ¹B_3u state. This finding is in agreement with earlier measurements by Vemulapalli and Cassen, but the report by these authors that such quantum yield variations also occurred in the rovibronic structure is not confirmed.     

Energy distribution in CF2(1B1) from the triplet-triplet annihilation of CF2(3B1) and from the vacuum ultraviolet photolysis of C2F4

Spectral analysis of the CF₂(¹B₁) → CF₂(¹A₁) transition showed that the energy distribution found in the v₂ bending vibration (v'₂ ? 3) of CF₂(¹B₁) produced from the triplet-triplet annihilation of CF₂(³B₁) and from the vacuum ultraviolet photolysis of C₂F₄ are approximately statistical and closely related to each other.     

Multiphoton ionisation spectroscopy of H2S: a reinvestigation of the 1B1-1A1 band at 139.1 nm

The 3 + 1 multiphoton ionisation (MPI) spectrum of the ¹B₁-¹A₁ transition in H₂S at 139.1 nm has been recorded in both linearly and circularly polarised light. The rotational structure shows marked differences from that of the one-photon absorption spectrum. Properties of the excited state revealed through analysis of this structure include confirmation of its ¹B₁ character, refined values for its A, B and C rotational constants and the operation of an energy-dependent predissociation mechanism. It is shown that the third-rank tensor component of the transition operator dominates over the first-rank component in this MPI band. The orbital nature of this ¹B₁ excited state is considered.     

Threshold energies for production of CH2(3B1) and CH2(1A1) from ketene photolysis. The CH2(3B1) ↔ CH2(1A1) energy splitting

By measuring the relative CO quantum yields from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25° for CH₂CO(¹A₁) → CH₂(³B₁) + CO(¹Σ⁺) to be 75.7 ± 1.0 kcal/mole. This corresponds to a value of 90.7 ± 1.0 kcal/mole for ΔH_f298⁰[CH₂(³B₁)]. By measuring the relative ratio of CH₂(¹A₁)/CH₂(³B₁) from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25°C for CH₂CO(¹A₁) → CH₂(¹A₁) + CO(¹Σ⁺) to be 84.0 ± 0.6 kcal/mole. This corresponds to a value of 99.0 ± 0.6 kcal/mole for ΔH_f298⁰[CH₂(¹A₁)]. Thus a value for the CH₂(³B₁) ? CH₂(¹A₁) energy splitting of 8.3 ± 1 kcal/mole is determined, which agrees with three other recent independent experimental estimates and the most recent quantum theoretical calculations.     

Solvent effects on relaxation of the 1B2u state of benzene

Values of triplet yields for dilute solutions of benzene in methylcylohexane, cyclohexane, methanol, ethanol, acetonitrile, perfluoro-n- hexane, and water, and also fluorescence yields and lifetimes in perfluoro-n-hexane are reported. The calculated rate constants for fluorescence and intersystem crossing are only slightly affected by solvent, except for water. The enhancement of “internal conversion” by increasing solvent polarity is interpreted as a result of an increase in the pre-exponential factor for a possible ¹(π,π^*) → ¹(σ,π^*) transition.     

CAS SCF Cl calculations for the 3Σ−g, 1Σ+g, 3Σ+u, and 5Δu states of Sc2

CAS SCF CI (SD) calculations have been carried out for the ³Σ^?_g, ¹Σ⁺_g, ³Σ⁺_u, and ⁵Δ_u states of Sc₂ using large gaussian basis sets. The ³Σ^?_g, ¹Σ⁺_g, and ³Σ⁺_u states arise from the ²D(4s² 3d¹) + ²D(4s² 3d¹) limit of Sc₂ and are found to be only weakly bound (D_c ≈ 0.06 eV and R_c ≈ 8.0a₀). The ⁵Δ_u state arises from the ²D(4s² 3d¹) + ⁴F(4s¹ 3d¹ 4p¹) atomic limit. This state is found to be strongly bound relative to its limits (D_c ≈ 0.8 eV and R_c ≈ 7.0a₀).     

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).     

Qualitative considerations of the T1 → S0 intersystem crossing in benzene

We consider the equivalent of the S₀(¹A_1g → T₁(³B_1u absorption spectrum of benzene obtained by Burland et al. On the basis of this spectrum we suggest that the theoretical rate of the T₁(³B_1u) → S₀(¹A_1g) intersystem crossing in benzene is faster by several orders of magnitude than that obtained in recent theoretical work. Furthermore, it is suggested that the rate of this process is not retarded drastically upon deuteration, as claimed in the literature. A new interpretation of the S_o(¹A_1g) → T₁(³B_1u absorption spectrum is also given.     

Benzene B2u → A1g emission in an argon afterglow

The benzene ¹B_2u → ¹A_1g emission spectrum is observed in a flowing argon afterglow by a low power microwave discharge. No emission of dissociation products was found in 2000–7000 Å region.     

Delayed fluorescence from the lowest 1B+3u state of anthracene,due to hetero-triplet—triplet annihilation of 3anthracene* and 3xanthone*

The P-type delayed fluorescence (DF) S_i→S_o of aromatic compounds results from the population of excited singlet states S_i by triplet—triplet annihillation (TTA) of molecules in their lowest and metastable triplet state T₁ : T₁ + T₁

S_i + S_o; S_i may be any excited singlet state whose excitation energy E(S_i ? 2 E(T₁). TTA of unlike molecules A and B (hetero-TTA) may lead to excited singlet states either of A or of B. In particular, if E(T_A¹) < E(T₁^B), hetero-TTA may lead to excited singlet states S_k^A which are not accessible by TTA of 2 T₁^A. In the present paper we report the first example of the detection of the DF from a very short-lived upper excited singlet state S_k^A which has been populated by hetero-TTA. The systems investigated are liquid solutions of A = anthracene-h₁₀ or anthracene-d₁₀ or 9,10-dimethylanthracene and B = xanthone in 1,1,2-trichlorotrifluoroethane at 243 K. S_k^A is the lowest ¹B_3U⁺ state (B_b state) of anthracene.