Reading
Reading Molecular
Molecular Clocks
Clocks with
with femtosecond
femtosecond laser
laser pulses
pulses
X.
X.M.
M.Tong
Tongand
andC.
C.D.
D.Lin
Lin
t0
t1
t3
t4
t2
E cos(ωt)
H+ + H+
σu ,
Tunneling
Ionization
Ion.
πu
Diss.
t1
Rescattering
Induced
Excitation
H2+: g
The
Idea
The Idea
The oscillating electric field of a laser
provides the “ticks” such that the time of
successive ionizations of a molecule can be
read. Using hydrogen molecules, the clock
starts with the first ionization and stops at
the second ionization. The time interval is
deduced from the measured kinetic
energies of the break-up ions.
e1
Tunneling
Ionization
H2
t0
Rescattering double Ionization: the
second ionization is from the
returned electron.
The second ionization can occur
when the first electron returns to
collide with the ion and ionizes it
(Rescattering, left) or when the
laser’s electric field reaches the
peak again (sequential, right).
By changing laser’s pulse
intensity and/or pulse duration,
we can control the time interval
between the two ionizations.
The sharpness of the kinetic
energy peaks shows that the
clock can be read to better than
1 fs accuracy.
Sequential
Ionization
Sequential Ionization
t0
t1
Seq. Ionization
E
H+ + H+
Energy
Rescattering
Ionization
Rescattering Ionization
H2+
H2
R
sequential double ionization: the
second ionization is from the laser’s
electric field.