The Vulcan Nd : glass laser at the Central Laser Facility is a Petawatt (10 15 W) interaction facility available to the UK and international user community. The facility came online to users in 2002 and considerable experience has been gained operating the Vulcan facility in this mode. The facility is designed to deliver irradiance on target of 10 21 W cm −2 for a wide-ranging experimental programme in fundamental physics and advanced applications. This includes the interaction of super-high-intensity light with matter, fast ignition fusion research, photon induced nuclear reactions, electron and ion acceleration by light waves and the exploration of the exotic world of plasma physics dominated by relativity.
The photodissociation of nitrobenzene and the nitrotoluene isomers at 375 nm, induced by a 90 femtosecond laser, is analyzed and compared with the fragmentation by a 10 nanosecond laser at the same wavelength. The molecular ionization is attributed to a nonresonant multiphoton process, and the observed fragmentation can be explained predominantly by an above ionization mechanism (ladder climbing). The mass spectra of the three nitrotoluene isomers show differences which can be used for analytical purposes. The molecular rearrangement taking place prior to the dissociation is also discussed. For nitrobenzene, it is suggested that most of the dissociation occurs from the nitrobenzene structure rather than that of phenyl nitrite. In the case of o-nitrotoluene, it seems that the hydrogen transfer from the -CH 3 to the NO 2 group (ortho effect) is favored in ionic states, while the rearrangement to a nitrite structure is possible in the excited electronic states.
The photochemistry of nitromethane has been studied extensively
for many years. Although it is generally
agreed that the principal photodissociative process is cleavage of the
C−N bond to yield the methyl radical
and nitrogen dioxide, there is some evidence of minor competing
dissociation channels. A number of different
groups have used lasers of different wavelengths, but the results of
these studies vary considerably and no
clear picture of the minor dissociative channels has yet emerged.
The use of femtosecond (fs) duration laser
pulses for photoionization of molecules is currently an area of
considerable interest, since the process can
lead to the efficient production of intact molecular ions. It was
felt that femtosecond laser mass spectrometry
(FLMS) could provide added information on the dissociation pathways of
nitromethane. Laser pulses of 90
fs time duration at wavelengths of 375 and 750 nm, coupled to a
time-of-flight mass spectrometer, have been
used in this study, and contrary to photoexcitation using nanosecond
(ns) pulses, a large parent ion, 61 (CH3NO2
+), is detected together with strong peaks
at m/e = 15 (CH3
+),
30 (NO+), 46 (NO2
+) as well as
a number
of other minor peaks. This fragmentation pattern can be explained
by a predominantly ID (ionization followed
by dissociation) route.
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