1. High Resolution Infrared Spectroscopy of Linear Cluster Ions
Harald Verbraak
Marcel Snels, Dorinel Verdes, Peter Botschwina, Harold Linnartz
Sackler Laboratory for Astrophysics
Leiden Observatory

3. 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

4. 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

5. 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

6. How do we measure these ions?
Tunable diode laser infrared spectrometer

7. 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

8. Measurements on Ar-DN2+
71 rovibrational transitions were measured.
Two series of rovibrational transitions overlap.
Trot=10 K
Verbraak et al. JCP 124 (2006)

9. 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

10. 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

11. Analysis of the perturbation
Relative shift = E(pert.) - E(unpert.)
E = E(1) - E(2+4s)
=PJ(1)-PJ(2+4s) or RJ(1)-RJ(2+4s)
The perturbation has a strong rotational character

12. 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

13. 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).

14. Acknowledgments Infrastructure Sackler Laboratory for Astrophysics
Leiden Observatory
Dutch fundamental organisation for fundamental research

15. cw-CRD using an OPO-system
Experimental setup
Measurements on 1 of HCO+