Vibrational inelastic and charge transfer processes in H
+
+H
2
system:
An ab initio study
Saieswari Amaran and Sanjay Kumar
a͒
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
͑Received 20 August 2007; accepted 2 October 2007; published online 5 December 2007͒
State-resolved differential cross sections, total and integral cross sections, average vibrational
energy transfer, and the relative probabilities are computed for the H
+
+H
2
system using the newly
obtained ab initio potential energy surfaces at the full CI/cc-pVQZ level of accuracy which allow for
both the direct vibrational inelastic and the charge transfer processes. The quantum dynamics is
treated within the vibrational close-coupling infinite-order-sudden approximation approach using
the two ab initio quasidiabatic potential energy surfaces. The computed collision attributes for both
the processes are compared with the available state-to-state scattering experiments at E
c.m.
=20 eV. The results are in overall good agreement with most of the observed scattering features
such as rainbow positions, integral cross sections, and relative vibrational energy transfers. A
comparison with the earlier theoretical study carried out on the semiempirical surfaces ͑diatomics in
molecules͒ is also made to illustrate the reliability of the potential energy surfaces used in the
present work. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2801858͔
I. INTRODUCTION
The treatment of charge transfer processes in ion-
molecule collisions has always been a challenging problem
theoretically as it involves generally two or more electronic
states and the corresponding coupling between/among them.
Because of these complications most of the charge transfer
collision dynamics have relied on the semiempirical diatom-
ics in molecules
1
͑DIM͒ potential energy surfaces ͑PESs͒ and
on a semiclassical approach. For example, the trajectory sur-
face hopping method has been used to study the charge ex-
change and chemical reactions in the H
+
+D
2
, the H
+
+HD,
and the D
+
+HD systems
2–4
employing the DIM PESs of
Tully and Preston
5
and Schilier et al.
6
Giese and Gentry ͑GG͒ obtained an analytical fit of the
ground electronic state ͑GS͒ PES for the H
+
+H
2
system
7
using the available ab initio data points
8
and studied the
inelastic vibrational excitations using the classical trajectory
method.
9
Exact quantum dynamical calculations for vibra-
tional excitation processes in the vibrational close-coupling
rotational infinite-order-sudden approximation ͑VCC-
RIOSA͒ framework
10,11
was performed using the GG PES
͑Ref. 7͒ at low energies ͑3–10 eV͒ by Schinke and
McGuire.
12
However, the comparison with the experiments
was not completely satisfactory and it demanded even more
precise potential energy surfaces for the title system. Subse-
quently, Schinke and co-workers computed a complete con-
figuration interaction ͑CI͒ ab initio GS PES and performed
the quantum dynamics within the VCC-RIOSA approach in
the same energy range
13
which gave better results.
Quantum mechanical calculations within the VCC-
RIOSA approach using the GG PES in the high energy range
͑E
c.m.
=70 eV͒ have also been performed by Gianturco and
Kumar.
14
Electronically nonadiabatic transitions in the col-
linear H
+
+H
2
system have been studied recently using both
quantum mechanical and trajectory surface hopping methods
by Ushakov et al.
15
Their studies employed ͑3 ϫ3͒ DIM
PESs involving the GS and the lowest two excited electronic
states ͑ES͒. The PESs were obtained by fitting the available
ab initio data points of Ichihara and Yokoyama.
16
A detailed information on the charge transfer processes
of the title system became available in 1987 from the experi-
ments of Niedner et al.
17
The vibrational state-resolved mea-
surements of differential cross sections ͑DCSs͒ at E
c.m.
=20 eV ͑Ref. 17͒ reveal the dependence of the vibrational
excitation ͑VE͒ and vibrational charge transfer ͑VCT͒ pro-
cesses upon scattering angle ͑
͒. Experimental observations
suggest that the VE on the lower H
+
+H
2
surface leads to
charge transfer in the outgoing collisions for only those H
2
molecules which are excited vibrationally high enough
͑
Ј
ജ4͒ to overcome the endoergic barrier ͑⌬E=1.83 eV͒.A
quantum mechanical study on the integral cross sections and
DCSs for vibrational state-to-state inelastic and charge trans-
fer processes at E
c.m.
=20 eV was performed for this system
within the VCC-RIOSA approach
18
using the DIM PESs.
5
A. Present focus
Although information on the nonadiabatic interactions
between the first two electronic states has become available
in the literature
5,6
a detailed insight into the title system from
the ab initio approach is still lacking. Also, in previous quan-
tum mechanical study
18
two serious discrepancies were
noted in comparison with the experiments.
17
The first was
related with the position of the rainbow angle, and in the
second discrepancy the theoretical VCT DCSs differed sig-
nificantly from the experimental cross section for small scat-
tering angles. Hence, in the present study we carry out an
elaborate time-independent quantum mechanical ͑TIQM͒
a͒
Electronic mail: sanjay@iitm.ac.in
THE JOURNAL OF CHEMICAL PHYSICS 127, 214304 ͑2007͒
0021-9606/2007/127͑21͒/214304/10/$23.00 © 2007 American Institute of Physics127, 214304-1