Electronic spectra and vibronic coupling of pyrazine

The absorption and fluorescence spectra of pyrazine have been observed in vapor, solution and crystal, and the vibrational structures have been analyzed in detail. The fluorescence spectrum consists of long progressions of the nontotally symmetric vibration ν₅(b_2g), and the absorption spectrum contains also the progression of the corresponding excited state vibration ν′₅(b_2g. However, the pattern of the latter progression is unusual because of the highly anharmonic nature of the potential of the ¹B^3u state, which may be expressed by a functional form containing a quadratic term in the normal coordinate. All the experimental results suggest that the known vibronic interaction between the ${}^1\text{B}{}_{\mathrm{3u}}\text{(n,π}{}^1\text{)}$ state and the ${}^1\text{B}{}_{\mathrm{1u}}\text{(π, π}{}^1\text{)}$ state through the vibration ν₂(b_2g) is strong. The vibronic coupling and the potential of the ¹B_3u state were found to be very sensitive to solvent.     

Electronic spectra of azanaphthalenes: Mixed crystal spectra of quinoxaline and quinoxaline-d6

The following spectra have been measured at 4 K, in three polarizations: ${}^1\text{B}{}_1\text{(n,π}{}^1\text{) ←}{}^1\text{A}{}_1$ absorption of quinoxaline in durene, durene-d₁₄, naphthalene and naphthalene-d₁₀, and of quinoxaline-d₆ in durene and naphthalene. The polarized phosphorescence of quinoxaline-d₆ in durene is also recorded.The analyses permit unusually extensive assignments of excited state vibrational frequencies. Also noted are various crystal effects: isotope shifts, modification of molecular frequencies, prominence of Herzberg-Teller origins, site splittings, effects of perdeuterated hosts, and polarization ratios. An unusual line-splitting seen in durene is analyzed as a resonance between coincident a₁ and b₁ vibrations of the quinoxaline molecule, coupled through the exciton bands of the host.     

Electronic spectrum of 1,5-naphthyridine: Vapor spectra

Analyses of the vapor spectra of 1,5-naphthyridine-d₀ and -d₆ are presented. The spectra are characterized by two principal origins, one the true electronic origin, magnetic dipole allowed, ${}^1\text{B}{}_{\mathrm{g}}\text{←}{}^1\text{A}{}_{\mathrm{g}}$, 27 598.5 cm^?1 (-d₆ 27 676.9), and the other a vibronic origin, electricd-dipole allowed, corresponding to the activity of a low-frequency vibration 6a_u, 183 cm^?1 (-d₆ 163). Extensive sequence structure is evident and the relative intensities of the sequence bands associated with the two origins provide the strongest argument for their assignments. The absence of 6a_u as a major source of intensity in the hot bands is in agreement with vibronic coupling calculations which propose that in absorption intensity “flows” to the lower-frequency vibrations.     

Thioformaldehyde: Vibrational analysis of the Ã1A2-X̃1A1 visible absorption system

The electronic absorption spectra of thioformaldehyde and thioformaldehyde-d₂ have been obtained. A vibrational analysis of the discrete band system in the 6100-4400-Å region is reported. The type A origin bands are at $\text{16 394}\text{16 484}\text{cm}{}^{\mathrm{?1}}$ for $\text{CH}{}_2\text{S}\text{CD}{}_2\text{S}$, and are magnetic dipole allowed. The electronic transition is $\text{A}\text{?}{}^1\text{A}{}_2\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ under the C_2v point group. Most of the intensity of the system is in type B bands, and is due to vibronic mixing with higher ¹B₂ states when the inversion mode ν₄ is excited. The molecule in the excited ¹A₂ state is “floppy-planar,” having a broad potential function with a barrier of the order of 20 cm^?1 to the inversion motion.     

Near-uv spectra with fully resolved rotational structure of naphthalene and perdeuterated naphthalene

The combination of a single-frequency, tunable uv source with a well-collimated supersonic molecular beam and sensitive fluorescence detection has been used to obtain spectra with rovibronic resolution for some large organic molecules. The results of analysis of the 0₀⁰ and $\text{8}$₀¹ vibronic bands of the ${}^1\text{B}{}_{\mathrm{3u}}\text{←}{}^1\text{A}{}_{\mathrm{g}}$ electronic transition for naphthalene and naphthalene-d_g are presented. The obtained spectra are assigned using a rigid asymmetric rotor Hamiltonian and the structure in both the ground and electronically excited states is determined. The rotational temperature of the molecules cooled in the beam has been determined. The influence of the nuclear spin statistics on the line intensities is observed and discussed.     

