A Reanalysis of theA1Π–X1Σ+Transition of AlBr

TheA¹Π–X¹Σ⁺transition of aluminum monobromide (AlBr) near 2800 Å was recorded using a Bruker IFS 120 HR Fourier transform spectrometer at nominal resolutions of 4 and 0.03 cm⁻¹. All bands showP,Q, andRbranches and all are degraded to longer wavelengths. The 0–0 band is the most intense and the Δv= 0 sequence dominates the observed spectrum. Each band appears doubled due to the natural isotopic abundances of⁷⁹Br (50.69%) and⁸¹Br (49.31%). Band origin shifts due to isotopic substitution of bromine were examined to confirm the assignments of isotopic species. The rotational structure of the 0–1, 1–2, 0–0, 1–1, and 2–2 bands was assigned and fitted. These data were merged with previously reported photographic data for the 1–0, 2–1, 3–2, 2–0, and 3–1 bands and also infrared and microwave measurements to provide an improved set of constants for both electronic states. The rotational constants for each vibrational level in theX¹Σ⁺state vary smoothly with increasing vibrational quantum number and thus an expansion of the constants in terms of equilibrium values is recommended. An expansion of theA¹Π rotational constants in terms of equilibrium values is not recommended as the distortion constants do not change smoothly with increasing vibrational quantum number. Therefore, the rotational constants for theA¹Π state were determined for each individual vibrational level. This approach leads to vastly improved vibrational constants for theA¹Π state by reducing correlations between rotational and vibrational constants. This problem is serious for theA¹Π state owing to severe departures from harmonic behavior in thev= 2 andv= 3 levels.     

A New Study of the Perturbations in theA1Π State of PN

TheA¹Π–X¹Σ⁺system of the PN molecule has been reexamined via high resolution conventional spectroscopy, at higher rotationalJ-values than those in previous studies. It is shown that perturbations occurring in theA¹Π (v= 0 to 3) levels give access to information concerning the (e³Σ⁻,a³Σ⁺,d³Δ,C¹Σ⁻,D¹Δ) valence states. Of particular interest are (1) the strong spin–orbit interaction between thee³Σ⁻andA¹Π (v_A= 2) levels, yielding 17 rotational energy levels of thee³Σ⁻state, and (2) the triple crossing occurring between theA¹Π (v_A= 3),d³Δ, andC¹Σ⁻levels.     

New band systems of NiBr molecule in visible region

Thermal emission spectrum of NiBr molecule excited by vacuum graphite tube furnace revealed the existence of ten new band sub-systems in the region λλ 5540-4720 Å which were attributed toA→X,B→X,C→X andD→X transitions. Vibrational analysis was carried out for each of the systems mentioned above.A ² Δ has been suggested as the ground state of NiBr molecule with an electronic interval of about 533 cm^?1. Transitions responsible for NiBr spectrum appear to be of the type²π–²Δ and²Δ–²Δ.     

The First Excited Ca–C–C Bending Vibration Levels in theÃΠ andXΣStates of the CaCCH Radical: The Renner–Teller Effect andK-type Resonance

The 5¹₀and 5¹₁bands of the CaCCHÃ²Π–X²Σ⁺transition, corresponding to the Ca–C–C bending mode, have been rotationally analyzed through cw dye laser excitation and dispersed fluorescence with a CCD array detector. The upper state is subject to Renner–Teller and spin–orbit couplings, and strongK-type resonance interactions were observed between the nearby²Δ and²Σ vibronic components. A model that invokes a full matrix treatment of these interactions was employed in a least-squares fit of a total of 708 rotational lines of the two bands, recorded with high precision. The fundamental bending frequencies have been determined as ν₅= 101.394(1) and 102.940(1) cm⁻¹for theÃ²Π andX²Σ⁺electronic states, respectively. The Renner–Teller parameter has been determined as ?₅ω₅= 3.528(14) cm⁻¹. The nonadiabatic parameter for the ν₅mode,g_K= 0.6542(10) cm⁻¹, is in accord with the observation that the²Δ_5/2vibronic component liesabovethe normally highest κ²Σ component.     

