Polyynes and Other Products from Laser-Ablated Graphite: A Time-of-Flight Mass Spectroscopic Study in Combination with One-Photon Ionization - Polyynes: Synthesis, Properties, and Applications - page 183

FIGURE 9.4 Mass spectra of laser ablated graphite in the H 2 buffer gas. One-photon
ionization with 10.5 eV/photon. Delay times ( m s) are shown to the right. Spectra
observed at 55 and 65 m s are expanded by a factor of 20 and those at 200 and 8000 m s
by 40. TOF mass peak assignment is given in the bottom spectrum. Vertical dotted
lines are to assist reading the subtle but significant shift of the peaks.
Notice that the upper four spectra of Figure 9.4 show a quick dis-
appearance of most of the TOF mass peaks as the delay time was increased
from the shortest of 35 m s. The rapid disappearance at longer delay times
implies that the neutral products giving the disappearing TOF mass peaks
passed the ionization region before the ionizing laser was fired, which, in
turn, implies a faster initial velocity of the products when they entered the
ionization region.
The relationship between the initial velocity of the incoming neutral
species and the TOF for the ionized species to reach the MCP detector is
well known for the set-up shown in Figure 9.1 , which is known as the Wiley-
McLaren-type TOF mass spectrometer [21]. The right and left ordinates
of the lower panel of Figure 9.6 show the relationship between the initial
velocity and the flight time under our experimental set-up. Detailed dis-
cussion on the relation will be made elsewhere [18]. Instead, we will focus on