Nicholas M. Lakin

Detection of C3 in Diffuse Interstellar Clouds

The smallest polyatomic carbon chain, C3, has been identified in interstellar clouds (Av ~ 1 mag) toward ζ Ophiuchi, 20 Aquilae, and ζ Persei by detection of the origin band in its A 1Πu-X 1Σ electronic transition, near 4052 A. Individual rotational lines were resolved up to J = 30, allowing the rotational-level column densities and temperature distributions to be determined. The inferred limits for the total column densities (~1-2 × 1012 cm-2) offer a strong incentive to laboratory and astrophysical searches for the longer carbon chains. Concurrent searches for C, C, and C were negative, but provide sensitive estimates for their maximum column densities.

Detection of C

The smallest polyatomic carbon chain, C3, has been identified in interstellar clouds (Av ~ 1 mag) toward ζ Ophiuchi, 20 Aquilae, and ζ Persei by detection of the origin band in its A 1Πu-X 1Σ electronic transition, near 4052 A. Individual rotational lines were resolved up to J = 30, allowing the rotational-level column densities and temperature distributions to be determined. The inferred limits for the total column densities (~1-2 × 1012 cm-2) offer a strong incentive to laboratory and astrophysical searches for the longer carbon chains. Concurrent searches for C, C, and C were negative, but provide sensitive estimates for their maximum column densities.

_{3}

The A2Πu–X2Πg electronic system of gaseous C7− is examined experimentally in the light of theoretical predictions. Ab initio calculations at the RHF, RCCSD(T) and MRCI levels using the aug-cc-pVQZ basis set indicate that the transition is accompanied by a small elongation in the molecule and a significant reduction in the spin-orbit coupling constant. On the basis of these predictions the band profiles of the 000, 101, 201 and 301 transitions were recorded using photodetachment spectroscopy. These spectra revealed the spin-orbit component bands for each transition as well as providing band contours which show partially resolved rotational structure. The experimental spectra are compared to simulations based upon the calculated spectroscopic constants and the possible causes of the main features in the band contours are accessed by least-squares fitting of the profiles for the 000 and 101 transitions. The implications for the recent observation of coincidences between the A2Πu–X2Πg vibronic bands of C7− an...

in Diffuse Interstellar Clouds

A six-dimensional potential energy surface (PES) for the singlet electronic ground state of C3H− has been generated by electronic structure calculations using the coupled cluster CCSD(T) approach. Two potential minima are located: the global one, corresponding to an aromatic, cyclic structure (1A1), and a local one, lying about 0.56 eV higher in energy, with a trans chain structure (1A′). Both minima are found to be separated by a relatively high barrier on the singlet surface but the isomerization process can also proceed via a coupling between the singlet and triplet PESs with a lower barrier. Variational calculations for the vibrational levels (J=0) up to 3000 cm−1 were carried out for both isomers, taking into account the full dimensionality of the problem. The present results allow a clear distinction between the singlet isomers by infrared experiments. The structure of the vibrational and rotational stacks in both isomers is very different. For some vibrational overtones and combination levels inver...

Theoretical and experimental study of the A2Πu–X2Πg band system of C7−

The intermolecular potential energy surfaces for the electronic ground states of the ammonium ion-rare gas dimers NH4(+)-He and NH4(+)-Ne are calculated at the MP2 and CCSD(T)/aug-cc-pVXZ (X = D/T/Q) levels of theory. The global minima of both potentials correspond to proton (vertex)-bound structures, Re = 3.13 A, De = 171 cm-1 (He) and Re = 3.21 A, De = 302 cm-1 (Ne). The face- and edge-bound structures are local minima and transition states for the internal rotation dynamics, corresponding to barriers of approximately 20 (He) and 50 cm-1 (Ne). The ab initio potentials are employed in numerical solutions to the rotation-intermolecular vibration Hamiltonian to determine the term values and the rotational and distortion constants for the lowest bound levels in the intramolecular ground vibrational state of both complexes. The results are used to assess the accuracy of two-dimensional (fixed-R) representations of the potentials for determining the internal rotor levels in the ground and nu 3 vibrational states. This model is employed to produce simulations of the IR nu 3 transitions, which are compared to the experimental spectra recorded using photofragmentation spectroscopy. In the case of NH4(+)-Ne the potential parameters are least-squares fitted to the experimental spectrum. The trends within the NH4(+)-Rg series (Rg = He, Ne, Ar) revealed by both the IR spectra and theoretical calculations are discussed.

