Vibrational Predissociation Spectra of C2 N- and C3 N- : Bending and Stretching Vibrations.

We present infrared predissociation spectra of C2 N- (H2 ) and C 3 N- (H2 ) in the 300-1850 cm-1 range. Measurements were performed using the FELion cryogenic ion trap end user station at the Free Electron Lasers for Infrared eXperiments (FELIX) laboratory. For C2 N- (H2 ), we detected the CCN bending and CC-N stretching vibrations. For the C3 N- (H2 ) system, we detected the CCN bending, the CC-CN stretching, and multiple overtones and/or combination bands. The assignment and interpretation of the presented experimental spectra is validated by calculations of anharmonic spectra within the vibrational configuration interaction (VCI) approach, based on potential energy surfaces calculated at explicitly correlated coupled cluster theory (CCSD(T)-F12/cc-pVTZ-F12). The H2 tag acts as an innocent spectator, not significantly affecting the C2,3 N- bending and stretching mode positions. The recorded infrared predissociation spectra can thus be used as a proxy for the vibrational spectra of the bare anions.

A vibrational action spectroscopic study of the Renner-Teller- and spin-orbit-affected cyanoacetylene radical cation HC3N.

The linear radical cation of cyanoacetylene, HC3N+ (2Π), is not only of astrophysical interest, being the, so far undetected, cationic counterpart of the abundant cyanoaceteylene, but also of fundamental spectroscopic interest due to its strong spin-orbit and Renner-Teller interactions. Here, we present the first broadband vibrational action spectroscopic investigation of this ion through the infrared pre-dissociation (IRPD) method using a Ne tag. Experiments have been performed using the FELion cryogenic ion-trap instrument in combination with the FELIX free-electron lasers and a Laservision optical parametric oscillator/optical parametric amplifier system. The vibronic splitting patterns of the three interacting bending modes (ν5, ν6, ν7), ranging from 180 to 1600 cm-1, could be fully resolved revealing several bands that were previously unobserved. The associated Renner-Teller and intermode coupling constants have been determined by fitting an effective Hamiltonian to the experimental data, and the obtained spectroscopic constants are in reasonable agreement with previous photoelectron spectroscopy (PES) studies and ab initio calculations on the HC3N+ ion. The influence of the attached Ne atom on the infrared spectrum has been investigated by ab initio calculations at the RCCSD(T)-F12a level of theory, which strongly indicates that the discrepancies between the IRPD and PES data are a result of the effects of the Ne attachment.

Strong ortho/para effects in the vibrational spectrum of Cl-(H2).

The predissociation spectrum of the Cl-35(H2) complex is measured between 450 and 800 cm-1 in a multipole radiofrequency ion trap at different temperatures using the FELIX infrared free electron laser. Above a certain temperature, the removal of the Cl-(p-H2) para nuclear spin isomer by ligand exchange to the Cl-(o-H2) ortho isomer is suppressed effectively, thereby making it possible to detect the spectrum of this more weakly bound complex. At trap temperatures of 30.5 and 41.5 K, we detect two vibrational bands of Cl-(p-H2) at 510(1) and 606(1) cm-1. Using accurate quantum calculations, these bands are assigned to transitions to the inter-monomer vibrational modes (v1,v2 l2 ) = (0, 20) and (1, 20), respectively.