The AB2-XA1 electronic transition of NO2: A rovibronic survey covering 14 300-18 000 cm

More than 250 rotationally resolved vibrational bands of the A²B₂-X²A₁ electronic transition of ¹⁵NO₂ have been observed in the 14 300-18 000 cm⁻¹ range. The bands have been recorded in a recently constructed setup designed for high resolution spectroscopy of jet cooled molecules by combining time gated fluorescence spectroscopy and molecular beam techniques. The majority of the observed bands has been rotationally assigned and can be identified as transitions starting from the vibrational ground state or from vibrationally excited (hot band) states. An exceptionally strong band is located at 14 851 cm⁻¹ and studied in more detail as a typical benchmark transition to monitor ¹⁵NO₂ in atmospheric remote sensing experiments. Standard rotational fit routines provide band origins, rotational and spin rotation constants. A subset of 177 vibronic levels of ²B₂ vibronic symmetry has been analyzed in the energy range between 14 300 and 17 250 cm⁻¹, in terms of integrated density and using Next Neighbor Distribution. It is found that the overall statistical properties and polyad structure of ¹⁵NO₂ are comparable to those of ¹⁴NO₂ but that the internal structures of the polyads are completely different. This is a direct consequence of the X²A₁-A²B₂ vibronic mixing.     

Calculated potential surfaces for the description of the emission spectrum of the C2H radical

Ab initio MRD-CI calculations are reported for the CC stretch and CCH bending potential surfaces for the lowest-lying electronic states of the ethynyl radical. It is found that the energies of the fully relaxed $\text{π → π}{}^1$ states of this system are as much as 2.3 eV below the corresponding values at the ²Σ⁺ ground state (linear) equilibrium structure. The fact that the energy minima for the C₂H excited states occur for widely different internuclear arrangements than that of the ground state is shown to be very consistent with the experimental observation that the electronic spectrum of C₂H has a much different appearance in emission than it does in absorption studies. On the basis of both calculated energy differences and oscillator strength results it is found that the most likely assignment for the broad emission feature in the 4000–6000 Å region of the C₂H spectrum is caused by vertical excitation from the lowest vibrational levels of the 3²A′ (and also 2²A″) upper states to the 2²A′, ²A′ and ²A″ species which correlate with X²Σ⁺ and A²Π for linear nuclear arrangements; the possible mechanisms for production of 3²A′ and 2²A″ upon photolysis of acetylene are also discussed. The possibility that spinforbidden transitions are involved at longer wavelengths in this spectral region is also considered. Finally it is pointed out that a potential crossing of the linear $\text{X}\text{?}{}^2\text{Σ}{}^{\mathrm{+}}$ and A²Π species calculated to occur in the neighborhood of R_CC = 2.6 bohr, which becomes sharply avoided upon molecular bending, would be expected to lead to the type of irregular vibronic structure in the 3600 cm^?1 region of the C₂H IR spectrum recently observed by Jacox.     

The uv spectrum of 18OD

The combined high-resolution ultraviolet (uv) absorption spectrum of ¹⁶OD and ¹⁸OD was obtained. State selective measurements of the transitions from the electronic ground state to the first excited electronic state, $\text{A}\text{?}{}^2\text{Σ}{}^{\mathrm{+}}\text{←}\text{X}\text{?}{}^2\text{Π}$, in the 0-0 vibronic band were performed by means of a narrow bandwidth dye laser system. Evaluation of these transition frequencies in wave-numbers yielded molecular constants as well as rotational term values for each of the isotopic species. A computer program based on a linearized least-squares procedure was used to determine the molecular constants and term values. The term value formulas which were employed for this purpose, take into account the Λ splitting and the centrifugal distortion of the diatomic species. The transitions, recalculated from the semiempirically determined term values agree with the measured absorption lines to better than 0.1 cm^?1. The following molecular constants are reported: B, D, H, the rotational constants of the ²Π and ²Σ⁺ states; O₀, P₀, Q₀, the constants of the Λ splitting of the ²Π state; A and A₁; the constants of the spin-orbit coupling of the ²Π state; and γ₀, the constant of the p doubling of the ²Σ⁺ state. Futhermore, term values up to J″ and J′ of 25.5 and the corresponding uv transitions are given.     

