Muriel Wyss

Electronic absorption spectra of linear C6, C8 and cyclic C10, C12 in neon matrices

The electronic absorption spectra of the even-numbered carbon molecules C6–C14 have been measured in neon matrices. Bare carbon anions were produced in a cesium sputter source, mass selected, codeposited with neon at 6 K, and neutralized. The spectra show, apart from the known (1) 3Σu−←X 3Σg− transition of linear C6, C8, and C10 in the visible, absorption bands in the UV region. The spectral data when considered in conjunction with ab initio calculations show that the linear forms of C6 and C8 have the next strong (2) 3Σu−←X 3Σg− transition with band maximum near 238 and 277 nm, respectively, whereas the band systems of C10, C12, and C14 at 316, 332, and 347 nm are due to the monocyclic species.

Electronic and infrared spectra of H2C3H+ and cyclic C3H3+ in neon matrices

The A 1A′←X 1A1 electronic transition of the propargyl cation H2C3H+ with the origin band at 267.8(2) nm has been identified in a neon matrix at 5 K. The frequencies of the two modes excited in the upper state are 667(50) and 1629(50) cm−1 and imply a reduction of symmetry from C2v in the ground state to Cs in the excited state. The most intense IR mode of the propargyl cation is observed at 2079.9(1.0) cm−1 and for the cyclopropenyl cation at 3130.4(1.0) cm−1. Ab initio calculations on the excited states of the two isomer cations support the assignment and explain why the electronic transition could not be observed for the cyclic species; it lies below 200 nm. The A 2A″←X 2B1 and B 2A′←X 2B1 absorptions of the neutral propargyl radical have also been observed with origin bands at 351.9(2) and 343.0(2) nm, respectively. These results provide the basis for the study of these astrophysically interesting C3H3+ species in the gas phase.

Electronic absorption spectra of C2nH−,C2n−1N− (n=4–7), and C2n−1N (n=3–7) chains in neon matrices

The 1Σ+←X 1Σ+ electronic transition of the C2nH− and C2n−1N− (n=4–7) anion chains has been observed following mass selection and codeposition with excess neon at 6 K. Photodetachment of the electron resulted in the detection of a band system due to the neutral C2n−1N radical. The spectra suggest that C7N, like C5N, has a 2Σ ground state. The B 2Π←X 2Π transition is detected for the larger C2n−1N (n=5–7) chains. These appear at slightly higher energies than those of the isoelectronic C2nH radicals and show similar spectral features. Several infrared transitions of the anions have also been observed.

Electronic absorption spectra of B3 and B3− in neon matrices and ab initio analysis of the vibronic structure

Mass selected B3− ions have been isolated in 6 K neon matrices and their absorption spectra measured. A band system with origin at 467 nm is assigned as the 1E′←X 1A1′ electronic transition of the cyclic anion. After photobleaching, the 1 2E′←X 2A1′ and 2 2E′←X 2A1′ band systems of neutral cyclic B3 are observed which start around 736 and 458 nm, respectively. Large scale ab initio calculations have provided potential energy surfaces for a variational treatment of the vibrational motion. Calculated band origins leave no doubt about the electronic symmetry assignments. The complex vibrational structure in the 1 2E′ state, which is due to relatively strong Jahn–Teller distortions, appears to be closely reproduced by the calculated vibrational energies and intensities, if the first observed stronger line is identified with the first vibrationally excited state, placing the “true” band origin of the 1 2E′ state at 775 nm where no signal with significant strength is apparent. The 2 2E′ state undergoes only a r...

Electronic spectra of long odd-number carbon chains C17–C21 and C13−–C21−

Abstract Electronic transitions of the odd-number neutral and anionic carbon chains C 2 n +1 ( n =8–10) and C 2 n +1 − ( n =6–10) have been observed in a neon matrix at 6 K after co-deposition with laser-vaporized graphite. The assignments are based on a comparison with the previously identified electronic transition of C 7 to C 15 and C 5 − to C 11 − , and the monotonic wavelength dependence of the origin band on the number of carbon atoms. The 1 Σ + u ← X 1 Σ + g transition of C 17 to C 21 appears between 455 and 544 nm. The A 2 Π ← X 2 Π transition of C 13 − to C 21 − falls in the 1052–1675 nm region.

