Vibronic structure of the 3s Rydberg state of the 2-methylallyl radical

Resonance-enhanced multiphoton ionization combined with electronic ground state depletion spectroscopy of jet-cooled 2-methylallyl (C₄H₇) radicals provides vibronic spectra of the 3s and 3p Rydberg states. Analysis of the vibronic structure following one-photon and two-photon excitation of rovibronically cold 2-methylallyl radicals and its isotopologues C₄H₄D₃ and C₄D₇ reveals transitions to more than 30 vibrational levels in the 3s Rydberg state that are identified and reassigned on the basis of predictions from ab initio calculations and results from pulsed-field-ionization zero-kinetic-energy photoelectron spectra obtained with resonant multiphoton excitation via selected intermediate states. Depletion spectroscopy reveals transitions to short-lived 3p Rydberg states that have a large oscillator strength.     

Microwave spectrum,dipole moment,and structure of 3-aminopropanol

The microwave spectra of 3-aminopropanol and three of its deuterium substituted isotopic species have been investigated in the 26.5 to 40 GHz frequency region. The rotational spectrum of only one conformer has been assigned in which presumably a hydrogen bond of the OH---N type exists. The rotational spectra of a number of excited vibrational states have been observed and assignments made for some of these excited states. The average intensity ratio for the rotational transitions between the ground and excited vibrational states indicates that the first excited state is about 120 cm^?1 above the ground state.and the next higher state is roughly 200 cm^?1 above the ground vibrational state. The dipole moment was determined from the Stark effect measurements to be 3.13 ± 0.04 D with its principal axes components as |μ_a| = 2.88 ± 0.03 D, |μ_b| = 1.23 ± 0.04 D and |μ_c| = 0.06 ± 0.01 D. The possibility of another conformer where the hydrogen bond could be of NH---O type was explored, but the spectra of such a conformer could not be identified.     

Spectral transitions from the Rydberg autoionization states of a Li-like Mg X ion

Satellite lines caused by radiative transitions from the Rydberg autoionization states of a Li-like Mg X ion in a plasma heated by radiation from a XeCl and a Nd laser are identified for the first time, and their wavelengths are measured precisely. Comparison of the experimental data with the atomic structures calculated by the method of relativistic perturbation theory shows that the accuracy of calculations of the energy of autoionization states is rather high even without the use of any semiempirical corrections and is of the order of 0.06%. The experimentally measured wavelengths can be used for a semiempirical estimate of the value of the leading order of perturbation theory among the orders that were neglected in calculations. It is shown that the simulation of the population kinetics of Rydberg autoionization states of Li-like ions in a dense plasma should take into account all possible channels of dielectronic capture, in particular, from the excited states of a He-like ion. The precision experimental wavelengths obtained for satellites of the He_ß and He_γ lines of the Mg XI ion make possible to use these satellites as reference lines in studies of complicated spectra of multielectron ions.     

The b-type rotational transitions of HNCO in the lowest excited vibrational state

In the microwave and millimeter wave spectra of HNCO, the b-type transitions between the K_a = 0 and 1 levels in the lowest excited vibrational state have been observed. Because of strong a-type Coriolis resonances among the three bending excited states the energy difference between the levels for K_a = 0 and 1 is much smaller in the lowest excited state than in the ground state. The subband origin of these b-type transitions has been found in the millimeter wave region at 275 697.309 MHz (9.1963 cm^?1). The effect of the Coriolis resonances is discussed in relation to the molecular quasi-linearity and is compared with the case of HNCS.     

The first s-Rydberg transitions of ethyl bromide and ethyl iodide as related to the corresponding methyl halides

The transitions observed in the first s-Rydberg region of the spectra of ethyl bromide, ethyl bromide-d₅, and ethyl iodide are reported and assigned. The assignments are made by examining the effect of symmetry reduction from C_3v to C_s on the observed electronic states and vibrational modes of the corresponding methyl halide spectrum. The calculated spectra obtained in this way give good agreement with the observed spectra. These resulting assignments are discussed in terms of a competition between spin-orbit coupling and hyper-conjugation.     

