Modern Experimental Techniques in Ultrafast Atomic …
279
supersonic molecular jet is obtained by the supersonic expansion of the gas through
a skimmer (of diameter ∼500µm) from a region of high pressure (few bars) to a
region of extremely low pressure (∼10
−8 mbar).
After the molecular (or atomic) beam enters the ionization chamber, it interacts
with the ultrashort laser pulses. The ions and electrons generated as a result of the
interaction are then separated by two sets of electrodes present on either side of the
ionization region. The positively charged ions are extracted with the help of negative potentials applied to the electrodes on one side of the ionization region while
the negatively-charged electrons are separated by positive potentials applied to the
electrodes on the other side. The potentials applied on the electrodes generate a uniform electric field throughout the length of the ions and electrons’ trajectory until the
MCP detector. The position-sensitive delay line detector (PS DLD) in combination
with a Microchannel Plate detector (MCP) is situated at the end of the COLTRIMS
chamber. The MCP detectors give the time-of-flight (TOF) information while the
DLDs give the position information (x,y) of each electron and ion in a correlated
manner.
Once the TOF and the positions (x and y values) on the detector where the ions
and electrons strike the DLD are known, the 3-D momentum of each particle can be
determined using the equations:
p x =
m(x − x 0 )
t
, p y =
m(y − y 0 )
t
, p z = qE s (t − t 0 ),
(23)
where m is the mass of the particle, q is its charge, (x − x 0 ) and (y − y 0 ) are the
position information of ions on the DLD in the x and y directions, t is the TOF, t 0
is the instant when ionization occurs, and E s is the electric field in the extraction
region.The kinetic energy is then related to the momentum components by,
E =
p
2
x
2m
+
p
2
y
2m
+
p
2
z
2m
.
(24)
Let us take an example, the study of fragmentation of triply ionized CO 2 using
the COLTRIMS. The triply ionized CO 2 is unstable and breaks into many fragments. Depending on the excited repulsive states, the precursor (CO
3
2 +) can fragment through various channels. Let us consider a three-body fragmentation of CO
3
2 +,
which produces C
+ , O
+ , and O
+ charged ions. By momentum map analysis, we can
get information about the sequential or concerted/nonsequential breakup of CO
3
2 +.
The momentum vectors of each charged fragments are obtained using the above
equations. In the molecular frame, the ejection of one fragment can be taken as a
reference (here O
+ ion), and the momentum components of the other ions parallel to
it and perpendicular to it can be plotted in a 2D map. This plot is also known as the
Newton plot or momentum map. Figure 20 shows the Newton plots for three-body
fragmentation of CO
2+
3 induced by the intense laser fields. The figure’s left panel
is due to concerted/nonsequential fragmentation, and the right panel of the figure is
due to sequential fragmentation. Detail of this study is given here [60].
279
supersonic molecular jet is obtained by the supersonic expansion of the gas through
a skimmer (of diameter ∼500µm) from a region of high pressure (few bars) to a
region of extremely low pressure (∼10
−8 mbar).
After the molecular (or atomic) beam enters the ionization chamber, it interacts
with the ultrashort laser pulses. The ions and electrons generated as a result of the
interaction are then separated by two sets of electrodes present on either side of the
ionization region. The positively charged ions are extracted with the help of negative potentials applied to the electrodes on one side of the ionization region while
the negatively-charged electrons are separated by positive potentials applied to the
electrodes on the other side. The potentials applied on the electrodes generate a uniform electric field throughout the length of the ions and electrons’ trajectory until the
MCP detector. The position-sensitive delay line detector (PS DLD) in combination
with a Microchannel Plate detector (MCP) is situated at the end of the COLTRIMS
chamber. The MCP detectors give the time-of-flight (TOF) information while the
DLDs give the position information (x,y) of each electron and ion in a correlated
manner.
Once the TOF and the positions (x and y values) on the detector where the ions
and electrons strike the DLD are known, the 3-D momentum of each particle can be
determined using the equations:
p x =
m(x − x 0 )
t
, p y =
m(y − y 0 )
t
, p z = qE s (t − t 0 ),
(23)
where m is the mass of the particle, q is its charge, (x − x 0 ) and (y − y 0 ) are the
position information of ions on the DLD in the x and y directions, t is the TOF, t 0
is the instant when ionization occurs, and E s is the electric field in the extraction
region.The kinetic energy is then related to the momentum components by,
E =
p
2
x
2m
+
p
2
y
2m
+
p
2
z
2m
.
(24)
Let us take an example, the study of fragmentation of triply ionized CO 2 using
the COLTRIMS. The triply ionized CO 2 is unstable and breaks into many fragments. Depending on the excited repulsive states, the precursor (CO
3
2 +) can fragment through various channels. Let us consider a three-body fragmentation of CO
3
2 +,
which produces C
+ , O
+ , and O
+ charged ions. By momentum map analysis, we can
get information about the sequential or concerted/nonsequential breakup of CO
3
2 +.
The momentum vectors of each charged fragments are obtained using the above
equations. In the molecular frame, the ejection of one fragment can be taken as a
reference (here O
+ ion), and the momentum components of the other ions parallel to
it and perpendicular to it can be plotted in a 2D map. This plot is also known as the
Newton plot or momentum map. Figure 20 shows the Newton plots for three-body
fragmentation of CO
2+
3 induced by the intense laser fields. The figure’s left panel
is due to concerted/nonsequential fragmentation, and the right panel of the figure is
due to sequential fragmentation. Detail of this study is given here [60].