Daniel Cossart

Emission spectra of seven fluorobenzene cations

The emission spectra of C6F6 +, C6F5H+, the three C6F4H2 + and two C6F3H3 + (1, 2, 4 and 1, 3, 5) cations were obtained in the gas phase, in a special discharge tube. Symmetry and mode correlation of the fundamental frequencies of the parent neutral molecules enabled the isodynamic molecule method to be used for achieving vibrational analysis of the ion spectra. The vibronic structure in each emission spectrum is interpreted. Dynamic Jahn-Teller effects are shown to occur in the degenerate ground state of C6F6 + and of 1, 3, 5-C6F3H3 +, in particular irregular ground-state progressions in mode 6 are observed. Extensive Jahn-Teller calculations were made for 1, 3, 5-C6F3H3 +; frequencies and intensities in its progressions are in good agreement with the results of calculations based on the linear coupling model of Longuet-Higgins et al., using a deperturbed frequency ω = 477–484 cm-1 and a dimensionless interaction parameter D = 0·13-0·10. A review of the vibrational data of photoelectron spectra and Rydbe...

Rotational band contours of the first electronic transition origin bands of p-xylyl and p-fluorobenzyl

The origin bands of the gas phase discharge emission spectra of the p-xylyl and p-fluorobenzyl radicals were photographed at high resolution. The observed band contours were fitted by computer-simulated type B rotational bands; this indicates that the transition moment in both cases is along the b in-plane short axis and corresponds to a 1 2 A 2-1 2 B 2 first electronic transition, as found previously in benzyl itself. For reasonable geometrical models of the ground state (methyl-group reduced to a mass-point in p-xylyl), A″, B″ and C″ rotational constants and their variations ΔA, ΔB and ΔC during the transition are determined. The band origins are found to be at 21700·5 ± 0·2 and 21524·0 ± 0·2 cm-1 respectively in p-xylyl and p-fluorobenzyl, with observed band maxima which are at 4606·57 A or 21702·05 cm-1 and 4644·14 A or 21526·48 cm-1, respectively. Four band sequences are identified in both spectra and assignments of the vibrational modes involved are discussed. In p-xylyl are also shown and discussed...

The optical spectrum of the doubly charged molecular nitrogen ion: Rotational analysis of the (0-0) and (1-1) bands of the D1Σu+-X1Σg+ transition of N22+: Comparison of observations with Ab-initio calculation results

Abstract Emission spectra obtained in the 1550–1650 A region with a 10-m vuv spectrograph are conclusively assigned to the N 2 2+ ion. The 1589-A band, previously observed by Carroll, and a new band of the same system, have been rotationally analyzed. Ab-initio calculations have been performed which support the assignment of these two bands to the D 1 Σ u + - X 1 Σ g + system. The calculations also explain the observed breaking-off points in the branch structure as well as weakening and broadening of the other expected bands. These phenomena arise from electron configuration changes and perturbation effects in the ground state.

Jahn—Teller effects in substituted benzene cations. II. Gas phase emission spectrum of 1, 3, 5-C6F3D+3 and comparison with 1, 3, 5-C6F3H+3

Abstract The emission spectrum of 1, 3, 5-C 6 F 3 D + 3 in the 4376–5435 A region has been photographed and vibronically analyzed. The transition is B 2 A″ 2 -X 2 E″. Vibrational frequencies were assigned using the isodynamic molecule method as previously done in analyzing the 1, 3, 5-C 6 F 3 H + 3 spectrum. Out-of-plane a″ 2 and e″ modes are active in even quanta only and are associated with sequence bands a′ 1 and e′ modes are active in both odd and even quanta. The e′ mode activity is shown to be due to dynamic Jahn—Teller effects in the X 2 E″ state. A brief review of the relevant theory leads to a discussion of criteria for identifying Jahn—Teller spectral effects. Two importants parameters enter into the linear coupling model calculations: D , the interaction constant, and ω, the deperturbed frequency of the Jahn—Teller active mode. These parameters are determined by fitting calculated band positions and intensities to experimental results. It is shown how to resolve possible ambiguities in these determinations, D and ω values were obtained for four e′ modes of 1, 3, 5-C 6 F 3 D + 3 and 1, 3, 5-C 6 F 3 + 3 , modes 6, 7, 8 and 9. The corresponding Jahn—Teller barrier heights for interconversion between the lowest energy asymmetric configurations were evaluated for the two ions.

Optical emission spectrum of the NO2+ dication

Abstract We recorded and rotationally analyzed a new emission band at 2563 A obtained from a low-pressure, hot cathode and magnetically confined electric discharge through pure NO vapor. The available experimental data from photoion-fluorescence photon coincidence and translational energy loss spectroscopy, in conjunction with ab initio calculation, allowed assignment of the new band to the B 2 Σ + - X 2 Σ + (0, 0) transition of the NO 2+ ion.

