The formaldehyde cation: Rovibrational energy level structure and Coriolis interaction near the adiabatic ionization threshold

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the 0⁰, 6¹ and 4¹ vibrational levels of the ground electronic state of the formaldehyde cation were recorded using a resonant three-color three-photon excitation scheme. The first adiabatic ionization energy of CH₂O (87793.33(1.30) cm⁻¹) and the rigid-rotor rotational constants (A⁺ = 8.874(8) cm⁻¹, B⁺ = 1.342(15) cm⁻¹, C⁺ = 1.148(18) cm⁻¹) of the vibronic ground state of CH₂O⁺ were derived. A strong a-type Coriolis interaction between the 6¹ and 4¹ vibrational levels was observed. The Coriolis coupling parameter and the deperturbed fundamental vibrational frequencies of the in-plane-rocking mode ν₆ and the out-of-plane bending mode ν₄ were determined to be 8.70(10) cm⁻¹, 823.67(30) cm⁻¹ and 1036.50(30) cm⁻¹, respectively. The intensity distribution of the photoelectron spectra was analyzed in the realm of a simple photoionization model.     

Laser excitation spectrum of C3 in the region 26 000-30 700 cm

The vibrational structure of the electronic state of C₃ in the region 26 000-30 775 cm⁻¹ has been re-examined, using laser excitation spectra of jet-cooled molecules. Rotational constants and vibrational energies have been determined for over 60 previously-unreported vibronic levels; a number of other levels have been re-assigned. The vibrational structure is complicated by interactions between levels of the upper and lower Born-Oppenheimer components of the state, and by the effects of the double minimum potential in the Q₃ coordinate, recognized by Izuha and Yamanouchi [16]. The present work shows that there is also strong anharmonic resonance between the overtones of the ν₁ and ν₃ vibrations. For instance, the levels 2 1⁺ 1 and 0 1 ⁺ 3 are nearly degenerate in zero order, but as a result of the resonance they give rise to two levels 139 cm⁻¹ apart, centered about the expected position of the 2 1⁺ 1 level. With these irregularities recognized, every observed vibrational level up to 30 000 cm⁻¹ (a vibrational energy of over 5000 cm⁻¹) can now be assigned. A vibronic level at 30181.4 cm⁻¹, which has a much lower B′ rotational constant than nearby levels of the state, possibly represents the onset of vibronic perturbations by the electronic state; this state is so far unknown, but is predicted by the ab initio calculations of Ahmed et al. [36].     

Fourier transform spectroscopy of chemiluminescence from the AΠ-XΣ system of SrO

The A^′1Π-X¹Σ⁺ near infrared system of strontium oxide (SrO) was observed at high spectral resolution by measuring the chemiluminescence from a Broida flow reactor using a Fourier transform spectrometer. In total, 32 bands from , , were measured within the spectral region at a resolution of . Vibrational levels of the upper state were observed up to v_A^′=4, and more than 5600 rotational lines were assigned. Incorporating previously published high resolution data for the A¹Σ⁺-X¹Σ⁺ system, a global fit to both data sets yields improved Dunham constants for the ground state and for the lower vibrational levels (v_A^′=0, 1, and 2) of the A^′1Π state. Because perturbations arising from interactions with the b³Σ⁺ and A¹Σ⁺ states affect the higher vibrational levels of the A^′1Π state more strongly, levels v_A^′=3 and 4 were represented by effective band constants in the fits. RKR potentials for the X¹Σ⁺,A^′1Π, and b³Σ⁺ states have been generated utilizing all the available data, Franck-Condon factors have been calculated for the A^′1Π-X¹Σ⁺ system, and A^′1Π∼b³Σ⁺ and A^′1Π∼A¹Σ⁺ perturbations are discussed.     

