Chapter 3
Ultrafast Dynamics of Hydrogen Atoms
in Hydrocarbon Molecules in Intense Laser
Fields: Hydrogen Atom Migration
and Scrambling in Methylacetylene
Tomoya Okino and Kaoru Yamanouchi
Abstract Characteristic ultrafast processes of hydrogen atom migration commonly
observed in hydrocarbon molecules in an intense laser field are introduced by
referring to our recent studies on three-body decomposition of methylacetylene
and methyl-d 3 -acetylene. Special emphases are placed on how dynamical pictures of hydrogen migration are extracted from coincidence momentum imaging
(CMI) maps and proton distribution maps constructed from the CMI maps and
how migrations of respective hydrogen atoms are correlated with each other, leading to ultrafast exchange and scrambling of hydrogen atoms within a hydrocarbon
molecule.
3.1 Introduction
When gaseous molecules are exposed to an intense laser field (> 10 13 W/cm 2 ),
they are multiply ionized and decomposed eventually into fragment ions through
Coulomb explosion processes [1, 2]. We have shown from our series of studies that rich information of molecules interacting with an intense laser field such
as ultrafast structural deformation and ultrafast hydrogen migration can be extracted from the analysis of the observed momentum vectors of fragment ions [3–
22]. Among the phenomena we have found so far, ejections of triatomic hydrogen molecular ions, H
+
3 , and ultrafast hydrogen migration are noteworthy. These
phenomena were commonly found when hydrocarbon molecules are exposed to
an intense laser field, and can be regarded as those characteristic of H and D
atoms having considerably smaller mass than other heavier atoms in hydrocarbon
molecules.
It is interesting to note that there are two different timescales during which hydrogen atoms move when hydrocarbon molecules are irradiated with an intense
laser field. The shorter timescale is characterized by the duration of ultrashort laser
T. Okino · K. Yamanouchi (B)
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
e-mail: kaoru@chem.s.u-tokyo.ac.jp
R. de Nalda, L. Bañares (eds.), Ultrafast Phenomena in Molecular Sciences,
Springer Series in Chemical Physics 107, DOI 10.1007/978-3-319-02051-8_3,
© Springer International Publishing Switzerland 2014
49
Ultrafast Dynamics of Hydrogen Atoms
in Hydrocarbon Molecules in Intense Laser
Fields: Hydrogen Atom Migration
and Scrambling in Methylacetylene
Tomoya Okino and Kaoru Yamanouchi
Abstract Characteristic ultrafast processes of hydrogen atom migration commonly
observed in hydrocarbon molecules in an intense laser field are introduced by
referring to our recent studies on three-body decomposition of methylacetylene
and methyl-d 3 -acetylene. Special emphases are placed on how dynamical pictures of hydrogen migration are extracted from coincidence momentum imaging
(CMI) maps and proton distribution maps constructed from the CMI maps and
how migrations of respective hydrogen atoms are correlated with each other, leading to ultrafast exchange and scrambling of hydrogen atoms within a hydrocarbon
molecule.
3.1 Introduction
When gaseous molecules are exposed to an intense laser field (> 10 13 W/cm 2 ),
they are multiply ionized and decomposed eventually into fragment ions through
Coulomb explosion processes [1, 2]. We have shown from our series of studies that rich information of molecules interacting with an intense laser field such
as ultrafast structural deformation and ultrafast hydrogen migration can be extracted from the analysis of the observed momentum vectors of fragment ions [3–
22]. Among the phenomena we have found so far, ejections of triatomic hydrogen molecular ions, H
+
3 , and ultrafast hydrogen migration are noteworthy. These
phenomena were commonly found when hydrocarbon molecules are exposed to
an intense laser field, and can be regarded as those characteristic of H and D
atoms having considerably smaller mass than other heavier atoms in hydrocarbon
molecules.
It is interesting to note that there are two different timescales during which hydrogen atoms move when hydrocarbon molecules are irradiated with an intense
laser field. The shorter timescale is characterized by the duration of ultrashort laser
T. Okino · K. Yamanouchi (B)
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
e-mail: kaoru@chem.s.u-tokyo.ac.jp
R. de Nalda, L. Bañares (eds.), Ultrafast Phenomena in Molecular Sciences,
Springer Series in Chemical Physics 107, DOI 10.1007/978-3-319-02051-8_3,
© Springer International Publishing Switzerland 2014
49