The lower excited states of CS: A study of extensive spin-orbit perturbations

Abstract We present previously unpublished data on the A1Π, e3Σ−, d3Δi, a′ 3 Σ + , and a3Π states of CS from uv emission and absorption transitions to the X1Σ+ ground state. Term values obtained from d3Δi ← a3Π ir emission bands are also included. Rotational analyses are presented for about 50 new fine-structure components in some 30 new vibrational levels, together with extended data and analyses for many of the previously observed levels. The data now availabe for these five electronic states more than triples that previously published. Vibrational numbering for the e, d, and a′ states is established by data for minor isotopes. In a Hunds case a-b basis, off-diagonal spin-orbit elements (incipient case c effects) produce extensive coupling among these levels, not only for perturbation crossings but also between levels widely separated in energy. A systematic deperturbation requires two stages, which are iterated. Term values computed from the spectral lines are used to fit parameters of Hamiltonian matrices for groups of nearby, coupled levels. Additional shifts are computed by second-order perturbation theory from the electronic interactions deduced from vibronic coupling elements. The resultant parameters satisfy certain tests for self-consistency; they conform to low-order polynomials in v + 1 2 , and vibrational overlap factors from wave-functions computed with RKR potential curves are proportional to the vibronic coupling elements, to within experimental error in most cases. To obtain this self-consistency, we have computed and applied normally neglected centrifugal distortion effects in the off-diagonal coupling elements and in the second-order perturbation sums. We also present and interpret the diagonal spin-orbit fine structure in the a and d states, including the centrifugal distortion parameters, AD, for the latter, and values for several fitted second-order elements. Possible assignments for three additional perturbations of the A1Π state and one faint band are discussed in view of 1Δ, 1Σ−, and 5Π states that are also expected to occur in the region studied. Tentative parameters for the D1Δ state are obtained from one possible set of assignments.

The vacuum uv emission spectrum of the 15N22+ molecular ion

Abstract The vacuum uv emission of the 15N22+ ion has been recorded for the first time. Rotational analysis of two bands, analogous to those already observed in the case of the natural isotope, confirm their assignment to the D1Σu+-X1Σg+ (0, 0) and (1, 1) bands. More precise data are also obtained for the 3Σg− state which perturbs ground state vibrational levels.

New electronic transitions in the N2+ ion: Rotational analysis of the D′2Πgr-A2Πui transition: Comparison of experimental and ab initio calculated molecular constants

Abstract Rotational analysis has been carried out for the first time for bands of the D ′ 2 Π g - A 2 Π u system of the N 2 + ion. At the same time, ab initio calculations were performed using the SCF method followed by full-valence configuration interaction calculations. Theoretical results are presented as required for interpretation of the spectra.

The first valence states of the SO+ ion: Rotational analysis of the A2Πi-X2Πr and b4Σ−-a4Πi transitions. Comparison of experimental and ab initio calculated molecular parameters

Abstract Rotational analysis has been carried out for the first time for bands of the A 2 Π i - X 2 Π r system of the SO + ion, as well as for newly discovered bands of the b 4 Σ − - a 4 Π transition of the same ion. At the same time, ab initio calculations were performed using the SCF method followed by configuration interaction. Theoretical results are presented as required for interpretation of the spectra. Important spin-orbit interactions are shown to occur between the four X 2 Π r , A 2 Π i , a 4 Π i , and b 4 Σ − states and neighboring unobserved states.

Jahn—Teller effects in substituted benzene cations. III. Gas phase emission spectrum of sym-C6Cl3H+3

Abstract The gas phase emission spectrum of 1, 3, 5-C6Cl3H+3 was obtained in a discharge tube. Vibronic analysis involving correlation of ion fundamental frequencies with those of parent molecules enables a detailed comparison to be made between the vibrational structure of the spectrum of the trichloro-ion and that of 1, 3, 5-C6F3H+3. Analogous Jahn—Teller effects are shown to take place in the ground state of the two ions. The same linear coupling model is used and two possible solutions for D6 and ω6 are obtained by fitting the first two intervals in the 60,0v 1 2 progression in 1, 3, 5-C6Cl 3H+3: (i) D6 = 0.05 - 0.08, ω6 = 455 - 447 cm−1, (ii) D6 = 0.39 - 0.49, ω6 = 399 - 392 cm−1. Arguments based on previously established criteria, and applied here to the case of the 1, 3, 5-C6Cl3H+3 ion are given to select set (ii) as being far more satisfactory in interpreting the experimental data. Fitting to a larger number of bands gave D6 = 0.45, ω6 = 395 cm−1. The Jahn—Teller potential energy barrier EJT6 = 178 cm−1 is about three times greater than in the 1, 3, 5-trifluorobenzene ions, but sufficiently small for the dynamic Jahn—Teller effect to be operative in 1, 3, 5-C6Cl3H+3. Some remarks are made concerning assignments in the excitation spectrum and matrix fluorescence analyses.