Experiment and Theory Elucidate the Pathways for
H3+ Formation in the Ultrafast Double
Ionization in Methanol
James M. Farrar
Department of Chemistry
University of Rochester
Rochester NY 14627 USA
The H3+ molecular ion has been
well-known in chemistry for over a century. The ion is the simplest
member of a class of protonated closed-shell molecules including
H3O+,
NH4+, and
CH5+ that can be formed by
ion-molecule reactions in high-pressure ion sources.[1] Such species
have been employed extensively in chemical ionization mass spectrometry
[2-4] as a means of soft ionization. The
H3+ ion has played a central role in
atomic and molecular physics dating back to the earliest days of mass
spectrometry. At the beginning of the 20th century, J.
J Thomson’s pioneering parabolic ray experiments with hydrogen gas
[5] confirmed the presence of ions with mass to charge (m/e) ratios
of 1 and 2, but also an unexpected peak corresponding to m/e of 3.
Thomson correctly identified this peak as H3, a result
that was later confirmed by Hogness and Lunn,[6] who employed mass
spectrometry with a high-pressure ion source. Their results showed
unequivocally that this species was produced by reaction between the
primary ionization product H2+ and molecular hydrogen.
More recently, H3+ produced by a fast
ion-molecule reaction between molecular hydrogen and
H2+ formed by cosmic ray ionization
has been established as the central ion that initiates chemistry in the
interstellar medium. In a seminal paper, Herbst and Klemperer [7]
proposed a detailed chemical kinetic scheme to account for the presence
of the large number of molecules detected by radioastronomy in the 1960s
and early 1970s.[8] Proton transfer reactions of
H3+ to abundant atomic and molecular
species CO, O, N, O2, and N2 initiate
this scheme. The formyl cation, HCO+, speculated to be
“X-ogen”,[9-11] the carrier of a prominent line observed in
radioastronomy, is a particularly important reaction product.
Protonation of CO on the oxygen end to form the isoformyl cation,
COH+, was shown to lead to significant concentrations
of this species as well.[12]
Laboratory studies of the high-resolution spectroscopy of
H3+, [13, 14] supported by
measurements of reaction rates of this species with numerous small
molecules,[15] provided excellent context to the report by Geballe
and Oka in 1996 [16] of the direct detection of
H3+ in interstellar space. The
preeminence of H3+ in astrochemistry
and astrobiology has been summarized recently in important
publications.[17, 18]
The original mass spectrometric findings of Hogness and Lunn have
withstood the test of time as the “standard model” to produce
H3+ in ionized gases, especially
interstellar space. In light of the extensive literature that highlights
both the terrestrial and interstellar chemistry of
H3+, it is remarkable to discover new
motifs for its production. The emerging field of multiply-charged ions
has provided the context for several studies of alcohols and
hydrocarbons in which electron impact and strong field laser ionization
have produced H3+ by Coulomb
explosion. The paper by Gope, Livshits, Bittner, Baer, and
Strasser,[19] the subject of the present Highlight, represents the
state of the art of such studies, reporting time-resolved ultrafast
pump-probe experiments on CH3OD with 3D coincidence
imaging of the products of Coulomb explosion. The experimental work is
complemented with ab initio molecular dynamics (AIMD)
computations that characterize the timescales and kinetic energy
releases for the two distinct pathways for product formation:
H3+ + COD+, and
H2D+ + HCO+.
The work of Gope et al. was preceded by a number of studies,
notably the one-color multiphoton double ionization (MPDI) experiments
conducted on CH3OD by Dantus and co-workers, [20,
21] in which near IR (NIR) laser pulses both ionized
CH3OD and initiated the
H+/D+ transfers leading to chemical
reaction. The experiments revealed that
H2D+ product formation was delayed
relative to the appearance of H3+,
leading to a proposal that reaction was controlled by the formation of a
nascent roaming H2 molecule [22] on the carboxyl
(CH3CO) moiety of methanol that could undergo rapidgeminal proton transfer from carbon to form the early
H3+ product, or the roaming molecule
could react with the deuterium ion on the vicinal oxygen to form
H2D+ after a measurable delay. Similar
experiments with longer chain alcohols led to the expected result that
the longer the distance between the proton-donating carbon atom and the
OD group, the longer the delay time between
H3+ and
H2D+ formation.
Gope et al . have performed single photon double ionization (SPDI)
experiments with low field ultrafast extreme-ultraviolet (EUV) pump
pulses, followed by NIR probe spectroscopy to overcome the strong
sensitivity of MPDI experiments to laser parameters. Three-dimensional
coincidence imaging provides reliable product detection with a
determination of the kinetic energy release. Complementary AIMD
calculations on both the CH3OD and CH3OH
systems play a critical role in revealing important dynamical signatures
of prompt and delayed H3+ formation.
Specifically, monitoring the time evolution of the H3 –
COH distance shows that delayed proton transfer (from oxygen) occurs
approximately 110 fs after the prompt proton transfer from carbon. Of
special interest is a comparison of the computed kinetic energy releases
for the prompt and delayed proton transfers in CH3OH
with the same processes in CH3OD. As expected, the
kinetic energy releases for prompt reactions (proton transfer from
carbon) in CH3OH and CH3OD are not
affected by deuteration. However, the kinetic energy release for delayed
proton transfer (from oxygen) in CH3OH to form
H3+ + HCO+ is
shifted to lower energies relative to delayed proton transfer
(from oxygen) in CH3OD to form
H2D+ + COH+. The
authors attribute this surprising result to a subtle kinetic isotope
effect rather than an energetic effect arising from the greater total
energy available to the HCO+ product.
Throughout the history of elementary reaction dynamics, the symbiosis of
experiment and theory has been a recurring theme. As experiments have
become more sophisticated, advancing from the specification and
measurement of asymptotic reactant and product quantum states to
time-resolved probes of detailed atomic motions, so also has theory
responded with advanced methodologies to probe ever-increasing detail.
The work of Gope et al. is an outstanding exemplar of the synergy
between theory and experiment, and illuminates the way to additional
advances in both disciplines.