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.     

Spectra and structure of organophosphorus compounds: XVII. Infrared and Raman spectra. vibrational assignment and barriers to internal rotation for t-butylphosphine

The infrared spectra of gaseous and solid tertiary-butylphosphine, [(CH₃)₃CPH₂], have been recorded from 50 cm^?1 to 3500 cm^?1. The Raman spectra of gaseous, liquid and solid (CH₃)₃CPH₂ have been recorded from 10 to 3500 cm^?1. A vibrational assignment of the 42 normal modes has been made. A harmonic approximation of the methyl torsional barrier from observed transitions in the solid state gave a result of 4.22 kcal mol^?1 and 3.81 kcal mol^?1 in the gaseous state. Hot band transitions for the phosphino torsional mode have been observed. The potential function for internal rotation about the C-P bond has been calculated. The two potential constants were determined to be: V₃ = 2.79 ± 0.01 kcal mol^?1 and V₆ = 0.07 ± 0.01 kcal mol^?1.     

High-resolution UV photoelectron spectrum of CO2

The highly-resolved HeI photoelectron spectrum of CO₂ is presented and its vibrational structure studied in detail. In the X? ²Π_g ionic state the v₃ antisymmetric mode is found to be excited in double quanta (v_1-v_2-v₃ = 0. 0. 2) with energy hv₃ = 181 meV. In the C? ²Σ_g⁺ state a single quantum of the same mode is found to be excited (hv₃ = 189 meV) in combination with a v₁ excitation. Vibronic interaction with vibrational levels in the B? ²Σ_u⁺ state of the ion is suggested to promote this (1, 0, 1) excitation. It is established that inelastic scattering processes contribute to the vibrational structure in the C? ²Σ_g⁺ band. The spin-orbit splitting in the X? ²Π_g is determined to be 19±1 meV and 10±2 eV in the ā²Π_u state. Vibronic structure is resolved in the X? ²Π_g band where the Renner-Teller coupling constant is determined to be ? = 0.21±0.02 and the vibrational energy of the v₂ mode as 60±7 meV. In the ā²Π_u state the v₂ energy is found to be hv₂ = 60 meV from the observed hot-band structure.     

Picosecond spectroscopic measurement of very fast intersystem crossing for 9,10-dioxa-anti-bimanes

Picosecond spectroscopy, following the buildup of T₁ → T_n absorption (maximum at 420 nm), shows that the T₁ state of 1,5-diazabicyclo[3,3,0]octa-3,7-diene-2,6-diones(9,10-dioxa-anti-bimanes) is formed within about 10 ps. The nature of the T₁ state was confirmed by decay rates of T₁ → T_n absorption in acetonitrile (n = 0.375 cP, k_nr = 4.5 × 10⁵ s^?1), 1,2-ethanediol (n = 26 cP, k_nr = 1.5 × 10⁴ s^?1 and glycerol (n = 1400 cP, k_nr = 1.3 × 10³ s^?1). The very fast intersystem crossing is ascribed to the proximity of a ³nπ* state to the ππ* (S₁ state produced by light absorption (El-Sayed rule).     

Effect of molecular weight on radiation chemical degradation yield of chain scission of γ-irradiated chitosan in solid state and in aqueous solution

Chitosan A₁, A₂ and A₃ with molecular weight of 471, 207 and 100 kDa respectively, produced from squid pen chitin was degraded by gamma rays in the solid state and in aqueous solution with various doses in air at ambient temperature. Effect of molecular weight on radiation chemical degradation yield of chain scission and degradation rate constants of γ-irradiated chitosan in solid state and in aqueous solution was investigated. The radiation chemical degradation yield G_(s) and degradation rate values were calculated. The molecular weight changes were monitored by capillary viscometry method and the chemical structure changes were followed by UV analysis. The results showed that, the degradation of chitosan was faster in solution, than in solid state. The values of G_(s) in solid state and in aqueous solution were respectively 1.1×10^?8 mol/J and 0.074×10^?7 mol/J for A₁, 4.42×10^?8 mol/J and 0.28×10^?7 mol/J for A₂ and 6.08×10^?8 mol/J and 0.38×10^?7 mol/J for A₃. Degradation rate constants values ranged from 0.41×10^?5 to 2.1×10^?5 kGy^?1 in solid state, whereas in solution they ranged from 13×10^?5 to 68×10^?5 kGy^?1. The chitosan A₃ was more sensitive to radiolysis than A₁ and A₂. The chain scission yield, G_(s) and degradation rate constants seems to be greatly influenced by the initial molecular weight of the chitosan. Structural changes in irradiated chitosan are revealed by the apparition of absorption peaks at 261 and 295 nm, which could be attributed to the formation of carbonyl groups. In both conditions the peak intensity was higher in chitosan A₃ than in A₁ and A₂, the oxidative products decreased with increasing molecular weight of chitosan.     

