Claudina Cossart-Magos

Two‐mode vibronic interaction between neighboring 1 2A2 and 2 2B2 excited electronic states of the benzyl radical

The 2 2B2–1 2B2 transition of the benzyl radical occurs in the same frequency region as the 1 2A2–1 2B2 transition. Vibronic coupling between the 2 2B2 and 1 2A2 excited electronic states via 6b and 18b vibrational modes is shown to be responsible for the unusual structure of the electronic spectrum of benzyl observed in the visible region. Calculations of vibronic interaction were carried out using a model constructed with crude adiabatic, harmonic oscillator basis functions. The model was expressed in terms of four dimensionless parameters: D, the energy separation between the deperturbed 2 2B2 and 1 2A2 states; Cα and Cβ, which are interaction constants for the α≡18b and β≡6b modes, respectively; the ratio ωα/ωβ of these zero‐order mode frequencies. The possible range of these constants is discussed in detail. The model reproduces well the vibronic energies and relative transition intensities of bands observed within 700 cm−1 of the 1 2A2–1 2B2 transition origin for isotopic benzyl radicals. The depert...

The second conformer of the benzene—argon2 van der Waals complex

Abstract From a rotational contour study of UV vibronic bands of the benzene—Ar 2 complex as well as from theoretical structures modelling, conclusive arguments are presented, demonstrating the existence of a second (2, 0) stable conformer of C s symmetry, in addition to the already known symmetric (1, 1) species.

High‐resolution gas phase emission and laser induced fluorescence excitation spectra of 1,3,5‐C6F3H3+ and 1,3,5‐C6F3D3+: Critical bands in the Jahn–Teller effect analysis

Cooled gas phase laser induced fluorescence excitation (LIFEX) spectra of 1,3,5‐C6F3H3+ (h3+) and 1,3,5‐C6F3D3+ (d3+) ions were obtained between 4600 and 4110 A at a resolution of 1 cm−1. Band intensities were corrected for laser power variations. Gas phase discharge emission spectra of the same ions were photographed at a resolution of 0.25 cm−1 from 4450 to 4800 A and at 0.1 cm−1, between 4500 and 4700 A. In emission, only the spectral region λ<4580 A, corresponding to the 000 band and bands to lower wavelengths, is reported here. Particular regions of the LIFEX spectra were also recorded at a resolution of 0.25 cm−1. Vibronic analysis of the B 2A″2–X 2E″ transition is made on the basis of five (seven) e′, three a′1, and one a″2 excited state fundamental frequencies which were assigned in the h3+ (d3+) ions. A few low‐lying ground state levels, among which the first j=3/2 (A1, A2) levels of mode 6 (e′), the most active Jahn–Teller (JT) mode, are particularly critical. Some discrepancies in band relati...

The [Btilde] 2 A″2 → [Xtilde] 2 E″ transition of 1,3,5-C6F3H3 + and 1,3,5-C6F3D3 + in discharge and supersonic free jet emission sources

Emission spectra of 1,3,5-C6F3H3 +(h 3 +) and 1,3,5-C6F3D3 +(d 3 +) ions were obtained by electron impact in seeded He and Ar supersonic free jets. The h 3 + (and d 3 +) spectra were recorded between 4570 (4445) A and 4820 (4810) A with a resolution R of the order of 0·5 cm-1. Gas phase discharge emission spectra of the same ions were photographed at R=0·25 cm-1 from 4450 to 4800 A and R=0·1 cm-1, between 4500 and 4700 A, and are compared to the jet cooled spectra. This comparison leads to some new or refined spectral assignments. Jet cooled and Ne matrix ion emission spectra are also compared, particularly with regard to frequencies and relative intensities of bands exhibiting the Jahn-Teller effect in the [Btilde] 2 A″2-[Xtilde] 2 E″ transition. Matrix dependent effects on Jahn-Teller levels and interaction parameters are observed and discussed. Rotational contour analysis of peak structures in the 00/0 and 61,1/2/0,0 emission bands in the discharge and jet spectra of the h 3 + ion indicates that ΔB ⋍ -...