The 341 nm band system of pyridine-N-oxide. Analysis of the in-plane vibrational structure

The near-ultraviolet absorption of pyridine-N-oxide has been photographed at high resolution in the range 295–365 nm, and a weak, long-axis polarized subsystem of A-type bands has been identified in the spectrum for the first time. The presence of both A- and B-type bands in the system establishes conclusively that the transition is $\text{π}{}^1\text{← π [}{}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1\text{]}$.The complete set of a₁ and b₂ fundamental frequencies has been identified in the ground and excited states of the transition, using infrared and Raman measurements to supplement the data from the electronic analysis. Physical structure in the excited state has not been determined but appears qualitatively to involve a generalized increase in the dimensions of the ring, coupled with a decrease in the NO distance. The structure of the band system is very similar to that of isovalent aromatic molecules, including phenol and chlorobenzene.     

Hyperfine spectra of bromine vapor near 633 nm

The hyperfine spectra of the (17-7)P(57) line of ⁷⁹Br₂ and the (12-4) P(129) line of ⁸¹Br₂, both of the electronic transition near 633 nm have been measured by the saturated absorption method. An analysis based on nuclear quadrupole and spin-rotation interactions is presented.     

The visible absorption spectrum of NO2: A three-mode nuclear dynamics investigation

The vibronic band origins of the visible absorption spectrum of NO₂ are calculated theoretically with the aid of a simple model Hamiltonian for the coupled electronic and vibrational motions. Including all three vibrational modes in the calculation and using ab initio values of the relevant parameters, we obtain satisfactory qualitative agreement with experiment. In particular, the observed high density and irregular intensity distribution of the band origins is reproduced correctly by the calculation. The present results confirm unambiguosly that the anomalous vibronic structure of the ${}^2\text{B}{}_2\text{←}{}^2\text{A}{}_1$ transition is caused by strong nonadiabatic interactions between the ²B₂ and ²A₁ electronic states of NO₂. They also show that simple deconvolution procedures, which are often used to deperturb irregular spectra, are not applicable to the ${}^2\text{B}{}_2\text{←}{}^2\text{A}{}_1$ transition of NO₂. To further explore the strength of the nonadiabatic effects in NO₂, we calculate the mixing of the different electronic species in the vibronic eigenstates and compare it to several relevant experimental quantities.     

The 341-nm band system of pyridine N-oxide: Assignment of the out-of-plane fundamentals

Analysis of the 341-nm bands of pyridine N-oxide vapor has been extended to the out-of-plane modes of the molecule in the $\text{X}\text{?}{}^1\text{A}{}_1$ and $\text{A}\text{?}{}^1\text{B}{}_2$ states. Data for the ground state are obtained primarily from infrared and Raman measurements; using this information, the excited-state out-of-plane manifold is then built up by the assignment of overtone, sequence, and cross-sequence bands observed in the electronic absorption spectrum. These assignments lead to a complete set of a₂ and b₁ fundamental frequencies in the $\text{A}\text{?}$ and $\text{X}\text{?}$ states, though some questions cannot be fully resolved, and the uniqueness of the assignments is briefly discussed.     

Polarized absorption spectra of diphenylnitroxide

A polarization study of the low-lying electronic states of the diphenylnitroxide radical imbedded in a benzophenone single crystal is presented. Assignments are made for the first two electronic excited states: ${}^2\text{B}{}_2\text{(n → π}{}^1\text{)}$ at 569 nm and ${}^2\text{B}{}_1\text{(π → π}{}^1\text{)}$ at 431 nm. Vibronic coupling is invoked in inducing the $\text{n → π}{}^1$ transition. The electronic structure of the nitroxide bears some resemblance to the corresponding carbonyl. The delocalization of the π electrons is also discussed.     

Laser excitation spectra of He2

Metastable $\text{a(2sσ)}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ He₂ molecules are produced by a dc discharge in a flowing He stream. Laser excitation downstream of the discharge produces excitation spectra for a number of He₂ states. LIF spectra are observed for the $\text{(npπ)}{}^3\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ series for n = 4–9, excepting 5 and the $\text{(npπ)}{}^3\text{Π}{}_{\mathrm{g}}$ series for n = 5–15.     