Gas phase electronic spectrum of propadienylidene C3H2

The rotationally resolved vibronic bands in the forbidden electronic transition of the cumulene carbene C₃H₂ have been observed in the gas phase by cavity ring down absorption spectroscopy through a supersonic planar plasma with allene as precursor. The band detected in the 16 223 cm⁻¹ region is a result of vibronic interaction and is assigned to a combination of a₁ and b₂ vibrations with a frequency around 2250 cm⁻¹. Another vibronic band near 15 810 cm⁻¹ has an unusual rotational structure because the K_a = 0-1 subband is absent. It is assigned to a combination of a₁ and b₁ vibrations, ∼1850 cm⁻¹, which borrow intensity from the near lying state due to a-type Coriolis coupling. A rotational analysis using a conventional Hamiltonian for an asymmetric top molecule yields molecular constants for the vibrational excited levels of the ùA₂ state, which were used for the determination of the geometry. The stronger transition of C₃H₂, measured in a neon matrix in the 16 161-24 802 cm⁻¹ range, was not detected. The reason for this is a short lifetime of the state, leading to line broadening.     

FTIR and diode laser spectroscopy of isobutylene: Analysis of the rotational structure in the v28 fundamental band

Rotational structure in the fundamental band of isobutylene has been examined at room temperature using a combination of FTIR and Pb-salt diode laser instruments. The highest spectral resolution for the FTIR measurements was 0.125 cm⁻¹. Even at this resolution however, rotational structure for the band could be observed and appeared to possess a very regular pattern. A preliminary spectral assignment was obtained using the Watson/Gora asymptotic approximation for a rigid oblate asymmetric rotor. Within this approximation, the band origin was determined to be 890.937 (4) cm⁻¹. Excited state rotational constants, without the inclusion of centrifugal distortions terms, are A = 0.3033(16), B = 0.2801(12) and C = 0.15362 (8) cm⁻¹ respectively. Finally, a full set of spectroscopic constants, including quartic centrifugal distortion constants, were obtained for the band by including the high resolution Pb-salt spectra.     

Dispersed fluorescence spectroscopy of S0 vibrational levels in formaldehyde-d

High resolution dispersed fluorescence (DF) spectra of excited vibrational levels in S₀ HDCO up to 10 000 cm⁻¹ energy were recorded in a free-jet expansion. Excitation to the 0₀₀ rotational level in 4⁰ and 4¹ S₁ HDCO yielded pure vibrational spectra that are free from rotational congestion. The 162 transitions (133 unique vibrational levels) assigned in these spectra have been fit to a multiresonant Hamiltonian model, which includes harmonic frequencies , anharmonic constants (x_ij), and resonance constants (K). The assigned vibrational states were fit to the model with a standard deviation of 4.02 cm⁻¹. Extensive vibrational mixing is observed throughout the spectra. Six harmonic constants, eight anharmonic constants, and four resonance constants (K_44,1, K_66,1, K_44,66, and K_33,5) were determined experimentally. The 18 experimentally determined spectroscopic constants, with the exception of and K_66,1, were found to agree within 6 cm⁻¹ of ab initio calculated values.     

The 313 nm band system of SeO2. Part 2: rotational structure

Rotational analyses of the first four bands in the ν₁ progression of the transition of SeO₂ at 312.7 nm are reported. The gas phase sample, which contained selenium isotopes in natural abundance, was formed in a free jet expansion from a heated nozzle source. The rotational constants for the state show a rather erratic dependence on v₁^′, consistent with the identification of small, local perturbations in the rotational structure. The A-rotational constant shows a particularly irregular dependence on v₁^′. The r₀ structure was therefore determined from the B and C values of ⁸⁰SeO₂ to be:

     