Millimeter wave spectra of OPF3 in the vibrationally excited state v6 = 2

Millimeter wave spectra have been recorded for the excited vibrational state v₆ = 2 of OPF₃. A full analysis of these spectra yields new rovibrational parameters and also the vibrational separation of the l = 0 and l = 2 levels, given by x_ll = 16 134 ± 390 MHz. The results are compared with the v₆ = 1 state.     

Photoelectron-photon coincidence studies of the A$̃and B$̃excited electronic states of X-CC-CN+, X = CH3, CD3, Cl,Br, I

Fluorescence quantum yields and lifetimes of the above given cations in selected levels within their lowest excited electronic states have been measured by a photoelectron—photon coincidence technique. These data, obtained under collision-free conditions, lead to the radiative and non-radiative rate constants as a function of the internal energy. The symmetry of the A$\text{\$}\text{?}$state is ²A₁ (X = CH₃, CD₃), ²Σ⁺ (X = Cl), but ²Π (X = Br, I) and the corresponding k_r values for these two groups, 1–2 × 10⁶s^?1 and 2 × 10⁷s^?1 respectively, reflect the different nature of the transitions. Other essential features of the results are discussed.     

The rotation-vibration spectrum and double-minimum ν12 potential function of propadienone: A semirigid bender analysis

Propadienone is an interconverting molecule having a pair of equivalent symmetry related conformers separated by an energy barrier rising well above the vibrational ground state. Microwave spectra of molecules in excited states of the large-amplitude in-plane bending mode ν₁₂, including intersystem lines, have been successfully represented using the semirigid bender model. The model reveals a double-minimum bending potential with a barrier rising 359 cm⁻¹ above the minima at C₁C₂C₃ = 142°. In the ground state the interconversion frequency is 3.7 GHz and the ν₁₂ fundamental frequency is predicted to be 160 cm⁻¹. Analysis of other vibrational satellites involving the lowest-frequency out-of-plane mode ν₈ indicates a vibrational frequency of 240 cm⁻¹. The inplane vibrational satellite and also the ground state substitution spectra are quite accurately reproduced by the model. Our generalized semirigid bender method offers a variety of approaches to fitting molecular parameters to the experimental data.     

Rydberg states of carbon dioxide and carbon disulfide

The electronic absorption spectra of carbon dioxide and carbon disulfide have been reexamined. Model potential calculations have been used to calculate the energies of excited states in Rydberg approximation, and (npσ) and (npπ) Rydberg series have been assigned. For both molecules, the lowest excited ¹Π_g and ¹Π_u states are identified as Rydberg states. The lowest ${}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ state is mainly Rydberg for CO₂, but contains some valence character for CS₂, There is no evidence for transitions to additional valence states of these symmetries.It is shown that $\text{LCAO}\text{MO}$ predictions about excited states can be misleading because of near-linear dependencies which arise in multicenter expansions. A consideration of the united atom orbitals for CO₂ and CS₂ predicts that there should be only the number of low-energy excited states which are found from the spectral analysis.     

The millimeter wave spectrum of tertiary butyl fluoride

Rotational spectra of tertiary butyl fluoride (TBF) in the ground and excited vibrational states have been recorded and analyzed. The excited state spectra show large splittings due to l resonance and the effect of the 2, -1 term r_t. Coriolis constants have been obtained for the three lowest degenerate states. An accidental resonance enabled the determination of the axial rotational constant of TBF.     

Study of the 3s Rydberg state of pyrimidine by multiphoton ionization spectroscopy

The 3s Rydberg state of pyrimidine is seen as a two-photon resonant structure on its three-photon ionization spectrum. The vibrational structure is analyzed and compared with the VUV absorption and photoelectron spectra.     

Study of the 3s Rydberg states of picolines by multiphoton lonization spectroscopy

The 3s Rydberg states of 2-, 3-, and 4-picolines are seen as a two-photon resonant structure on their three-photon ionization spectrum and their vibrational analysis is given.     

