A new approach to electron diffraction analysis of symmetric triatomic molecules with large-amplitude bending motion: The equilibrium geometry,force field and vibrational frequencies of BaI2 as determined by electron diffraction

Diffraction data on BaI₂, analyzed by a new approach, indicate an anharmonic potential with a barrier of 71(12) cm^?1 at a linear geometry. The structural and vibrational parameters were found to be r_e^h(Ba-I_o) = 3.150(7)Å, ∠_eIBaI = 148.0(9) °, f_q = 0.69(8) mdyn/Å,f_qq= 0.14(6) mdyn/Å, k₂ = ?0.0075(15) mdyn/Å, k₄ = 0.0025(9) mdyn/ų, v₁ = 106(12) cm^?1 and v₃ = 145(21) cm^?1. The bending frequency v₂ is predicted to be near 16 cm^?1.     

The first two excited 1Σg+ states of Cs2

Laser-induced fluorescence Of Cs₂ molecules in the infrared region (4000–9000 cm^?1) has been observed using several exciting wavelengths from an argon-ion laser and from a ring dye laser. Accurate molecular constants for the first two excited ¹Σ_g⁺ electronic states are derived from spectra recorded at high resolution by Fourier transform spectroscopy. Main molecular constants are: (2)¹Σ_g⁺: T_c = 12114.090 cm^?1, ω_e = 23.350 cm^?1, B_c = 7.4.5 × 10^?3 cm^?1, R_c = 5.8316 Å; (3)¹Σ_g⁺: T_e = 15975.450 cm^?1, ω_e = 22.423 cm^?1 , B_e = 8.23 × 10^?3 cm^?1, R_c = 5.5569 Å.     

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     

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.     

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*.     

CEPA calculations on open-shell molecules

Ab initio calculations at SCF and CEPA levels using large Gaussian basis sets have been performed for the two lowest electronic states,X ² Σ⁺ andA ² Π, of HeAr⁺. Spin-orbit coupling (SOC) effects have been added using a semiempirical treatment. The resulting potential curves for the three statesX,A ₁, andA ₂ have been used to evaluate molecular constants such as vibrational intervals ΔG(v + 1/2) and rotational constantsB _v as well as — by means of a Dunham expansion — equilibrium constants such asR _e , ω _e ,B _e etc. Comparison with the experimental data from UV emission spectroscopy shows that the calculated potential curves are slightly too shallow and have too large equilibrium distances:D _e = 242 cm^?1 andR _e = 2.66 Å compared to the experimental values of 262 cm^?1 and 2.585 Å, respectively, for theX ²Σ⁺ ground state. However, the ab initio calculations yield more bound vibrational levels than observed experimentally and allow for a more complete Dunham analysis, in particular for theA ₂ state. The experimental value of 154 cm^?1 for the dissociation energyD _e of this state is certainly too low; our best estimate is 180±5 cm^?1. For theA ₁ state our calculations are predictions since this state has not yet been observed experimentally.     

Electronic absorption spectrum of Fe3+ doped in ammonium chloride single crystal

The electronic absorption spectrum of the Fe²⁺ ion doped in ammonium chloride has been studied at room and liquid air temperatures. The observed bands have been assigned transitions from the ground ⁶A_1g(S) state to the excited ⁴A_1g(⁴E_g), ⁴T_1g(G) and ⁴T_2g(G) states. The cubic field approximation with D_q = 675 cm^?1, B = 645 cm^?1 and C = 4.4 B is found to give a good fit to the observed band positions.It is further concluded that the site symmetry of the Fe³⁺ ion in the crystal is lowered from O_h to C_4v symmetry at liquid air temperature.     

Laser raman spectra of the stable low-temperature phase of ferrocene

Laser Raman spectra of ferrocene crystal are recorded for the stable (orthorhombic) and metastable (triclinic) low-temperature phases, and the stable and undercooled (monoclinic) high-temperature phases.The skeletal vibrational modes [v₄(A_1g) and v₁₆(E_1g)], the CH bending (⊥e) mode [v₂₅(E_2g)], and the lattice modes below 100 cm^?1 show characteristic changes among these phases.     