The v6 = 1 and v6 = 2 vibrational states of DCF3

The high-resolution infrared spectra of DCF₃ were reinvestigated in the ν₆ fundamental band region near 500 cm⁻¹ and around 1000 cm⁻¹ with the aim to assign and analyze the overtone level of the asymmetric CF₃ bending vibration v₆ = 2.The present paper reports on the first study of both its sublevels (A₁ and E corresponding to l = 0 and ±2, respectively) through the high-resolution analysis of the overtone band and the hot and bands.The well-known “loop method”, applied to and , yielded ground state energy differences Δ(K, J) = E₀(K, J) − E₀(K − 3,J) for the range of K = 6 to 30.In the final fitting of molecular parameters, we used the strategy of fitting all upper state data together with the ground state rotational transitions.This is equivalent to that calculating separately the and coefficients of the K-dependent part of the ground state energy terms from the combination loops.All rotational constants of the ground state up to sextic order could be refined in the calculation.This led to a very accurate determination of C₀ = 0.18924413(25) cm⁻¹, , and also .In the course of analyzing simultaneously the overtone band together with the and ν₆ bands, the original assignment of the fundamental ν₆ band [Bürger et al., J. Mol. Spectrosc. 182 (1997) 34-49] was found to be incompatible with the present one. Assignments of the (k + 1, l₆ = +1)/(k − 1,l₆ = −1) levels had to be interchanged, which changed the value of Cζ₆ = −0.14198768(26) cm⁻¹ and the sign of the combination of constants C − B − Cζ in the v₆ = 1 level to a negative value.     

High-resolution infrared and theoretical study of gaseous 1,2,5-selenadiazole in the 600-1400 cm range

The Fourier transform gas-phase IR spectrum of natural isotopic 1,2,5-selenadiazole, C₂H₂N₂Se, has been recorded with a resolution of ca. 0.0025 cm⁻¹ in the wavenumber region 600-1400 cm⁻¹. The three a-type bands, ν₂ (A₁), ν₄ (A₁), ν₅ (A₁), the two b-type bands ν₁₁ (B₁), ν₁₂ (B₁), and the c-type band ν₁₄ (B₂) for each of the isotopologues C₂H₂N₂⁸⁰Se and C₂H₂N₂⁷⁸Se have been analyzed using the Watson model. Ground state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from the fits. The rotational constants, harmonic and anharmonic frequencies, and vibration-rotation constants (alphas, ) have been predicted by quantum chemical calculations using a cc-pVTZ basis at the MP2 and B3LYP methodology levels, and compared with the present experimental data. Although the rotation constants are marginally closer to experiment from the MP2 calculations, in general the B3LYP frequencies and alphas are closer to experiment.     

Optical characterization of Cu ion-doped zinc lead borate glasses

We have developed a new series of zinc lead borate (ZLB) glasses by varying ZnO content, to enhance UV transmission, in the chemical composition of xZnO-15PbO-(85−x)B₂O₃, where x=0, 5, 10, 15, 20, 25, 30, 35, 40 and 45 mol% ZnO. From the measurement of UV absorption spectra both the direct and indirect band gaps have been evaluated. Also different physical properties of a reference glass of 45ZnO-15PbO-40B₂O₃ have been studied. From the measurement of refractive indices at six different wavelengths, Cauchy's constants (A=1.578743209; and ) have been computed and a satisfactory correlation has been achieved between the theoretical and the experimental results. Absorption spectra of Cu²⁺(45−x)ZnO-15PbO-40B₂O₃ (where x=0.1, 0.2, 0.5 and 1.0 mol%) have shown two absorption bands at 428 nm (²B_1g→²E_g) and 777 nm (²B_1g→²B_2g). Emission spectra of (1.0 mol%) Cu²⁺:ZLB have revealed two emission transitions at 400 and 493 nm with excitations at 288 and 316 nm.     

Stretch-bend combination polyads in the ÃAu state of acetylene, C2H2

Rotational analyses are reported for a number of newly-discovered vibrational levels of the S₁-trans (ùA_u) state of C₂H₂. These levels are combinations where the Franck-Condon active and vibrational modes are excited together with the low-lying bending vibrations, and . The structures of the bands are complicated by strong a- and b-axis Coriolis coupling, as well as Darling-Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of (trans bend, a_g) and (in-plane cis bend, b_u). This anharmonicity presumably represents the approach of the molecule to the trans-cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of and . The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling-Dennison resonance starts to break down. The effective Darling-Dennison parameter, K₄₄₆₆, is found to increase rapidly with excitation of , while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2¹3¹B¹ and 3¹B³ polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S₁-trans state in order to search for possible S₁-cis (¹A₂) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm⁻¹.     