The (1) 3Σg+ electronic state of Cs2

Laser-induced fluorescence of Cs₂ molecules, recorded by high-resolution Fourier spectroscopy, has been used for the first spectroscopic identification of the lowest gerade triplet (1) ³Σ_g⁺ electronic state. This state can be described by the molecular parameters: T_e = 11602.10 cm⁻¹, B_c = 8.258×10⁻³ cm⁻¹, D_c = 2.56×10⁻⁹ cm⁻¹ and R_c = 5.5425 Å. Determination of the absolute vibrational numbering will require further experiments.     

A laser flash photo lysis-resonance fluorescence kinetic study: Reaction of O(3P) with O3

The laser flash photolysis of ozone at ≈ 6000 Å has been used to generate a clean kinetic source of ground state atomic oxygen, O(³P). The decay of O(³P) due to reaction with O₃ was monitored via resonance fluorescence at 1300 Å, under static reaction cell conditions. Over the temperature range of 220–353°K, the bimolecular rate constant, k₁, could be expressed in Arrhenius form as: k₁ = (2.02 ± 0.19) × 10^?11 exp[-(4522 ± 210 kcal/mole)/RT]. Units are in cm³molec^?1 sec^-1. A comparison of the results from this work with other recent investigations, indicates that the reliability of k₁ is now probably as good as 10–15% over nearly 300 degrees.     

Rotational relaxation in S1 glyoxal: Cross sections and a search for propensity rules

Details of rotational energy transfer from a few selected K′J′ levels in the zero point vibrational level of ¹Au(S₁) glyoxal vapor have been studied. The cross section for destruction of an initial K′J′ level by rotational relaxation in collision with ground electronic state glyoxal is about 240 A² or 4.5 times gas kinetic. Much of the rotational transfer within the S₁ state occurs with large ΔK′ and ΔJ′. No strong propensities for △K′ = 0, ± 1, ± 2, or ± 3 with small ΔJ′ changes occur in collisions with ground electronic state glyoxal. The study was made by examination of the rotational structure in the 5₁⁰ emission band at various pressures after excitation in the 0,0 band of the S₁—S₀ system with the 454.5 nm argon ion line.     

Microwave spectrum of the s-trans form of 3-pyridinecarbaldehyde

The microwave spectra of the ground state and one excited state of the ON s-trans form of 3-pyridinecarbaldehyde have been measured and assigned. The ground state rotational constants and dipole moment components are: A = 5417.8, B = 1583.289, C = 1225.389 (in MHz) and ¦μ_a¦ = 1.35, ¦μ_b¦ = 0.5 (in debye). The excited state most probably belongs to the C₃C torsion, for which the vibrational frequency is estimated to be 135 ± 30 cm^?1.     

Microwave spectrum,centrifugal distortion,internal rotation and excited torsional states of isobutene

The microwave spectrum of isobutene has been recorded from 10 to 35 GHz. From the analysis of the ground and first two excited torsional state splittings, the following internal rotation parameters were calculated: V₃ = 2170 cal mol.^?1, V'₁₂ = ?210 cal. mol.^?1, I_α = 3.18amu Ų and angle (methyl-top to b-axis) 58.21°. Centrifugal distortion parameters were also obtained for the ground state.     

