Observation of Na223Πg-13Σ +u bound-free emissions generated from Na(3p) in a reactive oxidation process

Article abstract of DOI:10.1016/S0009-2614(00)01051-4

The chemiluminescent reaction products of the (Na,Na₂)+Br halogenation system at concentrations in excess of 10¹² cm^-3 are considered. In a purely reaction driven environment evidence is obtained for the formation of the Na₂ 2³Π_g excited state and bound-free emission associated with the 2³Π_g-1³Σ_u⁺ band system.

Full text of DOI:10.1016/S0009-2614(00)01051-4

10 November 2000
Chemical Physics Letters 330 (2000) 417±422
www.elsevier.nl/locate/cplett
‡
Observation of Na223Pg±13Ru bound-free emissions generated
from Na(3p) in a reactive oxidation process
D.R. Grantier a, J.L. Gole b,*
a
Bell South Corporation, 1155 Peachtree St. N.E., Atlanta, GA 30309±3610, USA
School of Physics, Georgia Institute of Technology, Atlanta, GA 30332±0430, USA
b
Received 22 May 2000; in ®nal form 6 September 2000
Abstract
The chemiluminescent reaction products of the ꢀNa; Na2† ‡ Br halogenation system at concentrations in excess of
3
1012 cm are considered. In a purely reaction driven environment evidence is obtained for the formation of the Na2
‡
23Pg excited state and bound-free emission associated with the 23Pg 13Ru band system. Ó 2000 Elsevier Science
B.V. All rights reserved.
1. Introduction
formation of the Na2 violet band systems, ac-
‡
companied also by Na2 B1Pu X1Rg emission,
At densities which range from 1012 to
1016 cm 3, the interactions between atomic sodium
excited states and small sodium molecules dem-
onstrate surprising consequences. At the highest
number densities, these include the ®rst evidence of
a Raman pumping [1±6] process induced through
chemical reaction. This highly ecient process
which appears to involve light scattering from the
low lying vibrational levels of ground state sodium
dimer, correlates with the Na D-line components
(D1; D2) formed through the Na2 ‡ Br ! NaÃ
ꢀ3P† ‡ NaBr reaction [4±6] (Fig. 1). Equally in-
triguing is the observation of the optical signatures
for the bound-free Na2 violet emission band sys-
tems, onsetting also as the Raman emission fea-
tures develop in a completely reaction driven
environment. In order to observe the chemical
we have successfully operated a unique extended
path length supersonic expansion source, de-
scribed in detail elsewhere [5±7] under conditions
which produce high concentrations of primarily
1
sodium atoms and dimers.
The violet bands, ®rst observed in 1932 [8], have
been the subject of over 30 publications [9] as these
features have been observed in absorption and
especially in emission following electrical or single-
or multiphoton excitation generally at total pres-
sures exceeding 0.1 Torr. In fact, their use as a
laser transition has been proposed [10,11] and
signi®cant gain has been measured [12]. However,
our observation of emission associated with the
Na223Pg 13Ru and 21Ru X1Rg transitions,
®rst correctly correlated with the violet bands by
Pichler et al. [9], appears to be the ®rst monitoring
‡
‡
‡
* Corresponding author. Fax: +1-404-894-9958.
E-mail address: james.gole@physics.gatech.edu (J.L. Gole).
A very small concentration of trimers is also produced in
these experiments.
1
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 0 5 1 - 4