The high-resolution infrared spectrum of thiophene between 600 and 1200 cm: A spectroscopic and theoretical study of the fundamental bands ν6, ν7, ν13, and the c-Coriolis interacting dyad ν5, ν19

The Fourier transform infrared spectrum of gaseous thiophene, C₄H₄S, has been recorded in the 600-1200 cm⁻¹ spectral region with a resolution of ca. 0.0030 cm⁻¹. Five fundamental bands ν₁₃ (B₁, 712.1 cm⁻¹), ν₇ (A₁; 840.0 cm⁻¹), ν₆ (A₁; 1036.4 cm⁻¹), ν₅ (A₁; 1081.5 cm⁻¹) and ν₁₉ (B₂; 1084.0 cm⁻¹) have been analysed by the standard Watson model (A-reduction). Ground state rotational and quartic centrifugal distortion constants have been obtained from a simultaneous fit of ground state combination differences from four of these bands and previous microwave transitions. Upper state spectroscopic constants have been obtained for all five bands from single band fits using the Watson model. A strong c-Coriolis resonance perturbs the close lying ν₅ and ν₁₉ bands. We have analysed this dyad system by a model including first and second order Coriolis resonance using the theoretically predicted Coriolis coupling constant . From this analysis we locate the previously unobserved ν₁₉ band at 1083.969 cm⁻¹. The rotational constants, ground state quartic centrifugal distortion constants, anharmonic frequencies, and vibration-rotational constants (α-constants) predicted by quantum chemical calculations using a cc-pVTZ basis with B3LYP methodology, are compared with the present experimental data, where there is generally good agreement. A complete set of anharmonic frequencies and α-constants for all fundamental levels of the molecule is given.     

The high-resolution infrared spectrum of BF3 from 400 to 1650 cm

High-resolution infrared spectra of boron trifluoride, enriched to 99.5 at. % ¹¹B, have been measured from 400 to 1650 cm⁻¹. In that region we have identified and analyzed 16 absorption bands attributed to the three fundamental bands, two combination bands, 10 hot bands, and one difference band. All possible states were accessed in this region through direct transitions either from the ground state or as hot bands from thermally populated levels. The spectral resolution of the measurements varied from 0.0015 to 0.0020 cm⁻¹. An improved set of ground state rotational constants and rovibrational constants for the infrared-active fundamental vibrations have been determined from over 32 000 assigned transitions. This study resulted in the first direct characterization of the infrared-inactive ν₁ state of ¹¹BF₃ leading to values for ν₁, , and of 885.843205(24), 0.000678548(53), and 0.000337564(66) cm⁻¹, respectively. The Fermi resonance perturbation between the E′ states ν₃ and 3ν₄ (l = ±1) was further elucidated by observation of hot band transitions to both the 3ν₄ (l = ±1) and 3ν₄ (l = ±3) states. Several other resonances were also found including the weak rotational interaction, between the state 2ν₂ and the E′ state of ν₁ + ν₄.     

The high resolution infrared bands ν1, ν2, ν4 and ν2 + ν5 of FClO3

The high resolution infrared spectrum of mono-isotopic F³⁷Cl¹⁶O₃ has been studied in the regions of ν₁, ν₂, ν₄ and ν₂ + ν₅ bands, centered at 1060.20, 707.16, 1301.71 and 1292.15 cm⁻¹, respectively. The ν₁ and ν₂ parallel bands are unperturbed so their analysis was straightforward and 3355 and 2433 transitions were assigned, respectively. The band origins, the rotational and centrifugal molecular constants in the v₁ = 1 and v₂ = 1 states have been determined, with standard deviation of the fits σ = 0.00019 and 0.00018 cm⁻¹. The ν₄ fundamental is affected by an anharmonic resonance with the ν₂ + ν₅ combination band. The kl > 0 sublevels cross at kl ? 27 because of the opposite values of and . The anharmonic resonance constant  cm⁻¹ has been derived. The Δl = Δk = ±2 and Δl = 0, Δk = ±3 essential resonances have been found to be effective in ν₄, while in ν₂ + ν₅ only the Δl = Δk = ±2 one was active. A total of 5721 transitions have been assigned, 25% of them belonging to ν₂ + ν₅. The rovibrational parameters and the interaction constants of F³⁷Cl¹⁶O₃ have been obtained. The standard deviation of the fit is 0.0006 cm⁻¹, six times the estimated data precision. The equilibrium geometry of perchloryl fluoride has been determined from the A_e and B_e constants of F³⁵Cl¹⁶O₃ and F³⁷Cl¹⁶O₃. Using the A₀ and B₀ constants of all the symmetric species the r₀ geometry has also been derived.     