Non-radiative decay of the benzene 1B2u and 3B1u states

Optic-acoustic measurements on high pressure benzene are presented, and are used to analyse the nature of the decay channels form the highly vibrationally excited ³B_1u state. The vibrationally relaxed benzene ³B_1u state is deactivated by n-pentane with a collisional efficiency of 3 × 10^?5. A model, introducing an intermediate state close in energy to the ³B_1u state, is shown to be in good accord with the results.     

Population and deactivation of lowest lying barium levels by collisions with He,Ar, Xe and Ba ground state atoms

Excitation transfer between the barium low lying excited states 6s6p ³ P ₁ ⁰ , 6s5d ¹ D ₂ and 6s5d ³ D _J by collisions with He,Ar,Xe and Ba has been investigated. The population densities in all levels involved were probed by absorption or by fluorescence usingcw lasers. The depopulation cross sections of the Ba³ P ₁ ⁰ state by collisions with noble gases were found to be σ_He(³ P ₁ ⁰ )=5.5·10^?16 cm², σ_Ar(³ P ₁ ⁰ )=4.6·10^?16 cm², and σ_Xe(³ P ₁ ⁰ )=1.7·10^?16 cm². For Ar, the collisional depopulation of the³ P ₁ ⁰ level is exclusively due to the transition to the¹ D ₂ state. Under the assumption that the³ D _J metastable states are populated collisionally by¹ D ₂ →³ D _J transfer only, we have deduced the upper limit for the corresponding cross section σ ₁₃ ^Ar =1.5·10^?18 cm². From the Ba¹ D ₂ and Ba³ D _J steady-state diffusion distributions, collisional relaxation rates of the¹ D ₂ and³ D _J levels were evaluated. The collisional relaxation rates by Ar and Ba yielded total cross sections for the depopulation of metastable levels: σ_Ar(¹ D ₂)=1.5·10^?17 cm², σ_Ba(¹ D ₂)?1·10^?13 cm², σ_Ar(³ D _J)=7·10^?21 cm², and σ_Ba(³ D _J)=1·10^?15 cm². Furthermore, it was found that the main contribution of the collisional depopulation of the¹ D ₂ state by Ar is related to back transfer to the³ P _J ⁰ state, whereas the deactivation of the³ D _J metastable state is due to back transfer to the¹ D ₂ state. Taking into account other cross sections reported in literature we can conclude that collisional deactivation of both metastable levels by Ba ground state atoms can be attributed to their mutual collisional mixing.     

The emission spectrum of the 1B1(π*-n+) state of 1,2-cyclobutanedione excited at 488.0 nm

The spectrum of the emission from the ¹B₁(π^*-n₊) state of 1,2-cyclobutanedione excited at 488.0 nm has been measured. Wavelengths and vibrational assignments are reported for 24 bands between 490 and 550 nm, 12 of which can be identified with hot bands in the absorption spectrum. Prominent bands in the emission spectrum are associated with excitation of V^''₈, the symmetric in-plane carbonyl bend (281 cm⁻¹); v^''₁₂, the asymmetric carbonyl wag (488 cm⁻¹); and v^''₇, a symmetric ring distortion (522 cm⁻¹). Sequences in v₁₃, the ring-twisting vibration, are also prominent; the initial excitation lies in the 13³₃ absorption band, while the emission shows intensity maxima for v^'₁₃ = 0 and 2, and a bimodal vibrational relaxation is suggested.     

Kinetic study of electronically excited silicon atoms,Si(3p2(1S0)), by time-resolved attenuation of atomic resonance radiation