Absolute vacuum ultraviolet absorption spectra of some gaseous azabenzenes

Absolute extinction coefficient and oscillator strength of pyridine, pyrimidine, pyrazine, and s-triazine were recorded with moderate resolution in the region of 3.5–9.5 eV. Analogous to ${}^1\text{A}{}_{\mathrm{<mtext>1</mtext><mtext>g</mtext>}}\text{→}{}^1\text{B}{}_{\mathrm{<mtext>2</mtext><mtext>u</mtext>}}$, ¹B_1u, ${}^1\text{E}{}_{\mathrm{<mtext>1</mtext><mtext>u</mtext>}}\text{π → π}{}^1$ transitions of benzene, several $\text{n → π}{}^1$ and Rydberg transitions are presented and discussed.     

Excited vibrational state microwave spectra,structure, and force-field of trifluorosilane

The J = 2?1 microwave spectrum of six isotopic species of HSiF₃ has been observed and assigned in excited states of five of the six fundamental vibrations. The assignment is based on relative intensities, double resonance experiments, and trial anharmonic force constant calculations. Analysis of the spectra leads to experimental values for five of the $\text{α}{}_{\mathrm{r}}{}^{\mathrm{B}}$ constants, all three l-doubling constants q_t, one Fermi resonance constant φ₂₃₃, and one zeta constant $\text{ζ}{}_{\mathrm{6,\; 6}}{}^{\mathrm{(z)}}$.The harmonic force field has been refined to all the available data on vibration wavenumbers, centrifugal distortion constants, and zeta constants. The cubic anharmonic force field has been refined to the data on $\text{α}{}_{\mathrm{r}}{}^{\mathrm{B}}$ and q_t constants, using two models: a valence force model with two cubic force constants for SiH and SiF stretching, and a more sophisticated model. With the help of these calculations, the following equilibrium structure has been determined: $\text{r}{}_{\mathrm{e}}\text{(}\text{SiH}\text{) = 1.4468(±5)}\text{A}\text{?}$, $\text{r}{}_{\mathrm{e}}\text{(}\text{SiF}\text{) = 1.5624(±1)}\text{A}\text{?}$, ∠HSiF = 110.64(±3)°,     

The 3B1-1A1 band system of sulfur dioxide: Rotational analysis of the (010), (100), and (110) bands

The (010)-(000) band of the ${}^3\text{B}{}_1\text{←}{}^1\text{A}{}_1$ system of sulfur dioxide has been rotationally analyzed and constants for the magnetic interactions in the (010) vibrational level determined. Partial rotational analyses are given for the rotationally perturbed (100)-(000) and (110)-(000) bands of the system. The extensive perturbations in this band system are provisionally attributed to: (i) vibronic interaction with a ³A₂ state, visible (a) as a large negative anharmonicity in higher bands of the ³B₁ system, and (b) as a ΔK = 0 resonance perturbation in low-lying vibrational levels of the ³B₁ state; and (ii) a K, K ± 1N-dependent perturbation possibly due to rotational-electronic coupling with a ³B₂ state, though evidence on this point is incomplete. Evidence bearing on these conclusions is discussed.     

Absorption spectra of Ni2+ in (NH4)2Mg(SO4)2·6H2O and Co2+ in Na2Zn(SO4)2·4H2O

Optical absorption spectra of Ni²⁺ in (NH₄)₂Mg(SO₄)₂·6H₂O and Co²⁺ in Na₂Zn(SO₄)₂·4H₂O single crystals have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, a successful interpretation could be made assuming octahedral symmetry for both the ions in the crystals. The splitting observed for ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band in Ni²⁺ and ${}^4\text{T}{}_{\mathrm{2g}}\text{(F)}$ band in Co²⁺ at liquid nitrogen temperature have been explained as due to spin-orbit interaction. The extra band observed at 16,325 cm^-1 in the case of Ni²⁺ at low temperature has been interpreted to be the superposition of vibrational mode of SO^2-₄ radical on ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band. The observed band positions in both the crystals have been fitted with four parameters B, C, Dq and ζ.     

Zeeman effects in the visible 2B2 ← 2A1 system of NO2

The Zeeman effect facilitates certain rotational assignments in bands of the ${}^2\text{B}{}_2\text{-}{}^2\text{A}{}_1$ electronic system of NO₂. A partial analysis of two bands of this system is reported.     

Intensities and pressure-broadened widths of CO2 R-branch lines at 15 μm from tunable laser measurements

Intensities and half-widths of individual lines, over the temperature range 200–325°K in the 15 μm bands of ¹²C¹⁶O₂, have been determined with a tunable diode laser spectrometer. Measurements were made on pure CO₂ and on dilute CO₂-in-N₂ mixtures on the R-branches of the 01¹0-00⁰0 and 02²0-01¹0 transitions. Intensities are approximately equal to those listed in the AFGL compilation. The pressure-broadened half-widths follow the general relationship $\text{b}{}_{\mathrm{L}}{}^0\text{(T) = b}{}_{\mathrm{L}}{}^0\text{(T}{}_0\text{) [}\text{T}{}_0\text{T}\text{]}{}^{\mathrm{n}}$ where n varies considerably from line to line but is always greater than $\text{1}\text{2}$.     

