50
T. Okino and K. Yamanouchi
pulses, which is of the order of 10 to 30 fs, and the longer timescale is in the
rage of 0.1∼1 ps, which is of the order of a period of molecular rotation. Our
pump-and-probe study of two-body decomposition of methanol exposed to an ultrashort intense laser pulse showed that “fast” hydrogen atom migration proceeds
in the presence of the intense laser field whose duration is 40 fs and the “slow”
hydrogen atom migration proceeds after the light field disappears [15]. The slow
post-laser pulse migration can be regarded as a unimolecular decomposition process of an energized parent ion prepared by the irradiation of an ultrashort intense laser pulse. Our recent study on the ejection of H
+
3 from ethane (C 2 H 6 ) and
deuterated ethane (CH 3 CD 3 ) excited by an intense laser field showed that six hydrogen atoms are scrambled almost statistically within the relatively long lifetime
of a precursor parent ion prior to the ejection of a triatomic hydrogen molecular
ion [20].
In the present study, we focus on the “fast” hydrogen atom migration processes by referring to our recent study on the three-body fragmentation processes
of methylacetylene and methyl-d 3 -acetylene induced by ultrashort intense laser
pulses, and show on the basis of the coincidence momentum imaging maps how
four H/D atoms within a parent molecule migrate in an ultrashort intense laser
field.
3.2 Experiment and Data Analysis
The details of the experimental setup have been reported in our previous papers
[21–23]. Typically the output of a femtosecond laser system (790 nm, 40 fs, 5 kHz)
is focused with a quartz lens (f = 150 mm) at the point where they crossed
at right angles with a molecular beam of a sample gas introduced into the ultrahigh vacuum chamber through a nickel skimmer (∅0.48 mm) from a microsyringe (∅70 µm). The laser field intensity at the focal spot is in the range of
10 13 ∼10 14 W cm −2 .
The ion species ejected from the laser focal volume were extracted by three
equally spaced parallel-plate electrodes in the velocity mapping configurations [24]
and detected by a position sensitive detector with delay-line anodes [25]. The three
dimensional momentum vectors of the respective fragment ions were determined by
their time-of-flight and the position on the detector. False coincidence events originating from two or more parent molecular ions were discriminated from the true
coincidence events by imposing the momentum conservation conditions. The number of detected ions was kept to be about 0.5 events per laser shot in order to secure
the coincidence conditions and reduce the false coincidence events. The accumulated event number is of the order of 10 8 .
The spatial distribution of a proton or deuteron within a triply charged parent
molecule decomposing into three fragment ions including the proton or deuteron
can be mapped by converting the observed momentum vectors of the fragment ions
into the coordinate space, in which the position of the proton or deuteron with re-
T. Okino and K. Yamanouchi
pulses, which is of the order of 10 to 30 fs, and the longer timescale is in the
rage of 0.1∼1 ps, which is of the order of a period of molecular rotation. Our
pump-and-probe study of two-body decomposition of methanol exposed to an ultrashort intense laser pulse showed that “fast” hydrogen atom migration proceeds
in the presence of the intense laser field whose duration is 40 fs and the “slow”
hydrogen atom migration proceeds after the light field disappears [15]. The slow
post-laser pulse migration can be regarded as a unimolecular decomposition process of an energized parent ion prepared by the irradiation of an ultrashort intense laser pulse. Our recent study on the ejection of H
+
3 from ethane (C 2 H 6 ) and
deuterated ethane (CH 3 CD 3 ) excited by an intense laser field showed that six hydrogen atoms are scrambled almost statistically within the relatively long lifetime
of a precursor parent ion prior to the ejection of a triatomic hydrogen molecular
ion [20].
In the present study, we focus on the “fast” hydrogen atom migration processes by referring to our recent study on the three-body fragmentation processes
of methylacetylene and methyl-d 3 -acetylene induced by ultrashort intense laser
pulses, and show on the basis of the coincidence momentum imaging maps how
four H/D atoms within a parent molecule migrate in an ultrashort intense laser
field.
3.2 Experiment and Data Analysis
The details of the experimental setup have been reported in our previous papers
[21–23]. Typically the output of a femtosecond laser system (790 nm, 40 fs, 5 kHz)
is focused with a quartz lens (f = 150 mm) at the point where they crossed
at right angles with a molecular beam of a sample gas introduced into the ultrahigh vacuum chamber through a nickel skimmer (∅0.48 mm) from a microsyringe (∅70 µm). The laser field intensity at the focal spot is in the range of
10 13 ∼10 14 W cm −2 .
The ion species ejected from the laser focal volume were extracted by three
equally spaced parallel-plate electrodes in the velocity mapping configurations [24]
and detected by a position sensitive detector with delay-line anodes [25]. The three
dimensional momentum vectors of the respective fragment ions were determined by
their time-of-flight and the position on the detector. False coincidence events originating from two or more parent molecular ions were discriminated from the true
coincidence events by imposing the momentum conservation conditions. The number of detected ions was kept to be about 0.5 events per laser shot in order to secure
the coincidence conditions and reduce the false coincidence events. The accumulated event number is of the order of 10 8 .
The spatial distribution of a proton or deuteron within a triply charged parent
molecule decomposing into three fragment ions including the proton or deuteron
can be mapped by converting the observed momentum vectors of the fragment ions
into the coordinate space, in which the position of the proton or deuteron with re-