Kinetic study of gas-phase Lu(2D3/2) with O2 , N2O and CO2

Article abstract of DOI:10.1016/S0009-2614(00)01132-5

The second-order rate constants of gas-phase Lu(²D_3/2) with O₂,N₂O and CO₂ from 348 to 573 K are reported. In all cases, the reactions are relatively fast with small barriers. The disappearance rates

Full text of DOI:10.1016/S0009-2614(00)01132-5

17 November 2000
Chemical Physics Letters 330 (2000) 547±550
www.elsevier.nl/locate/cplett
2
Kinetic study of gas-phase Luꢀ D3=2† with O2, N2O and CO2
Mark L. Campbell *
Chemistry Department, United States Naval Academy, Annapolis, MD 21402, USA
Received 5 July 2000; in ®nal form 26 September 2000
Abstract
2
The second-order rate constants of gas-phase Luꢀ D3=2† with O2; N2O and CO2 from 348 to 573 K are reported. In
all cases, the reactions are relatively fast with small barriers. The disappearance rates are independent of total pressure
1
indicating bimolecular abstraction processes. The bimolecular rate constants (in molecule cm3 s 1) are described in
Arrhenius form by kꢀO2† ˆ ꢀ2:3 Æ 0:4†  10 10 expꢀ 3:1 Æ 0:7 kJ mol 1=RT†; kꢀN2O† ˆ ꢀ2:2 Æ 0:4†  10
10
expꢀ 7:1 Æ 0:8 kJ mol 1=RT†; kꢀCO2† ˆ ꢀ2:0 Æ 0:6†  10 10 expꢀ 7:6 Æ 1:3 kJ mol 1=RT†, where the uncertainties
are Æ2r. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
oxygen-containing oxidants. This study continues
our e€orts to further understand the dynamics of
gas-phase metal reactions. Although primarily
phenomenological, the increase in the database of
these reactions should yield greater insight into the
important factors a€ecting gas-phase metal atom
reactions.
The kinetics of gas-phase reactions involving
metal atoms with oxygen-containing oxidants has
recently received considerable attention [1±4].
Metal atom chemistry is an intriguing ®eld of
study due to the high multiplicities of the atomic
ground states and the large number of low-lying
metastable states.
In this Letter, we report a gas-phase kinetic
2
study of the ground D3=2 state of lutetium with
2. Experimental
O2; N2O and CO2. Lutetium is the rarest of the
lanthanides and is unique among the lanthanides
due to its f subshell being completely ®lled. Be-
cause of its ®lled f subshell, the condensed phase
reactivity and spectroscopy of lutetium is similar
to that of the Group 3 metals.
Pseudo-®rst-order
kinetic
experiments
ꢀ‰LuŠ ꢁ ‰oxidantŠ† were carried out in an appara-
tus with slowly ¯owing gas using a laser photolysis/
laser-induced ¯uorescence (LIF) technique. The
experimental apparatus and technique have been
described in detail elsewhere [5]. Brie¯y, the reac-
tion chamber is a stainless steel reducing 4-way
cross with attached side arms and a sapphire win-
dow for optical viewing. The reaction chamber is
enclosed within a convection oven (Blue M, model
206F) for temperature dependence experiments.
Lutetium atoms were produced by the 248 nm
photodissociation of tris(2,2,6,6-tetramethyl-3,5-
We are unaware of any rate constant measure-
ments involving lutetium in the gas-phase with
* Fax: +1-410-293-2218.
E-mail address: campbell@brass.mathsci.usna.edu (M.L.
Campbell).
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 1 3 2 - 5