The A 3Σ−–X 3Σ− electronic transition of HC6N

A combined matrix and gas phase study is presented to identify the A 3Σ−–X 3Σ− electronic transition of the linear triplet isomer of HC6N and isotopic derivative DC6N. Absorption spectra have been observed in a 6 K neon matrix after mass selective deposition and in the gas phase by cavity ring down spectroscopy through a supersonic planar plasma. The band origin of the 000 A 3Σ−–X 3Σ− electronic transition of HC6N is determined to be at 21 208.60(5) cm−1, shifted ∼30 cm−1 to the blue of the neon matrix value. Rotational analysis indicates that the chain is slightly stretched on electronic excitation, yielding B0′=0.027 92(5) cm−1. Transitions to vibrationally excited levels in the upper A 3Σ− state are observed as well. The results are compared with a rotationally resolved spectrum of the 000 A 3Σu−–X 3Σg− electronic transition of the isoelectronic HC7H species.

Electronic absorption spectra of CCS− and CCS in neon matrices

Mass selected C2Sions have been co-deposited with neon to grow a matrix at 6 K. The A 2 R þ X 2 P and B 2 P X 2 P elec- tronic absorption spectra of the linear CCSanion have been identified with origin band at 10 606 and 22 273 cm � 1 , respectively. After exposure to UV radiation a new electronic transitionðE 3 RX 3 R � Þ of CCS is observed (origin band at 30 563 cm � 1 )i n addition to its known A 3 P X 3 Rband system. Ab initio calculations provide support for the symmetry assignment of the upper electronic states of CCS � , CCS and of the vibrational structure in the spectra.

Large amplitude vibrations in the X 2A1 state of C2B

A three-dimensional potential energy function (PEF) of the 2A1 electronic ground state of C2B has been generated by electronic structure calculations. The PEF possesses a minimum in an isosceles triangular structure which lies 2204 cm−1 below two equivalent minima having linear equilibrium geometry. The barrier height between the minima relative to the triangular structure has been calculated to the 2383 cm−1. The nuclear motion problem has been solved variationally in Jacobi coordinates for J=0 and 1. Ten vibrational states of A1 and nine of B2 symmetry are calculated to lie below the linear minima. The permutational splitting between the (000)+ and (000)− states in the linear 12C2 11B has been calculated to be 0.064 cm−1, in 12C13C11B this is 0.530 cm−1. Above the energy of the barrier to linearity there are large amplitude vibrations with triangular structure character. In the dense stack of such states vibrational modes of the linear structure are discernible, including their permutational splittings.

The A1Π(u)←X1Σ Electronic Transition of CCN+ and CNC+

The electronic absorption spectra of the A 1Π(u)X 1Σ transition of CCN+ and CNC+ have been observed in a 5 K Ne matrix after mass selection of C2N+. CCN+ has the origin band at 462.0(2) nm. The vibrational structure with frequencies 1223(20) and 1725(20) cm−1 corresponds to the symmetric and antisymmetric stretching modes in the excited state. The origin band of CNC+ is observed at 325.7(2) nm, and the system shows extensive vibrational excitation. Calculations of the potential energy functions of CCN+ and CNC+ in their X 1Σ ground state and the A 1Π state of CCN+ followed by variational evaluation of the rovibronic energy levels allows the assignment of the observed spectra. These spectroscopic data open the way to gas-phase studies of the astrophysically important C2N+ ions.

Bound electronic states X1Σ+, a3Π and A1Π of C2B-

Large-scale electronic structure calculations were performed to generate a three-dimensional potential energy function for the X1Σ+ state of C2B-. Spectroscopic constants and anharmonic ro-vibrational levels were calculated variationally and by perturbation theory using this function. The ground state possesses an equilibrium geometry with ReCC=1.270 Aand ReCB=1.461 A, the fundamental vibrational transitions are predicted at ν1CB(J=0)=1014.7 cm-1, ν2(J=1)=125.4 cm-1 and ν3CC(J=0)=1935.4 cm-1 (exp. 1936.3 cm-1). The difference electron density plot showed that the negative charge is almost entirely localized in the boron lone pair and the BC bond region. The A1Π–XΣ+Te excitation energy was calculated to be 23722 cm-1 confirming the assignment made for the transition detected at T0=23131 cm-1 in a neon matrix. For the A1Π state an equilibrium geometry with ReCC=1.324 Aand ReCB=1.396 Awas obtained. The vertical excitation energy of the a3Π–X 1Σ+ transition is predicted at 15306 cm-1. Both excited states lie below the vertical detachment energy calculated at 25845 cm-1.