Supplementary Information for "Determining the Structure of the N2Ar van der
Waals Complex with Laser-Based Channel-Selected Coulomb Explosion"
Chengyin Wu,1,2,*, Cong Wu,1 Di Song,3 Hongmei Su,3,* Xiguo Xie,1 Min Li,1 Yongkai Deng,1
Yunquan Liu,1,2 and Qihuang Gong,1,2
1State
Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871,
People’s Republic of China
2Collaborative
3State
Innovation Center of Quantum Matter, Beijing, China
Key Laboratory of Molecular Reaction Dynamics, Beijing National Laboratory for Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
In the experiment, we combined a femtosecond laser amplifier and a newly-built reaction
microscope1 to reconstruct the structure of the N2Ar van der Waals complex with laser-based
channel-selected Coulomb explosion. The van der Waals complex was generated by expanding the
mixtures of N2 and Ar with a ratio of ~ 1:1 through a 30 μm nozzle with a driving pressure of 8
bars. The laser pulse with 25 fs duration centered at 780 nm was produced from a Ti:Sapphire
laser system (Femtolasers, GmbH). The ions produced in the laser-molecule interaction were
collected by a temporal and position-sensitive detector (RoentDek, Germany). To ensure that all
fragmental ions originate from the same target molecule, we controlled the reaction chamber
pressure to be lower than 3×10-10 mbar so that there is less than one ionization event within one
laser pulse. The ionization events were then recorded in the event-by-event list-mode file. One of
the advantages of the reaction microscope is that the data of all reaction channels can be recorded
in one experiment. In the off-line analysis, the data from different reaction channels can be
disentangled by designing some constraints to filter the experimental data. For example, when we
study three-body fragmentation channels of multiply charged N2Ar ions, we can design the
following constraints. 1) One argon atomic ion and two nitrogen atomic ions are detected in one
laser pulse. 2) The sum-momentum of the three detected ions is less than 10 atomic units to ensure
that the three atomic ions are generated from the same reactant target. 3) The momentum of each
ion is greater than 30 atomic units to rule out false coincidence. With these constraints to filter the
experimental data, we can obtain precise data for three-body fragmentation channels of multiply
charged N2Ar ions.
Figure S1 shows two-dimensional momentum distributions (P// and P) in the center-of-mass
coordinate frame for argon atomic ions and nitrogen atomic ions generated in the three-body
fragmentation of N2Ar ions with varied charge states. The laser pulse duration is 25 fs and the
central wavelength is 780 nm. The laser intensity was estimated to be around 1.3×1015 W/cm2
according to the yield ratio of Ar2+ to Ar+.2 The P// and P represent the momentum vectors parallel
and perpendicular to the laser polarization axis, respectively. The color represents the counting
number of the atomic ions with specified two-dimensional momentum vectors. According to these
raw experimental data, we can calculate the kinetic energy release distribution and plot the
Newton diagram for the three-body fragmentation of N2Ar ions. When we plot the Newton
diagram, we need to define a xoy plane and rotate the momentum vectors of the three atomic ions
to this plane. In addition, the momentum vector of the argon atomic ion is along the positive part
of the x axis. The definition of xoy plane and the rotation operation are purely a mathematical
operation. The operation doesn’t change the absolute value of each three momentum vector nor
the angle between either two of them. Then the momentum vectors of the three atomic ions are
normalized to the length of the argon atomic ion momentum vector. Thus formed Newton diagram
is shown in Figure 1 in the manuscript for the three-body fragmentation process of N2Ar ions.
Figure S1: (Color online) Experimentally measured two-dimensional momentum distributions of
argon atomic ions and nitrogen atomic ions produced in the three-body fragmentation process of
N2Ar ions.
Supplement references:
1
2
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Gong, J. Mod. Opt. 60, 1388 (2013).
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Urbasch, M. Vollmer, H. Giessen, and R. Dorner, J. Phys. B: At. Mol. Opt. Phys. 33, L127
(2000).