tof mass spectrometer & control and identification of strong field dissociative channels in co 2...
TRANSCRIPT
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:
• 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.
Ionization source(Laser field)
Analyzer(TOFMS)
Detector(MCP)
Linear Time of Flight Mass Spectrometer
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.
Repellerplate
Laser Beam
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.
Reflectron Type TOF Mass Spectrometer
MCP
Laser Beam
x
V1V2V3
Figure: Ethane(C2H6) Mass spectra.
Repellerplate
• 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.
Control and identification of strong field dissociative channels in CO2
+ via molecular alignment
CO2 Molecular Orbitals
Carbon has 4 and Oxygen has 6 valence electrons.
Carbon shares 2 electrons to form a double bond with one Oxygen
Figure: CO2 Molecular Orbitals.
HOMO
HOMO-1
HOMO-2
HOMO-3
Figure : Tunneling ionization
Figure : Inelastic recollison
In the strong field, the electron is emitted from the molecule and the electron is driven back to the parent molecule
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
Figure : Different pathways to reach the third excited state.
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.
M Oppermann et all. Control and identification of strong field dissociative channels in CO 2+ via molecular alignment(2014)
Experimental Methods
Figure : The pump probe setup.
Table: Experimental parameters used for the 1350 nm and 800 nm probe cases
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 CO 2+ via molecular alignment(2014)
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 .
Results for CO+ Channel
M Oppermann et all. Control and identification of strong field dissociative channels in CO 2+ via molecular alignment(2014)
• strong monotonic decrease of fragmentation yield most probably comes from the channel TI from HOMO-2 followed by photo excitation
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 CO 2+ 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 CO 2+ via molecular alignment(2014)
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.
Conclusions