The collisional behaviour of electronically excited silicon atoms in the 3p²(¹S₀) state, 1.909 eV above the 3p²(³P₀) ground state, is investigated by time-resolved attenuation of atomic resonance radiation at λ = 390.53 nm (4s(¹P^o₁)←3p² (¹S₀)). The optically metastable Si(3¹S₀) atoms were generated by the repetitive pulsed irradiation of SiCl₄ and their decay monitored in the presence of added gases. Absolute quenching rate constants (k_Q, cm³ molecule^?1 s^?1, 300 K) are reported for the following collision partners: He (?1.3 × 10^?15), SiCl₄ ((9.1 ± 1.4) × 10^?11), O₂ ((1.5 ± 0.2) × 10^?11) and N₂O ((4.3 ± 0.4) × 10^?11). The results for O₂ and N₂O are compared with analogous data reported hitherto for Si(3p²(³P_J)) and with those for the other np²(¹S₀) states of the group IV atoms C, Ge, Sn and Pb. The rate data for the silicon atoms are considered in terms of the nature of the potential surfaces arising from symmetry arguments based on the weak spin orbit coupling approximation.     

Collisional population of the 23 ∏ g state inK 2

We have measured the effect of potassium vapour pressure within the heat-pipe oven on the shape of the 2³ ∏ _g ?1³∑ _u ⁺ fluorescence induced by collision energy transfer from theC ¹∏ _u state which was optically excited by the 457.9 nm argon-ion laser line. The estimated cross section of that process is 300 Ų. The simulation of the diffuse band shape indicates a non-thermalised vibrational distribution in the 2³ ∏ _g state.     

High resolution photofragment spectroscopy of the gas phase methyl iodide ion

Photodissociation spectra of the molecular ion CH₃I⁺ were obtained with a three stage quadrupole mass spectrometer. Starting from the \(\tilde X^2 E_{3/2} \) ground state, theà ² E _1/2 state was excited with a stilbene 3 cw dye laser. This state predissociates to CH ₊ ³ +I. Measuring the intensity of the CH ₊ ³ fragment ions as a function of the wavelength of the exciting laser, a spectrum showing vibrational and rotational structure was obtained. The vibrational structure was assigned to three progressions ofv ₃ and new vibrational frequencies were determined. From a computer simulation of the (0, 1, 10) band rotational constants were derived. In particular, their dependence on the vibrationv ₃ was studied.     

Reversible intersystem crossing,fluorescence lifetime lengthening and collisionally induced phosphorescence in biacetyl

The emissions of biacetyl excited at 4200 Å were studied at pressures down to 10^?3 torr. Apart from the well-known nanosecond fluorescence, a new emission of the same spectral composition was found with a non-exponential decay in the microsecond range. Furthermore the phosphorescence, as defined by its spectral composition, was found to be collisionally induced.The results imply that after excitation, the molecule rapidly transfers (rate constant k_S→T) to the triplet state, giving rise to the nanosecond decay time; and can then transfer back to the singlet state (rate constant k_T→S), giving rise to the microsecond emission. At the same time internal conversion can occur (k_S→S0). From an analysis of the data we find for k_S→S0 = 2.4 × 10⁷ sec^?1, k_S→T = 7.6 × 10⁷ sec^?1, k_T→S = 1.9 × 10⁵ sec^?1. The kinetic treatment can be transformed to a quantum mechanical one, yielding values for the triplet level density (?_T), the coupling element V_ST and the number of triplet states (N) coupled to the singlet excited. At 4200 Å we find ?_T = 6.3 × 10⁵cm, V_ST = 1.0 × 10^?5 cm^?1, N = 400.Phosphorescence occurs only when the molecule is deactivated by collisions to a vibronic triplet state below the vibrationless excited singlet state. The efficiency of biacetyl collisions is 0.54.     