Electronic states of Al2

Ab initio multi-configuration self-consistent field and first-order configuration interaction (FOCI) calculations in an extended basis set have been carried out for the lower energy electronic states of Al₂. The ten core electrons of each Al atom were replaced by an accurate compact effective core potential. The FOCI calculated T_o value for the ³Σ_g^?-³Σ_u^? transition agrees with the experimentally observed emission band to within 90 cm^?1. ³Π_u is calculated to be the electronic ground state of Al₂. Based on FOCI energies and qualitative intensity arguments, the reported optical absorption spectrum of matrix isolated Al₂ also agrees best with a ³Π_u ground state. The ³Σ_g^?1 state is calculated (T_e) at only 324 cm^?1 above the ³Π_u state, and the ¹ΣE_g⁺ state is predicted to lie higher.     

A molecular electronic Raman effect of the ferricenium ion

A molecular electronic Raman effect is reported for the ferricenium ion. The transitions occur at 213 cm^?1 and 1580 cm^?1 to spin-orbit levels of the ground electronic configuration (a_1g)²(e_2g)³. The positions of the spin-orbit states as obtained from the Raman spectrum agree with EPR data.     

The absorption spectrum of carbon monoxide in the CO(3Π r,a) state between 215 nm and 285 nm

Bye-beam excitation of a He/CO mixture the CO(³Π _r ,a) state was sufficiently populated to allow the measurement of the absorption spectrum. The (0, 0), (1, 1), (2, 2) and (0, 1) bands of thec ³Π←a ³Π system of CO have been observed and the molecular constantsT _e =92036.0 cm^?1 (for the band head), ω _e =2249.5 cm^?1, ω _e x _e =29.5 cm^?1 have been derived for CO(c). A new electronic state withT _e =91854.3 cm^?1, ω _e =848.4 cm^?1, ω _e x _e =9.8 cm^?1,B _e =1.351 cm^?1, and α _e =0.021 cm^?1 was identified to be a³Σ state. It seems to be very likely that this state is the CO (3pσ,³Σ,j) state discussed in the literature. The results indicate a perturbation of the υ=1 levels of the new state by the CO (c,υ=0) levels. Another strong perturbation is found in the υ=4 levels. The three CO(³Σ,b,υ′=0,1,2)←CO (a,υ″=0) bands were also investigated yielding for CO(b):T _e =83778 cm^?1, ω _e =2335 cm^?1, ω _e x _e =59 cm^?1 andB _e =1.86 cm^?1.     

Electronic structure and vibrational frequency of Cr2

Cr₂ is produced by pulsed YAG laser vaporization of chromium metal and its fluorescence excitation spectrum is analyzed. The high value of vibrational frequency ΔG″_{$\text{1}\text{2}$} = 452.34 cm^?I (ω″^e ≈ 470 cm^?1) and short internuclear distance r″_e = 1.6788 Å are indicative of a very strongly bound ¹∑⁺_g ground state.     

A 340 nm emission system in I2 induced by a two-photon excitation process

Iodine vapour is optically excited in a two-step process populating selectively the first four vibrational levels of an ion-pair state having T_e = 47032 ± 6 cm^?1 and ω_e = 98 ± 2 cm^?1. Fluorescence from this state is observed at 340 nm with vibrational resolution and is found to terminate on levels of the intermediate B state.     

Temperature dependence of the bandwidths and frequencies of some anthracene phonons. High-resolution Raman measurements

