Electronic spectroscopy and ionization potential of UO
in the gas phase
Jiande Han, Vasiliy Goncharov, Leonid A. Kaledin, Anatoly V. Komissarov,
and Michael C. Heaven
Department of Chemistry, Emory University, Atlanta, Georgia 30322
͑Received 19 November 2003; accepted 18 December 2003͒
The electronic spectroscopy of UO
has been examined using multiphoton ionization with
mass-selected detection of the UO
ions. Supersonic jet cooling was used to reduce the spectral
congestion. Twenty-two vibronic bands of neutral UO
were observed in the range from 17 400 to
32 000 cm
. These bands originated from the U(5f
⌽(3u) states. The
stronger band systems are attributed to metal-centered 7p ←7s transitions. Threshold ionization
measurements were used to determine the ionization potentials of UO and UO
. These were found
to be higher than the values obtained previously from electron impact measurements but in
agreement with the results of recent theoretical calculations. © 2004 American Institute of Physics.
Methods for computing the properties of small actinide-
containing molecules are currently being investigated by sev-
eral research groups.
The challenge with these systems is
the large numbers of electronic conﬁgurations associated
with the presence of open f and d shells, coupled with the
need to provide adequate treatment of the relativistic terms.
Diatomic and triatomic heavy element molecules may now
be investigated using reasonably high-level theoretical mod-
els, but evaluation of the quality of these calculations is not a
straightforward matter. Gas phase spectroscopic data are es-
sential for this purpose, but there have been very few such
studies for actinide compounds.
The present paper is primarily concerned with the elec-
tronic spectroscopy of UO
. This molecule has been the fo-
cus of several recent theoretical calculations.
With the ex-
ception of the recent communication by Han et al.,
for gas phase UO
had not been reported previously. How-
ever, other properties of gas phase UO
have been investi-
gated. The ionization potential ͑IP͒ has been measured using
electron impact techniques,
yielding the generally ac-
cepted value of 5.4͑1͒ eV. Capone et al.
reported a bond
dissociation energy of 7.9 eV, which they deduced from frag-
ment ion appearance potentials. Allen et al.
photoelectron spectroscopy for a mixture of UO and
. They observed structure corresponding to excited
states of UO
, but were unable to determine the IP as the
relevant region of the spectrum was overlapped by a broad
feature attributed to UO. The geometry of ground state UO
was investigated in a molecular beam–electric ﬁeld focusing
A weak focusing effect was observed, indicat-
ing either that the molecule was slightly bent, or that the
ground state was linear ͑not focused͒ but that excited mol-
ecules present in the beam could be focused.
Ground state vibrational frequencies have been mea-
sured for UO
trapped in solid Ne and Ar matrices.
isotope shift data for matrix isolated UO
indicates that it is
a linear symmetric molecule.
Interestingly, the frequency
for the asymmetric vibrational mode exhibits an anomalously
large shift when the host is changed from Ne to Ar.
origin of this effect is an open question at present.
Consistent with the experimental results, theoretical cal-
culations for UO
predict a linear, symmetric ground state.
Earlier studies concluded that the unpaired electrons would
reside in the U 5 f orbital,
giving rise to a
ground state. However, more recent calculations have pre-
dicted that U(5 f
is lower in energy, yielding a
⌽(2u) ground state.
The calculations of Zhou et al.
Gagliardi et al.
yielded IPs of 6.27 and 6.17 eV, well outside
the range of the electron impact measurements.
argued that their calculations should not be in error by
such a large margin and called the experimental results into
question. This discrepancy was recently investigated by us-
ing resonantly enhanced multiphoton ionization ͑REMPI͒
techniques to measure the IPs of UO and UO
͑Ref. 8͒. The
results for both molecules exceeded the IPs measured by
electron impact techniques. The IP for UO
was in much better agreement with theory, providing an en-
couraging validation of the most recent calculations. To date
there have been very few attempts to calculate the IP of UO.
Malli et al.
obtained a value of 6.17 eV using a relativistic
density functional-SCF approach, while Boudreaux and
reported 5.71 eV using a relativistic extended
ckel model. As for UO
, the ab initio result exceeded the
IP obtained by electron impact techniques ͑5.6͑1͒ eV͒ but
was in reasonable agreement with the photoionization value
The present paper describes spectroscopic results for
and the IP measurements for U, UO, and UO
sequence of IP measurements was needed to arrive at an
accurate value for the IP of UO
. The IP of atomic U was
investigated to calibrate the apparatus. Photoionization efﬁ-
ciency ͑PIE͒ curves and mass analyzed threshold ionization
Present address: Chemistry Division, Brookhaven National Laboratory,
Upton, NY 11973.
Author to whom all correspondence should be addressed. Electronic mail:
JOURNAL OF CHEMICAL PHYSICS VOLUME 120, NUMBER 11 15 MARCH 2004
51550021-9606/2004/120(11)/5155/9/$22.00 © 2004 American Institute of Physics