1A1 Transition Research Articles

Rotational structure in the near-infrared absorption spectrum of ozone

Ozone absorption spectra near 1 μm exhibit fine structure indicative of rotational subbands, suggesting that the upper electronic state is metastable. Preliminary analysis supports a previous assignment to the 3A2←1A1 transition. A binding energy of ∼0.1 eV is inferred from the breakoff in the observed structure.

Theoretical studies of several electronic states of the NO ? 2 anion

The equilibrium geometries, excitation energies, force constants and vibrational frequencies for several lowlying electronic states X 1A1, 1B1, 3B1, 1A2, 3A2, 1B2 and 3B2 of the NO–2 anion have been calculated at the MRSDCI level with a double-zeta-plus polarization and diffuse s and p orbital basis set. Our calculated excitation energies and vibrational frequencies for these electronic states are in agreement with available experimental data. Some electronic transition properties for the 1B1→ X 1A1 and 1B2→ X 1A1 transitions are calculated based on the MRSDCI wavefunctions.

Direct laser-induced emission detection of the S1 and T1 states of germanium dichloride: Pyrolysis jet spectroscopy and ab initio studies

Spectra of jet-cooled germanium dichloride were obtained by pyrolysis of trichlorogermane in the throat of a supersonic jet. Laser-induced emission excitation spectra were recorded for the weak 450–400 nm and strong 320–300 nm band systems, both of which were vibrationally analyzed. Ab initio predictions of the excited state geometries, vibrational frequencies, and excitation energies were made to aid in assigning the spectra. The strong ultraviolet band system is assigned as à 1B1–X̃ 1A1 with upper state vibrational frequencies of ν1=354 cm−1 and ν2=104 cm−1. It is the direct analog of the 580–440 nm band system of dichlorocarbene. The weaker band system in the visible is shown to be the ã 3B1–X̃ 1A1 transition, with upper state vibrational frequencies of ν1=393 cm−1 and ν2=118 cm−1. This is the first report of direct laser-induced phosphorescence detection of the excited triplet state of any of the carbene or heavier carbene analogs.

Anomalous intense ultraviolet emission bands in the radio-frequency glow discharges of GeH4-fluorocarbon-H2 mixtures

Anomalous intense ultraviolet emission bands in the region 320–370 nm with a maximum at 340 nm were observed in radio frequency glow discharges of CF4-H2 mixtures containing 0.3%–1% of GeH4. The system of emission bands is prominent also in the discharges of GeH4-C2F6-H2 mixtures but the corresponding emission system was not observed in the discharge spectra of GeH4-CH4-H2 and SiH4-CF4-H2 mixtures. From the vibrational analysis of the bands, it was found that the emission bands almost agree with the weak emission system of GeF2, which has been tentatively assigned in earlier work as the 3B1–1A1 transition.

Experimental observation of a minimum in the generalized oscillator strength for the A(2A''2)X(1A1) transition of ammonia

A minimum has been observed in the generalized oscillator strength (GOS) for the A(2A''2) from X(1A1) transition of ammonia at K2=0.9 au. The GOS were determined from the inelastic differential cross sections measured with a previously described electron energy-loss spectrometer. The incident electron energy was 1 keV and the energy resolution 0.6 eV.

Jet spectroscopy and excited state dynamics of SiH2 and SiD2

Silylene radicals, SiH2 and SiD2, are generated in a supersonic free jet by ArF laser (193 nm) photolysis of phenylsilane and phenylsilane-α-d3, respectively. LIF excitation and dispersed fluorescence spectra are measured for the ν2 vibronic bands of the à 1B1−X̃ 1A1 transition. The heterogeneous predissociation to Si (3P)+H2 is proposed from the anomalous rotational structure in the excitation spectra; the rotational lines of the r(1) subbranch (K′a=0←K■a=1) have stronger intensity than those of the r(0) subbranch (K′a=1←K■a=0), though the latter is expected to be stronger due to the low temperature Boltzmann distribution in the jet. The time-resolved excitation spectra demonstrate shorter lifetime of K′a=1 rovibronic levels in the à 1B1 state. The heterogeneous predissociation is interpreted with the second order perturbation: à 1B1 –(a-type Coriolis)→X̃ 1A1 -(spin–orbit)→ã 3B1→Si(3P)+H2. It is demonstrated experimentally that there is a potential barrier associated with the dissociation path of ã 3B1→Si(3P)+H2, the height of which is estimated to be 1540–2160 cm−1 from the bottom of the à 1B1 state. The electronic transition moment of the à 1B1–X̃ 1A1 transition is estimated to be ‖μe‖2=0.26e2a20 from the Einstein equation for spontaneous emission using measured fluorescence lifetimes for single rovibronic levels with Ka=0 and calculated Franck–Condon factors. The onset of a second predissociation channel, à 1B1→Si(1D)+H2, at the (0,7,0) vibronic level of SiH2 à 1B1 is manifested as a sharp decrease in the observed fluorescence lifetime for the v′2=7, J′=0 level relative to that predicted for a pure radiative lifetime.

