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Preparation and characterization of state-selected molecular ions

The Weitzel groups employs laser ionization with a variety of scientific goals. Nanosecond lasers with relatively high spectral resolution are employed to prepare molecular ions in well-defined quantum states (electronic, vibrational, rotational). Femtosecond lasers, with broad spectrum, are employed to generate ions with wave packet properties for chemical analysis.

In this section we discuss the preparation of molecular ions in well-defined electronic, vibrational and rotational states. The goal is to use these ions in subsequent ion-molecule reaction studies.

In the past years we have mainly employed the resonance enhanced multiphoton ionization (REMPI) in order to prepare state-selected ions. As a first example we mention the REMPI spectroscopy of all four ammonia isotopomers [1].

Fig. 1              REMPI spectroscopy of normal ammonia (NH3) via the   band. Comparison of experimental and calculated spectrum. Selected transitions of the Q-branch are assigned. Taken from [1]

Further examples include the HCl, DCl, HBr and DBr molecules. Isotopic labeling is rewarding since it provides access to kinetic isotope effects in subsequent proton / deuteron transfer reactions. As a second example the REMPI spectrum of HCl recorded via the 3D2(v’’’=0)  ¬  1S(v’’’’=0)  transition is shown in Figure 2 [2].

Fig. 2              REMPI spectrum of HCl recorded via the f 3D2(v’’’=0)  ¬  1S(v’’’’=0) transition. Only the Q-, and part of the P-, R- and S- branch are shown. Taken from [2]

How can we determine the state distribution of molecular ions formed e.g. via a REMPI process? In general one could titrate the photoelectrons back. An even better resolution is achieved by projecting the state distribution of interest onto a dissociating state. By recording the fragment ion spectrum (action spectrum) we are able to derive accurate rotational state distributions. In Figure 3 we show rotational state distributions of HCl+ ions formed via a series of pump lines in the REMPI spectrum presented above.

Fig. 3  Rotational state distribution of HCl+ ions formed by REMPI formation on specific pump lines as indicated. Taken from [3]

As a conclusion we find that REMPI provides access to ions with very narrow rotational state distributions (in general 2 or 3 states dominating) where the center state can be tuned by tuning from one pump line to the next. We have successfully demonstrated this technique for the HCl+ and DCl+ ions and HBr+ ions.

 Literature:

[1]   M. Nolde , K.-M. Weitzel, C. Western, PCCP, 7, 1527, (2005)
The Resonance Enhanced Multiphoton Ionisation Spectroscopy of Ammonia – Isotopomers NH3, NH2D, NHD2 and ND3.

[2]   Martin Penno, Andrea Holzwarth, and Karl-Michael Weitzel, Molecular Physics, 97, 43± 52, (1999)
State Selective Predissociation Spectroscopy of Hydrogen Chloride Ion (HCl+) via the 2S+ ß2P3/2 Transition.

[3]   M. Michel , M.V. Korolkov  , K.-M. Weitzel, J. Phys. Chem. A, 108, 9924-9930, (2004)
State-selective predissociation spectroscopy of HCl+ and DCl+ ions.

[4]   M. Michel, K.-M. Weitzel, Chem. Phys. Chem., 5 , 1507-1512, (2004)
Two photon dissociation spectroscopy of state selected HCl+ and DCl+ ions.

[5]   Martin Penno, Andrea Holzwarth, and Karl-Michael Weitzel, J. Phys. Chem. A, 102, 1927-1934, (1998)
State Selective Predissociation Spectroscopy of Hydrogen Bromide Ions (HBr+) via the 2S+ ß2Pi(i=1/2, 3/2) Transition

[6]   M. Penno und Karl-Michael Weitzel, Z. Phys. Chem., 218 , 311-325, (2004)
Rotational State Distribution of HBr+ Ions Formed by Resonance Enhanced Multiphoton Ionization.