High-resolution infrared and microwave study of the ν20 and ν21 levels near 300 cm in 1,2,4-triazine

The gas phase far-IR spectrum of the ν₂₀ (A″, 367.88 cm⁻¹) and ν₂₁ (A″, 311.28 cm⁻¹) bands of 1,2,4-triazine, a five membered ring having the point group C_s, has been studied at a resolution ranging from 0.002 to 0.003 cm⁻¹. From the MW spectrum 58 transitions in the ν₂₀ level and 64 in the ν₂₁ level have been assigned. The ν₂₀ and ν₂₁ modes which are due to non-planar motions of the ring system are found to be nearly unperturbed. From a simultaneous analysis of IR and MW transitions band centers, rotational constants, and the quartic centrifugal distortion constants , and δ_K have been obtained using the Watson Hamiltonian, A-reduction, III^r-representation.     

Substantially low desorption barriers in recombinative desorption of deuterium from a Si(1 0 0) surface

We have studied desorption kinetics of deuterium molecules from a Si(1 0 0) surface by means of temperature-programmed desorption (TPD) spectra and isothermal desorptions.Three desorption components, denoted as β_1,A,β_1,B, and C, can be distinguished in semi-logarithmic plots of the TPD spectra.Their peak positions and intensities are strongly affected by the surface preparation methods employed, either with or without annealing to control the initial D coverage .Peak C appears at the leading edge of the TPD peak.It accounts for only about 5% of the TPD peak at and it diminishes rapidly with decreasing , vanishing at .In contrast, together the β_1,A and β_1,B peaks account for the whole TPD peak at any less than 1.0 ML. The maximum of the β_1,A peak is nearly constant at around the maximum temperature of the TPD peak.On the other hand, the β_1,B peak appears on the high-temperature side of the TPD peak and it systematically shifts to higher temperatures with decreasing .These results imply that first- and second-order kinetics are operating for the β_1,A and β_1,B desorptions, respectively.Isothermal desorption experiments confirm the above predicted kinetics for a limited region, namely .From the results for the rate curve analysis, the desorption barriers are evaluated to be 1.6 ± 0.1 eV and 1.8 ± 0.1 eV for the β_1,A and β_1,B desorptions, respectively.These values are substantially lower than the widely accepted value of ∼2.5 eV. To reproduce the measured TPD spectra we take the Arrhenius-type rate equation containing the first- and second-order rate terms for the β_1,A and β_1,B desorptions.The TPD spectra measured for can be reasonably fit with the proposed rate equation when the values given above for E_d,A and E_d,B are used. For , however, the TPD curves are not fit with the same values; rather, the best-fit curves require values for E_d,A and E_d,B larger than those given above. Combining the present kinetics results with those obtained by STM along with the studies, the β_1,A and β_1,B peaks may be attributed to desorption along the 2H path, while peak C may be attributed to desorption along the 4H path. The atomistic desorption mechanism as well as the energy relationship between the desorption barrier and isosteric heat of adsorption are discussed.     

The vibrational spectrum of thiazole between 600 and 1400 cm revisited: A combined high-resolution infrared and theoretical study

