Patrick Freivogel

ELECTRONIC ABSORPTION SPECTRA OF LINEAR CARBON CHAINS IN NEON MATRICES. III. HC2N+1H

Absorption spectra observed between 400 and 2500 nm in 5 K neon matrices have been assigned to electronic transitions of linear C−2n (n=2–10): 2Π←X 2Π, C2n (n=3–7): 3Σ−u←X 3Σ−g, and C2nH (n=3–8): 2Π←X 2Π chains. The species have been produced by mass selected deposition of cations or anions produced in a hot cathode discharge source (C2n, C−2n, C2nH) and by laser vaporization of graphite (C2n, C−2n). In addition to experimental and chemical evidence, the dependence of the absorption wavelength on the number of carbon atoms was used to assign the electronic transitions. Infrared absorptions which were recorded around 2000 cm−1 are attributed to asymmetric stretching frequencies of C8, C10, and C12. This is based on correlation of their intensities with the identified electronic bands. The possible relevance of the electronic spectra of these carbon chains to astrophysical observations of diffuse interstellar bands is discussed.

Absorption spectra of conjugated hydrocarbon cation chains in neon matrices

Abstract Absorption spectra for a series of mass selected C k H 2 + systems ( k = 4–16) isolated in neon matrices at 5 K have been measured in the visible and near infrared region. Application of the free-electron molecular-orbital model reveals two homologous series corresponding to the deposition of systems containing even or odd numbers of carbon atoms. With the aid of previous gas-phase data, even-numbered carbon systems are identified as cations of polyacetylenic chains. Positions of the A 2 Π←X 2 Π origin bands as well as fundamental vibrational frequencies in the excited state for the systems H-(CC) n -H + ( n = 4–8) have been obtained. For the deposition of ions containing an odd number of carbons, absorption transitions are tentatively assigned to HC 2 n +1 H + where n = 2–7.

Infrared bands of mass-selected carbon chains Cn (n = 8−12) and Cn− (n = 5−10, 12) in neon matrices

Abstract Infrared bands of carbin chains C n ( n = 8−12) and C − ( n = 5−10, 12) have been observed in neon matrices in the 1600–2100 cm −1 region. The matrices were grown at 5 K by codepositing mass-selected carbon anions with excess of neon. The neutral carbon chains were obtained from the anions by photodetachment. The assignments of the infrared transitions are based on mass-selection, photobleaching behaviour, intensity correlation with the known electronic transitions, and available ab initio calculations.

Electronic absorption spectra of cyano‐substituted polyacetylene cations in neon matrices

The A 2Π←X 2Π transitions and vibrational frequencies in the excited electronic states have been determined for the cyanopolyacetylene cations H–(C≡C)n–CN+ (n=2–6) and NC–(C≡C)n–CN+ (n=1–5). The spectroscopic information was derived by absorption measurements after cations from a mass selected beam were condensed together with excess of neon to form a 5 K matrix. Spectra have been also obtained for ions with odd number of π‐centers and are tentatively assigned to H–C2n−1–CN+ (n=3–6) and NC–C2n−1–CN+ (n=3–5). The assignments are based on trends observed for the homologous series of these cations, known gas phase data on three of the smaller species, and the free electron molecular orbital model.

Electronic absorption spectra of C4− and C6− chains in neon matrices

The absorption spectra of the electronic transitions A 2Σg+←X 2Πg, B 2Σu+←X 2Πg, (2) 2Πu←X 2Πg, and (3) 2Πu←X 2Πg of C4−, as well as A 2Σg+←X 2Πu, (2) 2Πg←X 2Πu, and (3) 2Πg←X 2Πu of C6− have been obtained in neon matrices. The spectra were measured after mass-selected deposition of the anions with excess of neon at 5 K. The assignments are based on mass-selection, spectroscopic evidence, photobleaching behavior, and ab initio calculations. Vibrational frequencies in the electronically excited states have been inferred.

The 3Σu− ← X3Σg− electronic absorption spectrum of linear C4 in a neon matrix

Abstract The electronic absorption spectrum of linear C 4 in a neon matrix has been detected. This was achieved by codepositing mass selected C 4 − ions with an excess of neon and UV irradiation of the matrix during or after deposition. Mass selection, experimental and spectroscopic evidence, available ab initio calculations and comparison with the spectra of C 2 n ( n =3–7) chains lead to the conclusion that the observed band system with origin at ≈ 380 nm is due to the 3 Σ u − ←X 3 Σ g − transition of linear C 4 . Vibrational frequencies of C 4 in the excited electronic state have been inferred.

Electronic absorption spectra of carbon chain anions C2n− (n=4–7) in neon matrices

Electronic transitions, (2)2Π←X 2Π (n=4–7), 2Σ←X 2Π (n=4,5) in addition to the known one C 2Π←X 2Π (n=4–7) of linear carbon anions C2n− have been observed in neon matrices. Absorption spectra were measured after mass-selected deposition of the anions with excess of neon at 5 K. The assignments are based on mass-selection, wavelength dependence of the 000 transition on size, photobleaching behavior, and ab initio calculations. Vibrational frequencies in the excited electronic states have been inferred. The band system attributed previously to the 3Σu− ←X 3Σg− transition of C10, C12, and C14 is shown to be the (2)2Π←X 2Π transition of their anions. Another set of weak bands is assigned to the 3Σu−←X 3Σg− system of C10 but none were discernible for C12 or C14. This is experimental evidence for the predicted change from linear to nonlinear geometries for the even numbered carbon species larger than C10.

Emission spectrum of mass-selected C4−:C2Πu → X2Πg in a neon matrix

Abstract The laser excited emission spectrum of the C 2 Π u → X 2 Π g electronic transition of mass-selected C 4 − embedded in a 5 K neon matrix has been observed. This is an extension of the technique coupling mass spectrometry and matrix isolation and allows information on the vibrational frequencies of the species in the ground state to be obtained. In the case of C 4 − , three vibrational frequencies ( ν 1 ( σ g + ) = 2047(20), ν 2 ( σ g + ) = 936(20) and ν 4 ( π g ) = 396(20) cm −1 ) have been inferred for the X 2 Π g state.

Diffusion of mass-selected carbon atoms and molecules in argon and neon matrices

Annealing reactions of carbon atoms, molecules, and their anions, trapped in noble gas matrices, were studied spectroscopically. To this aim Cn− (n=1–4) anions were mass-selected and codeposited with argon or neon to form matrices at low temperatures. The charged species were neutralized by uv irradiation. Carbon chain elongation is observed only if neutral or charged atomic carbon is present in the matrix. Spectra of linear carbon clusters up to C11 were observed after C− deposition, electron detachment, and subsequent annealing. The reaction between two Cn (n⩾2) species in solid argon or neon was studied and can be disregarded even at temperatures near the point of matrix sublimation.Annealing reactions of carbon atoms, molecules, and their anions, trapped in noble gas matrices, were studied spectroscopically. To this aim Cn− (n=1–4) anions were mass-selected and codeposited with argon or neon to form matrices at low temperatures. The charged species were neutralized by uv irradiation. Carbon chain elongation is observed only if neutral or charged atomic carbon is present in the matrix. Spectra of linear carbon clusters up to C11 were observed after C− deposition, electron detachment, and subsequent annealing. The reaction between two Cn (n⩾2) species in solid argon or neon was studied and can be disregarded even at temperatures near the point of matrix sublimation.