548
M.L. Campbell / Chemical Physics Letters 330 (2000) 547±550
heptanedionato) lutetium (III) [Lu(TMHD)3] us-
ing the focused output of an excimer laser
(Lambda Physics Lextra 200). The photolysis laser
output was focused using a lens (f ˆ 564 mm)
positioned approximately one focal length from
the detection zone. Lutetium atoms were detected
via LIF using an excimer-pumped dye laser
(Lambda Physics Lextra 50/ScanMate 2E) tuned
reported previously [7]. The following reagents
were used as received: O2 (MG Industries, 99.8%),
N2O (MG Industries, electronic grade, 99.999%),
CO2 (MG Industries, anaerobic grade, 99.9%) and
N2 (Potomac Airgas, Inc., 99.998%).
3. Data analysis and results
0
2
to the D5=2 2D3=2 transition at 465.802 nm and
0
2
2
The decay rates of the 2D3=2 state of lutetium as
a function of reactant pressure were investigated at
various temperatures and total pressures. The loss
of ground state atoms is described by the ®rst-or-
der decay constant, k1
the D5=2 ! D5=2 ¯uorescence at 513.509 nm was
isolated with an interference ®lter [6]. The ¯uo-
rescence was detected at 90° to the counterpropa-
gated laser beams with a three-lens telescope
imaged through an iris. A photomultiplier tube
(Hamamatsu R375) was used in collecting the LIF
which was subsequently sent to a gated boxcar
sampling module (Stanford Research Systems
SR250), and the digitized output was stored and
analyzed by a computer.
k1 ˆ 1=s ˆ k0 ‡ k2‰oxidŠ;
ꢀ1†
where s is the ®rst-order time constant for the
removal of the transition metal under the given
experimental conditions, k0 ꢀˆ 1=s0† the loss term
due to di€usion out of the detection zone and
reaction with the precursor and precursor frag-
ments, and k2 is the second-order rate constant.
Typical decay pro®les are shown in Fig. 1. A
time constant, s; for each decay pro®le was de-
termined using a linear least-squares procedure.
The second-order rate constant is determined
from a plot of 1=s vs. reactant number density.
Typical plots for obtaining second-order rate
constants are presented in Fig. 2; the slope yields
the observed rate constant. The relative uncer-
tainty (i.e., the reproducibility) of the second-
order rate constants is estimated at Æ20% based
on repeated measurements of rate constants un-
der identical temperature and total pressure
conditions. The absolute uncertainties are esti-
mated to be Æ30% and are based on the sum of
the statistical scatter in the data, uncertainty in
the ¯owmeter and ¯ow controller readings (5%)
and the total pressure reading (1%), and uncer-
tainties due to incomplete gas mixing and in-
complete relaxation of excited states produced in
the photolysis event.
Because of the low vapor pressure of the pre-
cursor at room temperature, the precursor re-
quired heating to get enough molecules into the
gas phase. Thus, the lowest temperature at which
kinetic experiments could be performed corre-
sponded to 348 K. Below these temperatures the
LIF signal was too weak to measure reliable values
of the rate constant.
The heated precursor was entrained in a ¯ow of
nitrogen bu€er gas. The precursor carrier gas,
bu€er gas and reactant gases ¯owed through cali-
brated mass ¯ow meters and ¯ow controllers prior
to admission to the reaction chamber. Each side-
arm window was purged with a slow ¯ow of ni-
trogen bu€er gas to prevent deposition of the
transition metal and other photoproducts. Pres-
sures were measured with MKS Baratron ma-
nometers, and chamber temperatures were
measured with a thermocouple.
The delay time between the photolysis pulse and
the dye-laser pulse was varied by a digital delay
generator (Stanford Research Systems DG535)
controlled by a computer. The trigger source for
these experiments was scattered pump laser light
incident upon a fast photodiode. LIF decay traces
consisted of 200 points, each point averaged for
four laser shots.
2
Second-order rate constants for the D3=2 state
reacting with O2; N2O and CO2 at 5 Torr from
373 to 573 K are listed in Table 1. Rate constants
could not be measured above 573 K due to thermal
decomposition of the precursor. The rate con-
stants at 398 K for these reactions were also
The Lu(TMHD)3 precursor was synthesized
from LuꢀNO3† and Li(TMHD) similar to that
3