Rotational contour analysis of selected absorption bands of benzyl produced by flash photolysis

Abstract Three bands of the flash-photolysis absorption spectrum of benzyl in the visible are rotationally analyzed: a type- B band, 6 a 0 1 , at 4456 A, and two type- A bands, A 1 and A 2 , at 4477 and 4465 A, respectively. The band notation is taken from the vibronic analysis of that region of the benzyl electronic spectrum by C. Cossart-Magos and S. Leach. The observed rotational contours were fitted by computer-simulated rotational contours, resulting in the determination of the variations, Δ A , Δ B , and Δ C of the rotational constants accompanying the vibronic transitions corresponding to each band. Different A ′, B ′, and C ′ values are obtained for the three upper levels and are discussed in light of the previous vibronic assignments. The application of the present results to the study of low-temperature rotational band contours in supercooled benzyl spectra is also discussed.

Rotational contour analysis of the electronic origin band in the emission spectrum of the orthoxylyl radical at 4683 Å

Abstract The rotational contour of the 4683 A emission band of the o-xylyl radical was studied at high resolution. Calculations of the rotational contour of this hybrid band were made in the rigid rotor approximation for various sets of values of the excited state rotational constants and directions of the transition moment μ. Matching of computed and experimental rotational features showed that μ is oriented at +37° or −37° with respect to the b inertial axis. The nature of the excited states of o-xylyl and the methyl-to-ring interaction are discussed with respect to these two possible assignments

Jahn—Teller effects in substituted benzene cations. IV. Intermode interactions and intensity anomalies in sym-trifluorobenzene ions

Abstract A theoretical treatment of coupling between Jahn—Teller modes is given which includes both linear and quadratic Jahn—Teller coupling. Effects of two and three mode interactions on the B2A″2 → X2E″ transition of the sym-trifluorobenzene ions are studied, within the linear coupling approximation, for modes 6, 7, 8 and 9. Quadratic coupling effects are considered in a companion paper (part V). The results show that discrepancies previously noted between observed relative band intensities and those calculated on the basis of single Jahn—Teller modes cannot be accounted for by multimode interactions or by Fermi resonance effects operating in conjunction with Jahn—Teller effects. Jahn—Teller combination bands are shown to have several components, the most intense of which are assigned. Particular attention is paid to various possible assignments of the 80,01, 1 2 and 60,03, 1 2 vibronic transitions in the light of the multimode interaction analyses.

Jahn—Teller effects in substituted benzene cations. V. Quadratic coupling

Abstract A calculation model is derived for taking into account quadratic, in addition to linear, coupling Jahn—Teller effects in determining vibronic energy levels and transitions. Procedures are developed for analysis of Jahn—Teller electronic spectra on this basis and the new features, with respect to the linear coupling approximation, brought about by introduction of quadratic coupling, are discussed. Vibronic analyses of the B 2 A″ 2 -X 2 E″ transitions of 1,3,5-C 6 F 3 H + 3 , 1,3,5-C 6 F 3 D + 3 and 1,3,5-C 6 Cl 3 H + 3 are carried out, in particular for bands involving excitation of the mode 6 vibration. Experimental evidence for quadratic Jahn—Teller effects is obtained for the sym-trifluorobenzene ions and the linear coupling parameters D 6 , ω 6 and quadratic coupling parameter q 6 are derived. Two possible orders of magnitude of the quadratic coupling strength are found to be compatible with the spectra of 1,3,5-C 6 Cl 3 H + 3 . The analyses are consistent between the three ions and are not in contradiction with the general findings based on the linear approximation alone.

Rotational band contour analysis in the B̃←X̃ system of chlorobenzene cation prepared by resonance enhanced multiphoton ionization

Abstract The rotational structure of some vibronic bands of the B←X system of the chlorobenzene cation (φCl + ) has been studied by computation of rotational band contours. For the first time contours for ions prepared by resonance enhanced multiphoton ionization (REMPI), with a restricted (or non-equilibrium) rotational excitation have been analysed. Using the mass selectivity of the REMPI method, the relative contributions from the two principal isotopes 35 Clφ and 37 Clφ have been taken into account. For this purpose the excited state rotational constants of 37 Clφ have been determined. The ionic bands are shown to be A type which agrees with our vibrational assignments.

The C2Δr-A2Πi transition of the 12CO+ and 13CO+ ions: New rotational analysis and isotope effect studies

Abstract Use of a special emission source allowed recording of high-resolution spectra for the C2Δ-A2Π transition of the CO+ ion. Relative to previous work, the new information on the C2Δ state was as follows: 1. (i) determination of the vibrational numbering from the isotopic spectra, 2. (ii) substantial corrections in the spin-orbit and rotational constants, 3. (iii) evidence for an electron configuration change with increasing vibrational quantum number.