The Fourier transform infrared gas-phase spectrum of thiazole, C₃H₃NS, has been recorded in the 600-1400 cm⁻¹ wavenumber region with a resolution around 0.0030 cm⁻¹. Nine fundamental bands (ν₅(A′) to ν₁₁(A′), ν₁₅(A″), and ν₁₆(A″)) are analysed employing the Watson model. Ground-state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from the fits. A detailed analysis of perturbations identified in the ν₁₁(A′) band at 866.5 cm⁻¹ enables a definitive location of the very weak ν₁₀(A′) and ν₁₄(A″) bands at 879.3 and 888.7 cm⁻¹, respectively. The three levels are analysed simultaneously by a model including Coriolis resonance using an ab initio predicted first order c-Coriolis coupling constant; second and higher order Coriolis parameters are determined. Qualitative explanations in terms of Coriolis resonances are given for a number of crossings observed in ν₅(A′), ν₆(A′), and ν₇(A′) at 1383.7, 1325.8, and 1240.5 cm⁻¹, respectively. The rotational constants, anharmonic frequencies, and vibration-rotation constants (alphas, ) calculated by quantum chemical calculations using a cc-pVTZ and TZ2P basis with B3LYP methodology, have been compared with the present experimental data. The rotation constant differences for each vibrational state, from the ground state values, are closer to experiment from the TZ2P calculations relative to those using cc-pVTZ. The values for Δ_J, Δ_JK, Δ_K, δ_J, and δ_K are close to experiment with both basis sets.     

Sub-Doppler high-resolution excitation spectroscopy of the S1 ← S0 transition of dibenzofuran

Rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the S₁ ← S₀ transition of dibenzofuran have been observed using the technique of crossing a collimated molecular beam and the single-mode UV laser beam. 3291 rotational lines of the band and 3047 rotational lines of the band have been assigned. The band has been found to be a b-type transition, in which the transition moment is along the twofold symmetry axis of this molecule, and only the ΔK_a = ± 1 transitions were observed. The excited state is identified to be the S₁¹A₁(ππ^∗) state. In contrast with this, the band has been found to be an a-type transition in which the transition moment is along the long axis in plane. It indicates that the intensity of this vibronic band arises from vibronic coupling with the S₂¹B₂(ππ^∗) state. We determined the accurate rotational constants and the molecule have been shown to be planar both in the ground and excited states.     

Experimental and theoretical study of the electronic states and spectra of PbLi

Gas phase emission spectra of the hitherto unknown free radical PbLi have been measured in the NIR range with a Fourier-transform spectrometer. The emissions were observed from a fast flow system in which lead vapor in argon carrier gas was passed through a microwave discharge and mixed with lithium vapor in an observation tube. Five electronic transitions have been found in the wavenumber range 3800-10 000 cm⁻¹. Bands from two excited states to the ground state were measured at high spectral resolution such that rotational analyses could be performed and accurate molecular parameters derived. In order to aid in the analysis of the experimental data, a series of relativistic configuration interaction calculations has been carried out to obtain potential curves for the low-lying states of PbLi and also electric dipole transition moments connecting them. As in the lighter molecules of this group, CLi and SiLi, the ground state of PbLi is found to be 3/2) with a spin splitting of about 2000 cm⁻¹. The first excited state is (A 1/2), and two observed band systems are assigned to the transitions A→X₁ and A→X₂. Two more excited states, (B 3/2) and C 1/2, are identified from the observed spectra with the help of the computed data, and their spectroscopic constants are determined. In contrast to PbH and PbF, the ab initio results indicate a very complicated low-energy electronic structure for the PbLi radical, with 19 bound electronic states calculated to lie below 3 eV.     

Time-resolved infrared diode laser spectroscopy of the ν1 (C-O stretch) band of the CoCO radical

Infrared spectrum of the cobalt carbonyl radical CoCO produced by the 193 nm excimer laser photolysis of cobalt tricarbonyl nitrosyl Co(CO)₃NO was observed by time-resolved diode laser spectroscopy. More than 600 lines were identified as belonging to the ν₁ (C-O stretch) fundamental band, consisting of the Ω=5/2 and 3/2 subbands, and the associated hot bands , , , and . The ²Δ_i electronic ground state of CoCO was experimentally confirmed. The ν₁ band origins are 1974.172582(93) cm⁻¹ and 1973.53178(14) cm⁻¹ for the Ω=5/2 and 3/2 subbands, respectively. The rotational constant in the ground state was determined as B₀=4427.146(50) MHz. The centrifugal distortion constant D₀=1.1243(68) kHz was obtained for the Ω=5/2 substate of the ground state. The equilibrium rotational constant B_e=4435.44(14) MHz was derived, together with the vibration-rotation interaction constants.