Using a coupled interferometer—spectrometer with a resolution of 0.02 cm^?1 we have measured the Raman band profiles of the four low-frequency anthracene phonons ω₁(a_g), ω₂(a_g), ω₆(b_g) and ω₇(b_g) in the temperature range 2–70 K. These phonons possess very narrow bandwidth at low temperature which are convinently measured under high resolution. In particular the two lowest-frequency phonons ω₁(a_g) and ω₆(b_g) have a bandwidth at 2 K of 0.045 cm^?1. The other two phonons ω₇(b_g) and ω₂(a_g) have bandwidths at 2 K of 0.165 and 0.4 cm^?1, respectively. A detailed analysis of the bandwidth variation with temperature was made in terms of three-phonon decay processes. The exrerimental variation of the bandwidth with temperature was correctly reproduced assuming a single down-and up-process. The following results were obtained: ω₁(a_g): 49.45 cm^?1 = 2×24.72 cm^?1, 49.45 cm^?1 = 98.45 cm^?1 ?49.0 cm^?1; ω₆(b_g): 57.50 cm^?1 = 2×28.75 cm^?1, 57.50 cm^?1 = 108.50 cm^?1 ?51.0 cm^?1; ω₇(b_g): 71.20 cm^?1 = 2×35.6 cm^?1, 71.20 cm^?1 = 120.20 cm^?1 ?49.0 cm^?1: ω₂(a_g): 82.40 cm^?1 = 57.50 cm^?1 +24.9 cm^?1, 82.40 cm^?1 = 138.4 cm^?1 ?56 cm^?1. The efficiency of the down- and up-processes is discussed in terms of the two-phonon density of states. The bandwidths at 2 K follows very closely the variation of the two-phonon sum density of states, whereas the relative importance of the up-processes follows well the two-phonon difference density of states. The anharmonic frequency shifts are corrected for the thermal expansion of the crystal using the Grüneisen single-phonon parameters and the thermal expansion coefficients given in the literature. This permits an estimation of the variation of the anharmonic shifts in the temperature range studied.     

Autoionizing rydberg states of. The Li2 molecule: Molecular constants for Li2+

Autoionizing Rydberg series of Li₂ have been observed in the two-step optical cxcitation of a supersonic lithium beam. The series limits are vibrational states of Li₂⁺. In the most probable assignment IP(Li₂) = 41236.4 ± 2.5 cm^?1 and for Li₂⁺ω_e = 263.45 ± 1.3 cm^?1; ω_eχ_e = 1.35 ± O.2 cm^?1; r_e = 3.032 ± 0.01 Å; D_e = 10807 ± 150 cm^?1.     

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.     

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.     

Quasi-resonant energy transfer in collisions: Na2(A1Σ u + )+K(4S)

Cross sections for electron energy transfer from the initial rotational stateJ′of the two lowest vibrational levelsv′=0 andv′=1 of excited dimers Na₂(A) to potassium atoms as described by Na₂(A¹Σ _u ⁺ ,v′J′)+K(4S)→Na₂ (X¹Σ _g ⁺ ,v″J″)+K(4P)+ΔE have been examined by laser-induced fluorescence. A strong increase of the cross section by as much as an order of magnitude has been observed for those dimerv′J′-levels for which the dipole transitions are close to resonance of the 4S-4P transitions in the atom (ΔE<4 cm^?1). The absolute cross sections for energy transfer have been calculated by the Rabitz approximation of first-order perturbation theory. In the case of closest energy resonance (ΔE=0.9 cm^?1) the cross section is Q=7.8×10^?13 cm².     

Combined analysis of microwave and infrared spectra of methyl iodide: rotational and l-type doubling constants of the v5, v3+v6 and v2 states

The rotational constant B and the l-type doubling constant q were determined for the v₅, v₃+v₆ and v₂, states of CH₂I from the microwave transition frequencies, in combination with the infrared data previously reported. Since these vibrational states were coupled through the Fermi resonance and the xy-type E-E and A₁-E Coriolis resonances, the analysis was made by setting up and solving the complete form of the secular determinants of the energy matrices. The rotational and l-type doubling constants were determined as B₅, = 0.250 173 cm^?1, B₃₆ = 0.247 600 cm^?1, B₂ = 0.249 369 cm^?1, q₅ = ?0.000 027 cm^?1 and q₃₆ = ?0.000 179 cm^?1, which are unperturbed by Fermi and Coriolis interactions. Other band constants for v₅ and v₃+v₆ were also refined in accordance with the new values of B₅ and B₃₆. The present study indicated that the combined analysis of microwave and infrared spectral data was useful for the precise determination of vibration-rotation, levels in the perturbed system.     

Theoretical evidence for multiple 3d bondig in the V2 and Cr2 molecules

Calculated CAS SCF potential curves are reported for the ³Σ_g^? state of V₂ and the ¹Σ_g⁺ state of Cr₂. At the CAS SCF level the ³Σ_g^? state of V₂ is calculated to be bound (R_c = 1.77 Å ω_c = 593.6 cm^?1, D_e 0.33 eV) and to involve a triple 3d bond; while the Cr₂ potential curve is not bound but shows a shoulder near the experimental R_e and the wave function shows multiple 3d bonding in this region.