M.L. Campbell / Chemical Physics Letters 330 (2000) 547±550
549
Fig. 3. Arrhenius plots for the collisional disappearance of
2
2
Fig. 1. Typical Luꢀ D3=2† decay curves in the presence of oxi-
Luꢀ D3=2†. Each solid line is a linear regression ®t of the rate
dant at 473 K, Ptotal ˆ 5:0 Torr. (a) PꢀO2† ˆ 97 mTorr; s ˆ
36:1 ls; (b) PꢀN2O† ˆ 194 mTorr; s ˆ 50:5 ls; (c) PꢀCO2† ˆ
89 mTorr; s ˆ 78:6 ls. The solid lines through the data are
exponential ®ts. The inset is a ln plot of the data.
Ea=RT
constants to the equation kꢀT† ˆ Ae
.
10
kꢀO2† ˆ ꢀ2:3 Æ 0:4†  10
 expꢀ 3:1 Æ 0:7 kJ mol 1=RT †;
ꢀ2†
ꢀ3†
ꢀ4†
10
kꢀN2O† ˆ ꢀ2:2 Æ 0:4†  10
 expꢀ 7:1 Æ 0:8 kJ mol 1=RT†;
10
kꢀCO2† ˆ ꢀ2:0 Æ 0:6†  10
 expꢀ 7:6 Æ 1:3 kJ mol 1=RT†;
where the uncertainties are Æ2r.
4. Discussion
The pressure independence observed for these
reactions indicates termolecular processes are un-
important; therefore, bimolecular abstraction re-
actions are indicated. The production of LuO on
the ground state surface is exothermic for all the
oxygen-containing reactants reported here. The
exothermicities for the reactions with O2; N2O
and CO2 are 193, 524, and 159 kJ/mol, respec-
tively; [8] thus, no thermodynamic barriers exist
for these reactions. The experimentally measured
2
Fig. 2. Typical plots for determining k2nd for Luꢀ D3=2† at 448
K. The solid line for each set of data is a linear regression ®t
from which k2nd is obtained.
measured at 20 Torr and are independent of total
pressure within experimental uncertainty.
The rate constants (in molecule cm3 s 1) are
described in Arrhenius form (see Fig. 3) by:
1

550
M.L. Campbell / Chemical Physics Letters 330 (2000) 547±550
Table 1
the gas-phase reactivities of these elements. The
1
Second-order rate constantsa(/10
state of lutetium with O2; N2O and CO2
mol cm3
s
1) for the 2D3=2
CO2
11
rate constants reported here for lutetium reacting
with O2; N2O and CO2 are very similar to the
rate constants reported for yttrium and scandium
with these oxidants while the rate constants for
lanthanum are about an order of magnitude
higher [11,12].
T (K)
O2
N2O
348
373
398
423
448
473
498
523
548
573
7.8
1.9
2.3
2.6
2.9
3.3
4.0
4.2
4.0
4.8
5.0
1.4
1.8
1.8
2.4
2.4
3.2
3.1
3.4
3.6
4.1
8.3
9.1
9.2
8.9
10
11
11
11
12
5. Summary
We have measured bimolecular reaction rate
constants and Arrhenius parameters for the reac-
a Uncertainties are estimated at Æ30%.
2
tions of the D3=2 state of lutetium with O2; N2O
and CO2. In each case, abstraction of an oxygen
atom to produce the metal oxide is the reaction
channel. The activation barriers for these reactions
are small (Ea < 8 kJ/mol).
barriers for all these reactants are small (<8 kJ/
mol), and in every case reaction occurs on average
in fewer than about 20 hard-sphere collisions.
Previous studies of the other lanthanides re-
acting with O2; N2O and CO2 indicated the acti-
vation energies for these reactions are correlated to
the energy required to promote an electron out of
the ®lled 6s subshell [2±4]. The lowest excited state
of lutetium with an s1 con®guration is the
Acknowledgements
This research was supported by the Camille and
Henry Dreyfus Foundation, Inc.
1
0
5d 6s16p14F3=2 state 17,427 cm above the ground
state [9]. This relatively large excitation energy
should result in higher activation energies com-
pared to those measured in this study; i.e., the
activation energies for O2; N2O and CO2 reactions
are predicted to be 11, 20 and 38 kJ/mol, respec-
tively. These calculated activation energies are
based on extrapolating the data from the other
lanthanide studies assuming a linear correlation
between the measured activation energy and the
excitation energy [2±4]. Thus, the activation energy
for lutetium does not appear to show the same
dependence on the s2 to s1 excitation energy as
observed for the other lanthanides.
The dynamics of the gas-phase reactions of
lutetium do not exhibit the same pattern as ob-
served for the other lanthanides. In common
practice, scandium, yttrium and lanthanum are
arranged together in Group 3 with the lanthanide
series following lanthanum. Yttrium is closer in
size and properties to lutetium so that an argu-
ment has been made that lutetium should be
placed in Group 3 preceded by the La±Yb ele-
ments [10]. This argument is also supported by
1
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