Polarized emission spectroscopy of photodissociating nitromethane at 200 and 218 nm

We report the polarized emission spectra from photodissociating nitromethane excited at 200 and 218 nm. At both excitation wavelengths, the emission spectra show a strong progression in the NO2 symmetric stretch; at 200 nm a weak progression in the NO2 symmetric stretch in combination with one quantum in the C–N stretch also contributes to the spectra. We measure the angular distribution of emitted photons in the strong emission features from the relative intensity ratio between photons detected perpendicular to versus along the direction of the electric vector of the excitation laser. We find the anisotropy is substantially reduced from the 2:1 ratio expected for the pure CH3NO2 X(1A1)→1B2(ππ*)→X(1A1) transition with no rotation of the molecular frame. The intensity ratios for the features in the NO2 symmetric stretching progression lie near 1.5 to 1.6 for 200 nm excitation and 1.7 for 218 nm excitation. The analysis of the photon angular distribution measurements and consideration of the absorption spectrum indicate that the timescale of the dissociation is too fast for molecular rotation to contribute significantly to the observed reduction in anisotropy. The detailed analysis of our results in conjunction with electron correlation arguments and previous work on the absorption spectroscopy and final products’ velocities results in a model which includes two dissociation pathways for nitromethane, an electronic predissociation pathway and a vibrational predissociation pathway along the 1B2(ππ*) surface. Our analysis suggests a reassignment of the minor dissociation channel, first evidenced in photofragment velocity analysis experiments which detected a pathway producing slow CH3 fragments, to the near threshold dissociation channel CH3 + NO2(2 2B2).

NH2 electron affinity

The 363.8 nm (3.408 eV) photoelectron spectrum of the NH2 (X̃ 2B1)+e−←NH−2(X̃ 1A1) transition of the amide anion is reported. The electron affinity of amidogen is found to be EA(NH2) =0.771 ±0.005 eV. P, Q, and R rotational branches are observed in the spectrum; a simple model which accounts for the band structure is presented.

Photochemistry of CCl3F and CCl2F2 in the 106–200 nm region

The photoabsorption cross sections and fluorescence excitation spectra of CCl3 F and CCl2 F2 were measured using synchrotron radiation at 106–200 nm. The observed absorption bands were accounted for as Rydberg transitions. The emitters produced from the CCl3 F and CC12F2 photodissociative excitations were attributed to CClF(Ã 1A″←X̃1 A′) and CF2 (Ã 1B1 →X̃ 1A1 ) transitions, respectively, and their radiative lifetimes were determined to be 626 ± 28 and 58 ± 2 ns. The emission from the CClF(A) starts at 140 nm and increases to 9.1 Mb at 106 nm, and that for CF2 (A) was deduced to be 0.9 Mb at 104.8 nm. The onsets of the fluorescence suggest that the electronically excited CClF and CF2 radicals are formed by the atomic Cl elimination but not by molecular Cl2 releasing process.

CCl2(Ã 1B1) radical formation in VUV photolyses of CCl4 and CBrCl3

In vacuum ultraviolet photolyses of CCl4 and CBrCl3 a diffuse emission band was observed in the region of 410–750 nm by Ar i resonance and H Lyman-α line irradiation. The band was attributed to a CCl2(Ã 1B1 → X̃ 1A1) transition from the measurements of the appearance energies of the emitters produced from photodissociative excitation of both CCl4 and CBrCl3 using synchrotron radiation. The fluorescence decay of the CCl2(Ã → X̃) transition showed a superposition of two lifetime components of 2.17±0.26 and 4.0±0.12 μs at pressures from 10 to 140 mTorr. The pressure dependence of the amplitudes for the two lifetimes suggests the occurrence of collision-induced intersystem crossing between 1B1 and 3B1 states of CCl2 radicals. The absorption cross section of CBrCl3 was measured for the first time in the 106–200 nm wavelength region and tentative assignments of Rydberg transitions are presented.