The potential energy surface and vibrational structure of C3H

The first gas phase observation of the species InOH is reported through the detection of its electronic spectrum in the near ultraviolet region, between 345 and 377 nm. The molecule was generated by the high temperature reaction between H2O and In metal or between H2 and In2O3, and cooled in a free jet expansion. Two separate electronic transitions have been identified and are tentatively assigned as α1A’←X1A’ and β1A‘←X1A’. Values for the vibrational wavenumbers ν2 (bending vibration) and ν3 (In–O stretching vibration) have been determined for InOH and InOD in all three electronic states involved. There is evidence that the molecule is quasilinear in its ground electronic state which somewhat complicates the values determined for ν2 in this state. Rotational structure was easily resolved at the lowest temperature achieved in this work (Trot≊12 K). Analysis of this structure shows that the molecule is bent in all of the electronic states studied, with a bond angle of about 132° in the X state and about...

Internal rotation in NH4+–Rg dimers (Rg = He, Ne, Ar): Potential energy surfaces and IR spectra of the ν3 band

Electronic transitions of C3- and C5- to states lying above the electron affinity of the neutral (EA) have been recorded in the gas phase by laser photodetachment spectroscopy. The excited states are identified by comparison with absorption spectra for the mass-selected ions deposited in neon matrices and with ab initio calculations. The C 2 sigma u (+)-X 2 pi g transition and two higher energy band systems are observed for C3-, corresponding to excitation energies more than 1.5 eV above the EA. In the case of C5- the strongest features, at about 0.6 eV above the EA, are attributed to close lying 2 delta g-X 2 pi u and 2 sigma g(-)-X 2 pi u transitions. The dominant configurations in these states identify them as long-lived Feshbach resonances. Lifetimes for these resonances in C3- are estimated to be between 200 fs and 3 ps from the band widths.

The identification of InOH in the gas phase and determination of its geometric structure

The molecular species In2O has been identified in the gas phase as a product of the high temperature reaction between water and indium (850 °C) or between indium trioxide (In2O3) and indium (950 °C) by the observation of an electronic transition in the near-ultraviolet. The spectra are simplified by supersonic cooling of the sample in a free jet expansion after it is formed. The vibrational structure shows that the molecule has a very similar geometry in the two states involved while the 18O/16O isotope shift suggests that the molecule is only slightly nonlinear in the excited electronic state. Rotational structure can be resolved at high resolution and shows an intensity alternation; the molecule thus has a symmetric In–O–In arrangement.

Spectroscopy of excited states of carbon anions above the photodetachment threshold

Abstract Three-dimensional potential energy functions for the A ″ and A ′ components of the X 2 Π g state of C 3 − have been generated ab initio using the internally contracted multi-reference configuration interaction method. The spin–orbit constant in this state has been calculated from a complete-active-space-self-consistent-field wavefunction. These data were used to solve the linear/linear Renner–Teller problem variationally for J = 1/2, 3/2 and 5/2. Rovibronic levels for K =0, 1 and 2 were determined for energies up to about 3800 cm −1 . The very large splitting between A ″ and A ′ was found to give rise to a strong mixing for the rovibronic Π and Δ bending states. The results are compared with those obtained experimentally and for similar molecular systems.

The identification of In2O in the gas phase by high resolution electronic spectroscopy

Abstract The gross features of the recently observed ultraviolet band system of InOH are explained in terms of transitions from the ground 1 A′ to components of an excited triplet state in which the molecule is also bent. The triplet state correlates with a 3 Π state in the linear configuration and is subject to a large Renner-Teller effect.