Del Mar Photonics
MCP detector for high resolution ion time-of-flight
analysis for measuring molecular velocity distributions
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Y. Kim, S. Ansari, B. Zwickel and H. Meyer,
High resolution ion
time-of-flight analysis for measuring molecular velocity distributions, Rev.of
Sc. Instr. 74, 4805-4811(2003).
A new electrode setup for high-resolution ion time-of-flight ~TOF! analysis is described. The setup
is used in combination with a counterpropagating pulsed molecular-beam scattering apparatus and
laser ionization to measure one-dimensional velocity distributions of low-energy molecular products
resulting from scattering or dissociation processes. In the case of ensembles characterized by
cylindrical symmetry with respect to the molecular-beam axis, measured TOF spectra represent the
angular distribution of the products. In the imaging of the ions onto the detector, this symmetry is
preserved by using a pair of electrostatic mirrors for the deflection. Combined with separate velocity
dispersion and acceleration fields, the present arrangement achieves superior resolution and
detection efficiency. Although the resolution of the setup is limited by the velocity distribution of the
molecular-beam pulses, changes in the average local velocity as small as 10 m/s have been
Y. Kim and H. Meyer,
REMPI detection of alignment in NO collisions, Chem.
Phys. 301, 273-281(2004).
The detection of collision induced alignment in ensembles of NO molecules scattered from He or Ne is investigated. The
alignment is probed through (2+ 1) resonance enhanced multiphoton ionization of NO via different Rydberg states: E 2R, F 2D, and
H 2R;H0 2P. Transitions belonging to the first two electronic systems yield identical polarization effects consistent with the fact that
the two-photon transition is characterized by a single second rank tensor component Zð2Þ
1 . The electronic system involving the Hstate
is considerably more complicated because of the state mixing due to L-uncoupling and the simultaneous presence of zeroth and
second rank tensor components for the two-photon transition. Alignment moments extracted from TOF spectra measured through
different electronic systems are identical within the experimental uncertainty. The alignment is consistent with predictions of either
the kinematic apse or the geometric apse models. These models indicate further that higher moments have a negligible influence on
the polarization effect if the alignment is derived from spectra measured with laser light polarized parallel and perpendicular to the
molecular beam axis.
Y. Kim and H. Meyer, Two-photon spectroscopy of aligned acetaldehyde, Chem. Phys. Lett. 387, 339-344 (2004).
We reinvestigate the two-photon absorption of the (3s-n) transition in
acetaldehyde and its implications for the observation of
collision-induced alignment. The two-photon spectroscopy of an aligned ensemble of acetaldehyde produced in collisions with He
provides clear evidence for the quantum interference of tensors of different rank. The observed polarization effect can be understood
in terms of the non-vanishing tensor components Zð0Þ
0 ¼ 1:0, Zð2Þ
0 ¼ 0:4 0:1 and jZð2Þ
2j ¼ 0:3 0:1. For a collision energy of
916 cm 1, the alignment is largely consistent with the predictions of the kinematic apse model.
Y. Kim, H. Meyer, and M.H. Alexander, Molecular beam scattering of NO + Ne: A joint theoretical and experimental study, J. Chem. Phys. 121, 1339-1343 (2004).
The collision dynamics of the NO1Ne system is investigated in a
molecular beam scattering
experiment at a collision energy of 1055 cm21. Employing resonance enhanced multiphoton
ionization of NO, we measured state-resolved integral and differential cross sections for the
excitation to various levels of both spin-orbit manifolds. The dependence of the scattered intensity
on the laser polarization is used to extract differential quadrupole moments for the collision induced
angular momentum alignment. The set of cross section data is compared with results of a full
quantum mechanical close coupling calculation using the set of ab initio potential energy surfaces
of Alexander et al. @J. Chem. Phys. 114, 5588 ~2001!#. In previous work, it was found that the
positions and rotational substructures for the lowest bend-stretch vibrational states derived from
these surfaces agree very well with the observed spectrum of the NOľNe complex. For the same
potential, we find that the calculated cross sections show a less satisfactory agreement with the
experimental data. While the overall J f dependence and magnitude of the integral and differential
cross sections are in good agreement, noticeable discrepancies exist for the angle dependence of the
differential cross sections. In general, the calculated rotational rainbow structures are shifted
towards larger scattering angles indicating that the anisotropy of the potential is overestimated in the
fit to the ab initio points or in the ab initio calculation itself. For most states, we find the measured
alignment moments to be in excellent agreement with the results of the calculation as well as with
predictions of sudden models. Significant deviations from the sudden models are observed only for
those fine-structure changing collisions which are dominated by forward scattering. Results of the
full quantum calculation confirm the deviations for these states.