interpretations
Equation 2.2. The derivation, from Newton’s equations of motion, is given in Append
4.
7;=V2,
Equation 2.2
where T, is the total time of flight of the ion from the point of origin to the detector, de is
the distance from the ion point of origin to the exit of the extraction field, d„ is the
distance across the acceleration field, d^ is the drift region length. Eg is the extraction
field, £;,is the acceleration field. Kgis the initial kinetic energy of the ion and mand qare
the mass and charge of the ion respectively.
offset NO % 15xt)' 00 7 2 3 6
Figure 2.12: A comparison of the total flight times for N2O parent and fragment ions with an
initial velocity of zero parallel to the TOF axis deduced from the simulation and from the experimental results. The offset is due to an electronic delay.
The gradient of a — versus T, graph is linear if the initial kinetic energy of the ion
(calculated from the velocity parallel to the TOF axis) is 0 eV. The linear gradient is
dependant on the extraction and acceleration fields. Thus, the linearity of such a graph
can be assured if the peak determination of the zero positions is correct. Figure 2.13
shows how the zero positions for some of the fragment ions vary with extraction field.
3 5 xt) 150V/cm 3ÛCD' 300 v/cm 450 V/cm 600 V/cm F toco- OX) 2 3 4 S (N/bss/Charge) 0
Figure 2.13: A calibration graph for the fragment ions of N2O for 4 different extraction fields.
2.6
Vibrational Excitation of Molecules
Collisions between a molecule and a surface can result in the transfer of energy from
one to the other, resulting in the excitation of the molecule to which the energy is
transferred. Equation 2.3 can be used to determine the temperature of the molecules
within the heated surface, such that their mean thermal energy, E, is enough to result
in excitation of the test gas molecules into vibrationally excited states, (v > 0).
E = - k T 2
Equation 2.3
where k is the Boltzmann constant and T is the temperature of the surface.
The method used to heat the gas molecules into a excited state was to introduce
them into the extraction region via a hypodermic needle around which a coil of
Kanthal heating wire was wrapped, as shown in Figure 2.4.
The temperature of the gas was monitored by wrapping a thermocouple around the
tip of the hypodermic needle that is situated 10 mm from the laser focus. The
thermocouple consisted of copper and constantan 32-gauge wire. The calibration of
the thermocouple was carried out by measuring the voltages, using a Keithly auto
Table 2.1; The relevant details of the Kanthal heating wire.
Parameter Value
Max continuous operating temp
. . .
1673 K
Nominal composition Cr (22%) AI (5.8%) Fe (72.2%)
Electrical resistivity at 20 °C 1.45 Q mm^ m'^
Melting point 1773 K
Coeff. of thermal expansion at 20b^C
...
14x lO^K"^
The thermocouple was placed in a container of boiling water that was allowed to cool, to obtain more data points for the calibration. The temperature of the water was measured using a Digitron thermometer (Model; 1758-K) and compared to the thermocouple voltage readings. For the highest temperature (148 °C) the thermocouple and the Digitron thermometer was placed on the filament of an electric fire. Figure 2.14 shows the results for the calibration.
Temperature f C) = Voltage (mV) - 0.744 -0.44 -2- -3- O) -4 - 0 20 40 60 80 100 120 140 Temperature (°C )
Figure 2.14: Temperature calibration for the thermocouple voltage readings.
2.7 TOF Mass Spectrometer Acceptance Angle Determination
When carrying out direct angular measurement experiments the angular resolution of the apparatus is limited by the angular acceptance of an ion of a particular energy. The angular resolution of the experiment was also limited by the magnitude of the fields in the extraction, acceleration and drift regions of the spectrometer. A visual basic program was written to determine the angular acceptance of the spectrometer
for an ion of a specific energy moving towards and away from the detector when the
potentials on the elements of the spectrometer were specified. The code for this
program is given in Appendix 3.
To achieve a very small acceptance angle for ions reaching the detector the
potentials on the spectrometer elements were set at a high value (500 V/cm
extraction field). Spectra were recorded for a range of laser polarization directions. In
this way the relative number of ions moving initially along a particular trajectory, within
the angular error set by the angular acceptance of the apparatus, could be found. A
small acceptance angle allowed the detection of ions only within that angle resulting in
better angular resolution of the final angular distribution spectra. The peak kinetic
energies of the ions were determined as described in Section 2.5 and using this value
the angular acceptance for each ion could be calculated using the visual basic
program. This method is limited in that if a TO F peak is produced by many
dissociation channels, and a distribution results, (see, for example, the dissociative
ionization of N2O in a picosecond laser field in Section 5.1) the peak energy of the
distribution will result in an incorrect determination of the angular acceptance of that
ion, but if the peaks are well defined (see, for example, the dissociative ionization of N2O in a femtosecond laser field in Section 5.2.1) the angular acceptance values are
more accurate.
(a)
(b) D etector axis Top extraction Bottom extraction i plate I ■ XV + X V 9(T 75- ^6(T §451 13 ® Forward moving N* ® Baclcward moving N* 10 Energy (eV)Figure 2.15: (a) Shows that the geometry of the extraction region means that the forward moving ion (red line) can escape from the region at a bigger angle with respect to the TOF axis than the ioackward moving ion (blue line) and (b) shows the acceptance angles of the TOF
The acceptance angles for the fonft/ard and backward moving ions of a particular
kinetic energy are slightly different due to the geometry of the TO F mass
spectrometer extraction region, as shown in Figure 2.15a. Figure 2.15b shows the
calculated acceptance angles of the TO F mass spectrometer for a forward and
backward moving ion. Figure 2.16 shows how the acceptance angles of the TO F
mass spectrometer for forward moving and vary for different initial kinetic
energies.
I"
13- N*- 10 Energy (eV)Figure 2.16: The acceptance angles for the TOF mass spectrometer found from the visual basic program given in Appendix 3 for forward moving AT and bns.
2.8
Detector and Circuit
Two multi-channel plates used in parallel form the active element of the detector. The
chevron channels, which are at 13° through the detector, are set such that the
channels of the two plates are in the opposite direction to each other, as shown in
Figure 2.17. A ring electrode is positioned in electrical contact either side of the multi
channel plate. The operating voltage drop across each multi-channel plate was varied