High resolution electronic study of ONO, ONO and ONO: A rovibronic survey covering 11 800-14 380 cm

The 11 800-14 380 cm⁻¹ frequency range has been scanned for rotationally resolved rovibronic transitions in the A²B₂-X²A₁ electronic band system of the symmetric (C_2v) ¹⁶O¹⁴N¹⁶O and ¹⁸O¹⁴N¹⁸O isotopologues and in the corresponding electronic band system of the asymmetric (C_s) ¹⁸O¹⁴N¹⁶O isotopologue. The rotational analysis—reflecting minor differences in mass—in combination with symmetry induced spectral differences allows an identification of 68 ¹⁶O¹⁴N¹⁶O vibronic levels, 26 ¹⁸O¹⁴N¹⁸O vibronic levels and 51 ¹⁸O¹⁴N¹⁶O vibronic levels. The bands are recorded using near infrared fluorescence spectroscopy and a piezo valve based pulsed molecular beam expansion of premixed ¹⁸O₂ and ¹⁴N¹⁶O in Ar. The majority of the observed bands is rotationally assigned and can be identified as transitions starting from the vibrational ground state of one of the isotopologues. Numerous hot bands have also been identified. A comparison of the overall spectroscopic features of C_2v vs. C_s symmetric species provides qualitative information on symmetry dependence of vibronic couplings.     

Ab initioRelativistic CI Calculations of the Spectroscopic Constants and Transition Probabilities for the Low-Lying States of the BiOH/HBiO Isomers

Spin-orbit MRD-CI calculations have been carried out for the potential energy surfaces of the seven lowest-lying electronic states of the BiOH molecule by employing relativistic effective core potentials. The HBiO isomer is found to be 4020 cm⁻¹less stable because of its inability to form multiple Bi–O bands. A bent³A″ BiOH ground state is predicted, which is split into all three of its components by spin-orbit coupling. The calculatedX̃₂A″–X̃₁A′ splitting is computed to be 5217 cm⁻¹, but the correspondingX̃₃–X̃₂value is only 29 cm⁻¹. Finket al.have observed spectral bands which appear with aT_evalue of 6200 cm⁻¹which are likely caused by BiOH. Since calculations at the same level for BiF underestimate the observedX̃₂–X̃₁spin–orbit splitting by 650 cm⁻¹, it appears that the present calculations are consistent with this experimental assignment. A vibrational progression with a 500 cm⁻¹frequency is also observed and this result fits in well with the computed Bi–O stretch ω_evalue of 527 cm⁻¹. The calculations also find a relatively large¹Δ splitting (600 cm⁻¹) because of the bent BiOH geometry, with comparatively strong transitions to theX̃₁A′ ground state, and it is suggested that the experimental BiOH assignment can be confirmed on this basis. Much stronger transitions to the¹Δ component should also be observed in emission in the 10 000 cm⁻¹range.     

The Stimulated Raman Spectrum of Cyanogen

The Raman spectrum of cyanogen¹²C₂¹⁴N₂has been investigated at nearly Doppler resolution by means of the Stimulated Raman technique. The regions around theQbranches of the ν₁(2330 cm⁻¹) and ν₂(845 cm⁻¹) vibrations have been recorded. Besides the fundamentals, hot bands arising fromv₅= 1,v₅= 2, andv₄= 1 have been observed. The spectra have been analyzed, and rotational constants for the excited states have been obtained. Computer simulations of the Raman contours have been carried out as a test of the assignments.     

Time-resolved Fourier transform emission spectroscopy of AΠ-XΣ infrared transition of the CN radical

The A²Π-X²Σ⁺Δv = −3 bands of the ¹²C¹⁴N radical have been observed by time-resolved Fourier transform spectroscopy in the 1850-3100 cm⁻¹ region with a wavenumber resolution of 0.025 cm⁻¹. The radical was produced in a pulsed positive column discharge in a cyanogene and helium mixture. Seven bands of v = 0-3, 1-4, 2-5, 3-6, 4-7, 5-8, and 6-9 were analyzed to give the molecular constants of each state by least-squares fitting of 801 lines. The pulsed discharge was found to be efficient for production of CN in the excited A²Π state. The vibrational excitation temperature was determined to be 6680 ± 835 K and 6757 ± 534 K for the A²Π and X²Σ⁺ states, respectively. The population of the A²Π was found to be 4% of that of the X²Σ⁺ state in the time after turning off the discharge.     

