58
T. Okino and K. Yamanouchi
along the diagonal line, representing the cases in which two singly charged ions
have equal momentum releases, could not be identified. Furthermore, obviously, the
pairs of the inner crescent feature in Figs. 3.3(a) and 3.3(b), showing the spatial distribution of the singly charged ion, which is supposed to have been produced from
the second ejection stage, is not isotropic.
Therefore, it can be said that the ejections of the light ion species could not be
separated from each other. This means that the three-body decomposition does not
proceed in a sequential manner but in a concerted manner. The concerted decomposition scheme can be supported by the cigar-type shape of Domain A and Domain B
in the 3D momentum space. When H + and H
+
2 are on the same side with respect to
C 3 H + , like in the original methyl group configuration, C 3 H + should receive large
momentum recoil. On the other hand, if H + and H
+
2 are located on the opposite
sides with respect to C 3 H + , C 3 H + should have much smaller momentum release
because C 3 H + is sandwiched by H + and H
+
2 . In Domain A, the largest value of
|p(C 3 H + )| can be taken when H + is ejected as soon as after H
+
2 is ejected so that
C 3 H + receive the largest momentum recoil from the same side, while the smallest
value of |p(C 3 H + )| can be taken when H + is ejected after H + is transferred to the
opposite side of C 3 H + with respect to the side of the ejecting H
+
2 . This transfer of
the position of H + can be realized by the overall rotation of H + –C 3 H + by about π
induced by the recoil momentum received from the ejecting H
+
2 at the earlier part
of the three-body decomposition. When H + is ejected from H + –C 3 H + after the π
rotation, the direction of the recoil momentum received by C 3 H + is in the opposite direction to that received at the earlier ejection of H
+
2 . The continuous event
distribution in Domain A indicates that there are a lot of events categorized into intermediate cases in which H + is ejected when the H + –C 3 H + moiety rotates by the
angle smaller than π . In the earlier part of this concerted three-body decomposition,
H
+
2 starts to leave and the rest of the molecule starts to rotate simultaneously, and
H + starts to leave before H
+
2 is still in the process of the ejection. Almost exactly
the same scenario can be drawn for the events in Domain B in which H + is ejected
“earlier” than H
+
2 .
3.4 Summary
In this chapter, our recent studies on ultrafast hydrogen migration processes occurring in the three-body decomposition processes of methylacetylene and methyl-d 3 -
acetylene induced by intense laser fields are introduced.
From the H/D distribution maps obtained for methyl-d 3 -acetylene, it was revealed that a variety of different types of the H/D migration processes coexist such
as (i) the migration of one H atom, (ii) the exchange between two H atoms, (iii) the
migration of two H atoms, and (iv) the exchange of two H atoms and additional
migration of one H atom (“hydrogen scrambling”).
These processes of hydrogen migration and hydrogen scrambling are considered
to be finished by the time when the enhanced ionization [26, 27] occurs from the
T. Okino and K. Yamanouchi
along the diagonal line, representing the cases in which two singly charged ions
have equal momentum releases, could not be identified. Furthermore, obviously, the
pairs of the inner crescent feature in Figs. 3.3(a) and 3.3(b), showing the spatial distribution of the singly charged ion, which is supposed to have been produced from
the second ejection stage, is not isotropic.
Therefore, it can be said that the ejections of the light ion species could not be
separated from each other. This means that the three-body decomposition does not
proceed in a sequential manner but in a concerted manner. The concerted decomposition scheme can be supported by the cigar-type shape of Domain A and Domain B
in the 3D momentum space. When H + and H
+
2 are on the same side with respect to
C 3 H + , like in the original methyl group configuration, C 3 H + should receive large
momentum recoil. On the other hand, if H + and H
+
2 are located on the opposite
sides with respect to C 3 H + , C 3 H + should have much smaller momentum release
because C 3 H + is sandwiched by H + and H
+
2 . In Domain A, the largest value of
|p(C 3 H + )| can be taken when H + is ejected as soon as after H
+
2 is ejected so that
C 3 H + receive the largest momentum recoil from the same side, while the smallest
value of |p(C 3 H + )| can be taken when H + is ejected after H + is transferred to the
opposite side of C 3 H + with respect to the side of the ejecting H
+
2 . This transfer of
the position of H + can be realized by the overall rotation of H + –C 3 H + by about π
induced by the recoil momentum received from the ejecting H
+
2 at the earlier part
of the three-body decomposition. When H + is ejected from H + –C 3 H + after the π
rotation, the direction of the recoil momentum received by C 3 H + is in the opposite direction to that received at the earlier ejection of H
+
2 . The continuous event
distribution in Domain A indicates that there are a lot of events categorized into intermediate cases in which H + is ejected when the H + –C 3 H + moiety rotates by the
angle smaller than π . In the earlier part of this concerted three-body decomposition,
H
+
2 starts to leave and the rest of the molecule starts to rotate simultaneously, and
H + starts to leave before H
+
2 is still in the process of the ejection. Almost exactly
the same scenario can be drawn for the events in Domain B in which H + is ejected
“earlier” than H
+
2 .
3.4 Summary
In this chapter, our recent studies on ultrafast hydrogen migration processes occurring in the three-body decomposition processes of methylacetylene and methyl-d 3 -
acetylene induced by intense laser fields are introduced.
From the H/D distribution maps obtained for methyl-d 3 -acetylene, it was revealed that a variety of different types of the H/D migration processes coexist such
as (i) the migration of one H atom, (ii) the exchange between two H atoms, (iii) the
migration of two H atoms, and (iv) the exchange of two H atoms and additional
migration of one H atom (“hydrogen scrambling”).
These processes of hydrogen migration and hydrogen scrambling are considered
to be finished by the time when the enhanced ionization [26, 27] occurs from the