418
D.R. Grantier, J.L. Gole / Chemical Physics Letters 330 (2000) 417±422
‡
‡
very weak Raman-like signal. The A1Ru ! X1Rg
molecular ¯uorescence is distinguished from the
Raman-like features by smaller peak separations
1
on the order of ꢁ114 cm which correspond to
vibrational frequencies associated with the
‡
Na2 A1Ru state.
As the ¯ux continues to increase to the inter-
mediate levels of the pure sodium expansion of
Fig. 1 and to the highest ¯uxes used in these ex-
‡
‡
periments, no Na2 A1Ru ! X1Rg molecular ¯uo-
rescence is detected and the emission is dominated
by Raman-like scattering. Here, the observed fea-
tures generated in the reactive environment can
exceed all other chemiluminescence by nearly two
orders of magnitude (Fig. 1). If the sodium vapor
is entrained in argon or helium and this mixture is
expanded through the extended slit nozzle, the
ratio of the Na2 B±X ¯uorescence to the Raman
scattering intensity increases. However, if we
compare the spectra obtained as one converts from
a helium to argon expansion, we observe a
quenching and red shifting of the vibrational band
Fig. 1. Survey spectrum of chemiluminescent emission and
Raman scattering involving various electronic states of di-
‡
atomic sodium. The Na2 23Pg ! 13Ru transition corresponds
to a triplet±triplet bound-free excimer-like emission process.
ꢀ
(Res. ± 12 A, Toven ± 875 K, Tnozzle ± 935 K).
‡
intensities associated with the Na2 B1Pu X1Rg
molecular ¯uorescence which would appear to re-
sult from di€erences in the formation [13] e-
ciency of the Na2 B state with argon versus helium
entrainment. Further, the bound-free molecular
¯uorescence which is virtually absent in the helium
expansion onsets with argon seeding, albeit not to
the extent of that for a pure expansion. This is also
demonstrated in Fig. 3 for the highest reactant
¯uxes obtained in a Na/Ar expansion where the
molecular scattering and ¯uorescence features are
accompanied by a cascade of Na atomic emission
features.
of these band systems associated with (1) a react-
ing supersonic expansion and (2) a purely che-
miluminescent reactive process.
2. Observations
The nature of the energy release in the
Na2 ‡ Br ! NaÃꢀ3p† ‡ NaBr reactive environ-
ment as displayed through a Raman-like e€ect is
intriguing in that the Stokes/antiStokes emission
features depicted in Fig. 1 begin to dominate at
moderately high particle densities. For a pure so-
dium expansion, they are accompanied by
Previous experimental observations [14±16]
‡
suggest that the B1Pu ! X1Rg emission spectrum
is produced in one of the following reaction se-
quences:
‡
B1Pu X1Rg and Na2 bound-free emission. By
Á
comparison, Fig. 2 demonstrates that the devel-
opment of emission in these systems at lower
densities corresponds to a thermalized Na2B1Pu
‡
Ã
Naꢀ3p† ‡ Na2ꢀX1Rg † ! Naꢀ3s† ‡ Na2 B1Pu
Á
NaÃ2 B1Pu ! Na2ꢀX1Rg † ‡ hm
ꢀ1†
ꢀ2†
‡
‡
X1Rg ¯uorescence which accompanies a weak Na
Naꢀ3p† ‡ Naꢀ3s† ‡ M ! NaÃ2ꢀB1Pu† ‡ M
D-line. As the source ¯ux increases, the relative
density of dimers in the reaction zone also in-
creases as we observe (Fig. 2b) collision induced
molecular ¯uorescence associated with the
NaÃ2ꢀB1Pu† ! Na2ꢀX1Rg † ‡ hm
‡
where the M in the associative reaction (2) corre-
sponds to a third body whose presence provides a
‡
‡
Na2A1Ru ! X1Rg transition superimposed over a

D.R. Grantier, J.L. Gole / Chemical Physics Letters 330 (2000) 417±422
419
Fig. 2. Survey scans of observed chemiluminescence resulting from sodium/halogen atom interactions at reduced halogen reactant
densities. The sodium oven is operated at a temperature close to ꢁ800 K. The relative intensities of the two spectra (a) and (b) are on
the same scale.
cm 3) the probability of process (2) is often rather
route to funnel excess energy. The rather large
energy defect (DE  3000 cm 1) which would be
low [14]. Nevertheless, energetics alone suggests
associated with process (1) suggests that this pro-
cess represents an unlikely mechanism for the ob-
that it is more likely that process (2) represents the
dominant mechanism for the population of and
subsequent chemiluminscence from the B1Pu state
of sodium dimer, particularly at the higher atom
number densities present in the current system.
Pichler et al. [9] have used the single mode 351.1
nm excitation from an argon ion laser and optical±
optical double resonance via mixed A1u ꢁ b3Pu
levels to obtain spectra which conclusively dem-
onstrate that the puzzling violet bands arise from
the superposition of two distinct continuum
‡
served molecular B1Pu ! X1Rg ¯uorescence.
At the highest concentrations in the present
study, the formation of an excited atom/dimer
complex might well enhance the probability that
process (1) contributes to the observed Na2 B±X
¯uorescence. However, Lam et al. [14] have con-
sidered the interaction between Na(3p) and
‡
Na2ꢀX1Rg ) and found the Na2A ! X band ¯uo-
rescence to dominate the B ! X band ¯uorescence
‡
‡
‡
for a Naꢀ3p† ‡ Na2ꢀX1Rg † collision process. Any
emission bands ± one singlet (21Ru ±X1Rg ), corre-
evidence for A±X ¯uorescence appears to be ab-
sent in Fig. 3. At lower sodium densities (ꢁ1013
sponding to the emision from the outer well of
‡
the double minimum 21Ru state, and one triplet