The Dushinsky effect and sum rules for vibronic transitions in polyatomic molecules

A method is presented for analyzing, in terms of sum rules, the intensity distribution among the vibronic bands in electronic spectra of polyatomic molecules, taking into account the rotation of the excited-state normal coordinates relative to those of the ground state (the Dushinsky effect). In the harmonic oscillator approximation, a quantitative criterion for the occurrence of the Dushinsky effect is obtained. The existence of this effect leads to non-product formulas for probabilities of the joint excitation of different vibrational modes. Expressions are obtained for the mean number of quanta excited in a given mode, and for its standard deviation, as well as for the correlation coefficients. The proposed method does not require a complete vibrational analysis of the spectrum of interest. As an example, the calculation of the correlation coefficient for the 3700-Å band system of the absorption spectrum of SO₂ is given.     

Electronic spectra of XeNe molecules in the range 77100-90100 cm-1 near Xe* (6p, 5d, 6p’, 7s, 7p, 6d) obtained by the (3 + 1) REMPI and (2 + 1) REMPI methods

The electronic spectra of XeNe molecules in the range of 77100-90100 cm^-1 are measured by the method of laser resonance multiphoton ionization in a supersonic jet. The photoionization spectra are obtained upon two- and three-photon excitations of molecules and their ionization by the next photon. In the range of 80300-90100 cm^-1 near Xe*(5d, 6p’, 6d, 7s, and 7p), the spectra are obtained for the first time. A whole number of vibrational systems are measured in this range. The majority of vibrational systems near Xe* (5d, 6d, 7p, and 7s) are located in the red range with respect to their dissociation limits. In the blue range with respect to the dissociation limits, continua corresponding to transitions of molecules from the ground state to repulsive potential curves of excited states are detected. For a number of excited states of XeNe molecules, the vibrational analysis is performed and molecular constants are estimated.     

Microwave spectrum of dimethylallene excited states and strong Coriolis coupling

The rotational spectra of six excited vibrational states of dimethylallene were measured and assigned to the corresponding vibrational levels, and for three more excited state spectra at least the rotational constants could be determined. Between the two lowest excited levels of symmetry species b₂ and b₁ of group C_2v a strong a-type Coriolis coupling was found to exist. The evaluation of the resulting perturbation by a diagonalization of the energy matrix yielded ζ^(a) = 0.36 and a precise value for the vibrational energy difference 48.761 GHz (1.6 cm^?1). The state b₂ is believed to be the first excited torsional substate (01, 10)₁ of methyl internal rotation, and the rotational transitions of this state as well as those of the strongly coupled state b₁ presented very irregular multiplet splittings. On the other hand, the splittings of the next-higher excited state of species a₂ which could be identified as the partner torsional substate (01, 10)₂, followed the regular pattern, yielding an internal rotation barrier V₃ (2079 cal/mole) not unlike that derived earlier from ground state splittings.     

Vibronic spectra, ab initio calculations, and structures of conformationally non-rigid molecules of oxalyl halides in the ground and lowest excited electronic states. Part II: Theoretical investigation of oxalyl chloride

The structures of the oxalyl chloride molecule (COCl)₂ in the ground and the four lowest (two singlet and two triplet) excited electronic states were investigated by means of the CASPT2(8-6)/cc-pVTZ technique. The equilibrium geometric parameters, harmonic vibrational frequencies, and adiabatic energies of the electronic transitions were determined for all states under investigation. The calculations predicted the existence of the trans- and gauche- conformers in the ground state and the trans- and cis-conformers in all excited states. For the ground electronic state, the conformer energy difference and the barriers to conformational transitions were estimated using extrapolation to the complete basis set within a Valence Focal-Point Analysis procedure. The internal rotation in the excited electronic states was found to be strongly coupled with the non-planar symmetric CCOCl wagging. Two-dimensional potential energy surface sections along internal rotation and non-planar coordinates were constructed, and the corresponding anharmonic vibrational problems for the trans-conformer were solved.     

