PHYSICAL REVIEW A 82, 023415 (2010)
Experimental investigation of dissociation pathways of cooled HeH
+
following valence electron
excitation at 32 nm by intense free-electron-laser radiation
H. B. Pedersen,
1,*
L. Lammich,
1
C. Domesle,
2
B. Jordon-Thaden,
2
O. Heber,
3
J. Ullrich,
2
R. Treusch,
4
N. Guerassimova,
4
and A. Wolf
2
1
Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark
2
Max-Planck-Institut f¨ ur Kernphysik, D-69117 Heidelberg, Germany
3
Department of Particle Physics, Weizmann Institute of Science, 76100 Rehovot, Israel
4
Hamburger Synchrotronstrahlungslabor at Deutsches Elektronen-Synchrotron, D-22607 Hamburg, Germany
(Received 2 July 2010; published 16 August 2010)
The dissociation pathways of HeH
+
have been investigated below the first ionization continuum by
photoabsorption at 32 nm, using fragment momentum imaging in a crossed-beams experiment at the free-electron
laser in Hamburg (FLASH). Investigations were done both for ions with several vibrational levels excited in the
ion source and for ions vibrationally cooled in an electrostatic ion trap prior to the irradiation. The product
channels He
+
(1s ) + H(nl ) and He(1snl ) + H
+
were separated and the He(1snl ) + H
+
channel was particularly
studied by coincidence detection of the He and H
+
fragments on two separate fragment detectors. At 32 nm
excitation, the branching ratio between the product channels was found to be σ
He
+
+H
/σ
He+H
+ = 0.96 ± 0.11 for
vibrationally hot and 1.70 ± 0.48 for vibrationally cold ions. The spectra of kinetic energy releases for both
channels revealed that photodissociation at 32 nm leads to high Rydberg states (n 3–4) of the emerging atomic
fragments irrespective of the initial vibrational excitation of HeH
+
. The fragment angular distributions showed
that dissociation into the He + H
+
channel mostly (∼70%) proceeds through
1
states, while for the He
+
+ H
channel
1
and
1
states are of about equal importance.
DOI: 10.1103/PhysRevA.82.023415 PACS number(s): 33.80.Gj
I. INTRODUCTION
Photoabsorption by molecular cations in the xuv and
soft x-ray region initiates dissociation through manifolds of
coupled excited states that arise from the excitation of the
molecular valence electrons. The pathways of the dissociating
system through these excited states can be strongly influenced
by nonadiabatic couplings between electronic and nuclear
motions [1]. Thus, in this region, the dissociation dynamics
goes beyond the Born-Oppenheimer approximation where a
separate description of the electronic and nuclear degrees
of freedom is assumed. Simple molecular cations are model
systems to study such interactions, since detailed aspects of
their nonadiabatic couplings can be theoretically isolated [2,3]
and compared to experimental observations.
In this paper, we report a kinematically complete investi-
gation of the photodissociation of the simplest heteronuclear
molecular ion HeH
+
at 32 nm (38.7 eV), addressing explicitly
the dissociation dynamics through the manifold of singly
excited states below the first ionization continuum, He
+
(1s ) +
H
+
, in a process schematically written as
HeH
+
(
1
) + 32 nm → [HeH
+
(
1
/
1
)]
∗
→
He(1snl ) + H
+
,
He
+
(1s ) + H(nl ).
(1)
Figure 1 illustrates the photoabsorption and dissociation
process through this excited state manifold. In a simplified
view, the photoabsorption instantaneously brings an electron
from the 1σ orbital of HeH
+
(1σ
21
) into an excited orbital
*
Corresponding author: hbjp@phys.au.dk
of σ or π symmetry. Ionization or double excitation of the 1σ
electrons only happens at photon energies beyond ∼40 eV
[4]. During the subsequent dissociation process, energy is
distributed between kinetic energy and internal energy of the
fragments in a process where nonadiabatic interactions can
play a significant role.
HeH
+
is astrophysically significant and it appears, for
instance, as a molecular coolant in models of the early universe
[5], and it is assumed to exist in astrophysical plasmas near
strong xuv sources like planetary nebulae [6]. Moreover, the
excited states of HeH
+
involved in the photodissociation pro-
cess are of relevance for evaluating the molecular contributions
to the electron energy spectrum of β -decaying T
2
[7,8] to be
used in upcoming measurements of the mass of the electron
neutrino [9]. Finally, HeH
+
is isoelectronic with the simplest
neutral molecule H
2
(D
2
) presently under intense study with
respect to molecular dynamics in strong laser fields. Thus,
while dissociation dynamics after strong-field ionization into
H
2
+
is being studied in great detail [10,11], the possible
strong-field excitation into the associated H(nl ) + H(n
′
l
′
)
state manifold is unexplored in these experiments since only
charged fragments are detectable. This dissociation dynamics,
however, is mimicked (albeit without the additional dressing
of the states by an intense laser field) by the dissociation
of HeH
+
through the excited-state manifold reached by xuv
photoabsorption as depicted in Fig. 1. Experimentally, we
observe the momenta of atomic fragments (He, H, and H
+
)
from isolated photoabsorption processes in a dilute fast-
moving beam of HeH
+
ions crossed by short pulses of xuv
radiation from the free-electron laser in Hamburg (FLASH)
[12,13]. The feasibility of the experiment relies largely on the
extreme intensity of the xuv pulses from this free-electron laser
source.
1050-2947/2010/82(2)/023415(8) 023415-1 ©2010 The American Physical Society