TOF Mass Spectrometer
&
Control and identification of strong field
dissociative channels in CO2+ via molecular
alignment
YAKUP BORAN
689 MOLECULAR PHYSICS
TOF Mass Spectrometer
• Mass Spectrometer is a device designed to identify the mass of the
individual atoms or molecules.
• Mass spectrometer can be divided 3 main parts:
Ionization source
(Laser field)
Analyzer
(TOFMS)
Detector
(MCP)
• TOF mass spectrometer use an electric field to accelerate the ions,
and then ions’ flight time is measured to separate different ions.
• Mass to charge ratio can be determined from TOF.
Linear Time of Flight Mass Spectrometer
Repeller
plate
Laser Beam
 Two different ions having the same mass to charge ratio
originated at different initial positions and they arrive at
detector at different times.
Figure: CO2 Mass Spectra.
Reflectron Type TOF Mass Spectrometer
V3
V2
Laser Beam
V1
MCP
x
Repeller
plate
Figure: Ethane(C2H6) Mass spectra.
 Blue and red colors show
two different ions having
the same mass to charge
ratio originated at different
initial positions and they
arrive at MCP at the same
time.
Control and identification of strong field dissociative
channels in CO2+ via molecular alignment
• Dissociative excitation of CO2+ in strong field has been studied
experimentally.
• 800 nm and and 1350 nm wavelengths have been used as a probe
pulse.
• Different laser intensities, ellipticities and polarizations have been
used.
CO2 Molecular Orbitals
 Carbon has 4 and
Oxygen has 6 valence
electrons.
 Carbon shares 2
electrons to form a
double bond with
one Oxygen
HOMO
HOMO-1
HOMO-2
HOMO-3
Figure: CO2 Molecular Orbitals.
Figure : Inelastic recollison
Figure : Tunneling ionization
 In an intense laser field, the potential barrier of an atom
or molecule is distorted and the length of the barrier
which electrons have to pass decreases so that electron
can escape from the atom or molecule
 In the strong field, the electron is emitted from the
molecule and the electron is driven back to the
parent molecule
 Channel 1 shows tunneling
ionization from HOMO-3
 Channel 2,3 and 4 consist of
tunneling ionization followed
by photo excitation
 Channel 5 consists of
tunneling ionization followed
by inelastic recollision.
Figure : Different pathways to reach the third excited state.
M Oppermann et all. Control and identification of strong field dissociative channels in CO2+ via molecular alignment(2014)
Experimental Methods
Table: Experimental parameters used for the 1350 nm and 800
nm probe cases

Figure : The pump probe setup.
Using mixture of gases has some advantages:
 it helps to increase rotational cooling of the
sample and thus the degree of molecular
alignment is increased.
 Here Ar+ signal can be used to monitor the laser
intensity fluctuations.
M Oppermann et all. Control and identification of strong field dissociative channels in CO2+ via molecular alignment(2014)
Results for CO+ Channel
Figure 7. Polarization dependence for CO+
for 800 nm and 1350 nm, linearly polarized
30 fs probe pulses at an average intensity of
2x1014 W.cm-2 .
Figure 8. Ellipticity scans at different θ values
at an average intensity of 2x1014 W.cm-2 .
• strong monotonic decrease of fragmentation yield most probably comes from the
channel TI from HOMO-2 followed by photo excitation
M Oppermann et all. Control and identification of strong field dissociative channels in CO2+ via molecular alignment(2014)
Results for O+ Channel
Figure 9. Polarization dependence for O+ for
800 nm and 1350 nm, linearly polarized 30
fs probe pulses at an average intensity of
2x1014 W.cm-2 .
Figure 10. Ellipticity scans at different θ values
at an average intensity of 2x1014 W.cm-2 .
M Oppermann et all. Control and identification of strong field dissociative channels in CO2+ via molecular alignment(2014)
Branching Ratio for CO+
Figure 11. Branching ratio R for 30 fs probe pulses
with wavelengths of 800nm and 1350nm
• R is the ratio between CO+ yields
and sum of the CO+ and O+ yields.
• For 800nm, R=0.8 for the peak
intensities less than 5x10^14
W/cm^2
• For 1350nm, R=0.9 and it is almost
constant over the intensity range.
• This results shows that the
production of CO+ strongly
dominates over O+.
M Oppermann et all. Control and identification of strong field dissociative channels in CO2+ via molecular alignment(2014)
Conclusions
The dominant contribution comes from the two step pathway.
 First, tunneling ionization from the HOMO-2 of CO2 takes place and
this leaves the parent ion in the second excited state. Second, this
second excited state is coupled to the third excited state via a parallel
dipole transition.
Weak ellipticity dependence has been observed and this means
inelastic recollision do not contribute significantly to the dissociation
mechanism.