Rotational analysis of the 8000–9000 Å bands of nitrogen dioxide

The rotational structure of bands of NO₂ vapor in the region 8300–9000 Å has been partially analyzed and the absorption assigned to the (000)-(000) and (000)-(010) vibronic bands of the $\text{A}\text{?}{}^2\text{B}{}_2\text{←}\text{X}\text{?}{}^2\text{A}{}_1$ electronic transition. Irregular weak perturbations in the N-structure of the upper-state manifold are accompanied by larger resonance-type crossings in the K-structure. The larger perturbation is attributed to vibronic coupling between the à state and excited vibrational levels of the ground state, characterized by a low density of ground state levels and a large vibronic coupling matrix element between the à and X? states. The reconstituted, deperturbed bands have blue-degraded N-structure and strongly red-degraded K-structure, indicating that the bond angle decreases sharply in the excited state. The physical structure of the ²B₂ state is uncertain but some suggestions are made. The electronic energy of the ²B₂ state is T₀ = 11 962.9 cm^?1.     

The laser-induced fluorescence study of AΣ-XΠi band system of CuS

The laser-induced fluorescence excitation spectra of jet-cooled CuS molecules have been recorded in the energy range of 17 200-19 500 cm⁻¹. Fourteen observed vibronic bands have been assigned as three transition progressions: A²Σ⁻ (v′ = 0-4)-X²Π_3/2 (v″ = 0), A²Σ⁻ (v′ = 0-4)-X²Π_3/2 (v″ = 1), and A²Σ⁻ (v′ = 0-3)-X²Π_1/2 (v″ = 0). Spectroscopic constants of both the X²Π ground state and the A²Σ⁻ excited state of ⁶³CuS and ⁶⁵CuS were determined by analyzing their rotationally resolved spectra. Furthermore, the lifetimes of most observed bands were measured for the first time.     

Rotational study and hyperfine effects of the (0,0) band of the B2Σ-X2Σ system of ScS

Rotational analysis of the (0,0) band of the B²Σ-X²Σ transition of ScS is reported. Spectrographic illustration of a hyperfine coupling transition in the ground state is demonstrated for the first time. This enables an order of magnitude to be obtained for γ″ (～0.003 cm^?1). The results for the other constants were: X state: B″ = 0.1971 cm^?1, D″ = 5 × 10^?8cm^?1, 4b = 0.23 cm^?1 (equal to that for ScO within the limits of measurement uncertainty); B state: B′ = 0.1853 cm^?1, D′ = 6 × 10^?8cm^?1, γ′ = ?0.0594 cm^?1, which can be compared with p_A²Π = 0.060 cm^?1. It was found that the two excited states A²Π and B²Σ constitute an excellent example of pure precession (p_pp = 0.058 cm^?1, and this enables the vibrational levels of A²Π to be numbered.     

Fourier Transform Infrared Emission Spectroscopy of thebΠ–aΣSystem of BN

The emission spectrum of BN has been investigated in the 1800–9000 cm⁻¹region using a Fourier transform spectrometer. BN was formed in a microwave discharge of He with a trace of BCl₃and N₂. The bands observed in the 3000–7800 cm⁻¹interval have been assigned as theb¹Π–a¹Σ⁺transition, with the 0–0 band at 3513.99040(43) cm⁻¹. This transition is analogous to theA¹Π_u–X¹Σ⁺_g(Phillips) system of the isoelectronic C₂molecule. The rotational analysis of the 0–0, 1–1, 1–0, 2–1, 3–2, 2–0, 3–1, 4–2, and 4–1 bands has been obtained and the molecular constants for theb¹Π anda¹Σ⁺states have been determined. A local perturbation has been observed in thev= 1 vibrational level of theb¹Π state nearJ= 18 caused by the interaction with thev= 3 vibrational level of thea¹Σ⁺state. The principal equilibrium constants for thea¹Σ⁺state are: ω_e= 1705.4032(11) cm⁻¹, ω_ex_e= 10.55338(52) cm⁻¹,B_e= 1.683771(10), α_e= 0.013857(16) cm⁻¹, andr_e= 1.2745081(37) Å. Although theb¹Π–a¹Σ⁺transition has recently been seen in emission from boron nitride trapped in solid neon matrices [J. Chem. Phys.104,3143–3146 (1996)], our work represents the first observation of this transition of BN in the gas phase.     

