Chemistry Reference
In-Depth Information
for the different ionized species are obtained by recording the TOF dis-
tributions at each selected m/e. Since the electron impact ionizer and the
QMS compactly reside in the rotating detector chamber, this setup allows
recording the intensity of distinct m/e as a function of the flight-time at
different laboratory angles in the scattering plane defined by the two
crossing beams (the detector can span the angular range from
25.0
Y
min
¼
72.0
, where
0
represents the direction of the CN beam and
to
Y
max
¼
Y
¼
Y
¼
90
represents the direction of the hydrocarbon beam). The detector
chambers are separated and differentially pumped (regions I and II) to
reduce the gas load from the main chamber; the innermost III region
contains the Brink-type electron impact ionizer (surrounded by a liquid-
nitrogen cold shield), the quadrupole mass filter, and the Daly-type scintil-
lation particle detector. Despite the differential pumping setup, molecules
desorbed from wall surfaces lying on a straight line with the electron impact
ionizer (straight-through-molecules) cannot be avoided, since the mean free
path of these species is of the order of 10
3
m compared to maximum detector
dimensions of a few meters. To reduce these straight-through-molecules, a
copper plate is attached to a two-stage closed cycle helium refrigerator and
cooled to
10K. Since the copper shield is located between the two
skimmers and the scattering region, the ionizer ''views'' a cooled surface
from which only H
2
and He desorb at 10K.
14.3 DATA ANALYSIS
The quantity which is measured in a CMB experiment with mass spectro-
metric and time-of-flight detection is the product intensity as a function of
the scattering angle and arrival time; what we call the time-of-flight spectra.
By integrating the TOF spectra with respect to time, we also obtain the inten-
sity as a function of the scattering angle, the so-called laboratory (LAB)
angular distribution. The measurements are carried out in the laboratory
system of coordinates, but for the physical interpretation of the scattering
data it is necessary to operate a coordinate transformation and move to the
center-of-mass (CM) reference frame. The velocity vector or Newton
diagram shown in
Figure 14.2
graphically represents the relation between
the LAB and CM quantities for an experiment where a typical CN beam
crosses at 90
an unsaturated hydrocarbon (RH) beam and reacts accor-
ding to CN
H. An observer in the LAB frame sees the
CM moving with the CM velocity vector, v
CM
, and the particle beams
approaching each other with v
CN
and v
RH
. An observer sitting on the CM
would see the CM at rest and the particles approaching along the relative
velocity vector, v
r
¼
þ
R-H
!
R-CN
þ
v
RH
, with the CM velocities u
CN
and u
RH
. After
the reactive collision takes place, the newly formed product R-CN and H are
scattered with a LAB velocity v
RCN
and v
H
corresponding to the CM
velocities u
RCN
and u
H
(in the Newton diagram of Figure 14.2 only v
RCN
and
v
CN
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