Analysis of the stability of finite subspaces in density functional theory

We have studied photodissociation of the A state of the H₂S⁺ ion using the quantum-chemical CAS methods, and the 1² A″ (X ² B ₁) and 1⁴ A″ states are involved in photodissociation of the 1² A′ (A ² A ₁) state (the electronic states in dissociation were studied in the C _s symmetry). The CASPT2 S-loss dissociation potential energy curve (PEC) calculations indicate that the 1² A″ and 1² A′ states correlate with the second limit [H₂ + S⁺(² D)] while the 1⁴ A″ state correlates with the first limit [H₂ + S⁺(⁴S)] and that there are a transition state and a local minimum along the 1² A′ PEC and the repulsive 1⁴ A″ PEC crosses the 1² A″ and 1² A′ PECs. The CASPT2 H-loss dissociation PEC calculations indicate that the 1² A″ and 1⁴ A″ states correlate with the first limit [HS⁺(X ³Σ^?) + H] while the 1² A′ state correlates with the second limit [HS⁺(a ¹Δ) + H] and that the repulsive 1⁴ A″ PEC crosses the 1² A′ PEC. For the crossing doublet and quartet states in pairs, we performed CASSCF minimum energy crossing point (MECP) calculations, and the CASSCF spin-orbit couplings and CASPT2 energies at the MECP geometries were calculated. We examined the two previously proposed mechanisms (mechanisms I and II) for dissociation of the A state to the S⁺ ion, based on our calculation results. We suggest processes for dissociation of the A state to the S⁺ ion (processes I and II, based on mechanisms I and II, respectively) and to the SH⁺ ion (process III) and conclude that photodissociation of the A state mainly leads to the S⁺ ion via the most energetically favorable process II: A ² A ₁ (1² A′) (2.38 eV) → barrier at the linearity (2.96 eV) → X ² B ₁ (1² A″) (0.0 eV) → the 1² A″/1⁴ A″ MECP (3.50 eV, large spin-orbit coupling) → H₂ $ (X^{ 1} \Upsigma_{\text{g}}^{ + } ) $  + S⁺(⁴S) (2.92 eV) (the CASPT2 relative energy values to X ² B ₁ are given in parentheses and the largest value is 3.50 eV at the MECP).     

Theoretical study of spin–orbit coupling and kinetics in spin-forbidden reaction between Ta(NH2)3 and N2O

The activation mechanism of the nitrous oxide (N₂O) with the Ta(NH₂)₃ complex on the singlet and triplet potential energy surfaces has been investigated using the hybrid exchange correlation functional B3LYP. The minimum energy crossing point (MECP) is located by using the methods of Harvey et al. The rate-determining step of the N–O activation reaction is the intersystem crossing from ¹ 2 to ³ 2. The reacting system will change its spin multiplicities from the singlet state to the triplet state near MECP-1, which takes place with a spin crossing barrier of 32.5 kcal mol^?1, and then move on the triplet potential energy surface as the reaction proceeds. Analysis of spin–orbit coupling (SOC) using localized orbitals shows that MECP-1 will produce the significant SOC matrix element, the value of SOC is 272.46 cm^?1, due to the electron shift between two perpendicular π orbitals with the same rotation direction and the contribution from heavy atom Ta. The rate coefficients are calculated using Non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM). Results indicate that the coefficients, k(E), are exceedingly high, k(E) > 10¹² s^?1, for energies above the intersystem crossing barrier (32.5 kcal mol^?1); however, in the lower temperature range of 200–600 K, the intersystem crossing is very slow, k(T) < 10^?6 s^?1.     

The predissociation of Li2 b3Πu by a3Σ+u

⁶Li₂ 1³Δ_g(F₁) → b³Π_u(F₁v = 0–11) rotationally resolved fluorescence spectra are recorded following perturbation-facilitated optical—optical double resonance excitation of 1³Δ_g via spin—orbit mixed A¹Σ⁺_u ∼ b³Π_u(F₁e) intermediate levels. The f-symmetry Λ-components of b³Π_u(F₁) are broadened above the 0.05 cm⁻¹ detection threshold owing to predissociation by the vibrational continuum of the a³Σ⁺_u state. The observed v = 0–11, N = 31f level widths were used to determine the potential energy curve for the Li₂ a³Σ⁺_u state in the region 2.35 < R < 2.60 Å and 11200 < E < 14900 cm⁻¹ (relative to E = 0 at the minimum of X¹Σ⁺_g). The a³Σ⁺_u ∼ b³Π_u curve crossing is at R = 2.57 Å and E = 11246 cm⁻¹ and the electronic part of the − BN·LL-uncoupling matrix element is 〈b Π¦L⁺ ¦aΣ〉 = 1.216H at an R-centroid Rv_bϵ_a = 2.61Å.