Analysis of perturbations in the A2Π-X2Σ+ “Red” system of CN

Deperturbed vibration-rotation constants of the A²Π(v′ = 0 to 12) and X²Σ⁺(v″ = 0 to 8) states of CN are obtained. Specroscopic data from several sources are combined using a weighted, nonlinear least-squares fitting routine. The diagonalized effective Hamiltonian matrix contains as many as two ²Π and four ²Σ⁺ mutually interacting vibronic levels. Perturbations of A²Π by both X²Σ⁺ and B²Σ⁺ are treated simultaneously. The deperturbed constants and interaction matrix elements obtained provide a significantly more accurate representation of all perturbed and unperturbed observed lines than the previously reported values. The electronic factors of the spin-orbit and rotation-electronic perturbation matrix elements for the A ～ X and A ～ B interactions are determined and several previously unreported perturbations are detected and analyzed. Merged constants and Dunham coefficients are calculated; a detailed statistical treatment of the parameters and error estimates has also been carried out.     

Observation of charged X−and X+excitons and metal-to-insulator transition in CdTe/CdMgZnTe modulation-doped quantum wells

The positively and negatively charged excitons,X⁺andX⁻, respectively, are identified by their magnetic circular dichroism in the absorption spectra of modulation-doped CdTe/Cd_0.69Mg_0.23Zn_0.08Te multiple quantum wells at small carrier densities (≈10¹⁰cm⁻²). In these quantum wells with a width of 8 nm the binding energies of the second electron of theX⁻exciton and of the second hole of theX⁺exciton are very similar, 2.9 meV and 2.6 meV, respectively. At larger hole densities a transition to the conducting state is observed. In a sample with intermediate hole density (low 10¹¹cm⁻²) the insulating phase is restored by application of a magnetic field.     

Hydrogen maser atomic beam resonances

Au^? ions at anionic places are formed in gold doped crystals by a reducing treatment withF centers. The ultraviolet absorption consists of 4 bands, which are namedA, B, C, andD in analogy to the isoelectronic centers of the s² type, like Tl⁺. TheB band oszillator strength strongly increases with temperature in accordance with a phonon allowed transition. The ratio of the dipole strength of theC band to that of theA band as a function of the relative position of theB band is compared with Suganos prediction. Zero phonon lines are found at helium temperatures for theA band in NaCl (2,985 Å), KCl (3,068 Å), and KBr (3,145 Å) and for theC band in KCl (2,329 Å). In KCl the Huang-Rhys factor isg=3.4 for theA band. The vibronic structure comes from the relatively large radius 6s ² state of the negative ion. Uniaxial stress splits the zero phonon line. The results definitely agree with the stress splitting behaviour of a degenerateΓ ₁→Γ ₄ transition. Inversion symmetry of the center is confirmed by the absence of a linear Stark effect.     

Electronic structure and reactivity of the CNO/NCO/CON isomers

Three-dimensional potential energy surfaces (PESs) have been calculated for the lowest electronic states of NCO, CNO and CON isomers, using internally contracted Multi Reference Configuration Interaction (MRCI) and Coupled-Clusters RCCSD(T) ab initio methods. For the low lying doublet and quartet excited states of the three isomers, the N–CO, O–CN, C–NO and C–ON collinear dissociation paths were mapped by the Complete Active Space SCF (CASSCF) approach and the energy variations with the bending coordinate have been explored. Several regions of conical intersections have been located and the spin–orbit interactions between states of different spin symmetry have been evaluated in the region of intersections of these states. The analysis of the PESs allows one to identify the main interactions governing the reactivity of the lowest electronic states. The NCO and CNO isomers have stable X²Π electronic ground states, for CON the X²Π ground state is separated from the dissociative [CO?+?N] asymptote by a barrier of 0.11 eV and crosses the dissociative ⁴Σ ^- state close to its minimum. At their equilibrium ground state geometries the spin–orbit interactions A _SO between the two electronic components of the X²Π states were calculated to be -95.6, -109.6 and -57.1 cm^?1 for NCO, CNO and CON, respectively. The predissociation of the vibrational levels of the A²Σ⁺ and B²Π states of NCO has been explained.