Infrared diode laser spectroscopy of the NS radical

The v = 1 ← 0 vibration-rotation bands of the NS radical in the $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electronic states were observed by using a tunable diode laser. From the least-squares analysis the band origins were determined to be 1204.2755(12) and 1204.0892(19) cm^?1, respectively, for $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$. The rotational and centrifugal distortion constants and the internuclear distance in the X²Π electronic state were obtained as follows: B_e = 0.775549(10) cm^?1, D_e = 0.00000129(33) cm^?1, and $\text{r}{}_{\mathrm{e}}\text{= 1.49403(4)}\text{A}\text{?}$, with three standard deviations indicated in parentheses.     

“Forbidden” rotational spectra of symmetric-top molecules: PH3 and PD3

A millimeter-wave spectrometer having a sensitivity of 4 × 10^?10 cm^?1 in the 2-mm region has been constructed for observation of extremely weak millimeter-wave spectra of gases. It has been used to measure J → J, K = 0 ← 3 transitions in PH₃ and J → J, K = 0 ← 3 as well as K = ±1 ← ±4 transitions in PD₃. The B₀ and C₀ spectral constants (in MHz) are: for PH₃, B₀ = 133 480.15 ± 0.12 and C₀ = 117 488.85 ± 0.16; for PD₃, B₀ = 69 471.10 ± 0.03 and C₀ = 58 974.37 ± 0.05. The effective ground-state values obtained for the bond angle and bond length are: for PH₃, $\text{r}{}_0\text{(}\text{A}\text{?}\text{) = 1.420}{}_0$ and $\text{α}{}_0\text{(}{}^{\mathrm{o}}\text{) = 93.34}{}_5$; for PD₃, $\text{r}{}_0\text{(}\text{A}\text{?}\text{) = 1.417}{}_6$ and $\text{α}{}_0\text{(}{}^{\mathrm{o}}\text{) = 93.35}{}_9$. The corresponding zero-point-average values were calculated to be: for PH₃, $\text{r}{}_{\mathrm{z}}\text{(}\text{A}\text{?}\text{) = 1.4269}{}_9\text{± 0.0002}$ and $\text{α}{}_{\mathrm{z}}\text{(}{}^{\mathrm{o}}\text{) = 93.228}{}_7$; for PD₃, $\text{r}{}_{\mathrm{z}}\text{(}\text{A}\text{?}\text{) = 1.4226}{}_5\text{± 0.0001}$ and $\text{α}{}_{\mathrm{z}}\text{(}{}^{\mathrm{o}}\text{) = 93.256}{}_7\text{± 0.004}$. For both species, the equilibrium values are $\text{r}{}_{\mathrm{e}}\text{(}\text{A}\text{?}\text{) = 1.4115}{}_9\text{± 0.0006}$ and $\text{α}{}_{\mathrm{e}}\text{(}{}^{\mathrm{o}}\text{) = 93.32}{}_8\text{± 0.02}$.     

Model calculation of the electronic structure and spectroscopy of Hg2

Energy curves and transition moments of the excited valence states of Hg₂ were obtained in a model calculation based on calculated Mg₂ energy levels and the assumption that the asymptotic spin-orbit matrix elements for the Hg atom are applicable to the molecular states. The spin-orbit and orbital-rotational interaction of the excited states of Hg₂ is analyzed in both a Hund's case (c) and (a) representation. The intermediate (a) → (c) transition moments are obtained as a function of the internuclear distance. The effect of the orbital-rotational interaction which introduces Hund's case (b) and (e) couplings is found to be small for transitions among excited states under the conditions normally encountered for populating excimer states.Using the energy level positions and transition moments, the observed spectra and predicted spectra are compared for both radiative transitions including the ground state and among the excited states. The lifetime of the $\text{1}{}_{\mathrm{u}}\text{(}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{)}$ excimer state is calculated to be 1.4 μsec with the 335 nm band assigned to the $\text{1}{}_{\mathrm{u}}\text{→ X}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ transition. The 485 nm bands cannot be assigned to any Hg₂ transitions. Strong bound-continuum absorptions are predicted for the 485 nm bands. On the other hand, the 335 nm emission is predicted to be absorbed by bound-bound transitions only.