420
D.R. Grantier, J.L. Gole / Chemical Physics Letters 330 (2000) 417±422
[9,14,18]. One such mechanism, involving the as-
sociative ionization process is [18]
‡
Naꢀ3p† ‡ Naꢀ3p† ! Na2 ‡ e
‡
Na2 ‡ e ! NaÃ2ꢀ23Pg†
ꢀ3†
NaÃ2ꢀ23Pg† ! Na2Ãꢀ13Ru † ‡ hm
‡
The results which we have obtained suggest a di-
rect correlation between the available excited
NaÃ(3p) and the formation of the Na2 23Pg state.
Both argon and helium supersonic expansions will
dilute the formation of the 23Pg state through
energy pooling processes involving the Na(3p)
state which can be enhanced by the trapping of
resonance radiation associated with the
Na 3p ! 3s transition. The enhancement of these
processes at higher concentrations suggests that
mechanism (3) may be operative. Figs. 3 and 4
depict the chemiluminscence observed under the
highest bromine atom and sodium ¯uxes. Here,
one observes a signi®cant energy pooling involving
the sodium 3p state and a resulting ¯uorescence
from higher atomic sodium states ꢀn ˆ 4; 5; 6; 7†
by way of the process,
Fig. 3. Chemiluminescent spectrum observed under conditions
of highest reactant ¯ux. We observe clear evidence of energy
pooling in the Na(3p) state as substantial emission from higher
atomic sodium states ꢀn P 4† is present. We also note the ap-
‡
pearance of a weak 23Pg ! 13Ru emission band.
Naꢀ3p† ‡ Naꢀ3p† ! Naꢀns; nd† ‡ Na ꢀ3s† ‡ DE
ꢀ4†
‡
(primarily 23Pg±13Ru ). We have compared the
emission features which we observe in an ex-
panded view of Fig. 1 with the data and analysis of
Pichler et al. (cf. Footnote 1) and with the spectra
obtained by Allegrini et al. [17] generated in a heat
pipe oven by pumping the Na D-line transition.
We conclude that the features observed in Fig. 1 in
the 425±460 nm region, display an intensity dis-
tribution closely paralleling that obtained by
Pichler et al. [9], which suggests that they result
where DE represents the energy defect for the
pooling interaction indicated. We compare the
spectrum of Fig. 4 to the laser excitation spectrum
produced by Allegrini et al. [17] under cw laser
excitation in a heat pipe oven. Under seeded ex-
pansion conditions we observe substantial energy
pooling accompanied by the onset of the
‡
23Pg ! 13Ru emission feature. This feature is even
more pronounced under pure expansion condi-
tions where the Na(3p) population is not diluted
by argon or helium. The remarkable similarity
between the experimental spectra in Fig. 4 would
seem to lend strong support to reaction mecha-
nism (3) as the route for production of sodium
dimer in the 23Pg state.
‡
predominantly from the 23Pg±13Ru band system
(mixing with 33Pg is possible) with some lesser
‡
‡
contribution from the 21Ru ±X1Rg system. It is
clear that a bound-free emission from Na2 has
been excited in an exclusively reaction based en-
vironment.
However, while process (3) may correspond to
the primary mechanism for production of the 23Pg
state of Na2, another possible mechanism has been
postulated by Pichler et al. [9]. This mechanism
involves a rapid dissociative recombination pro-
cess following the reaction sequence:
3. Discussion
Several mechanisms have been postulated for
the formation of the Na2 23Pg excited state

D.R. Grantier, J.L. Gole / Chemical Physics Letters 330 (2000) 417±422
421
‡
‡
Na2ꢀA1Ru † ‡ Naꢀ3p† ! Na3 ‡ e
ꢀ9†
These authors were able to con®rm that the sodi-
um trimer ions are produced in associative ion-
ization collisions between sodium dimer and
sodium atoms when both colliding partners are in
their lowest respective electronically excited states.
‡
They also suggest that Na3 can respresent a
dominant ion, especially for higher atom/dimer
densities, due to its relatively low ionization po-
tential (IP Na ꢁ 5:14 eV; Na2 ꢁ 4:87 eV; Na3 ꢁ
3:97 eV) [23±26]. The substantial Na(3p) sodium
atom excited state population and the excitation of
‡
both the A1Ru and the B1Pu sodium dimer states
would at lower expansion ¯ux suggest that mech-
anism (9) can constitute a route for formation of
‡
the Na3 ion, consistent also with the data pre-
sented in Figs. 3 and 4. Therefore, subsequent re-
combination, as in process (5), seems to represent
an equally likely route for production of the 23Pg
state of the sodium dimer.
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Fig. 4. Comparison of: (a) laser excitation spectrum (from Ref.
[14] with (b) the blue emission (400±560 nm) from the experi-
‡
mental spectrum in Fig. 3. Note the onset of the 23Pg ! 13Ru
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‡
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‡
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‡
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‡
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‡
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‡
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