The visible absorption spectrum of 1,2-cyclobutanedione in the gas phase

The visible absorption spectrum of 1,2-cyclobutanedione has been measured in the gas phase at wavelengths between 4000 and 5100 Å. The absorption is attributed to the allowed π* ← n+, 1B1 ← 1A1 transition corresponding to the first excited singlet state. The spectrum shows a complex well-resolved vibrational structure which has been analysed, with some 125 bands measured and assigned. The bands at the longer wavelengths show sharp rotational fine structure, not yet analysed. The strongest band in the spectrum at 4933 Å has been assigned as the 0–0 band, while a band almost as strong at 4820 Å is attributed to excitation of one quantum of [Formula: see text], the a2 out-of-plane carbonyl bending vibration, and it is suggested that this band owes its intensity to vibronic coupling. A number of symmetric vibrations are also excited in the spectrum, but with no long progressions. Sequence bands running to the blue with an interval of about 72 cm−1 are prominent throughout the spectrum, and are assigned to v13, the a2 ring-twisting vibration. Other hot bands were also observed involving v13 which permitted estimation of energy levels for this vibration both in the ground state and the excited state. The infrared spectrum was also measured and analysed in the gas phase between 600 and 4000 cm−1, and 14 bands were assigned to fundamental vibrations; some of these assignments, at the lower frequencies, are uncertain.

High-Spin(5T2) \ightleftharpoons Low-Spin(1A1) Transition in Mixed Ligand Complex [Fe(2,2′-bipyridine)(1,10-phenanthroline)(NCS)2

Abstract A new type of the mixed ligand complex [Fe(bpy)(phen)(NCS)2] (bpy=2,2′-bipyridine and phen=1,10-phenanthroline) has been prepared by two different methods. The influence of the mixed ligand coordination of bpy and phen to a central iron(II) ion is studied by measurements of the temperature variation of magnetic moments and Mössbauer parameters. It has been demonstrated that the mixed coordination tends to make the 5T2(high-spin) \ightleftharpoons 1A1(low-spin) transition more gradual. The spin-crossover transition temperature depends on the electronic property as well as the structure of the ligand coordinated to the central iron(II) ion.

ChemInform Abstract: The Magnetic Circular Dichroism of 1‐Azabicyclo[2,2,2]octane (ABCO) in the n0 ‐→ 3p Region.

An MCD and absorption spectra measured in the energy region of the second excited state of 1‐azabicyclo[2,2,2]octane (ABCO) reveal large positive A terms. Simple calculations of the magnetic moment of a 1E excited state yield also a positive sign for the A terms. A small negative B term is observed 355 cm−1 below the origin of the 1E state. Based on B term calculations we assign it as the 1A1→1A1 transition (n0→3pz). The results are compared with a recent MPI study in the same energy range.

Effects of Coriolis interaction on the rotational line intensities of symmetry-forbidden electronic transitions

The effect of Coriolis coupling on intensities in infrared rotation–vibration bands is here extended to vibronically allowed transitions between electronic states of a nearly symmetric top. Within the framework of the Herzberg–Teller treatment of vibronic interaction, equations are developed which describe the intensity distribution in the p- and r-form branches of a vibronic transition in the cases where Coriolis interaction perturbs one or both vibrational levels associated with the transition. The à 1A2–X̃ 1A1 transition in H2CO is used as an example to illustrate the theory.

The magnetic circular dichroism of 1-azabicyclo[2,2,2]octane (ABCO) in the n→3p region

An MCD and absorption spectra measured in the energy region of the second excited state of 1-azabicyclo[2,2,2]octane (ABCO) reveal large positive A terms. Simple calculations of the magnetic moment of a 1E excited state yield also a positive sign for the A terms. A small negative B term is observed 355 cm−1 below the origin of the 1E state. Based on B term calculations we assign it as the 1A1→1A1 transition (n0→3pz). The results are compared with a recent MPI study in the same energy range.