High resolution multiphoton ionization and dissociation of acetone via $\mathsf{3s\leftarrow n}$ Rydberg transitions

Measurements of multiphoton ionization and dissociation of acetone are reported in the wavelength range 582.60-585.80 nm at photon energy resolution of 0.3 cm^-1. To our knowledge there are not available results of (3 + 2) REMPI on acetone at 0.01 nm in this region. The experiments were performed using an Nd: YAG-OPO (optical parametric oscillator) laser system coupled to time-of-flight mass spectrometer. The ion yield and the single ion channel are presented. The high-resolution three-photon resonance multiphoton spectrum of the acetone Rydberg transition is also reported. The experimental results show three dissociation channels of the acetone ion, leading to the products: (CH₃CO ⁺ ), (CH₃ ⁺ ) and (COH ⁺ ); the channel CH₃COCH ₃CO ⁺ + CH₃ being the most favored. The acetone and acetyl ions are observed in all wavelength range investigated. In addition, we have measured the origin of the and transitions, and vibrational bands of the 3s state.Received: 21 March 2004, Published online: 11 May 2004PACS: 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 33.15.Ta Mass spectra - 33.80.Gj Diffuse spectra; predissociation, photodissociation     

Determination of the skeletal bending potential function for SiH3NCO from the microwave spectrum

The rotation-bending-torsion Hamiltonian (M. Kr?glewski, J. Mol. Spectrosc., in press) is used to simultaneously fit observed rotational transitions (J. A. Duckett, Ph.D. thesis, University of Reading, 1976) of silyl isocyanate SiH₃NCO in 18 vibrational states involving excited quanta of the ν₁₀ SiNC skeletal bending mode. The effective SiNC bending potential energy function and the geometry of the molecule in the small amplitude vibrational ground state are determined from the microwave data.     

The two-dimensional anharmonic oscillator: The CCC bending mode of C3O2

Newly observed data on the rotational constants of carbon su?ide in excited vibrational states of the low-wavenumber bending vibration ν₇ have been successfully interpreted in terms of the two-dimensional anharmonic oscillator wavefunctions associated with this vibration. By combining these results with published infrared and Raman spectra the vibrational assignment has been extended and a refined bending potential for ν₇ has been derived: this has a minimum at a bending angle of about 24° at the central C atom, with an energy maximum at the linear configuration some 23 cm^?1 above the minimum. From similar data on the combination and hot bands of ν₇ with ν₄ (1587 cm^?1) and ν₂ (786 cm^?1) the effective ν₇ bending potential has also been determined in the one-quantum excited states of ν₄ and ν₂. The effective ν₇ potential shows significant changes from the ground vibrational state; the central hump in the ν₇ potential surface is increased to about 50 cm^?1 in the v₄ = 1 state, and decreased to about 1 cm^?1 in the v₂ = 1 state. In the light of these results vibrational assignments are suggested for most of the observed bands in the infrared and Raman spectra of C₃O₂.     

Microwave spectrum,conformation, and dipole moment of 4-thiacyclohexanone

The microwave spectra of 4-thiacyclohexanone in the ground state and eight vibrationally excited states have been studied in the frequency region 18.0–40.0 GHz and the corresponding rotational constants have been determined. The following values of the ground-state rotational constants (MHz) were obtained from the analysis of the a-type transitions: A = 3935.149 (0.031), B = 1829.444 (0.001), and C = 1364.609 (0.001). Analysis of the Stark effect gives for the dipole components (in Debye units) μ_a = 1.409 (0.002), μ_c = 0.391 (0.064). These data are consistent with a chair conformation for the ring. A phisically reasonable set of structural parameters which reproduce the ground-state rotational constants has been derived. A qualitative estimate of the low-frequency vibrational modes was obtained from relative-intensity measurements. The lowest vibrational frequency is believed to be a ring-bending mode and it occurs at 77 ± 22 cm^?1 while the ring-twisting mode is at 204 ± 27 cm^?1.