High Resolution Infrared Spectroscopy of Linear Cluster Ions Harald Verbraak Marcel Snels, Dorinel Verdes, Peter Botschwina, Harold Linnartz Sackler Laboratory for Astrophysics Leiden Observatory
Why do we study cluster ions? (Cluster) ions are present in a wide variety of different media like e.g. plasmas, the upper layers of the atmosphere and most likely interstellar space Cluster ions can be seen as transition states in chemical reactions, e.g. proton transfer reactions, charge transfer reactions. To get more insight into the van der Waals equivalent of charged-neutral interactions zz Charge transfer reactions: Ar + N2+ [Ar-N2]+ Ar+ + N2 Linnartz et al. Science 297 (2002) 1166
How do we make these cluster ions? Experimental setup Trot ~ 20 K nion ~ 2x1010 ions/cm3 H. Verbraak et al. Int. J. Mass Spectrom. In press
What ions can be made with this source? Mass spectrum of a water plasma containing (H2O)nH+-cluster ions Mass spectrum of a CO/H2 plasma containing (CO)nH+-cluster ions Mass spectrum of a CO2/H2/N2 plasma H. Verbraak et al. Int. J. Mass Spectrom. In press
How do we measure these ions? Tunable diode laser infrared spectrometer
Calculations on the vibrations of Ar-H(D)N2+ Large frequency shifts are expected compared to free H(D)N2+. Large anharmonicity contributions are found for Ar-HN2+. For Ar-DN2+ anharmonic effects are less pronounced. Plot of the NN -and NH-frequencies of Ar-H(D)N2+ as a function of proton mass (CCSD(T)/219cGTO) Botschwina et al. JCP 113 (2000) 2736
Measurements on Ar-DN2+ 71 rovibrational transitions were measured. Two series of rovibrational transitions overlap. Trot=10 K Verbraak et al. JCP 124 (2006) 224315
Assignment of the rovibrational bands Combination differences calculated for the 1593 cm-1-band 2F”(J) = R(J-1) - P(J+1) Two bands, centered around 2436 cm-1, belong both to Ar-DN2+ Both bands are --transitions (no Q branch) Possible vibrations: 1, combination band of 2 and s CCSD(T)/219cGTO
Fitting of the spectra Hamiltonian for two interacting vibrations: H10 and H20 are the Hamiltonians for the unperturbed energies = H10 - H20 H12 = F0 + FJJ(J+1) (Fermi interaction term) Obtained constants reproduce line positions within experimental uncertainty
Analysis of the perturbation Relative shift = E(pert.) - E(unpert.) E = E(1) - E(2+4s) =PJ(1)-PJ(2+4s) or RJ(1)-RJ(2+4s) The perturbation has a strong rotational character
Additional Analysis Rotational constants: Vibrational frequencies: All vibrational frequencies are in good agreement with the calculations The large frequency shifts in Ar-DN2+ are due to the weaker DN-bond In Ar-HN2+ the shifts are caused by the anharmonicity in the HN-bond
Conclusions The experimental setup gives spectroscopic access to a large variety of complex ions. The Ar-H(D)N2+ is now the most complete studied ionic complex using high resolution spectroscopy (both IR and MW).
Acknowledgments Infrastructure Sackler Laboratory for Astrophysics Leiden Observatory Dutch fundamental organisation for fundamental research
cw-CRD using an OPO-system Experimental setup Measurements on 1 of HCO+