A b i n i t i o studies of the low-lying electronic states of ketene

Features of the potential energy surfaces of the X̃ 1A1, 3A2(3A″), 1A2(1A″), 3A1(3A′), 1B1, and 2 1As1 low-lying electronic states of ketene have been investigated using self-consistent-field (SCF) and configuration interaction singles and doubles (CISD) methods with double zeta plus polarization (DZP) and DZP+Rydberg (DZP+R) basis sets. The DZP+R CISD vertical excitation energies are in excellent agreement with observed transition energies and suggest assignments for the X̃ 1A1→1B1 and X̃ 1A1→2 1A1 transitions in the electronic spectrum of ketene. Stationary points have been located at the DZP SCF level of theory for the first four states listed above, and SCF quadratic force constants and harmonic vibrational frequencies have been computed analytically at these stationary points. The X̃ 1A1 geometry and vibrational frequencies compare favorably with experimental values, the agreement being typical of DZP SCF results. Due to curve crossings and conical intersections of potential surfaces, the four lowest theoretical excited state surfaces have only two valid (double) minima, corresponding to 3A″ and 1A″ electronic states. At the DZP SCF geometries, Davidson-corrected CISD adiabatic excitation energies of 16 700 and 19 000 cm−1 have been obtained for the 3A″ and 1A″ states, supporting the previous experimental T0 upper bounds of Laufer and Keller. Finally, the X̃ 1A1 state is predicted to lie only 5500–7000 cm−1 below the 1A″ state at the 1A″ optimum geometry and appears to have a significant effect on the 1A″ out-of-plane frequencies.

A new aspect of size effect on the high-spin(5T2)-low-spin(1A1) transition in supported ferrous complexes

The temperature-dependent Mossbauer studies of carbon or MgO-supported Fe(ppi)2(NCS)2 show that the high-spin fraction fhs vs wt% Fe loading plots obtained at 298 K and 78 K have a crossover point (we say the critical size) near 5 wt% Fe size (l0μ size). For the SiO2-supported Fe(2-pic)3Cl2.EtOH (pic=2-picolylamine), the room temperature Mossbauer spectra show that the high-spin fraction decreases steeply with decreasing iron-complex loading.

Pyrimidine, an Intermediate State Molecule?

The S1(1B1) ← S0(1A1) transition in pyrazine has been investigated. High resolution laser induced fluorescence spectra were obtained by crossing a well collimated molecular beam with UV radiation of an intra‐cavity frequency doubled ring laser. The extreme resolution enabled us to observe the molecular eigenstates of the electronically excited state. Most of the spectral features of the S1 state could be interpreted in terms of a slightly perturbed axis switched rotor spectrum. The perturbations show up both in the energy levels of the S1 and in the intensities of the observed transitions.

Configuration interaction study of the oscillator strenghts for the 1A11A1 and 1A11A1 transitions of the water molecule

Abstract Oscillator strengths for transitions between the X 1 A 1 ground state of water and its B 1 A 1 and D 1 A 1 excited states are computed employing two different theoretical approaches. In one series of calculations a common orthonormal one-electron basis is employed for all of the above states, while in the other type of treatment two different, mutually non-orthogonal, sets are used; the multireference single- and double-excitation (MRD CI) method is employed in each case, with configuration selection, to generate the various electronic wavefunctions. It is found that the use of ground-state SCF MOs leads to poor convergence in the wavefunctions of the (Rydberg-type) B 1 A 1 and D 1 A 1 excited states and consequently also for the corresponding B  X and D  X f -values; this behavior is seen to be closely related to the near degeneracy of the two excited states, each of which is a mixture of the 3a 1 → 3sa 1 and 1b 1 → 3pb 1 configurations. Analogous computations with the A 1 B 1 1 b 1 → 3sa 1 MOs show much better convergence properties, and the resulting f -values compare well with what is obtained when state-specific orbital sets are employed separately for ground and excited states and non-orthonormal techniques are applied to compute the desired transition moments. These results tend to confirm previous findings which indicate conceptual and computational advantages for the calculation of excited-state wavefunctions and properties within the context of a state-specific theory. They also show that although the goal of eliminating the dependence of MRD CI calculations on the choice of MO basis is very nearly approached for energy quantities, it is less satisfactorily achieved for other properties, especially when the existence of nearly degenerate electronic states is a critical factor.

The effect of pressure on the thermal hysteresis of the first-order spin transition in bis(1,10-phenanthroline-2-carbaldehyde phenylhydrazone) iron (II) complexes

The effect of pressure on the thermal hysteresis of the high-spin (5T2) ∏ low-spin (1A1) transition in [Fe(phy)2](BF4)2 and [Fe(phy)2](ClO4)2 has been studied (phy=1, 10-phenanthroline-2-carbaldehyde phenylhydrazone). An increase of the hysteresis width ΔTc as well as of the residual fractions nres5T2 and (1−n5T2)res with pressure is observed. The quantity (ΔTc)1/2 shows a linear dependence on pressure p as expected from the Landau theory. The increase of the residual fractions is explained by the assumption of decreasing size of the cooperative region with pressure.